Looking for HV fets for direct drive

[invisible force]

just comes to my mind : as it is the difference of the two step up primary-secondary caps which is causing the "fog current" it should be possible to reduce it by adding a cap to the smaller one...it needs to be a high voltage part and it will not be a perfect match because of the boring transformer complexity...sorry if I am a bit off topic, but it should be valid for most ESLs...
regards, Philipp

[maudio]

Back on topic

Originally Posted by invisible force

A suitable high voltage N channel FET for dd drive : STW3N150 (STP3N150 - STMicroelectronics) by STMicroelectronics..

Ciss almost 1000pf, that is a bit high. One of the most important parameters to look for is low gate capacitance (Ciss). You have to stack multiple mosfets to handle the voltage and to do so you need a resistor divider driving the gates. Those resistors together with Ciss limit your bandwidth. Lowering the value of those resistors makes your dissipation skyrocket.

The 1000V IR IRFBG20 is such a nice mosfet for this application because it has only 500 pf Ciss.
The BFC40 from Semelab has 550 pf and will even handle 1500V. Semelab also has the BFC60 with as little as 40 pf but that will only handle 100mA/20W and that is just not enough for a 4kV amp. Ideally would be a mosfet in between those 2 but I have never found one. Come on guys, start producing a bfc 50

The higher the current rating the higher Ciss will be. So you need to get the smallest available mosfet that will handle the power.

[invisible force]

O.K. STP2NK100Z (STP2NK100Z - STMicroelectronics) is a better choice (Ciss 500pF / Vds 1000V) and it is protected by build in zeners..
Maudio : I regard your feedback solution far more practicable than dd drive...would be great, if you find the time for a circuit diagram of electronics surrounding the power amp...

[invisible force]

If you look at Patent DE102008034456 (Espacenet - Original document)
you see a solution of dd drive with optocouplers..the inventor Fritz Linck is a very friendly man - I visited him to listen to his dd driven Quads...well it was a working solution but imho sound was worse than done with transformers..sorry to say that..For a short time it was offered as an Quad upgrade option (€ 10.000) by the official Quad distributer in Germany..but they removed it for sound reasons.

[invisible force]

No comments maudio..??

[jcx]

cascading AC performance can be improved with added C divider in parallel with the R

Cgs is bootstrapped by the device gain and appears reduced in value, Crss becomes an issue depending on the MOSFET type but the AC divider C may be smaller than you expect and don't add too much load

but even tweaked for better AC performance you can have catastrophic failure once one device goes, lower number of series devices is safer - KOSS E/90 electrostatic amp uses 3x 500 Vds MOSFET with +/-600 Vsupply and zener clamps them D-S

IXYS advertises up to 4 kV MOSFET

[alexberg]

Originally Posted by jcx

cascading AC performance can be improved with added C divider in parallel with the R

Cgs is bootstrapped by the device gain and appears reduced in value, Crss becomes an issue depending on the MOSFET type but the AC divider C may be smaller than you expect and don't add too much load

but even tweaked for better AC performance you can have catastrophic failure once one device goes, lower number of series devices is safer - KOSS E/90 electrostatic amp uses 3x 500 Vds MOSFET with +/-600 Vsupply and zener clamps them D-S

IXYS advertises up to 4 kV MOSFET

Every modern Si MOSFET has build-in quite powerful zener with energy up to few Joules, somewhat like avalanche diodes. Good luck with external clamp, unless you need to do so below Vds max. Moreover Zeners have smaller dies... Good HV MOSFET can sustain single overload around 20kW @ 25us and even more @ lesser time.
So no chain breakdown, just keep supply bypass capacitor value reasonable. And yes, biggest problem is Crss which feeds back to gate thus demanding rather low R values in ladder divider and/or capacitive shunts across the resistors. Such measures negatively affect achievable gain and HF gain in particular.
Have a look at
http://www.pes.ee.ethz.ch/uploads/tx..._Upload_01.pdf
Such circuit is even more demanding in regard to dynamic performance in respect to audio amp.

[invisible force]

gate voltage divider done with avalanching diodes is for switching purpose only...no linear application possible by doing this...interesting, but not for audio..have fun..

[maudio]

Originally Posted by invisible force

If you look at Patent DE102008034456 (Espacenet - Original document)
you see a solution of dd drive with optocouplers..

It is quite an achievement to get something like that to work at all, to make it performe good is almost impossible. There are several issues with optocouplers. First the are very noisy making it difficult to get sufficient s/n ratios. Second and worse they are not very linear. The linearity can be improved a lot by using linear optocouplers that use a second photodiode to compensate the non-linearity’s with a feedback loop controlling the led current, something I do not see in this design. But even then it is difficult to make a good sounding amplifier with optocouplers in the signal path. And there is the safety/reliability problem but that might be solvable choosing the right components.
I once built an experimental optically coupled output stage running from 2 kV with irfbg20mosfets, using hcnr 200 linear optocouplers. It did not work very well, in fact it proved almost impossible to get it to work at all. The output stage itself could oscillate with the inputs of both optical circuits shortened, no feedback whatsoever. Even when I managed to get it stable (at the cost of much needed bandwidth of course) I still had very poor control over the top fet, resulting in open loop performance of the output stage that was so horrible non-linear that I abandoned the idea altogether. No chance of building a good sounding amplifier from such a basis.
The reason is simple once you know it: we have a photodiode in the optocoupler that is generating photo currents in the nano and microampere range, coupled to a highly sensitive circuit that converts these minute currents to a control voltage for the mosfet . Now we have approx 0.5 pf capacitance between input and output side of the optocoupler (that no spice model takes into account btw). For the top optocoupler we have full output voltage swing over that capacitance, injecting error currents. Do the math yourself... Now add to the equation additional capacitances introduced by the DC/DC converters needed to power the top circuit and you don’t stand a chance to make it behave in any decent way.

Originally Posted by jcx

cascading AC performance can be improved with added C divider in parallel with the R

Cgs is bootstrapped by the device gain and appears reduced in value, Crss becomes an issue depending on the MOSFET type but the AC divider C may be smaller than you expect and don't add too much load

I experimented a lot with output stages with mosfets and found little room for improvement using compensated HV dividers. The internal capacities of the mosfets are not only rather large but behave highly "dynamic", they change radically with voltage changes. You can safely forget any values from the datasheet. In order to get any improvement from a compensated divider you need to outnumber the internal capacitances requiring very large capacitors, couple of nf at least. As all this extra capacity is basically parallel to the output device it adds so much load that things just get worse overall.

Originally Posted by jcx

but even tweaked for better AC performance you can have catastrophic failure once one device goes, lower number of series devices is safer

Not only safer but also faster. Reliability can be improved with fast diodes between gate and source, to prevent breakdown of the channel during negative transients. Zeners are not recommended as they are slower and not needed, positive transients are no problem as the fets will protect themselves in that case. Using simple 1n4148 I did not ever encounter reliability problems on output stages using 4 stacked irfbg20 mosfets for each device, even when processing square waves and other nasty waveforms.
Stacking more fets is not only ruining reliability but also bandwidth, even with 4 fets openloop bandwidth of the output stage was already far below 20 khz. Did some test with large number small 300V fets but found openloop bandwidths of 1-2 khz, no good starting point.

[invisible force]

Yes, linear optocouplers have been used (could be the hcnr 200..), the floating operational amps are types from AD (grabbing from my brain), specially designed for driving capacitive loads (remaining stable by enabling the internal compensation cap becoming bigger as capacitive load is heavy..)...the inventor told me he spent 10++ years on the project to get it work...I believed him...
Your feedback solution is more practicable, maudio...