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LM5146-Q1: Buck Converter

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Replies: 21

Views: 218

Part Number: LM5146-Q1

Hi Ti,

I am using LM5146-Q1 for 225W converter i.e VIN 40-60V and Out 15V and 15Aamp.

I have used high and low side mosfet in parallel.

need suggesting what will be the gate resistance ?

Do i need to use separate resistor to each MOSFET or 1 resistor is enough for parallel MOSFET? what is the resistor value need to be consider?

By the thermal camera i could see the gate trace for lower MOSFET feel temperature rise do i need to increase the resistance or any pull down resister need to use.

Another issue i have, at no load the low side 1 MOSFET have temperature rise more than other  , while i kept the some load both mosfet temperature came equal or start cool.

 

1 time the LM5146-q1 get hot and it went bed.

Please suggest.

  the upper mosfet is CSD19537Q3

and low: FDWS86068-F085

  • Hi Dinesh,

    See the LM5145 EVM (5V/20A/225kHz) design as it is similar to this applicable. Please use 5 x 6 mm MOSFET packages for this application as it provides much better thermal performance.

    Also, please complete and send the LM5146Q1 quickstart calculator so we can review power stage component power losses. The file is available in the product folder. In terms of gate resistance, a 1 or 2.2Ohm in series with the boot cap is sufficient (nothing on the low-side FET).

    Regards,

    Tim

  • In reply to Timothy Hegarty:

    Hi Tim,

    I have attached as pdf,

    waiting for the comment on my previous 3 questions:

    1. :>   Do i need to use separate resistor to each MOSFET or 1 resistor is enough for parallel MOSFET? what is the resistor value need to be consider?

    2.: >   By the thermal camera i could see the gate trace for lower MOSFET feel temperature rise do i need to increase the resistance or any pull down resister need to use.

    3:> Another issue i have, at no load the low side 1 MOSFET have temperature rise more than other  , while i kept the some load both mosfet temperature came equal or start cool.

     

    1 time the LM5146-q1 get hot and it went bed.

    LM5146-Q1 Quickstart Tool r2 - 48Vin 15Vout 15A 400kHz.pdf

  • In reply to Dinesh Gread2:

    Hi Dinesh,

    You can use separate gate resistors for the high-side FETs (if needed). No resistors are required for the low-side FETs. In my opinion, one correctly chosen high-side FET in 5 x 6 mm package is sufficient given the low duty cycle.

    Looking at your quickstart file, the loop is very much unstable. Please use the recommended compensation components. Also, update the MOSFET parameters so you can review the losses.

    The gate drive traces (HO and SW, LO) should be > 20 mils wide and should not have any temperature rise.

    Regards,

    Tim 

  • In reply to Dinesh Gread2:

    By the way, a 12mOhm inductor dissipates too much power at 15A. Consider using a 4.7uH and lower DCR. For the output cap, make sure any ceramic devices are correctly derated for the output voltage -- enter the correct cap value and ESR in the quickstart to accurately determine the compensation components.

  • In reply to Timothy Hegarty:

    Hello Tim,

    The compensation resistor and capacitor has been taken from WEBENCH and i am not sure which one have to follow excel sheet or webench.

    why at no load the low side 1 MOSFET have temperature rise, while i kept the some load both mosfet temperature came equal or start cool.

  • In reply to Dinesh Gread2:

    Hi Dinesh,

    Please forward the switching waveforms at no load for review. I suspect it is capacitive loss related to gate drive and hard switching at high-side turn off.

    In terms of compensation, please review the recommended compensation values from the quickstart and the associated bode plot to ensure stability.

    Regards,

    Tim

  • In reply to Timothy Hegarty:

    Hello Tim,

    i have take the resistor and capacitor value for companion  from Webench and it is not match to excel sheet .

     Do you suggest not to follow webnch ,used only excel sheet ?

    What about sync in pin , i did not connect to GND.

    i will send wave form soon.

    Thanks,

    Dinesh Gread

  • In reply to Dinesh Gread2:

    Dinesh,

    Use the quickstart as it matches the datasheet equations. Just ensure that any ceramic output cap is correctly derated for voltage.

    The is an internal 20k pulldown on SYNCIN, so the pin is effectively low if left open.

    Regards,

    Tim

  • In reply to Timothy Hegarty:

    Hi Tim,

    i am redesigning my converter and as per suggestion of webench i am using single Mosfet for Top and Single Mosfet for Lower The part number for Top : CSD19531Q5A And Lower is FDWS86068-F085

    Are you suggested same for the converter.

    or i need to use 2 Mos for uper or 2 for lower.

    i am attaching the excel file please share your view .MOSFET name is not able to edit rest of all data match to update given part number.

    Thankyou.LM5146-Q1 Quickstart Tool r2 - 48Vin 15Vout 15A 310kHz.pdf

  • In reply to Dinesh Gread2:

    Hi Dinesh,

    Here are some comments on the quickstart:

    1. Current limit should be ~150% of rated full load current.
    2. Output cap of 880uF is quite high for a 15V output. Use 3-4pcs of 22uF/25V ceramic should be fine.
    3. Note the ESR provided is very high - the ceramics will reduce this dramatically (a few mOhms).
    4. Input caps near the FETs should be ceramic, e.g. 3-4 pcs of 4.7uF/100V.
    5. Use 80V FETs for best efficiency given that Vin-max is 80V (see the LM5145 5V/20A EVM). This would allow one FET each for the HS and LS positions. However, this also depends on max ambient temp spec and availability of airflow.

    Regards,

    Tim