# Evaluate and Test Power-Factor Correction NeedsEvaluate and Test Power-Factor Correction Needs

September 10, 2009

Electronic and electromechanical equipment often uses switch-mode power supplies, motors, and other devices that “appear” as non-resistive loads to power-company generators that produce a sinusoidal voltage. These “reactive-load” devices often require power-factor correction, whereas purely resistive loads such as incandescent lamps, steam irons, and electric ranges do not.

The reactive-load devices create a phase difference between the power-line current and voltage. Remember “ELI the ICE man?” Voltage (E) leads current (I) in an inductor (L), and current (I) leads voltage (E) in a capacitor (C). In a perfect system with sinusoidal voltages and currents, you can mathematically relate the phase difference to the ratio of real power to apparent power. (For more on power-factor considerations and PFC needs, see the references below.)

Most switching supplies and motor controllers are anything but perfect and they create harmonics and non-linear conditions that can be difficult to work with, but which designers much take into account. Why should you care? Without power-factor correction (PFC), you cannot draw as much current from the power company as you might expect. And a power factor other than unity causes problems with the power-distribution network itself.

To give engineers a head start, Microchip Technology now supplies a Digital Interleaved Power Factor Correction Reference Design that lets engineers better understand how they might benefit from a digital PFC circuit in their power-supply designs. The reference design incorporates Microchip’s dsPIC33 “GS” series of digital signal controllers. The design operates as a switch-mode power supply with interleaved PFC. Here, the term “interleaved” means the reference circuit alternately operates two boost converters connected in parallel, thus interleaved power-factor correction, or IPFC

On its Web site, Microchip also furnishes a 46-page application note, “Interleaved Power Factor Correction (IPFC) Using the dsPIC DSC,” AN1278, by Vinaya Skanda and Anusheel Nahar. Visit: ww1.microchip.com/downloads/en/AppNotes/01278A.pdf to download the document.

The digital approach offers designers several benefits that, according to Microchip, include: smaller PFC inductors and transformers, higher power densities, lower ripple, and the flexibility of implementing control algorithms in software.

The reference circuit includes input EMI filter and rectifier, a dual-phase iPFC circuit, a fault-detection circuit that protects hardware, and a socket for a plug-in module that supplies a dsPIC33FJ16GS504 controller chip.

You can download files from the company’s Web site to build a complete unit, or you can contact a local Microchip salesperson who can provide a “loaner” unit (see photo). Microchip does not manufacture or sell a prebuilt power supply. The download files include a MATLAB model of the circuit and source code for the dsPIC digital signal controller.

More PFC references:

Wuidart, L., “Understanding Power Factor,” AN824, STmicroelectronics. www.st.com/stonline/products/literature/an/4042.pdf.

–, “Power Factor Correction Handbook,” HBD853/D, ON Semiconductor. www.onsemi.com/pub_link/Collateral/HBD853-D.PDF. Approximately 200 pages!