UCC25600: LLC MOSFET burst at light loads

Please use the calculator below to design your system.

https://www.ti.com/tool/download/UCC25600-DESIGN-CALC

Please share your power stage designer screenshot so I can understand where you get the waveform below 1.6A.

Do you have any measured waveform at burst mode and no load?

Hello Ning,

I've attached a word document below in which you can see the first image shows the input parameters as per our requirement and we have used the component values same as suggested in the tool itself.

And, In the second image for the magnetizing current the minimum load current we can vary is 1.6A and cannot be varied below this.

For the values which were calculated by the tool can we load the circuit below 1.6A?

Because we face issues at lighter loads.

The PDF file attached is the UCC25600 design calculation tool in which I have put the same specifications as in the power stage designer tool but the results of both are very different. 

Please help which document is to be referred.

Reards,

Dilip R

2_ LLC Query.docx  2630.sluc146.pdf

Dilip,

We will need some time to review the information before we can get back to you.

Please share the actual measured waveforms with us, instead of simulation by Power Stage Designer. 

Hello Ning,

I've attached our observation below for different loads.

As we are also debugging at our end, please let us know if you need some other waveforms as well to analyse.

It is seen from the waveforms that the switching frequency is near to the resonance i.e. 100kHz but still Half bridge MOSFETs heat-up to >80 degrees after few seconds for every load,why is it?

The dead time is set to 500ns

Resistor on the RT Pin is set such that :-

• Minimum Frequency is set to 60kHz 
• Maximum Frequency is set to 350kHz

Regards,

Dilip R

3_LLC Query to TI.pdf

Noted, we will let you know if we need more waveforms.

By any chance you have run the calculator to confirm the tank parameter?

Hello Dilip, 

MOSFET heating up over a few seconds time interval implies a thermal dissipation problem. 
What kind of heatsinking do you have for your half-bridge MOSFETs? 
Is it sufficient to cool the losses in your Fets to limit temperature rise?

Regards,
Ulrich 

Hello Ulrich,

We have designed heatsink plates with aluminum which are sufficient for heat dissipation. The heating is due to something else it seems.

• I have used the calculator for UCC25600 and the power designer tool aswell there are some discrepancy between them, the results are shared with you above in the same thread.
• After analysing the waveforms did you find any problem for the behavior of MOSFETS?

Regards, 

Dilip R

Hello Dilip, 

I was not able to determine any cause of overheating from the waveforms.  They look normal for the most part. 
The extra ringing of the current at no-load and 0.5A load suggest that there is another resonance from somewhere. 
I suspect it may be due to winding capacitance either of Lr or of Lm.

The excess power dissipation, even during light load, may be due to loss of ZVS which can be a big problem at your very high input voltage. 
Please check the Vds waveforms of your lower MOSFET to see if ZVS is present or lost.  

Note: I will be out-of-office tomorrow and not available until next week.  I'll try to have someone cover for me. 

Regards,
Ulrich