- Email: [email protected]
Fundamental frequency region-based thermal control of power electronics modules in high power motor drive
Microelectronics Reliability 88–90 (2018) 1242–1246
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Fundamental frequency region-based thermal control of power electronics modules in high power motor drive
T
⁎
Peng Fana, Shoudao Huanga, Huai Wangb, Derong Luoa, , Huimin Lia, Meidi Suna a b
School of Electrical and Information Engineering, Hunan University, Changsha, China Department of Energy Technology, Aalborg University, Aalborg, Denmark
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal control Power loss Switching frequency
The reliability of power modules in motor drive is closely related to its thermal cycling. Especially in low speed and high output power condition, the thermal performance can be even worse. In this paper, a fundamental frequency region-based thermal control strategy is presented. Firstly, the model for high power drive thermal cycling calculation is established. With the electro-thermal model, power switch junction temperature and its number of power cycles can be mapped with fundamental frequency. Then, the fundamental frequency region can be divided by the junction temperature fluctuation. Based on the obtained frequency region, a switching frequency control method is proposed to improve the thermal fluctuation and reliability. A comparison results with and without the proposed control method are given to validate the switching frequency hysteresis control strategy and the improvement on system power cycling capability.
1. Introduction
characteristics, a thermal control method based on fundamental frequency region is proposed to reduce temperature fluctuation in low speed and high output power. The matching of switching frequency reference value and the temperature fluctuation at specified fundamental frequency region is developed. Furthermore, the power switch junction temperature performance and its power cycling capability with and without the proposed control method is estimated and compared.
In industrial applications, semiconductors in power supplier are one of the most critical components for system reliable operation with high power rating. With the steadily increasing of high power requirement, their thermal management is a key factor for improving reliability and performance [1,2]. In traction drive application, it requires low speed under high power operating condition, resulting in high thermal cycling of the power modules [3]. In order to improve thermal cycling and increase lifetime of power modules, evaluating the loss and thermal behaviour of the semiconductors in the modules is a primary step [4,5]. Due to the paralleled configuration in high power module, the size of power switch including chip area and chip number is one of the loss/thermal determine factor. These parameters cannot be neglected in loss/thermal calculation. On the other hand, the thermal performance of high power drive is mainly related to the inverter operating parameters, including fundamental current [6,7], switching frequency [8], and DC-link voltage [9,10], etc. Plenty of freedoms to control the inverter loss and thermal distribution can be achieved by matching these electrical parameters. In this paper, an electro-thermal model correlated with chip area and number is built to calculate the switch loss and junction temperature. Then, the impact of fundamental frequency to inverter electrothermal performance is discussed. With the obtained loss/thermal
⁎
2. Electro-thermal model 2.1. Power loss model In high power drive, the power devices in a power module are generally parallel-connected to reach high output current. A power switch unit can be divided into multi-chips (chip 1- chip n), shown in Fig. 1. The total loss of a power switch consists of each individual power devices. The total conduction losses can be expressed as
pcon, T / D = n⋅vT / D (t )⋅iT / D (t )
(1)
where vT/D is the chips conduction voltage, iT/D is the chips current. The subscripts T stands for IGBT, D stands for diode. Then calculate the chip average current in a switching period, the power switch conduction loss can be considered as a function of switch
Corresponding author. E-mail address: [email protected] (D. Luo).
https://doi.org/10.1016/j.microrel.2018.07.036 Received 31 May 2018; Received in revised form 29 June 2018; Accepted 4 July 2018 Available online 30 September 2018 0026-2714/ © 2018 Elsevier Ltd. All rights reserved.
