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Old 19th December 2005, 12:06 PM   #41
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Kanwar,

Basically you are right that in case of dead short there may be cycling, although that depends on the exact implementation of the protection circuit. Even if there is no cycling, the prolonged overload will lead to temp rise.

There are several ways to deal with that. The simplest is a shut down in case of overtemp. You would need that anyway in professional amps, so this feature for the SOA protection comes for free

Another one is to make the protection system latching, so that after an overload, you have to reset it. I don't think that you would want that in pro audio.

Your proposd solution is in my view not optimal. For one, how do you decide if there is a dead short? Load 1 ohms? Load 0.1 ohms? There will always be border cases, and a dead short never exactly is a dead short. Experience tells us that when you try to define this kind of things, along comes a case that slips through the cracks. Another real issue is that if you built an ingenious system, it is almost impossible to test it under all conditions and you can end up with an amp that fails in order to protect the protection circuit...

Better is to build something that unconditionally protects the amp. The SOA will protect the output devices. An offset detector can protect the speakers in case something goes wrong in the amp anyway. A temp detector can protect against high temp either from prolonged heavvy duty in high ambient temp and as a bonus against prolonged overloads.

Wasn't it Einstein who said, "things must be made as simple as possible, but not simpler".

Jan Didden
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Old 20th December 2005, 03:25 AM   #42
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Hi Jan,

Yes you are right, the Dead Short only exists when its just happened at the output terminals of an amp...buts there is always a trap there if the short is encountered at speaker terminals then thats not the dead short because the Length of wire connecting speakers to amp has some finite resistance in order of few milliohms..which might ceases short circuit protection to function....
Secondly, Its also very difficult to make a difference in a set of conditions when a Short -Circuit or a Reactive load is encountered, in both cases..At Zero crossing of voltage during reactive loads..there is still finite current flowing through the device and similarly in case of dead short circuit the output voltage is zero, but current through device is finite...

Thirdly, Another problem is that when a short is encountered, The amp usually turns into open-loop type[as output is shorted to ground]..and if the power suppply rails are stiff...than the device which is activated to conduct the current would simply function as a power resistor and would now be conducting very high current with full Vds across it, which results in Very high dissipation and even failure because its unable to drive the Dead-Short Circuit ....

The SOA limiter if encountered with overcurrent or short circuit, then it starts cycling , which could give rise to High Temperature conditions, if not dealt with appropriate Thermal Breaker type protection..would eventually destrys the devices in prolonged stressful conditions....

But if we , further look into that matter there is still a point exists which shows us a difference between both short circuit and reactive load activity...and thats the output voltage..
If by some means we process the data obtained from Ids and Vout and ADD a Time Base Delay in order to overcome the False triggering during Reactive loads[Zero-Crossing voltage- finite current conduction similarity] then the short circuit protection could be made more effective....
Lets consider an example....
In an amp..if we implement another limiter favourably optocoupled one along with already present SOA limiter....
At idle the opto limiter would be engaged fully ON but the limiting wouldnot be activated as long as the gate voltage increases to overcome the Zener voltage[Vz is set to over come the usual gate bias which is 3 to 4 volts], But when ever a voltage signal is detected at the output , the gate voltage also rises but at same time the opto coupler is made to disengage thus no limiting......But whenever a short or severe over-current condition occurs ...2 things would happen at sametime..1.Gate voltage rises 2.output voltage drops to near zero..Simuntaneously the opto is engaged at that time and then limit the increase gate voltage to around Vz of zener and thus limits the Id conduction through the device...and its another feature is that its self-resetting..no cycling. no high temp rise during prolonged short circuit..self latching because as long as short exists there would be no or very low voltage at output hence opto limiter would be engaged thus limits indefinately.....though a Time Base Delay must be implemented to over come the false triggering during reactive loads ... Also SOA limiter is there to protect in case of excursion during reactive activities...

Correct me If i were wrong.....

here's the link to QSC short circuit limiting patent for reference....
http://patft.uspto.gov/netacgi/nph-P.../QSC&RS=AN/QSC

K a n w a r
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Old 20th December 2005, 07:15 AM   #43
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Hi Workhorse,
no. the SOAR limiter is there to prevent the output devices being driven beyond their specifications.

If the output stage is well designed it will be capable of what the designer chose for it to do. i.e. the designer decides on the load conditions and selects components accordingly.

If both stages of this design process are carried out competently then the limiter will never trigger for a valid load condition. i.e. never trigger early on any conventional signal condition nor on any loading within the design requirements.

The SOAR limiter can operate for both the excess load driving and short circuit/bad load conditions. It can also be made latching although very unusual.

Continuous and excessive DC offset (speaker protection) is better met by a separate detector and latching isolator (fuses even?).
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Old 20th December 2005, 10:32 PM   #44
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Quote:
Originally posted by AndrewT
Hi Workhorse,
no. the SOAR limiter is there to prevent the output devices being driven beyond their specifications.

If the output stage is well designed it will be capable of what the designer chose for it to do. i.e. the designer decides on the load conditions and selects components accordingly.

If both stages of this design process are carried out competently then the limiter will never trigger for a valid load condition. i.e. never trigger early on any conventional signal condition nor on any loading within the design requirements.

The SOAR limiter can operate for both the excess load driving and short circuit/bad load conditions. It can also be made latching although very unusual.

Continuous and excessive DC offset (speaker protection) is better met by a separate detector and latching isolator (fuses even?).
Well said AndrewT.....
But i am searching for some more Concrete solution , rather than relying on SOA Limiter & Thermal breaker to carry out the Task.........
Prolonged heating of Transistor Die....results in much less life expectancy.......IMHO

K a n w a r
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Old 22nd December 2005, 11:47 PM   #45
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