Unexpected ringing on the switch node of Buck converter with heavy load

Khai Lee

Prodigy 180 points

Other Parts Discussed in Thread: LM25088

Hi all.

I designed 20V to 15V,5A buck DC/DC converter with LM25088.

I found the weird waveform on the switch node when the haevy load- 3A above was applied.

Channel 1(yellow) is a switching node voltge and channel3(purple) is a inductor current.

It might look DCM,but it is not. - my guess is some kind of over current protection might be activated.

designed inductor is 6.8u, 7A .

Does anybody know why this happen and how to remove it ?

over 12 years ago

0 Frank De Stasi over 12 years ago

TI__Guru 72305 points

Looks like DCM; probably current limit. Your input supply is wringing. Try more input cap or a more stiff input supply.

0 Khai Lee over 12 years ago in reply to Frank De Stasi

TI__Prodigy 180 points

Thanks Frank.

I ve already checked DCM is normally operated on the light load. However this waveform appears when the heavy load is applied.

Some more details about this phenomenon is that input current suddenly went down from 2.4A to 1.7A as load changed from 3.4A to 3.5A- exactly when this phenomenon occurs

Also, big ripple voltage appears at the same time on both input and output voltage.

0 Jurjen Huisman over 12 years ago in reply to Khai Lee

Intellectual 510 points

When I look to the voltage on the switching node, I think there is an almost perfect sine superpositioned on the block wave of the gate driver. Even when the switch is off, you can see this sine wave continuing.

The frequency seems to be exactly twice the switching frequency.

Could it be some parts are resonating at this point?

Maybe an input inductor for EMC which resonates with the input capacitors?

0 Frank De Stasi over 12 years ago in reply to Jurjen Huisman

TI__Guru 72305 points

It could be; that is why I say maybe more input cap will help.

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Unexpected ringing on the switch node of Buck converter with heavy load