1 Introduction

An offline power supply requires power factor correction (PFC) when dealing with power levels greater than 75 W. The goal of PFC is to control the input current to follow the input voltage so that the load appears as if it is a pure resistor. With a sinusoidal AC input voltage, the input current also needs to be sinusoidal. In order to control the input current, it must be sensed.

Designers often use one of two current-sensing methods in a PFC application. The first method is to place a shunt resistor at the PFC ground return path (designated as R1 in Figure 1-1) to sense the input current, which is sent to an average current-mode controller [1] (shown in Figure 1-2) to force the input current to follow the input voltage. Because the shunt resistor senses the full boost inductor current, this current-sensing method provides a good power factor and low total harmonic distortion (THD). The shunt resistor does cause extra power losses, however, which could be a problem in applications that require high efficiency.

A second method is to place a current transformer in series with a boost switch to sense the switching current, designated as CT and R2 in Figure 1-1. This method is preferable when the use of a current shunt is not applicable, such as for interleaved PFC [2] and semi-bridgeless PFC [3]. Because the current transformer only senses the switching current (I_Q) (not the full inductor current), to control the full inductor current, a simple solution is to sample at the middle of the current transformer output (the middle of the pulse-width modulation [PWM] on-time). Sampling works because the middle-point instantaneous current value equals the average inductor current value in continuous conduction mode (CCM), as shown in Figure 1-3. This method has fewer power losses than the first method, but it also has limitations: the duty cycle for PFC varies from 0% to 100%. When the duty cycle is small, the PWM on-time is small; therefore, it is difficult to sample exactly at the middle of the PWM on-time. Any sample position offset can cause feedback signal errors and deteriorate both THD and the power factor.

This document introduces a new method – a special peak current mode to control PFC and achieve a unity power factor. This method does not need a current shunt, so power losses are eliminated. And although it still uses the current transformer to sense the switching current, there is no need to sample at the middle of the PWM on-time, so the sample position offset issue goes away. There are additional benefits as well.