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TPS53114: Why there is a sudden change in the slope of measured efficiency?

Jayden Li
Intellectual 2695 points
Part Number: TPS53114
Other Parts Discussed in Thread: TPS51116,

Why there is a sudden change in the slope of measured efficiency. For example, when the output current is 2A, the slope of the green curve is obviously changed.

  • 0 in reply to Jayden Li
    TI__Prodigy 400 points

    Hi Jayden,

    The question is tagged as TPS53114, but the plot you're showing is the TPS51116. As described in the TPS51116 datasheet (Section 8.3.1), the controller reduces its switching frequency under light load conditions to maintain high efficiency.

    The sudden change that you're seeing around 2 A is when the converter transitions from discontinuous conduction mode (light load conditions) to continuous conduction mode. The boundary between discontinuous and continuous conduction mode can be calculated from Equation 1 in the datasheet. Hope that helps!

  • 0 in reply to Kevin Bradford
    TI__Intellectual 2695 points

    Hi Kevin

    I'm sorry I mixed up the two products. And thank you so much for your reply.

    Could you please explain why the sudden change exsits around boundary between discontinuous and continuous conduction mode? Thanks.

  • 0 in reply to Jayden Li
    TI__Genius 14455 points

    Hi Jayden

         If the part works in Forced CCM, then efficiency would keep decreasing. At the boundary where the transition happens, DCM takes over and helps keep the efficiency high. As the switching frequency changes between the CCM and DCM mode and so does the nature of the efficiency curve.

    Regards,

    Gerold

  • 0 in reply to Jayden Li
    TI__Prodigy 400 points

    Hi Jayden,

    No worries - just wanted to make sure we were talking about the same thing!

    Basically, in discontinuous conduction mode (DCM), the switching frequency is proportional to the output current. So, for a lower output current, the converter operates at a lower switching frequency, and switching losses are reduced. Because the switching frequency is proportional to the output current, the switching losses represent approximately a constant percentage of output power.

    As the load current increases, the switching frequency increases until the output current reaches the critical conduction boundary (approximately 2 A in the graph above). Once the output current reaches the critical conduction boundary, the switching frequency is held constant, and switching losses no longer represent a fixed percentage of output power.

  • 0 in reply to Kevin Bradford
    TI__Intellectual 2695 points

    Hi Kevin,

    Thanks for your professional reply, I have understood.