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'1V to 1.8V 70A PSU Digitally controlled'
1999\10\21@084343 by McMeikan, Andrew

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Hi all,

Some time ago on the list someone was after an efficient 1V PSU, with the
resulting discussion on losses in the diodes.

There have also been discussions about using PIC's to PWM DC motors.

Having some interest in switching PSU's I've been looking around and found a
chip that provides 1V at 70A, looking at the circuit I noticed that rather
than use diodes they used FET's to clamp at the correct time.

No I have only used 16C84's so I don't know too much about those PIC's with
AtoD on them but it would seem that a PIC with a couple of FET's and an
inductor could make a nice PSU or motor driver?

       Anyone have comments as to why this would not be a good thing to
build?

       cya,    Andrew...

http://members.xoom.com/andrewmuck/MP3.htm

1999\10\21@091859 by Michael Rigby-Jones

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{Quote hidden}

Motor/servo control would be fine, and has been done many times.  However as
a Switched Mode PSU controler, I have my doubts.  You can already buy many
different PSU controllers, the manufacturers have done all the hard work for
you.  High speed control loops such as those used in SMPS are not trivial,
to get a good switching speed and good response times the PIC would have to
be running pretty quickly, and you still need som external circuitry for
over current/over voltage protection.

Cheers

Mike Rigby-Jones

1999\10\21@093956 by Andy Kunz

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>a Switched Mode PSU controler, I have my doubts.  You can already buy many

The AstroFlight http://www.astroflight.com Model 1xxD chargers are
essentially SMPS devices, and are controlled by a PIC16C71.

A friend of mine did the design.  I'll have to open it up again and review
the electronics, but as I recall the PIC does everything, including driving
the switch.

Andy

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1999\10\21@183346 by Russell McMahon

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Others have commented on the controller aspect.
I'll just comment on rectification.

For low output voltages rectifier voltage drop becomes a significant part of
total system voltage and a significant factor in power loss and efficiency.

Voltage drop for silicon rectifiers at 70A are liable to be 2v or so.
Even Schottky rectifiers are liable to have voltage drops of around 1 volt
at this current.
These are limitations of the technology (y' canna break the laws of physics)

FETs can have as low a forward voltage drop as you want - you just have to
pay more :-)
eg at an Rdson of 0.01 ohm at 70 A the voltage drop will be 0.7 volt.
Very best FETs available are down in the 0.005 ohm range (0.35 volts here)
and you can parallel as many as your bank balance allows. Lead drop,
mounting, bus bars etc are liable to be major features.

If your FETS have Rdson effective (1 or several in parallel) of over about
0.015 ohm you may be better off with Schottky diodes and at about 0.03 ohm
Silicon diodes may be no worse.

If efficiency is not an issue you could just use silicon diodes with
suitable power ratings.
Remember, 70A x 2v = 140 watt !!!!. Drop could be even higher than this.
Schottky is not too dear and will about halve power loss.





     Russell McMahon
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