mosfet variable B+

[6v6audio]

Using a mosfet, I would like to vary my B+ all the way down from about 300v to 50v using a pot to the gate of the mosfet somehow.


Anyone have any schematics or a little help with this?

Thanks

[anatech]

Hi 6v6audio,
How much current? This may generate a ton of wasted heat. Before starting this design, figure out how much power to dissipate with a linear pass transistor. I'm thinking mosfets.

Another consideration would be a switching supply, or "down converter" to use Carver slang. They were able to vary a +/- 125 VDC supply down to 12.5 VDC. The amp is rated at 1200W into a 2 ohm load, so they had to keep the dissipation down.

-Chris

[6v6audio]

max 500ma, I figure130 watts.

[anatech]

Hi 6v6audio,
That is an awful lot of heat to get rid of. It's also a scary amount of current at 300V.

At this point I'd be looking at a switching power supply. I am not qualified to design one, possibly another member would jump in here. Protection may be a major consideration here too. When things go wrong at those voltages and currents, it can really ruin your life.

-Chris

[aletheian]

Originally posted by 6v6audio
Using a mosfet, I would like to vary my B+ all the way down from about 300v to 50v using a pot to the gate of the mosfet somehow.


Anyone have any schematics or a little help with this?

Thanks

I just so happen to have one of those 3 feet away from me. It is my little ghetto bench supply. It gives 50V-800V, with a seperate little transformer that generates negative voltage for a bias supply. I used a 1000v, 1 or 2A mosfet rated at something like 200W... don't remember what brand... Fuji I think. Been using it for months with no problem. I'll draw up a schematic of the mosfet section for you

Here is a really bad picture

[aletheian]

Here is what I used. If you blow yourself up.. it is not my fault. ;-)
Pick a mosfet that meets your requirements. an IRF820 should be fine, and they are available all over for a buck or two. The mosfet should be very well heatsinked. I rigged up a little 120v AC fan that I pulled from a computer psu to the on switch, but that is probably unneccessary. There will be voltage on the mosfet mounting tab.. so hence there will be voltage on the heat sink unless you isolate it with silicone/mica spacers like I did. It's negative voltage in this case, and nott huge amounts of current, so you should be fine, but still isolate it. I actually bumped the tab a few times with my hand accidentally and didn't get zapped (more than I can say for when I bumped the 470u filter cap... grrr.)


PS If you exceed the ratings of the mosfet, the result is a rather "energetic" reaction, and if you have a cat, you'll be peeling him off the ceiling.

[Upupa Epops]

Don't forget protection zener diode to gate ( cca 10 V )

[aletheian]

Originally posted by Upupa Epops
Don't forget protection zener diode to gate ( cca 10 V )

Huh?

[Upupa Epops]

Zener diode, connected between gate and source protect gate against too high driving voltage, 'cos gate/source voltage is max cca 20-30 V. It is standard protection connection... huh

[anatech]

You may want to decrease the zener voltage to provide some crude current limiting. 6v6audio wants to get 0.5A out of this thing, which is 125W of dissipation. So max heatsink temperature (depending on ambient) will be high. A temperature cut out would be wise as well. Can you imagine a 100 V device when the pass device shorts? Over 300V applied faster than you can duck.

-Chris