Synchronous Rectification
Carrying the freewheeling current through the channel of the opposite MOSFET, turned on for the purpose, instead of through a diode. A diode drops typically 0.7–1.2 V, whereas the channel drops only around 0.2 V at 20 A; this reduces the loss in that phase by several times.
During the off phase of the PWM the motor current has to find a path. The simplest solution is the body diode, but the voltage drop across it is constant and it grows the loss linearly with current: 20 A at 0.9 V means 18 W of instantaneous loss.
In synchronous rectification the controller deliberately turns on the opposite MOSFET once the dead time has passed. The current now flows through the parallel channel. With a 10 mΩ channel resistance, 20 A drops only 0.2 V and produces 4 W of loss; less than a quarter of the diode loss.
The condition for this technique is precise timing. If the opposite switch turns on too early, a leg short circuit occurs; if it turns on too late, the diode loss continues. That is why synchronous rectification is always applied together with a correctly set dead time.
For the user, the result is a driver that runs cool. In high-current DC drivers, having this technique means a smaller heat budget at the same current and a longer life.
Where this term is used: DC Motor Driver Selection Guide