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Original question:

LM5156: LM5156-flyback converter schmatic review

LM5156: DCM design tool

DANIEL PATENAUDE
Prodigy 50 points
Part Number: LM5156
Other Parts Discussed in Thread: LM5155

Are there any tools to design a DCM flyback converter out of LM5156.

  • 0
    TI__Intellectual 2790 points

    Hi Daniel,

    Thanks for reaching out. If you wish to use a more specialized tool for the LM5156 that lets you determine the values needed for the tuning of the DCM control loop, the filter inductor and much more: I'd recommend you to look at the LM5156 Flyback Controller Quick Start Calculator.

    Furthermore, could try using Power Stage Designer for gaining insight into the topology. This is a very valuable tool for any power design.

    Thank you and best regards,
    Bryan

  • 0 in reply to Bryan Radix
    Prodigy 50 points

    Hi Bryan,

    The LM5156 Flyback Controller Quick Start Calculator is only for CCM designs.  Also, although the Power Stage Designer has a DCM topology, once you go in the loop calculator, it is limited to CCM design as well.  So this doesn't give me any insight on designing a DCM flyback with LM5156.

    Regards,

    Daniel

  • 0 in reply to DANIEL PATENAUDE
    TI__Intellectual 2790 points

    Hi Daniel,

    Excuse me, that was a wrong assumption on my end about the Quick Start Calculator. I'll get back to you once I've found some fitting resources.

    Thank you and best regards,
    Bryan

  • 0 in reply to Bryan Radix
    TI__Intellectual 2790 points

    Hi Daniel,

    I've found something else. This flyback calculator also supports DCM.
    It supports both isolated and non-isolated compensation.

    If this is still not what you're looking for, or if you have any specific questions about the design of a DCM flyback converter, please let me know

    Thank you and best regards,
    Bryan

  • 0 in reply to Bryan Radix
    Prodigy 50 points

    Hi Bryan,

       The calculator you provided is a generic flyback calculator.  I have no issue with figuring out the transformer I need, the FET and diode, and compensation components on the error amplifier.  My issue is with the LM5156 and its current mode compensation.  I'm not as used with current mode compensation and this chip seems to have a fix slope compensation and a programmable slope compensation.  I'd like to know how to set that up (or is it even required in a DCM flyback).  I figure I need to set the Rsense to a value that makes sense, but I'm not sure what to use.  That is where I need help.

       Thanks, and best regards,

    Daniel

  • 0 in reply to DANIEL PATENAUDE
    TI__Intellectual 2790 points

    Hi Daniel,

    Thanks for the clarification, now I know where to help you with exactly. I'll get back to you later today with some information.

    Thank you and best regards,
    Bryan

  • 0 in reply to Bryan Radix
    TI__Intellectual 2790 points

    Hi Daniel,

    Excuse me for the late reply.
    Like you can see in figure 9-18 of the datasheet, there are three optional elements on the CS pin.

    As described in section 9.3.7 of the datasheet, the fixed slope compensation is always present, but the programmable slope compensation can be programmed using Rsl. How the programmable slope compensation is added to the fixed slope compensation can be seen in figure 9-19.

    Furthermore, in 9.3.8 of the datasheet, the process for optionally picking Rf and Cf for creating an RC filter is described, to possible reduce the large leading edge spike of the current sense signal.
    So simply said, if you add footprints to your design for these three optional components, you will have an increased possibility of flexibility if you would need it.

    Talking about the sense resistor, this application note goes into the choice of a sense resistor in a similar device, the LM5155. Look at section 3.3.
    There you see it will depend on your peek inductor current. Which I assume you have already calculated in DCM.

    Thank you and best regards,
    Bryan