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Class E Troubles

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LAN_403

HV Enthusiast

Wed Aug 16 2006, 05:43PM

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

I'm helping a friend of mine build a Class E tesla coil and have been experiencing some troubles which i'm not sure how to fix at this point. This is looking at a drive stage (see photo below) and the gate (white) and drain (purple) waveforms. Looks like the falling edge of the gate is doing what it is supposed to be, but the rising edge is doing some funky stuff - looks like miller plateau stuff going on there . . .

Anyone have some ideas on how to "clean" this up?

The circuit is a basic transformer fed gate drive arrangement where the circuit is tuned so that the series inductance (including leakage inductance of transformer) cancels out capacitive reactance of mosfet gate, and the transformer is used primarily to match the low input impedance of the gate to that of my output stage.

Steve Ward

Wed Aug 16 2006, 05:58PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Maybe adding a discrete capacitor across the gate would help shape it up, of course you would need a different leakage inductance to resonate with. My thought here is that it would simply increase the available current, to push past the miller effect quicker than it is.

Also, looking at the actual sinusoid parts of the gate waveform, it looks like your LC on the output (driving the coil) is not matched with the natural Fo of the gate circuitry (the gate seems to be tuned too high).

Hazmatt_(The Underdog)

Wed Aug 16 2006, 07:17PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
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I'm not really sure what you want as a waveform, but I'll give it a shot anywho.

For the Gate pulse I would put a zener on the gate to flatten out the signal, which would give you more of a pulse. And possibly a resistor in series with the zener to set the 'depth' of the clamping.

Marko

Wed Aug 16 2006, 07:59PM

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

Zener is unneeded when resonant drive is set correctly (altough they can serve as a protection).

I think he just needs to adjust Lsec1 in resonance with gate as Steve already explained.

IRF520 has much lower gate capacitance than IRFP450 or 114N50 (used in Richie Burnett's output stage) so the transformer is going to need a bit twiddling with number of transformer turns and coupling.
Maybe a variable choke in series with secondary could help with adjustment..

Terry Fritz

Wed Aug 16 2006, 10:43PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
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Hi,

You probably won't clean it up much. Lower drive impedance...

Might see this patent:

USPTO # 05187580

The driver (#29) is another class E power driver, but just a conventional Nat Sokal thing...

But TC frequencies are far lower... So maybe you can do it. Be sure L8 is a really good inductor and is not saturating.

Cheers,

Terry

ragnar

Wed Aug 16 2006, 11:38PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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If the shunt capacitor is ringing too low (i.e. causing the body diode to conduct), you'll get ringing and ripples appearing on the gatedrive.

Similarly if the shunt capacitor is partially discharging into the MOSFET channel, you'll get spikies on the gate waveform at that point.

Have you tried tuning the drive frequency?

If you can afford to add more inductance to the circuit (might need to take some turns off the GDT to allow this), use a few turns of wire a round your finger, and squeeze or stretch it as appropriate until you get the maximum rise.

Just curious, who's your friend and what frequency are we talking here?

What does the gate waveform look like with no power applied? You probably want him ringing up to at _least_ +/- 20V without power, which will be damped to ~ +/- 12 to 15V when you let drain current flow.

You might also find it easier to put your primary where that RF choke is (straight between positive/drain). Means you have to retune less stuff when twiddling.

Steve Conner

Thu Aug 17 2006, 09:25AM

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

I think it's weirdness caused by the non-linearity of the device capacitances. The gate and Miller capacitances are both very strongly dependent on Vgs and Vgd, and they are part of your resonant gate circuit.

The fact that you have a f/2 subharmonic in the output tends to suggest that explanation too. If I remember right, the cure is to add damping with a resistor in the gate circuit, and/or to put a capacitor from gate to ground to swamp the non-linear capacitance.

*edit* I thought about it some more and this is completely the wrong explanation for the problem.

Marko

Thu Aug 17 2006, 12:03PM

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

Steve Conner wrote ...

Well putting a resistor in series with gate isn't a bad idea at all.

It can help regulating Q of the circuit in order not to overvolt the mosfet (easier than adjusting external power).

HV Enthusiast

Thu Aug 17 2006, 01:33PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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wrote ...

Well putting a resistor in series with gate isn't a bad idea at all.

It can help regulating Q of the circuit in order not to overvolt the mosfet (easier than adjusting external power).

The impedance transformer is what accomplishes this. Although its primary purpose is to transform the very low gate resistance of the IRF520 to match the output impedance of the input stage, it also acts like a series impedance to the gate. Remember, at these frequencies, we need to look at the circuit in an "RF" sense as opposed to traditional circuit theory.

We worked on the circuit a bit more last night and found that the drain choke on the IRF520 was corrupting the waveform. The drain choke was originally a 25 turn choke around a 0.25" x 1.00" DIA Type 77 material ferrite rod. With the ferrite rod core inserted in the coil, the waveform had that first notch in it (See attached image)

Now, removing the core, the waveforms cleaned up significantly and its working pretty good (IRF520 barely gets warm to the touch now)
(see attached image)

I also had to adjust the impedance matching transformer and the series tuning inductor to get a good amplitude on the gate (about 12V peak)

Dan

ragnar

Thu Aug 17 2006, 11:30PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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IRF520, eh? This must be some high frequency coil, EVR.

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