Re: SMPS, control loop?

"Terry Given" <my_name@xxxxxxxx> wrote in message
news:1146695138.395664@xxxxxxxxxx
In some constellations (small cap, >50% duty) this can result in
stability issues. You can read up on that under the topic "right half
plane zero" or "RHP".

50% duty? Erm, overshoot!? Or do you mean what is technically >25%? Why
would that be particularly special compared to any other duty cycle?

Current mode control prevents the inductor current to reach "kablouie"
levels. I usually employ current mode in my (often discrete) designs.

Mmmm, discrete.

Meh, I'm short on transistors and don't care to use that much breadboard
space, so I'd just as well use an LM393, 339, or better yet, a 3524 which
has everything rolled together for a free-running PWM bridge generator.

Either way, you need a PWM generator, and a 3524 has two op-amps inside it
for control, not inconvienient (though the 7500 is more universal as the
amps are identical).

As usual, Joerg is spot on. Jerald Foults' website (spelling?) has a whole
bunch more useful stuff. (P)CMC is the way to go. If only the UC384x was
developed before the poxy SG3524 or TL494....

http://www.acronymfinder.com/af-query.asp?acronym=pcmc ???

Personally I don't see an UC384x working for half bridge...

NB: real men dont use smps chips ;)

As long as the peak energy stored in the output cap(s) is >> that in the
choke, the overshoot is small. If electrolytic caps are used and ESR is
minimised, capacitance tends to be very high (multiple large cans are
often required for low ESR, resulting in huge capacitance), maximising
capacitive energy storage. Likewise if inductor size is minimised (eg
10-20% ripple) then so is inductor energy. Overall, overshoot is
minimised.


Yeah, Ec might be 8 * E_L, but it's still an overshoot of about a volt, bare
minimum, and far more if the circuit works theoretically (i.e., doesn't
explode or saturate from the excessive current) with delay dominated by the
filter.

IMO if the smps cant run continuously into a dead short, its not good
enough. A brutal test is to attach a pair of metal rods to an output, and
intermittently short them with a large, coarse file. If the smps can
handle that without evil overshoot (the filter inductor current ramps up
to the current limit value, even with (P)CMC) and/or explosions, then its
a good design.

Yeah, I want to make a bench supply for electrolysis, think 4 to 6V and
50-100A max. Short proof operation would definetly be a nice addition.
Regulation isn't critical as far as transients, but I can't have this thing
exploding every time the load changes quickly, either.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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