When is inrush relay necessary?

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Hello,

We are doing a non-PFC SMPS of 330W. (90-265VAC)

The input stage is as follows:-

http://i39.tinypic.com/29bmxci.jpg
(Electrolytic caps are 680uF, 200V

Capacitor Part Number = ESMQ201VSN681MQ30S

cap datasheet
http://www.chemi-con.co.jp/e/catalog/pdf/al-e/al-sepa-e/005-snapin/al-smqlug-e-110701.pdf


....How can we evaluate when the inrush is so much that the input electrolytics will be damaged and we therefore need an inrush resistor which is shorted out by a relay?

Also, are NTCs chosen on the basis of their RMS current withstand, as well as cold and hot resistance?
 
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I've never heard of using an NTC thermistor to limit the current to protect the capacitor, but I suppose it is possible. The purpose of the thermistor is to limit the current drawn from the power line, and also to prevent blowing the input fuse.

The thermistor needs to be able to handle the inrush energy and steady state RMS curreent, as well as have the required cold and hot resistances. See: http://www.ametherm.com/inrush-current/?gclid=CMa4rvOdo60CFQhjhwod8Dbr8w
 
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NTC thermistors (add heat shrink tube around teh device) are the standard way. Since it doesn't appear that you are busing the envelope on efficiency it should do fine.

We use a 4-10 ohm PTC in the signal path and bypass it with a relay once the line gets up high enough. That is usually 4-10 half cycles. Typically >500W is where I see them most often used. This is all under hardware control since usually nothing is awake downstream.

When using large capacitances, peak line voltage and 90 degree phase angle usually greatly exceeds the bridge rectifier surge rating so that is usually the weak point.


Tony
 
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