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LM51551: Soft Start Calculation

Part Number: LM51551
Other Parts Discussed in Thread: LM5155, LM5155EVM-FLY

Hello

We are designing with the LM51551 in a flyback configuration and have a question the on soft start calculation.

We have 2.2uF connected to SS pin, then going to a BJT gate connected to COMP_PRI (image below).

Normally we would have expected a soft start time of about 0.22usec, however we're seeing only 0.03 seconds. 

The calculation in the datasheet is tss = Css / Iss with Iss = 10uA.

Could you please let us know?  Thank you, Keith

  • Hello Keith, 

    Thanks for reaching out. I see that you are implementing your soft start on the primary side. Is this an isolated flyback? If yes, Is this bjt also connected to the optocoupler? Can you please share the complete schematic and your Vin, Vout and Iout values? 

    Kind regards, 

    Elisabetta 

  • Hello Keith, 

    Since we discussed this via email, I will now close this thread. 

    Kind regards

  • Hello, 

    We found that the ramp rate of the soft start in flyback configuration is not linear.

    It may be better to implement soft-start on the secondary side.

    The bottom diode is there to charge the cap as soon as the optocoupler starts conducting, the top one is there to discharge the cap to be immediately ready for a second SS event. The best approach would be to fine tune this circuit in the lab taking into account the way you are starting up the circuit (w or w/ load for example).  You need quite a large value for the resistor on top of the secondary ss cap.  I would suggest 100k (see the LM5155EVM-FLY), please try with this value and see if it gets better. You might want to check this with the PSPICE model of the LM5155 for example (if you do not have the EVM available), here the results I got some time ago simulating the test bench of the EVM available on ti.com

  • Hello Keith, 

    Thanks for sharing our discussion in this thread and making it available to other users! 

    Kind regards, 

    Elisabetta