Gap, Creepage and Clearance for HF HV

[D from BC]

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

 

[D from BC]

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

oops..Need to rewrite..

Aside from gap width, voltage and gas mix, does breakdown in air also
depend on frequency?


D from BC

 

[Wimpie]

Hello D from BC,

There are three things; Creepage, corona and distance through air.
According to "Reference data for Engineers" (Newnes):

Break down voltage for uniform E-field in air at sea level and room
temperature is 75kVp/inch, 30kVp/cm. Valid up to 300 MHz, use a safety
margin.

So you should evaluate the worst-case field strength for your
application. Highest field strength will probably occur at an air/
insulation transition on thin wire.

Corona discharge
When corona discharge occurs at an air/insulation transition,
frequency comes in play. High frequency will give higher corona effect
and erosion of the insulation will go faster. When the insulation
finally breaks down (under emission of material), the eroded material
helps to lower the air breakdown voltage significantly. A nice smooth
sounding arc will occur with nice smoke and color effects (I played
many times with that [with plenty of water nearby in case of fire]).

Also with high frequency, the zero voltage transition is too short to
extinguish the arc.

I use an RF receiver with high pass filter to search for corona
discharge in HV circuits.

Creepage is another thing. An arc is formed by miniscule discharges
over a (polluted) surface. Here evaporated material also helps the
ionization process to establish and maintain the arc. 300Vp requires
about 1.1mm insulation between traces on an uncoated PCB (functional
insulation, not safety insulation).

When breakdown of a barrier involves a safety hazard, there are
minimum requirements for distance trough air (clearance) and distance
over surface (creepage). Here transient over voltage must also be
taken into account (4kV for 230V mains building installation
equipment) and the type of insulation system (basic or double/
reinforced). To read more for free, go to
http://www.ecma-international.org/publications/standards/Standard.htm.
Go to ECMA287 it is like IEC950, but then for free.

300V/ 700 kHz does not arc that fast (I used same frequency in a power
circuit). Are you sure that there are no higher transient voltages
present in that waveform? Did you clean the board after soldering,
etc?

Best regards,

Wim
PA3DJS
www.tetech.nl

 

[ChairmanOfTheBored]

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

How long did it run before the failure mode occurred?

If it ran for a long period, then the solution is to conformally coat
the assembly (while it is new, of course).

Air ionization will eventually lead to carbon trail forming.

This is one reason why the long ceramic insulators on high tension
lines need to be changed out periodically. They eventually form carbon
trails on their surface, and all the creepage distance in the world wont
stop that resistor from conducting.

 

[[email protected]]

oops..Need to rewrite..

Aside from gap width, voltage and gas mix, does breakdown in air also
depend on frequency?

http://en.wikipedia.org/wiki/Corona_discharge

 

[neon]

GET SOME VARNISH AND GO TO WORK. PAINT THE THING. it doesn't have be air but because it is inert but air carry polutants and that is the problem. like pure water is inert but drinking water is not it has sodium-cloride and what not.

 

[D from BC]

http://en.wikipedia.org/wiki/Corona_discharge

I did a speed read of that and the only mention of frequency was that
corona can generate an audible or radio frequency noise.
The arcing I witnessed on my PCB did "sing" for about 2 seconds before
parts started blowing.
My arcing HF smps design probably got so chaotic that it managed to
produce spectrum in the audio range.
D from BC

 

[MooseFET]

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.)

As the frequency rises the needed gap also increases. A sharp edge
and RF frequencies are a bad combination. Both make it easier for the
electrons in the metal to get into the air and start trouble.

The corona effect creates ions and is bad in two ways. Firstly, these
ions can be the ones that start the spark. Secondly, these ions are
very reactive things and they eat the epoxy of the PCB. They slowly
turn the PCB into something that is conductive.

Since this is a new design you are working on, there could be another
problem. Did you forget to wash and dry the PCB before you applied
power? The drying step is very important. It is hard to wait while
you latest invention dries in the hot box but wait you must.

 

[John Larkin]

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

 

[D from BC]

Hello D from BC,

There are three things; Creepage, corona and distance through air.
According to "Reference data for Engineers" (Newnes):

Break down voltage for uniform E-field in air at sea level and room
temperature is 75kVp/inch, 30kVp/cm. Valid up to 300 MHz, use a safety
margin.

So you should evaluate the worst-case field strength for your
application. Highest field strength will probably occur at an air/
insulation transition on thin wire.

