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LM5022: LM5022 Flyback circuit

Charge Kuo1
Prodigy 70 points
Part Number: LM5022
Other Parts Discussed in Thread: PMP9253

Hi Sir

I used the LM5022 be the flyback converter.

Attached is my circuit.

SCHEMATIC1 _ 05. FLYBACK.pdf

When I saw the TI document, the D4 and D5 are DNP.

Here is the circuit which cut from PMP9253.

If there is no diode and when MOS is turn off, where the current flows?

So I added one resistor, one capacitor and a diode.

Also could you help me to check the whole circuit?

The input voltage is from 18~52V, and output V/A is 5V/4A.

BR

Charge

  • +1
    TI__Expert 7450 points

    Hi Charge,

    Thank you for selecting LM5022.

    The R24, C43, D19 you added is good. However, it is not for the main current flow during MOSFET off, but for the leakage current flow during the MOSFET turn off transient so that the voltage spike on the SW can be reduced. So in the PMP9253, the D4 and D5 is DNP but later if they want to reduce the SW spike they can put them on. The main current flow will be zero on the transformer primary side input when the FET is off. The secondary side output current will all come from the magnetizing current. This is also why the Flyback transformer has a smaller designed inductance rather than the large uncontrolled inductance in AC transformer.

    Please let me know if you have any question.

    Thanks,

    Yinsong