How to Implement the Soft Start in a Flyback

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.

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1 Overview

At the turn on of the SMPS, both the input and the output capacitors need to be charged. While the input capacitor is charged directly from the input supply, the output capacitors are charged through the DC-DC converter. The current needed to charge the capacitors at the output might be much higher than the maximum load current and therefore the overcurrent protection might be triggered causing the system to restart or shut-down.

The overcurrent condition and consequent restart will stress the circuit as the maximum allowable current is flowing through the power stage components. In addition, if the controller is enforcing a dead time before restarting, this might cause the discharge of the output capacitor leading to an endless re-start loop.

Figure 1-1 Soft-start Timing

Typically the soft-start mechanism allows to slowly ramp up the output voltage until the regulation target is achieved. This allows the output capacitors to charge with a defined current. In the case of the LM5155 the reference signal is modified by the voltage level of a charged capacitor (soft-start cap) connected to the soft-start (SS) pin of the device. During the start phase this capacitor is charged by a constant current source and as long as this voltage is below the internal reference voltage, the voltage on the SS capacitor is used for the adjustment of the output voltage. The start up time is defined via the value of the capacitor connected to the SS pin.

One mechanism is to limit the duty cycle of the MOSFET in order to limit the maximum peak current in the system and consequently avoid a big inrush current and onvershoot of the output voltage. Thanks to the soft-start, the MOSFET is not starting at the maximum possible pulse width.