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

4.4.2 Secondary Side Soft-Start Components

In the case of a secondary side regulated isolated flyback, see Figure 4-2, a mechanism on the secondary side is needed as well in addition to the capacitor on the SS pin and it comprehends:

  • One capacitor (CSS,Sec)
  • One resistor to slowly keep the capacitor charged during operation (RSS,Sec)
  • Two diodes: one to allow charging the cap and one to allow its discharge (DDIS, dual package)
  • One resistor to determine the RC time constant (ROPTO,Diode, in series with optocoupler)

Until the optocoupler is not conducting (Vout from 0 V to around 1.5 V) , the cap on the SS pin is limiting the peak current in the system. As soon as the optocoupler starts conducting then the soft start on the secondary side takes over. In this case the capacitor on the secondary side gets charged through the bottom diode and discharged thanks to the upper diode in order to be ready for a successive soft start event if needed.

The RC time constant is given by the capacitor on the secondary side itself (CSS,Sec) and the resistor in series with the optocoupler (ROPTO,Diode).

GUID-20230705-SS0I-KRDN-VWBS-18NQNFLNJJ8F-low.svg Figure 4-2 Secondary Side Soft Start on Flyback

Once current starts to flow through the opto, the time constant formed by the secondary SS cap and the resistance that is in series with the opto will control the remainder of the startup time. This is because these two elements then control the feedback current through the opto. Once the output voltage reaches the final value, the TL431 takes over and controls the feedback current through the opto. At this point, the resistor connected from the secondary SS cap to VOUT continues to charge the secondary SS cap so that it can no longer interfere with the loop. This resistance is usually very large, so that it does not affect the startup time.

When this circuit is not used, like in the PMP22151 for example, the primary SS cap will completely control the start up time. However, this often results in over-shoot of the output voltage during startup. (This is the main reason to use the secondary SS circuit.) The over-shoot happens because the TL431 amplifier is in saturation while the primary SS is in control of the loop.