Power amp VI limiting?

[AndrewT]

Hi all,
while reading this website www.angelfire.com/ab3/mjramp/protect.html
I found this titbit.
"Had we constructed some sort of protection circuit operating within the derated dc SOA however this would operate and cause heavy distortion. Including both temperature and time compensation however the protection would not operate, while at lower case temperatures higher short-term power into lower impedance loads would be possible. The added complexity compared to typical protection circuits need be little more than an extra capacitor and resistor, plus thermal contact between the protection circuit transistor and the heatsink ."
The classic VI limiter circuit to limit outputstage current usually has the protection transistors on the PCB. This writer is suggesting that we couple the transistors to the heatsink.

Q1. Is approximate temperature compensated VI limiting really this easy?

Q2. Could some simulation experts run some numbers and investigate what happens over a sensible temperature operating range?

Q3. Should the driver heatsink or the main output heatsink be monitored? The temp variation on the driver may be larger particularly when approaching max output into dangerously low loads.

[mikeks]

Originally posted by AndrewT
Q1. Is approximate temperature compensated VI limiting really this easy?

Not even close...the reduction in Vbe with temp. would need to possess the same slope as the anticipated reduction in SOA with temp.

Never happens...

[djk]

Get a copy of the Crown PSA2 schematic (in the Crown Legacy library).

This was their first amplifier with OPED, described as an output protection emulation device via an analog computer. It was to predict heating in output devices over time and adjust the SOA in the current limiter circuits. It uses a network of RC in a ladder circuit so that fast transients don't trip it, but long term overloads will.

ODEP was also used in most of the later MacroTech models.

[mikeks]

http://www.diyaudio.com/forums/showt...202#post637202