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WEBENCH-POWER-DESIGNER: Lower efficiency compared with the Webench generated design

Danilo A.
Mastermind 28590 points
Part Number: WEBENCH-POWER-DESIGNER
Other Parts Discussed in Thread: CSD18540Q5B

Hi Team,

According to our customer the Webench proposed design does not align with the real PCB design. He built the PCB and tested the board. Please see his findings below.

1. The efficiency of the device drops in the actual PCB by 7%. The PCB layout is the generated design in Webench.
2. If the output current is 19A, the Webench recommended two parallel CSD18540Q5B MOSFET but if the output current is set to 20A only one CSD18540Q5B MOSFET is recommended.
3. The efficiency of the device is lower using two MOSFETs compared to a single MOSFET by 2% and both doesn't meet the efficiency in the Webench simulation

Here is the Webench design file

webench.ti.com/.../SDP.cgi

The specifications are:
Vin = 30V
Vout = 5V
Iout = 20A

Please see attached excel file from our customer.

0523.30V_to_5V_20A_BUCK.xlsx

What could be the caused of this error?

Regards,

Danilo

  • 0
    TI__Expert 8365 points

    Hi Danilo,

    We will look into this. I will get back to you soon.

    Thanks & Regards,
    Praveen

  • 0 in reply to Praveen Badiger
    TI__Expert 8365 points

    Hi Danilo,

    Our product expert has answered your above queries through email. The query on FET selection on webench mentioned above in point 2 as follows.

    Q:If the output current is 19A, the Webench recommended two parallel CSD18540Q5B MOSFET but if the output current is set to 20A only one CSD18540Q5B MOSFET is recommended.

    The WEBENCH FET selection algorithm calculates the power dissipation of FETs for given conditions(Vin,Vout,Iout & Frequency). If FET hits the junction temperature threshold (IC Tj Max) then WEBENCH rejects the FET and go for parallel FETs in order to reduce power dissipation. If parallel FET still hits the junction temperature threshold then WEBENCH reduces the operating frequency and selects the FET.(Single or Parallel).

    For 20A load current, Single FET is selected with reduced frequency. i,e 286.63KHz.

    For 19A load current Parallel FETs are selected for maximum frequency( without frequency reduction).i.e 323.2KHz

    Please note that you can select the preferred frequency in advance user option as show below. Kindly let us know if you have any queries.

    Thanks & Regards,

    Praveen