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HF gate drive experiments

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ragnar

Mon Nov 06 2006, 01:54AM

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

I've found my "signal generator" (ha) really handy for sweeping each new setup... as soon as I put any kind of power into the FET, the Fres drops as does the Q, but I'm still above the 10MHz mark... I'd really like to be at the 13.56MHz mark, however, but it's still really nice to have a ballpark estimate of the Fres/inductance before I start each time ^^

When I'd finished work yesterday afternoon, I milled a small board to accommodate two IXDD414s in parallel, a divider to float the input voltage at Vcc/2 (so that my signal generator could swing it), tantalum+ceramic power bypass caps (soldered directly to legs) and two capacitors for the fixed end of the halfbridge.

With 12V, 0.535A in, I'm swinging an IRFP450 at 11MHz at > 8Vp/p through a 4:1 transformer. (Q~2.67, pah)... the usable frequency range is pretty wide, and I can get 8V or more on the FET between ~10MHz and 11.5MHz.

I'm very happy with the efficiency, considering it took me IIRC ~1.6A at 12V to hard-drive 12Vp/p on the same FET at 12MHz.

And now, if I want more volts, it's easier to increase the supply voltage than to try and make a fullbridge from more IXDDs. I'll be a happy chap if I can get 12Vp/p, 11MHz on an IRFP450 with only two chips =D

edit: update -- increasing the supply voltage to 18V caused a current draw of 1.06A and the waveform across the MOSFET is now 12Vp/p and I twiddled the freq to 11.5MHz.

The IXDDs do get warm though ^^

Now, time to build a boring variable-voltage power supply for more twiddling.

edit: update2 -- scrap the power supply, I wanna see some sparks (and hopefully not from the FET), which means winding a resonator of some description and making this mess rigid with a heatsink etc. =D

And my cheap thrills signal generator will prove mighty handy for sweeping the resonator Fres, too. Why didn't I build this years ago? ^^

Achoo.

Steve Conner

Mon Nov 06 2006, 10:08AM

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

balsamplack wrote ...

I guess you can't get stray inductance any lower than that!

Richie explained to me that you can get the frequency to go higher than the self-resonant frequency by connecting a capacitor in series with the primary, so that could be the next thing to try. It's pretty impressive as is though!

ragnar

Mon Nov 06 2006, 10:10AM

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

I was about to mention that putting a capacitor in series with the gate would lower the overall capacitance, albeit increasing the voltage you need to drive it to, right?

Steve Conner

Mon Nov 06 2006, 10:12AM

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

Yes, that's pretty much the idea. Capacitors in series have a lower capacitance, so the resonant frequency is pushed up. The capacitor goes in series with the primary though, ie the driver side not the gate side. It has the same effect electrically, but you don't have to worry so much about the physical size of it increasing stray inductance.

Avalanche

Mon Nov 06 2006, 11:15AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Awesome, I thought about putting a capacitor directly in series with the gate, but thought I must be missing something and it couldn't be that easy to reduce the gate capacitance

Only problem is it will form a potential divider, so the voltage will need a good boost. Won't the extra cap need to be a bit larger than the gate capacitance too?

Steve Conner

Mon Nov 06 2006, 11:43AM

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

It's only really worthwhile if it's done as part of tuning the gate drive circuit to resonance. The circuit will have a natural frequency that it resonates at, set by the gate capacitance and the transformer and wiring leakage inductance.

If you want to operate below this frequency, then you need to add a series inductor to move the resonant frequency down. If you want to operate above it, you need a series capacitor to move the resonant frequency up.

Avalanche

Mon Nov 06 2006, 12:23PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Yeah I get that bit how you can shift the resonant frequency up by adding series capacitance to the LC, but won't the overall charge accumulated in the mosfet gate be halved? I was under the impression that the mosfet gate had to be fully charged before the thing could conduct properly

I'm just going by what would happen if it weren't a resonant circuit, I'm probably totally wrong again anyway

Marko

Mon Nov 06 2006, 12:51PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Gate charge is same, except series cap makes a voltage divider vith mosfet's capacitance.

But, you also get some Q-factor increase so you may need just to adjust input power a bit.

With stronger drivers it shouldn't be a problem though.

Making a good matching transformer with low ohmic resistance may also help a lot to increase Q if we need that.

Avalanche

Mon Nov 06 2006, 02:46PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Aha, I think I see where this is going, hopefully with enough Q the voltage can resonate back up hopefully without having to change anything

So if I add roughly 4nF of caps to the primary, my total will be near the 1.76nF required to resonate with .1uH. My plan is to add a small variable inductor in series with the gate, so I can adjust it until it hits resonance.

The only problem is that I cannot add a capacitor on the primary, because I'm using a center tapped primary

But if it's the way forward I'll have to do some serious rethinking. Would there be any reason why I cannot put say a 1nF cap in series with the primary, and add a larger inductor to the gate? (apart from the fact that the voltage would be tiny, so I'd have to increase the input) I just want to know if i'm missing anything before I go and make a new high voltage driver that doesn't need a center tapped primary...

update! It does resonant back up enough!

I cobbled together a small board to hold an IRFP460, some series caps and an inductor. Because of my driver design I decided to put the caps directly in series with the gate, on the secondary side, and it works pretty well. I've used 4x1nF caps (rated at 400v each, I guess they're poly caps) and a small series inductor below that to bring the total inductance in the secondary side to .1uH, with as little of that as possible on the source side of the fet.

Without changing anything on my driver since the last photo, the voltage resonates back to to 7v p-p across g-s, which can't be that bad considering the 4:1 turns ratio on the transformer, and the capacitive divider formsed by the gate and the 'gate mmc'

This is with 15volts into my driver board.

(1v, .1uS/div)

ragnar

Tue Nov 07 2006, 12:36PM

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

Very nice, Avalanche! Though I'm not sure if it'd have been more straightforward to add the C to the primary ^^ (well, that's a given in my case

) I'll have to outpwn your IRFP460 with 12Vp/p, 11.5MHz for IRFP450 19.08W input.

I found my last LM317 today, too, and with some big heatsinks I can now make a variable power supply for safer tweaking. =D

Next target, 13.56MHz, and sparks, of course. ^^

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