Start-up resistor for offline flyback suffers overvoltage..but is it OK?

[coffeefet]

Hi,
I have been sent a flyback power supply by a customer to check out.
VAC input = 187-264VAC.
They have used a single 220k, MMB0207 resistor to act as the start-up resistor, which feeds the Vcc pin of the FSCQ1565 Flyback controller IC (as attached).
When VAC input is 264VAC, this means that this 350V rated resistor will keep suffering a peak voltage just above 350V, every 20ms.
(The MMB0207 datasheet says it can handle 500V for 1 minute, but this doesn’t seem relevant to the case here.)
Do you agree that we should really use startup resistors that aren’t going to get overvoltaged instead?

MMB0207 datasheet:

https://www.vishay.com/docs/28713/melfprof.pdf

 

[dick_freebird]

What does the overvoltage rating really mean?
It could be a real reliability-impact based rating
or it could just be a UL, arc / creepage sandbagged
number.

If there is a max duty associated with the 500V / 1 min
spec then you could criticize the waveform against
that. I imagine that the "effective duty cycle" (of V>350V)
is mighty low. Certainly there is a real duty cycle limit
(like, 500V for 1 min, 0 for 10 sec, 500V for 1 min... would
likely violate the test paradigm, if any). Whether Vishay
will provide that, who knows? Could be something in
the application notes for pulsed power applications.

The wording of the detail spec makes me think this
is not specifically about voltage across the resistor,
but voltage from the body to "ambient" (i.e. any close-
in conductor). You might want to dig for more clarity
about what it's really expressing:

Permissible voltage against ambient (insulation):​

If 500V (350V) is an end-end, terminal voltage value
then you could use 2 series devices and split the
stress.

Might also check the power dissipation which gets
a call-out as another reliability limit, possibly more
of a bind than voltage depending on value.

 

[KlausST]

Hi,

And 350V cause about half a Watt of heat. Be sure the heat is spread for the "ambient" temperature of the resistor to stay low enough.
It's a PCB and airflow problem.

Also a PCB layout problem may be the trace distances for that 350V. ... especially underneath the resistor.
Consider aging, increased temperature, humidity, dirt ....

Klaus

 

[coffeefet]

Thanks, the dissipation, even at 265VAC input, is only 139mW because of the connection to the half-bridge point of the mains rectifier.

 

[Easy peasy]

Use 2 resistors in series of equal or near equal value - @ 275Vac input worst case dissipation is 172mW as half wave rectified feed ...

 

[treez]

Use 2 resistors in series of equal or near equal value - @ 275Vac input worst case dissipation is 172mW as half wave rectified feed ...

Agreed, use two 1206 resistors in series.
I dont know why one MMB0207 resistor was used. They are more expensive than 2x1206....also, the continuous pulsing above 350V means this resistor could fail early.....the 500V rating is for very "now-and-again", one-off overvoltage transients.....not for repeated overvoltage like you have there.

Many engineers will tell you its ok and not to fuss...but you are operating "Outside the datasheet", and as such, nobody can truly tell you what will happen....you are in the "twilight zone"...that unknown area of potential doom for your product.

Do not try to ask the manufacturer if its OK what you do...they will not answer...why should they...its like asking a Mercedes car salesmen if its ok to drive the car too fast and with not enough oil and cooling water etc.