POWER FACTOR CORRECTIONPOWER FACTOR CORRECTION

Design considerations for optimizing performance & cost of continuous mode

boost PFC circuitsby

Supratim Basu,Tore.M.Undeland

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 filterInductor stores energy to maintain

conduction throughout half cycleHence reduces harmonic distortation

and improves power factorBut 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 CircuitsMagnetic Snubber CircuitsPower Switch types - IGBT or MOSFETBoost Diode Options

SiC Schottky DiodesSingle Package Series connected diodesPFC 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

Diode TypePart

NumberRating

TypicalRecovery

timeSupplier

Cost in USD

Sic Schottky SDT12S60 12A, 600V Zero Infineon 7.69

Single packageseries

connectedDSEE 808CC 10A, 600V 30ns IXYS 2.46

Single packageseries

connectedSTTH806TTI 8A, 600V 30ns ST Micro 1.82

PFC Specific 1SL9R1560P2 15A, 600V 25ns Fairchild 1.42

PFC Specific 15ETX06 15A, 600V 18ns IR 1.03

Experimental Results

PFC Specific DiodeSingle Package

Series ConnectedDiode

SiC SchottkyDiodes

Input AC Voltage(RMS)

85 – 264V 85 – 264V 85 – 264V

SwitchingFrequency

100kHz 100kHz 100kHz

Input Power (W) 652 1076 653 1078 642 1049

Output Power (W) 600 1006 598 998 597 1001

Efficiency 0.92 0.935 0.915 0.925 0.93 0.954

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 4dBV

Low frequency conducted emission

SiC DiodeSiC Diode Single package series connected diodeSingle package series connected diode

PFC Specific DiodePFC Specific Diode

High frequency conducted emission

Single package series connected diodeSingle package series connected diode

PFC Specific DiodePFC Specific Diode

SiC DiodeSiC 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