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Old 30th January 2005, 07:47 AM   #1
csl113 is offline csl113  Romania
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Question short-circuit protection in mosfet's amp...??

i have just build this amp:
http://www.astro.uu.se/%7Emarcus/private/m250.html

and i an thinking of building this amp:
www.castana.as.ro/300w.gif

or even this one
http://www.castana.as.ro/800w.pdf

problem is that they don't have short-circuit protection....and in the past i had some problem with amp's that didn't had protection......

could someone tell me how can i put protection on these amp's..???

i did not build any mosfet amp so far,so i don't have any expirience with mosfets....is the protection circuit identical in mosfet and bipolar amp's..???

could someone draw me the protection for these amp's......???
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Old 30th January 2005, 08:30 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the simple way to protect all those FET ouputs is to add a zener in parallel to r38 to limit the drive voltage to the FETs, or add a zener between output and the gate of each FET, or add a zener to the gate to source connections of the FETs. By limiting the gate to source voltage you automatically limit the maximum current through the FET. When the zener goes into conduction it draws current through the gate resistor and the driver. The gate resistor is the main voltage dropper from Vrail to output and you need to check current through the driver under short circuit condition.
I believe that the zeners protect better if located closer to the FET ie directly to gate & source and do their worst job located back at the driver resistor. You will find all three versions when you search, so others obviously do not agree with me.
Finally this protection mode only limits maximum current. For VI limiting you can copy the BJT system by detecting the voltage across the emitter (most designers) or the collector resistor (Bryston).
regards Andrew T.
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Old 30th January 2005, 08:36 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
your schematic shows a single voltage supply for all three stages of amplification, or worse a dropper diode to the VAS Vrail.
FET outputs operate more efficiently when the driver and the preceding VAS have a higher Vrail than the output devices.
Try the drivers at plus 5 to 8 volts and the VAS at plus 10 to 16 volts. You could regulate the lo current VAS supply for even better performance.
regards Andrew T.
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Old 30th January 2005, 09:01 AM   #4
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Zener diode protect Mosfet against breakdown gate ( cca +/- 20 V ). Safety value is practicaly ( by vertical types ) cca 15 V zener. But by this voltage are all fets full open and too high drain current can destroy this device. From this reason correct protection circuit must be based on current sensing principle, the same as by BJT case. Second reason, which is talking for current sensing, is complex load, where output current is not in phase with output voltage. At attach is example of this circuit.
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Old 30th January 2005, 10:21 AM   #5
csl113 is offline csl113  Romania
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i agree with Upupa Epops , and i also think that zener's protect only for gate overdrive,and not for overload...

does it work if i copy the protection from bipolar's......(i think this does not work but, but something like that does it work..?)
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Old 30th January 2005, 01:12 PM   #6
Eva is offline Eva  Spain
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Note that this linear current limiting approach is prone to oscillation when applied across two or more stages and may require frequency compensation. This is valid both for bipolar and MOSFET circuits
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Old 30th January 2005, 04:26 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi upupa epops,
is q10&20 the VI protection? what is the value of r42&49? and their purpose?
What is C14&20?
Is q21&24 a dynamic quiescent control? What does it achieve?
regards Andrew T.
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Old 30th January 2005, 10:47 PM   #8
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by csl113
................ i also think that zener's protect only for gate overdrive,and not for overload...

True....
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Old 31st January 2005, 06:10 PM   #9
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To AndrewT : Here is value of all devices. Q 18, 24 and all around devices make error correction.
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