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LM5160A: device can't work properly after change the primary side output capacitor

Part Number: LM5160A
Other Parts Discussed in Thread: LM5160, PMP10572


customer use LM5160 as flybuck, found a issue that when they change the primary output capacitor from 22uF*2 to 10uF*2, the output only 7V and ripple large to 1V. I want to know, if there any special requirement for output capacitors?

Meanwhile, this circuit is very typical for customer and they use LM5160 for many MP projects, after change capacitors, some boards can work, some are not. could you help on it?

below is the schematic: the ratio is 1:1, customer reference the PMP10572. customer change C281/C282/C267

  • Betty,

    A Fly-Buck works by transferring energy from the supply to the primary output capacitors on the high-switch cycle. Then the primary output capacitors transfer this energy to the secondary on the low-switch cycle. Reducing the primary output capacitance reduces the amount of energy that can be transferred to the secondary and reducing too much can mean lost regulation.

    The customer probably used an app note, datasheet, or quick-start tool to design the original Fly-Buck. They can use that same tool to determine the required output capacitance.

  • Hi Sam,

    thanks for your reply. I want to confirm that the primary output capacitors only can affect output ripple, and no other functions? 

    customer use 3*22uF before, the circuit can work properly. Can they use 7*10uF instead?

    meanwhile, what's the meaning of lost regulations? 

  • Betty,

    If your output capacitors are too low you may see:

    • Higher primary and/or secondary ripple
    • Output voltage will not be able to hold at the desired regulation voltage.
    • The design may go unstable

    7*10uF should be a fine replacement for 3*22uF.

    Lost regulation means the output is no longer the desired DC voltage. This can mean the voltage is drooping, overshooting, or rippling too much.


  • Hi Sam,

    customer replace the capacitors with 10uF/X7R, it can work properly and X5R can't work. But when the Vin less than 42V, the output only 7V, and when the Vin higher than 42V, circuit can output 12V. do you have some ideas it?
  • Betty,

    Flybucks have limitations on maximum duty cycle based on the leakage inductance of the transformer. See this document, especially figure 4. This may be your issue more than the output capacitance.