rjtucke (Ross) wrote...
I have a tube power supply that is putting out 520VDC. I need 350VDC
(at ~80mA) maximum. I am trying to think of a creative way to drop
the voltage. So far, here's my list:
* find a really big regulator IC
* massive resistive voltage divider
* massive diode voltage dropper
Phil suggested a string of zener diodes in series. But dropping
a fixed voltage exacerbates poor voltage regulation by keeping all
of your source variations, compared to your lower output voltage.
John asked if the transformer had taps, or other secondaries that
you could use to modify the ac before rectification. That's good.
I'll suggest you use a HV regulator with a feedback loop. It has
to be mentioned you could make it using a large vacuum tube.
I'll be conventional (I know you asked for a creative way, but...)
and suggest a power MOSFET pass element. The FET should be rated
for 600V and be able to safely handle 180V * 80mA = 15W with a hot
heatsink, and much more power under short-circuit conditions.
Checking the DigiKey catalog for an inexpensive part, I spied the
IRFPC40 at $3.30 (it's a lead-free version, but you can solder it).
http://www.irf.com/product-info/datasheets/data/irfpc40.pdf (Yes,
there are much more capable MOSFETs, but they cost a lot more too.)
The irfpc40 is rated for 150W max with a 25C heatsink, but lowering
the maximum junction temperature to 125C, and assuming a 60C max
heatsink temperature, the dissipation capability is reduced to
(125-60)/(0.83+0.24+0.33) = 46 watts. The last two terms come from
adding an insulator with heat-sink grease (the FET's case will be at
+520V, whereas you should have the heatsink bolted to the chassis).
To insure operation at 520V 80mA, we'll pick a 150mA current limit.
Passing 150mA while dropping 180V creates 27 watts of dissipation,
nicely under our 46 watt limit. But during a short-circuit your
regulator will have to limit the current to under 46/520 = 88mA.
We'll choose a nice cool 60mA, which means we'll need a "foldback"
current limiting circuit with a 2.5 ratio, which isn't very hard.
We discuss foldback current-limit designs in AoE pages 316-317.
There are multiple safety margins in all the specs above, so your
regulator should work well under abnormal conditions, like high
ac-line voltage, etc., or other mistreatment.
Because you're regulating down from a voltage considerably higher
than the output, your design can benefit from two simplifications:
You can use an N-type FET source follower, for easy feedback-loop
compensation, and you can use a simple resistor pullup to drive
the FET.
Over the years I've posted several suitable ASCII circuits here on
s.e.d., but Googling for a few minutes this morning didn't reveal
one, and I have to leave for work now. But if you're interested
in this approach I'll look further later on, or type up a new one.
