Re: Need input on high voltage regulator design

From: Ken Smith (kensmith_at_green.rahul.net)
Date: 11/28/04 

Date: Sun, 28 Nov 2004 22:57:55 +0000 (UTC)

In article <10qkdhad5k7rmd7@corp.supernews.com>,
Scott Miller <scott@3xf.com> wrote:
>I got bored yesterday and started working again on a Geiger counter design I
>came up with a couple of years ago. The high voltage power supply was
>mostly cribbed from a couple of other designs I found, since I'm a digital
>guy and high voltage isn't my thing. As it stands now, the microcontroller
>puts out a square wave that drives the primary of a transformer via a
>MOSFET. A voltage doubler on the secondary gets the voltage into the
>required range (500+ volts in this case), and a couple of varistors in
>series form a shunt regulator.
>

What does the ADC use as a reference?

How good does the regulator have to be?

Two ideas:

You could sample the primary windings voltage some time after the MOSFET
is switched off. The waveform will look like this:

   A B C D E
   ! ! ! ! !
   V V V V V
...*.......................
...**......................
...***.....................
...************............
...***........*............
...**.........*............
...*..........**...........
...*..........**...........
...*..........*****........
...*..........**..*........
...*..........*...*........
***...............*********

(A) the MOSFET turns off and the voltage goes rocketing up, overshoots and
rings a bit.

(B) the voltage flattens out at some highish level and droops slightly as
we go towards (C)

(C) the energy is all out of the inductance and the voltage drops
suddenly, undershoots and rings.

(D) the voltage settles down to Vcc (or maybe it doesn't get time to.

(E) the MOSFET turns on again.

The voltage you want for this sort of feedback is the one at (C). The one
at (B) is almost as good. From (B) to (C) is almost a straight line.

You should be able to get fairly good regulation. The tricky bit is that
the micro needs to fiddle with the ADC timing to get the right point for
the feedback. The nice thing is that the load doesn't change much so the
software can take a bit of time.

2nd Idea:

       R1 R2 R3
  ----/\/\/----/\/\/----/\/\/\----+-------
                                  ! !
                                  ! !
                                  / --!+\
                                  \R5 ! >----+-- To ADC
                                  / --!-/ !
                                  ! ! !
                                 GND ! !
                                          +--/\/\----
                                          ! R4
                                          \
                                          / R6
                                          \
                                          !
                                         Vref

The ADC will give a zero output up to some voltage and then go up towards
full scale as the volatge increases above that point. This allows the
ADCs bits to be put to better use since you know that the voltage must be
near 500V

R1, R2, R3 are a string of however many resistors you need to make the
impedance high enough.

(1+ R4 / R6) sets the gain for the op-amp

The divider of R5 and (R1+R2+R3..) sets the fullscale point.

Decide on a full scale.

Determine R5 and (R1+R2...) to make R5 have Vref on it at full scale

Decide on a bottom value for the ADC Lets say 450V

Figure out the voltage on R5 at 450V

The op-amp's input see the impedance of R5 in parallel with the (R1+R2)

You want the parallel combination of R4 and R6 to equal that impedance and
you want them to make the op-amp have a gain of (Vref-VR5)/VR5.

The TL071,2,4 type op-amp should work for you if you don't need to
accurate of voltage. 

-- 
--
kensmith@rahul.net   forging knowledge

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