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UCC25600: UCC25600 as LLC controller where resonant elements are in the secondary side

Caner Arslan
Prodigy 10 points
Part Number: UCC25600

Hello,

I want to design a LLC converter which will step up the voltage. Due to high currents circulating in the primary side I want to carry LLC resonant part to secondary side which will be high voltage low current so I can lower the number of capacitors and wire gauge of the inductors. My question is this:

When I move resonant components to secondary side  does the control scheme change ?  I found some papers with conflicting claims.Do we operate in inductive region like in the normal LLC converter in which resonant components are in the primary side and as the power increases do we lower switching frequency. Is the analysis and the design changes? If it is then I can use existing LLC controllers like UCC25600.

I am newly graduated EE engineer and have very little real world experience. 

Here is the paper I am looking at right now: Design of Wide Input Voltage Range High Step-Up DC-DC Converter based on Secondary-side Resonant Tank Full Bridge LLC by Arief Noor Rahman and Yao-Ching Hsieh

Thanks for your time

  • 0
    TI__Guru 71105 points

    Hello,

    I don't see the paper you are discussing.  Could you attach it please?

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    1-llc step up 2kw.pdf3-llc step up 500w.pdfThese are the papers which confused me I dont have experience with these type of converters a nice guide will very helpful. The first one is full bridge at a power of 2kW and which looks like its design is conventional, the second one is the rogue one its push pull with 500W of power which increases the frequency as power increases and and recommends operation below resonance frequency

  • 0 in reply to Caner Arslan
    TI__Guru 71105 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • +1 in reply to Mike O'
    TI__Guru 71105 points

    Hello,

    Thankyou for sharing the papers.  I have not seen this technique used.  I can see why you would be interested in using it your application.

    I have designed a few LLC converters and will give you my thoughts on your questions.  Please see the responses below.

    When I move resonant components to secondary side  does the control scheme change ? 

    >Yes

    >According to the paper you sent the gain is inverse to a normal LLC.

    I found some papers with conflicting claims.Do we operate in inductive region like in the normal LLC converter in which resonant components are in the primary side and as the power increases do we lower switching frequency. Is the analysis and the design changes?

    >The problem with any LLC is how the gain changes with frequency.

    >In a normal LLC you want the gain to increase as the frequency decreases and you do not want to operate in the capacitor region.

    a.) If the design were allowed to go into the capacitor region the control would latch.  That is why in the UCC25600 you program the minimum switching frequency to avoid this.

    >In the secondary side resonant converter the gain will increase as the frequency increases.

    a.) TI has not design LLC converters for this application.

    b.) The paper you sent does not show the capacitor region.  However, more than likely you would have to avoid the capacitive region in this design.  

    Here is the paper I am looking at right now: Design of Wide Input Voltage Range High Step-Up DC-DC Converter based on Secondary-side Resonant Tank Full Bridge LLC by Arief Noor Rahman and Yao-Ching Hsieh

    The normalized gain curve looks inverse compared to the normal LLC where the half bridge is on the primary.  So for this scheme to work you will need the frequency to increase to increase the gain.  The UCC25600 was not specifically designed to operate in this way.  So the traditional feedback will not work.

    a.) Your are correct.

    b.) You will need to find a controller or develop your own control scheme that increases the frequency to increase the gain.

    Regards,