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HF gate drive experiments

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ragnar

Tue Oct 31 2006, 08:02PM

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

Fantastic!

I'm impressed that you can get away with the ferrites -- I found that a single turn around anything exceeded the 53nH permissible for resonance at 12MHz with the IRFP450... but you've got two turns =P

What's the secret? ^^

Avalanche

Wed Nov 01 2006, 01:03PM

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

Probably the double 'binocular' core if anything... I found using a single ferrite ring I could barely get any decent coupling. Using the double I get a lot less leakage inductance (and it's easier to wind). I've calculated that I need .059uH for resonance, which is tiny! I'm not sure what grade the ferrite is, but they are the 'suppress noise on wires' type. I haven't actually tuned the gate drive for resonance yet, I just fired it up and it looked alright

I'm going to have a go at fixing it back up now.

I can increase the voltage on the final output stage up to about 28 volts safely, and wind the transformer to suit, to get crazily low output impedences. The only problem is that the current draw on the primary then increases beyond what the transistors can handle, so they fizz out.

Have you measured the gate capacitance on a MOSFET, as they vary wildly especially between different brands.

Marko

Wed Nov 01 2006, 01:22PM

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

Gate capacitances are exactly defined in mosfet's datasheet.
IRFP450 from fairchild has about 2nF, IRF's has 2,6 and etc. (it seems to vary somewhat).

You can try using thin strip instead of wire soldered as close as possible to mosfet, use highest coupling possible, or try some of those low-permeability HF powder cores and see if they are a good trade for inductance/coupling.

I admit I got bored of high frequency hunt since it's hard to get big sparks at >10Mhz. Efficiency drops quite fast over there due to mosfet's delays!

Avalanche

Wed Nov 01 2006, 02:25PM

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

Yeah it looks like there is still way too much leakage afterall, even on that transformer.

Here's some waveforms anyway, at 12Mhz into the gate of one IRFP460. The overall voltage is only about 2v peak to peak, so it isn't ideal. I was hoping to get at least 4v. However, the MOSFET itself is getting warm - about as warm as the heatsinks on the amplifier, so at least some power is going to the right place

I don't think I am driving the final power output transistors hard enough just from the TTL, i'm going to try and push them a bit more. I will probably also need to add a DC bias to the gate of the MOSFET.

Gate waveform at 12Mhz

perfect layout and component choice, ready for mass production

ragnar

Sat Nov 04 2006, 12:26PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Using my freshly acquired cheap thrills signal generator I had a little play around with some ferrite combinations.

At the (careful) expense of a short length of quality coax, I had a length of crosswoven braiding, and a hollow tube of thick, fingercutting copper foil.

If you look at the FIRST picture, the LEFT transformer is formed with two small chokes. The primary is three turns of twisted-pair (wires in parallel). The secondary is a single turn of thick copper braiding (folded double to make it 2x as thick). After connecting an IRFP450 across (comparable input capacitance I measured for this particular FET is 2.67nF) to the secondary, and sweeping the primary with newfangled signalgen, it resonated at 14.35MHz, (implying ~46nH in a perfect world.)

If you look at the FIRST picture, the RIGHT transformer is wound with two turns of green twisted-pair (wires in parallel). The secondary is a single turn of hollow copper foil which *perfectly conforms (sizewize) to the inside wall of the ferrite. If you look carefully, you can see that the two-turn primary actually runs INSIDE the foil, and I've spliced it out of the foil at the ends, and to tighten the second turn. After connecting the same IRFP450 on the secondary, it was revealed to resonate at 13.6MHz, (implying ~51nH in a perfect world.)

Tomorrow I'll try combining all four ferrites into a circlar loop (though this may be heading the wrong way). Failing that... parallel secondaries, drive the primaries in series. =)

Avalanche

Sat Nov 04 2006, 01:10PM

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

woh, getting closer now! I put a single turn copper wire secondary on my transformer above, it improved things slightly but still nowhere near good enough. Going by your results, it must have been fairly close afterall, as it's a very similar transformer. Maybe I just need to stand well back and push more power into the primary

Marko

Sat Nov 04 2006, 01:22PM

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

If you have enough copper foil you can try putting a ferrite toroid on a double-sided PCB; drilling hole in the middle and then envelopint entire toroid with foil for lowest possible inductance (wider, flatter winding = less inductance) and then simply wind primary winding on it.

I hope you got what i'm trying to say ^^

Avalanche

Sat Nov 04 2006, 05:06PM

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

Just gave litz wire a whirl, and it seems to perform nicely. Probably no real advantage except it's easier to push onto the core

When I get around to it I'll work out the inductance properly like BP did, into a known capacitance value.

Something which has surprised me is how much difference the straight wire inductance makes going to the MOSFET G-S, it's easy to overlook it when probing. Even though there are huge thick solder joints you still have to be careful where the probes go. It probably isn't helped by the fact I am using a tacky 1:1 probe also...

In the pics nothing has changed except the point of probage... I'll soon be drilling holes in the FET to try and hook up to the die

slight progress update...

I think it's resonant now, all I've done is extend the length of the gate slightly and that seems to add enough

However, all I can get is around 1.5x the voltage I would get from the turns ratio, by resonant action. I think the q-factor is so bad it makes it pretty much impossible to achieve any decent amount of rise, but I could be wrong. I've also changed a few component values in my rf driver which has helped a lot! Scope shot is 100nS, 1v/div. I'm wondering if (when I add gate bias) this is 'enough'?

ragnar

Sun Nov 05 2006, 12:03AM

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

Of course, what you see on the scope isn't what's actually on the gate, but should be near enough... if the gate is actually swinging 5.5Vp/p, it might be worth trying class-E (the swing of voltage/current helps the gate to turn on/off at the right times...?) you'd notice that the gate impedance will change (wildly throughout the cycle) as you start to switch actual volts. ^^

Assuming this isn't your last FET, by all means hook it up to some power!

After some more twiddling, I'm going to recreate my two-IXDD414 driver (remember the one with the nasty inverted waveforms) in the hope of now driving my little mess of transformers. In fact, scrap the inverted signals, I'll put two in parallel instead.

edit:
I made three identical copies of the orange-twisted-pair-and-braid ferrite pair... with the exception that the secondary now exits out the opposite side to the primary.

One by itself: Resonates at 12.2MHz, implying ~63nH
Two in parallel: Resonates at 14.1MHz, implying ~47nH
Three in parallel: Resonates at 15.2MHz, implying ~41nH

The Q is sufficiently low that the acceptable 'peak' ranges ~0.5MHz

Ack.

edit2:
For the sake of it I tried the four-ferrite combo: Three primary turns, each made from a parallel twisted pair. The secondary is a close-fitting copper foil. It's crap -- resonates at 11.8MHz, implying 68nH.

Splat.

---

Firkragg, I know what you mean -- but instead of copper foil (which gets messy), I meant to try it with pipe through the middle and half-torus shaped washers. Even after trying copper foil, it all seemed to be in vain.

Avalanche

Sun Nov 05 2006, 12:28PM

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

Excellent stuff! So with the improved setup you should be able to add some real inductance and get some resonant rise in the gate!

I found a 10Mhz crystal in an old cable modem yesterday... so I think I will probably 'cheat' and use that with my current setup

edit: scrap that idea, I ruined it trying to remove it. Back to 12Mhz

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