The MOS-Controlled Thyristor (MCT) is emerging as a powerful switch that combines the better characteristics of existing power devices. A study of switching stresses on an MCT switch under zero voltage resonant switching is presented. The MCT is used as a bidirectional switch in an ac/ac pulse density modulated inverter for induction motor drive. Current and voltage spikes are observed and analyzed with variations in the timing of the switching. Different snubber circuit configurations are under investigation to minimize the effect of these transients. The results will be extended to study and test the MCT switching in a medium power (5 hp) induction motor drive

Elbuluk, Malik E.

Lee, Tony

Zinger, Donald S.

English

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NASA-CR-192230
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CHARACTERIZATION AND SNUBBING OF
IN A RESONANT AC
Tony Lee
LINK CONVERTER
Malik E. Elbuluk* Donald S. Zinger
Department of Electrical Engineering
University of Akron
Akron, OH 44325-3904
Phone (216) 972-7649
FAX (216) 972-6487
A BIDIRECTIONAL MCT
ABSTRACT
The MOS-Controlled Thyristor (MCT) is emerging as a powerful switch that combines
the better characteristics of existing power devices. This paper presents a study of switching
stresses on a the MCT switch under zero voltage resonant switching. The MCT is used as a
bidirectional switch in an ac/ac pulse density modulated inverter for induction motor drive.
Current and voltage spikes are observed and analyzed with variations in the timing of the
switching. Different snubber circuit configurations are under investigation to minimize the effect
of these transients. The results of this work will be extended to study and test the MCT switching
in a medium power (5 hp) induction motor drive.
* Correspondence Author
(NASA-CR-192230) CHARACTERIZATION N93-20258
AND SNUBBING OF A BIDIRECTIONAL MCT
IN A RESONANT AC LINK CONVERTER
(Akron Univ.) 9 p Unclas
G3/33 0146047
https://ntrs.nasa.gov/search.jsp?R=19930011069 2020-03-17T07:57:08+00:00Z
2CHARACTERIZATION AND SNUBBING OF A BIDIRECTIONAL MCT
IN A RESONANT AC LINK CONVERTER
DIGEST
INTRODUCTION
The recent development of new hybrid power semiconductor devices such as the Insulated-
Gate Bipolar Transistor (IGBT) and the MOS-Controlled Thyristor (MCT) have lead to devices
that combine the power handling capability of bipolar junction transistors and the fast switching
characteristics of power MOSFETs. In particular the MCT, a newly emerging device that
combines the high current density and ruggedness of a thyristor with the ease of control of a MOS
gate promises to be the promising power switch for the next decade. Though the fabrication and
development of the MCT as a bidirectional switch is slowly coming up, device when fully
developed will probably dominate high power applications. Researchers continue to develop the
static and dynamic characteristics of the MCT [1]-[5].
B. Contribution of this 0aper
This paper addresses the issues related to a practical application of the MCT device in a
resonant ac link converter [6]. Because of the ac characteristics bidirectional switches are required.
Since the presently available MCT has only forward blocking capability, a bidirectional switch is
implemented with MCTs and diodes as shown in the circuit of figure 1. The circuit is an ac
resonant link for 3-phase PDM converter for induction motor drive. In order to keep the devices
operating in their SOA, snubber circuits are used and improvement in stresses are observed. In
particular, snubber circuits different from the conventional ones are designed for these ac/ac
converters. In an effort to obtain an optimum snubber, different circuits are under investigation
and will be presented. The results are extended to the design and implementation of an MCT, (5
hp) 1-F to 3-F induction motor drive using PDM technique.
2. THE PROPOSED CIRCUITS
A. The snubber circuits
The 1-phase ac/ac converter of figure 2 consists of two bidirectional switches. From
previous work [6], it was found that the turn-off delay of the device causes a current shoot-through
which might constitute damage at higher power levels. Previously the shoot-through was avoided
by delaying turn-on of the incoming switch and adding a snubber capacitor across each switch (see
figure 3(b)). In addition, thecapacitorcancauselargevoltage spikesacrossthe deviceduring
turn-off. As adualof theconventionalsnubber,aninductorin parallelwith afreewheelingdiode
asshownin figure 3(a) is employed. This snubberguaranteesthat nocurrentshootthroughthe
devicesand the major power lossof the snubberis in the freewheelingdiodes. In addition,
becausethetun-onsignaldoesnothaveto delayed,thegatecontrol circuit iseasierto implement.
Theimpedanceof the snubberinductorshouldbesmallcomparedto theload sothat thevoltage
dropacrosstheinductorbecomesinsignificant.
In most applications, loadsare inductive andload current hasto be continuous. The
proposedsnubbercircuit in figure 3(a)mightnot beasidealbecausetheinductorbeingin series
with the load would causea reductionin thecurrent response.By combining figures 3(a)and
3(b), thesnubbercircuit shownin figure 3(c)hasimprovedcharacteristicsbecausethecapacitor
