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
| Active PFC |
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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)
| Passive PFC |
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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!
