Coilgun experiments / switching pulsed inductive loads

[Da_Stier]

When I was around 14, I was really fascinated by the concept of a coilgun and built quite a few of them (very crude ones).
I guess the main reason for this fascination might have been the gauss rifle from the STALKER series, which I found to be the coolest thing ever. 

However during the lockdown I became somewhat interested in coilguns again and did a few experiments.
This time around I wanted to use all my testgear, that I have now and do it more "scientificly".

I build a coil with 3 different tabs to experiment with.
This is how it looks:



The coildata is the following:
1 -> 12.7uH    |   30mOhm
2 -> 55.6uH    |   80mOhm
3 -> 136.1uH  |  140mOhm

For driving the coil I used the following circuit:



The diode across the coil is a BY500 in my real circuit, otherwise it is identical.

Feeling way to confident I directly started by blowing up both an IRPF250N MOSFET, as well as a K100T60 IGBT.
After this incident I started to do some low voltage (21V) experiments, as well as some more simulations.

In the simulation I get the following:



This is about what I would expect the circuit to do as well.
My real measurement of the same circuit looks like this:



The pulses of the coil voltage and -current are quite a bit longer but the amplitude is about right.
However when switching off, I get around -34V across the coil.
This seems strange to me, since the diode across the coil should limit this voltage, shouldn't it?
(The peak is on the scope screenshot, however it is propably hard to see)

The voltage across the (in this case IGBT) goes up to the supply voltage plus the induction voltage of the coil to about 55V.
(again, the peak is very hard to see in this timescale but I took a closer look at it and it is definately there and correct)

I guess this spike is what kills my IGBT / MOSFET, when operating at higher voltages.

The gate is driven by a MCP1407 gate driver via a twisted pair (which is still a little bit long, I know)



So now my question is, what I missed, I tought a flyback diode across the coil should be all I need to limit the voltage peaks?
Or the other way around, what would be the "correct" way of driving a pulsed load like this?
I guess a halfbridge would eliminate the need of flyback paths?



Best regards,
Michael

BY500-600_BY500-800_BY500-1000_HY.pdf
Infineon-IKQ100N60T-DS-v02_03-EN.pdf
irfp250npbf.pdf
MCP1407-EP_MIC.pdf

[davekni]

There will be a little voltage spike due to forward-recovery time of the diode.  I think most of your spike is due to parasitic wiring inductance.  The FET/IGBT, diode, and bulk capacitor should all be on a double-sided copper board.  Copper foil tape on either side of a thin piece of plastic is fine.  I'd use one side for ground (FET source or IGBT emitter), with the other side cut into two sections, one for drain/collector and diode anode, and the other for VBus+ (diode cathode).  Then wiring inductance to your coil isn't critical.

All my simple coil guns use either thyristors or mechanical switching.  Pulse width is controlled by capacitor value and coil inductance.  The more rapid and controlled pulse end should be more efficient.  One advantage of thyristors (including TRIACs) is that parasitic inductance doesn't burn anything out, as there is no rapid current fall to make a voltage spike.

[Da_Stier]

Hi David,

thanks for your reply.
I thought, that wiring inductance will be a part of it, but I propably underestimated it. 
I will build one of your example layouts for low inductance bridge designs, as you described. 

All my simple coil guns use either thyristors or mechanical switching.  Pulse width is controlled by capacitor value and coil inductance.  The more rapid and controlled pulse end should be more efficient.  One advantage of thyristors (including TRIACs) is that parasitic inductance doesn't burn anything out, as there is no rapid current fall to make a voltage spike.

Thats exactly, what I did for my old stuff.
I got a box of some east german thyristors back in the day, they worked quite well.
(In the picutre of the coil, I actually used a thyristor as well)

So thanks for your suggestions, now I can try some stuff again. 


Greetings,
Michael