short-circuit protection in mosfet's amp...??

csl113 · 30th January 2005, 07:47 AM

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......???

AndrewT · 30th January 2005, 08:30 AM

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.

AndrewT · 30th January 2005, 08:36 AM

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.

Upupa Epops · 30th January 2005, 09:01 AM

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.

csl113 · 30th January 2005, 10:21 AM

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..?)

Eva · 30th January 2005, 01:12 PM

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

AndrewT · 30th January 2005, 04:26 PM

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.

mikeks · 30th January 2005, 10:47 PM

Quote:

Originally posted by csl113
................ i also think that zener's protect only for gate overdrive,and not for overload...

True....

Upupa Epops · 31st January 2005, 06:10 PM

To AndrewT : Here is value of all devices. Q 18, 24 and all around devices make error correction.

30th January 2005, 07:47 AM	#1
csl113 diyAudio Member Join Date: Nov 2004 Location: in the home	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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30th January 2005, 08:30 AM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

30th January 2005, 08:36 AM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

30th January 2005, 01:12 PM	#6
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	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

30th January 2005, 04:26 PM	#7
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.