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Does UCC28070 PFC really need two CTs?

Mike Leiss
Prodigy 10 points
Other Parts Discussed in Thread: UCC28180, UCD3138

Hello,

I am working on an interleaved PFC circuit.  I found this component, but I don't like the fact that the current transformers (CTs) are located in the switching path.  Would it be possible to use one CT in the general return or is there a different controller IC that would be better suited to utilize only one CT in this configuration?

Thanks!

  • 0
    TI__Expert 4970 points

    Mike,

    Unfortunately you cannot put a current transformer in the general return. Please allow me to explain. Presume for this argument that you are using a single phase PFC that looks something like this:

    When the FET (Q) is on, a positive voltage is seen across the Current Transformer (CT). Current in the CT begins to ramp up, and the magnetic field begins to build. When the FET is turned off, the current through the CT causes a voltage with the opposite charge to be seen across the CT, and the current ramps back down. During the whole switching the average voltage across the CT is equal to 0.

    However, if the CT is placed in the return path, the story is a little different:

    When the FET (Q) is on, a voltage is seen across CT in the same way as before, and current again ramps up. However, when the FET is off, and current flows through RL, there is still a positive voltage seen across CT. ICT ramps up more. Therefore there is a net positive voltage on the CT throughout the whole switching cycle. Volt-Second balance is not met, and current continues to ramp in the current transformer until it saturates.

    Take a look at the UCC28180. This device controls a single phase PFC with a single shunt resistor. However, if you need an interleaved PFC, you will need two current transformers in the switching path to ensure proper current balancing between the two separate phases.

    There are solutions to only using one current transformer, but it will still be in at least one of the switching paths, and most of those solutions will need a digital controller such as the UCD3138 to allow additional calculation processing.

    I hope this helps to answer your question. Please let me know if you have more!