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Min pulse width to trigger SCR

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Turkey9

Sat Mar 27 2010, 06:57AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

This thread is as the tittle suggests. I'm wondering what is the minimum pulse width to properly trigger a large stud type SCR. It is rated 130 amp 1200v and 3.5kA pulse. I'm using a transformer to simultaneously trigger 4 SCRs, should I use any resistance on the SCR side of the transformer? The primary is driven by a pair of the ucc27322/ucc27321 drivers with a 1uf cap in series with the output. The transformer is like a usual GDT with about 20 turns on it.

Thanks!

Proud Mary

Sat Mar 27 2010, 08:37AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If you want to have a gate pulse width less than a few tens of microsecs, then you'll have to increase the gate drive amplitude, because it takes time to get full area conduction. Without a generous gate over-charge, the steepness of your wave front will suffer in the first few thousand nanosecs when only a teeny bit of the active device area is conducting.

Steve Conner

Sat Mar 27 2010, 12:44PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

SCR datasheet should tell you. Too short a pulse could lead to incomplete triggering and blow the SCR, if you're trying to switch a high pulsed current.

Turkey9

Sat Mar 27 2010, 08:01PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

The data sheet for my devices is less than helpfull; it only has the main ratings, such as voltage and current related stuff. I'm planning on using a 100 usec pulse, would that be plenty?

EDIT* I found a better data sheet, but can't understand a lot of the terms. Can you help me out? Below are the one's I don't quite understand:

V sub (to)
r sub T
t sub q
V sub GT --- I'd imagine this is the gate voltage but the value under it is 1.4... is that a minimum?

also the gate current is 150 mA. Does that seem like a max rating?

the data sheet is attached for reference. My SCR is a T130

Turkey9

Sat Mar 27 2010, 11:33PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Here it is... Sorry for double post couldn't figure how to atatch it to previous post.
]1238982_1.pdf[/file]

MinorityCarrier

Sun Mar 28 2010, 03:22AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Igt of 150mA is probably maximum (I looked up some 300-600A GE scr's and that's about what their maximum Igt is.
Vgt of 1.4 is maximum volts, and that is a compliance voltage, not something you apply. Gate to Cathode is a PN junction, has to be at least ~.6V to forward-conduct.

GE has an excellent handbook on SCR's, there may be a PDF of it you can download and read.

Patrick

Sun Mar 28 2010, 04:13AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

what does "compliance voltage" mean? cani exceed it without damaging the device?

Steve Conner

Sun Mar 28 2010, 10:55AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That's not the full datasheet, keep looking!

If triggering several SCRs at once, you should use a series resistor for each SCR, on the SCR side of the transformer. This ensures the gates share current and all four get triggered.

That shows two separate GDTs, but it would be exactly the same if you used one GDT with multiple secondaries. Note the 100 ohm resistors to keep the gates clamped in the absence of any drive, this was recommended in my SCR datasheet to avoid spurious turn-on.

The gate-cathode junction of a SCR behaves the same as the base-emitter junction of a bipolar transistor, and you drive it the same way. Except of course you can only turn it on and not off again...

MinorityCarrier

Sun Mar 28 2010, 03:59PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

The forward current from gate to cathode will result in a voltage drop across the gate and cathode. That voltage drop for a given current from gate to cathode is the compliance voltage, and if I'm correct, (Steve McConner will B-slap me if I'm not) the maximum voltage drop, gate to cathode, will be 1.4 volts for the maximum forward current of 150mA. It will be a minimum of ~.6 volts because of the PN junction work function.

Hope you're staying sober enough at Chico to attend to your studies. When I was there, CSUC was rated #3 Party Campus in the U.S. by Playboy Magazine.

Turkey9

Sun Mar 28 2010, 09:45PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

It sounds like the value I have to be wary of is the gate current. I found the full datasheet finally... Half German or something and half English. It says the max current is 150mA and the min is 5mA. As long as I limit it to say, 75mA I should be fine, right?

It says the typical zero capacitance is 3nF, is that the capacitance that the drive circuitry will see? Would I just have to ensure that the capacitance would be charged with my trigger pulse using RC time constants?

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