SCR Selection

I'm trying to design an SCR based overvoltage crowbar circuit for a 100V to 24V DC/DC converter...I'm having some trouble finding the right part, here's my specs

My max load current is 1A. I have a 3A fast acting fuse at my input to the DCDC. I want an SCR that can handle 4A long enough for my fuse to pop, I want this to happen if my output goes to 30V...

so, I went off to go look for such a part... seems straight forward enough... I found this

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but the part says 4Arms, 2.6Aavg.... I'm just working in DC so I imagine RMS is the way I want to go here....

I also don't get the spec on SCRs that's called Vt which is the voltage across the anode to cathode when it's on... and it's usually 1.5V or so... if you have

30V across it how is it's votlage 1.5V or am I reading this wrong?

my last question is, how much voltage is it safe to put at the gate to turn these things on... I've seen 0.8V on datasheets as turn on voltage... is it ok to give them 1,2, or 3 volts at their gate?

much thanks!

Reply to
panfilero
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24V DC/DC converter...I'm having some trouble finding the right part, here's my specs

DCDC. I want an SCR that can handle 4A long enough for my fuse to pop, I want this to happen if my output goes to 30V...

I found this

is the way I want to go here....

the anode to cathode when it's on... and it's usually 1.5V or so... if you have

30V across it how is it's votlage 1.5V or am I reading this wrong?

these things on... I've seen 0.8V on datasheets as turn on voltage... is it ok to give them 1,2, or 3 volts at their gate?

The most critical rating is the "Circuit Fusing (t = 8.3 msec)" i^2*T" rating. You are discharging a capacitor (power source + load) which will produce a large current; if that is less than the current rating, you could still have a destroyed part when it is triggered.

As far as gate voltage, 0.7V turns on a transistor or SCR; DO NOT use a low-Z voltage source (!); 5V with 1K series to gate (ie: 5mA) will do for that.

Reply to
Robert Baer

Pulling 4A through a 3A fuse is going to make the fuse fail slowly, or perhaps not at all. You know your circuit, but you probably want to pull more than that.

Somewhere in the data sheet there should be information on pulse currents through the device. It's usually a plot of the allowable current vs. the duration of the pulse. If it's not in the data sheet -- find another part.

That's the rated maximum voltage drop at some specified on current. Presumably when you turn the SCR on the voltage will drop below 30, depending on the impedance seen by the SCR anode.

I'm not sure what's done in practice, but a 300m-ohm resistor would limit the SCR current to 10A -- that's enough to blow your fuse, and it would make the whole fuse-blowing event more predictable at the cost of making it slower and having one more component in your crowbar circuit to make it fail to do its job.

Think current, not voltage. The gate voltage rating is at room temperature, and is basically one diode drop. At other temperatures it's still one diode drop -- but it's not 0.8V any more.

There should be a rated gate current. You should be able to exceed this. It doesn't matter much, because usually what happens when you put enough current into an SCR gate to make it trip, but not enough to make it trip promptly, it fails short due to local heating turning parts of the device into (conductive) slag.

Keep in mind that the usual expectation for an SCR crowbar circuit like this is that the SCR pops off, fails short, and takes the fuse out. If you are guaranteed that the SCR is going to fail short and not open, then you do not care if you're putting in enough gate current to kill it -- only that it fails _short_, and takes the fuse out.

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Tim Wescott 
Control system and signal processing consulting 
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Reply to
Tim Wescott

Naaaah! You pick an SCR with an I^2t rating substantially higher than the fuse. The last one I built would do around 100A into a 5A fast-blo fuse. Didn't hurt the SCR at all. Don't be a weenie and limit the current to a low value... guaranteed to kill the SCR, and maybe not blow the fuse quickly enough to save the payload. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
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Reply to
Jim Thompson

100V

part,

to

imagine

voltage

so...

this

voltage...

You should really go for an SCR with a much higher current rating, such as:

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.pdf

These are about $2.50 each and handle 40A RMS and over 600A for 20 mSec for capacitor discharge duty. These devices have an I^2t rating of 1122, so you just need to be sure that the fuse has a lower value. The 3A fast acting

fuse will probably open in less than 10 mSec at 30A or more, which corresponds to an I^2t of about 10.

If cost is a major factor, you can get this 25A device for less than $1:

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45J0826

If I read you correctly, your fuse is at the input of your 100V-24V DC-DC converter, but you want to crowbar the 24V output at 30V peak. There is no guarantee of what current will be drawn at the input in case of output overvoltage failure, unless it is caused by a series component failing short and imposing the 100V input on the output. So the fuse should be on the

24 VDC output, and the SCR crowbar from output to GND. There will probably be enough capacitance on the output to provide a surge current sufficient to blow the fuse and protect the 24 VDC load.

The fuse at the input of the DC-DC should be chosen to protect against overcurrent and short circuits, and for a 24V 1A output, nominal input current will be about 250-300mA. So your 3A fuse should instead be about

1A. You can also add a crowbar to the input to protect against overvoltage.

I do not agree with the idea that one should rely on a crowbar SCR failing shorted to blow the fuse.

Paul

Reply to
P E Schoen

D'oh. Thanks Jim, that's obvious in hindsight.

--
Tim Wescott 
Control system and signal processing consulting 
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Reply to
Tim Wescott

It's a DC circuit, he needs to confirm the fuse is rated to interrupt that large of a current at that large of a VDC voltage.

Reply to
bloggs.fredbloggs.fred

That's the way I've always done it.

If the PSU has foldback, it's necessary to pick an SCR with a holding current an order less than the foldback current.

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Reply to
Fred Abse

then

Yes. Perfectly valid point. This SCR is too weak for the task as described.

?-)

Reply to
josephkk

In my world, the SCR has to survive. The only failure should be a fuse at the PSU *output*.

I've made rigs to test multiple trips, preceded with, and followed by, parameter measurements on the SCR.

The only time I would countenance relying on an input fuse is in the case of a series buck regulator, and then I'd do my best to dissuade the client.

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"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
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Reply to
Fred Abse

A bit too sensitive, too. Igt in the microamps range.

MCR12DCM (It(av)7.8A, I^2t 41A^2.sec, 40mA max holding, DPAK) would seem a more likely candidate. Max Igt is a bit high at 20 mA, but a simple zener trigger circuit is still feasible.

The OP stated a 1 amp load. A Littelfuse 370 series (1 amp rated, 0.2 A^2.sec) PCB fuse *at the output* should do nicely.

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"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
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Reply to
Fred Abse

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