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Inrush into offline SMPS EMC filter is too much for inrush resistor?

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treez

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Hello,

We have the following EMC filter for our 60W offline Flyback SMPS LED driver prototype(attached).
(LTspice Simulation of inrush event also included)

Do you think the 2R7 ROX3SJ2R7 inrush resistor will be able to survive the inrush at power on, which happens once per day? (I mean, will it survive for many years of this?)

The inrush event is only 30us long with peaking to 800W and 1000W

ROX3SJ2R7
**broken link removed**
 

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The mfr doesn't see fit to rate anything for pulsed stress,
other than terminal voltage. I wouldn't speak for them.

I would probably start by looking at the simulation result
(not shown) for the Joule energy deposited in the resistor,
figure "thermal time" is milliseconds and use the resistor
mass and some WAG at specific heat to figure a temp
rise, and go back to the datasheet's derating curves
which seem to top out at 200C for long term reliable.
Maybe figure a 100C rise as allowable for the pulse
event (70C to 170C) having margin at both ends.

Then see what you see.

Or go to a vendor who provides pulsed current
stress ratings.
 

Really shouldn’t be hard to find an alternate with a pulse rating or maybe better, an inrush NTC.

Also in general the circuit and filtering there seems a bit complicated..
 
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I agree with post #2, a quick sim ( 300VDC applied for 2mS ) will tell you the joules in the R, and from this you can approximate the Trise per pulse, also the DC bus caps for your 60W converter seem small....?

to a first approximation, C = 660nF, therefore energy in R = 34.8mJ ( 1/2 C V^2), in one cycle of mains this is 1.74W - so very likely survive - for a long time.
 
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Thanks, Yes the bus caps are small because we needed PFC since it is for driving LEDs.
 
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Well, if the input capacitance to the led converter is too small - you will have issues with your control loop ...
 

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