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HF gate drive experiments

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Avalanche

Tue Nov 07 2006, 03:10PM

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

I'm starting to consider winding a resonator too and making some sparks, I almost forgot about that part

outpwnage, doh!

I think I can boost the p-p value on my gate a bit more, just by turning up the input, but i'm max'd out at 15v in at the moment. I'll just have to find a bigger transformer to run the driver, then see (or smell) what happens...

I'm quite happy with the performance of the capacitance on the secondary side, so will probably stick with it and my current driver setup. Lots of small caps in parallel seem to perform better than just one large cap at HF, and I have hundreds of these 1nf's anyway. Just need to get on with the class-e part now!

So far this has kept me entertained a lot longer than my medium frequency sstcs. I was looking at HV mosfets the other day, and various techniques to run the things in parallel. Still haven't dropped the idea of base feeding...

Sulaiman

Tue Nov 07 2006, 04:19PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Have you tested the gate drive with a load/power on the drain yet?
The miller (feedback) capacitance will change the load quite a bit I think.
At such high frequency you may need to consider 'cascode' operation

Avalanche

Tue Nov 07 2006, 05:13PM

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

I haven't tried that yet, but i'm about to... I'm guessing I will probably have to spend a while getting the nice gate drive back again

I briefly ran across the 'cascode' thing when I was looking into running mosfets in series for higher voltage, I never thought of the advantage of dividing the mosfet capacitance as well by doing that so I suppose that's good news.

-Update- 10v p-p, 12Mhz

ragnar

Thu Nov 09 2006, 12:10AM

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

For a better impedance ratio, I've decided to go back to four IXDDs, two in parallel for each leg of a fullbridge (24V/turn, anyone? =P)

And so I can outpwn j00 all, my target remains at 13.56MHz (ISM band). ^^

Avalanche

Thu Nov 09 2006, 12:19PM

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

pwned

I'm having some minor problems with my driver now, after increasing the input voltage. As it heats up, the drive waveform slowly deteriorates, so I have to re-adjust the bias to get it back. I've made some huge heatsinks to combat it though.

One thing I did find when switching an actual load with the mosfet was that my gate waveforms actually seemed to improve - which was a nice surprise.

bp, you have the same calculator as me

Marko

Thu Nov 09 2006, 12:56PM

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

I'm having some minor problems with my driver now, after increasing the input voltage. As it heats up, the drive waveform slowly deteriorates, so I have to re-adjust the bias to get it back. I've made some huge heatsinks to combat it though.

You definitely have bad caps somewhere in the circuit.

I had some of those little blue ones and after I used them as shunt cap they melted, spilled their guts and embedded into surrounding plastic objects and themselves mutually (philadephia experiment?)

You have plenty about that in my and bp's old threads.

All caps must be polypropylene, same for gate drive, almost anything else will heat up dramatically and change capacitance with that.

Also try to move them away from mosfet and heatsink physically (little inductance won't bother you since you aare already tuning the circuit).

Once it's working properly your circuit shouldn't draw any singificant current without load nor get too hot either.

O_o

WaveRider

Thu Nov 09 2006, 03:11PM

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

Been lurking for a while now.. I am amazed by the ways you all find to torture various components!

A tip on the capacitors: Where high RF voltages and currents are encountered, nothing beats silver-mica caps. They cost a bit, but the performance is worth it.. (Example: my HF coil uses mica caps in the output matching.. It must pass amperes of RF current at 200-300 volts and it does not even get warm. On the other hand, the series matching coil, made out of 2 sq mm wire, gets noticeably hot, indicating significant current flowing in the circuit.)

If I may throw a suggestion in, I would shy away from the IXDD414 at much more than 10MHz. Its switching speed is not good enuf. Instead of going to all the trouble to parallel the things, you could cook up a nice simple class-C or class-BC bipolar or mosfet amplifier ( the BLF175 would loaf along in this service) to feed the gate resonant drive of the final PA mosfet. Of course, it costs a bit (about USD25 from Digikey), but 10, 13.56 or why not 29 MHz can be within reach. The DEIC420 will also work well, but will set you back 30-40 dollars...

Check out this site for hams building high power class-E rigs at respectible frequencies...

Avalanche

Thu Nov 09 2006, 03:23PM

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

I'm using a class-AB amplifier to produce my gate drive now, and with a bit of persuasion I've found it is possible to get ordinary easily available power transistors up to 12Mhz, which is what i'm doing now. It's up to a point now where it's fairly reliable, so I can draw up and post the schematic if anyone's interested

I started off with a class-A amp and a cheap-ish RF transistor (BLY89C) producing a couple of watts, but they are tricky (and relatively expensive) to get hold of. You also don't want to go blowing those things up, because they contain a toxic substance

The class A is probably the easiest way to get some power out of a single device, but I found you need a good RF transistor for it to do any good.

Sulaiman

Thu Nov 09 2006, 07:39PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Heatsinks;
I used to use them by the kilogramme, but now I prefer CPU-cooler types
an extra power connection/use but worth it for the size/weight/cost
(not suitable at very high voltages)

Silver-mica capacitors are good but incredibly expensive and bulky
Up to 1 MHz I haven't found anything better than polypropylens foil/film
(MKP, MMKP, MK/MMKP etc.) (less lossy than silver-mica)

My small experience at tens of MHz makes me lean towards 'Transmitter' 'Doorknob' ceramic capacitors due to cost/availability.
If you've got thousands of pF at tens of kV - it's probably a lossy/poor dc-rated ceramic

WaveRider

Thu Nov 09 2006, 08:08PM

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

Silver micas are more expensive than polypro caps, but the ones I buy from Digikey are about 1.50-3.00 each and they are not terribly bulky...not to bad if you only need a few of them. Loss factors for teflon, polypro and polyethylene are slightly lower, but the dielectric strength of mica sheets(breakdown field strength) is very high in comparison.. They also have excellent temperature stability.

If I had to use plastic dielectric caps in high power CW RF applications, I would probably use teflon, as it's breakdown properties and loss performance are very good and it is also pretty temperature stable.. (In fact, I usually use teflon based PCBs for my microwave projects above 3-4 GHz... Excellent electrical properties, but can be a bit soft and bendy..)

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