The design I am testing is based on the Linear Tech LTC3731 three-phase synchronous buck controller. An example circuit from the data sheet is shown below. I am shooting for similar values, but this diagram shows the components of interest. A more detailed diagram of the control loop can be found in the data sheet. Linear Tech, as well as other vendors, also offer demo boards that can be modified to suit your tastes and allow you to prototype your design before you build your own.
How do we test for stability?
The first thing we want to do before we get very far into evaluating our prototype circuit is test the response of the power supply with a network analyzer. In the design phase, it’s a good idea to put a 50Ω resistor in the voltage divider (near the 9.09kΩ), so that we can attach a transformer to allow the network analyzer to inject signal into the control loop to verify performance. The network analyzer can inject a sweep into the controller to perturb the loop, and measure the gain and phase response of the power supply. This will allow us to measure the 0dB frequency of the power supply, and the phase margin at that point.
If we have low phase margin, or even negative phase margin, our power supply will be underdamped and may oscillate. We should shoot for a phase margin at 0dB of better than 45°. And if our 0dB point crosses at a frequency that is too low, even though we have plenty of phase margin, our power supply may be too slow in responding to transient load changes.
Read More: Stability Testing a Switching Power Supply