Synchronous rectifier can improve the transient response of switching power supply?

Have you ever designed a power supply that has a good load transient response in a light load state? 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. This results in poor transient response and requires a large number of output filter capacitors.

Have you ever designed a power supply that has a good load transient response in a light load state? 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. This results in poor transient response and requires a large number of output filter capacitors. A simpler approach is to make the power supply continuous in all load states.

Figure 1 is a simple synchronous buck converter that demonstrates the load transient response of continuous and discontinuous currents in the output inductor. The output inductor current is maintained continuously in the low to no load state, because the synchronous rectifier allows the inductor current to flow in a light load state. Simply replace the bottom FET (Q2) with a single diode, and the circuit can be converted to discontinuous. Although this article describes the difference between a buck topology, you will notice that all power topologies have similar responses.

Figure 1 A simple buck converter for demonstrating transient response

Figure 2 shows the two transient load responses for a 700mA step change in the output current. The traces on the left are continuous, while the traces on the right are non-contiguous. In discontinuous cases, the transient response is more than three times times worse than the continuous condition. The synchronous FET is used to force continuous operation. However, there are other ways to get better transient response, including pre-loaded outputs or using oscillating inductors. The oscillating inductor is used to increase inductance at low currents. This goal is achieved mainly through two core materials: low-current saturated high ferrite, and low-current unsaturated powder ferrite.

Figure 2 Synchronous operation (left) with optimal transient response

During discontinuous operation, the reason for the poor transient response is a sharp change in the loop characteristics, as shown in Figure 3. The curve on the left shows the loop gain during continuous operation. The control loop has a bandwidth of 50kHz, with a phase supplementary angle of 60 degrees. The curve on the right is the response when the power level transitions to discontinuous. The power stage changes from a pair of repolarization during continuous operation to a single low-frequency real pole in the discontinuous operation. The frequency of the pole is determined by the output capacitor and the load resistor. Compared to the continuous situation, you can see the phase shift process caused by low frequency poles at lower frequencies. At low frequencies, the gain drops sharply because the poles cause a lower crossover frequency, which reduces the transient response.

Figure 3 Large loop gain loss in discontinuous operation (right)

In summary, synchronous rectification improves efficiency and can also greatly assist in transient load regulation. It provides an efficient method for power preload. In addition, it has a more stable control loop characteristic than the oscillating inductor. It improves the dynamics of traditional buck converters, as well as all other topologies that can use synchronous rectification.