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Steve Conner

Thu Oct 12 2006, 12:01AM

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

Hi all

After chatting to Blackplasma, WaveRider and Firkragg about Class-E Tesla coils, I decided that I ought to try building one myself. My goal is to get as much power as possible at 27.12MHz using IRFP460s. (OK, so maybe the amount of power possible is "None", but I'll still have fun finding that out.)

Driving MOSFET gates at these frequencies is about the most difficult part, so I thought I would start with that. The gate looks like a resistance of a few ohms in series with a capacitor so big it might as well be a short-circuit.

I have a HF transmitter that can put out 100W at 27.12MHz into a 50 ohm load, so my first thought was to make a step-down transformer that would transform 50 ohms to "A few" ohms. As a first attempt, I made a broadband transformer, using a ferrite core with 4 turns of coax. The core went through all 4 turns in series, and the screen was cut into 4 parts that I soldered in parallel. This gives a voltage ratio of 4:1, hence an impedance ratio of 16:1, which would transform 50 ohms to about 3 ohms. The core I used was an EMC suppressor I found in a junked printer.

The transformer functioned as I expected up to 10MHz. I could feed it directly from my HF rig and light two 12V 20W halogen bulbs in parallel. But as I increased the frequency, the SWR began to get worse. By the time I reached 27MHz, the transmitter could barely light the bulbs before going into protection. I guessed this was due to leakage inductance, as it's not a true transmission-line transformer, even though it's made out of coax. I tried adding a variable capacitor in series with the input to resonate it, but the dip I got was very shallow, and the transmitter still wasn't happy.

So I used the antenna tuner to match it and could get both bulbs lit brightly at 27.12MHz. This worked great for about 20 seconds, then the bulbs went out as the ferrite core overheated and passed its curie point, and the coax sort of melted too...

1160611275 30 FT0 Gdt Attempt1 Test 2712

But I managed to deliver about 40W of "gate drive" into 3 ohms at 27MHz, which is a start.

ragnar

Thu Oct 12 2006, 12:11AM

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

Here's the autotransformer I used to obtain synchronised inverted signals for my two-ended power gatedriver.

I regret that I'm having great difficulty matching the driver into an IRFP450 (even the transmission-line transformer seems to have too much inductance.. or something =-P)

I'll try again with a binocular core.

Below: the four-turn autotransformer is driven between turns #2 and #3. Turn #2 is grounded. Inverting and noninverting outputs can be drawn between #2 and #0, and #2 and #4 at 2x the input voltage.

Matching the outputs of the IXDD414s into an IRFP450 gate is proving difficult however -- GDT, transmission-line transformer/balun... none of it wants to work, no matter how much tweaking I can apply.

Later today I'll see if some sheer brute force can do it.

WaveRider

Thu Oct 12 2006, 08:20AM

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

Steve: I've found that ferrites don't really like high-power high frequency for very long.. The powdered iron core (e.g. Amidon mix 2 or 7) toroids work better, altho leakage inductance can be higher.. (Low permeability mix). This seems to be what most hams use in their HF antenna baluns.

BP: The IXDD414 should not require matching...it already has a low impedance output.....of course, if you want resonant gate drive, that's another thing..

ragnar

Thu Oct 12 2006, 09:11AM

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

Driving an IRFP450 with an IXDD414 at 10+ MHz straight of an IXDD414 isn't possible, which is why I transformer the impedance down a few times, and ensure the voltage resonates back up to a usable level.

WaveRider

Thu Oct 12 2006, 09:25AM

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

Have you tried putting an inductor in parallel with the gate to "tune out" the gate capacitance.

from IXDD414                        To MOSFET gate
                     |  |
 --------------------|  |--------------------
                     |  |             |
                                      )
                     C                )  Inductor
                                      )  (for parallel resonance
                                      )    with gate and C)
                                      |
                                  -------
                                    ----
                                     --

The IXDD414 should then see a purely resistive load (at who knows what value...I should try a simulation with spice)

Marko

Thu Oct 12 2006, 12:33PM

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

Inductor is actually rather put in series (or leakage inductance of GDT is used) so the driver doesn't 'see' anything more than his supply voltage, and resonant rise boosts gate voltage for a Q factor.

For this reason it's generally better to use a GDT, make driver run at relatively high voltage wich is stepped down with GDT, to just few volts. Resonant rise with gate capacitance lifts it to reasonable level, while reducing lload on the chip (since most of power is resonantly conserved with each cycle instead being shorted to ground).

It is completely normal for a common EMI ferrite core to blow up at 27Mhz 100W.

At that power level I knew to heat up those cores at 100kHz O_O

ragnar

Thu Oct 12 2006, 01:14PM

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

wrote ...
Have you tried putting an inductor in parallel with the gate to "tune out" the gate capacitance.

I've only ever tried putting a parallel in series with the gate. I guess my flagrant ignorance is showing through, here

WaveRider

Thu Oct 12 2006, 01:50PM

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

By paralleling the inductor ant the gate, it should transform the gate resistance to a higher value.. parallel resonance should give an impedance proportional to 1/Rgate... in fact, it should be around

Rin_parallel approx L / (C * Rgate)

there will be a reactive (usually capacitive) component, but by judicious choice of L, you can make it small compared to Rin.

The main problem is the inductor will have a fairly small value... A couple of small turns of fat wire (50-60nH) should do the trick...
This may pose practical problems with tune-up, tho'...

ragnar

Thu Oct 12 2006, 02:15PM

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

Since I managed to rip a 12MHz crystal off a dead CRT monitor board, I have a 12MHz fixed SSTC... not 13.56.

Edit: I canned the idea of any inductors after them not doing much... but now I have a nice 0V - 12V sinewave drive on the gate of my IRFP450... at 12MHz I'll attach pics soon.

My gatedriver assembly is drawing 1.4A at 12... 16W.. probably at least 10-12W being dissipated in the gate, I'd guess?

ragnar

Fri Oct 13 2006, 05:14AM

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

OK, working a little more reliably now, limited only by the fact I haven't heatsinked the IRFP450.

It probably would have been better value for my time to try and find someone who would sell me a DEIC420, but even then I couldn't afford it if I wanted to.

Here's the proof:

Ham radio not included. Or required

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