The design of a high voltage scr pulse generator for ultrasonic pulse echo applications

The design of a high voltage scr pulse generator for ultrasonic pulse echo applications J.G.

OKYERE

and A.J.

COUSIN

This paper presents the design of a high voltage scr pulse generator that is useful for testing highly attenuative materials in the pulse echo mode. Electrical and ultrasonic considerations are discussed in detail followed by an example of a practical circuit design that can operate from a supply voltage of 1000 V at a pulse repetition frequency of 1000 Hz.

Introduction There are two different methods commonly used for exciting ultrasonic transducers in the pulse echo mode. The first involves the generation of a medium voltage, pulsed, radio frequency sinusoid using a gated oscillator, and the second involves the generation of a high voltage spike by a sudden discharge of a capacitor charged to several hundreds of volts. Comparatively, the first method is the more complex and the maximum pulsed voltage is typically limited to tens of volts. On the other hand, the second method, referred to as shock excitation, has the virtue of being s’mple, being easier to build and producing a maximum pulsed voltage capability of many hundreds of volts. A well-known shock excitation curcuit that is still in common use constists of a capacitor initially charged to a high voltage which is suddenly applied to a transducer using a thyratron as a switch.’ The advantages of using a thyratron as a switch are the following: rise time less than 20 ns, current capability in the many tens of amperes and breakdown voltage in excess of I kV. On the other hand, the thyratron consumes a significant amount of power and is bulky and quite fragile. In applications such as testing highly attenuative materials where high voltage pulses are required, avalanche transistors” 3 or silicon controlled rectifiers (scr)4, ’ can be used, since they can withstand hundreds of volts (singly or in cascade), pass switching currents in the tens of amperes and have fast turn-on time (generally less than 10 ns). As the attenuation coefficient of the material to be tested increases, higher value voltage pulses are required for adequate penetration. This constraint implies either higher breakdown voltage capability of the switching element or circuit techniques that employ series connected elements. Since scr’s are able to withstand higher voltages before breakdown occurs than avalanche transistors, they are more readily employed for applications involving highly attenuative materials. The authors are at the Department of Electrical Engineering, University of Toronto, Toronto, Ontario M5.S 1 A4, Canada. Paper received 10 August 1978.

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Although shock excitation circuits have appeared previously in the literature (for example references ‘-‘), the design considerations of high voltage pulse generators have not been reported. In this paper, the electrical and ultrasonic considerations for and the design of a practical scr pulse generator are presented. Circuit operation The basic circuit used to generate a high voltage pulse is shown in Fig. 1. It consists of a dc voltage source V,, resistance R,,storage capacitor C, a damping or load resistance R and an scr THl . An understanding of the operation of the circuit is important for a proper appreciation of the analysis that follows. Assuming that the scr is initially off, capacitor C is charged to the supply voltage V, through resistances R, and R.When a trigger signal is applied to the gate of the scr, the latter turns on. Storage capacitor C then discharges through the scr and the resistance R,and a pulse voltage appears at the output of the load resistance. When the charge stored in capacitor C has dissipated, the scr turns off since its current has become less than the required holding current. The storage capacitor is then recharged to the supply voltage Vs. Analysis of the basic circuit The discharge operation of the storage capacitor is initiated by triggering the ser. At this instant, the anode of the scr does not return to ground potential due to the finite turn-on

0 TO

transducer R

Trigger

SIgnal

Fig. 1

0041-624X/79/020081-04/$02.00

Basic scr pulse generator

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1979

circuit

IPC Business

Press

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time of the ser. If the storage capacitor was initially charged to the supply voltage, V,, and if t,, is the turn-on time of the scr, it can be shown that the output voltage across resistance R during the turn on period is given by =_!!i~c(I

_,-*lRC)

VO

(1)

ton

Equation (1) shows that as the turn-on time, t,, , increases, the voltage at the output of the load resistance decreases. From the frequency domain viewpoint, the shorter the turn-on time of the scr, the more higher frequencies are contained in the falling edge of the output pulse voltage and the easier it is to excite proportionately higher frequency transducers. During turn-on, an scr can be subjected to a very steep rise of current, the rate of which may be faster than the rate of activation of the gate junction so that current concentration occurs in an area of the junction forming a hot spot.6 To prevent formation of hot spots that might result in the failure of the component, the critical rate of rise of the on-state anode current (&/dt) must not be exceeded. Using (1) and the fact that the output pulse current is the same as the anode curent of the scr during turn-on, it can be shown that the maximum rate of rise of anode current is given by: die dt

,,,=

-- v, = Rt,,

(2)

There are two stages in the recharging operation of the scr pulse generator. During the first stage, the output voltage, vo, that was at a maximum at time t = t, returns to zero exponentially. This is attained by the switching current decaying towards zero through the R, C and TH 1 circuit. The second stage begins with the scr turned off and is followed by the recharging of the storage capacitor to the supply voltage, VS. An important feature of the pulse generator circuit under consideration is that a commutation circuit is not required to turn off the switching element. The scr is turned off when the magnitude of the anode current is reduced to a level below the holding current, ZH. This is facilitated by choosing

To prevent the scr from re-firing when the storage capacitor is being charged, it is necessary that the rate of rise of the anode voltage, dV,/dt, be less than the critical rate of rise of the anode voltage, dV/dt,,,, of the ser. The maximum rate of rise of anode voltage can be calculated to be

I

V

max = (e)c.

