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Overheating MOSFET w/ LM5072

James Autry
Prodigy 30 points
Other Parts Discussed in Thread: LM5072

I have put together a POE PD design based on the 12W LM5072 reference design and a custom transformer which would allow for a 24v secondary which I regulate on.  The basic functionality works, but the MOSFET SUD15N15 overheats on a very basic load of 100ma let alone the 500ma which I need it to operate it at.  Just looking for some suggestions on how to attack this, I know I must have something set up wrong, and was considering moving to the 25W design.  I am including the gate-source and source-drain waveforms for reference.  The gate source is at 5v/div .05us/div, the drain-source is 50v/div .05us/div.

  • 0
    TI__Mastermind 40100 points

    Hi James,

    What reference design are you referring to? Do you have a schematic? What topology is this?

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Prodigy 30 points

    I was basing it on the AN1455 reference design, but I needed 24v .5ma output instead of the 12v.  I am using a custom transformer provided by XFMR Inc. who claimed it was compatible with the LM5072 but noted the resistances were higher which indicated lower gauge wire in all the windings.  I tried to add the pnp transistor on the gate to compensate for the gate capacitance as is used in the 25W reference design TI released for the IP camera, but it did not seem to want to work at all using the SUD25N15 MOSFET instead of the SUD15N15, not sure why yet.  In any case, using the PNP at the gate of the SUD15N15 seemed to clean the signal, but the unit still gets almost too hot to touch when 100ma is drawn from the secondary                     

  • 0 in reply to James Autry
    TI__Mastermind 40100 points

    Hi James,

    Changing the output voltage thus changing the transformer, thus changing the turns ratios, thus changing the primary currents and so on means you may have to change some components on your board.

    I recommend you look at one of our design app notes SLVA305C. This is a step by step guide to a simple flyback (non synch) design that will help when you do component selection.

    www.ti.com/lit/an/slva305c/slva305c.pdf

     

    ‎
  • 0 in reply to Darwin Fernandez
    Prodigy 30 points

    That was a good read, thanks.  An issue I am facing is that the reference winding is typically reverse polarity to ground, but my supply seems to only operate with the connection reverse from the reference design, not sure why.