Simplistic Mosfet HV Shunt Regs

[Bas Horneman]

Yes. Will do. Thanks Salas.

[Salas]

Originally Posted by Bas Horneman

Just did some quick measurements:

200v=36mA
180v=43,5mA
160v=50mA

But you Vin remains steady? How high input DC you feed?

[Bas Horneman]

The Vin drops as well.

[Salas]

Its a balancing act because you have small sinks and a Vin that sags with increasing CCS. I would say, determine a Vout B+ target first. Simulate your load consumption with proper dummy. Then set R1 and Vout (trimmer) until you get 1.Target B+ 2.Target CCS. 3.Vin remains equal or more than (Vout+15V).

[rolls]

Hello
I have come across your pages because I am looking for a shunt reg for my tube power amp. I have already tried a CCS to see what was going on. It seems that a CCS blocks the mains dirt, and since then it was able to listen the whole day, not just during the night. Of course the shunt is missing, so output transformer, driver transformer and CCS are fighting against each other.
I have gone nearly through all the pages and I am impressed to notice how nice people are, there has not been a bad word.
I think about the pnp transistor, because I need at least 360V, and I would feel uneasy pushing a 375V transistor nearly to its limit. I have found a Fairchild Darlington, FJPF9020TU, it can stand 15W, and 550V up to 7mA. It is an end of life product, but there are still more than 400 available at Mouser.
Last I would like to thank Salas to be such a creative genius and for his endless patience.

Andre

[Salas]

You would need the #967 circuit. Has an HV PNP that drives a 500V NMOSFET. It uses low current defined by the MOSFET's VGS/BJT's collector load resistor. An MJE5731A should hold well since MJE350 has been reported to hold at 330V in an example. I would be careful with the Darlington not to ring in that position, because it has to modify the OLG and PM of the circuit probably. Also the steep hfe curves are not inspiring for mA region linearity. If it will work, its higher Vbe is welcome for stabilizing even better the K170 Jfet as CCS that is nesting in it.

[rolls]

Hi Salas
I see darlingtons could be troublesome. Ringing seems to be the main problem with this shunt, even if I think that it is not intrinsic in this circuit.
I have also the idea to move the FET to the negative side. This way the transistor could be a npn, and the following a p-ch Mosfet.
Personally I am going through all the options theoretically in the first place, because when I am working on the amp I have to be very careful, this one starts with 750V, but the tubes are stacked, there is one ccs for both on the negative side, then one shunt for the driver, about 200V, another one for the output tube, from 360V- 400V. I think nobody has done this before, but I don't want to remove the ccs any more, so I have to go the full monty.
Thank you
Andre

[Salas]

The #967 has 90deg phase margin. In shunt the problems start when the load becomes inductive. You could try cascoding MJE350s since NMOS HV is easier to get is another option. It shortens PM by 10deg only theoretically. I use NPN drive & PMOS output in the LV circuits, its certainly feasible if you have PMOS with enough Vmax for your application. See it has as much transconductance as the IRF840 at least, else the Zo is going to rise. Good luck.

[rolls]

Thank you for your help. I think it is time to start now with a trial board.

[Tony]

@salas,

what is your take on connecting a zener in series with the collector of the MJE350 to address Vceo issues when output voltage is greater than 300 volts?