Picking The Right Power Supply: What You Should Know

Of Power Factors, Apparent Power And Effective Power

PFC (Power Factor Correction): It Does What It Says

Don’t worry, you won’t need your high school physics text book for this part. We just want to point out another characteristic that can be used to tell a high-quality PSU from a bad one. Once you know the basics, you’re a lot less likely to make a bad purchasing decision. So, let’s dive right in.

Idle Current And Idle Power

One problem that plays an especially big role in the case of switching power supplies is the so-called idle current, which results from inductivity. Note that idle current has nothing to do with the idle state of your computer. This idle current only transports energy between a generator and a load, but doesn’t end up benefiting the load in any way. Again, load in this context has nothing to do with a PC at full throttle, but refers to a component that uses power. Think of it as a power shuttle that isn’t involved beyond its role as a transporter. This idle current has to be reduced as much as possible and as early as possible, since it causes power loss in conjunction with ohmic resistors, manifesting as heat. This idle power consumption is a waste, and should be kept as low as possible through appropriate circuitry.

Effective Power And Apparent Power

Unlike idle power consumption, effective power measures the power that is actually used, while apparent power refers the sum total of the effective power and the cumulative idle power.

Power Factor

This factor is the result of the ratio between effective power and apparent power, falling somewhere between 0 (worst) and 1 (ideal). Thus, the higher the power factor the less energy goes wasted back to the mains network. Although residential consumers do not have to pay for apparent power, in order to minimize apparent power usage, the EU standard EN61000-3-2 states that all switched mode power supplies with output power of more than 75W must include a passive PFC converter. In addition, 80 PLUS certification requires a power factor of 0.9 or more. Some years ago, many PSU manufacturers used passive PFC in their products. PPFC uses a filter that passes current only at line frequency, 50 or 60Hz, so the harmonic current is reduced and the nonlinear load is transformed to a linear load. Then, with the usage of capacitors or inductors, the power factor can be brought close to unity. The disadvantage of PPFC is that it attains smaller power factors than APFC and requires a voltage doubler for the PSU to be compatible with 115/230V. On the contrary, PPFC has higher efficiency than APFC, something that probably most of you didn’t know. But that doesn't mean PPFC-equipped PSUs are more efficient than APFC ones, since PPFC units are often based on older designs that cannot compete with the performance of modern APFC PSUs.

Practical Application

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Active PFC

As the name implies, active power factor correction (APFC) employs a circuit that actively corrects the power factor. As we just explained, the power factor is an important parameter in the world of PSUs, since it describes the ratio between effective power and apparent power:Pros:• An almost ideal power factor close to unity• Very stable current output even under input voltage variations• Demands for smaller parts (transformer) Cons:• Higher initial cost• More susceptible to faults• Energy losses occur onto the APFC’s boost diode (and the FETs)

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Passive PFC

Passive PFC attempts to reduce idle currents by using large choke coils. While this method is simpler and cheaper, it is also much less effective.Pros:• Cheaper (depending on the power handling range) • Practically no electro-magnetic interference•    More efficient than APFC (beware: this doesn’t mean though that PPFC PSUs are more efficient that APFC ones)Cons:•    No automatic universal voltage input•    Not suitable for higher power levels •    Heavier due to the larger PFC choke •    Lower power factor ranging from 70% to 80%Power supplies with passive PFC should be considered outdated and can be considered inferior. And don't confuse efficiency and PFC!

Contributing Editor

Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.

  • abryant
    Archived comments are found here: http://www.tomshardware.com/forum/2916-56-picking-power-supply-know
  • pjmelect
    A few more words about active power factor correction. APFC won't save you money on your electric bill although the electric companies will love you for it as it minimizes loss over the power lines saving them money, it does however enable you to use a much lower rated battery backup system. A hypothetical example a computer that uses say 200W without APFC would require a backup system of 700W or much more to cope with the large peaks in current where as a power supply with APFC would require a backup system of 250W or so.
    Therefore APFC is only worthwhile if you were to use it with a battery backup system.
  • redgarl
    Toms knows everything... yeah right.
  • turkey3_scratch
    Very well written article! Just one thing. You say:
    Regardless of whether the PC is idling or under full load, voltages may not deviate from their spec by more than five percent according to the ATX spec.
    But the ATX specification seems to disagree. According to the spec, full load or "peak loading" allows 10% deviation from the nominal voltage for the 12V rail.

    Also, Q about the power factor correction. It's probably the most difficult topic to understand. In this case, you say the load would be anything that used power. Are you talking about hardware like a GPU or the internals of the PSU like capacitors and such? Also, say the computer is putting load on the PSU. How is there idle current then?
  • JackNaylorPE
    Voltage Stability ?

    Ripple ?
  • Aks_X
    Unaligned tables :|
  • jossrik
    There have been quite a few instances in the past where you could get an XFX PSU 550w or so for 40$ or less with rebates. I know compared to more modern PSUs they may not stack up, but they used to be pretty decent. Ya, more often than not, the cheaper the PSU the worse the quality, but you really do need to do your homework.
    Budget PSU
  • Aris_Mp
    The newest ATX spec defines 5% at peak load as well. The 10% is only for the -12V rail which is now optional. The newest ATX spec is confidential (dont know why)
  • cats_Paw
    While I can understand that having a beefy power supply on idle state wont be too efficient, its on loads where you want it as efficient as possible.
    Somehow, having a low efficiency under a 65W load is less expensive than low efficiency at 500W load, go figure :D.

  • cats_Paw
    In all fairness, a PC is not a self-maintenance Robot.
    If you want a PC to last a good 10-15 years you need to take care of it:
    Clean dust, replace fans when they fail, replace thermalpaste, check your temperatures from time to time, not turn it on-off-on too fast, keep your Hard drives with some spare space and defraged if they are HDDs....

    There is quite some work for a PC to keep their form, but its not like a human can lay down in bed eating cheese and drinking cola looking like a model either.

    PSUs however have this strange aura of magic around them since some people vastly overestimate what power supply they need (I got a 700W TT one for a load of 320, go figure) and others buy things that are simply bad products, no matter how high the W are.

    I did once burn a PC due to a bad PSU (and I even OCed the damn PC, went down in smoke.. I gotta say it was quite fun, but expensive), so I stay on the safe side (I just simply add an extra 20% for 12v rail amps as long as the price of a quality supply is not doubling).