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Original question:

LM3429: LM3429 running hot

LM3429: efficiency 80% and low on frequency more than 300khz

Ivan Pupkin
Prodigy 10 points
Part Number: LM3429

I have I problem with over temperature  LM3429 and switching MOSFET and system work with efficiency above 80%. Maximal efficiency is only on 300khz (Rt=82k) on more high frequency,  efficiency is very low.

Topology: buck-boost

Vin: 36V

Iout: 0.23A

MOSFET: Sir696DP

Inductor: CDRH127/LDNP-680MC SUMIDA 2.6A

schematic:

topology:

termal zones:

board:

  • 0
    Prodigy 10 points

    Signal on MOSTET's gate

    Signal on Induction:

  • 0 in reply to Ivan Pupkin
    TI__Mastermind 18890 points

    Hello Ivan,

    The last scope capture that you have is where in the schematic?  I do not understand induction...Is this on the drain of the FET connecting to the inductor?  It looks like you are boosting the voltage to about 55V is that correct?

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 10 points

    Thanks, Tuan

    1) last scope into connection mosfet's Q1 drain and inductor L1

    2) Inductor's CDRH127/LDNP-680MC datasheet: products.sumida.com/.../CDRH127LD (en).pdf

    3) Yes, drain connext to inductor

    4) Yes, boost voltage is 55V

    5) Schematic in other variant:

  • 0 in reply to Ivan Pupkin
    TI__Mastermind 18890 points

    Hello Ivan,

    I have an issue with the last scope capture that shows negative 38V.  There should not be any negative voltage on the board at all.  The FET Q1 when turn ON should show the voltage on the Drain to be close to 0V (small positive voltage).  There is something wrong here that does not make sense to me.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 10 points

    Hello, Tuan

    Sorry, I remeasure diagrams with the help of other equipment. Amplitude measure must be X10, because I use 10 devided probe

    1) Point mosfet Q1 drain and Inductor (amplitude measure must be X10):

    2) Point mosfet gate (amplitude measure must be X10):

    3)Yellow - Point mosfet gate (amplitude measure must be X10)

    Blue - Point gate and Inductor (amplitude measure must be X10):

    4) Point LED + AC mode, filtered DC (amplitude measure must be X10):

    5) Yellow - Point LED + AC mode, filtered DC (amplitude measure must be X10)

    Blue - Point LED + AC mode, filtered DC (amplitude measure must be X10):

    6) Yellow - Point LED + AC mode, filtered DC (amplitude measure must be X10)

    Blue - Point gate and Inductor (amplitude measure must be X10):

  • 0 in reply to Ivan Pupkin
    TI__Mastermind 18890 points

    Hello Ivan,

    The FET that you use is a 125V FET and that is high voltage FET with higher Rdson and Gate Charge for this 55V design.  Please look for a 75V or 80V FET that has lower Rdson for this design.  The lower voltage FET with lower Rdson and lower gate charge will give you better IR loss and switching loss.  The other thing is the layout needs to have a way to get the heat away from the FET otherwise it will heat up more and have higher IR losses due to increase Rdson at temperature.

    Thanks Tuan