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

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

PWM Stage

FAN480x offers two operate modes for PWM stage; voltage mode and current mode. Voltage mode could provide more stability for the system but response is slower than current mode. Current mode could provide a fast response but is easily influenced by noise. Voltage mode is operated by setting PWM duty cycle that FBPWM voltage is compared with RAMP pin internal triangle waveform as shown in Figure a. And the duty cycle of current mode is determined by feedback voltage FBPWM and current sense signal on sense resistor which is below the PWM MOSFET as shown in Figure b. Figure c shows the output voltage compensation circuitry. The small signal analysis of the compensation is calculated as equation (17). And the compensation method is also similar to the PFC stage.

Click on image to enlarge.

Click on image to enlarge.