1 Introduction

In a control system, if you want to control something, you need to sense it; this applies to power factor correction (PFC) applications as well. In offline power supplies with power levels >75W, PFC controls the input current to create a sinusoidal waveform (in other words, following the sinusoidal input AC voltage). In order to control the input current, it needs to be sensed.

The most common current-sensing method places a shunt resistor at the PFC ground return path (designated as R in Figure 1) to sense the input current. The sensed input current signal (I_SENSE) is then sent to an average current-mode controller [1] (shown in Figure 2). Because the current reference (I_REF) is modulated by the input voltage (V_IN), it is a sinusoidal waveform. The control loop forces the input current to follow I_REF, thus achieving a sinusoidal waveform.

Almost all continuous conduction mode (CCM) PFC controllers use traditional average current-mode control. Although traditional average current-mode control achieves a good power factor and has low total harmonic distortion, it also has some limitations, especially in totem-pole bridgeless PFC. This article presents a brand-new control algorithm: charge-mode control [2].