Re: High voltage lab supply for tube experimenting (follow up..)

Peter Andersen wrote...
>
> Winfield Hill wrote:
>> Whoa! You don't have a gate-source zener protection diode.
>> Anything from 8V to 15V should work fine -- that'll probably
>> solve your blown FET problem.
>
> Ok, I have added that one now. New pdf is oploaded.
>
>> Another point, you've paralleled two MOSFTEs, but in reality
>> one will dominate in a linear circuit like this. If you need
>> the extra power capability of a second FET you can use a volt
>> or so of source-degeneration, or you can use the active MOSFET
>> current-divider circuit I posted here last year or so. Or you
>> can use a series connection for the MOSFETs, dividing the extra
>> unused input voltage between them, that's a common approach.
>
> Well, I have read some discussions about this subject and there
> seems to be a some disagreement on how to parallel MOSFETS.

Switched MOSFETs (with high gate voltages, each device fully
turned on) may be paralleled and will share current per each
transistor's conductance, I ~ 1/Ron. Since Ron goes up with
temperature this is a safe scheme. Modern power MOSFETs are
not so often used as linear elements, and some mis-information
circulates about this mode of operation. For example, at any
reasonable power level, for a fixed gate voltage, the current
increases with temperature, so direct paralleling is not safe.

> But I would like to but do both things: series and parallel so
> I can use som less expensive mosfets and still have a high
> current out (max 500mA).

I bought a few of these Xantrex 0-600V 1.7A switchers on eBay.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=6814980234
The seller might have a few more of them.

OK, that was an aside. Have you set a reasonable lower voltage
at which you'd want a full 0.5A, such as 300V? That's still a
high 175W of hot-air wasted heat, if your DC input is 550V. If
you want a 0.5A rating down to zero volts (e.g. a short circuit)
you'll need to dissipate 350 watts. Ouch! Space-heater-city.

> As for now I have removed one of them, and then I think I will
> try to do experiment with it...

A second little-known issue if that it's not the FET's Ron rating
you should be looking at, but the Pmax rating, or more accurately
its thermal resistance. Another factoid, you'll find that some
of the lowest thermal resistances available are for high-voltage
power MOSFETs. Ahem, another eBay opportunity is the STW8NB100,
rated at 190W with an impressive thermal resistance of 0.66 C/W,
http://www.st.com/stonline/books/pdf/docs/6509.pdf and eBay link:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3844593888
This guy only wants $2.99 for these great MOSFETs. Note their
large surface area, which improves heat-sink conduction. Nice!

You can ignore the 1kV rating, it's as if the voltage rating was
infinite. Given typical heatsink issues and saftey margins, you
might end up dissipating only 40W or so max in each MOSFET, which
means you'd want at least five or six as regulators. You could
wire them in series, with gate dividers as we show in AoE figure
6.52, page 372. Or, if you want to use them in parallel, you'll
have to provide a current-sharing scheme to deal with the positive
drain-current temperature coefficient. A simple way would be to
use 22-ohm 1/2-watt degenerating source resistors, these will drop
2V at 100mA, and effectively prevent any one FET from taking more
than its share of the current. You could use a little servo for
each FET to lower the voltage drop, but 2V is not a problem.


--
Thanks,
- Win
.

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