Microelectronics Reliability 88–90 (2018) 1242–1246
P. Fan et al.
ic
n
ic
i0
v
v
v Fig. 2. Switch unit junction-case thermal network.
ic
n
ic The fitting function of switching energy can be described by
i2 i i Esw ⎛ o ⎞ = ⎜⎛c1 o + c2 o2 ⎟⎞ (c3 + c4 AT / D + c5 Tj ) n ⎠ ⎝n⎠ ⎝ n
where the coefficients c1 and c2 are the polynomial approximation of current dependences. And the coefficient c3, c4 and c5 are the linear approximation of chip area and junction temperature dependences. From Eq. (6), the switching energies Esw is the function of mean output current. In the IGBT modules datasheet, the switching energies Esw are usually given with reference to two junction temperatures Tj (in cold and warm condition). Therefore, it can be calculated with first order approximation of the transistors or diodes junction temperatures. Similarly, the chip area is also approximated in the equation by datasheet.
n
Fig. 1. Power switch unit in the high power drive.
duty cycle dT/D and mean fundamental current i o/n.
pcond, T / D = n⋅dT / D (t )⋅vT / D (t )⋅
i 0 (t ) n
2.2. Thermal cycling model
(2)
The thermal network of a switch unit (n chips) is indicated in Fig. 2, in which the thermal impedance from junction to case is modelled as a four order foster equivalent circuit. Assuming that each paralleled chip has the same junction temperature, the junction to case impedance ith layer RC parameters of the power switch is equal to the chip RC lump in parallel. The switch RC elements can be expressed as
with
d (t ) =
1 + ζm 2
where m is modulation index. ζ = 1 for calculate the devices in upper bridge and ζ = −1 for lower bridge in the inverter legs. Considering the multi-chips structure, the relationship between chip area, number, junction temperature and conduction voltage can be approximately obtained by interpolating data sampled in the datasheet.
vT / D (t ) = a0 + a1 AT / D + a2 Tj _ T / D
i (t ) + a3 0 n
Ri =
psw, T / D
∑ Esw
Vdc i (t ) ⋅Esw ⎛ 0 ⎞ Vref ⎝ n ⎠
(8)
4
Zjc (t ) =
n j=1
(7)
In these equations, the thermal resistance is reduced by n times, and the thermal capacitor is expanded by n times. It should be noted that the thermal time constant τ remains unchanged. Therefore, the junction to case transient thermal impedance of a switch unit is
∑ Ri (1 − e−t / τi) =
Zjc _ T / D (t ) n
i=1
(4)
(9)
with
In order to process the difference in individual switching characteristic, the solution is chosen an average energy-current curve, and then summed up to obtain the total loss. The switching loss of the power switch can be calculated as
psw, T / D = nfs⋅
RT / Di n
Ci = nCT / Di
(3)
where the polynomial coefficients is a0, a1, a2, a3. AT/D is chip area of power devices. The Infineon High Speed 3 IGBT modules and bare dies can be chosen as the reference database. The switching loss of a power switch is the sum of each individual chip switching loss, which calculated as
V = fs dc Vref
(6)
τi = Ri Ci By the thermal impedance function, the junction temperature Tj can be calculated as
Tj _ T / D (t ) = Tc + PT / D (t )⋅
(5)
where the equivalent switching frequency is nfs.
Zjc _ T / D (t ) n
The mean value of junction temperature Tm can be written as
1243
(10)
Microelectronics Reliability 88–90 (2018) 1242–1246
P. Fan et al.
5600
120 100 Tm_90 Tm_60
60
ΔΤ_90 ΔΤ_60
40 20 0
10
20
30
40 50 f0 / Hz
60
70
80
fs 8kHz
4000
Pinv / W
T/
80
0
fs 10kHz
4800
fs 6kHz
3200
fs 4kHz
2400
fs 2kHz
90
1600
(a)
800
300
400
500
600
nAchip / mm2
700
800
Nf
Fig. 4. Switch unit average power loss related to parallel chip numbers and area having different switching frequency.
