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Step Down- LM5117

JoaoSarrico
Prodigy 30 points
Other Parts Discussed in Thread: LM5117, LM5117EVAL

Hello,

I am trying to do a step down buck converter that connects a battery pack to a output charge. My battery works from 60 V to 40V as it goes discharging, and I need to get 24V in the output charge, the maximum current is 12A and the working frequency is 100kHz.

When using the lm5117 IC, I only get in the output charge something like 3.5 V instead of the 24V expected.

I changed the working frequency to 20kHz and the charge too, with different values. I start with 100R then 20 R, 8R and 2R and the output had little variation.  I change the inductor value  between 240uF and 20uF and still get the same 3.5V in the output.

I added resistors to the mosfet (IRF451) gates of 4R.

The values were calculated using WEBENCH and the LM5117 Quick Start Calculator given by TI and are described in the figure below. Almost all the components were bought regarding the Bill of Materials generate by the software. The ones that weren´t are Mosfet 451, inductor and Css. The schematic were import to Altium and the board was made from the beginning.

The components that aren´t illustrated on the picture are Cin=10uF, Cinx=100nF and Cout=300uF.

In the last picture I show the gate and Vds of the high mosfet, I think that Vg  is correct but the mosfet isn´t polarized. Can you help me finding a solution?

Best regards,

João

 

 

 

 

 

  • 0
    TI__Guru 58391 points

    Hi

    Need Cvin and Cvcc placed close to the IC.
    Please check RES pin. Your converter might be in hiccup operation.
    Please check input power supply. Your Vin might drop during the start up due to not enough capability.
    Connect AGND to the analog ground (please refer LM5117EVAL EVM layout)
    Negative side of Rsense should be connected to your Cin directly.

    Regards,

    EL

  • 0 in reply to EricLee
    Prodigy 30 points

    Hi, 

    Thanks for the quick answer.

    I replaced Cvcc and put more close to the IC and connect negative side of Rsense to Cin directly.

    The AGND pin is connected to analog ground and I am not using Cvin neither Rvin.

    I’m still getting the same output voltage values 3,5V and the same signal waves in mosfet gate and drain to source. In the picture below is the signal of the Res pin.

    What do you suggest?

    Best regards

  • 0 in reply to JoaoSarrico
    TI__Guru 58391 points

    Hi 

    Please test following the steps below

    #1: Measure VCC voltage. Check the voltage level. Check if any drop during the operation 

    #2: Measure SS voltage. Check the voltage level. Check if any drop during the operation 

    #3: Measure COMP voltage. Check the voltage level. Check if any oscillation during the operation

    #4: Measure V(HO-SW) and V(SW-GND). MOSFET turns on when the driver output is high state? 

    #5:Measure V(SW-GND)and the inductor current. The inductor current rise when the driver output is high state? 

    Regards,

    EL

  • 0 in reply to EricLee
    Prodigy 30 points

    Hi,

    #1: Didn´t noticed a voltge drop during operation.

    #2: Noticed a voltage drop when increase the input voltage.

    #3: Noticed the oscillation represented in figures 6) and 7) bellow.

    #4: Seeing figure 10 I think that MOSFET isn't turning on properly.

    #5: Yes, the inductor current rise when the driver output is high state.

    I tested using two diferent MOSFETs, the IRF451 and 2N6130 MOSFET and with the second one the output voltage rise to about 10V but the signals are with very noise as is possible to see in figure12.

    Below I send the waveforms of the following signals:

    1)Vcc voltage:

    2)Vcc voltage amplified :

    3)Vcc voltage when the IC starts

    4)Vss voltage with 40V input

    5)Vss voltage with 60V input

    6)Vcomp voltage

    7)Vcomp voltage amplified

    8)V(HO-SW) voltage

    9)V(SW-GND) voltage

    10)V(DRAIN-SOURCE) voltage

    11)V(SW-GND) and Inductor current with IRF451 MOSFET

    12)V(SW-GND) and Inductor current with 2N6130 MOSFET

    Regards,

    João

  • 0 in reply to JoaoSarrico
    TI__Guru 58391 points
    Hi
    I think you should check your PC board if any error and check all the component values. Especially, as you mentioned your switch is not working well now. The waveforms in #8 and #9 should be in phase.
    Regards,
    EL