MOSFET Inrush and Short Circuit Limiter

jackinnj · 15th June 2005, 12:04 PM

The following circuit appeared in the design ideas section of EDN:

the description can be found here:
http://a330.g.akamai.net/7/330/2540/...es/60905di.pdf

ilimzn · 15th June 2005, 02:42 PM

Q2 is not going to live very long...

Steven · 15th June 2005, 06:02 PM

Insert a resistor in the base of Q2 to protect it.

Steven

ilimzn · 15th June 2005, 07:10 PM

Of course - my point was, it's rather strange for a specialised magazine to publish this not noticing such an obvious mistake, let alone in such a simple circuit.

jackinnj · 15th June 2005, 07:29 PM

they do print corrections at EDN

tlf9999 · 16th June 2005, 01:38 AM

I don't know why Q2 will be killed - I think Q1 has a better chance of getting killed.

The "flaw" in the design is that the voltage drop over Q1 is always Vbe of Q2, and if there is a lot of current going through Q1, it will get hold quickly.

Also, the design would be a lot more interesting if it is done on N-channels.

peranders · 16th June 2005, 04:26 AM

Guys, I'll guess you don't have notice that this mosfet is connected backwards (we use this also at work) and the circuit may not need any gate resistors. This has to be determined from case to case.

sivan_and · 16th June 2005, 04:49 AM

Therez no base current limit for Q1..in a practical circuit any short or overload will damage Q1 ......

tawen_mei · 16th June 2005, 05:15 AM

You may have Vgs breakdown for the kind of voltage solid-state amp will see.

Q2 is fine. It should turn Q1 off fast enough before its base-emitter junction melts.

tlf9999 · 16th June 2005, 10:18 AM

Quote:

Originally posted by peranders
Guys, I'll guess you don't have notice that this mosfet is connected backwards (we use this also at work)

are you sure about that?

15th June 2005, 12:04 PM	#1
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	MOSFET Inrush and Short Circuit Limiter The following circuit appeared in the design ideas section of EDN: the description can be found here: http://a330.g.akamai.net/7/330/2540/...es/60905di.pdf

15th June 2005, 06:02 PM	#3
Steven diyAudio Member Join Date: Oct 2003 Location: The Netherlands	Insert a resistor in the base of Q2 to protect it. Steven __________________ The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973

16th June 2005, 04:26 AM	#7
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Guys, I'll guess you don't have notice that this mosfet is connected backwards (we use this also at work) and the circuit may not need any gate resistors. This has to be determined from case to case. __________________ /Per-Anders (my first name) or P-A as my friends call me

16th June 2005, 04:49 AM	#8
sivan_and diyAudio Member Join Date: Dec 2004 Location: Dxb / India	Therez no base current limit for Q1..in a practical circuit any short or overload will damage Q1 ...... __________________ SivanandBalan

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15th June 2005, 02:42 PM	#2
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	Q2 is not going to live very long...

15th June 2005, 07:10 PM	#4
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	Of course - my point was, it's rather strange for a specialised magazine to publish this not noticing such an obvious mistake, let alone in such a simple circuit.

15th June 2005, 07:29 PM	#5
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	they do print corrections at EDN

16th June 2005, 01:38 AM	#6
tlf9999 Account Disabled Join Date: May 2005 Location: none	I don't know why Q2 will be killed - I think Q1 has a better chance of getting killed. The "flaw" in the design is that the voltage drop over Q1 is always Vbe of Q2, and if there is a lot of current going through Q1, it will get hold quickly. Also, the design would be a lot more interesting if it is done on N-channels.

16th June 2005, 05:15 AM	#9
tawen_mei diyAudio Member Join Date: Jan 2005 Location: California Republic	You may have Vgs breakdown for the kind of voltage solid-state amp will see. Q2 is fine. It should turn Q1 off fast enough before its base-emitter junction melts.

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