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  1. #1
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    Flybuck Design Secondary Voltage not correct

    Hi,

    I'm trying to design an isolated flybuck similar to the example design shown in AN-2292 by TI. My simulation is not turning out with the secondary voltage correctly though.
    My design criteria:
    Need isolated 40W (currently set to 11.5V, 3.5A). Ideally I need a controllable signal also, but starting with the basics...

    My LTSpice circuit looks like this:
    Click image for larger version. 

Name:	FlyBuck Help.png 
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    I am subbing in a similar schottkey for the one listed. I want to use a Coilcraft coupled inductor with low DCR. Set to operate at 900kHz currently.

    A few questions:

    I am not getting 11.5V or my 3.5A on the secondary output and am not sure why. Not sure if it has to do with the current ratio or not. Any suggestions would be appreciated there.
    I (in theory) am going to lose something like 1.4W over the diode. Is there any way around this or am I just stuck with that?

    Thanks in advance.

    •   Alt17th March 2016, 19:37

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  2. #2
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    Re: Flybuck Design Secondary Voltage not correct

    coilcraft coupled inductors are 1:1....and by the looks of your buck circuit, you are not going to get enough current flowing in the primary to enable the current that you want for your secondary with a 1:1 coupled inductor.



    •   Alt17th March 2016, 19:55

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  3. #3
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    Re: Flybuck Design Secondary Voltage not correct

    Quote Originally Posted by treez View Post
    coilcraft coupled inductors are 1:1....and by the looks of your buck circuit, you are not going to get enough current flowing in the primary to enable the current that you want for your secondary with a 1:1 coupled inductor.
    Is there any way to get that kind of power out of the secondary? The LT3690 I like because I can place an input at the SS pin and have control over the output. Are there any other methods or architectures that will accomplish the same thing? The only other thought was to have a 28V isolated and then feed it into a normal synchronous buck. This is pretty efficient-but uses a huge amount of board space for the DC-DC converter and the buck with all its large capacitors and inductor.



    •   Alt17th March 2016, 22:35

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  4. #4
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    Re: Flybuck Design Secondary Voltage not correct

    Yes just use a flyback or two transistor forward to get the isolated 11.5V, 3.5A.
    Stick with a separate buck for your other output whatever that is?
    Alternatively use a single flyback with two isolated outputs to give you each output.
    I wasn't sure what your actual vin was?



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