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

4.4.1 PSR Flyback: Bipolar Transistor as Pull Down for COMP Pin

In a Primary Side Regulated Flyback (PSR) two soft-start mechanisms are necessary:

  • The capacitor on the soft start pin
  • The PNP bipolar transistor between the soft start pin and the COMP pin as shown in Figure 4-2

The PNP bipolar transistor is necessary to limit the maximum peak current that can be delivered during start up and therefore avoid an overshoot of the output voltage.

GUID-20221215-SS0I-9V47-ZNJQ-3NWFD5KRFMLC-low.svg Figure 4-1 PNP Bipolar Transistor Between the Soft Start Pin and the COMP Pin