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Original question:

PFCLLCSREVM034: Regarding the LLC gain curve selection

PFCLLCSREVM034: Modified to output 30V/5A/150W

Sean Hung
Prodigy 60 points
Part Number: PFCLLCSREVM034

Dear Sir/Madam,

I have bought a development board PFCLLCSREVM034 and tested its performance. It is very good. Then I would like to modify it to output 30V/150W (it was 12V/168W). I didn't change anything of PFC stage,  but I used the design tool, UCC25640x Design Calculator Rev4.0, to calculate the parameters of LLC stage.

After replacing some components, such as transformer, resonant capacitor, resonant inductor (removed), output capacitors (higher rated voltage) and feedback resistor, the development board can output 30V while input voltage is 90Vac, 110Vac and 230Vac.

However, the modified development board is not 100% successful. I am hoping that you might able to give me some advises.

Here are some issues:

1) The transformer of LLC stage worked with high frequency noise. This high frequency occurred while the system output 30V (Vin=90Vac, 110Vac, 230Vac), load - 0~1A.

2) The PFC stage seems not worked well or correctly. Please refer to the measured waveform below: (Tested condition: input: 230Vac, output: 30V 1A). System efficiency is 77.8%

I also measured waveforms of the 400V DC bus and 30V output. they are both stable. The ripple are 3.1% and 3.6% respectively (measured by AC coupling).

 

The replaced components list is here:

Transformer is Wurth Eleltronik #760895641. According to the transformer SPEC, Lp=500uH, LLK=95uH, then using the formula from design tool,

I can obtain: k=0.9, Lr1=50uH, Lm=450uH.  

The LLC Gain curve from the design tool is here:

Could you please have a look if I made any mistake on design and calculation? any advise and opinion are welcome and appreciate!

Thank you!

  • +1
    TI__Genius 9140 points

    Hello Sean,

    can you separated PFC stage and LLC stage during the debug?

    I mean you can use external DC source for PFC VCC supply, and directly add load on PFC bus cap to see whether it is still normal?

    the same process can be used in LLC stage debug and you do not need external DC source for LLC because LLC controller has high voltage start up feature.(for LLC stage debug, you can just remove the PFC mosfet turn on resistor and directly use ac input from input filter circuit with 270VAC,means at this condition we have 270*1.414=382V bus cap).

    if the abnormal is on PFC stage, you can probe PFC inductor current waveform and Vds of mosfet and ZCD/CS pin waveform together and share with us for further analysis.

  • 0 in reply to David Huang33
    Prodigy 60 points

    Hi David,

    Thank you for your suggestion. I was thinking I could do as much test as I can before separating PFC stage and LLC stage. So I have tested the modified board and found some interesting results. could you have a look my test results below?

    1) the measured efficiency and PF curves are below (30V/6A):

    NOTE: I changed the rated output current from 5A to 6A. All components including MOSFET and transformer are safe under this operation. 

    2) the measured efficiency and PF of original development board PFCLLCSREVM034 (12V/14A) are below:

     

    From the above test results, I feel the performance of the modified board including the efficiency and PF are acceptable. Generally, the efficiency of modified board is 2% less than the original board (86%-90% VS 88%-92%). But their rated output are different (high voltage low current and low voltage high current). Maye this might affect the efficiency? And the measured PF values of modified board and original board are very close. then it is not a problem.

    It is very clear that the most serious issue is the high frequency noises from the transformer of LLC circuit. Moreover, here are two minor issues, I have no idea what is the causes

    A) when input voltage is 90Vac, the original board is not able to output rated 14A (12.5A maximum) and the modified board is not able to output 6A (5.2A maximum).

    B) when the input voltage is 264Vac, the PF value of modified board is sometimes between 0.5 and 0.6 only. But this issue didn't happened every time and it can bed solved by turning off input power and turning on again. 

    Regarding the high frequency noise and above two minor issues, any opinions and advises are welcome and appreciate.

    Thank you very much!