40 °C, and when f0 is > 30 Hz, ΔT is reduced to 15 °C or even lower. The average junction temperature Tm is less affected by the output frequency f0, which is mainly related to the output current i0. When i0 = 90 A, Tm is about 110 °C. When i0 = 60 A, Tm is about 85 °C. As f0 increases, the value of Tm remains stable. In Fig. 3(b), the power cycling capability of IGBT chip is presented under the 90 A fundamental current. When the output frequency is 30 Hz, the junction temperature fluctuation is reduced to 15 °C, and the number of power cycles can be as high as 1010, resulting in an increased lifetime expectancy of power switch. Therefore, this paper chooses 30 Hz as the standard to divide the output frequency into regions. With 30 Hz as the critical standard, the output frequency is lower than the standard to be determined as the low frequency region, and vice versa to the high frequency region. In order to implement the thermal management of the high power inverter, the effect of output frequency on power switch junction temperature is required to be analyzed. At low output frequency, the junction temperature fluctuation ΔT should be reduced. At high output frequencies, the average junction temperature Tm should be lowered. This is the main idea to achieve region-oriented thermal management control.
fo/Hz
(b) Fig. 3. (a) IGBT Junction temperature as a function of the inverter output frequency, (b) IGBT Power cycling numbers as a function of the inverter output frequency.
Tjm _ T / D = Tc + PT / D⋅Rjc _ T / D
(11)
With the mean temperature, the fluctuation of junction temperature ΔTj can be analytically taken by
ΔTj _ T / D = 2 × (Tj max _ T / D − Tjm _ T / D )
(12)
where the temperature maximum values Tjmax are extracted by filter. The lifetime of power device is closely related to its average junction temperature Tm and junction temperature fluctuation ΔT. CoffinManson failure model is employed to evaluation the IGBT lifetime.
E Nf (Tm , ΔT ) = A⋅ΔT α⋅exp ⎛ a ⎞ k B ⎝ ⋅Tm ⎠ ⎜
4. Thermal control system
⎟
(13)
where Nf refers to the total number of power cycles in the condition of a specific average junction temperature Tm and junction temperature fluctuation ΔT. A is a device-dependent constant, Ea is the activation energy constant, and kB is Boltzmann constant. The key to improving device reliability is to reduce the average junction temperature Tm and the temperature fluctuation ΔT as much as possible, thus increase the total number of power cycles Nf.
In the high power inverter total loss, switching loss is a dominant component. With the proportion of switching loss dramatically pumped by fs increased, the total loss in three phase inverter Pinv at different switching frequency is shown in Fig. 4, which focuses on 100 A parallel chip in a 200 kW inverter. With increasing of the total chip area nAchip, the inverter power loss is reduced due to the loss average distribution. But in the high switching frequency condition, the loss is increased because of the equivalent switching frequency nfs. Therefore, change switching frequency is a direct method to decrease inverter losses substantially. In order to improve the thermal cycling in low speed condition, the switching frequency can be real-time adjust according to the fundamental frequency. At nominal speed, reducing switching frequency may cause higher current harmonics in output current because of the carrier wave ratio decreased. But fortunately at low speed condition, it is feasible to decrease the switching frequency while keep the carrier wave ratio high. Therefore, a fundamental frequency region-based switching frequency control method can be proposed in Fig. 5. That is the switching frequency fs reduced according to the junction temperature fluctuation ΔT in the low fundamental frequency (< 30 Hz).
3. Fundamental frequency regional division Combining the loss calculation formula and the heat transfer model, the effect of the fundamental frequency f0 on the individual chip (100 A chip) average junction temperature Tm, the junction temperature fluctuation ΔT and number of power cycling at different chip current amplitudes (e.g. 90 A and 60 A) can be obtained in Fig. 3. It can be observed that the junction temperature peak value and thermal cycling rise exponentially in motor acceleration/deceleration process. The output frequency f0 has a significance influence on the junction temperature fluctuation ΔT, especially when the output frequency is lower. The data in the figure shows that when f0 = 5 Hz, ΔT is as high as
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Microelectronics Reliability 88–90 (2018) 1242–1246
P. Fan et al.
Thermal Model
f0
fs w* i*
Thermal Control
10
Loss Model
T f0
fs
fs / kHz
i Plos s Tj
6 4
Vce Es w
fs
8
2 0
d
1.0
0.5
2.0
1.5
t/s (a)
fs
fs0 ΔT f0
Compare with 30 Hz
fs0
low
ΔΤ /
f0 region-based guidance
f4 f3 f2 f1
fs T1 T2 T3 T4
t/s (b)
T
high
140
fs0
120 100
Fig. 5. Thermal control system diagram.
