Gap, Creepage and Clearance for HF HV

[D from BC]

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

 

[John Larkin]

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

 

[Paul Mathews]

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 coathttp://www.duplicolor.com/products/engine.html

I wonder which one to use...
D from BC- Hide quoted text -

- Show quoted text -

acrylic or if available styrene both have excelent film forming and HV
breakdown properties
Paul Mathews

 

[Paul Mathews]

[snip]
You can add some 'blanking' time by paralleling the current sense
resistor with a fairly large capacitance (needed because Rsense is
quite small) and/or adding an RC filter to the current sense input.
This will affect the minimum load for stability (increase it), but the
undesirable transient also has that effect.
It's easy to add some capacitance to your Spice model to generate the
switch-on transient that is the bane of current feedback designs.
Think about it.
Rectifier reverse recovery characteristics, leakage inductance and
layout inductance (loop area), and switch turn-on time are all
important determiners of this type of problem.
Paul Mathews
I think you're saying, assume the worst...
Yup...thought about RC filtering too..
But just the minimal amount to help out the following circuits I use
which also help reduce reaction to spikes.
I can easily make a blanking circuit but RC filtering is easier to add
to an existing prototype pcb.
Simply:
/\/\/\/\/\/\/
spice signal for current feedback
| | | | | |
/|\|/|\|/|\|/
| | | | | |
real world for current feedback (spikes and ringing)
D from BC

You need to add a 3rd picture, which is between the other two in
amplitude and relabel them as follows:

Spice (least amplitude)
As seen by control chip (medium amplitude)
As seen by oscilloscope (highest ampliltude) that you're calling 'real
world'

This is because it's difficult to setup a scope probe that doesn't add
to the spike amplitudes. One thing that helps quite a bit sometimes:
wind several turns of the scope coax through a large ferrite toroid or
clamp-on core to improve scope CMRR. Always check what you get when
you 'short' the probe tip using probe ground (whatever you're using).
Paul Mathews

Sounds easier than my other idea....
I have a 17Mhz analog optoisolator IC I got from Digikey..
It's a HCPL-4562-000Ehttp://www.avagotech.com
I thought about using that IC for a battery powered opto probe.
D from BC- Hide quoted text -

- Show quoted text -

Get or make yourself a proper HV differential probe (like Tek P5205)
if you're going to keep doing this type of work. There's nothing much
to them if you want to build your own: a good fast diff amp chip and a
balanced input attenuator driving a 50 Ohm buffer amp. However, the
input networks have to be well matched or CMRR will be poor.
Paul Mathews

Paul Mathews

 

[D from BC]

Polyurethane. But the clearance is still scairy.

John

Yeah..I know. I plan on redoing the layout for the power section of
the smps with industry clearances.
Then if nothing blows up...I'm into EMI emission problems... Oh joy
700khz could be somebodys favorite country/western AM radio station..

D from BC

 

[Paul Mathews]

Yeah..I know. I plan on redoing the layout for the power section of
the smps with industry clearances.
Then if nothing blows up...I'm into EMI emission problems... Oh joy
700khz could be somebodys favorite country/western AM radio station..

D from BC- Hide quoted text -

- Show quoted text -

Most offline switchers are still operating down under 150 kHz, largely
for EMI reasons. Differential mode EMI is more easily filtered at 700
kHz compared to lower freq. Common mode components at 700 kHz and
harmonics and much higher freq stuff at the parasitic ring freqs will
predominate. Easiest way to reduce your EMI without lowering clock
rate: slow down switching transitions and take an efficiency penalty.
It's all about I = C dv/dt and V = L di/dt with the parasitic Ls and
Cs, so reduce di/dt, dv/dt and/or get those parasitics down. For the
higher freq components (ringing), softer rectifiers and optimal
snubbing.
Paul Mathews

 

[D from BC]

[snip]

Most offline switchers are still operating down under 150 kHz, largely
for EMI reasons. Differential mode EMI is more easily filtered at 700
kHz compared to lower freq. Common mode components at 700 kHz and
harmonics and much higher freq stuff at the parasitic ring freqs will
predominate. Easiest way to reduce your EMI without lowering clock
rate: slow down switching transitions and take an efficiency penalty.
It's all about I = C dv/dt and V = L di/dt with the parasitic Ls and
Cs, so reduce di/dt, dv/dt and/or get those parasitics down. For the
higher freq components (ringing), softer rectifiers and optimal
snubbing.
Paul Mathews

Lower the dv/dt & di/dt... ... noooooooooooo....

The fall time on the power mosfet drain is immeasurable with my 20Mhz
scope.. It's a vertical line on the screen at max sweep.
It's beautiful

(Note: My smps prototype functions ok at 50% Vin. Parts burned out at
100% Vin. I'm nibbling at the problem..)

I'll give it a shot 1st to tame components and layout parasitic's for
less buzz for those country/western AM listeners..
Failing that..Then I'll make everything sluggy..(reduce the I&V rates)
That should keep the black helicopters away

Thanks for the pointers..Good stuff..

D from BC

 

[Paul Mathews]

[snip]

Most offline switchers are still operating down under 150 kHz, largely
for EMI reasons. Differential mode EMI is more easily filtered at 700
kHz compared to lower freq. Common mode components at 700 kHz and
harmonics and much higher freq stuff at the parasitic ring freqs will
predominate. Easiest way to reduce your EMI without lowering clock
rate: slow down switching transitions and take an efficiency penalty.
It's all about I = C dv/dt and V = L di/dt with the parasitic Ls and
Cs, so reduce di/dt, dv/dt and/or get those parasitics down. For the
higher freq components (ringing), softer rectifiers and optimal
snubbing.
Paul Mathews

Lower the dv/dt & di/dt... ... noooooooooooo....

