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HV dicharge through inductor killing my irf840!

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NoName

Fri Feb 17 2012, 06:02PM

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

I have a capacitor bank that needs to be discharged through an inductive load. The inductive load is a small solenoid. It is attached to a 300V 480uF capacitor bank. The 300V enters the solenoid and then the other end of the solenoid goes to the drain of the IRF840. The source of the IRF840 is connected to ground, and also the negative side of the cap bank. I am using a 100 Ohm resistor at the gate of the MOSFET. The gate is also protected with a 15V reversed biased zener diode.

I originally had just a uf4007 diode across the solenoid leads to protect the mosfet from the inductive load. The mosfet was still being killed on the first use. I then added a 100 Ohm and 0.1 uF cap as a snubber from the drain to ground. The mosfets are still dying. I dont know what I am doing wrong. I thought about adding a zenner that is somewhere between my 300V cap values, and the 1000V limit if the irf840. i also thought about a relay. Any advice?

Weston

Fri Feb 17 2012, 06:33PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Have you measured the pulse current? It is quite possible you are killing the irf840 through over current.

Dr. Dark Current

Fri Feb 17 2012, 07:26PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
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Posts: 3384

"300V 480uF" and "small solenoid" sounds to me like you're way over the pulse current rating of the poor FET... SCRs or IGBTs are usually used in such applications as they can handle large peak currents.

NoName

Fri Feb 17 2012, 11:02PM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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Ok, I have done some calcs and my peak current considering my inductor is only 4.6A. I have been told that the resistor and zener may be inhibiting my turn on time, which can fry it. So, I may be removing those and possibly adding a schottky. I do have some SCRs but I was getting very frustrated with it locking up.

Dr. Dark Current

Sat Feb 18 2012, 12:00AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Just out of curiosity, what is the inductor you are using? If my calculations are right, you get a resonant frequency just of a few Hertz. Maybe your inductor is saturating?

NoName

Sat Feb 18 2012, 12:07AM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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It is actually a solenoid I wound up on my coil winder. It is 250 turns of 21 gauge wire. I am just trying to make some electrical-mechanical parts.

Patrick

Sat Feb 18 2012, 12:48AM

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

NoName wrote ...

It is actually a solenoid I wound up on my coil winder. It is 250 turns of 21 gauge wire. I am just trying to make some electrical-mechanical parts.

whats the cross section of the magnetic core? you should probly put a power resistor in series with the current feeding the coil. I have a feeling that you killing it from overheating, remember the more sophisticated silicon die (mosfets) dont reallly do well with high current high speed swithching. As others have said that what the SCR's and vacuum tubes are for.

Carl Pugh

Sat Feb 18 2012, 02:20AM

Registered Member #1064 Joined: Tue Oct 16 2007, 05:04PM
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I think that Weston gave the answer.
If you ignore the voltage drop across the IRF840, The peak current is 300volt/Rcoil.
If Rcoil=5 ohm, then the peak current is 60 amp.

NoName

Sat Feb 18 2012, 05:49AM

Registered Member #4451 Joined: Fri Feb 17 2012, 05:51PM
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Hmmm....300V at 480uF comes to about 21.6J. Also, my solenoid has an inductance of 2500uH. Using:

E = (L/2)xI^2

I = (2XE/L)^1/2

That gives me a peak current of 4.17A. How do you get the 60A value? Is there a way to get an SCR to unlatch without shutting power off?

Wolfram

Sat Feb 18 2012, 09:57AM

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

Your equation looks right but you have used the wrong numbers. You said that the coil was 2500uH but you've calculated the current with an inductance of 2500mH. Assuming 2500uH is correct, I get a peak current of right under 130 amperes.

I just did a simulation with the numbers you provided, this should give more realistic results. I assumed that the coil had a DC resistance of around an ohm (have you measured it?) and that the MOSFET had a series resistance of 0.85 ohms (taken from the datasheet). I got a peak current of almost 80 amperes, which is well past twice what the MOSFET can handle according to its datasheet. I would suggest either an SCR like others in this thread have mentioned, or a beefier MOSFET or an IGBT if you need to turn the current of before the capacitor has fully discharged.

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