4.1 Sensing Primary (Bus) Voltage

The dsPIC33C that is used as a power controller is located on the secondary side. V_BUS is on the primary side, so an isolated sense approach must be used.

A PIC16F on the primary side measures V_BUS via its on-board ADC. This value is used to modulate the duty cycle of a PWM on the PIC16F with a period of 100us. This signal is transferred across the isolation barrier via an optocoupler. On the secondary side, this signal is passed through a low-pass filter (shown in Figure 4-2) and digitized by an ADC on the main dsPIC33C power controller.

Figure 4-2. Isolated sensing of V_BUS from secondary and low-pass filter for PWM