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  1. #1
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    Double LLC resonant converter

    Hello all, I am designing a bidirectional DC DC converter comprising of two resonant tank. My topology is like two H bridge giving two square wave output to two LLC resonant tanks. I am employing PFM to maintain constant output voltage.
    I have designed single core dual primary dual secondary HF transformer using PQ50/50 core with eqaul turn ratio of 4.2. The magnetizing inductance is around 570uH. The total resonant inductance is 136 uH and resonant capacitance is 111.86 nF.
    However, the two square wave input from H bridge are not identical. Kindly look at the waveform. Blue and Pink are two input square waveform. and Cian n green is the secondary output of both transformer.

    There is a DC offset in the H-bridge output waveform.
    What could be the reason for it?

    I employed a roughly designed RCD snubber too but of no use.

    Please help. Thanks

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    Re: Double LLC resonant converter

    Full schematic...? to appear to have a DC offset in your drive from one or both of the H bridges ...



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    Re: Double LLC resonant converter

    Quote Originally Posted by Easy peasy View Post
    Full schematic...? to appear to have a DC offset in your drive from one or both of the H bridges ...
    Thanks for the reply.
    Sorry, Can't show the topology schematic here.It's confidential at this stage.

    The offset occurs in both H-bridge output as can be seen from blue and pink waveforms.
    Can the mismatch of 10uH in magnetizing inductance result in such offset?



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    Re: Double LLC resonant converter

    A H-bridge is switching DC bus voltage and can't generate DC offset as long it's actually switching. The shown waveform is either caused by a silly measurement error, an AC coupled measurement, or specific properties of your unknown topology.


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    Re: Double LLC resonant converter

    Um - it can if the dead times are different - or the control signals are not symmetric

    - - - Updated - - -

    the ringing looks to be due to insufficient LoZ capacitance on the busses - or too much lead inductance between fets and bus caps - without photo's impossible to be specific ...


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    Re: Double LLC resonant converter

    Thanks alot FvM and Easy Peasy.
    I troubleshooted the problem. I had been using optocoupler based gate drive with variable gate resistor. The problem existed with the gate driver. Need to vary the gate resistor to get the square wave from H bridge.

    Moreover, the ringing issue is supressed by using RC snubber.

    Thanks again. Will post more threads for more doubts n shortcomings.



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    Re: Double LLC resonant converter

    also -feel free to post some large clear shots of the H bridges working too ...!



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    Re: Double LLC resonant converter

    Hello all.

    The H-bridges are working fine. I employed phase shift modulation for voltage regulation. However the transformer output voltage shows lots of ringing and distorted waveform. The attached picture is the two LLC resonant tanks. The two transformers share a common core.
    Click image for larger version. 

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    The waveforms are as in the attached pictureClick image for larger version. 

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    Blue: Input square voltage
    Cian: transformer secondary voltage
    Green: Vds
    Pink: Vgs

    The phase shift modulation results in lots of ringing and distorted output voltage.
    What could possibly the reason?

    How important is shielding between primary and secondary winding of the transformers? I have used masking paper between the windings for isolation. It seems there is alot of stray inductance that could possibly one reason.

    Kindly have a look and give suggestions. Thank you.



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  9. #9
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    Re: Double LLC resonant converter

    I'm afraid your layout may be too "open" to avoid ringing - also a schematic would help - are there 2 full H bridges? or only one, i.e. two half bridges?



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