MOSFET Inrush and Short Circuit Limiter - diyAudio
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Old 15th June 2005, 12:04 PM   #1
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Default MOSFET Inrush and Short Circuit Limiter

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

Click the image to open in full size.

the description can be found here:
http://a330.g.akamai.net/7/330/2540/...es/60905di.pdf
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Old 15th June 2005, 02:42 PM   #2
ilimzn is offline ilimzn  Croatia
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Q2 is not going to live very long...
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Old 15th June 2005, 06:02 PM   #3
Steven is offline Steven  Netherlands
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Insert a resistor in the base of Q2 to protect it.

Steven
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Old 15th June 2005, 07:10 PM   #4
ilimzn is offline ilimzn  Croatia
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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.
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Old 15th June 2005, 07:29 PM   #5
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they do print corrections at EDN
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Old 16th June 2005, 01:38 AM   #6
tlf9999 is offline tlf9999  United States
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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.
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Old 16th June 2005, 04:26 AM   #7
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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.
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Old 16th June 2005, 04:49 AM   #8
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Therez no base current limit for Q1..in a practical circuit any short or overload will damage Q1 ......
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Old 16th June 2005, 05:15 AM   #9
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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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Old 16th June 2005, 10:18 AM   #10
tlf9999 is offline tlf9999  United States
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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?
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