Janis M. Niedra

QSS Group, Inc., Cleveland, Ohio

A Current Source Method For tq Measurement

of Fast Switching Thyristors

NASA/CR—2006-214260

November 2006
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NASA/CR—2006-214260

November 2006

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared under Contracts NAS3–98008 and NAS3–00145

Janis M. Niedra

QSS Group, Inc., Cleveland, Ohio

A Current Source Method For tq Measurement

of Fast Switching Thyristors
Acknowledgments

This work was sponsored by the NASA Glenn Research Center under NAS3–98008 and NAS3–00145

with G.E. Schwarze as the Project Manager.

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NASA/CR—2006-214260 1

A Current Source Method For tq Measurement of 

Fast Switching Thyristors 

Janis M. Niedra 

QSS Group, Inc. 

Cleveland, Ohio 44135 

Abstract 

A current source driven circuit has been constructed to measure the turn-off time (tq) of fast-switching 

SiC thyristors. This circuit operates from a single power supply and a dual channel pulse generator to 

provide adjustment of forward current, magnitude and duration of reverse applied voltage, and rate of rise 

of reapplied forward voltage. Values of tq down to 100 ns can be resolved. 

Introduction 

This report describes the operation of a circuit developed to measure the so-called turn-off time (tq) of 

fast-switching thyristors. When a specified forward current through a thyristor is interrupted by a sudden 

application of a reverse voltage, a time tq must elapse before the thyristor can recover its ability to block a 

reapplied forward voltage. Since the forward current conduction mechanism of a thyristor depends on 

minority carrier injection at internal junctions, tq is a measure of the decay time of these carriers, after 

normal injection has ceased. Once the minority carrier density falls below a certain value, the regenerative 

action necessary for forward conduction cannot be maintained. Thus tq depends on the magnitude of the 

forward current, the magnitude of the applied reverse potential that helps to sweep out the carriers and 

also on the rate of reapplication of forward voltage. The latter rate induces capacitive displacement 

currents and minority carrier injection. Bias conditions at the gate junction affect injection and hence also 

influence tq. A superior tq measuring circuit would provide for independent and perfect control of all 

influential magnitudes and rates. The circuit presented here is not superior in every way, but rather 

provides simplified control of forward current, magnitude and duration of reverse applied voltage and rate 

of rise of reapplied forward voltage, all from a single power supply and a single timing generator that 

drives a single MOSFET switch. For comparison, test circuits such as used by A. Frasca (ref. 1) need 2 

transistor switches, 2 pulse sources and use 2 power supplies, making it more difficult to compact; 

however, a broader range of bias conditions can be obtained. 

Principles of Operation 

The tq of the thyristor under test (TUT) is measured repetitively in a cycle that consists of 3 phases. 

First, a gating pulse from a pulse generator turns on the TUT in order to establish a steady state forward 

current (~1 A). This current also stores energy in an inductor for use later in the cycle. Simultaneously, a 

10-μF capacitor gets charged to serve as the source of a reverse voltage, to be applied to the TUT during 

the next phase. 

After a steady state current is achieved through the inductor and the TUT, a signal from a second 

channel of the pulse generator closes the MOSFET switch, starting the second phase. This action connects 

the precharged 10-μF capacitor across the TUT, such that the sudden reverse bias cuts off its forward 

conduction current. A return path for the current is provided through this capacitor and the MOSFET, 

since the inductor enforces current continuity. The action of the current is to discharge the capacitor, but 

only a negligible amount during the short time (typically a μs or less) of the application of the reverse 

bias. At the start of the reverse bias, there is a fast-rising reverse current pulse that reaches a peak and 

decays, as the stored minority carriers in the TUT recombine and conduction ceases. The duration of the 
NASA/CR—2006-214260 2

reverse bias state is essentially the duration of the pulse that switches the MOSFET. Normally this 

duration is sufficiently long to avoid immediate turn-on of the TUT at reapplication of a forward voltage. 

Such conditions as magnitude of the reverse bias and rate of rise and limiting value of the reapplied 

forward voltage have a great effect on this minimum duration, which is essentially the tq. For Si-based 

thyristors, the tq is usually greater than a few μs, but for SiC thyristors, values below 100 ns have been 

observed (ref. 1). Times below 100 ns are difficult to resolve accurately with a circuit made of discrete 

components, because of the presence of spurious oscillations and the limits to switching speed (~30 ns) of 

MOSFETs. In that regard, the circuit presented here may have the advantage of improved response, due 

to compactness. 

