Electronics > Projects, Designs, and Technical Stuff
High voltage SMD capacitors and High power resistors. Do you use them ?
Miyuki:
Hi fokls,
I need to place some snubbers.
"Problem is" 800V and 4W dissipated
There is 1kV rated 1206 Capacitors and 2W rated 2512 Resistors
Is it any problem or is it all fine to apply such stress to this tiny components (2512 is not so tiny I know) ?
Mark:
Yes, they can be problematic. Read the datasheets carefully.
With high voltage SMT capacitors you might find mention of a conformal coating being necessary to meet the voltage rating.
Even if coating is not necessary, bear in mind that handling (sweaty hands) and environment (damp) can increase the risk of arcing. If an SMT cap arcs across the body it will need to be replaced because it will be much more likely to arc in the future.
Also note that HV SMT caps can typically only withstand 20% overvoltage, where as leaded HV parts can normally take 200% rated (briefly).
As for SMT high power resistors, check the power vs temperature derating curves carefully and add lots of copper to act as a heatsink, and possibly lots of vias to transfer heat to copper on other layers.
Also check the pulse rating of the resistors.
Mechanical stress is another thing to bear in mind, especially with the SMT caps, PCB flexing can cause cracking.
tszaboo:
Read up on X and Y capacitors, decide which one you want, if you need it at all.
For resistors, place many in series, that will reduce the power and voltage rating. It might be worth to place two in series, in case one would turn into charcoal.
daqq:
We do use PULSED SMD power resistors for snubbers. You can get away with a LOT more power (~1+kW for a short pulse for a 2512 resistor designed for it). The average power on the resistors is relatively low (0.5W). See: http://www.farnell.com/datasheets/2140397.pdf , page 5.
Can't you use more resistors? Divide the power into a larger area, rather than using only one.
Miyuki:
I can use 2-3 smaller ones, that is basically my question what will be better, in size, parasitic and reliability
I need to burn in resistor 800V ~1kW pulses with average power up to 4W
And capacitor to withstand up to 8A pulses
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