- Email: [email protected]
A compact SiC photovoltaic inverter with maximum power point tracking function
Solar Energy 141 (2017) 228–235
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
A compact SiC photovoltaic inverter with maximum power point tracking function Yuji Ando a, Takeo Oku a,⇑, Masashi Yasuda b, Yasuhiro Shirahata a, Kazufumi Ushijima c, Mikio Murozono d a
Department of Materials Science, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan Collaborative Research Center, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan c ArumoTech Corporation, 572 Sanboh-Nishinotohincho, Nakagyou-ku, Kyoto 604-8277, Japan d Clean Venture 21 Corporation, 38 Ishihara Douno-Ushirocho, Kissyouin, Minami-ku, Kyoto 601-8355, Japan b
a r t i c l e
i n f o
Article history: Received 21 September 2016 Received in revised form 18 November 2016 Accepted 23 November 2016
Keywords: Silicon carbide Solar cell Inverter Photovoltaic device Maximum power point tracking Lithium-ion battery
a b s t r a c t A compact 150 W photovoltaic inverter was developed using SiC devices, which integrated a maximum power point tracking charge controller and a direct current (DC) - alternating current (AC) converter into a single module. The DC-AC converter circuit was built with four SiC metal-oxidesemiconductor fieldeffect transistors, while the DC-DC converter circuit built with four SiC Schottky barrier diodes. An increase of the switching frequency led to the module of a reduced size (250 180 28 mm3), which is just one third volume of a commercial Si-based inverter available today. Besides being compact, the conversion efficiency of the DC-AC converter was approximately 3% higher than that of the commercial Si-based inverter. In addition, the MPPT controller showed a conversion efficiency exceeding 96%, which raised the total efficiency under practical operation conditions up to 86%. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Advantages of wide bandgap semiconductor materials Recent power device requirements include a high blocking voltage, a low ON resistance, a high switching frequency, and good reliability. These requirements have led to great interest in power devices based on wide bandgap semiconductors such as GaN and SiC (Labedev and Chelnokov, 1999; Monroy et al., 2003; Okumura, 2006). The main advantage of wide bandgap semiconductors is their very high electric field capability. Wide bandgap semiconductors possess high critical field strengths. This means that a thinner epi layer is required to block the same voltage compared with Si devices. Thus, switching devices with much lower ON resistances can be fabricated using GaN or SiC. A lower ON resistance improves the efficiency of inverters due to reduced conduction and switching losses, and also decreases the module size due Abbreviations: DC, direct current; AC, alternating current; MOSFET, metaloxidesemiconductor field-effect transistor; SBD, Schottky barrier diodes; MPPT, maximum power point tracking; PWM, pulse width modulation; JFET, junction field-effect transistor; BJT, bipolar junction transistor; IGBT, insulated gate bipolar transistor; HEMT, high electron mobility transistor. ⇑ Corresponding author. E-mail address: [email protected] (T. Oku). http://dx.doi.org/10.1016/j.solener.2016.11.041 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
to the increased power density. The high electron mobility of GaN allows switching operations with higher frequencies, which also decreases the module size because of the smaller passive components. The excellent thermal stability of SiC and GaN should enable devices based on these materials to operate at high temperatures. 1.2. Survey of SiC-based photovoltaic inverter development The first photovoltaic inverter using SiC diodes was reported by Frank and Bruno (2001), while that using SiC transistors was reported by Stalter et al. (2007). At present, SiC Schottky barrier diodes (SBDs), metal-oxide-semiconductor field-effect transistors (MOSFETs), junction field-effect transistors (JFETs), and bipolar junction transistors (BJTs) are available in the market. There have been considerable reports on the applications of SiC devices on power converters (Chinthavali et al., 2009; Yamane et al., 2013, and Li et al., 2013). Concerning direct current (DC)-DC converters, Hensel et al. (2011) and Ho et al. (2011) reported interleaved boost converters built with SiC BJTs and SiC SBDs, respectively. Concerning DC-alternating current (AC) converters single-phase inverters, using SiC JFETs and SiC MOSEFTs were reported by Kranzer et al. (2008) and by Burger et al. (2008), respectively. Three-phase inverters using SiC JFETs and SiC MOSFETs were reported by Stalter et al. (2010) and by Burger et al. (2009), respectively.
Y. Ando et al. / Solar Energy 141 (2017) 228–235
Recently, all SiC inverters were reported by De et al. (2013) and by Nashida et al. (2014). A comparative study of Si- and SiC-based power converters was given by some authors (Burkart and Kolar, 2013; Sintamarean et al., 2013; Ho et al., 2013). A comprehensive review on SiC-based photovoltaic inverters was given in Kim et al. (2013). In the literatures, most of power converters using SiC devices were focused on multi-kW class applications. This is because advantages of a lower ON-resistance of SiC devices were considered to be lost in sub-kW class inverters. In addition, most of developed power converters were examined as a stand-alone equipment. Concerning photovoltaic power applications, however, performances of total systems using these converters should be also discussed. 1.3. Aim of this article Previously, the authors reported a photovoltaic power storage system comprising a SiC-based DC-AC converter (Oku et al., 2015, 2016; Matsumoto et al., 2016). This SiC-based inverter was prepared by replacing Si power MOSFETs in a Si-based inverter by SiC MOSFETs, but the overall circuit remained optimized for Si MOSFETs. In this work, we have newly developed a photovoltaic inverter optimized for SiC devices. This photovoltaic inverter integrates a DC-AC converter and a maximum power point tracking (MPPT) controller (Lauria and Coppola, 2014) into a single module. The capacity of handling power was set at 150 W to enable portability. The aim of this study is to evaluate the feasibility of SiC devices for sub-kW class photovoltaic inverters. This sub-kW class photovoltaic inverter is available for the applications where compactness and efficiency are of tremendous importance, such as portable electronic devices, solar-powered cars, and emergency power supply systems. This study also aimed to compare the SiC- and Sibased inverters with respect to the performance of a total photovoltaic power generation system. The next section describes setup of the developed photovoltaic system and the power module. In Section 3, the total efficiency and the electric power stability are investigated for the photovoltaic power generation system including spherical Si solar cell panels (Oku et al., 2014), an MPPT controller, and a storage battery as well as the inverter. In Section 4, the efficiency and the electric power stability are compared between the SiC- and Si-based systems. 2. Experimental
229
input and output terminals of the inverters were monitored by power meters (Hioki, PW3336). The measurement interval was 200 ms and reported data are the average measurements over each minute. The temperature, humidity (Hioki, LR5001), and solar radiation power (Uizin, UIZ-PCM01-LR) were monitored simultaneously during measurements. Filament lamps were used as the load. Fig. 1(b) illustrates setup of a photovoltaic power storage system using a newly developed SiC-based inverter (SiC inverter 2). In this inverter, an MPPT controller and a SiC-based inverter were integrated into a single module. The MPPT circuit is driven by the input voltage regulation (Texas Instruments, 2016). The inverter circuit consisted of a front stage DC-DC converter followed by a second stage DC-AC converter. The DC-DC part was a push-pull converter where four SiC Schottky barrier diodes (SBDs) (Rohm, SCS210AJ) (ROHM Semiconductor, 2015b) were used as rectification diodes. The DC-AC part was a single-phase full-bridge inverter, where four SiC MOSFETs (Rohm, SCT2120AF) were used as switching transistors. The switching frequency of the pulse width modulation (PWM) signal was increased from 20 kHz for the conventional inverters to 100 kHz. The rated output power was set to 150 W. As the power source, spherical Si solar cell panels wired in parallel were also used unless otherwise stated. The current and voltage at the output terminals of the solar cell, the Li-ion battery, and the inverter were monitored synchronously by power meters (Hioki, PW3336). The measurement interval was 200 ms and reported data are the average measurements over each minute. The temperature, humidity (Hioki, LR5001) and solar radiation power (Uizin, UIZ-PCM01-LR) were monitored simultaneously during measurements. Also, filament lamps were used as the load. 2.2. SiC-based inverter module Fig. 2(a) and (b) are photographs of the developed photovoltaic power module (SiC inverter 2) and the DC-AC converter circuit with an MPPT controller, respectively. The module of a reduced size (1260 cm3) achieved just one third volume of the conventional Si-based inverter (SXCD-300: 2037 cm3, Tracer-2215BN: 1738 cm3). Weight of the module was just 1.25 kg. This small module size was resulted predominantly from the increase in the switching frequency. Besides, the weight of a spherical Si solar cell panel was 2.13 kg. The total weight including the inverter module, a single solar cell panel, and a 20 Ah battery was 5.96 kg.
