Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!ucsd!sdcsvax!ucsdhub!hp-sdd!hplabs!hp-pcd!hplsla!tomb
From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: DC-DC inverter
Message-ID: <5170026@hplsla.HP.COM>
Date: 26 Apr 89 16:41:24 GMT
References: <10837@bloom-beacon.MIT.EDU>
Organization: HP Lake Stevens, WA
Lines: 58

> tytso@athena.mit.edu (Theodore Y. Ts'o) / 11:52 am  Apr 25, 1989 /
>I am trying to design an alternative power supply for a photometer (the
>GR1501-A, if anyone carse).  The problem is that it requires 3 45 Volt
>batteries which are almost impossible to obtain, and when they are
>available, they are extremely expensive.

I'll bet!!

>
>So what I'm trying to design is a DC-DC inverter that will take power
>from a 6V latern battery (or something similar) and covert it to a +90
>and -45 volt output.  Since it's going to be used to power a light
>meter, the output current is going to have to be pretty will filtered.
>Ideally, it should be regulated, but I have no idea how to regulate
>voltages this high.  Can you get IC's that will do something like
>this?  

I won't make any value judgements on the merit of doing this.  But
sure, it should be not only possible but fairly easy.  First, if the
thing is at all a power hog, you probably ought to use a rechargable
battery.  Either nickel-cadmium or sealed lead-acid ("gel cell")
could be appropriate, depending on operating time/charge and desired
size and weight.

The DC-DC converter should be fairly easy with a switching regulator
IC, a small transformer, possibly an additional transistor to handle
the current, and a handful of passive parts.  The IC would monitor
one of the output voltages through a voltage divider, and regulate
its pulsewidth (or freq) to maintain a constant voltage.  Proper
design of the supply should allow use of the other output with no
further regulation.  I'm a little out of touch with state-of-the-
art sw-regulator IC's, but would check on what Linear Technology,
National, Silicon General, and maybe TI have to offer.  They usually
publish fine apnotes that will get you very close to what you want.
My recollections are that the biggest problems have to do with practical
transformer design.  One nice thing is that transformers at 50 kilohertz
even for the voltages you want have rather few turns.  A final note:
for the rectifiers on the output, be sure to use high-speed devices!
Common rectifiers designed for low frequency work will blow up.

Hope these thoughts help -- folk have written whole books on this topic,
but you should be able to get something acceptable without having to
take a course on it...
>
>The other constraint is that it would be best if it could be designed to
>use mostly common parts, but that's not that important.

I'm sure I could do this with all parts except the transformer obtained
from a place like DigiKey or Active; the transformer core could be a
problem...maybe some kind soul in your area could help you out.
>
>I'd appreciate any suggestions on how to design such a beast or
>pointers to books or articles on the subject.  Thanks!
>=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
>Theodore Ts'o				bloom-beacon!mit-athena!tytso
>3 Ames St., Cambridge, MA 02139		tytso@athena.mit.edu
>   Everybody's playing the game, but nobody's rules are the same!
>----------