allowcontinuouscurrentatturn-onandcurrent cannot shootthroughthedevices.
B. The gate drive circuit
It was found from an early experiment that the device could be damaged by a gate signal a
slow rise time. A safe rise time is found to be around 200ns _ 250ns. The static capacitance
measured at the gate input is about 0.91.tF; therefore each switch has about 1.8t.tF.
Theoretically, the gate drive circuit should be able to supply a high current. For a gate voltage
swings from -10V to +I5V the gate drive circuit needs more than what small signal devices can
handle. The gate drive circuit shown in figure 4 consists of power BJTs at the output stage so
that the gate current is supplied by both of the collectors of the power BJTs and the emitters of the
small signal BJTs. With this circuit rise times closer to 200 ns were obtained which guarantee a
safe switching of the device.
3. EXPERIMENTAL WORK
To study the turn on and turn off characteristics of an MCT, a simple dc/dc converter with
an inductive load and a turn-off snubber was built. The MCT ratings are 600 V and 60 A. A
few MCTs were damaged during this early experiment. Later it was found that in order to switch
an MCT without damage, the drive voltage should have a fast rise time. Therefore, the gate drive
circuit of figure 4 is designed and built for each MCT.
The ac/ac converter of figure 2 was built using two MCT switches. A Mapham inverter
[7] was used as a 20KHz voltage source for this ac/ac converter. The output voltage was
modulated to 4KHz. Since the load is inductive, it is represented by a low frequency current
source as compared to the 20 kHz input, figure 5 shows the idealized control tinting and the output
4voltageandcurrentand figure 6 showstheexperimentalwaveformscorrespondingto the load
voltageandcuri'¢nt.
Ideally in thisconfigurationthedevicesswitchatzerovoltagewith oneswitchturnson the
otherturnsoff. Conventionally,to preventshootthrough,the switch turningon is often delayed
anda capacitoris requiredto keepthe currentcontinuousduring turn on. This approachhas
beenappliedto similarcircuits usingIGBTs [6]. At high powerthis could createlargevoltage
spikesthat coulddamagethedevice. Experimentallywith a 20kHz sourceof 90v pp theturn on
wasdelayedto about4 Its. Under thisconditionvoltagespikescauseddamageto theMCTs with
switching currents as low as 1.5A. Although a delay of 4 l.tsec seems awfully high, it is
introduced here to show the potential voltage spikes that will occur with a small delay in a high
power circuit.
To observe the effect of shoot through on the device, the turn-on was advanced to have up
to 4us period in which both switches are on. Without any snubbing on the devices, current spikes
of about 12A occurred (see figure 7) but it did not constitute any damage to the devices. A simple
snubber circuit consisting of a diode parallel with a 50 ItH inductor was used to prevent current
shoot-through as shown in figure 3(a). It was found that the current spikes were reduced to about
1.5A as shown in figure 8 and the voltage drop across the inductor was significantly small.
The full paper will discuss details of these experimental results along with results of
switching at higher power levels.
REFERENCES
[1] Frank Goodenough, "A New Breed of Power Switch," Electronic Design, November, 1988,
[2] Harris Semiconductor Inc. "User Guide to MCTs," 1989.
[3] T.M. Jahns, R.W. De Doncker, J.W.A Wilson, V.A.K. Temple and D.L. Watrous, "
Circuit Utilization Characteristics of MOS-Controlled Thyristors," Proceedings of the IEEE
Industry Applications Society, Oct. 1989, pp. 1-6.
[41 R.W. De Doncker, T.M. Jahns, A.V. Radun, V.A.K. Temple and D.L. Watrous, "
Characteristics of MOS-Controlled Thyristors under zero voltage soft-switching conditions,"
Proceedings of the IEEE Industry Applications Society, Oct. 1989, pp. 1-6.
[5] C. Braun, "Circuit-Level Modeling of MOS Controlled Thyristors," Proceedings of the 19th
Power Modulator Symposium, 1990, pp. 436-440.
[6] S.K. Sul, F. Profumo, G.H. Cho and T.A. Lipo, "MCTS and IGBTS: A Comparison of
Performance in Power Electronic Circuits," IEEE PESC '89 Record, June 1989, pp 163-169
[7] N. Mapham, "An SCR Inverter with Good Regulation and Sine-Wave Output", IEEE Trans.
on Ind. Gen. Application, Vol. IGA-3, Mar./Apr. 1967, pp. 176-187.
51
Figure 1 MCT bidirectional Switch
G
a
4kHz
20 kHz
Figure 2 1-phase PDM ac resonant link converter using MCTs
(a)
Figure 3
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(c)
Different snubber circuits under investigation
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Figure 4 Improved MCT gate-drive with rise time of "_ 200 nsec
Figure 5 Timing and Control Waveforms for
the Circuit of figure 2
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Figure 6 Experiment load current and voltage waveforms
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Figure 7 Current shoot-through due to turn-on advance
Iw/snubber I
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Fig. 8 Current shoot-through due to turn-on advance
with the snubber of figure 3(b)


Characterization and snubbing of a bidirectional MCT in a resonant ac link converter

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