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Subject: Choosing the right transformer
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Sat, 18 Jan 2003 23:19:19 -0500
NNTP-Posting-Date: Sat, 18 Jan 2003 23:19:23 EST
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 really
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 (50/8
= 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?
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?
Thank you very much for your help.
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