2.1. Photovoltaic power storage systems
3. Results
Fig. 1(a) illustrates setup of conventional photovoltaic power storage systems that we reported in Oku et al. (2015, 2016) and in Matsumoto et al. (2016). There are two conventional systems capable of operating independently. One system employs a commercial Si-based DC-AC converter (Meltec, SXCD-300) denoted by ‘‘Si inverter”, while another using an in-house SiC-based DC-AC converter denoted by ‘‘SiC inverter 1”. The SiC inverter 1 was prepared replacing four Si MOSFETs (Fairchild, FQPF16N25C) (Fairchild semiconductor, 2013) in SXCD-300 by four SiC MOSFETs (Rohm, SCT2120AF) (ROHM Semiconductor, 2015a). Spherical Si solar cell panels (Clean venture 21, CVFM-0540T2-WH) wired in parallel were used as the power source. Principle features of these cell panels are lightness, flexibility, and economical efficiency. The maximum operating current and voltage for a single solar panel were 3.34 A and 16.2 V, respectively. The operating point was controlled to maximize the output power with the aid of an MPPT controller (EPsolar, Tracer-2215BN). The MPPT stabilized the electric power by charging and discharging a 12.8 V Li-ion battery (capacity: 20 Ah) (O’Cell, IFM12-200E2). The current and voltage at the
3.1. Conversion efficiencies for individual circuits First, we have characterized conversion efficiencies for the MPPT controller and the DC-AC converter of the developed SiC inverter, separately. The conversion efficiency of the MPPT controller was measured by connecting an alternative power supply to the photovoltaic DC terminal while making the inverter circuit inactive. Fig. 3(a) presents an MPPT efficiency measured with respect to DC supplied power. The MPPT efficiency exceeded 95% when the photovoltaic power ranging between 15 and 65 W, and the peak efficiency reached 96.4%. The conversion efficiency of the DC-AC converter was measured under a battery operation, while changing the load power. This eliminated influence of the MPPT controller. Fig. 3(b) presents efficiencies measured with respect to AC output power. The inverter efficiency exceeded 85% when the output power ranging between 60 and 150 W, and the peak efficiency reached 86.8%. The current and voltage at a connecting node between the DC-DC and DC-AC circuits were monitored by means of a shunt resistor connected
230
Y. Ando et al. / Solar Energy 141 (2017) 228–235
Si inverter or SiC inverter 1 Solar cell (spherical Si)
DC-AC Inverter (Si or SiC device)
MPPT controller
Load
(a) Power meter
Li-ion battery
180 28 mm3)
SiC inverter 2 (250 Solar cell (spherical Si)
DC-AC Inverter (SiC device)
MPPT controller
PC
Load
(b) Power meter
Li-ion battery
PC
Fig. 1. Schematics of the photovoltaic power storage systems using (a) conventional inverters (SiC inverter 1 and Si inverter) and (b) developed inverter (SiC inverter 2).
(a)
(b) USB
Inter-stage monitor
180 mm 28 mm 250 mm
PV input
Battery input
AC output
Fig. 2. Photographs of (a) the developed photovoltaic power module and (b) the DC-AC converter circuit with an MPPT controller (SiC inverter 2).
(b) 100
Inverter efficiency (%)
MPPT efficiency (%)
(a) SiC inverter 2 95
90
85 0
20
40
60
80
100
120
PV output power (W)
100 80
SiC inverter 2 DC-DC DC-AC TOTAL
60 40 20 0
20
40
60
80 100 120 140 160
AC output power (W)
Fig. 3. (a) Efficiency of the MPPT controller part versus DC supplied power and (b) DC-AC conversion efficiency of the inverter part versus AC output power (SiC inverter 2).
between them. This enabled us to separate the measured efficiency into two components originating from the DC-DC and DC-AC circuits. As shown in Fig. 3(b), peak efficiencies exceeded 87% and
98% for the DC-DC and DC-AC parts, respectively. This indicates power losses caused by the DC-DC converter to be a main factor to determine the inverter efficiency.
231
Y. Ando et al. / Solar Energy 141 (2017) 228–235
3.2. Total efficiency of SiC-based inverter module
100
Pac : P pv þ Pbat
gtot ¼
Efficiency (%)
To study a substantial conversion efficiency of photovoltaic inverters, the total efficiency was introduced by Haeberlin et al. (2005). Modifying this approach, the total efficiency (gtot) of the present photovoltaic inverter module was calculated by
ð1Þ
In this expression, Ppv, Pbat, and Pac are a DC power supplied from the solar cell, that from the battery, and an AC power supplied to the load, respectively. Also, the efficiency of MPPT controller (gMPPT) was expressed by
gMPPT
PMPPT ðPpv Þ ¼ : P pv
ð2Þ
In this expression, PMPPT is a DC output power of the MPPT controller, which can be determined with respect to Ppv based on the relationship shown in Fig. 3(a). Then, the efficiency of the DC-AC converter (ginv) was calculated by
ginv ¼
P ac : PMPPT ðPpv Þ þ Pbat
ð3Þ
To evaluate the output power dependence of the total efficiency, we have carried out the step load measurement of the present SiC inverter. In this measurement scheme, the load power was increased stepwise from 5 to 144 W, and held at each step for 5 min. As the power source, spherical Si solar cell panels were connected to the photovoltaic DC terminal. Fig. 4(a) and (b) present variations in measured power at the three terminals (Ppv, Pbat, and Pac) and the evaluated efficiencies (gtot, gMPPT, and ginv), respectively. Presence of MPPT controller losses reduced gtot by several % as compared to ginv. However, difference between gtot and ginv became smaller at a high output power. This result was explained as follows. As Pac exceeded 54 W, contribution of Ppv was decreased and that of Pbat was increased. Since Pbat was supplied from the battery without interposing the MPPT controller, this led to a decrease in the MPPT losses, and hence, gtot approached to ginv at a high output power. Fig. 5 plots gtot, gMPPT, and ginv with respect to Pac (lines), which were obtained from the step load measurements. The total efficiency exceeded 85% when the AC output power ranging between 74 and 144 W, and the peak gtot reached 86.5%. To investigate the efficiencies under more realistic operation conditions, we have also carried out the steady state measurement. In this measurement scheme, the load power was kept constant, and the DC-AC conversion efficiency was determined by time average for 1 or 2-h duration measurements after the initial variation. In Fig. 6, also
160 140
Power (W)
120 100
Ppv
80
Pbat
60
Pac
40 20
Total (step load) Total (steady state) MPPT (step load) MPPT (steady state) Inverter (step load) Inverter (steady state)
40
0
40
80
160
Fig. 5. Comparison of efficiencies measured under step load and steady state measurements (SiC inverter 2).
plotted are results of the steady state measurements (symbols). Two different measurement schemes showed a satisfactory agreement. This indicated the initial variation was not so important in this SiC-based inverter.
3.3. Electric power stability of photovoltaic systems To investigate the electric power stability, we have performed continuous operation measurements with the load power kept constant for the present SiC inverter. The measurement time was chosen between 2 and 8 h, which depended on the output power. Fig. 6(a), (b), (c), and (d) show an example of variations in the voltage, current, power and efficiency values, respectively. The load power was set at 30 W. In these figures, subscripts pv, bat, and ac to the voltage U and current I mean the photovoltaic terminal, battery terminal, and AC terminal, respectively. Fig. 7 (a) and (b) show corresponding variations in solar radiation power and those of temperature and humidity, respectively. Before 14:30, a great majority of the electricity was supplied from the solar cell, but a small amount of it was supplied from the battery. In accordance with a drop of the solar radiation power after 14:30, the current (Ipv) and power (Ppv) supplied from the solar cell steeply deceased, while the current (Ibat) and power (Pbat) from the battery increasing. Voltages of the solar cell (Upv) and the battery (Ubat) were, however, kept almost constant. Concerning the AC output electricity, no fluctuation was seen in the voltage (Uac), current (Iac), or power (Pac). A gradual increase in gtot after 14:30 was resulted from the increased contribution of Pbat (or the decreased contribution of Ppv), since Pbat was supplied from the battery without interposing the MPPT controller, as described above.
100
80 SiC inverter 2 Step load Δt = 5 min
60
Total
40
MPPT
0 -20
120
AC output power (W)
(b) SiC inverter 2 Step load Δt = 5 min
SiC inverter 2
60
20
Efficiency (%)
(a)
80
Inverter
0
10
20
30
Time (min)
40
20
0
10
20
30
40
Time (min)
Fig. 4. Variation in (a) power at solar cell, battery, and AC terminals and (b) efficiencies of MPPT controller, DC-AC converter, and total module. The load power was increased stepwise from 5 to 144 W, and held at each step for 5 min (SiC inverter 2).
Y. Ando et al. / Solar Energy 141 (2017) 228–235
16
160
12
120
8
80
Pload = 30 W
12
(b)
200
Release load
Apply load
1.2 Pload = 30 W Ipv Ibat Iac
9 6
0.9 0.6
Release load
Apply load
3
0.3
0
0
Iac (A)
20
Ipv, Ibat (A)
Upv, Ubat (V)
(a)
Uac V
232
Upv
4
40
Ubat Uac
0 9:00
11:00
13:00
-3 9:00
0 17:00
15:00
11:00
100
Pbat Pac Apply load
Release load
25 0 -25 9:00
11:00
13:00
-0.3 17:00
15:00
Apply load
Release load
90
Efficiency (%)
Power (W)
(d)
Ppv
Pload = 30 W
75 50
15:00
Time
Time
(c) 100
13:00
80 70 Pload = 30 W Total MPPT Inverter
60 50 9:00
17:00
11:00
13:00
15:00
17:00
Time
Time
Fig. 6. Variation in (a) voltage, (b) current, and (c) power respectively at solar cell, battery, and AC terminals. (d) Variation in efficiencies of MPPT controller, DC-AC converter, and total module. The load power was set at 30 W (SiC inverter 2).
