There is an article in next month's (Feb 2009) issue of Australian PC User Magazine which compares the real power and VA consumption of several ATX PSUs under various load conditions.
Some PSUs appear to incorporate APFC while others do not.
A Huntkey 80Master 700W PSU consumes 100W and 114VA when idling in WinXP desktop mode. However, in standby (S3) mode, with power applied to system RAM, the figures are 5W and 44.5VA. In the "off" state the numbers are 3.3W and 43.4VA.
The other PSUs also have similar results for the standby and off states.
Why are the VA figures so high?
- Franc Zabkar
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Please remove one \'i\' from my address when replying by email.
The light power load results in a very short conduction period / angle of the AC input rectifier bridge which results in the high VA (apparent power) figure.
On Fri, 02 Jan 2009 21:41:13 +0000, Eeyore put finger to keyboard and composed:
I'm trying hard to understand why a PSU that incorporates APFC would have such a bad VA rating when doing nothing. I can only assume that APFC is not applied to the +5VSB standby rail, or that it is difficult to design an APFC circuit that can accommodate a wide range of loads. In order for a real power of 3.3W to appear as 30VA or more, the PF would have to be around 0.1. According to my calculations, this is feasible, but only if the conduction duty cycle is very low, approximately 0.005 if we assume a square current pulse centred on the voltage peak.
I don't have access to any PSU circuits with APFC, but here is such a circuit in an LG TV:
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There is an 0.22uF EMI suppression cap across the mains terminals. This would contribute about 4VA to the figures. I wonder how *it* behaves at standby. BTW, I realise that it is not just a matter of adding the VA contributions of each section of the circuit.
AFAICS, anything connected to the APFC side of the supply would appear as a resistor. So an ideal PSU would have an "active" resistor in parallel with an EMI cap.
- Franc Zabkar
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Please remove one \'i\' from my address when replying by email.
It definitely does ? Yes it must I guess for C tick.
Not at all unlikely. It doesn't have to because it falls below the 75W IEC
61000-3-2 (IIRC) limit in that situation.
Also quite likely. Read an app note on the subject. I could speak VOLUMES on the subject. IEC1000-3-2 as it started life had to be withdrawn PDQ when certain industry sectors (including ME) pointed out certain appliances could never conform.
In standby mode PFC isn't required because the power level drawn is below the
75W threshold as defined in IEC61000-3-2 regarding harmonic currents and the threshold for (serious or any ?) correction.
It happens to be a standard I became VERY familiar with since it would (as originally draughted) have effectively caused ALL audio power ampliers of a decent wattage to HAVE to use SMPS supplies with PFC. NO more toroids etc, unless you had PFC on the secondary side !
I was one of the first to notice this and alerted John Woodgate at PLASA immediately. Curiously IEC1000-3-2 1st Edition strangely went out of print just after we got a copy and its very existence was formally denied. There was something else in too that was IMPOSSIBLE.
I could ramble at great length on this one - the bureaucratic 'fixes' required to make the standard workable were typically Byzantine in typical EU manner when producing the equivalent ENs.
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