System Demystification II: Understanding Power Supply Inrush Current
Knowledge of a power converter’s input current characteristics can prove useful for determining how many devices can be powered by a given electrical service/outlet without tripping breakers in the power system. Input current ratings given on switch-mode power supply (SMPS) nameplates are worst-case steady state figures and therefor can only provide useful insight into the devices input current characteristics during worst-case steady state operation. SMPS input currents can momentarily exceed these steady-state figures during transient events such as load steps or, more notably, during start up. The transient input current spike that occurs when an SMPS is first energized is known as inrush current. Understanding a devices inrush current is another critical element of determining how the device will interact with protective features in the power system such as breakers and fuses. Excessive inrush currents can also cause premature power supply failures. This month’s technical bulletin will provide an overview of what causes SMPS inrush currents, how they are typically minimized, and what implications they may have on the greater power system.