UPSs and Active PFC PSUs.

[Scott Sullivan]

In the recent UPS thread a major question came up about Active PFC PSUs 
and UPSs.
I've take some time to research the issue and provide a little clarity 
at the best of my ability.

# My Current Setup
I recently got an APC 1500VA Back-up UPS myself.
I have two older PCs (both doing server duty), network gear connected 
and a single 15" LCD attached.
Currently they draw 105W of the 850W the UPS can provide, run time is an 
estimated 53 min.
One of the machines has a 350W Active PFC PSU.
I HAVE tested this load with a simulated power failure. (I pulled the 
plug).


# Definitions:

Active PFC:
     "Active power factor corrector" is a power electronic system that 
controls the amount of power drawn by a load in order to obtain a power 
factor as close as possible to unity.
http://en.wikipedia.org/wiki/Active_power_factor_correction#Active_PFC

Plain Language:
     As far as I've wrapped my head around it, it adjusts it input 
current to reduce the wasted power that non purely resistive load (like 
that of a computer) presents.

# Why is Active PFC not good on some UPS models?

The general gist has been that Active PFC should not be used with a UPS 
that does not Supply a pure sine-wave which is most consumer UPS models.
Anecdotal evidence to support this claim include:
- UPS shutting off when switching to battery with a Active PFC PSU
- Strange Humming or Resonant noises while on battery power.

# What APC has to say on the matter.

http://www.apcmedia.com/salestools/RMUZ-7DTKRC_R1_EN.pdf

Active PFC *CAN* be used as long as you have enough spare Wattage from 
the UPS.
This is because Active PFC compensated for the micro loss of power 
during battery switch over by Drawing more current. The consumer UPS 
models respond to this load by shutting down when it exceeds their 
operating limit.
According to APC's documentation, 500W UPS and greater is about the 
magic number that will cause this failure reaction. YMMV.

As for the noise, that just the Active PFC working harder compensate for 
the Stepped Sine Wave that these consumer units create. It does not 
appear to actually be harmful to the PSU but is annoying.



== Disclaimer ==
I am only sharing my experiences and details I have collected. I can not 
be held responsible for how this information is used. Any mistakes or 
omissions are my own.
-- 
Scott Sullivan



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