W>>> W>>>
>>>> This active-zener method works well with low-voltage power MOSFETs,
>>>> such as under 100V, but it's dangerous with high-voltage FETs, 200V
>>>> and up, because they have a bad tendency to go into RF oscillation.
>>>> This is a high-power RF oscillation at frequencies of 15 to 40MHz,
>>>> which is very difficult to damp with external parts such a ferrite
>>>> beads, gate resistors, etc. That's because the RF oscillation is
>>>> internal to the FET, employing its inductance and self capacitance.
>>>> The required linear properties occur whenever a high current flows
>>>> while the drain-source voltage is higher than 10 to 20V. The latter
>>>> condition causes the FET capacitances to drop to the levels where RF
>>>> amplification is efficient.
>>>
>>> If the RF feedback path is wholy internal how would this affect the
>>> method of using a higher gate drive resistance to slow the current
>>> fall to limit the voltage to less than the breakdown voltage?
>>
>> Two different effects... The slowing of the turnoff means the
>> coil can flyback and dI/dt discharge as it's doing so, without
>> reaching the avalanche voltage, if carefully done.
>>
>>> or does the zener just add more parasitics to make the difference?
>>
>> You're asking if oscillation doesn't happen in the event of a
>> slowed transition, as in the zener case? It certainly can with
>> high-voltage MOSFETs, although the dV/dt slewing output helps to
>> hide it, on the one hand, and perhaps to dampen it, on the other.
>
> Yes thanks thats what i was asking, as both cases have the vds>20v
> at high curent. Trying to think of a way of avoiding it yet still
> using a more deterministic way of setting the peak voltage.
>
> Actually i was wondering if a cascode mosfet arangement would behave
> any better, again it might make it less noticable as the bottom device
> would stay more in control of the current, although i would be worried
> about this as long ago I had some nasty oscilations when i was trying
> to make a high voltage power supply with several series mosfets (600v
> mosfets were very limited at the time), but unfortunatly i never had
> the time (or the experience back then) to get to the bottom of all
> the diferent modes of oscilations.
Hmm, oscillation for a high-voltage string of MOSFETS in series, due to the series connection, you think? As opposed to just the bottom MOSFET by itself? How high was the FET operating current when you observed oscillation?
Let's evaluate the scene.
For a series-connected MOSFET the current gain is unity from DC to a frequency f_T = g_m / 2pi Ciss, where the gate capacitance robs the ac signal current away from the FET's source path. For a BJT, the transconductance gm = Ic/Vt = 40 Ic. It's lower for power MOSFETs, g_m = Id/nVt in the subthreshold region, where n = 3 to 5, according to my measurements. So here a MOSFET has 3 to 5x lower g_m than a BJT at the same current. Above the FET's threshold gate voltage, where the currents are from 5 to 100% of the FET's maximum operating current, g_m still rises with current, but at a much slower rate.
I would think the bottom line is, you need to work within say 20% or higher of the FET's maximum current to get its g_m, and thus f_T, high enough to take part in serious RF oscillation. While operation at such a high voltage and current is practical for a few milliseconds, I imagine it'd create too much power dissipation to do continuously.
This means most continuous linear use of power MOSFETs occurs in the subthreshold region, where the g_m/Id ratio is higher, but where the transistor's f_T remains low, say under 20MHz.
For example, I'm using fqd2n100 surface-mount 2A 1kV FETs in a series-connected amplifier. At the maximum current of 4mA with 400V across the FET, it dissipates about 1.6W, pushing the junction temperature up by about 90C, which is as high as I'm comfortable to go. This FET has Ciss = 400pF. At 4mA it has g_m = 32mS, which means its f_T = 13MHz. Oops! that's getting into a dangerous region. If I was using a similar MOSFET, with heatsinks, at currents higher than 4mA, there could be trouble.