Power Supply Design Basics: Passive PFC Design

By Nuvation | Jul 18, 2013

Previously we introduced the concept of the Power Factor. Now we’re discussing what Power Factor Correction (PFC) is, and one of two types of PFC for your power supply design.

Power Factor Correction

Power Factor Correction (PFC) allows AC power distribution to operate at its maximum efficiency. A power factor corrected load appears resistive to its source. This implies that the input current must differ from the sinusoidal source voltage by only a scaling factor. Their waveforms must be identical, though scaled by the effective input resistance of the PFC, by Ohm’s Law. There are two types of PFC: Active PFC and Passive PFC.

Passive PFC

For small power supplies, 100W, the most common type of PFC is Passive. Passive PFC uses a harmonic filter at the AC input to correct a poor power factor. It may be used when environmental vibration occurs. Passive PFC requires that the AC input voltage be set manually. However, it does not use the full energy potential of the AC line.

As stated above the passive PFC is basically a low pass filter. If there were no economic or size restrictions the break point of the filter would be set slightly above 60Hz. L1 and C1 form a series resonant circuit, as shown below.

Passive PFC is great for low power (below 100W) and only one input voltage at a time. Under these conditions both the inductor and capacitors used can be fairly small, relatively inexpensive and efficient.

Following the passive PFC and bridge (BR1), is usually a DC/DC converter. The normal tolerance range on the AC line voltage is 15% which implies that the DC/DC converter must operate at an input voltage with a variance of 30%. This makes the DC/DC converter less efficient, more complex and expensive. Once it is tuned to 60 Hz, and a 50 Hz input is provided, the power factor goes down roughly by 1%.

Overall, Passive PFCs are simple, reliable, and robust for low power requirements. They don’t generate EMI and they offer a low-cost alternative for complying with harmonic current limit specifications. The major limitation of passive PFCs is their size and weight, which is primarily due to the associated inductor.