(b)
1
100
50 Apply load
Apply load
Release load
0.8 0.6 0.4 0.2 0 9:00
11:00
13:00
15:00
17:00
Time
Release load
40
80
30
60
20
10 9:00
40
Temp. Humid. 11:00
13:00
15:00
Humidity (%)
1.2
Temperature (ഒ)
Solar radiation power (kWm-2)
(a)
20 17:00
Time
Fig. 7. Variation in (a) solar radiation power and (b) temperature and humidity, during the measurements shown in Fig. 6.
Fig. 8(a), (b), (c) and (d) show another example of variation in the voltage, current, power, and efficiency values, respectively. The load power was set at 74 W. In this case, a considerable part of the electricity was supplied from the battery throughout the whole measurement, since the photovoltaic power supplied from the solar cell (around 40 W) was insufficient to bear the load power. In this case, gtot was as high as 85% and was almost constant during the measurement.
4. Discussions 4.1. Comparison with conventional inverters Fig. 9 compares DC-AC conversion efficiencies in the present and conventional inverters. The peak efficiency of the developed
SiC inverter was 86.8%. This value was comparable to the conventional SiC inverter (86.5%), while being 3% higher than the Si inverter (83.7%). We are currently engaged in analyzing power losses in the present inverter. A preliminary study indicated a smaller switching loss of SiC MOSFETs and a smaller reverse recovery loss of their body diodes were responsible for the 3% higher efficiency of the SiC inverters. It is worthy to note that the SiC inverter 2 exhibited an efficiency comparable to the SiC inverter 1, even though the increased switching frequency should cause the increase of the switching losses. The power loss analysis also indicated a reduction of the DC-DC converter losses led to the high efficiency for the increased switching frequency of SiC inverter 2. The stability characteristics were compared between the SiC and Si inverters. Concerning the output power, these inverters equally showed a reasonable stability. However, with regard to the DC-AC conversion efficiency, the SiC inverters exhibited more
233
Y. Ando et al. / Solar Energy 141 (2017) 228–235
16
160
12
120
8
80
Pload = 74 W
12 9
1.2
0.6
3
0.3
0
40
Ubat
Pload = 74 W
Uac
0 9:00
10:00
11:00
-3 9:00
0 13:00
12:00
10:00
Apply load
Release load
(d) Efficiency (%)
75
Power (W)
50 25 Pload = 74 W
0
Ppv Pac
10:00
11:00
100
Apply load
12:00
0 -0.3 13:00
Release load
90 80 70
Pload = 74 W Total MPPT Inverter
60
Pbat
-25 -50 9:00
11:00
Ipv Ibat Iac
Time
Time 100
0.9
6
Upv
4
(c)
Release load
Apply load
Iac (A)
Upv, Ubat (V)
(b)
200 Release load
Apply load
Ipv, Ibat (A)
20
Uac V
(a)
12:00
13:00
50 9:00
10:00
11:00
12:00
13:00
Time
Time
Fig. 8. Variation in (a) voltage, (b) current, and (c) power respectively at solar cell, battery, and AC terminals. (d) Variation in efficiencies of MPPT controller, DC-AC converter, and total module. The load power was set at 74 W (SiC inverter 2).
Inverter efficiency (%)
90
80
70 SiC inverter 2 SiC inverter 1 Si inverter
60
50
0
40
80
120
160
200
AC output power (W) Fig. 9. Comparison of DC-AC conversion efficiencies for the developed and conventional inverters.
stable characteristics. Fig. 10(a) and (b) compare the stability in the DC-AC conversion efficiency for the Si inverter and the SiC inverter 2. In the Si inverter, a gradual decrease of the efficiency was observed for the load power of 90 and 120 W, while it was not observed for the load power of 54 W or less. Besides, this kind of efficiency degradation was less significant in the SiC inverters. Therefore, the gradual decrease in the efficiency would be due to the self-heating effect of Si MOSFETs. 4.2. Influence of defects in SiC devices Poor reliability resulted from defects in the SiC epitaxial wafer and in the gate oxide film has been one of the critical issues of SiC MOSFETs. Quality of the gate oxide film directly affects reliability of SiC MOSFETs. The charge to breakdown QBD determined by the time dependent dielectric breakdown test is a quality indicator of the gate oxide. Recently, quality of oxide on SiC has been
improved by optimizing the growth process. The QBD value of the SiC MOSFETs used in this study is 15–20 C/cm2 that is equivalent to that of Si MOSFETs (ROHM semiconductor, 2014). Even with a high quality gate oxide layer, there still remains crystal defects in the epitaxial wafer that may cause initial failures. Several authors reported correlation between the breakdown voltage and density of screw dislocations (Wahab et al., 1999; Fukuda et al., 2010) or micropipes (Dmitriev and Spencer, 1998). Also, the firstprinciples calculation verified that point defects such as silicon and carbon substitutionals and oxygen interstitials significantly degrade the channel electron mobility (Iskandarova et al., 2015). These effects may increase the conduction loss of SiC MOSFETs, and hence, affect the conversion efficiency. To identify and exclude such defective devices, we have introduced a screening DC test before mounting devices in the circuit.
4.3. Advantages over other power devices While SiC MOSFETs were used to replace 250 V planar MOSFETs in the photovoltaic inverter, we still have the option of choosing another device, too. The alternatives are advanced Si MOSFETs, Si insulated-gate bipolar transistors (IGBTs), and GaN high electron mobility transistors (HEMTs), and so on. Properties of these power devices are summarized in Table 1. The advanced Si MOSFETs include trench MOSFETs and super-junction MOSFETs. These advanced MOSFETs have lower ON resistances as compared to planar MOSFETs. Si IGBTs have very high breakdown voltages (600– 6000 V) and low ON-resistances. An existing issue of IGBTs is the collector current tailing due to accumulation of minority carriers and the consequent limited switching frequency. GaN and SiC devices have very high breakdown voltages, low ON resistances, small reverse recovery losses, and a negligible tail current effect. An unsolved problem of GaN HEMTs is difficulty in normally-OFF
234
Y. Ando et al. / Solar Energy 141 (2017) 228–235
(a)
(b) 90
95
54 W 90 W 120 W
Inverter efficiency (%)
Inverter efficiency (%)
Si inverter
85
80
54 W 90 W 120 W 90
85
SiC inverter 2 75
80 0
1
2
3
4
5
0
1
2
3
4
5
Time (h)
Time (h)
Fig. 10. Variation in the inverter efficiency for (a) the conventional Si inverter and (b) the developed SiC inverter 2.
Table 1 Comparison of power devices.
