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Class E SSTC problems

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Andrew L.

Fri Apr 27 2007, 11:51AM

Registered Member #515 Joined: Sun Feb 11 2007, 02:06PM
Location: Slovenia/Ljubljana
Posts: 94

Ok so I built this circuit:

But I modofied it just a litle bit, instead of C6 being connected in series with the primary coil I connected it paralel to the primary coil. And usualy with this circuit I blow a mosfet prety fast. is this because the mosfet is rated for only 500V, and when the mosfet turns off the high voltage spike feeding back from the primary coil exceedes the voltage rating of the mosfet or is there something else?
And usualy when I blow a mosfet the gate driver also blows, why is that?
So do you have any suggestions on how I could improve the circuit so that the mosfet and gate driver wouldn't blow.

r3d

Fri Apr 27 2007, 12:26PM

Registered Member #532 Joined: Sat Feb 17 2007, 05:00PM
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Posts: 11

Try to connect the cap as it is in the schematic?

Zum Beispiel

Fri Apr 27 2007, 12:50PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

I don't know much about class E, but I'm fairly sure the cap should be tuned so that it's at 0V when the mosfet switches back on. If it has a lot of charge, it will be shorted by the mosfet. Do that, say a million times a second, and you have a blown mosfet.

Oh, btw, what is your supply voltage? Try running it of a lower supply voltage and see if that helps.

And definately put the cap across the Fet.

Andrew L.

Fri Apr 27 2007, 01:13PM

Registered Member #515 Joined: Sun Feb 11 2007, 02:06PM
Location: Slovenia/Ljubljana
Posts: 94

Why is it so important that I put the capacitor across the fet?

HV Enthusiast

Fri Apr 27 2007, 01:33PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Andrew L. wrote ...

Why is it so important that I put the capacitor across the fet?

The shunt capacitance there works with the L and load (R) to cause a damped oscillation and make the switch close precisly when voltage and current are zero. The value of this capacitance, as well as the load will determine how the damping works.

For low Vds, the MOSFET has a pretty large Cds and doesn't need much external capacitance, but as you increase the Vds, the Cds of the MOSFET does decrease, so you need to add more.

The capacitance doesn't necessarily have to be across the MOSET, but in practice, is generally is placed there.

Also, your C6 doesn't have to be polypropylene. Usually, it may only require a small few hundred pF from a ceramic capacitor - of course depending on your load and power level. I run my Class-E supply at about 300-400W and use only a few hundred pF across my power stage MOSFET.

ragnar

Fri Apr 27 2007, 02:46PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

The capacitor across the MOSFET catches the EMF from the primary; the shunt capacitor and the primary are a resonant circuit. Place the shunt capacitor as close as you can to the MOSFET drain and source.

If this is anything like your last coil, your shunt cap should probably be less than 1nF.

When tuning this kind of coil, it really is important to use a signal generator or a variable frequency circuit to drive the MOSFET. Slightly-under or slightly-over F(res) may give the most efficient operation. It's also important to be able to see how your drain waveform changes with varying frequency.

Also critically important is that the MOSFET duty cycle is LESS than 50%. 35-40% is a good figure to start with.

Do you have an oscilloscope? After you set it up with a variable-frequency drive, perhaps show us some photographs of your drain waveform, and we can help. Tweaking the duty cycle is another variable to play around with, but remember to keep it away from 50%!

Dr. Shark

Fri Apr 27 2007, 04:24PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Good to see more people play with Class E! Almost everything has been said, one addition though:

Andrew L. wrote ...

And usualy when I blow a mosfet the gate driver also blows, why is that?
So do you have any suggestions on how I could improve the circuit so that the mosfet and gate driver wouldn't blow.

Do you have a resistor between the driver and the gate? If you dont, the driver will discharge into a short and overheat in very little time. I usually omit that resistor for maximum current into the gate and pay for it with a lot of dead TC4422s

Andrew L.

Fri Apr 27 2007, 05:07PM

Registered Member #515 Joined: Sun Feb 11 2007, 02:06PM
Location: Slovenia/Ljubljana
Posts: 94

Ok I forgot to say that this is my first SSTC

and that I am a total beginner.

BlackPlasma wrote ...

Do you have an oscilloscope?

Yes I do have an oscilloscope, but it is quite old and not working correctly.

joe wrote ...

Do you have a resistor between the driver and the gate?

Yes I do have a resistor between the driver and the gate.

WaveRider

Fri Apr 27 2007, 05:24PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Andrew,
Class-E requires precise phasing between drain current and voltage waveforms.. I would need to think about your circuit a bit before commenting on whether or not you can get Class-E.

Moreover, usually, you want the generator to look like a current source during the transistor off-periods..

Have you tried any spice simulations of this circuit?

Cheers,
Bill

Andrew L.

Fri Apr 27 2007, 05:49PM

Registered Member #515 Joined: Sun Feb 11 2007, 02:06PM
Location: Slovenia/Ljubljana
Posts: 94

I don't have spice program.

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