short circuit , over I protection

hi,
modern av receivers use emitter resistor voltage sensing and trigger micro controller to trip relay to disconnect speaker from amplifier , mute the amp input via micro controller . i heard relay is not fast enough to protect output transistors in over current situation .SOA says temperature rise also reduces current carrying capacity with respect to VCE EVEN FURTHER.

is it possible to implement av receiver type over current protection with relay to disconnect speaker load in (dead)short circuit situation without destroying output transistors (BJT):cuss::cuss:

thank you
 
Sensing voltage at the speaker out is the easiest way to detect DC offset to the speaker, using the emitter resistors means summing and differencing several voltages (there can be several pairs of output devices), and requires accurate resistor values.


Protecting the output devices has to be fast, so direct current limiting circuitry is good for that - then the amp is safe against short circuit without needing a microcontroller.



Adding a micro-controller should be the icing on the cake I think, rather than being part of the basic protection. It can indicate status, handle mute at switch on/off, diagnose problems and shutdown power supply if there's a continuous fault, as well as monitoring temperatures.
 
And how does the microcontroller measure operating point of the transistors without using as much or more circuitry as the limiter itself?

You have to measure the small voltage across a pair of swinging shunt resistors, and the output voltage, suitably scaled, which suggests at least 3 opamps and a multi-channel ADC, and it may force you to add a power control relay under microprocessor control so the amp doesn't get power until the microcontroller is ready.

Not sure that's less complex.

What I think would be ideal is packaging SOA detection circuitry in the same package as the output device. After all voltage regulators have built-in protection circuitry, why not a power transistor? Thermal compensation diodes have been co-located like this, so its not a big jump really.
 

wg_ski

Member
2007-10-10 5:21 pm
SOA protection inside the package? They already do that - it’s called a chip amp. If you don’t need more than 40 watts.

The problem with putting real SOA protection in an AV receiver is that it would be tripping all the time. Unless they doubled or tripled up the typical output stage. A single output pair on +/- 50 volts is barely good enough for an “8 ohm” load (25^2/6 = 104, leaving no room for temperature derating, if you play by the book). What they count on is the fact that typical audio signals don’t dwell at the maximum dissipation long enough to be a problem, statistically. But When you crank it up loud, with lots of bass (ie, greater than 10 millisecond signals), and get it hot enough it’s going to die.