Si planar MOSFET Si advanced MOSFET Si IGBT SiC MOSFET GaN HEMT
Breakdown voltage
ON resistance
Switching frequency
Normally OFF
Cost
Good Good Excellent Excellent Excellent
Good Excellent Excellent Excellent Excellent
Good Good Fair Excellent Excellent
Easy Easy Easy Easy Difficult
Excellent Good Good Fair Fair
operation. Totally, SiC has the most attractive features as compared to other options. However, the main application of SiC power devices is multikW level converters. It is considered that a 100-W class converter cannot fully benefit from the low ON resistance of SiC devices. Therefore, SiC MOSFETs are commercially available only for 600 V voltage or more to date, and the SiC MOSFETs employed here own a drain-source breakdown voltage of 650 V. This ‘‘wastefully” high breakdown voltage of the SiC MOSFETs should be disadvantageous for reducing the ON resistance as compared to 250 V Si MOSFETs. In spite of this, the developed SiC inverter exhibited an efficiency superior to that of the conventional inverters. As described above, reduction of switching and reverse recovery losses of SiC devices would be responsible for this improvement. While we have made some progress in this first trial, there still remain some issues that warrant more attention. In this work, four identical MOSFETs were used to build the inverter. However, the switching loss is much important for high-side transistors, while the conduction loss is much important for low-side transistors. Therefore, careful consideration should be given to optimize the combination of low-side and high-side devices. Passive components and circuits should be also optimized to improve the efficiency. In addition, examination of other power devices (Si trench MOSFETs, Si super-junction MOSFETs, GaN HEMTs, and so on) may be tried out. 5. Conclusions We have developed a compact 150 W photovoltaic inverter, where an MPPT controller and a DC-AC converter were integrated into a single module. The DC-AC converter circuit was built with four SiC MOSFETs, while the DC-DC converter circuit built with four SiC SBDs. An increase in the PWM switching frequency up to 100 kHz resulted in the module of a reduced size (250 180 28 mm3), which was just one third volume of a commercial Si-based inverter available today. First, conversion efficiencies for the MPPT controller
and the DC-AC converter were characterized separately. The DC-AC converter exhibited the peak efficiency ginv of 86.8%, while the MPPT controller showing the peak efficiency gMPPT of 96.4%. The ginv value was approximately 3% higher than that of the conventional Si-based inverter. The power loss analysis indicated a smaller switching loss of SiC MOSFETs and a smaller reverse recovery loss of their body diodes were responsible for this improvement. Next, gtot under practical operation conditions were estimated with spherical Si cell panels connected as the power source. Although gtot values depended on the contribution of Ppv and Pbat to the total input power, a measured peak gtot reached 86.5%. We expect the sub-kW class SiC-based inverters would provide an optimal solution for self-sustaining energy systems used for portable electronic devices, solar-powered vehicles, emergency power supply, and so on. Acknowledgements This work was supported by Super Cluster Program of Japan Science and Technology Agency (JST). References Burger, B., Kranzer, D., Stalter, O., 2008. Cost reduction of PV-inverters with SiCDMOSFETs. In: 5th Int. Conf. Integrated Power Systems (CIPS 2008), pp. 1–5. Burger, B., Kranzer, D., Stalter, O., 2009. Efficiency improvement of PV inverters with SiC-DMOSFETs. Mater. Sci. Forum 600–603, 1231–1234. Burkart, R.M., Kolar, J.W., 2013. Comparative evaluation of SiC and Si PV inverter systems based on power density and efficiency as indicators of initial cost and operating revenue. In: 2013 IEEE 14th Workshop on Control and Modeling for Power Electronics (COMPEL) (Salt Lake City, UT, 23–26 June 2013), pp. 1–6. Chinthavali, M., Zhang, H., Tolbert, L.M., Ozpineci, B., 2009. Update on SiC-based inverter technology. In: Electronics Conf. (COBEP ’09), pp. 71–79. De, D., Castellazzi, A., Solomon, A., Trentin, A., Minami, M., Hikihara, T., 2013. An all SiC MOSFET high performance PV converter cell. In: 15th Euro. Conf. Power Electronics and Applications (EPE 2013), pp. 1–10. Dmitriev, V.A., Spencer, M.G., 1998. SiC fabrication technology: growth and doping. In: Park, Y.S. (Ed.), SiC Materials and Devices (Semiconductors and Semimetals 52). Academic Press, San Diego, pp. 21–76. Fairchild Semiconductor, 2013. Datasheet of FQPF16N25C – N-Channel QFET MOSFET.
Y. Ando et al. / Solar Energy 141 (2017) 228–235 Frank, P., Bruno, B., 2001. A new high voltage Schottky diode based on siliconcarbide (SiC). In: 9th Euro. Conf. Power Electron. Appl. (EPE 2001), p. 00259. Fukuda, F., Kinoshita, A., Ohyanagi, T., Kosugi, R., Sakata, T., Sakuma, Y., Senzaki, J., Minami, A., Shimozato, A., Suzuki, T., Hatakeyama, T., Shinohe, T., Matsuhata, H., Yamaguchi, H., Nagai, H., Harada, S., Ichinoseki, K., Yatsuo, T., Okumura, H., Arai, K., 2010. Influence of processing and of material defects on the electrical characteristics of SiC-SBDs and SiC-MOSFETs. Mater. Sci. Forum 645–648, 655– 660. Haeberlin, H., Borgan, L., Kaempfer, M., Zwahlen, U., 2005. Total efficiency gtot – a new quantity for better characterization of grid-connected PV inverters. In: 20th European Photovoltaic Solar Cell Energy Conf., pp. 1–4. Hensel, A., Wilhelm, C., Kranzer, D., 2011. Development of a boost converter for PV systems based on SiC BJTs. In: Proc. 2011–14th European Conf. Power Electronics and Applications (EPE 2011), pp. 1–7. Ho, C.N.M., Breuninger, H., Pettersson, S., Escobar, G., Coccia, A., Oikonomous, N., 2011. Practical implementation of an interleaved boost converter using SiC diodes for PV applications. In: 8th Int. Conf. Power Electron. (ECCE Asia), pp. 372–379. Ho, C.N.-M., Breuninger, H., Pettersson, S., Escobar, G., Canales, F., 2013. A comparative performance study of an interleaved boost converter using commercial Si and SiC diodes for PV applications. IEEE Trans. Power Electron. 28, 289–299. Iskandarova, I., Khromov, K., Knizhnik, A., Potapkin, B., 2015. The role of neutral point defects in carrier mobility degradation in bulk 4H-SiC and at 4H-SiC/SiO2 interface: first-principles investigation using Green’s functions. J. Appl. Phys. 117. 175703-1-7. Kim, T., Jang, M., Agelidis, V.G., 2013. Current status of silicon carbide power devices and their application in photovoltaic converters. In: IEEE ECCE Asia Downunder (ECCE Asia) (Melbourne, Australia, 3–6 June 2013), pp. 555–559. Kranzer, D., Wilhelm, C., Reiners, F., Burger, B., 2008. Application of normally-off SiC-JFETs in photovoltaic inverters. In: 13th Euro. Conf. Power Electron. Appl., pp. 1–6. Labedev, A.A., Chelnokov, V.E., 1999. Wide-gap semiconductors for high-power electronics. Semiconductors 33, 999–1001. Lauria, D., Coppola, M., 2014. Design and control of an advanced PV inverter. Sol. Energy 110, 533–542. Li, L., Li, C., Cao, Y., Wang, F., 2013. Recent progress of SiC power devices and applications. IEEJ Trans. Electric. Electron. Eng. 8, 515–521. Matsumoto, T., Oku, T., Hiramatsu, K., Yasuda, M., Shirahata, Y., Shimono, A., Takeda, Y., Murozono, M., 2016. Evaluation of photovoltaic power generation system using spherical silicon solar cells and SiC–FET inverter. AIP Conf. Proc. 1709. 020023-1-6.
235
Monroy, E., Omnès, F., Calle, F., 2003. Wide-bandgap semiconductor ultraviolet photodetectors. Semicond. Sci. Technol. 18, R33–R51. Nashida, N., Hinata, Y., Horio, M., Yamada, R., Ikeda, Y., 2014. All-SiC power module for photovoltaic power conditioner system. In: IEEE 26th Int. Symp. Power Semiconductor Devices & IC’s (ISPSD 2014), pp. 342–345. Oku, T., Kanayama, M., Ono, Y., Akiyama, T., Kanamori, Y., Murozono, M., 2014. Microstructures, optical and photoelectric conversion properties of spherical silicon solar cells with anti-reflection SnOx: F thin films. Jpn. J. Appl. Phys. 53. 05FJ03-1-7. Oku, T., Matsumoto, T., Hiramatsu, K., Yasuda, M., Shimono, A., Takeda, Y., Murozono, M., 2015. Construction and characterization of spherical Si solar cells combined with SiC electric power inverter. AIP Conf. Proc. 1649, 79–83. Oku, T., Matsumoto, T., Hiramatsu, K., Yasuda, M., Ohishi, Y., Shimono, A., Takeda, Y., Murozono, M., 2016. Construction and evaluation of photovoltaic power generation and power storage system using SiC field-effect transistor inverter. AIP Conf. Proc. 1709. 020024-1-10. Okumura, H., 2006. Present status and future prospect of widegap semiconductor high-power devices. Jpn. J. Appl. Phys. 45, 7565–7586. ROHM Semiconductor, 2014. SiC Power Devices and Modules – Application Note. ROHM Semiconductor, 2015a. Datasheet of SCT2120AF – N-Channel SiC Power MOSFET. ROHM Semiconductor, 2015b. Datasheet of SCS210AJ – SiC Schottky Barrier Diode. Sintamarean, C., Blaabjerg, F., Wang, H., 2013. Comprehensive evaluation on efficiency and thermal loading of associated Si and SiC based PV inverter applications. In: IECON 2013–39th Annual Conf. IEEE Industrial Electron. Society (Vienna, Austria, 10–13 November 2013), pp. 555–560. Stalter, O., Burger, B., Lehrmann, S., 2007. Silicon carbide (SiC) D-MOS for gridfeeding solar inverters. In: 12th Euro. Conf. Power Electron. Appl. (EPE 2007), pp. 1–10. Stalter, O., Kranzer, D., Rogalla, S., Burger, B., 2010. Advanced solar power electronics. In: Proc. 22nd Int. Symp. Power Semicon. Devices & ICs (Hiroshima, Japan, 2010), pp. 3–10. Texas Instruments, 2016. Datasheet of BP24650 – Synchronous Switching-Mode Battery Charge Controller for Solar Power with Maximum Power Point Tracking. Wahab, Q., Ellison, A., Hallin, C., Henry, A., Janzen, E., 1999. Study of breakdown limiting defects in 4H-SiC substrates and epilayers on the performance of high voltage Schottky diodes. In: Abstracts of the Int. Conf. on Silicon Carbide and related Materials (NC, October 1999), p. 298. Yamane, A., Koyanagi, K., Kozako, M., Fuji, K., Hikita, M., 2013. Fabrication and evaluation of SiC inverter using SiC-MOSFET. In: IEEE 10th Int. Conf. Power Electronics and Drive Systems (PEDS 2013), pp. 1029–1032.
