How is this thyristor unlatched in this pulsed power circuit?

[zigunov]

Hi, fellow enthusiasts! I was trying to build a trigger circuit for a Xenon flashlamp (Yes, I know it is dangerous, I am taking significant precautions!).

The issue I'm facing is likely because I am not familiar with SCRs. See a circuit similar to what I'm trying to build in the attachment (comes from Les' lab teardown of this laser power supply https://youtu.be/8khFcicKwJ4?si=zSY34p-erosQiF_e&t=522).

After building some simpler versions of this circuit (say, a low-voltage capacitor crowbar with the SCR), I'm facing the problem that the SCR remains latched after the trigger signal at the gate. As far as I understood, this is expected behavior and it's ok. The thing I'm not quite appreciating by reading the attached circuit is how the SCR is unlatched. If I gather correctly, I need to make the current between Anode and Cathode close to zero for it to unlatch. But the only thing that could do that is the TVS diode D1. But that diode will not conduct until the voltage between A&K exceeds 550V, so it should be open throughout the operation, is that not right?

I thought that maybe the trigger transformer would produce a negative voltage transient that would unlatch the SCR, but that doesn't happen in my circuit. I also tried at a higher voltage (1000V) and it triggers once, latches, and then the current through the SCR is so high that it breaks.

Perhaps you could point out what I'm missing here?

Thanks!
 

[Kim Christensen]

All I can think of is that R320 (268K) should be of a high enough value that there's not enough current through it to keep the SCR on after C311 (0.47uF) has been discharged into T1 by the SCR.
ie: SCR should have a higher hold current than HT/R320.

EDIT: Are you testing this circuit without the Xenon flashlamp installed? That may be the issue since the Xenon flashlamp will partially discharge the HT supply which will temporarily lower the current through R320.

[Terry Bites]

Yes. A typcial small SCR will have an Ih in the 1-5mA range.

For a flashlamp, HT is 400V or so. That limits the current to about 1.5mA when charging. After discharge C311 starts to recharge keeping the anode voltage low long enough for the SCR to reset. The availble current via R320 drops to uA levels.

[mag_therm]

The magnetizing inductance of T1 and capacitor C311 form a resonant LC.
When the scr is fired, there will be a sine half pulse of current into the anode of scr. That will ignite the lamp.
After the positive half pulse the LC will try to ring for a negative half pulse.
This reverse current in the scr will cause it to switch off, with a transient reverse bias voltage to hold it off
until is recovers to off-state.
So your LC has to have a larger enough pulse width to be > the reverse recovery time (tot) of the scr
We can guess the scr would have a tot of less than 20 us.
So you can adjust LC to have a pulse width > 40 usec or so. ( My guess)
pw = pi * sqrt(L*C) However the lamp acts to damp this so  the transformer jWL ( Ohm) should be > 4 * equivalent lamp R (Ohm)

[zigunov]

Thanks for the replies, they were very helpful. I did not think about the holding current having a threshold. That actually makes sense and is evidently reason that R320 resistor is there. I'll review my circuit to see if that is the problem. I tested using an appropriate limiting resistor on a low-voltage bench setup and it unlatches properly (I'm using the 70TPS16 SCR for my circuit, which has a holding current of 40mA).

Also, another thing to think about; is the fact that the microcontroller could potentially just shut off the HT power supply after the flash trigger signal is sent. That would guarantee that the SCR opens (eventually).

Thanks for the help!