Hi Kevin,
I had a rather bad experience between UL and CSA in
the older days when there wasn't so much discussion
and agreement between them. I had set up an MOU
between them with UL as the test location.
Went like this ...
Switching power supply. Has a transformer.
Must do abnormals on it. UL does the abnormals.
Temp probe on the *windings* of the transformer.
CSA said, "No way. Since the real concern is
the PCB flaming from over temps
from the transformer, we want the
probes on the *bobbin*."
UL said, "No way. Since the real concern is
what generates the heat, we want the
the probes on the *windings*."
To this day, I can't say absolutely which way
is the better.
Guess I didn't help you much either.
Regards, Doug
> From: Kevin Harris <[email protected]>
> To: EMC-PSTC (E-mail) <[email protected]>
> Subject: EN 60950 and component heating
> Date: Monday, September 15, 1997 6:14 PM
>
> Hello All,
>
> In testing some product for excessive temperatures I have come up
> against the following problem. Consider a diode (part of a bridge
> rectifier circuit) and the PCB underneath the component. If one measures
> the temperature of the diode it does not come close to the specification
> for the part. However if we place a thermocouple on the pad where the
> diode is attached to the PCB and we consider that as a temperature
> measurement for the PCB material itself ,then the temperature obtained
> is above the board manufacturers spec of 110 C (when we take into
> account our maximum permissible ambient temperature of 49 C). By the by
> all this is NOT operator accessible if that makes any difference.
>
> Questions.
>
> 1. Is this a valid temperature measurement for the PCB? I'm of two minds
> on this. It could said that I'm really measuring the diodes temperature
> and not the PCB. On the other hand the diode pad does touch the PCB .
>
> 2.Would it be more reasonable to measure the temperature in the same
> neighborhood as the pad but make sure that the probe does not touch the
> PCB pad? Would a notified body be of the same opinion?
>
> 3.If you feel that the first method is a valid measurement technique
> then do you know of any ways to work around the problem? There are many
> power devices that can easily and safely exceed a PCB material spec of
> only 110 C.
>
> Thanks for your opinions!
>
>
> Best Regards,
>
>
> Kevin Harris
>
> email [email protected]
>
