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HV dicharge through inductor killing my irf840!

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NoName

Sun Feb 19 2012, 04:25PM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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Posts: 16

Thanks for posting that calculator! Pretty cool. I will more than likely add more turns at some point. 2 final questions:

-no diode on the caps to prevent them from going negative and blowing up. What should I do here?

-is there a very basic way to keep a thyristor/SCR from locking up?

Wolfram

Sun Feb 19 2012, 07:22PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

NoName wrote ...

Thanks for posting that calculator! Pretty cool. I will more than likely add more turns at some point. 2 final questions:

-no diode on the caps to prevent them from going negative and blowing up. What should I do here?

Add the diode.

-is there a very basic way to keep a thyristor/SCR from locking up?

The SCR will stay conducting until the current through it goes to zero (or at least below the minimum holding current). If you need to be able to turn off the current before the capacitor is discharged, you'll be better off with a MOSFET or an IGBT instead. Since you have a high pulse current in your circuit, an IGBT would probably be the cheapest solution. Check out the HGTG20N60A4D for example.

NoName

Sun Feb 19 2012, 08:09PM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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Thanks for all the help guys! I think I have everything working. I may post another problem later (knowing how things go). I found that I have ten or so 2n6509 SCRs. They nothing has blown. Thats good news. As for the latching, I am sure it happens because I am not shutting off charging of the caps upon discharge. I will momentarily shut it down and then kick it back on once it falls far enough. I am monitoring the voltage with the ADC.

NoName

Sun Feb 19 2012, 10:21PM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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Ok, everything is great with the scr. Can you guys tell me:

-should I have a snubber between the inductor and the scr?
-as far as the flyback diode goes, I have a uf4007. It is rated for 1000v but only a peak of 30A. Is assume this is not suitable? If not, what should I use? Never used a bigger diode.
-finally, it was mentioned above to use a diode across the cap bank to prevent reverse potential on the cap bank. What type of diode is good for such situation, I assume the similar to the inductor. Is this to be reverse biased?

m4ge123

Sun Feb 19 2012, 10:31PM

Registered Member #4118 Joined: Mon Oct 03 2011, 04:50PM
Location: MD
Posts: 140

If you have a bunch of uf4007s you can parallel them up, but its easier and better to buy an ultra-fast diode with specs similar to your SCR.

Pinky's Brain

Mon Feb 20 2012, 02:23AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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wrote ...
What type of diode is good for such situation

Assuming you are charging relatively slowly (10s of ms or more) it can be a standard recovery one, reverse recovery isn't really relevant in that case. You put it parallel to the capacitor so it blocks when charging, when discharging it clamps the capacitor voltage to -Vforward.

The free wheeling diode across the inductor isn't really relevant with a SCR, when the SCR shuts off there is no current to go through the free wheeling diode. It's only important with a MOSFET or IGBT.

NoName

Mon Feb 20 2012, 03:26PM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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Ok, so the SCR method is working great. The only drawback is that my pulsed current has to go to 0 before I can shut it off. There are some huge benefits to controlling the pulse length. With that said, I ordered a bunch of mosfets that have huge current ratings. Well, 200+A pulsed. My inductor will not create more than 135A. So, my question is about flyback diode ratings. I honestly have only done projects where the IN4001 would do fine. So, I was curious...

If I have 300V and 135A, what ratings should I have on my flyback? Should they have the same peak current ratings? I ordered some that are rated at 400V and 600V, 30A continuous current, and 300A pulsed. Is this about right, or overkill?

NoName

Mon Feb 20 2012, 03:32PM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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And one more question....Sorry

It was mentioned that I should add a diode across the cap bank. Should I also have a high peak current rating on this too. To me it just seems as though I need to make sure the break down voltage is over the 300V on the bank and that it will see little current flow.

Pinky's Brain

Mon Feb 20 2012, 04:43PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

The free wheeling diode can be a regular diode as well, reverse recovery isn't really relevant for it (unless you turn on the MOSFET while the diode is free wheeling). P600x or 6Ax would suffice, and your 30A diodes will certainly suffice.

The current the diode across the capacitor sees depends on how damped the oscillation is, or in other words how much resistance the coil has. If it's critically damped it will so no current, if the resistance is zero it will see max current ... probably will be somewhere in between.

NoName

Mon Feb 20 2012, 05:03PM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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Posts: 16

So, if I do place a diode on the cap bank as suggested, wont it also be serving as a flyback when the mosfet is conducting? In other words, are 2 separate diodes really needed?

Also, would it be smart to keep a snubber by the inductor? I have a 0.1uF cap in series with 100 ohm resistor.....but dont know if that will help or even interfere with the field produced in the inductor.

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