SNVAA44 july   2023 LM5155 , LM5155-Q1 , LM51551 , LM51551-Q1 , LM5156 , LM5156-Q1 , LM51561 , LM51561-Q1 , LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581

 

  1.   1
  2.   How to Implement the Soft Start in a Flyback
  3.   Trademarks
  4. 1Overview
  5. 2Design Considerations
  6. 3Soft-Start Time Calculation Considerations
    1. 3.1 Soft-Start Capacitor Selection for the LM5155x
  7. 4Soft-Start Time Within Different Topologies
    1. 4.1 Buck Topology
    2. 4.2 Boost Topology
    3. 4.3 Sepic Topology
    4. 4.4 Flyback Topologies
      1. 4.4.1 PSR Flyback: Bipolar Transistor as Pull Down for COMP Pin
      2. 4.4.2 Secondary Side Soft-Start Components
  8. 5Fixing Soft-Start Issue
  9. 6Summary
  10. 7References

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.

GUID-20221025-SS0I-WPBX-GN0S-8XQZ3BQX2R0Q-low.svg 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.