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Inrush resistor into offline flyback blew up.

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Flyback

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Hello,

We are doing a 30W PFC’d Flyback SMPS with an input stage as in the attached. We have a 10R inrush resistor which is ROX1S (datasheet below).

Just to see what would happen, I changed the X2 capacitor, C1 from 220nF to 2u2.
The 10R inrush resistor was damaged the next time I switched the power supply on, obviously due to inrush into the 2u2. (there was also a 1A fuse [not shown in schem] which did not blow). The resistor did not appear physically damaged, but was open circuit when checked.

I would not expect a surge rated 1W resistor to blow under these circumstances.

Nothing in the datasheet says it would blow up due to such a short term power spike…..i mean, as you can see in the LTspice simulation, the power peak is 10kW and the waveform shape is like a 1.5us/25us ‘spike’.
I mean, surely this is far too short a time to heat up such a resistor to the point of damage?

(the power supply worked fine when I replaced the 10R and changed back to a 220n X2 capacitor)

When the 220n X2 capacitor is fitted for C1, then the power peak is 8kW and the spike is of a 500ns/2.5us shape. How would we know that this wouldn’t cause wear out over time and blow up?

[we are now trying to come away from NTC’s for inrush limitation as they don’t work well when hot, and also, inrush relays etc are too expensive for us…as such, we now want to have an inrush resistor, sized to dissipate no more than 400mW for the sake of efficiency…after this , I confess I will probably put an NTC in aswell].

The resistor cannot have heated up much at all in the 26.5us of the power surge……so this actually makes me wonder, what is the long term effect of inrush into the X2 capacitors is? …..what is the time wear out process for them, years, decades?

ROX1S resistor datasheet:
http://www.te.com/commerce/Document...pdfEnglishENG_DS_1773271_E.pdf3-1625892-1
 

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dknguyen

Well-Known Member
Most Helpful Member
Hello,

We are doing a 30W PFC’d Flyback SMPS with an input stage as in the attached. We have a 10R inrush resistor which is ROX1S (datasheet below).

Just to see what would happen, I changed the X2 capacitor, C1 from 220nF to 2u2.
The 10R inrush resistor was damaged the next time I switched the power supply on, obviously due to inrush into the 2u2. (there was also a 1A fuse [not shown in schem] which did not blow). The resistor did not appear physically damaged, but was open circuit when checked.

I would not expect a surge rated 1W resistor to blow under these circumstances.

Nothing in the datasheet says it would blow up due to such a short term power spike…..i mean, as you can see in the LTspice simulation, the power peak is 10kW and the waveform shape is like a 1.5us/25us ‘spike’.
I mean, surely this is far too short a time to heat up such a resistor to the point of damage?

(the power supply worked fine when I replaced the 10R and changed back to a 220n X2 capacitor)

When the 220n X2 capacitor is fitted for C1, then the power peak is 8kW and the spike is of a 500ns/2.5us shape. How would we know that this wouldn’t cause wear out over time and blow up?

[we are now trying to come away from NTC’s for inrush limitation as they don’t work well when hot, and also, inrush relays etc are too expensive for us…as such, we now want to have an inrush resistor, sized to dissipate no more than 400mW for the sake of efficiency…after this , I confess I will probably put an NTC in aswell].

The resistor cannot have heated up much at all in the 26.5us of the power surge……so this actually makes me wonder, what is the long term effect of inrush into the X2 capacitors is? …..what is the time wear out process for them, years, decades?

ROX1S resistor datasheet:
http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet1773271EpdfEnglishENG_DS_1773271_E.pdf3-1625892-1
Why couldn't it blow up after such a short time? I don't see the connection if the surge during that time is high enough. Shorter time just means whatever heat build up that does occur can't travel fast enough to equilibriate so it just stays where generated, stressing the imperfections with localized heating producing hot spots without affecting the overall temperature of the material (i.e. not measurable). Weakest link.

It's almost like you were working under the assumption that you can get away continuing to double the power but halve the duration ad infinitum?
 
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