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

3 Soft-Start Time Calculation Considerations

The soft-start ramp time can be calculated with Equation 1

Equation 1. C * U = I * t

Reordered to tSS gives Equation 2:

Where:

Equation 2. t s s =   C O U T * U s s _ S t e p I s s _ R a m p

Uss_step: Softstart voltage ramp = Expected voltage after Softstart - Precharge voltage

Expected voltage after Soft-start: typically VOUT

Precharge voltage: For Boost the input voltage for most other topologies 0 V

C_out: total output capacitance

I_ss_ramp: current available for the charging of the output capacitors during soft-start. If there is a contribution of the target load during the soft-start time this needs to be subtracted first. for example, if the SMPS supplied a circuit which only gets enabled after power good is signaling the power up has finished, the load current can be fully used for the startup. In case of an ohmic load which is connected all the time only the Delta been target load current and max load current can be used for that. But in this case still some margin can be used to avoid the system triggers overcurrent