Tm /
80 60 40
In the low frequency region, a third-order hysteresis controller is used in this paper. For example, when the junction temperature fluctuation exceeds T1 (set to 15 °C), fs can be decreased from f4 (10 kHz) to f3 (8 kHz); if ΔT is lower than T2 (25 °C) and higher than T1, switching frequency will fall from f3 (8 kHz) to f2 (6 kHz). When the temperature is always higher than T4 (45 °C), fs remains unchanged. Until ΔT decreases, fs will increase gradually. In order to avoid the harmonic introduction, the minimum switching frequency is set to f1 (4 kHz). Taking the 200 kW inverter for example, the switching frequency control process is shown in Fig. 6(a). The fundamental current in the paralleled chip is 100 A. When the control is not introduce in the system, the switching frequency is 10 kHz and the inverter fundamental frequency is increased from 5 Hz to 100 Hz. Eqs. (9)–(13) can be used to calculate the junction temperature fluctuation ΔT, the average junction temperature Tm, and the number of power cycles Nf before and after using the fundamental frequency region guidance control, as shown in Fig. 6(b)–(d). It can be seen that the initial switching frequency is high, but as the switching frequency hysteresis control works, the switching frequency changes significantly with the junction temperature. At 0.25 s, the fundamental frequency is about 10 Hz, and the junction temperature fluctuates 70 °C. In a result, the system triggers hysteresis control of the switching frequency. Then, the junction temperature fluctuation is gradually decreases to 25 °C, and the average junction temperature also decreases from 93 °C to 77 °C. The average temperature and its fluctuation eventually become stable in 100 °C and 7 °C. From Fig. 6(d), the number of power cycles is increased by nearly 10 times with the fundamental frequency based thermal control, extending the power switch lifetime effectively.
20 0
0
1.0
2.0
4.0
3.0
t/s (c) 1010
Nf
108 106 104
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
t/s (d) Fig. 6. With fundamental frequency based control (red line) and without control (black line) comparison (a) Switching frequency, (b) Temperature fluctuation, (c) Average junction temperature, (d) Power cycle capability. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements This work was supported by the China National Key R&D Program (2016YFF0203400), the Centre of Reliable Power Electronics, Aalborg University, Denmark, and the China Scholarship Council. References
5. Conclusion [1] J. Lemmens, P. Vanassche, J. Driesen, Optimal control of traction motor drives under electro-thermal constraints, IEEE J. Emerg. Sel. Top. Power Electron. 2 (2014) 249–263. [2] G. Zhang, D. Zhou, J. Yang, Fundamental-frequency and load-varying thermal cycles effects on lifetime estimation of DFIG power converter, Microelectron. Reliab. 76 (2017) 549–555. [3] M. Andresen, M. Liserre, Impact of active thermal management on power electronics design, Microelectron. Reliab. 54 (2014) 1935–1939. [4] J. Ye, K. Yang, H. Ye, A. Emadi, A fast electro-thermal model of traction inverters for electrified vehicles, IEEE Trans. Power Electron. 32 (2017) 3920–3934.
