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Thread: 350W Half-Bridge SMPS with PFC

  1. #1
    diysmps Member
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    350W Half-Bridge SMPS with PFC

    Hi! I'm starting to build a 350W Half-Bridge SMPS with a PFC stage using the UCC28019A from Texas Instruments. I've built other Half-Bridge SMPS with the two input capacitor forming a voltage divider. The problem is that now I don't have the voltage divider I only have 390VDC from the PFC. How do I connect the MOSFETs now?

    Searching over internet I found http://www.smpspowersupply.com/atx-power-supply-pfc-schematic.pdf design that uses only one MOSFET connected to one end of the primary and the other end connected directly to the PFC output. Also https://www.fairchildsemi.com/reference-designs/RD-268.pdf on page 3 it uses 2 MOSFETs one connected to each end of the primary with the diodes D8 and D9 that are used to clamp high voltage spikes I think. And finally this http://www.st.com/st-web-ui/static/active/en/fragment/product_related/rpn_information/product_circuit_diagram/circuit_diagram_14821_thumbnail.png using 2 MOSFETs with the blocking capacitor but this is a ZVS Half-Bridge and I want a hard-switching Half-Bridge.

    What's the correct way to connect it? Are this topologies still Half-Bridge when using just one MOSFET and no blocking cap?

  2. #2
    Those links you posted are broken.

    You will not be able to use a current mode half-bridge with the PFC output because the half-bridge requires a voltage divider/blocking cap on one side of the transformer's primary. Current mode control will walk this side of the transformer into one of the supply rails and the supply will stop working (or possibly give subharmonic oscillation, I forget which). You can try using a balance winding or other tricks to prevent this problem but I have never had good luck with them.

    In summary, you will not have good luck with current mode control on the half-bridge topology. Voltage mode control could work for you but is antiquated and will not perform that well.

    I find the full bridge easiest to design and work with, though some people will disagree. You should consider the LLC half-bridge, the flyback, and the full-bridge.

    You will get current mode control with the flyback and full-bridge. The LLC controllers I have seen provide current limiting but may not give cycle by cycle current control.

    If you really want a hard switched half bridge, perhaps because you think it is simple, then consider the UC3525A. Be warned that with voltage mode controllers like the UC3525A the compensation network will be harder to design than a current mode controller would be. You may well have to lag it out (lots of integral) to get it stable.

    I suggest you try to copy a reference design for an LLC converter for example then modify it to meet your needs.

  3. #3
    diysmps Member
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    Ok, so is not as simple as I imagined. Can you recommend any book to design and learn about LLC converters?

  4. #4
    For a simple current mode control switching stage running from the output of a PFC. Look into doing a 2-Switch Forward converter. These are very easy to implement, hard switched, simple to regulate, and will easily handle 350W. To make things even easier you can control them with a UC3844/3845 driving a GDT.

  5. #5
    diysmps Senior Member
    Join Date
    Oct 2012
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    The 3 schematic links you posted are single switch forward, single switch flyback and LLC respectively. It's perfectly fine to do a half bridge after a PFC, just split the PFC output capacitor into 2 series capacitors or if the PFC is a separate module, have the 2 caps in parallel with the PFC output but on the half bridge board. As said before, half bridge doesn't like current mode control. 2 switch forward is a reasonable topology at that power level, can use CMC and is relatively easy for a beginner, not that any of these are easy or trivial in any way. LLC is about as difficult as you can get.
    Buck derived converters in regulated power supplies require output inductors
    and the following control modes (voltage or current mode control; bold is recommended).

    Buck (CMC/VMC) | Forward (CMC/VMC) | 2-switch forward (CMC/VMC) | Half Bridge (VMC) | Push-pull (CMC) | Full Bridge (CMC). Click for basic schematics.
    ON Semiconductor TL431 design guide

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