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TPS23751 3,7V modification

Jeroen Nonnekes
Prodigy 50 points
Other Parts Discussed in Thread: TPS23751

Hello,


we have a TPS23751 circuit, which is the same as the reference model exept for the feedback circuit. our circuit is designed for 3,8V.

our problem now is that the IC stops gate operation when the input voltage rises above 15V, which makes it unusable for both PoE and aux power input(24V).

i hope someone can help me with this problem, because at this moment we have no idea where this shutdown comes from.

Thanks!

  • 0
    TI__Mastermind 41760 points
    Hi Jeroen,

    Did you change the power transformer as well to accommodate you new output voltage?

    In any case, for 3.3V solutions, in your design 3.8V, at type 2 power levels, you will typically see active clamp forward topologies. I recommend looking PMP7452 to start with, then changing the feedback resistor.
    http://www.ti.com/tool/PMP7452
  • 0 in reply to Darwin Fernandez
    Prodigy 50 points
    no i have not changed the transformer. i did however change the eval board of the TPS23751 to match our output of 3,8V. (changed R36 on eval)

    on our board this configuration does not work. the eval board on 3,8V works on an input of 15V, 24V and PoE. our board stops working when the voltage rises above 14,9V. under the 14,9V we have an output of 3,8V but it is unstable.

    i hope you can help
  • 0 in reply to Jeroen Nonnekes
    TI__Mastermind 41760 points
    Hi Jeroen,

    It seems there might be something different between your boards and the eval boards. I recommend rechecking the components, their values, connections, and impedances with the eval board to make sure they are the same as something might have been connected or placed incorrectly.

    Note also that the EVM/transformer was not designed for 3.8V or an input of 15V. For a custom design I recommend looking at the SLVA305C design guide that has step-by-step calculations for designing a simple flyback.
  • 0 in reply to Darwin Fernandez
    Prodigy 50 points

    thanks for the anwser, but it is not really an anwser on my question. All components have allready been triple checked. so our board is definitely the same as your reference board. so i want to know why it can work for the non spec 15V and not with the spec 24V.

    we know that it was not designed for 15V but for some reason the converter works at 15V and not with your specified 21.6V min.
    there is a very stange threshold there that for some reason disables the converter. because all inputs, responseble for keeping the converter working (CTL, VC) , all indicate more power is needed on the secondary side but the converter is still stalled.

    the only way i get 24V to work is to short R28 and let Vin first drop under the 15V threshold and then i can succesfully turn it to 24V.

    i hope you can tell me what is wrong here. because my measurements all say that it should be working.

    is it possible to simulate this IC in spice??