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When is inrush relay necessary?
When is inrush relay necessary?
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Old 27th December 2011, 02:43 PM   #1
eem2am is offline eem2am
Join Date: Jun 2011
Default When is inrush relay necessary?


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

The input stage is as follows:-

(Electrolytic caps are 680uF, 200V

Capacitor Part Number = ESMQ201VSN681MQ30S

cap datasheet

....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?

Last edited by eem2am; 27th December 2011 at 02:47 PM. Reason: forgot datasheet
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Old 27th December 2011, 08:56 PM   #2
sawreyrw is offline sawreyrw  United States
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Join Date: Apr 2006
Location: Minnesota
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-curre...FQhjhwod8Dbr8w

Last edited by sawreyrw; 27th December 2011 at 08:59 PM. Reason: Add info on NTC
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Old 28th December 2011, 04:42 AM   #3
! is offline !  United States
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Join Date: Jun 2005
Location: Midwest
Not necessary for these caps. Their ripple is rated RMS over a 120Hz period and always with the capability of power-on inrush up to the max spec'd voltage.

Last edited by !; 28th December 2011 at 04:45 AM.
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Old 28th December 2011, 07:42 AM   #4
dtproff is offline dtproff  United States
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Join Date: Oct 2008
Location: Plano, TX, USA
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.

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Old 28th December 2011, 08:10 AM   #5
Kv3audio is offline Kv3audio  India
Join Date: Sep 2011
Best thing is to use resistors, 10W or 2x5W, around 47 Ohm

Thats an advice
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