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
A compact SiC photovoltaic inverter with maximum power point tracking function Yuji Ando a, Takeo Oku a,⇑, Masashi Yasuda b, Yasuhiro Shirahata a, Kazufumi Ushijima c, Mikio Murozono d a
Department of Materials Science, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan Collaborative Research Center, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan c ArumoTech Corporation, 572 Sanboh-Nishinotohincho, Nakagyou-ku, Kyoto 604-8277, Japan d Clean Venture 21 Corporation, 38 Ishihara Douno-Ushirocho, Kissyouin, Minami-ku, Kyoto 601-8355, Japan b
a r t i c l e
i n f o
Article history: Received 21 September 2016 Received in revised form 18 November 2016 Accepted 23 November 2016
Keywords: Silicon carbide Solar cell Inverter Photovoltaic device Maximum power point tracking Lithium-ion battery
a b s t r a c t A compact 150 W photovoltaic inverter was developed using SiC devices, which integrated a maximum power point tracking charge controller and a direct current (DC) - alternating current (AC) converter into a single module. The DC-AC converter circuit was built with four SiC metal-oxidesemiconductor fieldeffect transistors, while the DC-DC converter circuit built with four SiC Schottky barrier diodes. An increase of the switching frequency led to the module of a reduced size (250 180 28 mm3), which is just one third volume of a commercial Si-based inverter available today. Besides being compact, the conversion efficiency of the DC-AC converter was approximately 3% higher than that of the commercial Si-based inverter. In addition, the MPPT controller showed a conversion efficiency exceeding 96%, which raised the total efficiency under practical operation conditions up to 86%. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Advantages of wide bandgap semiconductor materials Recent power device requirements include a high blocking voltage, a low ON resistance, a high switching frequency, and good reliability. These requirements have led to great interest in power devices based on wide bandgap semiconductors such as GaN and SiC (Labedev and Chelnokov, 1999; Monroy et al., 2003; Okumura, 2006). The main advantage of wide bandgap semiconductors is their very high electric field capability. Wide bandgap semiconductors possess high critical field strengths. This means that a thinner epi layer is required to block the same voltage compared with Si devices. Thus, switching devices with much lower ON resistances can be fabricated using GaN or SiC. A lower ON resistance improves the efficiency of inverters due to reduced conduction and switching losses, and also decreases the module size due Abbreviations: DC, direct current; AC, alternating current; MOSFET, metaloxidesemiconductor field-effect transistor; SBD, Schottky barrier diodes; MPPT, maximum power point tracking; PWM, pulse width modulation; JFET, junction field-effect transistor; BJT, bipolar junction transistor; IGBT, insulated gate bipolar transistor; HEMT, high electron mobility transistor. ⇑ Corresponding author. E-mail address: [email protected] (T. Oku). http://dx.doi.org/10.1016/j.solener.2016.11.041 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
to the increased power density. The high electron mobility of GaN allows switching operations with higher frequencies, which also decreases the module size because of the smaller passive components. The excellent thermal stability of SiC and GaN should enable devices based on these materials to operate at high temperatures. 1.2. Survey of SiC-based photovoltaic inverter development The first photovoltaic inverter using SiC diodes was reported by Frank and Bruno (2001), while that using SiC transistors was reported by Stalter et al. (2007). At present, SiC Schottky barrier diodes (SBDs), metal-oxide-semiconductor field-effect transistors (MOSFETs), junction field-effect transistors (JFETs), and bipolar junction transistors (BJTs) are available in the market. There have been considerable reports on the applications of SiC devices on power converters (Chinthavali et al., 2009; Yamane et al., 2013, and Li et al., 2013). Concerning direct current (DC)-DC converters, Hensel et al. (2011) and Ho et al. (2011) reported interleaved boost converters built with SiC BJTs and SiC SBDs, respectively. Concerning DC-alternating current (AC) converters single-phase inverters, using SiC JFETs and SiC MOSEFTs were reported by Kranzer et al. (2008) and by Burger et al. (2008), respectively. Three-phase inverters using SiC JFETs and SiC MOSFETs were reported by Stalter et al. (2010) and by Burger et al. (2009), respectively.
Y. Ando et al. / Solar Energy 141 (2017) 228–235
Recently, all SiC inverters were reported by De et al. (2013) and by Nashida et al. (2014). A comparative study of Si- and SiC-based power converters was given by some authors (Burkart and Kolar, 2013; Sintamarean et al., 2013; Ho et al., 2013). A comprehensive review on SiC-based photovoltaic inverters was given in Kim et al. (2013). In the literatures, most of power converters using SiC devices were focused on multi-kW class applications. This is because advantages of a lower ON-resistance of SiC devices were considered to be lost in sub-kW class inverters. In addition, most of developed power converters were examined as a stand-alone equipment. Concerning photovoltaic power applications, however, performances of total systems using these converters should be also discussed. 1.3. Aim of this article Previously, the authors reported a photovoltaic power storage system comprising a SiC-based DC-AC converter (Oku et al., 2015, 2016; Matsumoto et al., 2016). This SiC-based inverter was prepared by replacing Si power MOSFETs in a Si-based inverter by SiC MOSFETs, but the overall circuit remained optimized for Si MOSFETs. In this work, we have newly developed a photovoltaic inverter optimized for SiC devices. This photovoltaic inverter integrates a DC-AC converter and a maximum power point tracking (MPPT) controller (Lauria and Coppola, 2014) into a single module. The capacity of handling power was set at 150 W to enable portability. The aim of this study is to evaluate the feasibility of SiC devices for sub-kW class photovoltaic inverters. This sub-kW class photovoltaic inverter is available for the applications where compactness and efficiency are of tremendous importance, such as portable electronic devices, solar-powered cars, and emergency power supply systems. This study also aimed to compare the SiC- and Sibased inverters with respect to the performance of a total photovoltaic power generation system. The next section describes setup of the developed photovoltaic system and the power module. In Section 3, the total efficiency and the electric power stability are investigated for the photovoltaic power generation system including spherical Si solar cell panels (Oku et al., 2014), an MPPT controller, and a storage battery as well as the inverter. In Section 4, the efficiency and the electric power stability are compared between the SiC- and Si-based systems. 2. Experimental
229
input and output terminals of the inverters were monitored by power meters (Hioki, PW3336). The measurement interval was 200 ms and reported data are the average measurements over each minute. The temperature, humidity (Hioki, LR5001), and solar radiation power (Uizin, UIZ-PCM01-LR) were monitored simultaneously during measurements. Filament lamps were used as the load. Fig. 1(b) illustrates setup of a photovoltaic power storage system using a newly developed SiC-based inverter (SiC inverter 2). In this inverter, an MPPT controller and a SiC-based inverter were integrated into a single module. The MPPT circuit is driven by the input voltage regulation (Texas Instruments, 2016). The inverter circuit consisted of a front stage DC-DC converter followed by a second stage DC-AC converter. The DC-DC part was a push-pull converter where four SiC Schottky barrier diodes (SBDs) (Rohm, SCS210AJ) (ROHM Semiconductor, 2015b) were used as rectification diodes. The DC-AC part was a single-phase full-bridge inverter, where four SiC MOSFETs (Rohm, SCT2120AF) were used as switching transistors. The switching frequency of the pulse width modulation (PWM) signal was increased from 20 kHz for the conventional inverters to 100 kHz. The rated output power was set to 150 W. As the power source, spherical Si solar cell panels wired in parallel were also used unless otherwise stated. The current and voltage at the output terminals of the solar cell, the Li-ion battery, and the inverter were monitored synchronously by power meters (Hioki, PW3336). The measurement interval was 200 ms and reported data are the average measurements over each minute. The temperature, humidity (Hioki, LR5001) and solar radiation power (Uizin, UIZ-PCM01-LR) were monitored simultaneously during measurements. Also, filament lamps were used as the load. 2.2. SiC-based inverter module Fig. 2(a) and (b) are photographs of the developed photovoltaic power module (SiC inverter 2) and the DC-AC converter circuit with an MPPT controller, respectively. The module of a reduced size (1260 cm3) achieved just one third volume of the conventional Si-based inverter (SXCD-300: 2037 cm3, Tracer-2215BN: 1738 cm3). Weight of the module was just 1.25 kg. This small module size was resulted predominantly from the increase in the switching frequency. Besides, the weight of a spherical Si solar cell panel was 2.13 kg. The total weight including the inverter module, a single solar cell panel, and a 20 Ah battery was 5.96 kg.
