[SOLVED] Misbehaving thyristor with ltspice simulation

Trying to simulate circuit with a thyristor in it. I am unsure why thyristor isn't conducting far more current. I guess its because I haven't any experience simulating 3rd party thyristors and I don't know spice to find issues.


Greatly appriciated.

Hi,

Can't you simply show the simulation and it's results?

Klaus

Don't need this simulation.

First issue is wrong SCR connection (A-K flipped), second issue is different pin order of Ltspice SCR symbol (A-G-K) and SCR model file (A-K-G). Also a real SCR should be never triggered by an unlimited voltage source.



After fixing this points, the actual circuit problems reveal. Most serious problem is that the SCR is never extinguished after being triggered once.

FvM said:

...by an unlimited voltage source...

Click to expand...

Source had series resistance to imitate choke.

FvM said:

...never extinguished after being triggered once...

Click to expand...

I will use an IRFP4668 fet to dump cap energy into coil instead of SCR. However I did get the scr model to work by simply rearranging pins.

I would really appreciate if someone would let me know whether the fet is up for the task. AFAIK exceeds circuit demands which is ~120amp spike ~8ms long.
**broken link removed**

Hi

I would really appreciate if someone would let me know whether the fet is up for the task. AFAIK exceeds circuit demands which is ~120amp spike ~8ms long.

Click to expand...

Usually there is an SOA chart in the datasheet. There you should find the answer.

Klaus

Source had series resistance to imitate choke.

Click to expand...

I referred to unlimited gate trigger circuit rather than anode circuit.

The MOSFET selection problem has been already discussed in your previous thread.
https://www.edaboard.com/showthread.php?376985-MOSFETs-handling-high-current-within-short-duration

KlausST said:

...an SOA...

Click to expand...

I know very well what an SOA is but Im confused about load parameters. Presently I measure very negligible volts & amps yet the fet dissipates ~4watts on average when discharging the cap @ 11hz which is confusing. What coordinates do I reference on the SOA? Are measured parameters correct?



Greatly appreciated.

Hi,

Are measured parameters correct?

Click to expand...

Are you talking about "measured" or "simulated" currents?

A simulation result is only as good as the informations you input in the simulation tool.
Thus I already recommended to use realistic values and a realistic circuit.

--> you used an "unrealistic" ideal switch not taking care about voltage_drop and not taking care about switch_off_conditions.
Thus I've already talked about "useless" simulation.

And there are other unrealistic things in your system: ideal capacitor, maybe the power supply, gate current limitation....

I almost never use simulations. I mostly rely on the datasheet specifications and on the application informations.
This doesn't mean that simulations are useless. You can play around with varying part values without the risk of explosion and fire. And it gives a "visual" idea about voltage and current waveforms.

Especiall the switch OFF behavior of an SCR makes that the waveform stops when the current becomes zero.
But with a Mosfet you may expect the current going negative .... causing ringing.

Klaus

It's easy to see that the discharge current waveform is defined by circuit LRC, in so far it's realistic, if the assumed circuit parameters are correct.

You did not show the related simulation circuit, I presume it's similar to the post #1 circuit, replacing the SCR with a MOSFET. Presume also you notice the high flyback diode current, should use a 5 or 10A rectifier diode.

Hi,

In simulations, I got caught out a month or two ago: The 5457 JFET model is great, works well because the simulations are very similar in the real world with real components and circuit conditions - however, I forgot to check what VGSth off the JFET model had set. It was -1.37V. My two little test boards all had JFETs with -2.7, -1.7, -2.3, -2.8. The test boards were for voltage sources that depended on a specific current which depends entirely on the JFET VGSth off which sets a dubious but workable voltage regulation - they were all like the simulation but none were like the exact transistor in the simulations so not one of the references was the voltage required... You live and learn. Design for worst case manufacturing parameters - imagine something turning on when you'd thought it should be off based on real-world manufacturing variations and minute differences in e.g. gate/base voltages and temperature affecting that.