(4)

Therefore, the above value of the rate of rise of anode voltage must be less than the specified critical rate of rise of the anode voltage to assure reliable firing of the ser.

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The average electrical power, PAvE, delivered to the output circuit (including the damping resistor) is given by: 1 cv,” = - --

P*"E

2

T

= ;- CV,2f where T is the period of the pulse repetition the firing circuit.

frequency

of

If the turn-on time of the scr is very small ( and usually attempts are made to achieve this in any practical design), it is reasonable to assume that the output voltage exhibits a step edge followed by an exponential decay depicted by: Vomax e-*‘ICR 1 1

(7)

where tr ZZ t - t,, Att=t,,,v,= (1) as

Vomx,

V Omax Transforming V,(jw)

-

=

6

which can now be calculated from

cR(1

_

e

*on/CR)

t on

(7) into the frequency =

domain yields

v, maX L1 +jwr

where 7 = CR.

P = Kvo(jw)v;(jo)

An scr may be triggered on by a rapidly rising anode voltage in the absence or presence of a gate signal. A fast changing anode voltage produces a pulse of current which flows through the relatively small reactances of the junction capacitances of the scr and is large enough to promote an avalanche effect and thus turn on the scr.6

dt

pulse current of the ser.

The power of the pulse can then be expressed as I,.

RS

s

is the maximum

where I,,,

ve = -

Therefore, the rate specified by (2) should be less than the guaranteed critical value of the ser.

3 <

In pulse echo systems, it is desirable that the damping resistance be small such that the pulse energy transmitted to the transducer is dissipated quickly to allow the transducer to receive echoes from close flaws or interfaces. The lowest value of damping resistance depends essentially on the maximum pulse current that the scr can deliver and the supply voltage magnitude as

= Iw;,,,

L

1 t cIJ272

where K is a constant. At the angular frequency, CJ* = l/7, the 3dB point occurs. Presuming that the piezoelectric crystal is modelled as a pure resistor, it can be seen from (8) that 50% of the pulse energy is delivered to the output circuit and contained within the band from zero to 0,. It should be noted that since the pulse power is spread over the whole frequency spectrum, a low Q transducer, like that usually used in pulse echo applications, is excited by more energy than a high Q transducer. Thus, if the central frequency of a low Q transducer circuit is f,, then from (9) 50% of the electrical power will be coupled to the output circuit if f,

=

_-1

(10)

27rRC

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Rate generator

Pulse amplifier

Pulse shor&ng

cwut

f High I 1 “5

voltage pulse generotor

56 k 47nF 25 V

t PI 611 B Hammond

2 2M

I

I 56 IN4006

620 k

DI

’ 2

t: 47k

16 nF

Fig. 2

0033==

oc

Circuit

diagram

of the scr pulse generator

If RC < 1/(27rf,), there is more energy m frequency bands higher thanf, and if RC > 1/(27rfc), there would not be enough energy to excite a higher frequency transducer but more energy would be concentrated at lower frequency bands. Equation (10) can therefore give an optimum value of the time constant RC for a particular transducer. If the transducer is modelled as a parallel RC equivalent circuit, the bandwidth calculations given above will vary depending on the effective bandwidth change caused by the crystal capacitance. Choice of the components

The choice of the time constant, RC, involves some conflicting requirements. The larger RC is, the less charge is lost by the capacitor C while the scr is turning on, as evidenced by (1). On the other hand, the smaller the time constant RC, the more energy there is to excite high frequency transducers as shown by (10). Although the damping resistance is required to be small to dissipate the pulse energy quickly, it cannot be made smaller than the value that exceeds the scr’s capabilities (5). The value of the capacitor, C, is required to be small not only for its small size but also to keep its self-inductance to a minimum value. A high self-inductance would reduce the rate of rise of the scr anode current to an unacceptable value. The scr should have a fast turn-on time, high switching current capability and be able to withstand the required

MARCH

supply voltage. The reason for the fast turn-on time is evident from (1). A high switching current is needed to allow the use of a small-valued damping resistance (5). It should be noted that this requirement becomes more stringent as the supply voltage is increased. In applications where the voltage requirement exceeds the voltage rating of available high current, fast switching scr’s two or more scr’s of the same type may be connected in series. The supply voltage is made to divide across them with a resistive divider, so that the breakdown voltage of the individual scr’s is not exceeded.

of the basic circuit

For most industrial flaw detection applications where the pulse repetition frequency ranges from 50 to 2500 Hz, the time constant CR, must be kept small so that the storage capacitor will be charged to the supply voltage at the end of the pulse period. The resistance, R,, should be chosen such that the current flowing through it is less than the holding current specified by (3).