A fundamental region-based thermal control is proposed in this paper. In the operating condition of the low motor speed, which means low fundamental frequency, the thermal cycling can be improved by adjusting the switching frequency in the specific region. Using this switching frequency hysteresis control in the drive system, it can regulate losses and keep the temperature fluctuation in the ideal range, as well as extend the inverter lifetime. 1245
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Reliab. 51 (2011) 1903–1907. [8] M. Andresen, G. Buticchi, M. Liserre, Study of reliability-efficiency tradeoff of active thermal control for power electronic systems, Microelectron. Reliab. 58 (2016) 119–125. [9] M. Andresen, K. Ma, G. Buticchi, et al., Junction temperature control for more reliable power electronics, IEEE Trans. Power Electron. 33 (2018) 765–776. [10] J. Lemmens, J. Driesen, P. Vanassche, Dynamic DC-link voltage adaptation for thermal management of traction drives, Proc. ECCE, 2013, pp. 180–187.
[5] P.L. Evans, A. Castellazzi, C.M. Johnson, Automated fast extraction of compact thermal models for power electronic modules, IEEE Trans. Power Electron. 28 (2013) 4791–4802. [6] Y. Liu, H. Abu-Rub, B. Ge, Front-end isolated quasi-Z-source DC–DC converter modules in series for high-power photovoltaic systems—part I: configuration, operation, and evaluation, IEEE Trans. Ind Electron. 64 (2017) 347–358. [7] C. Busca, R. Teodorescu, F. Blaabjerg, An overview of the reliability prediction related aspects of high power IGBTs in wind power applications, Microelectron.
1246
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Fundamental frequency region-based thermal control of power electronics modules in high power motor drive
T
⁎
Peng Fana, Shoudao Huanga, Huai Wangb, Derong Luoa, , Huimin Lia, Meidi Suna a b
School of Electrical and Information Engineering, Hunan University, Changsha, China Department of Energy Technology, Aalborg University, Aalborg, Denmark
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal control Power loss Switching frequency
The reliability of power modules in motor drive is closely related to its thermal cycling. Especially in low speed and high output power condition, the thermal performance can be even worse. In this paper, a fundamental frequency region-based thermal control strategy is presented. Firstly, the model for high power drive thermal cycling calculation is established. With the electro-thermal model, power switch junction temperature and its number of power cycles can be mapped with fundamental frequency. Then, the fundamental frequency region can be divided by the junction temperature fluctuation. Based on the obtained frequency region, a switching frequency control method is proposed to improve the thermal fluctuation and reliability. A comparison results with and without the proposed control method are given to validate the switching frequency hysteresis control strategy and the improvement on system power cycling capability.
1. Introduction
characteristics, a thermal control method based on fundamental frequency region is proposed to reduce temperature fluctuation in low speed and high output power. The matching of switching frequency reference value and the temperature fluctuation at specified fundamental frequency region is developed. Furthermore, the power switch junction temperature performance and its power cycling capability with and without the proposed control method is estimated and compared.
In industrial applications, semiconductors in power supplier are one of the most critical components for system reliable operation with high power rating. With the steadily increasing of high power requirement, their thermal management is a key factor for improving reliability and performance [1,2]. In traction drive application, it requires low speed under high power operating condition, resulting in high thermal cycling of the power modules [3]. In order to improve thermal cycling and increase lifetime of power modules, evaluating the loss and thermal behaviour of the semiconductors in the modules is a primary step [4,5]. Due to the paralleled configuration in high power module, the size of power switch including chip area and chip number is one of the loss/thermal determine factor. These parameters cannot be neglected in loss/thermal calculation. On the other hand, the thermal performance of high power drive is mainly related to the inverter operating parameters, including fundamental current [6,7], switching frequency [8], and DC-link voltage [9,10], etc. Plenty of freedoms to control the inverter loss and thermal distribution can be achieved by matching these electrical parameters. In this paper, an electro-thermal model correlated with chip area and number is built to calculate the switch loss and junction temperature. Then, the impact of fundamental frequency to inverter electrothermal performance is discussed. With the obtained loss/thermal
⁎
2. Electro-thermal model 2.1. Power loss model In high power drive, the power devices in a power module are generally parallel-connected to reach high output current. A power switch unit can be divided into multi-chips (chip 1- chip n), shown in Fig. 1. The total loss of a power switch consists of each individual power devices. The total conduction losses can be expressed as
pcon, T / D = n⋅vT / D (t )⋅iT / D (t )
(1)
where vT/D is the chips conduction voltage, iT/D is the chips current. The subscripts T stands for IGBT, D stands for diode. Then calculate the chip average current in a switching period, the power switch conduction loss can be considered as a function of switch
Corresponding author. E-mail address: [email protected] (D. Luo).
