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Cascaded DCDC Converters

Discussion in 'Power Electronics' started by stoui333, May 16, 2019.

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  1. stoui333


    May 14, 2019

    I was wondering what are the benefits and drawbacks of the below 2 configurations.
    Let`s assume that:
    - Both DCDC Converters power up different devices.
    - Input and output voltage and current ratings are sufficient.

    Does 2nd configuration provides more filtering on the output?



  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    A general answer can not be given. It depends on the specific voltages from the AC/DC converter and the two DC/DC converters.
    Configuration 2 (bottom) makes sense if the Y output voltage is much lower than the output voltage of the AC/DC converter.

    In Configuration 1 both DC/DC converters must deal with the output voltage of the AC/DC converter which typically would be a bit higher than the X output voltaeg to allow the DC/D converter for X work as a comparatively simle buck converter. However, the converter for Y must then deal with a comparatively high input voltage.

    In configuration 2 the DC/DC converter for Y operates on the already lowered output voltage of the X converter. This allows using transistors with lower Vdsmax and accordingly lower Rdson, thus reducing losses within the DC/DC converter.

    In practise the best suited configuration will have to be evaluated taking into account the specific voltages and currents and also the topologies of the converters.
  3. stoui333


    May 14, 2019
    Thanks for your reply.
    If i got correct the logic the only reason to have configuration 2 is in case Y output voltage is much lower than the ACDC Output one since it reduces the DCDC losses by using better specs components...

    As an example.

    ACDC output: 28v
    DCDC X output: 12v
    DCDC Y output: 5V

    In the above example it makes sense to use configuration 2 but it does not mean configuration 1 won`t work...

    In case DCDC Y is handling devices sensitive to DC noise still conf. 2 is better approach?
    In case DCDC X is handling high power devices is conf. 2 still better choice?

    Not sure why with conf. 2 it allows using transistors with lower Vdsmax and accordingly lower Rdson since the DCDC Y converter will still provide same voltage value either configuration...
  4. BobK


    Jan 5, 2010
    Actually, if you are talking about switch mode converters, I see little reason to ever put them in series. The efficiency is lower since you are going through 2 conversions, and the current capacity of the first converter has to be higher than necessary since it has to handle the power at both voltages.

    The only time I would do it would be if the input voltage was out of the range of the typical DC to DC converter you can get, say 90V. It would be reasonable to use only one converter handling that high voltage. If they are more like your example, the typical low cost converters you can get will handle the input and I would just use two in parallel.

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