2.1. Photovoltaic power storage systems
3. Results
Fig. 1(a) illustrates setup of conventional photovoltaic power storage systems that we reported in Oku et al. (2015, 2016) and in Matsumoto et al. (2016). There are two conventional systems capable of operating independently. One system employs a commercial Si-based DC-AC converter (Meltec, SXCD-300) denoted by ‘‘Si inverter”, while another using an in-house SiC-based DC-AC converter denoted by ‘‘SiC inverter 1”. The SiC inverter 1 was prepared replacing four Si MOSFETs (Fairchild, FQPF16N25C) (Fairchild semiconductor, 2013) in SXCD-300 by four SiC MOSFETs (Rohm, SCT2120AF) (ROHM Semiconductor, 2015a). Spherical Si solar cell panels (Clean venture 21, CVFM-0540T2-WH) wired in parallel were used as the power source. Principle features of these cell panels are lightness, flexibility, and economical efficiency. The maximum operating current and voltage for a single solar panel were 3.34 A and 16.2 V, respectively. The operating point was controlled to maximize the output power with the aid of an MPPT controller (EPsolar, Tracer-2215BN). The MPPT stabilized the electric power by charging and discharging a 12.8 V Li-ion battery (capacity: 20 Ah) (O’Cell, IFM12-200E2). The current and voltage at the
3.1. Conversion efficiencies for individual circuits First, we have characterized conversion efficiencies for the MPPT controller and the DC-AC converter of the developed SiC inverter, separately. The conversion efficiency of the MPPT controller was measured by connecting an alternative power supply to the photovoltaic DC terminal while making the inverter circuit inactive. Fig. 3(a) presents an MPPT efficiency measured with respect to DC supplied power. The MPPT efficiency exceeded 95% when the photovoltaic power ranging between 15 and 65 W, and the peak efficiency reached 96.4%. The conversion efficiency of the DC-AC converter was measured under a battery operation, while changing the load power. This eliminated influence of the MPPT controller. Fig. 3(b) presents efficiencies measured with respect to AC output power. The inverter efficiency exceeded 85% when the output power ranging between 60 and 150 W, and the peak efficiency reached 86.8%. The current and voltage at a connecting node between the DC-DC and DC-AC circuits were monitored by means of a shunt resistor connected
230
Y. Ando et al. / Solar Energy 141 (2017) 228–235
Si inverter or SiC inverter 1 Solar cell (spherical Si)
DC-AC Inverter (Si or SiC device)
MPPT controller
Load
(a) Power meter
Li-ion battery
180 28 mm3)
SiC inverter 2 (250 Solar cell (spherical Si)
DC-AC Inverter (SiC device)
MPPT controller
PC
Load
(b) Power meter
Li-ion battery
PC
Fig. 1. Schematics of the photovoltaic power storage systems using (a) conventional inverters (SiC inverter 1 and Si inverter) and (b) developed inverter (SiC inverter 2).
(a)
(b) USB
Inter-stage monitor
180 mm 28 mm 250 mm
PV input
Battery input
AC output
Fig. 2. Photographs of (a) the developed photovoltaic power module and (b) the DC-AC converter circuit with an MPPT controller (SiC inverter 2).
(b) 100
Inverter efficiency (%)
MPPT efficiency (%)
(a) SiC inverter 2 95
90
85 0
20
40
60
80
100
120
PV output power (W)
100 80
SiC inverter 2 DC-DC DC-AC TOTAL
60 40 20 0
20
40
60
80 100 120 140 160
AC output power (W)
Fig. 3. (a) Efficiency of the MPPT controller part versus DC supplied power and (b) DC-AC conversion efficiency of the inverter part versus AC output power (SiC inverter 2).
between them. This enabled us to separate the measured efficiency into two components originating from the DC-DC and DC-AC circuits. As shown in Fig. 3(b), peak efficiencies exceeded 87% and
98% for the DC-DC and DC-AC parts, respectively. This indicates power losses caused by the DC-DC converter to be a main factor to determine the inverter efficiency.
231
Y. Ando et al. / Solar Energy 141 (2017) 228–235
3.2. Total efficiency of SiC-based inverter module
100
Pac : P pv þ Pbat
gtot ¼
Efficiency (%)
To study a substantial conversion efficiency of photovoltaic inverters, the total efficiency was introduced by Haeberlin et al. (2005). Modifying this approach, the total efficiency (gtot) of the present photovoltaic inverter module was calculated by
ð1Þ
In this expression, Ppv, Pbat, and Pac are a DC power supplied from the solar cell, that from the battery, and an AC power supplied to the load, respectively. Also, the efficiency of MPPT controller (gMPPT) was expressed by
gMPPT
PMPPT ðPpv Þ ¼ : P pv
ð2Þ
In this expression, PMPPT is a DC output power of the MPPT controller, which can be determined with respect to Ppv based on the relationship shown in Fig. 3(a). Then, the efficiency of the DC-AC converter (ginv) was calculated by
ginv ¼
P ac : PMPPT ðPpv Þ þ Pbat
ð3Þ
To evaluate the output power dependence of the total efficiency, we have carried out the step load measurement of the present SiC inverter. In this measurement scheme, the load power was increased stepwise from 5 to 144 W, and held at each step for 5 min. As the power source, spherical Si solar cell panels were connected to the photovoltaic DC terminal. Fig. 4(a) and (b) present variations in measured power at the three terminals (Ppv, Pbat, and Pac) and the evaluated efficiencies (gtot, gMPPT, and ginv), respectively. Presence of MPPT controller losses reduced gtot by several % as compared to ginv. However, difference between gtot and ginv became smaller at a high output power. This result was explained as follows. As Pac exceeded 54 W, contribution of Ppv was decreased and that of Pbat was increased. Since Pbat was supplied from the battery without interposing the MPPT controller, this led to a decrease in the MPPT losses, and hence, gtot approached to ginv at a high output power. Fig. 5 plots gtot, gMPPT, and ginv with respect to Pac (lines), which were obtained from the step load measurements. The total efficiency exceeded 85% when the AC output power ranging between 74 and 144 W, and the peak gtot reached 86.5%. To investigate the efficiencies under more realistic operation conditions, we have also carried out the steady state measurement. In this measurement scheme, the load power was kept constant, and the DC-AC conversion efficiency was determined by time average for 1 or 2-h duration measurements after the initial variation. In Fig. 6, also
160 140
Power (W)
120 100
Ppv
80
Pbat
60
Pac
40 20
Total (step load) Total (steady state) MPPT (step load) MPPT (steady state) Inverter (step load) Inverter (steady state)
40
0
40
80
160
Fig. 5. Comparison of efficiencies measured under step load and steady state measurements (SiC inverter 2).
plotted are results of the steady state measurements (symbols). Two different measurement schemes showed a satisfactory agreement. This indicated the initial variation was not so important in this SiC-based inverter.
3.3. Electric power stability of photovoltaic systems To investigate the electric power stability, we have performed continuous operation measurements with the load power kept constant for the present SiC inverter. The measurement time was chosen between 2 and 8 h, which depended on the output power. Fig. 6(a), (b), (c), and (d) show an example of variations in the voltage, current, power and efficiency values, respectively. The load power was set at 30 W. In these figures, subscripts pv, bat, and ac to the voltage U and current I mean the photovoltaic terminal, battery terminal, and AC terminal, respectively. Fig. 7 (a) and (b) show corresponding variations in solar radiation power and those of temperature and humidity, respectively. Before 14:30, a great majority of the electricity was supplied from the solar cell, but a small amount of it was supplied from the battery. In accordance with a drop of the solar radiation power after 14:30, the current (Ipv) and power (Ppv) supplied from the solar cell steeply deceased, while the current (Ibat) and power (Pbat) from the battery increasing. Voltages of the solar cell (Upv) and the battery (Ubat) were, however, kept almost constant. Concerning the AC output electricity, no fluctuation was seen in the voltage (Uac), current (Iac), or power (Pac). A gradual increase in gtot after 14:30 was resulted from the increased contribution of Pbat (or the decreased contribution of Ppv), since Pbat was supplied from the battery without interposing the MPPT controller, as described above.
100
80 SiC inverter 2 Step load Δt = 5 min
60
Total
40
MPPT
0 -20
120
AC output power (W)
(b) SiC inverter 2 Step load Δt = 5 min
SiC inverter 2
60
20
Efficiency (%)
(a)
80
Inverter
0
10
20
30
Time (min)
40
20
0
10
20
30
40
Time (min)
Fig. 4. Variation in (a) power at solar cell, battery, and AC terminals and (b) efficiencies of MPPT controller, DC-AC converter, and total module. The load power was increased stepwise from 5 to 144 W, and held at each step for 5 min (SiC inverter 2).
Y. Ando et al. / Solar Energy 141 (2017) 228–235
16
160
12
120
8
80
Pload = 30 W
12
(b)
200
Release load
Apply load
1.2 Pload = 30 W Ipv Ibat Iac
9 6
0.9 0.6
Release load
Apply load
3
0.3
0
0
Iac (A)
20
Ipv, Ibat (A)
Upv, Ubat (V)
(a)
Uac V
232
Upv
4
40
Ubat Uac
0 9:00
11:00
13:00
-3 9:00
0 17:00
15:00
11:00
100
Pbat Pac Apply load
Release load
25 0 -25 9:00
11:00
13:00
-0.3 17:00
15:00
Apply load
Release load
90
Efficiency (%)
Power (W)
(d)
Ppv
Pload = 30 W
75 50
15:00
Time
Time
(c) 100
13:00
80 70 Pload = 30 W Total MPPT Inverter
60 50 9:00
17:00
11:00
13:00
15:00
17:00
Time
Time
Fig. 6. Variation in (a) voltage, (b) current, and (c) power respectively at solar cell, battery, and AC terminals. (d) Variation in efficiencies of MPPT controller, DC-AC converter, and total module. The load power was set at 30 W (SiC inverter 2).
