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Inrush into 4.3mF capacitor at output of Boost PFC (240VAC mains)?

Flyback

Well-Known Member
Hi,
Do you know of any super small 20 Ohm resistors that can handle this?
There also a 5W flyback running throughout the inrush, and drawing its current thru the inrush res.

Resistor datasheets give nothing that pertains to the one-off-ness of inrush
 

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  • Inrush to 4300uF.jpg
    Inrush to 4300uF.jpg
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ronsimpson

Well-Known Member
Most Helpful Member
Why D7?
For inrush, use a thermistor. At power up maybe 20 ohms. Then it gets hot and reduces down to 2 ohms. This is good for the first-time power up but needs a minute to cool off before works well again. These are often used on old TV sets.
 

Flyback

Well-Known Member
Oh sorry i should have said, D7 is acting as the boost diode of the Boost PFC...just representative of it.

In fact, the only way to be sure, is to use two HSC100 47R resistors in parallel.(no need for heatsink)..for inrush into 4.3mF from 240VAC..

Literally no datasheet deals with the "energy per piffelingly short intervals of time" parameter that we need for inrush investigation.

BTW, i like the "idt" feature of LTspice...as in the attached sim.
 

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shortbus=

Well-Known Member
Most Helpful Member
All these questions. I think if this is how stuff is engineered in the UK it explains why you complained before about all the power supplies being made in China.
 

ronsimpson

Well-Known Member
Most Helpful Member
It is common for a flyback boost PFC to have an extra diode across it. The inrush current should not go through the inductor because it drives the inductor into saturation. The PWM sees a saturated core and is not happy.
 

kubeek

Well-Known Member
Most Helpful Member
Why do you even bother with a PFC if you have a 4300uF cap right there after the rectifier? Your power factor will be horrible and no PFC circuit after that can help you. Datasheet of the controller says the primary cap should be 100uF which still seems rather high, but could be inside the regulations.
 

Flyback

Well-Known Member
Thanks, though the caps are shown downstream of the "Boost diode".....at least thats what its to be representative of. I didnt draw the boost PFC in there as it wasnt necessary for the purposes of the post.
 

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