PMP41081: How to ensure ZVS when Fs > Fr?

Wayne Huang

Part Number: PMP41081

Tool/software:

Hi champs,

I am asking this for our customer.

The user wonders how TI ensures our LLC reference design operates in ZVS (zero-voltage switch) when Fs (switching freq.) > Fr (resonant freq.) for the primary-side MOSFET especially at light loads or zero loads?

Wayne

over 1 year ago

0 Wayne Huang over 1 year ago

TI__Genius 17155 points

Hi champs,

Any update?

0 Sheng-yang Yu1 over 1 year ago

TI__Expert 4840 points

Wayne,

Just set the minimum switching frequency to be higher than resonant frequency.

Regards,

Sheng-Yang Yu

0 Sheng-yang Yu1 over 1 year ago in reply to Sheng-yang Yu1

TI__Expert 4840 points

Sorry I didn't read the question right. When your operation frequency is much higher than resonant frequency, you lost ZCS on the output synchronous rectifier and the magnetic current not only need to discharge/charge Coss on the primary side MOSFET to achieve ZVS but also need to discharge transformer intra-winding capacitance. So to ensure ZVS and light load or zero load, you need to design/model the LLC taking transformer capacitance and all the Coss into account.

Please refer to slide 30 in my PSDS presentation: https://www.ti.com/seclit/ml/slup377/slup377.pdf

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PMP41081: How to ensure ZVS when Fs > Fr?