Distributed power systems can oscillate, especially when driving one power supply with another power supply, which could wreak havoc on a system. Thus, keeping a distributed power system operating in a stable manner is one of the main concerns of power system designers and engineers.

There are numerous causes for stability issues in distributed power systems, but many are caused by a lack of understanding of source and load impedance interactions. For example, power supplies look like negative resistors at DC. The power supplies’ job is to keep a constant power output over a range of input voltages. Therefore, as the input voltage is increased, the input current decreases. The phase of the power supply tends toward -180° at low frequencies. If the source impedance is resistive, the phases could be 180° different, and oscillation is possible.

In this example, the source and load have impedances with the same magnitude of 13mΩ and the phases are opposite around 250 Hz, which means the system will oscillate with just a slight change in DC voltage.

Avoiding Oscillation Issues
To proactively avoid oscillation issues, test the impedance of each piece of the system using a frequency response analyzer and visually inspect the curves. You can inspect the source and load individually or combine them, but the easiest analysis is to mathematically add the source and load impedances together and inspect the combined impedance. If the combined impedance goes close to zero, you have a problem.

If you prefer to inspect the source and load individually, see if the magnitude of the impedances ever come close or cross. If this occurs, look at the phases. If the phases are almost opposite (180° different) you may have a problem. If the magnitudes of the impedances cross when the phases are 180° different, you definitely have a problem.

To better understand the causes of oscillation and how to avoid them, download our Distributed Power: What Causes These Systems to Oscillate whitepaper.