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SCR and snubber LTSpice simulation

I have been working on a high current test set which uses SCR Solid State Relays to drive the primary of a transformer with 480 VAC mains power. There is a contactor which closes to apply power to the SSR, and then at a later time it is energized at the peak voltage point to reduce DC offset into the inductive load. The output of the transformer is about 2 volts and is capable of supplying 20,000 amperes or more into a low impedance load (circuit breaker), or a shunt for the purposes of calibration.

The problem has been that, sometimes, the SCR will conduct for a half-cycle or so when the contactor closes. We have tried all sorts of snubbers across the SCRs and also across the primary of the transformer, with some success, but it still occasionally triggers, which sometimes causes a very high inrush and it has blown 100 ampere LPS fuses.

I made an LTSpice simulation that is roughly the same as the actual circuit, and it mostly performs as expected. But the SCR triggers on for 1/4 cycle when the contactor is closed at the voltage peak (but also at other phase angles). I have tried various snubbers and even series inductance to reduce the dV/dt which seems to be triggering the SCRs. But nothing worked. However, when I put a resistance of less than about 1 MegOhm across the contactor, only a very small spike occurs at turn-on. I will be doing work on the actual unit tomorrow, but I'd like to understand what may be going on before then. Any ideas will be appreciated.

Paul

Here is the ASCII file:

=============================================== Version 4 SHEET 1 1756 680 WIRE 0 -96 -48 -96 WIRE 96 -96 80 -96 WIRE 64 -16 -112 -16 WIRE 96 -16 96 -96 WIRE 400 -16 96 -16 WIRE 560 -16 544 -16 WIRE 704 -16 560 -16 WIRE 16 16 -208 16 WIRE 64 16 64 -16 WIRE 288 16 272 16 WIRE 416 16 368 16 WIRE -48 64 -48 -96 WIRE -16 64 -48 64 WIRE 0 64 -16 64 WIRE 96 64 96 -16 WIRE 96 64 80 64 WIRE 128 64 96 64 WIRE 256 64 128 64 WIRE 272 64 272 16 WIRE 272 64 256 64 WIRE 416 64 272 64 WIRE 512 64 480 64 WIRE 560 64 560 -16 WIRE 560 64 512 64 WIRE 704 64 704 -16 WIRE 960 64 832 64 WIRE 704 112 704 64 WIRE 832 112 832 64 WIRE 960 112 960 64 WIRE -208 128 -208 16 WIRE -48 160 -48 64 WIRE 256 160 256 64 WIRE 288 160 256 160 WIRE 512 160 512 64 WIRE 512 160 352 160 WIRE 128 176 128 128 WIRE 384 208 352 208 WIRE 512 208 512 160 WIRE 512 208 464 208 WIRE -208 240 -208 208 WIRE -112 240 -112 -16 WIRE -112 240 -208 240 WIRE -48 256 -48 240 WIRE 128 256 -48 256 WIRE 704 256 704 192 WIRE 704 256 128 256 WIRE 832 256 832 192 WIRE 832 256 704 256 WIRE 960 256 960 192 WIRE 960 256 832 256 WIRE -48 288 -48 256 FLAG -48 288 0 FLAG -16 64 Vin FLAG -208 240 0 FLAG 704 64 Vrly SYMBOL res 112 160 R0 SYMATTR InstName R1 SYMATTR Value 1 SYMBOL cap 112 64 R0 SYMATTR InstName C1 SYMATTR Value 6µ SYMBOL voltage -48 144 R0 WINDOW 3 103 175 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 22 174 Left 0 WINDOW 0 -45 9 Left 0 SYMATTR Value SINE(0 720 60 0 0 0 240) SYMATTR SpiceLine Rser=.1 SYMATTR InstName V1 SYMBOL sw 96 64 R90 WINDOW 3 49 -5 VRight 0 WINDOW 0 24 92 VRight 0 SYMATTR Value MySwitch SYMATTR InstName S1 SYMBOL ind2 688 96 R0 WINDOW 3 21 114 Left 0 SYMATTR Value 500m SYMATTR InstName L1 SYMATTR SpiceLine Rser=100u Rpar=1Meg SYMATTR Type ind SYMBOL voltage -208 112 R0 WINDOW 3 -219 162 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PWL(0 0 .054167 0 .054168 5 .55 5 .551 0 .9 0 .91 5 1.6 5 1.61

