Power Design Tip 39: Synchronous Rectification brings more than just high efficiency (PDF with circuit diagram!)

GW MB10F 0.8A 1000V rectifier bridge

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Have you ever designed a power supply with a good load transient response under light load conditions? If so, and you also allow the power to be discontinuous, you may find that the gain of the control loop drops sharply under light load conditions. This results in a poor transient response and requires a large number of output filter capacitors. An easier way is to keep the power supply continuous under all load conditions.

Figure 1 is a simple synchronous buck converter that demonstrates the load transient response of continuous and non-continuous currents in the output inductor. The output inductor current remains continuous during load conditions as low as no load, because the synchronous rectifier allows the inductor current to flow in reverse under light load conditions. Simply replace the bottom FET (Q2) with a diode and the circuit can be turned discontinuous. Although this article describes the differences in buck topology, you will notice that all power supply topologies have similar responses.

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