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thyristor control circuit technical question

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motorhomer:
Hello All, I have a technical question about a circuit which has two thyristors which switch on/off to power two transformers which then drop down the voltage from 750v to 110vdc.

The part that fails over a short period of time are the thyristors which go to short circuit. Attached is the schematic but I cannot show the controlling circuitry to this part.

My main two questions are.

1. If they are shorting out then I assume switching the thryristors on permanently will not cause these to fail and I can assume there is no short to earth like a H bridge would do as this would cause an open circuit. With the Thyristors on permanently the transformers will be like an inductor in the circuit.

2. What would cause a thyristor to short across. Could this be the snubber circuit not working or the bank of capacitor not doing there job properly which are designed to smooth out any spikes, or could switching on/off to quickly cause the thyristor to overheat and fail.

I will assume over switching of the thyristors will increase the output frequency at 110v side which i assume will mean less magnetic flux cutting secondary coil and therefore either less voltage or no voltage on output.

The top right circuit is the 100Vdc rectified output and the bottom right is the 370Vdc output which goes on to be inverted to 240vac

Will update when I have more information on the circuit. What i can say is this is a flyback converter

T3sl4co1l:
Yikes, where's the commutation?

Tim

PartialDischarge:
At first glance I’d say the snubber is not working properly. Check if the thyristor is mounted properly with enough clamping force. Overvoltages are the achilles heel of thyristors and will short them easily, if its an old machine snubber caps do fail, so check or replace it, also check the parallel diodes

motorhomer:
Thanks i did read over voltage could short thyristors, so i will get these parts tested. To add to the circuit D1 is there to stop any reverse spike on 750v side when switching on/off. So if this was shorted out then this might cause a reverse spike through the thyristor and short it, but this is a theory as the reverse diodes D2 and D3 should take this.

NiHaoMike:
Have you considered upgrading to a modern design based around IGBTs? Thyristor DC/DC converters for less than 1kV input have practically disappeared due to the difficulty getting proper switching.

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