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Power Factor Correction (PFC) Magnetics Checklist

(This design checklist is also available in printable format.)

In recent years, power factor correction (PFC) for the lagging power factor has been gaining momentum. While the U.S. does not have any existing regulatory requirements, in Europe, the employment of PFC in power supplies is necessary to meet the European EN61000-3-2 regulatory standard. As the U.S. government continues to emphasize energy conservation and efficiency, the domestic need for PFC is on horizon. While many design considerations exist for a PFC circuit, the design of PFC magnetics hinge upon a few key parameters. Precision, Inc. has created a checklist to demystify these parameters and ensure that key design considerations are addressed.

Power level in Watts
• Output of the PFC
• Do not fail to include the efficiency of the downstream DC/DC converter when determining the required PFC output power.
Lacking better information, assume 85% efficiency for the DC/DC.
Input voltage range
• Usually 85-265VAC
85-265V input range includes worst-case for a label rating of 100-240V
Input frequency range
• Usually 50-60 Hz
Some applications require 400 Hz. This doesn't usually affect the magnetics appreciably but it does have some implications for the control method.
Operating frequency
• Fixed or variable
Fixed frequency operation is only available in
Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM) operation.  Boundary Conduction Mode (BCM)) is variable frequency.

Output voltage
• Fixed, usually 390VDC or tracking

Tracking output voltage may offer some advantage at low mains voltages, but it requires a wider input range on the DC/DC. If this method is chosen, verify that the efficiency of the DC/DC does not degrade at
low input.
Conducted EMI requirements
• Class A or Class B
EMI requirements will mostly affect the design of the input filter. Normally, at least one common-mode inductor will be required.
• Single-switch boost
• Two-phase interleaved boost
Two-switch interleaved topology may result in smaller individual components, but there will be more of them and statistical reliability should be considered. Also the choice of controllers is not as great.
Operating mode
• Continuous inductor current
• Boundary-mode
• Discontinuous inductor current
Discontinuous and boundary mode control both
produce high ripple current in the inductor (2x the average current) which increases the inductor loss and also the conducted EMI. But at low power levels (usually under 100W), these items are manageable and the control method is simple and robust. The inductor usually requires a sense winding for zero-current detection. At high power levels, CCM is the only realistic choice.
Average vs. maximum output power Many PFC's are used in applications where the
maximum rated power output is not continuously required. This may allow some relaxation of the thermal requirements on the inductor.
Auxiliary output requirements
• Voltage
• Number of aux windings
• Isolation
CCM PFC's can very effectively use an auxiliary
winding of a few turns to generate a surprisingly stable DC voltage for operating the controller after startup. Also, it is relatively easy to make the aux winding comply with SELV requirements so a secondary-side control voltage can be provided.
Stray field from the PFC inductor In some crowded applications, the stray field from the inductor may cause a problem. Toroids are not always the best solution in this regard, and some ferrite shapes can be considered (potcore, RM core).
Physical size constraints and mounting
Size can be partly driven by (9) above. Also, when calculating the inductance, it is important to understand that it is OK for the value to swing
downward as much as 50% at maximum current. This is why materials other than ferrite are usually chosen. Ferrite saturates abruptly where powdered
iron, Kool Mu, MPP and related materials do not.
Temperature rise constraints
• Usually the magnetics are not the issue
The inductor in a well-designed PFC is not usually a major loss item. Also, allowing a fair amount of swing may allow a smaller core which will have less core loss.
Cost objective Cost is driven mostly by choice of core material and mounting hardware

The checklist above presents a list of parameters generally applicable to most PFC inductor applications. Contact Precision, Inc. for custom requirements specific to the desired application.

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