LM5180: LM5180 light load unstable switching frequency

Morty Xu

Intellectual 1640 points

Part Number: LM5180

Customer feedback LM5180 input 48V output 15v & 7.5v,

the following waveform is loaded at 15V&30mA +7.5V&100mA SW voltage waveform,

the switching frequency is not fixed at 12kHz, but mixed with about 350kHz, what is the reason?

ps: When 15v outputs 100mA current, the switching frequency can be stabilized

Transformer parameters: primary side inductance 21uH, leakage inductance 0.5uH, turns ratio 13：4：4.

Below is the actual schematic from the customer：

over 2 years ago

0 Andrew Kutzler over 2 years ago

TI__Mastermind 23935 points

Hi Morty,

My initial suspicion is that the customer may need to add a minimum load in order to stabilize the design. Could you have the customer fill out the calculator tool?

https://www.ti.com/tool/LM5180DESIGN-CALC

Thanks,

Andrew

0 Morty Xu over 2 years ago in reply to Andrew Kutzler

TI__Intellectual 1640 points

I have noticed that the primary side value of the customer's transformer is 21uH < the calculated 39uH, try to ask the customer to replace the transformer.

Close this question for now.

Thank you for your answer.

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LM5180: LM5180 light load unstable switching frequency