0 1.90 0 1.91 5 2.85 5 2.851 0 3 0) SYMATTR InstName V2 SYMBOL Misc\\SCR 480 48 R90 SYMATTR InstName U1 SYMBOL Misc\\SCR 288 176 R270 SYMATTR InstName U2 SYMBOL res 560 -32 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 20 SYMBOL cap 464 -32 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C2 SYMATTR Value .47µ SYMBOL ind2 848 96 M0 SYMATTR InstName L2 SYMATTR Value 50µ SYMATTR SpiceLine Rser=50u Rpar=1Meg SYMATTR Type ind SYMBOL res 944 96 R0 SYMATTR InstName R3 SYMATTR Value 100µ SYMBOL res 96 -112 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 250k SYMBOL voltage 368 208 R270 WINDOW 3 291 -216 VRight 0 WINDOW 123 0 0 Left 0 WINDOW 39 24 -12 Left 0 SYMATTR Value PWL(0 0 .1041667 0 .1041700 5 .5041667 5 .50417 0 1.00 0 1.001 5 1.50 5 1.501 0 2.004167 0 2.004168 5 2.5 5 2.75 5 2.7501 0 3 0) SYMATTR SpiceLine Rser=10 SYMATTR InstName V3 SYMBOL voltage 384 16 M270 WINDOW 3 128 -1014 VRight 0 WINDOW 123 0 0 Left 0 WINDOW 39 24 -12 Left 0 SYMATTR Value PWL(0 0 .1041667 0 .1041700 5 .5041667 5 .50417 0 1.00 0 1.001 5 1.50 5 1.501 0 2.004167 0 2.004168 5 2.5 5 2.75 5 2.7501 0 3 0) SYMATTR SpiceLine Rser=10 SYMATTR InstName V4 TEXT -408 200 Left 0 !.tran 3 startup TEXT -392 344 Left 0 !.model MySwitch SW(Ron=.01 Roff=100Meg Vt=2 Vh=-.5 Lser=10n Vser=0) TEXT -24 160 Left 0 !.inc SCR.SUB TEXT 720 16 Left 0 !K1 L1 L2 1

================================================= The SCR model (SCR.sub) modified from

formatting link
is:

  • SCR:
  • Extracted from PONT_DIPH.CIR file from EDN website.
  • I renamed that file as SCR_CKT.CIR
*
  • Anode
  • | Cathode
  • | | Trigger
  • | | | .subckt SCR 1 3 2
  • (I had to change the above line to match the pins of the SCR symbol)
  • Models used by the SCR model: .model STH VSwitch (ROn=0.1,VOff=0.5) .model DLG D (Rs =25m)
*
  • We assume that the SCR in the on-state is equivalent to 0.1 ohm: SSCR 1 4 5 0 STH VSCR 4 2 0V
*
  • Trigger to cathode impedance is a 1k ohm resistance - for simplification: RTrig 2 3 1k
*
  • The following circuit locks in the on-state when a 1mA current flows
  • through the gate to the cathode with a positive anode to cathode voltage.
  • This circuit locks in the off-state when the current through ths SCR tends
  • to become negative: VPlus 6 0 2V SiTrig 6 5 3 2 STH CState 5 0 1uF IC=0 DLogic 5 8 DLG HCur 8 0 poly(1) VSCR 1 10000
*
  • The potential of node 5 reflects the state of the SCR. It is less than 1V
  • for the off-state and between 1V and 2V in the on-state. .ends
Reply to
Paul E. Schoen
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Did you lower the gate to K impedance? dV/dt on the anode couples to the gate as well as you having to meet risetime dV/dt spec on anode?

Sorry, I'm not looking at the sim right now.

Grant.

Reply to
Grant

I first did the simulation with th gates shorted to cathode. Now I have a PWL 5V voltage source with 10 ohms resistance.

Thanks for the idea.

I might also try simulating an SCR with two cross-connected transistors PNPN.

Paul

Reply to
Paul E. Schoen

I don't know how well a sim is going to match real life with power SCRs.

Way back we used low gate resistance to a trigger transformer, the bigger SCRs with low sensitivity gates we fired reliably with pilot SCRs ;)

Grant.

Reply to
Grant

You didn't specify the parameters of the SCR relay you are using.

Here's a subcircuit that you might find useful, it's the SKT100 (Semikron) stud thyristor, but it can be tailored to many other devices by "plugging in" datasheet parameters. I've had good results on several occasions modeling various devices with variations of it.

SKT100.lib:

.SUBCKT SKT100 anode gate cathode PARAMS:

* *SCR *MODEL FORMAT: PSpice
  • Vdrm=1200v Vrrm=1200v Idrm=1u
  • Ih=150ma dVdt=500e6
  • Igt=150ma Vgt=1.75v
  • Vtm=1.75v Itm=100
  • Ton=10u Toff=20u

  • Where:

  • Vdrm => Forward breakover voltage
  • Vrrm => Reverse breakdown voltage
  • Idrm => Peak blocking current
  • Ih => Holding current
  • dVdt => Critical value for dV/dt triggering
  • Igt => Gate trigger current
  • Vgt => Gate trigger voltage
  • Vtm => On-state voltage
  • Itm => On-state current
  • Ton => Turn-on time
  • Toff => Turn-off time

