Fail-safing Bleeder resistors for high voltage inputs/outputs

Thread Starter

SiCEngineer

Joined May 22, 2019
299
Hi All,

Pretty simple question here but can't find an answer on the board.

I am designing high voltage power supply and am currently choosing bleeder resistors. I had originally selected single resistors that are rated for the appropriate power and resistance and drain from 300V and 6kV to safe voltage of 50V in 5 minutes.

I have seen multiple reference designs from Texas Instruments which, rather than using single resistors across the input and output capacitors, use approximately 10 in series each rated lower than the power and voltage on a single resistor basis, but together meet the required criteria.

My question is, what is the point of having 10x200V 56k resistors, rather than having a single 500V resistor rated at 560kOhm? Is it for fail-safe proofing in case any one of the resistors fails? I would think that if this was the case, they would be connected in parallel. Because a series failure in a resistor, should mean the entire string fails, meaning the high voltage capacitors will not drain rendering the bleeder resistors unsafe and unable to drain the HV. In parallel, however, if one resistor failed, the other 9 or so would still be safe to discharge the capacitor.

Please let me know if there is any fundamental flaw in my thinking.

Best wishes,
J
 

wayneh

Joined Sep 9, 2010
16,531
I don't know anything about this but I speculate that it's simply cost and availability. A string of ten, 1/4W resistors is cheaper than a single, 2.5W resistor. Pick and place machines are more likely to handle the standard part easily.
 
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