Re: Switching Regulator for Audio Amplifier

On Sep 19, 5:14 pm, Jan Panteltje <pNaonStpealm...@xxxxxxxxx> wrote:
On a sunny day (Fri, 19 Sep 2008 13:46:29 -0700 (PDT)) it happened Fred
<frederick.br...@xxxxxxxxx> wrote in
<7c7028ac-aade-436b-999c-321e881d0...@xxxxxxxxxxxxxxxxxxxxxxxxxxx>:

So here are some pictures from the PA3000 inside, the transformer is 220VA.

Discrete is simple :-) and no, it is not MOSFETs, the power transistors seem to be
2SA1943 (PNP) and 2SC5200 (NPN).

And I payed 65 Euro (about 91 dollars) for it..... some years ago, see they are now 87.95 Euro...
with more LEDs :-)
Lookup 'PA3000' atwww.conrad.nl

Pictures, what pictures?

Oops, seems I forgot the link:
ftp://panteltje.com/pub/PA3000/

What is this equation called V * T = C * I, and what is is supposed
to do?

Well, we say 'U' and you say 'V', but apart from that, it says
that if you charge capacitor C with current i, then after t seconds the voltage on that
cap will be U (or V ;-) ) volts.
Actually it is Q = C .U = i . t, where Q is the charge.

I is supposed to make sure the output maintains a certain amp
capacity?

I only used it to see how big the voltage drop would be between 2 consequentive
mains tops in your rectifier for a given line frequency and a given cap, and
a given current.

I've been reading that calculations of ripple current a really just a
technical exercise, because the ESR of both the filter caps and the
transformer will only be know after build time. The rule of thumb
that was presented was to size the filter caps with an ESR rating of
about 3.5 to 4.5 the output current depending on the size of the
transformer.

I dunno about rule of thumb, you can simply calculate it, or, if you
want a visual, run it in spice, or stick a scope on it.
That last thing rules.
Electrolytic caps have tolerances that can be quite large anyways, say 20%.

On the capacity side, were the sightly contradictory suggestions of
aminimum capacity to reduce ripple voltage to <= to 10% of supply
voltage, and 2000uf per ampere of output current.

It is up to you, again if you want to run 100% sine waves all the time,
then you need more C, I would think, then for normal music.
100% sine waves is _not_an audio application, but you could be driving a motor or
whatever.

Somebody suggested thermal protection in the transformer, I think that is a cool idea.

I've got the chip up and running again with the old transformer:
384VA 10% reg -> 2x24.

I resoldered the wiring and removed some caps. The voltage now is
about +/-37 volts. it drops to about +/-35 under load.

A interesting thing I noticed is that it only puts out about 7.5V into
a 5R power resistor before the positive side starts to clip. If I
raise the input signal, the top rail will continue to clip, while the
bottom rail will go to about -18V pk before the spike protection
starts activating.

Also I reviewed the thermal considerations, and realized that my 3C/W
heat sink is only half of what it should be. I can see on the scope
the thermal protection activate as the heat sink gets hot.

The data *** states at +/35V the max dissipation of the chip is
65W. I found a spice model for it and have been studying various
transformer models and capacitor values with it. I modeled the
transformer I actually have and the chip dissipation was 56W at max
output of 88W. The data *** says 68W continues into 4 ohms with
+/-35 Volts. I can plainly see that thermal management is a priority
here.

I have to deal with over a hundred watts of dissipation with two
chips. I've been looking at heatsinks and fans. I found a neat
control circuit that provides linear voltage response in regard to
temperature with a floor that will just keep the fan turning with the
heat sink is cool.

Fred
.