POWER FACTOR CORRECTION Design considerations for optimizing performance & cost of continuous mode boost PFC circuits by Supratim Basu,Tore.M.Undeland

High circulating currents All rectified ac sine wave voltages with capacitive filtering draw high amplitude discontinuous current pulses rich in harmonics , causing: Low input power factor High circulating currents

There are many approaches to mitigate this problem : Passive and Active power factor correction Passive and Active filtering of network Accepting non-sinusoidal voltage / current in the system.

Passive Power Factor Correction Simple inductive input filter Inductor stores energy to maintain conduction throughout half cycle Hence reduces harmonic distortation and improves power factor But size, weight and cost limits it’s application upto 200W

Active high frequency power factor correction Makes load behave like a resistor Near unity load power factor Load generating negligible harmonics

Types of active PFC circuits with Boost converter topologies Hard switched Soft switched using ZVT Discontinuous Conduction mode (DCM) Critical Conduction mode (CRM) Continuous Conduction mode (CCM)

Block Diagram of an active PFC Circuit of CCM Boost converter

Mosfet & Diode switching waveforms showing switching losses

Switching loss reduction strategies RCD Snubber Circuits Magnetic Snubber Circuits Power Switch types - IGBT or MOSFET Boost Diode Options SiC Schottky Diodes Single Package Series connected diodes PFC specific single diodes

RCD Snubber Circuits

Magnetic Snubber Circuit

Comparison of recovery time of various diodes

Comparision of RECOVERY TIME and COST of various diodes

Experimental Results

Effect of Diode Recovery Current on the Switching Current at turn-on The switch turn-on peak current was the lowest for the SiC Schottky Diode and highest for the Single Package Series Diode

Effect of Diode recovery current on Mosfet drain current with a SiC Diode

Effect of Diode recovery current on Mosfet drain current with a PFC specific diode

Effect of Diode recovery current on Mosfet drain current with a single package series connected diode

Conducted EMI generated by the PFC board was measured separately for each of the three diode types: Measurements were made at 90V AC input, 600W output load with a 3mH common mode EMI filter connected at the input circuit Low freq part of conducted emission spectrum ( 150kHz - 1 MHz) is almost unaffected by different diode types High freq part of conducted emission spectrum ( 1 MHz - 30MHz) is affected by diode behavior SDT12S60 SiC Schottky diode generates lower noise Increased EMI caused by STTH806TTI single package series connected diodes is only about 4dBV

Low frequency conducted emission SiC Diode Single package series connected diode PFC Specific Diode

High frequency conducted emission SiC Diode Single package series connected diode PFC Specific Diode

OPTIMIZING PERFORMANCE BY DESIGN - A SUMMARY Power levels < 200W - Critical conduction mode PFC may be considered Power levels > 200W - Hard switched CCM PFC is preferred Power levels < 1000W & sw. freq of 100kHz - PFC specific is the best choice Power levels > 1000W & sw. freq > 100kHz - Higher initial costs of SiC Schottky diodes are justified Higher efficiency or higher sw. freq - ZVT resonant mode boost converter may be considered Power levels < 600W - Older generation Mosfets like IRF460N(IR) could reduce costs w/o affecting performance significantly