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 itself 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, you should always ensure you are buying a power supply with a high power factor, since the factor is an extremely good indicator of the quality of the PSU circuitry.

Practical Application

Active PFC

As the name implies, active power factor correction (PFC) 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:

• An almost ideal effective power of nearly 99%
• Very stable current output
• Very low power loss
• Very low heat dissipation
• Does without iron, leading to reduced weight

• Higher initial cost
• More susceptible to faults
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.

• Cheaper
• Practically no electro-magnetic interference

• Requires better cooling
• Not suitable for higher power levels
• Increased power consumption due to power loss
• Heavier due to the choke coil
• Lower effective power ranging from 70% to 80%

Power supplies with passive PFC should be considered outdated and can be considered inferior.

Note: Do not confuse efficiency and PFC!