Abstract

During the definition of a switched mode power supply (SMPS) the most typical application scenario is the steady state to calculate and define the operation parameters and the configuration, for example, the inductor, output capacitors, operation mode. But before the DCDC converter reaches its first steady state operation point it needs to power up. Without taking certain considerations the high inrush current to charge up the output capacitor would typically trigger the overcurrent limit and force the system to restart. This can lead to a system which could even not start up if the output capacitor fully discharge till the next restart cycle is initiated.

The inrush current needs to be split into the current which flows into the input capacitor and the current which is going to the output side of the SMPS. The output side then again splits into the current to charge the output capacitor and the 2nd part which is required for the application. Esp. due to the relative high output capacitors typically required, a high current is flowing into them during the charge up phase until the voltage has reached the required output voltage level.

To overcome this a so called soft-start circuit should be utilized. In below example the LM5155 will be used but this will apply to most SMPS (Switched Mode Power Supply) devices.

The report will discuss several aspects on how to define the soft-start time.