Current-Loop Compensation

The FAN480x employs two control loops for power factor correction: a current control loop and a voltage control loop. The current control loop shapes current based on the reference signal from the IAC Pin 2. The voltage loop stabilizes output voltage and defines THD balance. The figure below shows a simplified block diagram of the current control loop. The PWM block is comprised of a comparator, flip-flop and output MOSFET driver. The voltage-controlled voltage source combines input voltage source, rectifier, MOSFET, and boost diode. The current control loop is closed around the L₁R₅ pole, essentially eliminating the inductor from consideration during voltage control loop analysis.

Current Control Loop Click on image to enlarge.

The system plant of the current loop could be calculated by small signal analysis as equation (2).

Where V_RAMP is 2.55V.

The figure below is the frequency responses of current-loop where G_{PWM_Boost} is the system plant of the current-loop, G_{PWM_Boost_fc} is the frequency response of current error amplifier compensation and G_Close is loop gain of the current-loop. The current-loop bandwidth is determined by the crossover frequency (f_c) of loop gain G_Close. The gain of current-loop compensation in f_c is calculated as shown in equation (3).

The Frequency Responses of Current-loop Click on image to enlarge.

The figure below shows the bode plot of current-loop loop gain in 300W PC Power.

Bode Plot of Current-loop Loop Gain in 300W PC Power Click on image to enlarge.

The current loop compensation network includes an original pole that the system has no steady-state error; a zero to increase the bandwidth and phase margin; and a pole to reduce the high frequency disturbance. The crossover frequency (f_c) of loop gain should be 1/6 to 1/10 of switching frequency. Adjusting the zero (f_Z) and the pole (f_P) to a proper value could stabilize the system and has a better transient response, so that the zero is recommended 1/10 of crossover frequency and the pole is about 10 times of crossover frequency.