Corona discharge
When corona discharge occurs at an air/insulation transition,
frequency comes in play. High frequency will give higher corona effect
and erosion of the insulation will go faster. When the insulation
finally breaks down (under emission of material), the eroded material
helps to lower the air breakdown voltage significantly. A nice smooth
sounding arc will occur with nice smoke and color effects (I played
many times with that [with plenty of water nearby in case of fire]).

Also with high frequency, the zero voltage transition is too short to
extinguish the arc.

I use an RF receiver with high pass filter to search for corona
discharge in HV circuits.

Creepage is another thing. An arc is formed by miniscule discharges
over a (polluted) surface. Here evaporated material also helps the
ionization process to establish and maintain the arc. 300Vp requires
about 1.1mm insulation between traces on an uncoated PCB (functional
insulation, not safety insulation).

When breakdown of a barrier involves a safety hazard, there are
minimum requirements for distance trough air (clearance) and distance
over surface (creepage). Here transient over voltage must also be
taken into account (4kV for 230V mains building installation
equipment) and the type of insulation system (basic or double/
reinforced). To read more for free, go to
http://www.ecma-international.org/publications/standards/Standard.htm.
Go to ECMA287 it is like IEC950, but then for free.

300V/ 700 kHz does not arc that fast (I used same frequency in a power
circuit). Are you sure that there are no higher transient voltages
present in that waveform? Did you clean the board after soldering,
etc?

Best regards,

Wim
PA3DJS
www.tetech.nl

Errr...noooo... I didn't clean the board...Doh!
I used a flux pen on the pcb for better soldering and then used flux
core solder.
There's visible rosin between the HV and gnd traces spaced 15mil to
20mil apart..

Your post should help. Thanks.
D from BC

 

[D from BC]

How long did it run before the failure mode occurred?

If it ran for a long period, then the solution is to conformally coat
the assembly (while it is new, of course).

Air ionization will eventually lead to carbon trail forming.

This is one reason why the long ceramic insulators on high tension
lines need to be changed out periodically. They eventually form carbon
trails on their surface, and all the creepage distance in the world wont
stop that resistor from conducting.

1 to 2 seconds. Probably closer to 1 second.
The arc started when I switched on the power.

It's probably a combo of traces too close together with flux on the
pcb.

But from I've read so far, it looks like I have to do my trace spacing
not only to prevent arcing but also to prevent eventual arcing.

More complications for the pcb layout. Arrrghhh
D from BC

 

[ChairmanOfTheBored]

1 to 2 seconds. Probably closer to 1 second.
The arc started when I switched on the power.

It's probably a combo of traces too close together with flux on the
pcb.

But from I've read so far, it looks like I have to do my trace spacing
not only to prevent arcing but also to prevent eventual arcing.

More complications for the pcb layout. Arrrghhh
D from BC

Our multiplier stages had either fully buried traces where the only
thing exposed was the via/component mount hole, or they were all on one
side, and the assembly was vacuum potted.

You may want to bake your assembly at approx 60C for an hour, and even
place it under a vacuum for about ten minutes. PCB material is
hygroscopic (soaks up water). Both FR4 and G10 are hygroscopic.

The way we cleaned our boards, we could detect more moisture trapped in
a masked PCB than in the unmasked multiplier areas of our designs. The
unmasked area was also more receptive of the potting primer, and thereby
caused less potting detachment issues. Definitely the way to go with
potted HV assemblies. We even grit blasted the glassy finish off our
dipped capacitors to get superior potting adhesion. One fingerprint
blows the whole multiplier though.

You should watch out for sharp corners in your traces too. We even
used arced (sp) traces when it yielded a benefit.

 

[ChairmanOfTheBored]

Hello D from BC,

There are three things; Creepage, corona and distance through air.
According to "Reference data for Engineers" (Newnes):

Break down voltage for uniform E-field in air at sea level and room
temperature is 75kVp/inch, 30kVp/cm. Valid up to 300 MHz, use a safety
margin.

So you should evaluate the worst-case field strength for your
application. Highest field strength will probably occur at an air/
insulation transition on thin wire.

Corona discharge
When corona discharge occurs at an air/insulation transition,
frequency comes in play. High frequency will give higher corona effect
and erosion of the insulation will go faster. When the insulation
finally breaks down (under emission of material), the eroded material
helps to lower the air breakdown voltage significantly. A nice smooth
sounding arc will occur with nice smoke and color effects (I played
many times with that [with plenty of water nearby in case of fire]).

Also with high frequency, the zero voltage transition is too short to
extinguish the arc.