  • Main conduction path Scr anode anode0 control 0 Vswitch ; controlled switch Dak1 anode0 anode2 Dakfwd OFF ; SCR is initially off Dka cathode anode0 Dkarev OFF VIak anode2 cathode ; current sensor

  • dVdt Turn-on Emon dvdt0 0 TABLE {v(anode,cathode)} (0 0) (2000 2000) CdVdt dvdt0 dvdt1 100pfd ; displacement current Rdlay dvdt1 dvdt2 1k VdVdt dvdt2 cathode DC 0.0 EdVdt condvdt 0 TABLE {i(vdVdt)-100p*dVdt} (0 0 ) (.1m 10) RdVdt condvdt 0 1meg

  • Gate Rseries gate gate1 {(Vgt-0.54)/Igt} Rshunt gate1 gate2 {0.54/Igt} Dgkf gate1 gate2 Dgk VIgf gate2 cathode ; current sensor

  • Gate Turn-on Egate1 gate4 0 TABLE {i(Vigf)-0.95*Igt} (0 0) (1m 10) Rgate1 gate4 0 1meg Egon1 congate 0 TABLE {v(gate4)*v(anode,cathode)} (0 0) (10 10) Rgon1 congate 0 1meg

  • Main Turn-on EItot Itot 0 TABLE {i(VIak)+5E-5*i(VIgf)/Igt} (0 0) (2000 2000) RItot Itot 0 1meg Eprod prod 0 TABLE {v(anode,cathode)*v(Itot)} (0 0) (1 1) Rprod prod 0 1meg Elin conmain 0 TABLE

  • {10*(v(prod) - (Vtm*Ih))/(Vtm*Ih)} (0 0) (2 10) Rlin conmain 0 1meg

  • Turn-on/Turn-off control Eonoff contot 0 TABLE

  • {v(congate)+v(conmain)+v(condvdt)} (0 0) (10 10)

  • Turn-on/Turn-off delays Rton contot dlay1 825 Dton dlay1 control Delay Rtoff contot dlay2 {290*Toff/Ton} Dtoff control dlay2 Delay Cton control 0 {Ton/454}

  • Reverse breakdown Dbreak anode break1 Dbreak Dbreak2 cathode break1 Dseries

  • Controlled switch model .MODEL Vswitch vswitch

  • (Ron = {(Vtm-0.7)/Itm}, Roff = {Vdrm*Vdrm/(Vtm*Ih)},
  • Von = 5.0, Voff = 1.5)

  • Diodes .MODEL Dgk D (Is=1E-16 Cjo=50pf Rs=5) .MODEL Dseries D (Is=1E-14) .MODEL Delay D (Is=1E-12 Cjo=5pf Rs=0.01) .MODEL Dkarev D (Is=1E-10 Cjo=5pf Rs=0.01) .MODEL Dakfwd D (Is=4E-11 Cjo=5pf) .MODEL Dbreak D (Ibv=1E-7 Bv={1.1*Vrrm} Cjo=5pf Rs=0.5)

  • Allow the gate to float if required Rfloat gate cathode 1e10

.ENDS

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)
Reply to
Fred Abse

"Fred Abse" wrote in message news: snipped-for-privacy@invalid.invalid...

Thanks for the subcircuit. I may try it next. I simulated an SCR with an NPN/PNP pair but I had to reduce the applied voltage because the models were for 60 volt devices. The simulation is now much faster, and I still get the initial unwanted turn-on. I tried various means to reduce the turn-on dV/dt but even at 14 V/uSec I get the initial spike. Here is the simulation with the two transistor SCR model.

