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UCC24612: ucc24612

Part Number: UCC24612
Other Parts Discussed in Thread: UCC28780

One more clarification on the use of UCC24612 .

One would conclude from Fig 17 of the datasheet that in every cycle when the main switch goes OFF, there is an implicit assumption that there is a diode( in this instance, the body diode) that has to conduct for a short time to begin the process of turning ON the SR.

In such a case, what would be requirements imposed on the body diode?

If the implicit assumption is not true, then how does the sr ON time begin if at time =0 which marks the beginning of the secondary current there is no conductive return path ( assuming SR on the low side) at t=0

We have a design in the works that will have a load of 20 amps. Including the capacitor charging, peak currents will be high. So there needs to be attention paid on this body diode.

thnx

robin

  • Hello Robin,

    You are correct; there is an implicit assumption that the SR MOSFET is a silicon MOSFET and that it has the so-called "body-diode" as a reverse conduction path from source to drain.

    The UCC24612 relies on the "forward voltage drop" of this body-diode to trigger turn-on of the SR gate drive. The VD-VS voltage must exceed the turn-on threshold of Vthvgon (which is typically -240mV). This threshold is easily met by almost any current level of significance at the start of the "flyback" interval (demagnetization).

    In almost all cases, the Si-MOSFET that is chosen to reliably handle the maximum peak and rms currents of the flyback output will intrinsically have a body-diode that is capable of reliably handling that same peak for the duration of the turn-on delay. It is true that the power loss of the peak current through the voltage of the body-diode at that current during the turn-on delay must be included in the total MOSFET loss when calculating junction temperature rise.

    Regards,
    Ulrich
  • Good to hear from you Ulrich!
    So then, can I add an external body diode just to handle this very short time(<50ns?) which begins the flyback power delivery every switching period as the OFF time starts in the primary?
    It is not a rhetorical question! It is loaded with implications of using other devices but enhanced by a fast diode without reverse recovery issues and is active for 50 ns every cycle. After that, the entire OFF time is handled by the SR under control of UCC24612.
    that would be cool.
    thnx
  • Hi Robin,

    Yes, you definitely can add a parallel diode (such as a schottky diode, for example) across the SR MOSFET to reduce the peak voltage during the turn-on delay time. Some customers do this while most do not because they wish to avoid the extra cost. Make sure that the diode is not so big (super-low Vf) that the worst-case SR turn-on threshold voltage can't be reached. Also be sure that the parallel diode connections do not interfere with the SR controller VD and VS sense paths to the MOSFET drain and source pads. Please avoid adding any stray inductance into the current path between the VD and VS sense points, since any L*di/dt offset will affect the SR turn-off point.

    Be aware, in the context of a UCC28780 ACF design, that the parallel diode will add parallel capacitance with the SR FET. This will have to be accounted for in the overall switched-node capacitance calculations. Whether it makes a significant difference or not I can't say, but better to be aware of it than to be surprised later on.

    Regards,
    Ulrich
  • thnx much.
    I follow your point.
    Cost is not an issue in our products!
    Performance is.
    Will apply due diligence here.