about LLC Startup waveform question

[KUOWENYUNG]

Hello Sir,

I have a question about IFX Primary Side MOSFET Selection for LLC Topology application node.

What is the reason for the red circle below? looks the second duty cycle since Cr charge and discharge current none balance induce hard commutation.

[AZIZ_HASSAN]

Hi,

Thank you for posting your query at Infineon community.

Before answering the query, can you please confirm the following questions,

1) VDS2 is across Q2 switch and Vds1 is across Q1 switch?

2) Is this the steady state waveform or during every starting instance the current is flowing through both switches. 

3) Can you please share the complete waveform with capacitance and inductance values to analyze the shoot through and oscillations.

BR,

AZIZ HASSAN

[KUOWENYUNG]

Hi AZIZ HASSAN, 

Sorry, I was just stud the IFX application note(page.11) and got a question about this wave, hope someone can answer why.

https://www.infineon.com/dgdl/Infineon+-+Application+Note+-+Power+MOSFETs+-+CoolMOS+-+Primary+Side+M...

Thanks!

KUO

[Anshika_G · Accepted solution]

Hi Kuo,

The reason behind your query is in the app note, here the high side power MOSFET Q1 turns on to instantaneously set the voltage of node A and charges the output capacitance of Q2. During the start of the first cycle when Q1 turns on, a large current flows through the two power MOSFETs to set the voltage at node A.
The red color current waveform that you are seeing in first cycle is not passing through the Mosfet, but from the Q2 capacitance, acting as charging current for the output capacitance of Q2.

 

I hope it helps. Please let me know if you have further queries.

Thanks and Regards,
Anshika