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LM5160A: LM5160A has large drop when output is 5V/1A in flybuck topology

Part Number: LM5160A
Other Parts Discussed in Thread: LM5160

Hi team,

customer use LM5160A for flybuck. the input is 12VDC and output is 5V/1A. there is a problem is that when the load current is 0.8A, output will drop to 4V. 

could you help on it?thank you!

the transformer spec is below:

the schematic is below:

  • Hi Berry,

    Please complete the LM5160A quickstart calculator available by download from the product folder. I note from your schematic that the primary-side output is set at 5.02V. Thus, if the transformer turns ratio is 1 : 1.4, the secondary winding voltage should be ~7V. The diode voltage drop and resistive drop of the secondary winding is then subtracted to obtain the secondary-side output voltage.

    I recommend changing to a 1 : 1 turns ratio transformer or coupled inductor (with a sat current greater than the peak current limit of the LM5160A) and set the primary side output to ~5.6V. The secondary side output should then equal ~5V. Also, it's preferable to use a Schottky rectifier diode to reduce the voltage drop on the secondary side.

  • Hi Tim,

    the turn ratio of transformer is 1.14, not 1.4, so the secondary-side voltage is about 5.7V. Do you have some ideas about large drop voltage?

    I found that in the datasheet of LM5160A, figure 29 load regulation, when the load current is 0.4A, the output voltage has drop about 1.2V. I wanT to know if the LM5160A has a large drop by itself when it work as flybuck?


  • Hi Betty,

    The Fly-Buck converter regulates the primary-side output voltage, so the secondary-side voltage is the primary voltage reflected minus the various voltage drops. You may want to check the rectifying diode forward voltage drop and also the drop related to the secondary winding DCR. Are you using a Schottky diode with current rating of 2A or more?

    Also, lower leakage inductance is helpful (as higher leakage inductance gives a more resonant shape to the secondary current and may increase the effective RMS value).

    Please complete the LM5160 quickstart calculator for the Fly-Buck topology (available in the product folder) so we can review the primary and secondary peak currents.