Question on the PCB clearance
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Question on the PCB clearance

 
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Justin2003mail



Joined: 16 Feb 2007
Posts: 1
Posted: Fri Feb 16, 2007 8:13 pm    Post subject: Question on the PCB clearance Reply with quote
Question I believe it is not a question only for me, according to UL standard, if the primary circuit has a peak working voltage 800V, it asks for a 3mm clearance even for a functional insulation.

It is very normal situation that the main MOSFET in an AC/DC power supply get 800Vpeak (for leakage...), but it is diffcult to select a MOSFET with pin to pin clearance 3mm or more.

Does I misunderstand the UL standard, or It is not applicable in our real design so no one will check it.

Please help me if anyone know the answer.


Thanks

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neon



Joined: 25 Feb 2006
Posts: 586
Posted: Mon Feb 19, 2007 12:15 am    Post subject: Reply with quote
I don't think that there is requirement of testing a device that usualy see 120 ac RMS or 120ac x1.41 peak or twice that for 120 v ac. you probably miss interpreter what you read. A hi-pot test probably is in order whereby a 800 volts is applied to the unit once without causing damage. that is not a sustainable test but once shot to see if it the unit survive then it passes the test for all other units as approved.Military they go further "g" strain and shock tesing and RF interference and more depends on requirements and applications and lets face it money.
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ronsimpson



Joined: 30 Apr 2007
Posts: 11
Posted: Mon Apr 30, 2007 4:53 pm    Post subject: Reply with quote
UL has an exception for spacing when the part does not meet spacing. Away from the part keep the spacing! As you approach the part you can reduce the spacing. In the case of TO-220 you can pull the middle leg out to make a triangle. I do not.

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neon



Joined: 25 Feb 2006
Posts: 586
Posted: Mon Jun 04, 2007 8:01 am    Post subject: Reply with quote
you may do as you want but the leeds when soldered to an eyelet is the critical arcing criteria i think
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