(b)
1
100
50 Apply load
Apply load
Release load
0.8 0.6 0.4 0.2 0 9:00
11:00
13:00
15:00
17:00
Time
Release load
40
80
30
60
20
10 9:00
40
Temp. Humid. 11:00
13:00
15:00
Humidity (%)
1.2
Temperature (ഒ)
Solar radiation power (kWm-2)
(a)
20 17:00
Time
Fig. 7. Variation in (a) solar radiation power and (b) temperature and humidity, during the measurements shown in Fig. 6.
Fig. 8(a), (b), (c) and (d) show another example of variation in the voltage, current, power, and efficiency values, respectively. The load power was set at 74 W. In this case, a considerable part of the electricity was supplied from the battery throughout the whole measurement, since the photovoltaic power supplied from the solar cell (around 40 W) was insufficient to bear the load power. In this case, gtot was as high as 85% and was almost constant during the measurement.
4. Discussions 4.1. Comparison with conventional inverters Fig. 9 compares DC-AC conversion efficiencies in the present and conventional inverters. The peak efficiency of the developed
SiC inverter was 86.8%. This value was comparable to the conventional SiC inverter (86.5%), while being 3% higher than the Si inverter (83.7%). We are currently engaged in analyzing power losses in the present inverter. A preliminary study indicated a smaller switching loss of SiC MOSFETs and a smaller reverse recovery loss of their body diodes were responsible for the 3% higher efficiency of the SiC inverters. It is worthy to note that the SiC inverter 2 exhibited an efficiency comparable to the SiC inverter 1, even though the increased switching frequency should cause the increase of the switching losses. The power loss analysis also indicated a reduction of the DC-DC converter losses led to the high efficiency for the increased switching frequency of SiC inverter 2. The stability characteristics were compared between the SiC and Si inverters. Concerning the output power, these inverters equally showed a reasonable stability. However, with regard to the DC-AC conversion efficiency, the SiC inverters exhibited more
233
Y. Ando et al. / Solar Energy 141 (2017) 228–235
16
160
12
120
8
80
Pload = 74 W
12 9
1.2
0.6
3
0.3
0
40
Ubat
Pload = 74 W
Uac
0 9:00
10:00
11:00
-3 9:00
0 13:00
12:00
10:00
Apply load
Release load
(d) Efficiency (%)
75
Power (W)
50 25 Pload = 74 W
0
Ppv Pac
10:00
11:00
100
Apply load
12:00
0 -0.3 13:00
Release load
90 80 70
Pload = 74 W Total MPPT Inverter
60
Pbat
-25 -50 9:00
11:00
Ipv Ibat Iac
Time
Time 100
0.9
6
Upv
4
(c)
Release load
Apply load
Iac (A)
Upv, Ubat (V)
(b)
200 Release load
Apply load
Ipv, Ibat (A)
20
Uac V
(a)
12:00
13:00
50 9:00
10:00
11:00
12:00
13:00
Time
Time
Fig. 8. Variation in (a) voltage, (b) current, and (c) power respectively at solar cell, battery, and AC terminals. (d) Variation in efficiencies of MPPT controller, DC-AC converter, and total module. The load power was set at 74 W (SiC inverter 2).
Inverter efficiency (%)
90
80
70 SiC inverter 2 SiC inverter 1 Si inverter
60
50
0
40
80
120
160
200
AC output power (W) Fig. 9. Comparison of DC-AC conversion efficiencies for the developed and conventional inverters.
stable characteristics. Fig. 10(a) and (b) compare the stability in the DC-AC conversion efficiency for the Si inverter and the SiC inverter 2. In the Si inverter, a gradual decrease of the efficiency was observed for the load power of 90 and 120 W, while it was not observed for the load power of 54 W or less. Besides, this kind of efficiency degradation was less significant in the SiC inverters. Therefore, the gradual decrease in the efficiency would be due to the self-heating effect of Si MOSFETs. 4.2. Influence of defects in SiC devices Poor reliability resulted from defects in the SiC epitaxial wafer and in the gate oxide film has been one of the critical issues of SiC MOSFETs. Quality of the gate oxide film directly affects reliability of SiC MOSFETs. The charge to breakdown QBD determined by the time dependent dielectric breakdown test is a quality indicator of the gate oxide. Recently, quality of oxide on SiC has been
improved by optimizing the growth process. The QBD value of the SiC MOSFETs used in this study is 15–20 C/cm2 that is equivalent to that of Si MOSFETs (ROHM semiconductor, 2014). Even with a high quality gate oxide layer, there still remains crystal defects in the epitaxial wafer that may cause initial failures. Several authors reported correlation between the breakdown voltage and density of screw dislocations (Wahab et al., 1999; Fukuda et al., 2010) or micropipes (Dmitriev and Spencer, 1998). Also, the firstprinciples calculation verified that point defects such as silicon and carbon substitutionals and oxygen interstitials significantly degrade the channel electron mobility (Iskandarova et al., 2015). These effects may increase the conduction loss of SiC MOSFETs, and hence, affect the conversion efficiency. To identify and exclude such defective devices, we have introduced a screening DC test before mounting devices in the circuit.
4.3. Advantages over other power devices While SiC MOSFETs were used to replace 250 V planar MOSFETs in the photovoltaic inverter, we still have the option of choosing another device, too. The alternatives are advanced Si MOSFETs, Si insulated-gate bipolar transistors (IGBTs), and GaN high electron mobility transistors (HEMTs), and so on. Properties of these power devices are summarized in Table 1. The advanced Si MOSFETs include trench MOSFETs and super-junction MOSFETs. These advanced MOSFETs have lower ON resistances as compared to planar MOSFETs. Si IGBTs have very high breakdown voltages (600– 6000 V) and low ON-resistances. An existing issue of IGBTs is the collector current tailing due to accumulation of minority carriers and the consequent limited switching frequency. GaN and SiC devices have very high breakdown voltages, low ON resistances, small reverse recovery losses, and a negligible tail current effect. An unsolved problem of GaN HEMTs is difficulty in normally-OFF
234
Y. Ando et al. / Solar Energy 141 (2017) 228–235
(a)
(b) 90
95
54 W 90 W 120 W
Inverter efficiency (%)
Inverter efficiency (%)
Si inverter
85
80
54 W 90 W 120 W 90
85
SiC inverter 2 75
80 0
1
2
3
4
5
0
1
2
3
4
5
Time (h)
Time (h)
Fig. 10. Variation in the inverter efficiency for (a) the conventional Si inverter and (b) the developed SiC inverter 2.
Table 1 Comparison of power devices.
