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Driving a magnet at 150A - snubbers, design, etc?

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GeordieBoy

Tue May 19 2015, 07:35PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The load is the DC resistance of the electromagnet and the resistance of the wires going to and from it. This is where you will be dissipating most of your power.

As others have suggested I would recommend a synchronous buck converter with multiple paralleled switching devices for the "switch" and for the "diode" positions in your buck topology.

Keep the switching frequency as low as you can whilst still meeting the current ripple specification for the electromagnet in your application.

If you feel sufficiently skilled in power electronics, you could make a setup of two or four interleaved buck converters to share the load current. 150A isn't really so high that you need to share the current between interleaved converters, however interleaving does reduce the ripple current seen at the load (electromagnet) and the DC bus capacitor, which is probably worthwhile!

Average current-mode control is the way to go with determining the pulse-width for PWM. There are some excellent application notes from Lloyd Dixon (Unitrode) on the subject.

Good luck,

-Richie Burnett,

Dr. Slack

Wed May 20 2015, 09:45AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

GeordieBoy wrote ...

If you feel sufficiently skilled in power electronics, you could make a setup of two or four interleaved buck converters to share the load current. 150A isn't really so high that you need to share the current between interleaved converters, however interleaving does reduce the ripple current seen at the load (electromagnet) and the DC bus capacitor, which is probably worthwhile!

If the OP wanted to make a buck power supply to drive the magnet coil as a load, this would certainly be a nice way to go, but each phase of a polyphase converter needs its own inductor. I think he is trying to use the inductance of the magnet coil load itself as the buck inductor, which will restrict him to a single phase.

Of course using the load itself as the inductor means that there will be a finite current swing in the load, which is OK for picking up bits of metal and holding doors closed, but not OK for doing (say) mass spectrography. A steady DC current (ie defined and relatively small ripple) would need to use a separate PSU.

GeordieBoy

Wed May 20 2015, 12:01PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Yeah, the load may have enough inductance to keep the current ripple acceptably low.

I was thinking more of two interleaved buck converters with their own inductors, then driving the load just as a resistive load. These inductors don't need to be particularly large indutance wise because the ripple current now sloshes back and forth between the two interleaved converters, but largely cancels at the load. You can also put a capacitor across the output at this point as you would normally do in a SMPS which will reduce the voltage ripple further. (There should in fact be 3-poles between the switched inverter voltage and the electromagnet current because it will form an LCL T-filter.)

Of course it's wasteful of the load's inherent inductance though, and that 3-pole T-filter might make loop compensation that bit more challanging.

-Richie,

Artlav

Wed May 27 2015, 10:42PM

Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94

So, project report time.
Put the thing together solidly, and was testing at 50A from a limited PSU for some time, going through various situations and probing everything.
It runs at around 7KHz, with little variation.
I had to strengthen up the RC filter to make it run reliably.

Then, i finally tried running it for real.

100A, no problems.
With the new layout the spikes on the FET never went above twice the input voltage, which is fine by me.
Max input voltage is 30V, spike tops at 50V, FET is 100V rated.

The spike on the diode remained unchanged, but it never exceeded 40V, regardless of current or voltage.
200V diode, not a concern.

Everything is metal cold, except for the magnet.

120A.
Magnet overheats rapidly, and is not exactly easy to cool down, but everything looks about the same as at 100A.
11V output into it's hot copper, which gives 1.3KW of utilized power.

130A, 12V, 1.5KW.
Same thing.

140A, 12.7V, 1.8KW.
Same thing.

150A, 14.2V, 2.1KW.
Nothing blew up!
Nothing is hot.
Nothing bad on the scope.

The FET and diode are metal cold... Might have overdone the heatsink.
The magnet gets very hot very quickly. But that is expected.

It's kind of freaky to see a 30Wh battery go from full to flat within 10 seconds in a controlled manner.
0.1T of safe magnetism is also quite fun to play with.

The final test was to see what would happen on full power undervoltage event...
Which was whole lot of nothing,
At around 18V input it went to 100% duty cycle and the current started to drop off.
The FET was dissipating about 150W, which at the given time scale is not enough to even warm it up.

All in all, i call that a success.
Thanks everyone for helping!

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