Solid State Relay (SSR) with peak firing SCRs for inductive loads

how about an FETs/IGBTs?

I have been thinking about a short circuit protected master switch for a triac controller board

A sil dc-dc converter, optocoupler, ir2127 and a pair of FETs or IGBTs back to back

-Lasse

"P E Schoen"

I have used SSRs with peak firing for inductive loads (transformers), but they are generally non-stock items and are rather expensive:

I would like to duplicate the drive circuits of the peak firing SSRs, which must be fairly simple. I have considered using optoisolators with SCR outputs, such as:

This is one of the better ones:

Despite the term "optotriac" in the title, the output uses SCRs - the various dv/dt figures are all 5 or 10kV /uS.

and simply using the anode voltage of the main SCR to drive its gate through the opto-SCR. But I am worried that the sudden application of the peak voltage (up to 375 volts) to the gate might cause damage,

although when the gate triggers, the anode voltage will drop almost instantly to 1-2 volts. And at that voltage, I'm not sure the opto-SCR can maintain gate drive after the initial firing.

With the opto-SCRs on for the remaining period of time determined by the control signal, there will be times during the zero crossing where there will be no voltage to trigger,

With an inductive load, zero current will not coincide with zero volts - SCRs and Triacs remain on until the current is below holding conditions.

and there may be some distortion until enough voltage is present to cause the main SCR to conduct.

Since it is a highly inductive load, there will be current in the SCR at the time of the applied voltage zero crossing, and if the gate is not triggered the current will keep it in conduction. If it does stop conducting and voltage once again appears on the anode, the opto-SCR will trigger the gate.

Are these peak-firing (and the ubiquitous zero-crossing firing) SSRs really so simple as to use opto-SCRs to drive the gated from the anode voltage?

Or do they use some sort of trigger pulse transformer for the gates? We have found that it is necessary to maintain DC gate drive on both SCRs throughout the entire cycle of the sine wave.

... Phil

IGBTs

I have thought about something like this, but SCRs are generally less expensive and better suited to high power non-PWM applications. After another search I found the following:

It shows two SCRs connected in anti-parallel and a simple switch closure

between the gates causes the module to turn on. Then they show the use of a rather complex opto-transistor driving another SCR and a FWB for bidirectional operation of the switch. But it seems that this would require several volts to maintain gate drive after turn-on. It may not be too critical, but it seems that it may be enough just to use a resistor to limit the current to the gate at turn-on.

I considered using a small inductor so that the current would not have such a high peak at initial turn-on. For some 55A 1kV SCRs I may use:

Thanks for the suggestions.

Paul

At the risk of depleting the supply, they are often (but unreliably) available by each, at short leadtimes and low cost if you peruse a certain popular action site that rhymes with "Be Prey." You have to do your own lookup most of the time - the sellers often don't know much about them. Some of the buyers may not either. Keep an eye out, and you might find something that works for you...

be

you

Thanks for the idea. I found a possible source for 30A 240 VAC random fired units here:

And there is a website for the manufacturer with higher current models:

This is for an OEM product so the onesey twosey items won't work for us, but the sources above seem to be able to supply in quantity.

Still, I think it may be best to roll our own, and avoid any possibility of shortages and long lead times.

Paul