Claimed OU circuit of Rosemary Ainslie

[Groundloop]

@TinselKoala,

I have read the papers of RA. She noted that the frequency of the HEXFET was different (higher) than the oscillator frequency. Have you noticed such behavior in your circuit?

GL.

[poynt99]

The input Gate drive parameters are not critical. It's there only to elicit and maintain parasitic oscillation in the MOSFET.

.99

[TinselKoala]

@TinselKoala,

I have read the papers of RA. She noted that the frequency of the HEXFET was different (higher) than the oscillator frequency. Have you noticed such behavior in your circuit?

GL.

Well, there's that ringdown frequency, which is quite a bit higher than the 2.4 kHz drive. But without seeing a scope trace I don't know if that's what she's talking about. So far, except for the spikes, my mosfet tracks the input frequency exactly, up to 2 MHz, which is where my FG tops out.
I don't quite understand what's happening in her circuit to produce the "random oscillations" that she talks about in the paper. She says she has to turn the gate drive down?? to get that? I guess I'll have to build the 555 portion to see what it introduces into the mix. That will be tomorrow, though.
Maybe the particular MOSFET does make a difference. I hate waiting for stuff. I wish I could find one locally.

[poynt99]

I don't quite understand what's happening in her circuit to produce the "random oscillations" that she talks about in the paper. She says she has to turn the gate drive down??

The Gate drive parameters may not critical in terms of frequency and duty cycle. In fact each device type and even between batches there will be variances. Again, the input is only a stimulus to excite parasitic oscillation natural to the MOSFET. This is of course the opposite effect one normally wants to achieve.

Parasitic oscillations come about when the MOSFETs are allowed to operate in their analog (or linear) region for a long enough time for a parasitic oscillation to get going. This region exists between the MOSFET gate threshold voltage and saturation voltage. MOSFETs have extremely high voltage gain, combined with very high capacitances, which makes the devices very prone to parasitics unless steps are taken to prevent them.

There is an optimum value of series gate resistance with all setups. Variance from optimum will result either in spiky edges as seen in your scope shots, or oscillation.

"Turning down the Gate Drive" means introducing more and more series gate resistance, Rg, until the things breaks into continuous parasitic oscillation.

Maybe the particular MOSFET does make a difference.

Almost all will oscillate, especially the higher voltage and current devices. Even obtaining the same part number she specified may not guarantee you'll get the same results. You may have to tweak the duty cycle, frequency, and Rg.

.99

[TinselKoala]

The Gate drive parameters may not critical in terms of frequency and duty cycle. In fact each device type and even between batches there will be variances. Again, the input is only a stimulus to excite parasitic oscillation natural to the MOSFET. This is of course the opposite effect one normally wants to achieve.

Parasitic oscillations come about when the MOSFETs are allowed to operate in their analog (or linear) region for a long enough time for a parasitic oscillation to get going. This region exists between the MOSFET gate threshold voltage and saturation voltage. MOSFETs have extremely high voltage gain, combined with very high capacitances, which makes the devices very prone to parasitics unless steps are taken to prevent them.

There is an optimum value of series gate resistance with all setups. Variance from optimum will result either in spiky edges as seen in your scope shots, or oscillation.

"Turning down the Gate Drive" means introducing more and more series gate resistance, Rg, until the things breaks into continuous parasitic oscillation.

Almost all will oscillate, especially the higher voltage and current devices. Even obtaining the same part number she specified may not guarantee you'll get the same results. You may have to tweak the duty cycle, frequency, and Rg.

.99

Yes, thanks for the review.
You will note that Ainslie uses a 100 ohm pot to vary the gate drive. I didn't know that when I put my circuit together, so I used 200 kilo ohms. So I can "turn down" my gate drive through her available range and much further. Plus I can vary the output attenuation of the FG.
The parasitic oscillations that she generates in her circuit are absent in mine. When I turn the gate drive down with short duty cycles, the MOSFET simply turns off and stays off. Did you watch my video? You can see me doing this several times. No wild parasitic oscillations evident. With longer duty cycles even the full 200K isn't enough (with 10V p-p on the FG output) to shut off the mosfet, and it happily amplifies, relatively cleanly, the 2.4 kHz input pulse. Still no parasitic oscillations. So I'll put more resistance in there, and I'll continue to wonder about the numbers in Ainslie's paper. 3.7 percent duty cycle? 100 ohms gate resistance? 2.4 kiloHertz? With these numbers I get no heating of the load. If I increase the duty cycle to 30 percent or more I get plenty heat of load and mosfet. But at 3.7 percent I get no parasitic oscillation, and no heat on the load.
So perhaps I have a "good" mosfet, or perhaps my circuit layout is "better", that is, less prone to oscillations, or perhaps her 555 driver is forcing the oscillations to happen.
I just can't get my circuit to misbehave properly, and yes, I've explored the parameter space, up to 2 MHz, as I said, and from 3 to 97 percent duty cycles, and 0 to 200K Rg (Vgs=10V)