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WEBENCH® Tools/LM5175: LM5175 Problems with lead battery and supercapacitor charging

Andrey Yakimov
Prodigy 190 points
Part Number: LM5175
Other Parts Discussed in Thread: LM5176

Tool/software: WEBENCH® Design Tools

Hi Team,

I'm developing lead battery and supercapacitor charging device based on LM5175.

10-16V IN   …..  0-15V 6A OUT

To make preliminary calculation I use WEBENCH® Designer LM5175.

CCM - mode; DITHERING - disable; aux. BIAS supply - 10V; Frequency 100 KHz;

Overcurrent Protection 5mOhm; Output Current Limiting 6mOhm; 

If I connect resistive load (lamp or resistor) - the device works correctly in all modes.

Efficiency 92-97% and minimum power dissipation of all active components - they are a little bit warm.

But if I connect discharged lead battery 12V 60AH or  discharged supercapacitor 300F 16.8V the device is working, but power dissipation of transistor HDRV2 and DIODE in BUST SIDE becomes 10 times more!  (VD8 and VT3 in my schematic )

As a result - transistor and diode are incredible HOT! After 15 seconds I have turn off the device - otherwise the diode will be destroyed!   I see this effect in all 3 modes  buck, bust and buck-bust. My schematic is attached.

Can you help me to arrange this problem?

Thanks!

TEST-4.pdf

  • 0
    TI__Mastermind 21720 points

    Hi Andrey,

    I think that when you connect the super capacitor or the battery the average current loop is engaged. The average output current loop is engaged because the output looks like a short for both of these loads. When this happens during star-up the LM5175 will operate in diode emulation mode (DEM) and the boost leg high side FET will never switch. This means the that current is flowing through the body diode of the MOSFET and explains why it is getting hot. There are two solutions.

    1. Add a schottky diode across the boost leg high side MOSFET. The one you have selected is only rated for 2A of forward current. This is probably why it is getting hot. 

    2. Change the IC to the LM5176, which is pin compatible with the LM5175. This has the functionality  operaterate where the boost high-side MOSFET will switch when the super capacitor or the battery are connected.

    Please let me know if you have any questions.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 190 points
    Hi Garrett,

    Thank you very much for detailed answer.
    I added a schottky diode across the boost leg high side MOSFET. Nice, big and strong - VS-42CTQ030SPBF 40A 30V 480mV . I placed this part close to the MOSFET to avoid long routes.

    Then I start to charge the super capacitor 300F from 0 to 15V with 6A Output Current Limiting as at previous time. MOSFET and DIODE are not so hot, they are a little bit warm - OK.
    The device in BUCK mode works perfectly.
    The device in BUCK-BUST mode works perfectly.
    The device in BUST mode works perfectly.

    But when output voltage is about 11.55 V, BUCK-BUST mode should switch to the BUST mode. (Vin12V) At this moment the device is unstable, output current drops from 6 to 4A. After third charging during such unstable situation BUCK leg was destroyed and LM 5175 was also destroyed. What can be a problem?
    May be it’s necessary to use only LM 5176 when the super capacitor or the battery are connected?

    Thank you and
    best Regards,
    Andrey
  • 0 in reply to Andrey Yakimov
    TI__Mastermind 21720 points

    Hi Andrey,

    I sounds like the inductor peak current limit is probably being triggered. In transition mode (VIN ~= VOUT) the LM5175 current limit is normally really close limiting at heavy and full loading conditions. This can be fixed by decreasing the value of the current sense resistor to increase the peak current limit. The LM5176 has a slightly different current limiting implementation, and should allow for the full load current to be provided when VIN is close to VOUT. Based on this I would suggest using the LM5176 for a charging application such as the one you are working on.

    Thanks,

    Garrett