The fall time on the power mosfet drain is immeasurable with my 20Mhz
scope.. It's a vertical line on the screen at max sweep.
It's beautiful

(Note: My smps prototype functions ok at 50% Vin. Parts burned out at
100% Vin. I'm nibbling at the problem..)

I'll give it a shot 1st to tame components and layout parasitic's for
less buzz for those country/western AM listeners..
Failing that..Then I'll make everything sluggy..(reduce the I&V rates)
That should keep the black helicopters away

Thanks for the pointers..Good stuff..

D from BC- Hide quoted text -

- Show quoted text -

Life is short. Get a better scope soon. Your rectifer circuits are
probably ringing at 80 Mhz or more.
Paul

 

[D from BC]

[snip]

Yeah..I know. I plan on redoing the layout for the power section of
the smps with industry clearances.
Then if nothing blows up...I'm into EMI emission problems... Oh joy
700khz could be somebodys favorite country/western AM radio station..

D from BC- Hide quoted text -

- Show quoted text -

Most offline switchers are still operating down under 150 kHz, largely
for EMI reasons. Differential mode EMI is more easily filtered at 700
kHz compared to lower freq. Common mode components at 700 kHz and
harmonics and much higher freq stuff at the parasitic ring freqs will
predominate. Easiest way to reduce your EMI without lowering clock
rate: slow down switching transitions and take an efficiency penalty.
It's all about I = C dv/dt and V = L di/dt with the parasitic Ls and
Cs, so reduce di/dt, dv/dt and/or get those parasitics down. For the
higher freq components (ringing), softer rectifiers and optimal
snubbing.
Paul Mathews

Lower the dv/dt & di/dt... ... noooooooooooo....

The fall time on the power mosfet drain is immeasurable with my 20Mhz
scope.. It's a vertical line on the screen at max sweep.
It's beautiful

(Note: My smps prototype functions ok at 50% Vin. Parts burned out at
100% Vin. I'm nibbling at the problem..)

I'll give it a shot 1st to tame components and layout parasitic's for
less buzz for those country/western AM listeners..
Failing that..Then I'll make everything sluggy..(reduce the I&V rates)
That should keep the black helicopters away

Thanks for the pointers..Good stuff..

D from BC- Hide quoted text -

- Show quoted text -

Life is short. Get a better scope soon. Your rectifer circuits are
probably ringing at 80 Mhz or more.
Paul

Is that a ring after reverse voltage?

You mean the power rectifiers as part of the converter circuit..
Yeah...I've considered SiC diodes and currently using a hyperfast
power diode..
mmm Hyperfast = hyperEMI ?
Maybe I can do the dum thing and just assume it rings and do a
standard fix whatever that might be..dunno..

Yeah...gotta get a better scope...
The scope I have is so old...(punchline).. the manual can be read in 5
minutes.
D from BC

 

[Paul Mathews]

[snip]
Yeah..I know. I plan on redoing the layout for the power section of
the smps with industry clearances.
Then if nothing blows up...I'm into EMI emission problems... Oh joy
700khz could be somebodys favorite country/western AM radio station..

D from BC- Hide quoted text -
- Show quoted text -
Most offline switchers are still operating down under 150 kHz, largely
for EMI reasons. Differential mode EMI is more easily filtered at 700
kHz compared to lower freq. Common mode components at 700 kHz and
harmonics and much higher freq stuff at the parasitic ring freqs will
predominate. Easiest way to reduce your EMI without lowering clock
rate: slow down switching transitions and take an efficiency penalty.
It's all about I = C dv/dt and V = L di/dt with the parasitic Ls and
Cs, so reduce di/dt, dv/dt and/or get those parasitics down. For the
higher freq components (ringing), softer rectifiers and optimal
snubbing.
Paul Mathews
Lower the dv/dt & di/dt... ... noooooooooooo....
The fall time on the power mosfet drain is immeasurable with my 20Mhz
scope.. It's a vertical line on the screen at max sweep.
It's beautiful
(Note: My smps prototype functions ok at 50% Vin. Parts burned out at
100% Vin. I'm nibbling at the problem..)
I'll give it a shot 1st to tame components and layout parasitic's for
less buzz for those country/western AM listeners..
Failing that..Then I'll make everything sluggy..(reduce the I&V rates)
That should keep the black helicopters away
Thanks for the pointers..Good stuff..
D from BC- Hide quoted text -
- Show quoted text -

Life is short. Get a better scope soon. Your rectifer circuits are
probably ringing at 80 Mhz or more.
Paul

Is that a ring after reverse voltage?

You mean the power rectifiers as part of the converter circuit..
Yeah...I've considered SiC diodes and currently using a hyperfast
power diode..
mmm Hyperfast = hyperEMI ?
Maybe I can do the dum thing and just assume it rings and do a
standard fix whatever that might be..dunno..

Yeah...gotta get a better scope...
The scope I have is so old...(punchline).. the manual can be read in 5
minutes.
D from BC- Hide quoted text -

- Show quoted text -

When the rectifiers turn off, their junction capacitance forms a tank
circuit with any connected stray inductances. This can have quite a
high Q, and it repeats at 700 kHz in your case. Also, the ring
amplitude adds to the voltage stress on your rectifiers and on the
switching transistors(s), reflected through the turns ratio of the
transformer. Soft recovery rectifiers help. If the inductances are
low, very small RC snubbers can damp down the ringing to a single
critically damped overshoot with minimal EMI consequences. There's no
excuse for not having a high BW scope these days.
Paul Mathews