Welcome back to the second part of our 3-part series on power supply design principles.  At Nuvation, we have been called upon to design power circuits and supplies to provide the sometimes large array of DC voltages required by a particular system.  Often times, our clients have a limited understanding of the complexity of these designs and as a result assume that the process is simple and consumes little time or talent.  In this series of articles I hope to provide a better understanding of the complexities of design and some of the thought processes involved in the design of power supplies. 

In part one, I discussed power supply design requirements, Power Factor, and an introduction to Power Factor Correction (PFC).  If you haven’t already read part one, please start with this link.  In this issue, I will provide a better understanding of the complexities of PFC, specifically Passive PFC and an introduction to Active PFC.

Power Factor Correction

Power Factor Correction (PFC) allows power distribution to operate at its maximum efficiency. A PFC 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 Power Factor Correction (Passive PFC). Passive PFC uses a harmonic filter at the AC input to correct 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.  

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. 

The primary advantages of passive PFC are:

1. Simplicity
2. Cost
3. Efficiency

The primary disadvantages of passive PFC are:

1. Size and weight
2. Lack of voltage regulation capabilities
3. Narrow range of allowable input voltages
   

Active PFC
For any design over 100W, the preferable type of PFC is Active Power Factor Correction (Active PFC) since it provides a lighter and more efficient power factor control.  Active PFC is comprised of a switching regulator operating at a high switching frequency, being able to generate a theoretical power factor of over 95%.

Active Power Factor Correction automatically corrects for AC input voltage, and is capable of a wide range of input voltage. One disadvantage of Active PFC is the extra cost resulting from the additional complexity required in its implementation.  I will be discussing the details of Active PFC in the next issue. Stay tuned.

For more information or to engage Nuvation on a Power Supply design or review, please email us at sales@nuvation.com.