Synchronous buck circuit diagram with isolated power supply

The example below shows an IC developed for a simple isolated bias supply. Any synchronous buck circuit that allows sink operation can be used. This circuit is called an asymmetrical half-bridge flybuck and operates in a very similar way to a synchronous buck regulator. The FET totem pole output connected to the input voltage supplies the LC filter. The filter output is then regulated via a voltage divider and the negative input of the error amplifier. The error amplifier controls the duty cycle of the FET totem pole output to maintain the DC voltage at the sense point.

The voltage across C6 is equal to the duty factor multiplied by the input voltage. As with the buck power stage, the voltage-second of the inductor must be zero. However, this circuit adds a coupled winding to the inductor and uses a diode to correct the reflected inductor voltage when the low-side FET is turned on. Since the inductor voltage during this period is equal to the output voltage, the output of the circuit will be regulated. However, the difference in voltage drop between the primary and secondary sides will reduce the regulation effect. In this circuit, the voltage regulation of the load will be affected by the forward voltage drop of diode D1. If the diode is replaced with a FET, the load regulation effect can be improved.

Figure 1 Synchronous buck circuit provides isolated power supply