Piglit wrote...
>
> Winfield Hill wrote:
>> Are you aware of the inherent difficulty in stabilizing high-
>> side current-amplification circuits? The regulator has an
>> internal high-gain feedback loop, whose gain drops 20dB/decade
>> with frequency, with an associated 90-degrees phase shift.
> (snip comprehensive reply)
>
> Great answer (kinda restores faith in usenet), thanks Win.
> cheers
As you can see, I compared two ways of enhancing high-voltage
low-current three-terminal programmable regulators, one with
excess gain inside the feedback loop, and a second without but
with hopefully-modest output voltage errors. There's a third
attractive approach using low-voltage high-current regulators,
like the LM317 or LM317L, and cascoded series voltage-dropping
MOSFET(s) that always present the low-voltage LM317 with about
7V more than its output, for safe cool operation. I, and others,
have written about these in earlier s.e.d. posts, complete with
ASCII drawings. Here's an example from January 7th, 2002,
From: Winfield Hill (hill@rowland.org)
Subject: Re: regulated power supply 50 volt to 400 100mA
Date: 2002-01-07 15:27:16 PST
Radioman wrote:
>
> I am looking for a schematic for a regulated power supply
> 50 volt to 400 volt that can deliver 100mA or more.
The ST VB408 etc., parts that Xenos suggests are nice, but
they may be hard to get. You may be interested in a simple
high-voltage regulator that uses standard LM317 or LM317L
chips plus an easy-to-get reliable high-voltage MOSFET.
600V FET _____ LM317L 5 to 500V
IN o----+----, ,-+-----+--| |--+---+----+---o OUT
| | | | s | | | | | | to 100mA
| _|_V_|_ | |_adj_| 1.2k | |
2.2M ----, | | | | | 0.47uF
1/2 W | '-||--+-----+ | ===
| | , 68pF | | | 630V
'--------+---|<|-----------|---' |
' 10V zener | 2.7 ohms
500k |
pot gnd
|
gnd
The FET needs a very serious heat sink, because 450V at
100mA is 45 watts, and under a short circuit condition the
LM317L may allow even more current to flow. You can add
a single-transistor foldback current limit if you want.
Add a 1n4002 diode backwards across the LM317 if you plan
on ever shorting out the input filter-storage capacitor.
(Always use drain resistors with high-voltage capacitors.
And always approach the open circuit with one hand behind
your back.)
The LM317L needs as much as 2.5mA to operate, and the 1.2k
resistor only takes 1mA, so this sets a 1.5mA minimum load
requirement. (BTW, the VB408 has a 1.2mA minimum load spec.
Note, you could use 470-ohms instead of 1.2k, but this would
require a 200k pot with up to 1W of dissipation, instead of
the 0.4W maximum dissipated in the 500k pot when its set to
about 400k for +400V out.)
---------
> (Looking forward to the next edition of "The Art..")
Groan.
--
Thanks,
- Win
