short protection for AV400

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To thread starter.

If the Mosfets are verticals such as Hexfets or Trench Gate than they require thermal compensation, but if the Mosfets are Lateral they donot require thermal compensation.

The type of protection such as these V-I limiters donot work efficiently with vertical mosfets. You are likely to blow up the devices when you apply short circuit during openload fullsignal conditions.

regarding external flyback diodes, you donot need them because mosfets have them already as reverse body diodes.



K a n w a r
 
Try using latch type protection, once the output is shorted the amp wil go into indefinate mute position until the user resets it. Also consider the activation time within which the protection must sense and mute the amp. By muting i mean, either mute the input audio signal or shutdown the driver stage or both. Though i use both.
 
Can I offer some belated thoughts on this thread starter’s questions.

My calculated values (from*) are as shown in the attachment, but note that Re, the emitter resistor, cannot be 0R22 otherwise the protection locus will be seriously violated. It must be no less than about 0R68. for the derated locus given. This alone would give one cause to reconsider this VI limiter scheme, Fig A, or heavily reduce the rail voltages from 56V.
This VI limiter action can be seen to be very inefficient especially when thermal derating is assumed. To reach an output reasonably near the supply voltages, with, for example the 4R@60 reactive load illustrated, and a Tc of 60 deg C, requires 10 pairs of output devices.
Fig B is better, but uses a zener diode string of 85.6V fed from the supply rail to give the -29.6 ref voltage.
Better schemes are available.

* Michael Kiwanuka “Transparent VI Protection in Audio Power Amplifiers” Electronics World Oct 2002.

Brian.
 
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