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LM2743-BJT transistor and Power FET heat up to 140 Celsius

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1886332
Posted by
on Apr 02 2012 08:00 AM
Intellectual695 points

Hi,

I have designed board with National LM2743 regulator with the Webench tool.

You can see the schematics below.

We found that there is a missing short between U17 pin 10 and between C114 capacitor (the right pin).

After the correction, we see that the voltages in the board are OK, and the board is up.

In addition, we see that components Q6 (BJT transistor) and U10 (Power FET) heat up to 140 Celsius.

Can you please advise how can I fix the high temperature issue?

  

Power Mosfet - http://www.infineon.com/dgdl/BSC020N03LS_rev1.25.pdf?folderId=db3a304412b407950112b408e8c90004&fileId=db3a304412b407950112b427641b3c1f

NPN General Purpose Amplifier - http://www.fairchildsemi.com/ds/MP/MPSA06.pdf

THANKS,

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  • EricLee1880267
    Posted by
    on Apr 02 2012 15:16 PM
    Expert5740 points

    Hi

    Would you please post a higher resolution image ?

    Thanks

    Eric

    LM2743
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  • 1886332
    Posted by
    on Apr 03 2012 01:04 AM
    Intellectual695 points

    Hi Eric,

    Is this better?

     

    Thanks,

     

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  • EricLee1880267
    Posted by
    on Apr 03 2012 16:42 PM
    Expert5740 points

    Hi

    Two MOSFET require about 50mA average driving current at 800kHz. Assuming 7V of Q6 voltage drop, it is 350mW and 350mW is enough to heat up Q6 over 100 degreeC.

    Also output load is not mentioned in your post though, I guess your MOSFET selection is now focusing on reducing conduction losses only. Switchig loss might be high enough to heat up your U10.

    Please check the temperatures again with reduced switching frequency and see if any improvement.

    Thanks

    Eric

    LM2743
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  • 1886332
    Posted by
    on Apr 04 2012 02:13 AM
    Intellectual695 points

    Hi,

    The output is 4.7A/0.8V.

    This is the preffered Frequency, do you think that 140 Celsius is a normal temp?

    thanks,

     

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  • CWilson
    Posted by
    on Apr 04 2012 03:47 AM
    Verified Answer
    Verified by
    Expert1730 points

    140 degrees Celcius will never be a good temperature for semiconductors! Normally 125 Celciius is the absolutely maximum for the core of the semiconductor.

    Find a MOSFET which has less Cgs and lower t_rise and t_fall times in order to bring your switching losses down.

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  • Aleksey Tereshchenko
    Posted by
    on Jun 15 2012 11:08 AM
    Prodigy10 points

    From datasheet 

    What voltage you get on TP302?

    Operating Ratings of U17 : LM 2743
    Supply Voltage Range (VCC) 3V to 6V and you pushing 12Volt

    r29 10K closes gate of Q6 when power is on

    D16 regulates gate power for Q6 but if i am correct when gate is closed on Q6  your 12VOLT going to Pin12/SD when it should be 3v to 6v max

    I may be wrong but try to use LM1117MPX-5.0/NOPB http://www.digikey.com/product-detail/en/LM1117MPX-5.0%2FNOPB/LM1117MPX-5.0CT-ND/660149 to regulate Vcc input for U17

    Thanks

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