Q: cascode FETS for linear control

Just assume two identical FETs in cascode configuration, with equal value resistors in divider from top FET drain to gate, then to bottom FET gate which is driven by an adjustable voltage. Assume this is used as an adjustable shunt regulator in the zero to

15mA drain current region and the open circuit voltage from the supply is more than twice the avalanche rating of the FETs. Question: will any step change in the gate drive (bottom FET) cause a large Vgs change in the top FET - sufficent to zap its gate? If so, isn't it sufficent to mitigate the problem by using a G-S capacitor (instead of a zener)? Does the use of a FQD2N100 give more problems than use of a IRFBG20? ** Useage: with opamp and load current monitor for forcing a settable current thru load (the resulting voltage is read using a 10E9 resistor). Very nice to measure points on high voltage zeners.

Well, as I said, MOSFETs have huge built-in gate capacitors, which don't need to be supplemented with more! My calculations and spice measurements show that Ciss dominates and prevents the gates from seeing dangerous voltages, without further protection, provided that certain conditions are met, such as adding drain and output resistors to limit fault currents, and avoiding the temptation to add parallel capacitors across the gate-resistor stack. I'm assuming low-speed modest-current applications.

I haven't found high-voltage zeners (10V and up) to be leaky, even at the low currents I've been working at. OTOH, without any current going through them, they don't heat up at all, so my measurements (in the sub-nA region) are at ambient temp.

Still... Adding gate-source zener diodes (not back-to-back, BTW) is comfort food for the designer, and helps him sleep at night. So I continue to recommend it on that basis. It'll also help you get through any design reviews.

Ahem. What's this about a 185C temperature? How's that happen? You aren't designing down-hole instrumentation, are you?

--
 Thanks,
    - Win

That would be "ground". Why would thiss be better?

Thanks. Why i mentioned using a capacitor instead of a (back-to-back) zener, is the fact that a zener would be very leaky at 185C, and the resistor current is 15uA for a different application.

OK, Robert, spill the beans. More detail, please.

High-voltage MOSFET drain-source leakage current also looks like a resistor, about 1.5G in the case of my FQD2n100 1kV FETs.

--
 Thanks,
    - Win

In the very early '70's Schlumberger was one of my best customers.

I was building them a hybrid in a gold-plated metal dual-in-line with double-stacked-alumina boards populated with a variety of transistors and OpAmps for, IIRC, measurement of alpha radiation down-hole.

Schlumberger was so concerned about failures that they would throw away a unit after ONE USE down hole.

So I asked for return of the units for examination of possible failure mechanisms.

Not one single device had failed. The only observable deficiency was discolored gold plating.

But I let them keep throwing them away... better business than toilet paper ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Ayup! Downhole tiz. Measured 1nA leakage at 9V on a 10V back-to-back zener. At 185C, that would appear to translate to too many microamps (at 9V). The leakage appeared to be resistive, as compared to semi-constant current above some small voltage.

Or the AnaLog Services' Codatron? Ahem, have you seen their patent?

The MOSFET leakage resistance is modeled in parallel with the textbook FET channel, hence its value has no relationship to the operating current. As I mentioned, I measured 1.5G for the FQD2n100, but I've also measured the IRFBG20, getting about 7G. It would seem one can safely ignore it at room temp. As for high temperatures, 150C or 185C, you'd better take some measurements!

--
 Thanks,
    - Win

I already have a working design for a shunt regulator that replaces the Victoreen Corotron. Presently, i limit each one to a max of 900V (using a 1000V FET) and stack them in series to get the required PMT regulation at a shunt current of 600-100uA (industry standard). Was thinking of a way to reduce parts and size via the cascode scheme.

** At what current levels did you see that resistive component?

Cute. What's the patent number?

Good.

--
 Thanks,
    - Win

I might have seen the patent; i wrote it. I guess that you got the pun on the Titan regulator (our "TitanTwo" shunt regulator). Put the patent write-up on the TitanTwo into the public domain - where it belongs.

In process aka pending. I might move the Codatron to PD as well. What do you think?

Have you torn apart Titan's "regulator"? I have "regulation" voltage VS input voltage step and temperature step (wide pulse in both cases); the output is worse than the input and there is severe temperature hystersis. That is why i made a better design; do not need to "tweak" a divider on FETs either. Makes the regulators very repeatable and allows automated assembly.

Look at the Oil 4 Less LLC site; the patent apps are now officially in the public domain, and i refer to the Titan regulator patent as well.

How are your corrected HV shunt regulators used, what are they for?

--
 Thanks,
    - Win