The final phase involves the application of a forward voltage at a controlled rate to the TUT to test 

whether or not it will spontaneously go into forward conduction. This phase is initiated by the turning off 

the MOSFET switch at the end of the switching pulse. Now the inductor does its job as a current source to 

feed some low value, selectable capacitors, thereby generating a voltage ramp across the TUT. The 

internal capacitance of the TUT, which can be significant, is of course included also. The voltage across 

the TUT will rise roughly linearly with time until either it causes the TUT to break down into forward 

conduction, or else a limit preset by transient absorbing Zener diodes (“tranzsorbs”) is reached. It may 

even happen that the TUT breaks down after a short lapse of time on the tranzsorb limited voltage 

plateau. For a given reapplied voltage limit and ramp rate and reverse bias voltage, the tq is measured by 

progressively decreasing the reverse bias duration until the TUT breaks down into forward conduction. 

The time tq is measured from the time when the forward current crosses zero, near the start of the reverse 

bias, to the time when the reapplied voltage goes positive (ref. 2), under incipient forward breakdown 

(c.f., the cursor lines in figs. 1 and 2). 

A circuit diagram is drawn in the appendix. The additional circuit elements needed to control various 

details of circuit function are also described there. 

Sample Waveforms 

The upper trace in figure 1 shows the current through an experimental SiC thyristor during a forward 

conduction, turn-off and re-triggering cycle and the lower trace is the simultaneous voltage across the 

device. Initially, a steady current of 1 A has been established. Closure of the MOSFET switch causes a 

very abrupt reversal of the current through the thyristor, in response to the suddenly applied reverse bias 

of about 80 V. This reverse bias has here a risetime of about 30 ns, due to the finite switching speed as 

well as thyristor and circuit capacitances. At least until the minority carriers recombine, the reverse 

impedance of the thyristor can be sufficiently low to permit a large reverse current pulse peak. The circuit 

presented here uses an adjustable resistor to limit this peak to safe values. In theory (refs. 3 and 4), the 

reverse current pulse has a step or plateau structure, not resolved in figure 1. 

After about 250 ns, a forward voltage is reapplied, rising above zero at a controlled rate. The rate 

shown in figure 1 is roughly 250 V/μs. While climbing at this rate, the thyristor turns on again, when the 

forward voltage has reached 60 V. There is a brief current spike, perhaps due to capacitive effects, and 

then the current settles to 1 A again. 

If the duration of the 80 V reverse bias had been longer than 250 μs, then instead of retriggering on 

the slope, the reapplied voltage would have continued to rise up to a limit preset by the tranzsorbs. This 

critical duration of the reverse bias, indicated as 248 ns in figure 1, is the tq, defined above as the time 

from the current zero crossing to the reapplied voltage zero crossing, when the device is just at the edge of 

re-triggering, as the duration of the reverse bias is gradually reduced. 

The scenario in figure 2 is similar, except that now the thyristor re-triggers on the forward voltage 

plateau of 70 V, as set by the tranzsorbs. The increased tq, shown as 604 ns, is to be expected, because the 

reapplied voltage was set to rise at a faster 670 V/μs. 
NASA/CR—2006-214260 3

Discussion 

The thyristor tq measurement circuit presented here is an alternative to a more common arrangement 

that uses 2 sources of power and 2 coupled pulse sources to drive 2 MOSFET switches. Typical of a 2-

switch circuit is the one used by A. Frasca (ref. 1), in a SiC thyristor dynamic characterization study. In 

such a 2-switch version, it is easier to obtain a wide range of forward current, reverse and reapplied 

forward voltages, but at the cost of increased equipment and switching complexity. Also, the switching 

from one MOSFET to the other has to be quick, but with no overlap. In contrast, the inductive current 

source circuit described here reduces peripheral equipment count and switching complexity by making a 

single MOSFET switch do the job. At high switching speeds, a more compact circuit has an advantage. It 

should be noted too, that instead of trying to minimize all inductive effects, this circuit makes good use of 

inductance to achieve its functions. Thus with substantial inductive energy storage, it is easy to obtain a 

linear ramp of reapplied voltage. 