Paul ==============================================================

Version 4 SHEET 1 1756 680 WIRE 256 -256 208 -256 WIRE 448 -256 336 -256 WIRE 480 -256 448 -256 WIRE 656 -256 576 -256 WIRE 448 -176 448 -256 WIRE 208 -112 208 -256 WIRE 400 -112 208 -112 WIRE 528 -112 528 -192 WIRE 528 -112 496 -112 WIRE 64 -16 -160 -16 WIRE 208 -16 208 -112 WIRE 400 -16 208 -16 WIRE 560 -16 544 -16 WIRE 656 -16 656 -256 WIRE 656 -16 560 -16 WIRE 704 -16 656 -16 WIRE 16 16 -112 16 WIRE 64 16 64 -16 WIRE -112 64 -112 16 WIRE -16 64 -48 64 WIRE 0 64 -16 64 WIRE 128 64 80 64 WIRE 208 64 208 -16 WIRE 208 64 128 64 WIRE 384 64 352 64 WIRE 560 64 560 -16 WIRE 560 64 464 64 WIRE 704 64 704 -16 WIRE 960 64 832 64 WIRE 704 112 704 64 WIRE 832 112 832 64 WIRE 960 112 960 64 WIRE 208 128 208 64 WIRE 240 128 208 128 WIRE 352 128 352 64 WIRE 352 128 336 128 WIRE 432 128 352 128 WIRE -160 144 -160 -16 WIRE -160 144 -208 144 WIRE -112 144 -160 144 WIRE -48 160 -48 64 WIRE 128 176 128 128 WIRE 432 176 432 128 WIRE 288 240 288 192 WIRE 384 240 288 240 WIRE 560 240 560 64 WIRE 560 240 480 240 WIRE -48 256 -48 240 WIRE 128 256 -48 256 WIRE 704 256 704 192 WIRE 704 256 128 256 WIRE 832 256 832 192 WIRE 832 256 704 256 WIRE 960 256 960 192 WIRE 960 256 832 256 WIRE -48 288 -48 256 FLAG -48 288 0 FLAG -16 64 Vin FLAG -208 144 0 FLAG 704 64 Vout SYMBOL res 112 160 R0 SYMATTR InstName R1 SYMATTR Value 20 SYMBOL cap 112 64 R0 SYMATTR InstName C1 SYMATTR Value 6µ SYMBOL voltage -48 144 R0 WINDOW 3 -261 54 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 30 83 Left 0 WINDOW 0 -45 9 Left 0 SYMATTR Value SINE(0 35 60 0 0 0 240) SYMATTR SpiceLine Rser=1 SYMATTR InstName V1 SYMBOL sw 96 64 R90 WINDOW 3 49 -5 VRight 0 WINDOW 0 24 92 VRight 0 SYMATTR Value MySwitch SYMATTR InstName S1 SYMBOL ind2 688 96 R0 WINDOW 3 21 114 Left 0 SYMATTR Value 500m SYMATTR InstName L1 SYMATTR SpiceLine Rser=100u Rpar=1Meg SYMATTR Type ind SYMBOL voltage -112 48 R0 WINDOW 3 -813 -83 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PWL(0 0 .054167 0 .054168 5 .55 5 .551 0 .9 0 .91 5 1.6 5 1.61

0 1.90 0 1.91 5 2.85 5 2.851 0 3 0) SYMATTR InstName V2 SYMBOL res 560 -32 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 1µ SYMBOL cap 464 -32 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C2 SYMATTR Value .47µ SYMBOL ind2 848 96 M0 SYMATTR InstName L2 SYMATTR Value 50µ SYMATTR SpiceLine Rser=50u Rpar=1Meg SYMATTR Type ind SYMBOL res 944 96 R0 SYMATTR InstName R3 SYMATTR Value 100µ SYMBOL voltage 368 64 R270 WINDOW 3 232 -1656 VRight 0 WINDOW 123 0 0 Left 0 WINDOW 39 -21 94 VRight 0 WINDOW 0 24 93 VRight 0 SYMATTR Value PWL(0 0 .1041667 0 .1041700 5 .5041667 5 .50417 0 1.00 0 1.001 5 1.50 5 1.501 0 2.004167 0 2.004168 5 2.5 5 2.75 5 2.7501 0 3 0) SYMATTR SpiceLine Rser=10 SYMATTR InstName V3 SYMBOL voltage 352 -256 M270 WINDOW 3 -9 198 VRight 0 WINDOW 123 0 0 Left 0 WINDOW 39 -50 52 VLeft 0 WINDOW 0 -48 101 VLeft 0 SYMATTR Value PWL(0 0 .1041667 0 .1041700 5 .5041667 5 .50417 0 1.00 0 1.001 5 1.50 5 1.501 0 2.004167 0 2.004168 5 2.5 5 2.75 5 2.7501 0 3 0) SYMATTR SpiceLine Rser=10 SYMATTR InstName V4 SYMBOL npn 384 176 M90 WINDOW 0 46 31 Left 0 WINDOW 3 37 59 Left 0 SYMATTR InstName Q1 SYMATTR Value 2N3055 SYMBOL pnp 336 192 M270 WINDOW 0 39 30 Left 0 WINDOW 3 41 61 Left 0 SYMATTR InstName Q2 SYMATTR Value D45H11 SYMBOL npn 496 -176 R90 WINDOW 0 46 31 Left 0 WINDOW 3 37 59 Left 0 SYMATTR InstName Q3 SYMATTR Value 2N3055 SYMBOL pnp 480 -192 R270 WINDOW 0 39 30 Left 0 WINDOW 3 41 61 Left 0 SYMATTR InstName Q4 SYMATTR Value D45H11 TEXT -544 56 Left 0 !.tran 3 startup TEXT -784 -64 Left 0 !.model MySwitch SW(Ron=.1 Roff=100Meg Vt=2 Vh=-.5 Lser=10n Vser=0) TEXT -24 160 Left 0 !.inc SCR.SUB TEXT 720 16 Left 0 !K1 L1 L2 1
Reply to
Paul E. Schoen

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