Re: Gap, Creepage and Clearance for HF HV
- From: "Steve" <sjburke1@xxxxxxxxxxx>
- Date: Tue, 4 Sep 2007 14:20:33 -0400
"D from BC" <myrealaddress@xxxxxxxxx> wrote in message
news:6b0rd3h9f19a3gu8bmdc2p2oviaqpu9kc8@xxxxxxxxxx
On Tue, 4 Sep 2007 08:46:48 +0200, "Frithiof Andreas Jensen"
<frithiof.jensen@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
"D from BC" <myrealaddress@xxxxxxxxx> skrev i en meddelelse
news:3ebnd3dj8ae0jq8rlivl1kh88mm5tql5r0@xxxxxxxxxx
Huhhhh...My smps design did a little plasma show....zzzzzzzzz
then the power mosfet shorted out..
then the fuses blew.. :(
I suspect arcing somewhere perhaps due to not enough PCB trace to
trace clearance. (Just trying to increase PCB density...)
"Somewhere" .... What kind of weak crap is that? The burn mark is flaming
obvious!
I have about 300Vpk @ 700Khz square wave areas on the PCB....
EN 60950 sez: 2 - 4 mm creapage distance, 2 mm for FR4 fiber-glass boards
and the 4 mm for paper-board (the brown stuff).
I know air breakdown depends on gap and voltage, but does air
breakdown voltage also depend on frequency?
Nah - your "about" is what kills: there is likely a needle on your 300Vpk
going to maybe 600 V?
(Assume humidity, pollutants or gases like N2,CO2,O2 are constant.)
Why worry about third-order effects if you do not care enough about 1'st
order to describe/measure?
D from BC
It's not obvious so far where the arc occurred.
I saw it.. I heard it, but it was so quick I can only say it's within
a square centimeter.
I have to remove some parts that are in the way to get a closer with a
magnifying glass.
I'm not sure but I think I see tiny holes in big SMD power resistor.
Hot spots?
I put a 2A fuse on the bulk capacitor (200uF charged to line peak
about 170V). Without it, I think I'd have obvious destruction.
recti >----+-----2A fuse--->to smps
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200uf,200V
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com
It's been suggested in other posts, that my design could be "spiking"
beyond expected and could start an arc.
Perhaps an unexpected spike beyond 300V (my design is supposed to work
without snubbers) across a flux gobbed 20mil gap provided conditions
for an arc. That or an SMD part failed with an arcing effect.
Besides...I should practice including creepage distances in my pcb
artwork. Which I'm just learning about.
2 mm creepage... Does that mean a 2mm distance between two parallel
traces (top side) on a FR4 pcb for 300V 700khz. Or is that the spacing
between top side and bottom side traces for 300V 700khz?
D from BC
Definition of creepage is "the shortest distance along a surface of an
insulating material between two conductive parts. "
It doesn't matter if the conductive parts are parallel traces or just IC
pads or any other copper feature (like ground fills). Its the shortest
distance, regardless of the path.
Creepage distance for a top/bottom pair is usually pretty long, because the
path is to the outer edge of the PCB, down the thickness of the board and
back across to the other conductor. An .060 board already gives you ~1.5 mm
vertically. In the earlier posts regarding slots , its assumed the worst
case problems occur on a common surface, and the creepage path is then
around the edge of the slot. Beware that the IEC creepage distance rules
say that if the slots are too narrow - they can't be considered in total
distance. I forget what the actual slot width is before they are considered
effective.
Steve
.
- References:
- Gap, Creepage and Clearance for HF HV
- From: D from BC
- Re: Gap, Creepage and Clearance for HF HV
- From: Frithiof Andreas Jensen
- Re: Gap, Creepage and Clearance for HF HV
- From: D from BC
- Gap, Creepage and Clearance for HF HV
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