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Original question:

TPS23753A: TPS23753A flyback design

TPS23753A: TPS23753A flyback design

Pjs
Intellectual 890 points
Part Number: TPS23753A

Hi, 

I've been using the SLVA305C app note to calculate the required transformer parameters for a design. If I run through the application example then I get very similar values to those calculated. 

However, the application I'm using does not need the voltage adaptor and will just be using PoE voltages of 37V-57V and so I've adjusted the Vflyback(min) voltage to be 37V to reflect this. I noted that doing so caused the transformer primary inductance defined in 2.6.5 to increase substantially.

What I don't understand is, given 2.6.5 suggests the inductor calculation is for a minimum inductance, why does decreasing the input voltage span that the flyback needs to work over cause the inductance to go up and be more constrained? 

It sounds like a similar issue may have been reported in the post I've linked so is there an error in the equations or have I messed up the calculations somewhere?

Thanks

  • 0
    TI__Mastermind 41760 points

    Hi Pjs,

    The higher minimum input means a higher di/dt. This means for example, if we use the same transformer (with the same inductance), the minimum output load to keep the flyback in CCM will be higher for 37V than in 20V. This is why the design example suggests to increase the inductance (so that it doesn't operate in DCM at a higher load).

    However, looking at the design with the same transformer as the design example and using a 37V input, the converter wouldn't go into DCM until ~500mA. If you're load is around 2A in normal operation you should be okay.

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    TI__Expert 5615 points
    PJs,

    We think this has resolved your issue. If not, please follow up with additional questions/concerns. If so, please click "Resolved" so that we know (and other readers can know) that this was helpful.

    Thank you for your interest in PoE products from Texas Instruments,
  • 0 in reply to Darwin Fernandez
    Intellectual 890 points

    Hi Darwin,

    Many thanks for your response on this.

    That makes sense but if at higher voltage you get higher di/dt then why does the equation 18 in SLVA305C not consider V(flybackMax) instead of V(flybackMin)? 

    I assume there are no issues with the flyback topology running in DCM but it's just more efficient in CCM? 

    Thanks

  • 0 in reply to Pjs
    TI__Mastermind 41760 points
    Hi Pjs,
    The flyback min is used because you get a largest peak primary current at min input.

    For DCM and CCM that's correct, you will have a higher RMS current (more loss, less efficient) at the lighter loads.

    Thanks!
    Regards,
    Darwin