User`s guide

Introduction

FIGURE 1-9: BUCK/BOOST PFC

FIGURE 1-10: BUCK/BOOST PFC INPUT CURRENT SHAPE

Regardless of the input line voltage and output load variations, input current drawn by

the Buck converter and the Buck/Boost converter is always discontinuous. However,

when the Boost converter operates in Continuous Conduction mode, the current drawn

from the input voltage source is always continuous and smooth as shown in Figure 1-8.

This feature makes the Boost converter an ideal choice for the Power Factor Correction

(PFC) application. In PFC, the input current drawn by the converter should be

continuous and smooth enough to meet Total Harmonic Distortion (THD) specifications

for the input current (ITHD) such that it is close to unity. In addition, input current should

follow the input sinusoidal voltage waveform to meet displacement factor such that it is

close to unity.

1.2.2 Phase-Shift ZVT Converter

A Full-Bridge converter is a transformer isolated Buck converter. The basic schematic

and switching waveform is shown in Figure 1-11. The transformer primary is connected

between the two legs formed by switches Q1,Q2 and Q4,Q3. Switches Q1,Q2 and Q4,

Q3 create a pulsating AC voltage at the transformer primary. The transformer is used

to step down the pulsating primary voltage, as well as to provide isolation between the

input voltage source and the output voltage V

OUT. A Full-Bridge converter configuration

retains the voltage properties of the Half-Bridge topology, and the current properties of

push-pull topology. The diagonal switch pairs, Q1,Q3 and Q4,Q2, are switched

alternately at the selected switching period. Since the maximum voltage stress across

any switch is V

IN, and with the complete utilization of magnetic core and copper, this

combination makes the Full-Bridge converter an ideal choice for high input voltage,

high-power range SMPS applications.

Input Current

Input Voltage