Re: Adapting this NSC app note for my +150V regulator

Winfield Hill wrote:
On Mar 25, 8:09 pm, Joerg wrote:

electronic Dave in New York wrote:

Fred Bloggs wrote:

electronic_dave wrote:

Hi all. I have the need for a linear regulator to supply +150V at 0 to
50mA from a 260VDC unregulated input. There's this National app note,
"High Voltage Adjustable Power Supplies", with which you may be
familiar: http://www.national.com/ms/LB/LB-47.pdf
It seems that using something like, say, TIP50 for Q1 and Q2, and
reducing R3 should do the trick. Is there anything I have overlooked,
though? Regulated power supply design is not my area of expertise.
http://www.fairchildsemi.com/ds/TI/TIP50.pdf

The TIP50 is not good enough. Taking the approach of short-circuit
limiting at 2x the maximum regulated current rating, as in LB-47,
exceeds the SOA of that particular transistor. Even strict limiting at
50mA would be too much.

Good point. I know there are some transistors intended for horizontal
output service in TV sets that are pretty beefy, with high C-E
breakdown voltage ratings. I'll look into those.

Also, take a look at big FETs. They have become rather inexpensive.


Power MOSFETs avoid the 2nd-breakdown problem, which
can be a big deal in HV regulators. However, high-voltage
power MOSFETs love to RF oscillate at from 10 to 30MHz
when used in the linear mode at even modest currents, like
5mA, and with more than say 25V across their drain-source.
The easiest way to prevent this is to use two ferrite beads
and slip one over both the gate and source leads of a
TO-220 part. Sometimes a gate resistor alone can spoil
the oscillation (they're always recommended anyway, and
especially if you use a gate-source zener, isolate the gate
with a resistor), but I have better results and am more
comfortable with two ferrite beads.
Kind of reminds of the bad batch of MMBT2222's we have sitting
in our bins that are good for nothing but saturated switching
circuits.
If you use this batch in a linear circuit, there is a point on
the curve that it actually oscillates. Go above or below this point
and we're back in business again. This was found to happen on 2
different test circuits to insure it was not bad board and component
design.
We found this out after debugging a circuit where these were
being used to control small servo's. Putting some padding in the
circuit removed the higher frequency component and thus still effected
the end results by causing low freq distortion in the curve at that same point.


A new batch was sent to us and all was well once again.


--
http://webpages.charter.net/jamie_5";

.

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