At present, this tq circuit has been pressed into service, without much effort to correct some manifest 

problems. Thus a larger storage inductor would make for a steadier current source and a smaller inductor 

in series, based on a lower permeability core, might speed up the response. Also, the zero crossing 

behavior of reapplied voltage is better in figure 2 than in figure 1. The circuit elements controlling this 

function are mentioned in appendix A. And there is an annoying amount of spurious high frequency 

oscillation, which may be indicative of errors in wiring layout and grounding. 

References 

1. A. Frasca, Wittenberg University, private communication. 

2. SCR Manual, (The General Electric Co.), 6th Edition, Prentice-Hall, Inc., Englewood Cliffs, NJ, has 

an excellent diagram of the definition of tq on p. 124. 

3. S.K. Ghandi, “Semiconductor Power Devices,” Wiley-Interscience Publishers, New York, 1977. 

4. B.J. Baliga, “Power Semiconductor Devices,” PWS Publishing Co., 1996. 
NASA/CR—2006-214260 4

Figure 1.—Typical current (upper) and voltage (lower) 

waveforms observed during tq measurement. Here the 

thyristor turns on spontaneously on the rising slope of the 

reapplied voltage. Except for the spurious oscillations, the 

reapplied voltage ramp starts at essentially zero volts. 

Figure 2.—Spontaneous turn-on of a thyristor on the plateau of 

maximum reapplied voltage. Due to the longer duration of the 

reverse voltage, the reapplied voltage ramp starts at negative 

values. 
NASA/CR—2006-214260 5

Appendix A 

Details of the tq circuit 

The diagram drawn in figure 1A, together with the descriptive comments on some of its elements, is 

sufficient for construction of the tq measurement circuit. As shown, the circuit has been used with input 

voltages Vin from 50 to 100 V and forward currents up to 2 A. But depending on the current and voltage 

ranges desired, the ratings of some of the components may have to be altered. 

List of components: 

C1 1000 μF, 200 V, electrolytic capacitor 

C2 10 μF, 100 V polypropylene film capacitor 

C3 ten selected ceramic disc and mica snubber type capacitors, ranging from 100 pF to 0.01 μF each, 

connected by a 10-section DIP switch 

D1 Type BAV21 silicon diode (200 V, 250 mA) 

D2 type NTE580 fast recovery silicon diode (3 A, 600 V, trr=250 ns) 

D3 type 1N6628 fast recovery silicon diode (2 A, 600 V, trr=30 ns) 

D4, D6 one section of Motorola type MGR2025CT gallium arsenide rectifier. (250 V, 10 A, trr=12 ns) 

D5 in-house fabricated SiC Schottky diode (~200 V, 0.5 A) 

L 1.6 mH inductor, based on a Magnetics type 55436-M4 MPP core (μr = 160) 

R1 350 Ω, 25W, wire-wound rheostat 

R2 1 kΩ, 1.5 W, 10-turn pot (Bourns ‘knobpot’ type 3600S-1-102, or similar) 

R3 2.7 kΩ, 1⁄4 W, carbon resistor 

R4 1 MΩ, 1⁄4 W, carbon resistor 

R5 2.7 Ω, 1⁄2 W, carbon resistor 

R6 100 Ω, 1 W, 25-turn cermet trimpot (Bourns type 3252W-1-101, or similar) 

R7 270 Ω, 1⁄4 W, carbon resistor 

R8 1.0 Ω, 1⁄2 W, carbon resistor, trimmed to better than 5% accuracy 

S MOSFET switch (types IRFF 320, IRF720, or similar) 

Z series array of Zener diodes for transient power absorption (“tranzsorbs”), selectable by jumpers 

Notes on component functions: 

R1 and R2: 

The adjustable R1, together with the source voltage Vin, sets the steady state forward current of the 

TUT. Once Vin is set, the reverse bias voltage to be applied to the TUT is set by the tap on R2, as a 

fraction of Vin. Hence to go to a higher reverse bias, the Vin may need to be increased. Consequently, 

the R1 will have to be increased in order to maintain the same forward current. This defines the setting 

of the forward current and the reverse bias voltage. 