Si planar MOSFET Si advanced MOSFET Si IGBT SiC MOSFET GaN HEMT
Breakdown voltage
ON resistance
Switching frequency
Normally OFF
Cost
Good Good Excellent Excellent Excellent
Good Excellent Excellent Excellent Excellent
Good Good Fair Excellent Excellent
Easy Easy Easy Easy Difficult
Excellent Good Good Fair Fair
operation. Totally, SiC has the most attractive features as compared to other options. However, the main application of SiC power devices is multikW level converters. It is considered that a 100-W class converter cannot fully benefit from the low ON resistance of SiC devices. Therefore, SiC MOSFETs are commercially available only for 600 V voltage or more to date, and the SiC MOSFETs employed here own a drain-source breakdown voltage of 650 V. This ‘‘wastefully” high breakdown voltage of the SiC MOSFETs should be disadvantageous for reducing the ON resistance as compared to 250 V Si MOSFETs. In spite of this, the developed SiC inverter exhibited an efficiency superior to that of the conventional inverters. As described above, reduction of switching and reverse recovery losses of SiC devices would be responsible for this improvement. While we have made some progress in this first trial, there still remain some issues that warrant more attention. In this work, four identical MOSFETs were used to build the inverter. However, the switching loss is much important for high-side transistors, while the conduction loss is much important for low-side transistors. Therefore, careful consideration should be given to optimize the combination of low-side and high-side devices. Passive components and circuits should be also optimized to improve the efficiency. In addition, examination of other power devices (Si trench MOSFETs, Si super-junction MOSFETs, GaN HEMTs, and so on) may be tried out. 5. Conclusions We have developed a compact 150 W photovoltaic inverter, where an MPPT controller and a DC-AC converter were integrated into a single module. The DC-AC converter circuit was built with four SiC MOSFETs, while the DC-DC converter circuit built with four SiC SBDs. An increase in the PWM switching frequency up to 100 kHz resulted in the module of a reduced size (250 180 28 mm3), which was just one third volume of a commercial Si-based inverter available today. First, conversion efficiencies for the MPPT controller
and the DC-AC converter were characterized separately. The DC-AC converter exhibited the peak efficiency ginv of 86.8%, while the MPPT controller showing the peak efficiency gMPPT of 96.4%. The ginv value was approximately 3% higher than that of the conventional Si-based inverter. The power loss analysis indicated a smaller switching loss of SiC MOSFETs and a smaller reverse recovery loss of their body diodes were responsible for this improvement. Next, gtot under practical operation conditions were estimated with spherical Si cell panels connected as the power source. Although gtot values depended on the contribution of Ppv and Pbat to the total input power, a measured peak gtot reached 86.5%. We expect the sub-kW class SiC-based inverters would provide an optimal solution for self-sustaining energy systems used for portable electronic devices, solar-powered vehicles, emergency power supply, and so on. Acknowledgements This work was supported by Super Cluster Program of Japan Science and Technology Agency (JST). References Burger, B., Kranzer, D., Stalter, O., 2008. Cost reduction of PV-inverters with SiCDMOSFETs. In: 5th Int. Conf. Integrated Power Systems (CIPS 2008), pp. 1–5. Burger, B., Kranzer, D., Stalter, O., 2009. Efficiency improvement of PV inverters with SiC-DMOSFETs. Mater. Sci. Forum 600–603, 1231–1234. Burkart, R.M., Kolar, J.W., 2013. Comparative evaluation of SiC and Si PV inverter systems based on power density and efficiency as indicators of initial cost and operating revenue. In: 2013 IEEE 14th Workshop on Control and Modeling for Power Electronics (COMPEL) (Salt Lake City, UT, 23–26 June 2013), pp. 1–6. Chinthavali, M., Zhang, H., Tolbert, L.M., Ozpineci, B., 2009. Update on SiC-based inverter technology. In: Electronics Conf. (COBEP ’09), pp. 71–79. De, D., Castellazzi, A., Solomon, A., Trentin, A., Minami, M., Hikihara, T., 2013. An all SiC MOSFET high performance PV converter cell. In: 15th Euro. Conf. Power Electronics and Applications (EPE 2013), pp. 1–10. Dmitriev, V.A., Spencer, M.G., 1998. SiC fabrication technology: growth and doping. In: Park, Y.S. (Ed.), SiC Materials and Devices (Semiconductors and Semimetals 52). Academic Press, San Diego, pp. 21–76. Fairchild Semiconductor, 2013. Datasheet of FQPF16N25C – N-Channel QFET MOSFET.
Y. Ando et al. / Solar Energy 141 (2017) 228–235 Frank, P., Bruno, B., 2001. A new high voltage Schottky diode based on siliconcarbide (SiC). In: 9th Euro. Conf. Power Electron. Appl. (EPE 2001), p. 00259. Fukuda, F., Kinoshita, A., Ohyanagi, T., Kosugi, R., Sakata, T., Sakuma, Y., Senzaki, J., Minami, A., Shimozato, A., Suzuki, T., Hatakeyama, T., Shinohe, T., Matsuhata, H., Yamaguchi, H., Nagai, H., Harada, S., Ichinoseki, K., Yatsuo, T., Okumura, H., Arai, K., 2010. Influence of processing and of material defects on the electrical characteristics of SiC-SBDs and SiC-MOSFETs. Mater. Sci. Forum 645–648, 655– 660. Haeberlin, H., Borgan, L., Kaempfer, M., Zwahlen, U., 2005. Total efficiency gtot – a new quantity for better characterization of grid-connected PV inverters. In: 20th European Photovoltaic Solar Cell Energy Conf., pp. 1–4. Hensel, A., Wilhelm, C., Kranzer, D., 2011. Development of a boost converter for PV systems based on SiC BJTs. In: Proc. 2011–14th European Conf. Power Electronics and Applications (EPE 2011), pp. 1–7. Ho, C.N.M., Breuninger, H., Pettersson, S., Escobar, G., Coccia, A., Oikonomous, N., 2011. Practical implementation of an interleaved boost converter using SiC diodes for PV applications. In: 8th Int. Conf. Power Electron. (ECCE Asia), pp. 372–379. Ho, C.N.-M., Breuninger, H., Pettersson, S., Escobar, G., Canales, F., 2013. A comparative performance study of an interleaved boost converter using commercial Si and SiC diodes for PV applications. IEEE Trans. Power Electron. 28, 289–299. Iskandarova, I., Khromov, K., Knizhnik, A., Potapkin, B., 2015. The role of neutral point defects in carrier mobility degradation in bulk 4H-SiC and at 4H-SiC/SiO2 interface: first-principles investigation using Green’s functions. J. Appl. Phys. 117. 175703-1-7. Kim, T., Jang, M., Agelidis, V.G., 2013. Current status of silicon carbide power devices and their application in photovoltaic converters. In: IEEE ECCE Asia Downunder (ECCE Asia) (Melbourne, Australia, 3–6 June 2013), pp. 555–559. Kranzer, D., Wilhelm, C., Reiners, F., Burger, B., 2008. Application of normally-off SiC-JFETs in photovoltaic inverters. In: 13th Euro. Conf. Power Electron. Appl., pp. 1–6. Labedev, A.A., Chelnokov, V.E., 1999. Wide-gap semiconductors for high-power electronics. Semiconductors 33, 999–1001. Lauria, D., Coppola, M., 2014. Design and control of an advanced PV inverter. Sol. Energy 110, 533–542. Li, L., Li, C., Cao, Y., Wang, F., 2013. Recent progress of SiC power devices and applications. IEEJ Trans. Electric. Electron. Eng. 8, 515–521. Matsumoto, T., Oku, T., Hiramatsu, K., Yasuda, M., Shirahata, Y., Shimono, A., Takeda, Y., Murozono, M., 2016. Evaluation of photovoltaic power generation system using spherical silicon solar cells and SiC–FET inverter. AIP Conf. Proc. 1709. 020023-1-6.
235
Monroy, E., Omnès, F., Calle, F., 2003. Wide-bandgap semiconductor ultraviolet photodetectors. Semicond. Sci. Technol. 18, R33–R51. Nashida, N., Hinata, Y., Horio, M., Yamada, R., Ikeda, Y., 2014. All-SiC power module for photovoltaic power conditioner system. In: IEEE 26th Int. Symp. Power Semiconductor Devices & IC’s (ISPSD 2014), pp. 342–345. Oku, T., Kanayama, M., Ono, Y., Akiyama, T., Kanamori, Y., Murozono, M., 2014. Microstructures, optical and photoelectric conversion properties of spherical silicon solar cells with anti-reflection SnOx: F thin films. Jpn. J. Appl. Phys. 53. 05FJ03-1-7. Oku, T., Matsumoto, T., Hiramatsu, K., Yasuda, M., Shimono, A., Takeda, Y., Murozono, M., 2015. Construction and characterization of spherical Si solar cells combined with SiC electric power inverter. AIP Conf. Proc. 1649, 79–83. Oku, T., Matsumoto, T., Hiramatsu, K., Yasuda, M., Ohishi, Y., Shimono, A., Takeda, Y., Murozono, M., 2016. Construction and evaluation of photovoltaic power generation and power storage system using SiC field-effect transistor inverter. AIP Conf. Proc. 1709. 020024-1-10. Okumura, H., 2006. Present status and future prospect of widegap semiconductor high-power devices. Jpn. J. Appl. Phys. 45, 7565–7586. ROHM Semiconductor, 2014. SiC Power Devices and Modules – Application Note. ROHM Semiconductor, 2015a. Datasheet of SCT2120AF – N-Channel SiC Power MOSFET. ROHM Semiconductor, 2015b. Datasheet of SCS210AJ – SiC Schottky Barrier Diode. Sintamarean, C., Blaabjerg, F., Wang, H., 2013. Comprehensive evaluation on efficiency and thermal loading of associated Si and SiC based PV inverter applications. In: IECON 2013–39th Annual Conf. IEEE Industrial Electron. Society (Vienna, Austria, 10–13 November 2013), pp. 555–560. Stalter, O., Burger, B., Lehrmann, S., 2007. Silicon carbide (SiC) D-MOS for gridfeeding solar inverters. In: 12th Euro. Conf. Power Electron. Appl. (EPE 2007), pp. 1–10. Stalter, O., Kranzer, D., Rogalla, S., Burger, B., 2010. Advanced solar power electronics. In: Proc. 22nd Int. Symp. Power Semicon. Devices & ICs (Hiroshima, Japan, 2010), pp. 3–10. Texas Instruments, 2016. Datasheet of BP24650 – Synchronous Switching-Mode Battery Charge Controller for Solar Power with Maximum Power Point Tracking. Wahab, Q., Ellison, A., Hallin, C., Henry, A., Janzen, E., 1999. Study of breakdown limiting defects in 4H-SiC substrates and epilayers on the performance of high voltage Schottky diodes. In: Abstracts of the Int. Conf. on Silicon Carbide and related Materials (NC, October 1999), p. 298. Yamane, A., Koyanagi, K., Kozako, M., Fuji, K., Hikita, M., 2013. Fabrication and evaluation of SiC inverter using SiC-MOSFET. In: IEEE 10th Int. Conf. Power Electronics and Drive Systems (PEDS 2013), pp. 1029–1032.