I use an RF receiver with high pass filter to search for corona
discharge in HV circuits.

Creepage is another thing. An arc is formed by miniscule discharges
over a (polluted) surface. Here evaporated material also helps the
ionization process to establish and maintain the arc. 300Vp requires
about 1.1mm insulation between traces on an uncoated PCB (functional
insulation, not safety insulation).

When breakdown of a barrier involves a safety hazard, there are
minimum requirements for distance trough air (clearance) and distance
over surface (creepage). Here transient over voltage must also be
taken into account (4kV for 230V mains building installation
equipment) and the type of insulation system (basic or double/
reinforced). To read more for free, go to
http://www.ecma-international.org/publications/standards/Standard.htm.
Go to ECMA287 it is like IEC950, but then for free.

300V/ 700 kHz does not arc that fast (I used same frequency in a power
circuit). Are you sure that there are no higher transient voltages
present in that waveform? Did you clean the board after soldering,
etc?

Best regards,

Wim
PA3DJS
www.tetech.nl

Errr...noooo... I didn't clean the board...Doh!
I used a flux pen on the pcb for better soldering and then used flux
core solder.
There's visible rosin between the HV and gnd traces spaced 15mil to
20mil apart..

Your post should help. Thanks.
D from BC

Rosin, RMA, Aqueous?

 

[Tim Williams]

Line-op stuff I see often has wide spacing (>0.2", I guess) and notches
punched out of the board to eliminate creepage as such entirely. At least,
the better stuff.

True, you may not have the luxury of that much space.

Tim

 

[D from BC]

As the frequency rises the needed gap also increases. A sharp edge
and RF frequencies are a bad combination. Both make it easier for the
electrons in the metal to get into the air and start trouble.

The corona effect creates ions and is bad in two ways. Firstly, these
ions can be the ones that start the spark. Secondly, these ions are
very reactive things and they eat the epoxy of the PCB. They slowly
turn the PCB into something that is conductive.

Since this is a new design you are working on, there could be another
problem. Did you forget to wash and dry the PCB before you applied
power? The drying step is very important. It is hard to wait while
you latest invention dries in the hot box but wait you must.

Ohhh..there's ooddles of flux I didn't clean off..
oops...
And I have to review the trace-trace separation with respect to
frequency, corona and voltage. All new stuff for me. The posts are
helping.

Now I'm wondering about something...

I'm using a power mosfet in a TO220 package.
It's got a 600Vds rating. (IIRC ..That's a DC spec.)
Could there be a frequency restriction due to the TO220 lead spacing?
In other words..
Could a mosfet be driven at such a frequency and voltage to get TO220
package lead to lead arcing?
Even worse... I make fat PCB pads for TO220 leads. Now there's even
less spacing.


D from BC

 

[Tim Williams]

I've seen (and have, I think) some TO-220's rated to 1500Vcbo. Most such
devices extend epoxy around the center pin (collector), but still. Makes ya
wonder...

(On the other hand, I have some deflection transistors that are TO-247
sized, ISO-TO-220 shaped (i.e. covered in black plastic), and the center pin
has extended plastic. Now that I would trust. Uh, 2SC5041 or something
like that.)

Tim

 

[David L. Jones]

Hello D from BC,

There are three things; Creepage, corona and distance through air.
According to "Reference data for Engineers" (Newnes):

Break down voltage for uniform E-field in air at sea level and room
temperature is 75kVp/inch, 30kVp/cm. Valid up to 300 MHz, use a safety
margin.

So you should evaluate the worst-case field strength for your
application. Highest field strength will probably occur at an air/
insulation transition on thin wire.

Corona discharge
When corona discharge occurs at an air/insulation transition,
frequency comes in play. High frequency will give higher corona effect
and erosion of the insulation will go faster. When the insulation
finally breaks down (under emission of material), the eroded material
helps to lower the air breakdown voltage significantly. A nice smooth
sounding arc will occur with nice smoke and color effects (I played
many times with that [with plenty of water nearby in case of fire]).

Also with high frequency, the zero voltage transition is too short to
extinguish the arc.

I use an RF receiver with high pass filter to search for corona
discharge in HV circuits.

Creepage is another thing. An arc is formed by miniscule discharges
over a (polluted) surface. Here evaporated material also helps the
ionization process to establish and maintain the arc. 300Vp requires
about 1.1mm insulation between traces on an uncoated PCB (functional
insulation, not safety insulation).

