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From: email@example.com (Ken Smith)
Subject: Re: Choosing the right transformer
Date: Mon, 20 Jan 2003 00:43:09 +0000 (UTC)
Organization: a2i network
NNTP-Posting-Date: Mon, 20 Jan 2003 00:43:09 +0000 (UTC)
X-Newsreader: trn 4.0-test76 (Apr 2, 2001)
Originator: firstname.lastname@example.org (Ken Smith)
In article ,
>I need to design a power supply for a classD amplifier (audio). The power
>supply must be ripple-free so I'll use a regulator.
I think you should check to see if a big capacitor will get you close
enough to "ripple free". If the class "D" amplifier design is a good one
it should be able to handle a small ripple.
> However, I don't really
>know how to choose the transformer
Generally the transformer is the last item you specify. You need to know
how much current and voltage the rectifier will be producing and for how
long before you can really get the transformer's specs.
Transformers are limited by the thermal rise and the voltage drop.
The thermal rise is controlled by the RMS current, the winding resistance
and the thermal mass. The thermal mass of a transformer is usually enough
that you can get away with the 1/8th power rating. For constant duty, the
VA rating of the transformer needs to be 1.5 times to power.
The voltage drop issue is fairly complex because (a) you normally have to
put some small impedance in series with the rectifier to protect them
against the power being connected when the AC is at its peak and (b) the
transformer's impedance is a combination of resistance and inductance.
These days the easiest way to get a good answer would be to use something
like SWCAD-II to model it. In the past. I've covered lots of paper with
lots of math to solve this.
> and regulator ratings
You need the regulator to be rated for the peak power. The heat sink it
is bolted to can be rated for less based on the 1/8th power assumption.
email@example.com forging knowledge
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