https://doi.org/10.1016/j.microrel.2018.07.036 Received 31 May 2018; Received in revised form 29 June 2018; Accepted 4 July 2018 Available online 30 September 2018 0026-2714/ © 2018 Elsevier Ltd. All rights reserved.
Microelectronics Reliability 88–90 (2018) 1242–1246
P. Fan et al.
ic
n
ic
i0
v
v
v Fig. 2. Switch unit junction-case thermal network.
ic
n
ic The fitting function of switching energy can be described by
i2 i i Esw ⎛ o ⎞ = ⎜⎛c1 o + c2 o2 ⎟⎞ (c3 + c4 AT / D + c5 Tj ) n ⎠ ⎝n⎠ ⎝ n
where the coefficients c1 and c2 are the polynomial approximation of current dependences. And the coefficient c3, c4 and c5 are the linear approximation of chip area and junction temperature dependences. From Eq. (6), the switching energies Esw is the function of mean output current. In the IGBT modules datasheet, the switching energies Esw are usually given with reference to two junction temperatures Tj (in cold and warm condition). Therefore, it can be calculated with first order approximation of the transistors or diodes junction temperatures. Similarly, the chip area is also approximated in the equation by datasheet.
n
Fig. 1. Power switch unit in the high power drive.
duty cycle dT/D and mean fundamental current i o/n.
pcond, T / D = n⋅dT / D (t )⋅vT / D (t )⋅
i 0 (t ) n
2.2. Thermal cycling model
(2)
The thermal network of a switch unit (n chips) is indicated in Fig. 2, in which the thermal impedance from junction to case is modelled as a four order foster equivalent circuit. Assuming that each paralleled chip has the same junction temperature, the junction to case impedance ith layer RC parameters of the power switch is equal to the chip RC lump in parallel. The switch RC elements can be expressed as
with
d (t ) =
1 + ζm 2
where m is modulation index. ζ = 1 for calculate the devices in upper bridge and ζ = −1 for lower bridge in the inverter legs. Considering the multi-chips structure, the relationship between chip area, number, junction temperature and conduction voltage can be approximately obtained by interpolating data sampled in the datasheet.
vT / D (t ) = a0 + a1 AT / D + a2 Tj _ T / D
i (t ) + a3 0 n
Ri =
psw, T / D
∑ Esw
Vdc i (t ) ⋅Esw ⎛ 0 ⎞ Vref ⎝ n ⎠
(8)
4
Zjc (t ) =
n j=1
(7)
In these equations, the thermal resistance is reduced by n times, and the thermal capacitor is expanded by n times. It should be noted that the thermal time constant τ remains unchanged. Therefore, the junction to case transient thermal impedance of a switch unit is
∑ Ri (1 − e−t / τi) =
Zjc _ T / D (t ) n
i=1
(4)
(9)
with
In order to process the difference in individual switching characteristic, the solution is chosen an average energy-current curve, and then summed up to obtain the total loss. The switching loss of the power switch can be calculated as
psw, T / D = nfs⋅
RT / Di n
Ci = nCT / Di
(3)
where the polynomial coefficients is a0, a1, a2, a3. AT/D is chip area of power devices. The Infineon High Speed 3 IGBT modules and bare dies can be chosen as the reference database. The switching loss of a power switch is the sum of each individual chip switching loss, which calculated as
V = fs dc Vref
(6)
τi = Ri Ci By the thermal impedance function, the junction temperature Tj can be calculated as
Tj _ T / D (t ) = Tc + PT / D (t )⋅
(5)
where the equivalent switching frequency is nfs.
