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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Choosing the right transformer
Date: Sun, 19 Jan 2003 04:58:58 GMT
NNTP-Posting-Date: Sat, 18 Jan 2003 20:58:58 PST
Phil Allison wrote:
> "Greg" wrote in message
> > Hi,
> > 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. However, I don't
> > know how to choose the transformer and regulator ratings a since in audio
> > you don't really need the full power all the time.
> > I read in TI datasheet
> > that if you want a 19V supply delivering an average power of 60W (25W per
> > channel + 15% of heating) in 4 ohm, the peak power required is 120W (twice
> > the average power). The peak current needed is then around 6.6A.
> > Then they say that a power supply delivering 1/8 of the average power
> > = 6.25W) is good enough for audio. My question is: when choosing the
> > transformer and regulator ratings, what do you need to satisfy?Average
> > demand, 1/8 of average demand or Peak demand?
> ** There are several issues - the regulator must supply the peak curent
> demand though an output filter cap could reduce this figure a lot - it
> depends on the signal - unclipped music, clipped music or even square waves.
The TI application note would seem to be based on nearly distortion free
reproduction of typical music programs where peak to average ratio is
10:1 to 20:1. This makes the 8:1 for the ratio of rated power to average
power make sense.
> The heatsinks for the regulator may be OK at the 1/8 power, if
> thermal protection is provided in case this rather artificial figure is ever
> The AC transformer will need to supply the peak too but the long
> term average is way lower. If it is too small the regulation will be poor
> and make your linear regulator design very difficult and lossy.
Not really. It is a no brainer to size the capacitors needed after the
rectifier to avoid these problems.
> > Approximately, using 15V for the secondary winding (20V/1.4) I get:
> > Peak demand is : 15V*6.6A = 100VA with a regulator able to supply 6.6A
> > Average demand is around 15V*4.5A = 67.5VA
> > 1/8 of average is : 15V*0.56A = 8.5VA
> > Those are quite different numbers in term of size and cost so I'd like to
> > know what are the common design techniques. Are the decoupling capacitors
> > there to supply that peak demand when it is needed? Are there better than
> > average regulators that comes to mind for this kind of application?
> ** I suspect the 67 VA figure is nearer the go - as a compromise between
> good regulation and long term capacity.
> Class D amps do not normally need a regulated supply - that kinda
> ruins the whole benefit of it don't you think ?
For that matter, a class A audio service amplifier does not need
regulated power either as long as power supply variation is below the
frequencies of interest. This goes back to the power supply filter
capacitors again. One reason for using a regulator would be running
output devices BTW according to their ratings.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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