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magnetic pulse compression

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2Spoons

Mon Nov 01 2010, 01:41AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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An excellent result - nice work!

cedric

Mon Nov 01 2010, 09:04AM

Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
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about the sharing of voltage in between the igbt...

in this configuration it does not mater to much because of the tvs diode,I like to think it is ideal with 400 volt at the borne of each igbt.
the tvs diode in between the collector and the emitter are rated for about 400 volt breakdown voltage,witch give a nice safety margin with the rating of the igbt.

Proud Mary

Mon Nov 01 2010, 09:55AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
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Good stuff, Cedric!

Steve Conner

Mon Nov 01 2010, 10:21AM

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

Yes, congrats

This is the first DIY magnetic pulse compression I've seen, certainly the first on this forum. And real experimental results, no "quantum bull-sheet"...

The capacitors in the pulse compressor, are they just those squares of PCB material? Was that really all the capacitance it needed?

cedric

Mon Nov 01 2010, 10:35AM

Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
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Steve McConner wrote ...

Yes, congrats

yes in deed ,but in this case the pcb is 0.4 millimeter witch give to the biggest capacitor a value of about 0.8 nf in comparison the capacitor in input is 7.4 nf,( I am really aiming at short pulse)
beside ,as i said ,I found those value empirically ,it might be that different value of inductance and capacitor give better result.

Pinky's Brain

Mon Nov 01 2010, 12:25PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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cedric wrote ...
the tvs diode in between the collector and the emitter are rated for about 400 volt breakdown voltage,witch give a nice safety margin with the rating of the igbt.

How can you be sure you're running the devices within rating? (Not important to just get something running, but still nice to know.) AFAICS there is nothing limiting the peak pulse current through the TVS other than stray inductance ... and they can't handle very much at all (P4KE 400 Watt TVS for instance, extrapolating the peak pulse power vs. pulse duration graph a bit, could only take around 40 ampere for a 10 ns pulse).

cedric

Mon Nov 01 2010, 01:42PM

Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
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Pinky's Brain wrote ...

cedric wrote ...
the tvs diode in between the collector and the emitter are rated for about 400 volt breakdown voltage,witch give a nice safety margin with the rating of the igbt.

the tvs are just here to make sure the voltage on the leg of the igbt don't go above rating ,if one of the igbt is a bit more slow than the other ,the tvs should absorb the spike until the igbt take over.

for the power rating of the tvs ,you are right ,it's not much ,on the other hand the gate drive system may be primitive but insure a fairly good synchronization of all the gate ,since the resistance of a close igbt is much smaller than the one of a close tvs,the current should go to the igbt as soon as it start closing>
I try to find some tvs of higher rating but I could not find them ,those one look ok so fare...

from the difference of voltage I get in between the input voltage and the maximum peak amplitude of the pulse with out mag pulse compression ,I ll say the impedance of the system is higher than the on of the cable I use to measure the pulse ( 75 ohm),probably around 100 ohm,witch give me a peak current of 40 A .also the way how the igbt are connected in between them make the electric path in zigzag,witch I am sure does help keeping the inductance high...

Pinky's Brain

Mon Nov 01 2010, 03:48PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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I understand the why, I'm just not so sure that it is a robust solution (not that it makes much difference here, the TVS's are cheap and they fail short). Especially at turn off where delay can vary on the order of 100 ns according to the sheets.

BTW, can someone explain to me how there can be an order of magnitude difference between peak power at 1 us pulse time between ST's and Littlefuse's 1.5ke TVS's according to their datasheets?

cedric

Mon Nov 01 2010, 05:39PM

Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
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Pinky's Brain wrote ...

I understand the why, I'm just not so sure that it is a robust solution (not that it makes much difference here, the TVS's are cheap and they fail short). Especially at turn off where delay can vary on the order of 100 ns according to the sheets.

BTW, can someone explain to me how there can be an order of magnitude difference between peak power at 1 us pulse time between ST's and Littlefuse's 1.5ke TVS's according to their datasheets?

I agree with you ,I haven't been digging in to it , for the turn of ,it is basically the capacitor discharge witch terminate the pulse I believe ,I do like tvs for the fact that if rated correctly they should not suffer from continuous duty unlike some other voltage protection,also they are very fast,for all purpose nearly instantaneous..

Pinky's Brain

Mon Nov 01 2010, 07:54PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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A nice paper on the rather iffy behaviour of TVS's with short duration high current pulses (clamping voltage goes up non-linearly with current) :

Once you start really making full use of the IGBTs (even small ones can push 100s of amps for short pulses) you are going to need quite beefy TVS's (lots in parallel perhaps).

PS. I might be underestimating the ability of IGBTs themselves to deal with avalanche though (at least NPT ones).

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