# TechPowerUp PSU Review Methodology



## crmaris (Apr 15, 2011)

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## fmh3x (Apr 3, 2013)

*How is inrush-current defined and measured?*

Thank you for your extremely useful reviews and clear explanations of equipment and methods used.

How is inrush-current defined and measured? From your reviews a recently measured value of 27.1 A is very good and values below 40 A acceptable. This must refer to a peak current with very short duration, or all ordinary household fuses would be tripped, not to mention a UPS with maximum rating of 1000 VA (about 670 Watt). How short?  Is the area under the curve the important parameter?

A UPS may shut down on a power outage, instead of protecting the PC. This can happen even the UPS has a high enough capacity for the actual PSU (disregarding inrush-current). High inrush-current is one of many factors likely to contribute, but this has not been analysed quantitatively (?).


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## crmaris (Apr 3, 2013)

the inrush current is the peak current (applied for very short, usually ms) that a PSU draws during a cold start-up. It is measured automatically by my Chroma AC source. Usually for 230VAC all values below 50 A are acceptable but of course the lower the inrush current the better. 

A large inrush current of course will trigger even a UPS device, besides fuses etc. But a good even an acceptable quality UPS won't be threatened by below 50 A inrush currents, according to my personal experience at least.


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## fmh3x (Apr 5, 2013)

*Does APFC cause higher inrush currents (was: How is inrush-current defined and measur*

Thank you, I hoped so, and this should be enough for practical purposes. But what do you think of the following reflections, from discussions in UPS user fora:

Charge depletion of the PSU circuitry while the UPS is switching to battery will cause inrush current to the PSU after the switch has been completed. Likely the inrush current will increase with the switchovertime of the UPS and the PC's power consumption PC in the moment that the power to the UPS is cut. You might even be able too measure and plot the relationship between these parameters .

In any case, a PSU with the  very low inrush current of 27,1 A, which you recently reviewed, should not bring down a UPS with reasonable reserve capacity because of a too high inrush current. And the very high power factor correction and efficiency of such a PSU would further reduce the risk.

And here comes the question: Active power factor correction is said to cause stronger inrush currents, so if your UPS is tripped immediately after switching to batteries the recommendation is to get a PSU without APFC. Is it really true that although APFC reduces the load on its source over time,  it increases inrush currents? Or is it simply that  PSUs differ in capacitances and current-limiting circuitry, independently of their type of PFC?


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## crmaris (Apr 5, 2013)

Yeap APFC increases inrush current because of the hold-up cap involved in its design. But an NTC thermistor can effectively lower the inrush current that the hold up cap(s) create.


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