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UCC27714: MISSING DIODE 2-SWITCHFORWARD CONVERTER

Eddie Loy
Intellectual 850 points
Part Number: UCC27714

REFERENCE PAGE 23 -

"The circuit in Figure 49 shows a two-transistor forward converter circuit driven by the UCC27714. This circuit requires both outputs to be high or low simultaneously. The bootstrap capacitor would be charged with LO high state only (HO low). As this would decrease overall system efficiency two additional diode and two additional transistors are required to charge the bootstrap capacitor during LO and HO low period.)

QUESTION  : (Figure 49. Two-Transistor Forward Converter Circuit)  I SEE THE TWO ADDED SWITCHES FOR THE BOOTSTRAP, BUT ONLY ONE ADDED DIODE.  WHERE IS THE PROPER PLACEMENT OF THE SECOND ADDED DIODE?

  • 0
    TI__Mastermind 33790 points
    Hello Eddie,

    I am an applications engineer supporting UCC27714. I think that "two" may be an error, as the circuit in Figure 49 should be able to function without additional modification. It also might be referring to the bootstrap diode, but this interpretation is somewhat confusing since the bootstrap diode is normally used with half-bridge circuits.

    Upon review, I think it would be helpful to split Rboot into two resistors: one with the typical low value of <20Ω charging HB through the boot diode, and one in the range of 1kΩ to 10kΩ for activating the bootstrap clamp transistor without consuming a lot of current. The time constant of the charging path for the bootstrap clamping transistor may become a limiting factor at high frequency, and the 1kΩ to 10kΩ resistor could be replaced with a P-channel transistor driven by LO for considerably faster clamp turn-on speeds.

    I will make a note to update this diagram for the next revision of the datasheet.

    Regards,