Re: Gap, Creepage and Clearance for HF HV

On Fri, 07 Sep 2007 04:44:36 GMT, D from BC <myrealaddress@xxxxxxxxx>
wrote:

On Fri, 07 Sep 2007 00:33:33 -0300, YD <ydtechHAT@xxxxxxxxxx> wrote:

Late at night, by candle light, D from BC <myrealaddress@xxxxxxxxx>
penned this immortal opus:

On Mon, 03 Sep 2007 10:07:06 -0700, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Mon, 03 Sep 2007 07:25:37 GMT, D from BC <myrealaddress@xxxxxxxxx>
wrote:

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...)

I have about 300Vpk @ 700Khz square wave areas on the PCB....

I know air breakdown depends on gap and voltage, but does air
breakdown voltage also depend on frequency?

(Assume humidity, pollutants or gases like N2,CO2,O2 are constant.)

D from BC

300 volts peak isn't really "high voltage" and is unlikely to cause
corona or arcing in air. I suppose it's possible, but just barely,
that dielectric losses heated up a spot on the pcb, and cooked the
epoxy to failure.

But likely something else went wrong.

What sorts of clearances do you have in the regions where you have the
300 volts? Post a pic if you can; schematic too.

John

Are you saying that it's possible for the epoxy between traces
(top side trace to top side trace
or top trace to bottom ground plane)
to get fried like too much current through a lossy capacitor?
Cool! A possible headache but still cool. :)
But dunno yet if that's the case..
Working on it..

I have the 300Vpk at 700Khz between two top traces separated by 20mil.
Beneath these traces is the ground plane separated by 40mil of FR4.
I've yet to do the math if this is ok at this voltage and frequency.


Yikes, that's ~0.5 mm. Still below 1 kV/mm but much too close for
comfort. I suggest something like 80 mil minimum for that kind of
voltage.

I've seen a PCB burned up by 150 V on traces separated by 3 mm (~120
mil). It was phenolic, and got dirty, humid, whatever. The leakage
heated the PCB at some spot and then the charring made it increasingly
worse. Burned it through for the whole length the traces ran together.

- YD.

I might spray the board to prevent arc over due to junk landing on the
pcb.. (The pcb is not in a sealed box.)
Perhaps something from a paint store might be a cheap and convenient
solution..
Let's see...
polyurethane
acrylic
lacquer
high heat engine clear coat
http://www.duplicolor.com/products/engine.html

I wonder which one to use...
D from BC

Polyurethane. But the clearance is still scairy.

John

.

Relevant Pages

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