Power Tip 39: You get more than just better efficiency by going synchronous
(Editor’s note: to see a linked list of entries #1 to #37 in the Power Tips series, click here.)
Have you ever been asked to design a power supply with good load-transient response at light load? If so, and you allowed the power supply to go discontinuous, you probably discovered that the gain in the control loop decreases greatly at light loads. This can result in poor transient response and the need for a massive output filter capacitor. A simpler approach is to make the power supply continuous at all loads.
Figure 1 is a simple synchronous buck converter. It was used to demonstrate the load transient response with continuous and discontinuous current in the output inductor. The current in the output inductor remains continuous down to no load, because the synchronous rectifier allows the inductor current to flow in the reverse direction at light loads.
This circuit was made to go discontinuous by simply replacing the bottom FET (Q2) with a diode. While this article shows the differences in a buck topology, you will note similar responses in all power supply topologies.
A simple buck was used to demonstrate transient response.
(Click here to see enlarged image)
Figure 2 shows two transient-load responses to a 700 mA step change in output current. The trace on top is the continuous case and the trace on the bottom is the discontinuous case. In the discontinuous case, the transient response was over three times worse than the continuous case. A synchronous FET was used to force continuous operation.
However, there are alternate ways to accomplish good transient response, including preloading the output or with the use of swinging inductors. A swinging inductor is an inductor that is designed to increase in inductance at low current. This is mainly accomplished with the use of two core materials: a high ferrite, which saturates at low currents; and a powdered iron one, which does not.
Figure 2a and 2b: Synchronous operation (upper image, 2a)
provides best transient response.
The reason the transient response suffers during discontinuous operation is that the loop characteristics change drastically. This is shown in Figure 3. The curve on the top shows the loop gain during continuous operation. The control loop has a 50 kHz bandwidth and crosses with 60 degrees of phase margin. The curve on the bottom shows the response when the power stage goes discontinuous. The power stage changes from a pair of complex poles during continuous operation into a single low-frequency real pole during discontinuous operation.
The frequency of this pole is set by the output capacitor and load resistor. You can see how the phase shifts at a lower frequency when compared to the continuous case due to the low-frequency pole. The gain drops significantly at lower frequencies due to the pole resulting in a much lower crossover frequency, which degrades transient response.
Figure 3a and 3b: Significant loop gain is lost in discontinuous operation (lower image, 3b).
To summarize, synchronous rectification improves efficiency and aids immensely in transient load regulation. It provides a high-efficiency alternative to preloading a power supply. It also provides a more consistent control loop characteristic when compared to swinging inductors. It improves the dynamics of a traditional buck as well as all topologies where synchronous rectification can be used.
Please join us next month when we will discuss common-mode noise in a non-isolated power supply.
For more information about this and other power solutions, visit: www.ti.com/power-ca.
About the author