Zjc _ T / D (t ) n
The mean value of junction temperature Tm can be written as
1243
(10)
Microelectronics Reliability 88–90 (2018) 1242–1246
P. Fan et al.
5600
120 100 Tm_90 Tm_60
60
ΔΤ_90 ΔΤ_60
40 20 0
10
20
30
40 50 f0 / Hz
60
70
80
fs 8kHz
4000
Pinv / W
T/
80
0
fs 10kHz
4800
fs 6kHz
3200
fs 4kHz
2400
fs 2kHz
90
1600
(a)
800
300
400
500
600
nAchip / mm2
700
800
Nf
Fig. 4. Switch unit average power loss related to parallel chip numbers and area having different switching frequency.
40 °C, and when f0 is > 30 Hz, ΔT is reduced to 15 °C or even lower. The average junction temperature Tm is less affected by the output frequency f0, which is mainly related to the output current i0. When i0 = 90 A, Tm is about 110 °C. When i0 = 60 A, Tm is about 85 °C. As f0 increases, the value of Tm remains stable. In Fig. 3(b), the power cycling capability of IGBT chip is presented under the 90 A fundamental current. When the output frequency is 30 Hz, the junction temperature fluctuation is reduced to 15 °C, and the number of power cycles can be as high as 1010, resulting in an increased lifetime expectancy of power switch. Therefore, this paper chooses 30 Hz as the standard to divide the output frequency into regions. With 30 Hz as the critical standard, the output frequency is lower than the standard to be determined as the low frequency region, and vice versa to the high frequency region. In order to implement the thermal management of the high power inverter, the effect of output frequency on power switch junction temperature is required to be analyzed. At low output frequency, the junction temperature fluctuation ΔT should be reduced. At high output frequencies, the average junction temperature Tm should be lowered. This is the main idea to achieve region-oriented thermal management control.
fo/Hz
(b) Fig. 3. (a) IGBT Junction temperature as a function of the inverter output frequency, (b) IGBT Power cycling numbers as a function of the inverter output frequency.
Tjm _ T / D = Tc + PT / D⋅Rjc _ T / D
(11)
With the mean temperature, the fluctuation of junction temperature ΔTj can be analytically taken by
ΔTj _ T / D = 2 × (Tj max _ T / D − Tjm _ T / D )
(12)
where the temperature maximum values Tjmax are extracted by filter. The lifetime of power device is closely related to its average junction temperature Tm and junction temperature fluctuation ΔT. CoffinManson failure model is employed to evaluation the IGBT lifetime.
E Nf (Tm , ΔT ) = A⋅ΔT α⋅exp ⎛ a ⎞ k B ⎝ ⋅Tm ⎠ ⎜
4. Thermal control system
⎟
(13)
where Nf refers to the total number of power cycles in the condition of a specific average junction temperature Tm and junction temperature fluctuation ΔT. A is a device-dependent constant, Ea is the activation energy constant, and kB is Boltzmann constant. The key to improving device reliability is to reduce the average junction temperature Tm and the temperature fluctuation ΔT as much as possible, thus increase the total number of power cycles Nf.
In the high power inverter total loss, switching loss is a dominant component. With the proportion of switching loss dramatically pumped by fs increased, the total loss in three phase inverter Pinv at different switching frequency is shown in Fig. 4, which focuses on 100 A parallel chip in a 200 kW inverter. With increasing of the total chip area nAchip, the inverter power loss is reduced due to the loss average distribution. But in the high switching frequency condition, the loss is increased because of the equivalent switching frequency nfs. Therefore, change switching frequency is a direct method to decrease inverter losses substantially. In order to improve the thermal cycling in low speed condition, the switching frequency can be real-time adjust according to the fundamental frequency. At nominal speed, reducing switching frequency may cause higher current harmonics in output current because of the carrier wave ratio decreased. But fortunately at low speed condition, it is feasible to decrease the switching frequency while keep the carrier wave ratio high. Therefore, a fundamental frequency region-based switching frequency control method can be proposed in Fig. 5. That is the switching frequency fs reduced according to the junction temperature fluctuation ΔT in the low fundamental frequency (< 30 Hz).
