2 Charge-mode control

The charge-mode control algorithm is a new control concept: to control an object, you don’t really need to sense it – you can sense its consequence and then indirectly control the object. For PFC, instead of controlling the input current directly, this control algorithm controls how much electric charge is delivered to the PFC output in each switching cycle, and employs a special control law such that the input current becomes a sinusoidal waveform by controlling the electric charge.

There are a few ways to obtain the electric charge information. Figure 3 shows an example of using a current shunt and an operational amplifier (op amp) circuit, with the op amp configured as an integrator. When the PFC boost switch turns off, the inductor current starts to charge the PFC bulk capacitor. The shunt resistor senses this current, which is then integrated through the integrator. The peak value of the integrator output represents the total electric charge delivered to the PFC output in each switching cycle. This electric charge (V_CHARGE) is sampled by the controller as a control-loop feedback signal. The integrator discharges to zero through Q1 before the boost switch turns off.

Figure 4 shows another method, which employs a current transformer (CT) on the PFC output side. The CT output connects to capacitor C1. When the PFC boost switch turns off, the inductor current starts to charge the PFC bulk capacitor. The CT senses this current and its output charges C1. The voltage on C1 rises up; its peak voltage represents the total charge delivered to the PFC output. The controller samples the peak voltage V_CHARGE as a control-loop feedback signal. C1 discharges to 0V through Q1 before the boost switch turns off.

Figure 5 shows the typical signal waveform for charge-mode control.