Hello All!

I have an 850 VDC electrolytic capacitor bank which I want to discharge “rapidly” using an SCR. This is part of some older experimental equipment which needs repair. The repair involves designing/building a suitable pulse circuit to trigger an SCR (PDF datasheet attached). The trigger circuit I envision would be based around a microcontroller which receives an optical signal from external equipment. Upon receipt of the optical signal (a 5V pulse from a Broadcom HFBR-2521Z, link below) the microcontroller would send a 5 VDC rectangular 50 uS pulse to the trigger circuit which would deliver a 12V 150 mA rectangular pulse to the SCR gate, to discharge the capacitor bank.

I am admittedly out of my depth in regard to experience designing a suitable circuit to trigger the SCR, and would appreciate some guidance, especially in view of the 850 V charge held on the capacitor bank (a parallel/series arrangement using 4-450V, 820uF Kemet ALC10A-series capacitors) . My internet searching has not yet revealed approaches I can readily start from.

I am also concerned with recommendations on how to best isolate the low-voltage microcontroller circuit from the high-voltage SCR discharge section of the system. I have looked at some 1:1 pulse transformers which I think could be appropriate for this.

The main component I am concerned about is the Powerex "Fast Switching SCR T7SH1646". This is now obsolete but functional. I hope I have posted the PDF datasheet correctly below.

Any guidance on where to look for some design theory/background on this project would be very much appreciated.
Thanks All!
-Steve

 

I would encourage you to consider modern alternatives to the SCR like SiC FET or a GaN device.

ETA: Do you have a schematic of the existing circuit?

 

For ISOLATED triggering of an SCR, which I certainly recommend, the classic method of using a pulse transformer is what I recommend. That method has been used since the SCR was invented. For this application the pulse would need to switch on a transistor that will discharge a capacitor into the primary of the pulse transformer. A pulse transformer can easily provide the adequate isolation as well as enough current from a low impedance source to properly trigger the SCR.
Please respond if this method of triggering seems reasonable and not too complex.

 

I would encourage you to consider modern alternatives to the SCR like SiC FET or a GaN device.

ETA: Do you have a schematic of the existing circuit?

I have attached a Word document, hope it is readable on the AAC forum. The schematic is a simplistic overview. A complete schematic does not exist. I inherited this project with very little information to go on.

Can you suggest a SiC FET or a GaN device I could go look at the data sheets? Concerns are relatively high-ish voltage and high discharge current.

Thanks!

 

For ISOLATED triggering of an SCR, which I certainly recommend, the classic method of using a pulse transformer is what I recommend. That method has been used since the SCR was invented. For this application the pulse would need to switch on a transistor that will discharge a capacitor into the primary of the pulse transformer. A pulse transformer can easily provide the adequate isolation as well as enough current from a low impedance source to properly trigger the SCR.
Please respond if this method of triggering seems reasonable and not too complex.

Hi Bill, thanks for the response. Isolation with a pulse transformer is what I'm leaning toward. Cheers!

 

I also would suggest a pulse transformer, as an isolated trigger.

EDIT: you would be discharging almost 150 joules. That is an immense amount of energy. Include a current limiting resistor.

 

I also would suggest a pulse transformer, as an isolated trigger.

EDIT: you would be discharging almost 150 joules. That is an immense amount of energy. Include a current limiting resistor.

Quite a few of those capacitor discharge circuits ARE INTENDED TO DELIVER A LARGE AMOUNT OF ENERGY! Impulse welding and magnet charging are the two that come to mind first, followed by capacitor-discharge ignition systems for IC engines. Of course, the system must be designed to only deliver the energy required.