3. Fundamental frequency regional division Combining the loss calculation formula and the heat transfer model, the effect of the fundamental frequency f0 on the individual chip (100 A chip) average junction temperature Tm, the junction temperature fluctuation ΔT and number of power cycling at different chip current amplitudes (e.g. 90 A and 60 A) can be obtained in Fig. 3. It can be observed that the junction temperature peak value and thermal cycling rise exponentially in motor acceleration/deceleration process. The output frequency f0 has a significance influence on the junction temperature fluctuation ΔT, especially when the output frequency is lower. The data in the figure shows that when f0 = 5 Hz, ΔT is as high as
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P. Fan et al.
Thermal Model
f0
fs w* i*
Thermal Control
10
Loss Model
T f0
fs
fs / kHz
i Plos s Tj
6 4
Vce Es w
fs
8
2 0
d
1.0
0.5
2.0
1.5
t/s (a)
fs
fs0 ΔT f0
Compare with 30 Hz
fs0
low
ΔΤ /
f0 region-based guidance
f4 f3 f2 f1
fs T1 T2 T3 T4
t/s (b)
T
high
140
fs0
120 100
Fig. 5. Thermal control system diagram.
Tm /
80 60 40
In the low frequency region, a third-order hysteresis controller is used in this paper. For example, when the junction temperature fluctuation exceeds T1 (set to 15 °C), fs can be decreased from f4 (10 kHz) to f3 (8 kHz); if ΔT is lower than T2 (25 °C) and higher than T1, switching frequency will fall from f3 (8 kHz) to f2 (6 kHz). When the temperature is always higher than T4 (45 °C), fs remains unchanged. Until ΔT decreases, fs will increase gradually. In order to avoid the harmonic introduction, the minimum switching frequency is set to f1 (4 kHz). Taking the 200 kW inverter for example, the switching frequency control process is shown in Fig. 6(a). The fundamental current in the paralleled chip is 100 A. When the control is not introduce in the system, the switching frequency is 10 kHz and the inverter fundamental frequency is increased from 5 Hz to 100 Hz. Eqs. (9)–(13) can be used to calculate the junction temperature fluctuation ΔT, the average junction temperature Tm, and the number of power cycles Nf before and after using the fundamental frequency region guidance control, as shown in Fig. 6(b)–(d). It can be seen that the initial switching frequency is high, but as the switching frequency hysteresis control works, the switching frequency changes significantly with the junction temperature. At 0.25 s, the fundamental frequency is about 10 Hz, and the junction temperature fluctuates 70 °C. In a result, the system triggers hysteresis control of the switching frequency. Then, the junction temperature fluctuation is gradually decreases to 25 °C, and the average junction temperature also decreases from 93 °C to 77 °C. The average temperature and its fluctuation eventually become stable in 100 °C and 7 °C. From Fig. 6(d), the number of power cycles is increased by nearly 10 times with the fundamental frequency based thermal control, extending the power switch lifetime effectively.
20 0
0
1.0
2.0
4.0
3.0
t/s (c) 1010
Nf
108 106 104
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
t/s (d) Fig. 6. With fundamental frequency based control (red line) and without control (black line) comparison (a) Switching frequency, (b) Temperature fluctuation, (c) Average junction temperature, (d) Power cycle capability. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements This work was supported by the China National Key R&D Program (2016YFF0203400), the Centre of Reliable Power Electronics, Aalborg University, Denmark, and the China Scholarship Council. References
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A fundamental region-based thermal control is proposed in this paper. In the operating condition of the low motor speed, which means low fundamental frequency, the thermal cycling can be improved by adjusting the switching frequency in the specific region. Using this switching frequency hysteresis control in the drive system, it can regulate losses and keep the temperature fluctuation in the ideal range, as well as extend the inverter lifetime. 1245
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