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Gate-drive for a large (325 A) thyristor.

Hello everyone,

I'm faced with an electronics problem. It's about a capacitor discharge spotwelder I'm building. For those who don't know what it is, these links explain best:

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(these products make me drool)

It basically discharges a large capacitor switched by a large thyristor, to the two electrodes.

My design is mostly based on the schematic as found in the 2nd link, with some modifications. First of all, it uses more capacity, about 2 F at 25 V (30 pcs of 68.000 uF/25 V elcos, total ~600 Ws). I have modified the trigger by adding a NE555 to debounce the switch and to make sure that continuous pressing of the trigger gives just one (~ .5 s) fired pulse to the thyristor. The .5 s time is pretty long (the thyristor will discharge the capacitor in milliseconds, if not microseconds) to make sure that the relay will properly disconnect the PSU as otherwise the thyristor (SCR) would remain conductive due to the large supply current (~13 A) of the PSU.

Schematic of how I intend to build mine can be found here:

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The thyristor that I intend to use is a large beefy P202CH12, rated at

325 A continuous and 3500 A pk. It should be more than fine for this application. However, the thing that I can't figure out is how to properly drive the gate.

In the datasheet

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(warning, it's an 8 MB .pdf file)) on page 3 it says, under item nr. 4 'gate drive': 'the recommended gate drive is 20 V, 20 ohm'. Sounds pretty straightforward. But, when I look at the graph on page 7, 20 V at 1 A seems to lie way outside the maximum-rated boundaries. Also, on page

2, it says that maximum forward peak gate voltage is 12 V. Obviously I'm missing something here... but what... ?

To the point, the questions I have:

  1. using 12V, what resistor value R would you recommend so that the thyristors turns fully on in the shortest amount of time, yet doesn't exceed its maximum ratings ? 2. would a snubber network over the K-A be needed for this application ? The datasheet recommends 22 ohm & .22 uF but I don't think one would be needed in this application. But I may be wrong.

Any advice will be greatly appreciated.

Peter.

Reply to
peter_dingemans
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(these products

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They probably mean 20 V across a 20 ohm resistor into the gate, with the gate setting its own voltage (about 1-3 V) by being forward biased.

The gate drive paragraph mentions 1 us max rise time and the 20v/20ohm to get the maximum rating out of the device. With a resistor of 20 ohms, you'd get less than 1 us, as long as the voltage applied to the resistor was stiff and rose fast enough. I guess that's for a short gate pulse, because the maximum average gate power is 1.5 W; if you limit to less than that, it should be safe. If the gate drop is as much as 4 V, a 12 V supply and a resistor of 21.3 ohms would give 1.4 W; if the gate voltage were only 2 V, you would then have less than 1 W. If the gate is staying on only 0.5 s, that should be safe, so maybe

20 ohms?

Maybe you'd still want that to control dv/dt. For example, what happens if the device is charged up and then you make the external contact before welding - there could be a sharp change in voltage that might trigger the device immediately.

-- John

Reply to
John O'Flaherty

Thanks for confirming my suspicion.

That sounds like a sensible thing to do. I hope it still turns on the thyristor fast enough.

Hadn't thought of that yet. Will add a snubber network then. It's not as if the components are expensive or hard to mount anyway. But it should give some more ease of mind.

Thanks for the help,

Peter.

Reply to
peter_dingemans

Thanks for confirming my suspicion.

That sounds like a sensible thing to do. I hope it still turns on the thyristor fast enough.

Hadn't thought of that yet. Will add a snubber network then. It's not as if the components are expensive or hard to mount anyway. But it should give some more ease of mind.

Thanks for the help,

Peter.

Reply to
peter_dingemans

Thanks for confirming my suspicion.

That sounds like a sensible thing to do. I hope it still turns on the thyristor fast enough.

Hadn't thought of that yet. Will add a snubber network then. It's not as if the components are expensive or hard to mount anyway. But it should give some more ease of mind.

Thanks for the help,

Peter.

Reply to
peter_dingemans

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so that the

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There probably is a worse way to do this but it escapes me for the moment. The biggest problem is you have no control over the current output. Your caps wont last long unless they are high discharge types (photo flash for example). Triggering the tyristor is the least of your problems almost any pulse will do that very effectively. No worries about dv/dt as there is no voltage anywhere when the thyristor turns off.

Reply to
cbarn24050

skrev i en meddelelse news: snipped-for-privacy@v46g2000hsv.googlegroups.com...

I would use a pulse-train instead of a single pulse. It might take a while i.e. some us for a big SCR to turn on so you want to make sure.

You do not want "the shortest time". SCR's have a dV/dT rating with respect to how fast the current is allowed to rise because it takes time to turn the device fully on. You can fix that with a simple inductor in series or a saturating inductor or even a Pulse Forming Network if you want to get creative.

I dont think it matters much at 25 V anyway because the impedance of the capacitors will limit the current and the risetime in any case.

Reply to
Frithiof Andreas Jensen

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