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PMP20367: Heatsinking Considerations for Output Rectifying Diodes

Mikha Waghmare
Intellectual 410 points

Part Number: PMP20367

Hi Expert,

I am using the PMP20367 reference design for one of my projects. My questions are:

1. Which heatsink part number is recommended for use with the rectifying diodes(D23,D24,D25 and D26) used in the circuit.

2. Do I need to use insulation to isolate the tab and the heatsink from each other.

3. If insulation is not used should heatsink's mounting terminals be connected to the rectifying diode's cathode through the PCB.

Thank you,

Regards,

Mikha.

  • 0
    TI__Intellectual 2610 points

    Hi Mikha,

    I have contacted the designer for this project. He will be in touch.

    I would suggest first that you review the Power Supply Layout topic from TI's Power Supply Designer Seminar. You can find it at www.ti.com/powerseminars. Here is the link to this specific topic. http://www.ti.com/lit/slup230 It goes into discussion on removing the heat from semiconductor devices.

    For the specific heat sink used for D23, D24, D25 and D26, the schematic and BOM suggest 578622B03200G from Aavid.

    More info to come.

    Ryan

  • 0 in reply to Ryan Manack
    TI__Expert 4750 points

    Mikha,

    Yes, you will need insulator to isolated diodes from each other as this design uses full bridge rectifier structure with non-isolated tab TO-220 package.

    Or you can use diode with isolated tab package like MBRF10200.

    If isolator isn't used with a non-isolated tab TO-220 package, you could tie heatsink to D25 and D26 cathode with heatsink shorted to output. You will need to use separate floating heatsinks for D23 and D24.

    I think using isolated tab package diode should work better for you.

    Regards,

    Sheng-Yang Yu

  • 0 in reply to Sheng-yang Yu1
    Intellectual 410 points

    Sheng-Yang-Yu,

    I have the following queries:

    1. The VS-MUR2020CTPBF has an average forward current rating of 20A(both anode terminals combiner) whereas the isolated tab package (MBRF10200) has an average current rating of 10A and

    2. RthJC for MBRF10200 is 4.5C/W as compared to 1.25C/W for VS-MUR2020CTPBF.

    Will this affect the thermal as well as electrical performance of the circuit. Also reverse current of MBRF10200 is higher compared to VS-MUR2020CTPBF.

    Thank you.

    Regards,

    Mikha.

  • 0 in reply to Sheng-yang Yu1
    Intellectual 410 points

    Hi Sheng-Yang Yu,

    I have the following queries:

    1. The Average Rectifed Forward Current IF(AV) for VS-MUR2020CTPBF is 20A(both anode terminals combined) whereas for MBRF10200 it is only 10A.

    2. The Junction to Case Temperature Resistance (RthJC) for VS-MUR2020CTPBF is 1.25C/W whereas for MBRF10200 it is 4.5C/W.

    How will this affect the electrical and thermal performance of the part. Also what changes would be considered for the heat sink.

    Thank you.

    Regards,

    Mikha

  • 0 in reply to Mikha Waghmare
    TI__Expert 4750 points

    Hi Mikha,

    Use the isolated tab package will make the diode junction temperature higher. Using 20A diode could help with reducing power loss on the diode because or a little bit lower VF.

    However, the average current of each diode on an LLC converter is about half of the output current. For 5A output current in PMP20367 design, you have 2.5A average on each diode. Assume VF=0.9V, you have 2.25W loss on each diode. With 4.5degC/W thermal resistance from junction to case, you can expect 10.125degC difference between diode junction and its case. If you check the thermal image of PMP20367 test report, the diode case temperature is about 65degC at 25degC ambient temperature. So the diode junction temperature is ~75degC, which still look good to me. But it really depends on what's the maximum ambient temperature in your design.

    Regards,

    Sheng-Yang Yu

  • 0 in reply to Sheng-yang Yu1
    Intellectual 410 points

    Hi Sheng-Yang Yu,

    Sorry for the delayed response. Yes I will use the Isolated Tab package. Thank you for the help.

    Thank you.

    Regards,

    Mikha.