When breakdown of a barrier involves a safety hazard, there are
minimum requirements for distance trough air (clearance) and distance
over surface (creepage). Here transient over voltage must also be
taken into account (4kV for 230V mains building installation
equipment) and the type of insulation system (basic or double/
reinforced). To read more for free, go to
http://www.ecma-international.org/publications/standards/Standard.htm.
Go to ECMA287 it is like IEC950, but then for free.

300V/ 700 kHz does not arc that fast (I used same frequency in a power
circuit). Are you sure that there are no higher transient voltages
present in that waveform? Did you clean the board after soldering,
etc?

Best regards,

Wim
PA3DJS
www.tetech.nl

Errr...noooo... I didn't clean the board...Doh!
I used a flux pen on the pcb for better soldering and then used flux
core solder.
There's visible rosin between the HV and gnd traces spaced 15mil to
20mil apart..

Your post should help. Thanks.
D from BC

It's quite common in HV PCB designs to route out a slot in your PCB to
separate high voltage traces.
Also commonly used for low leakages designs too.

Dave.

 

[David L. Jones]

Ohhh..there's ooddles of flux I didn't clean off..
oops...
And I have to review the trace-trace separation with respect to
frequency, corona and voltage. All new stuff for me. The posts are
helping.

Now I'm wondering about something...

I'm using a power mosfet in a TO220 package.
It's got a 600Vds rating. (IIRC ..That's a DC spec.)
Could there be a frequency restriction due to the TO220 lead spacing?
In other words..
Could a mosfet be driven at such a frequency and voltage to get TO220
package lead to lead arcing?
Even worse... I make fat PCB pads for TO220 leads. Now there's even
less spacing.

Well, don't make fat pads!
You can bend the leads before mounting to give you a staggered pin
spacing which increases the pin spacing a lot.

Dave.

 

[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.

I'll try to prep a pcb layout and schematic for my website.
Both need some spiffing up first for presentation for less confusion
and quicker viewing..
I'll let you know when it's uploaded and provide a link.
D from BC

 

[D from BC]

Hello D from BC,

There are three things; Creepage, corona and distance through air.
According to "Reference data for Engineers" (Newnes):

Break down voltage for uniform E-field in air at sea level and room
temperature is 75kVp/inch, 30kVp/cm. Valid up to 300 MHz, use a safety
margin.

So you should evaluate the worst-case field strength for your
application. Highest field strength will probably occur at an air/
insulation transition on thin wire.

Corona discharge
When corona discharge occurs at an air/insulation transition,
frequency comes in play. High frequency will give higher corona effect
and erosion of the insulation will go faster. When the insulation
finally breaks down (under emission of material), the eroded material
helps to lower the air breakdown voltage significantly. A nice smooth
sounding arc will occur with nice smoke and color effects (I played
many times with that [with plenty of water nearby in case of fire]).

Also with high frequency, the zero voltage transition is too short to
extinguish the arc.

I use an RF receiver with high pass filter to search for corona
discharge in HV circuits.

Creepage is another thing. An arc is formed by miniscule discharges
over a (polluted) surface. Here evaporated material also helps the
ionization process to establish and maintain the arc. 300Vp requires
about 1.1mm insulation between traces on an uncoated PCB (functional
insulation, not safety insulation).

When breakdown of a barrier involves a safety hazard, there are
minimum requirements for distance trough air (clearance) and distance
over surface (creepage). Here transient over voltage must also be
taken into account (4kV for 230V mains building installation
equipment) and the type of insulation system (basic or double/
reinforced). To read more for free, go to
http://www.ecma-international.org/publications/standards/Standard.htm.
Go to ECMA287 it is like IEC950, but then for free.

300V/ 700 kHz does not arc that fast (I used same frequency in a power
circuit). Are you sure that there are no higher transient voltages
present in that waveform? Did you clean the board after soldering,
etc?

Best regards,

Wim
PA3DJS
www.tetech.nl

Errr...noooo... I didn't clean the board...Doh!
I used a flux pen on the pcb for better soldering and then used flux
core solder.
There's visible rosin between the HV and gnd traces spaced 15mil to
20mil apart..

Your post should help. Thanks.
D from BC

Rosin, RMA, Aqueous?

Kester flux pen # 951 low solids no clean and then I used Multicore
solder which I believe contains rosin.

I haven't looked it up yet but I bet the breakdown voltage of this
flux combo is probably much less than air for a 20mil separation
between two top pcb traces with a potential diff of 300vpeak..
And I'm thinking about the frequency and square waveform making it
easier to arc through the flux too..
D from BC