On Sat, 31 Mar 2007 19:12:00 +0100, Eeyore
wrote:



John Larkin wrote:

Eeyore wrote:

I've seen this phenom elsewhere. Somebody gets an idea, falls in love
with it, and bends the entire design around it, usually badly.

It was Pat Quilter's trademark for ages. They've finally adopted emitter followers
now.

Horray! Give them another 20 years, and they may discover fets!

Personally I love the lateral 'audio fets' that Hitachi originated. Hitachi no longer
make them although there are some other suppliers. They are *very* expensive though
compared to bipolars..


I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Digitizing is cheap, especially when it lets you safely get, say,
twice the usable power from a given mass of transistors and heatsinks.

You don't. Audio amps already push output devices pretty much as far as they're happy
with.


But they don't do it intelligently. A proper protection scheme can
deliver a lot more usable power for the same amount of "push."


Here's a question for you.

Given a classic single channel of Class AB audio amplification with a rated output
power of say 600 watts, what kind of thermal resistance would you require to allow it
to continue to operate safely in typical use with an ambient air temp of up to 40C. Use
any sensible 'assumptions' that you feel are required to calculate this..

Graham

Assume n=60%, 600 watts RMS out, the transistors dissipate 400 watts.
For Tj of, say, 140C, we need a composite Tja of 0.25 K/w.

But audio is not RMS sine waves, any more than NMR gradient pulses
are. A smart protection circuit knows the true transistor dissipation
and the actual heatsink temperature, and understands the thermal time
constants. It has benefits like allowing lots of low-duty-cycle power,
or protecting the parts at high ambients or if a fan fails or
something. There's nothing that's a better thing to limit than actual
junction temperature; anything else is a bad guess of varying badness.

John