MOSFET B+ reducer

[ViperDoc]

I mad a mistake in the document I made. The part number for the 12v Zener diode was incorrect. I have updated the doc.

I realized my error when I started to order more parts and realized it was mis-labeled.

Sorry for any inconvenience.

I'm curious if any of these or other similar boards are still in the wild? Thanks.

[Stevem]

Sounds like it’s time for implementation of a Peltier device!

[dorrisant]

I have more of these boards if needed.

[craftyjam]

Sorry I'm replying to an old thread, but one could place several MOSFETs in parallel to lessen the power dissipation of each, correct? Like using pass-transistors with a linear regulator?

I know that MOSFETs of the same type won't be perfectly matched, but I've seen them used in parallel in power amplifiers without the different rds values causing balancing issues.

Seems that it could save spending big on a large power MOSFETs.

[trinity]

Hello, I built the Aiken circuit using an IRFPE50 instead of the STW20NK50Z. I used the resistor values with a 36V zener.

https://www.vishay.com/docs/91248/91248.pdf

I ran it up to a low voltage just to test it and it has no effect on the B+ . This is in a repurposed Marshall JCM800 Lead, modern combo and I want to drop the B+ down by about 10-15% . I used new new parts.

It’s so simple but its acting like it is fully on and conducting. Any test ideas? Thoughts?

In the picture, the zener was clipped to the terminal strip. There is a jumper between the lugs.

B9B4A685-3A30-474B-9698-72870F22BC08.jpeg

***PROBLEM SOLVED. I WAS USING THE WRONG CENTRE TAP. SEEMS TO BE PERFORMING PROPERLY NOW UNDER LOW V. TESTING***

[craftyjam]

The better way to do that is to have a pre-filter cap, which has a + terminal to the rectifiers and to the actual first grounded filter cap.
The negative side of this first pre-filter cap goes to the rectifier negatives, and the amplified zener goes between that point and the actual circuit ground on the first filter cap. This lets the zener setup handle the average current, not the rectifier pulses and helps keep it cooler.

I know this is an old thread, but I'm trying to figure out how to connect the pre filter cap in this instance, since my particular zener seems to be behaving oddly and having current spikes as the power tubes heat up.

bplus reducer.PNG

[LOUDthud]

You basically got it right except for the components around the MOSFET.

Pre_Filt_1.GIF

[craftyjam]

Another question, if I were to connect two of these in series, each with half of the zener voltage of the original zener amplifier, this would share the power dissipation between both, no? An amp that I"m building right now keeps destroying my irfp460's using the chassis as a heatsink. I don't want to stick a tiny fan in there or make room for a large heatsink, so it would be nice if I could share the load between two devices.

[craftyjam]

Something like this?

[pompeiisneaks]

I thought to share the load you parallel them no?

~Phil

[R.G.]

I thought to share the load you parallel them no?

Spot on. This circuit doesn't need series MOSFET. It would be unlikely to need paralleled ones, but if it did, you'd have to worry about paralleling them, not stacking them.

The voltage rating of the MOSFETs just have to be "big enough". In the ground to CT position, this is never more than the peak of the AC voltage from CT to one side of the AC winding. That only happens if you want to turn the MOSFET off for some reason. In a setup to just subtract voltage from the B+, the voltage won't ever get over maybe double the volts you're trying to subtract. 200V or so is likely to be overkill. An IRF820 or its bigger brother the 840 is probably fine.

[maint_tech]

It would seem that putting them in series would reduce the power dissipation per device.
With two, each would drop half the volts and would each have the same current thru them so half the power, no?

[pompeiisneaks]

It would seem that putting them in series would reduce the power dissipation per device.
With two, each would drop half the volts and would each have the same current thru them so half the power, no?

This article explains it I think pretty well:

https://resources.altium.com/p/should-y ... ets-series

~Phil

[R.G.]

It would seem that putting them in series would reduce the power dissipation per device.
With two, each would drop half the volts and would each have the same current thru them so half the power, no?

Yes, it would. So would paralleling two of them, as it's the same voltage and half the current. Parallel MOSFETs are far easier to bias than a series string, IMHO. Just apply the same bias voltage to both.

[craftyjam]

It would seem that putting them in series would reduce the power dissipation per device.
With two, each would drop half the volts and would each have the same current thru them so half the power, no?

Yes, it would. So would paralleling two of them, as it's the same voltage and half the current. Parallel MOSFETs are far easier to bias than a series string, IMHO. Just apply the same bias voltage to both.

My reason for having them in series is based off of merlin blencowes book on power supply design, specifically the section on the zener cascode.

Heres a picture of it. Not sure if this design has a strict advantage over the parallel method, but the biasing seems straightforward. However, in this instance, would RS be the winding resistance of the transformer?