The switch S: 

S is implemented by a fast-switching MOSFET, such as the IRFF 320. Best performance was 

obtained when using a gate driver IC, such as the type TC4425CPA. Power for this gate driver can be 

easily derived from Vin by means of a Zener diode to regulate the voltage. 

D3 and R5: 

The function of these elements is to ensure that C3 retains no positive charge when S opens and a 

voltage ramp is applied to the TUT. These elements add extra capacitance to the anode of the TUT 

and may not be needed. 
NASA/CR—2006-214260 6

D1 and R3: 

These elements serve to charge C2 to a voltage that is to be the reverse voltage applied to the TUT. 

The value of this voltage is derived from Vin by adjusting R2 and is limited by Vin. C2 gets charged 

during the forward conduction time of the TUT. 

D2:

This diode ensures that C2 can never get charged reverse from normal. 

R4: 

This resistor provides a slow discharge path for C2. 

D4 and R6: 

This combination of elements provides for essentially unimpeded current flow in the forward 

direction of the TUT. But in the reverse direction, the current is limited by R6. Thus R6 controls the 

peak height of the reverse current pulse when S closes and a reverse bias is suddenly applied to the 

TUT. For this reason, D4 should be a very fast recovery and low capacitance type. The specified 

gallium arsenide type power diode seemed to be acceptable. 

C3, D5, R7 and D6: 

When S opens to end the reverse bias of the TUT phase, the only paths for the inductor current (still 

essentially the same as the initial forward current of the TUT) are the C3 and the capacitances of the 

TUT and the Zener diode chain. This defines the anode voltage VA ramp. D5 exists to keep R7 from 

interfering with the linearity of this ramp. During the initial forward conduction of the TUT phase, C3

has only a low positive charge (at most a few volts). But during the reverse bias phase, C3 gradually 

acquires a negative voltage, since the reverse bias is also applied to C3 in series with R7. Hence this 

effect depends on the C3R7 time constant, as compared to the duration of the reverse bias. The effect 

is negligible in figure 1, but is evident in figure 2. A small amount of negative precharge on C3 may 

be desirable at the end of the reverse bias phase, because that avoids the erratic zero crossing, due to 

oscillations, of the reapplied forward voltage ramp in figure 1. Possibly R7 could be made adjustable, 

but this introduces complications. If R7 is made too small, then there is risk of damage to the TUT 

when it breaks into forward conduction, with high positive voltage on C3. A better way to provide a 

small, negative charge on C3 should be implemented. The extremely fast recovery SiC Schottky diode 

D5 was found to be beneficial in reducing the high forward current spike when the TUT breaks into 

forward conduction under reapplication of voltage. In figures 1 and 2, the height of this spike is about 

1 A above the 1 A steady state forward current. Capacitances other than C3 contribute to this spike. 

Thus the fast recovery D6 reduces the contribution coming from the capacitance of the power Zener 

diode chain Z. 

R8: 

The 1 Ω carbon resistor R8 is used for sensing the current IAK through the TUT. All such sensing 

must be done differentially, as at high switching speeds, there may not be a uniform ground potential 

throughout the circuit. 
NASA/CR—2006-214260 7

Figure 1A.—Diagram of the current source circuit for tq measurement of fast recovery thyristors. Specific 

values are provided in the included list and particular functions are explained in the text. 

IAK, VK

S 

C1

C2

C3

L 

D1 R3

D2

D4

D3

D5

R1

R2 R4

R5

R6

R7

D6

Z

R8

TUT

VA

Vin
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E–15543

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13

A Current Source Method For tq Measurement of Fast Switching Thyristors

Janis M. Niedra

t

q test circuit; Turn-off time tq; Fast SiC thyristor; Reapplied voltage rate

Unclassified -Unlimited

Subject Category: 33

QSS Group, Inc.

21000 Brookpark Road

Cleveland, Ohio, 44135

Project Manager, Gene E. Schwarze, Power and Propulsion Division, NASA Glenn Research Center, organization

code RPE, 216–433–6117.

A current source driven circuit has been constructed to measure the turn-off time (tq) of fast-switching SiC thyristors.

This circuit operates from a single power supply and a dual channel pulse generator to provide adjustment of forward

current, magnitude and duration of reverse applied voltage, and rate of rise of reapplied forward voltage. Values of tq

down to 100 ns can be resolved.