In a simple flyback converter used as a voltage booster, a low-side switch operates at a duty cycle that sets the output current on an output section as shown in Figure 1. In this idealized form, the rectifier diode conducts during the time that the switch is off, and allows output current to flow in the inductor as magnetic energy is transferred to the output capacitor in a unidirectional fashion. When regulating, the switch experiences a flyback peak voltage dV above the 12V supply, where dV is on the order of the supply voltage in most designs.
Figure 1. Basic Flyback Circuit Generating Boost of dV Volts
To make the converter isolated, we replace the inductor with a transformer as in Figure 2 so that the output appears on the secondary side. While the output is now isolated, the magnetic energy transfer is just the same as an inductor. The transformer turns-ratio N is selected to optimize the operation with the specific input and output voltages desired. Here again, the switch experiences a flyback peak voltage dV above the 12V supply. Notice that this circuit cannot prevent the output voltage from being forced above the set-point by an external current (this only supports one quadrant of operation).
Figure 2. Basic Isolated Flyback Circuit Generating dV/N Volts
A synchronous version is created when the rectifier is replaced by another switch as shown in Figure 3. This both improves the efficiency, since the switch will dissipate less power than a forward-conducting diode, and creates a second quadrant of operation because now the circuit has symmetry. This circuit can accept reverse current in the secondary