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Old 15th December 2004, 01:03 PM   #1
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Arrow Project:OPTIMUM SHORT CIRCUIT PROTECTION IN MOSFET AMPS

Hi everybody,
Recently I came across bassbins fitted 18" Peavey Lowrider speakers along with Passive 24dB/octave linkwitz - Riley passive crossovers.
Our further experiments shows that there is lot of Reactive energy [due to passive crossover]fedback to amplifier, thus turning the overcurrent protection ON without any coniderable power output to load.The normal trip level of protection sensing was 28A peak, but the measurement shows 40A peak of destructive current was present without the protection circuit involved.
This event triggers us to think a new way of activation of shortcircuit protection involved than using traditional approach of sensing overcurrent through a series resistor.

We want to propose a new idea of protection which includes :

1 Sensing the gate to source voltage overdrive during overcurrent conditions, then limit the gate drive or if necessity arises even mutes/limits the input which also avoid harsh clip conditions.

2. A comparator scheme which involves:Sensing the input voltage at the input of amp, sensing the corresponding voltage at output of amp. If the input voltage is present and output voltage is not present, which clearly indicates the conditions of short-circuited output, the the comparator latches or activates the mute circuitry, thus protects the amp.

Kindly , give your valuable suggestions so that we can further continue the discussions involved and a new project will be started soon involving various parameters to be catered along.

Regards,
ampman
 
Old 15th December 2004, 01:28 PM   #2
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Arrow Proposed Idea

Hi Everyone

Here are the simple image illustrations involved.


Regards
ampman
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File Type: jpg combo.jpg (32.4 KB, 1528 views)
 
Old 15th December 2004, 02:35 PM   #3
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Limiting MOSFET GATE :
I WOULD suggest a low current,high voltage mosfet as the sense element fed thru a zener, which will conduct as soon as the drive level exceeds a pre set level,The Mosfet should switch an optocoupler for isolation,the output of Optocoupler should turn on a small signal FET connected across the input of amplifier.Of course the amplifier input should have a resister in series with the signal sourse.This resistor and FET will form an attenuator or even mute.
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Old 15th December 2004, 03:14 PM   #4
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See the circuit...The fet wrongly shown as P-channel>>N-channel.
It would have been easier if source resisters are present...Read else where that u dislike the idea of source resisters..
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Old 15th December 2004, 03:48 PM   #5
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Hi ampman, your contemplation is uncorrect : every amp have phase shift, which bring to the comparator circuit big fault. Also is uncorrect to evaluate input vs output voltage - how will be function of this circuit, when will be any signal on input ? In mosfet amp must be current limiting circuits quite the same as in bipolar amp, but must have still " zener " protection for overvoltage on gates, that's all
 
Old 15th December 2004, 04:24 PM   #6
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For "better" current limit protection, i'd probably go with some sort of multistage approach:

- keep the existing super-fast current protection, which limits instantaneous peaks. Make it less sensitive than before - keep it as an "oh @#$^" circuit that will perform instantaneous protection from dead shorts across the output.

- add a second, similar circuit which has a lower current-limit threshold. But low-pass filter the current sample going into it, so that it doesn't act as fast. This protects from low-resistance loads that can damage the amp from sustained high current, but shouldn't clamp down on a quick current spike coming from a funny crossover reactance.

As for the implementation, that's an exercise left to the reader

I've designed RF amplifier protection circuits that work a similar fashion, and they work well.
 
Old 15th December 2004, 06:19 PM   #7
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hallo amp_man1
as Upupa Epops said amp has a phase shift, but there are also other problems with simple compairing: amplitude distortion('filtering'), overdriving (even slight), you also should know EXACTLY the voltage amplification factor.

for me it is much simpler (maybe I am missing something):
protection shuold turn on when
1.current via source resistor is to big
AND (even ttl or cmos and)
2. output voltage is zero (null, ground, gnd...)
 
Old 15th December 2004, 11:14 PM   #8
Eva is offline Eva  Spain
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This reactive overcurrent effect happens when passive crossovers :

- Are not designed properly
- Have capacitors failing in an intermittent short due to excessive voltage
- Have iron core coils saturating due to excessive current
- Have any component with bad solder joints losing contact intermintently
- Have any of their outputs not loaded with a suitable impedance or not loaded at all
- Include impedance compensation for a particular loudspeaker but this driver is not the one used
- Etc..

So I would rather not waste my time trying to make my amplifiers capable of driving a defective passive crossover. I see it pointless. When passive crossovers are working properly there are no reactive impedance drops

Also, loudspeakers only are reactive at frequencies where they show high impedances, so if the problem persists without the passive crossover then the loudspeaker is defective [probably shorted turns on the voice coil]

Your protection is already optimum enough. It trips just because the load is defective, so fix the load instead of the amplifier

Passive crossovers for subs are very critical, that's why almost nobody uses them
 
Old 16th December 2004, 02:07 PM   #9
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Arrow Reply

Hi everybody,

ThanX all of you for your valuable suggestions.

I myself feel that the comparator scheme is not good as there is some amount of phase shift still present at output.
Hafler also uses this comparator scheme but he senses the drive signal and output instead of input signal.

Now I will work on following aspects:

1 Gate overdrive voltage,
Overdrive on gate will be dealt with appropriate design.


Sivanand , your suggestion regarding optocoupler is good but it is not as fast because optocouplers are slow.

2 Traditional method of sensing current through resistor is still good but there must be some tradeoff value for peak current requirements for limiting.


EVA
your suggestion is absolutely right, but some customers will never want to put blame on their speakers rather they blame the amps for not driving them properly.


Any more suggestions are most welcomed

regards
ampman
 
Old 16th December 2004, 02:28 PM   #10
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amp_man_1 !!

Why u need high speed !! Opto's are usable upto 50Khz.(Your not in the RF arena).I 've tried a current limit scheme in a UPS system having 25KVA it was a PWM sinewave output. With Mosfets, I noticed the system would trip whenever a small surge appears the output of UPS,then i deliberately slowed down the response.The Mosfets can and will sustain short duration overloads w/o damage.
GOODLUCK!!
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