This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5088-2 not regulating properly at no load

Hi,

I designed a DCDC converter using lm5088-2 with specs:

Vin: 44-56V

Vout: 30V

Iout: 1.6A

The converter regulates properly when I apply a load ranging from 0.01A to 1.6A at any input voltage.

however at no load, it only regulates at 23V.

here are some comparisons between significant values at loaded and no load condition.

Vcc wrt gnd: Same

Vboot wrt Vsw: 7.4V (L) drops to 4.3V (NL)

Vss wrt gnd: 1.44V (L) drops to 1.09V (NL)

Frequency: Same

Venable: Same

Vfb: adjusts accordingly when Vss drops -- 1.28V (L) drops to 1V (NL)

Vcomp: 4.8V (NL) drops to 2.2V (L)

I already placed an Rramp as advised in another thread, but it does not improve matters.

It seems that the problem resides in the Vss level. As I understand from the datasheet, the Vref will revert to the Vss level once it drops below 1.205V. Any ideas on why the Vss drops?

Any advice on additional tests?

Thanks

  • Hi 

    My recommendation is to add a dummy load to keep the switching at every cycle. 

    Regards,

  • Thanks for the response.

    Yup, adding a dummy load has already crossed my mind. However I'm being very watchful of the buck's efficiency which is why I've decided against it.

    I have no problems in switching the MOSFET since I am using a logic type which has 3.5V Vgs threshold. My switching frequency also does not change at no load. 

    Here are additional information about my components.

    The divider for my feedback is 1.6k and 39k.

    Output Filter:

    L = 174uH    

    C = 100uF

    Compensation network: 

    R = 180k

    Ccomp = 1.5n

    Chf = 47p

    Switching Freq : 125kHz

    Cramp: 1n

    Rramp: 61k

     

    I really believe that the problem is related to voltage reference that is dependent on the soft start voltage. However, I have no solution for this problem. If you have any other speculations, please let me know.

    Is there any solution besides adding a dummy load?

    Regards