• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 3 answers
  • Subscribers 62 subscribers
  • Views 242 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

LM5146-Q1: Temperature Rise at Low side MOSFET in Parallel Condiation

Dinesh Gread2
Intellectual 330 points
Part Number: LM5146-Q1
Other Parts Discussed in Thread: LM5145,

Hello TI,

i am working on DC-DC converter , VIN : 30 to 60 

Vout : 15 V  @ 15 Amp

i am facing issue on temperature at low side mosfet  more than 10 amp out

While i am use single MOSFET at low side the temp rise on it  approx 89 degree at 14 amp

while i tried to reduce the temperature by adding another mos parallel to lower mosfet , now the condition is worst, it is not helpful to reduce, at 12 amp only the temp rise at low side is more than 90 degree

Could you please let me know what is the reason, and how to solve this issue.

LOw side mos :FDWS86068-F085

  • 0
    TI__Mastermind 49597 points

    Hello, please complete the LM5146-Q1 quickstart calculator for this disgn so you can examine power losses. Also, please see the LM5145 5V/20A225kHz EVM design. Your schematic image is unreadable.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 330 points

    Hi Tim,

    Please find the attached quick start guide.

    Please let me know the resion due to what the LOW side mosfet heat , also suggest the solution

    LM5146-Q1 Quickstart Tool r2 - 48Vin 15Vout 15A 215kHz.pdf

  • 0 in reply to Dinesh Gread2
    TI__Mastermind 49597 points

    Hi Dinesh,

    A few points on the schematic/quickstart:

    The current limit should be ~50% above the full-load current rating to deal with transients, setpoint variation, etc.

    Choose the compensation values based on the quickstart with Cout = 440uF. That amount of Cout seems high for a 15V output. Consider using one electrolytic with numerous 22uF/25V ceramics to conduct the ripple current and reduce noise.

    Tie NC pins of the controller to GND for better heat spreading.

    50kHz crossover is too high for Fsw = 215kHz. Typical is 10-15% of Fsw, max is 20% Fsw.

    Use 80V FETs for best performance ginve Vin-max = 60V. Consider the FETs used in the LM5145 EVM.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 330 points

    Hi Tim,

    i have seen the MOSFET that is 80 V, As ringing will be observed at the drain of lower mosfet and that may damage the MOSFET if it is 80 V.

    Do you still suggest 80V MOSFET ?

    Also let me know why IC is not able to capable to drive the parallel mosfet at low side, with single it is working , i am experiencing high heat while i tried to parallel 2 mos at low side.

  • 0 in reply to Dinesh Gread2
    TI__Mastermind 49597 points

    The controller can drive parallel MOSFETs as long as the total gate drive current (Qg*Fsw) is less than the VCC current limit. I suspect you increased switching losses by adding the paralleled FETs (twice the capacitance) and the increase was more than the reduction in conduction losses. Please confirm the MOSFET part numbers and check the power losses using the quickstart calculator

    My understanding is that 80V FETs should be adequate here. 100V FETs have a big reduction in performance, so 80V is preferable. You can minimize the spike by keeping the input caps close to the FETs (see app note snva803 for power-stage layout best practices).

    Regards,

    Tim