ULTRASONICS.

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Practical circuit Fig. 2 shows the scr pulse generator circuit. The rate generator is realized by operating a 5.55 timer in an astable mode. The output of the timer is connected both to the input of a pulse amplifier and to the time base sync of a scope. The inverter (CD 4049) and the transistor Tl drive the dual secondary transformer to make up the pulse amplifier. The pulse transformer, PI, is used to couple the pulse amplifier to the pulse sharpening circuits. The two secondary windings provide isolation between the gates of the two scr’s and also allow simultaneous triggering of the two series-connected scr’s. In order to obtain minimum turn-on time of the seriesconnected scr’s, it is desirable not only to trigger the gates with a strong drive but also to produce a triggering pulse with a fast rise-time.’ To accomplish this, the slowly rising pulse produced at the secondary of the pulse transformer is shaped to give a fast rise-time using a pulse sharpening circuit first introduced by General Electric.’ The ensuing pulse delivered to the scr has a rise time of about 50 ns.

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for damping transducers having central frequencies in the 1 MHz range. For damping resistors of 33, 56 and 120 R. the pulse widths of the output pulse are 0.61, 1.3 and 2 ps respectively. Fig. 3 shows the output waveform of the scr pulse generator.

Conclusion Design considerations for a high voltage scr pulse generator have been given along with a practical circuit realization. The treatment has been general in approach and is applicable to other switching elements and voltage requirements. The pulse generator circuit in Fig. 2 has been useful in ultrasonic defectoscopy of wood with shock excited transducers spanning the range from 0.25 to 2.2 MHz.~ Fig. 3 Output voltage wavetorm. Vertical scale 200 V div-’ , horizontal scale 1 us div-‘. R = 120 a. vs = 800 V

References 1

Scr 2N4203 was used because of its high peak forward blocking voltage (700 V), high switching current capability (100 A) and reasonably fast turn-on time (100 ns). High valued resistors have been connected across each of the scr’s to ensure that the supply voltage divides equally between the scr’s. With the holding current, IH, of the scr being about 20 mA, the resistance R, was found using (3).

2

3

4

The storage capacitor, C, was chosen to be 4.7 nF. This value allows the time constant CR, LO be small enough to enable the capacitor to charge to the supply voltage V, when the rate generator is operated at a pulse repetition frequency of 1000 Hz. At the same time, the chosen value of the storage capacitor (refer to (10)) enables the values of the damping resistance to be tens or a few hundred ohms

5 6 7 8

Connolly, C.C. An Ultrasonic Generator Giving Widely Variable Parameters, Bio-Medical Engg, 3 (1968) 72-75 Cheney, S.P., Lees, S., Gerhard, Jr., Kranx, P.R. Step Excitation Source for Ultrasonic Pulse Transducers, Ultrasonics, 11 (1973) 111-113 Myers, G.H., Thumin, A., Feldman, S., Santis, G., Lupo, F.J. A Miniature Pulser-preamplifier for Ultrasonic Transducers‘ Ultrasonics, 10 (1972) 87-89 Krautkramer, J. Ultrasonic Testing of Materials, SpringerVerlag, New York (1969) Wells, P.N.T. Physical Principles of Ultrasonic Diagnosis, Academic Press, London (1969) Dewan, S.B., Straughen, A. Power Semiconductor Circuits, John Wiley and Sons, New York (1975) ‘SCR Manual’, 5th Ed, Semiconductor Products Department, General Electric, Syracuse, New York (1972) Okyere, J.G., Cousin, A.J. OnFldw Detection in Live Wood, IEEE Tram Sonics and Ultrasonics, in press

The Evaluation and Calibration of Ultrasonic Transducers conference organized by the Materials and Testing Group of the lnstitu te of Physics The Geological Society, London, 11-12 May 1977 The proceeding of a two-day meeting concerned with various aspects of the standardization of ultrasonic transducers. The meeting attracted over a hundred deiegates, from the UK, Austria, Belgium, Denmark, France, West Germany, Italy, The Netherlands, Norway and Switzerland, bringing together workers from universities, medical establishments and from nondestructive testing laboratories.

The meeting was organized by the Materials and Testing Group of the Institute of Physics in collaboration with the Hospital Physicists Association, the Institute of Acoustics, and the Institute of Quality Assurance. It represents an invaluable survey of current transducer technology 240 x 170mm/200 pages /illustrations November 19781lSBN 0 902852 84 1 Price f 16.OOB41.60 further

l l l l

84

theoretical considerations practical evaluation development and design features absolute calibration

information

and complete catalogue available from

IPC Science and Technology Press Ltd Westbuly House, Bury Street, Guildford, England, GU2 5AW

Surrey,

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1979