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TPS55010: Confused as to the direction of the transformer in the datasheet

Robert Helvestine
Intellectual 275 points
Part Number: TPS55010

I'm a little confused as to why the transformer winding "dots" in the TPS55010 datasheet are the way they are:

I believe current flows "down" the primary winding from the PH pin to the VSENSE pin. Because the transformer primary "dot" is at the bottom of the primary winding, this means in the secondary winding current flows into the "dot", as in from cathode to anode of the diode, which won't happen unless breakdown occurs. I was taught regarding transformer dots, current flows into one dot, and out of the other dot. Is this not correct? I believe current should flow down the primary (PH to VSENSE) and up the secondary (anode to cathode) of the diode, which would put both dots either at the tops or bottoms of the windings, but not alternating as shown.

Can someone correct my thinking? I'm using a custom variant of the Coilcraft LPD5030V transformer and need to make sure I implement it correctly so I don't ruin a board spin. 

www.coilcraft.com/lpd5030v.cfm

Thanks,

Rob

  • 0
    TI__Mastermind 25590 points

    Rob,

    Your understanding of transformers is correct. Let me clarify the operation of the flybuck.

    The flybuck works in two states:

    1. PH is high which charges Cpri through the primary winding of the transformer. The secondary has no current flowing. The 200mA load is supplied from the charge stored on Co.

    2. PH is low which discharges Cpri through the primary winding which transfers current into the secondary. In this case the current is going into the dot on the primary and going out of the dot on the secondary, as you described.

    Check out this video for more info.

    -Sam

  • 0
    TI__Mastermind 35115 points

    Hi Rob,

    The dots in the datasheet are correct. For the flybuck you need to pay attention to the voltage across the transformer instead of the current. It's not exactly like a flyback where you think mainly of the current in the transformer. Here are the two states:

    1. When the HS FET is on the voltage across the primary is Vpri-Vin. The voltage across the secondary is then N*(Vpri-Vin) and this is a negative voltage so the secondary diode is off.
    2. When the LS FET is on the voltage across the primary is Vpri. The voltage across the secondary is then N*Vpri and this is a positive voltage. If the voltage is higher than Vout+Vd, the secondary diode is on and the output cap is charged up. The current through the secondary into the output cap reflects current to the primary winding.

    Best Regards,
    Anthony