Hello,
I am working in the SMPS design and development department of a huge global consumer goods company (you will know the name, but I can’t say it).
The SMPS’s are offline, and of powers from 5W to 200W.
I am just designing a 42W offline, isolated flyback. The product is likely to be permanently connected to the mains, so it has to have a standby power draw of less than 300mW.
I had just finished the schematic , when the boss came over , and handed me a 10 Ohm, 2Watt, Wirewound, flameproof resistor (“Bathtub” type…you know the ones, they’re white and look like they’re made from the same stuff that they make space shuttle tiles out of )
…..the boss told me to put this resistor after the PFC Bus capacitor, (at 400V) in series with the flyback.
-At max load/minimum line, this resistor will constantly dissipate 1.8W of power, due to the high RMS current of the flyback.
-I am told that adding such a resistor is essential because if the flyback FET goes short circuit, then something could blow off the board and potentially start a fire……..however, with the said resistor in place, there will be no such fire, because the PFC bus voltage will be dropped across the flameproof resistor, which will harmlessly go open circuit.
-But the product has a plastic case, and I wonder how any device blowing off the board could possibly start a fire?
In standby Mode, the Boost PFC stage is not running, and the PFC Bus is just the rectified mains peak voltage of 330VDC.
Is it really necessary to add such a huge, resistor?
There is after all, a 5Amp, slow-blow fuse at the mains input.
It seems like a waste of power to me…..is it?