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PMP12072: PMP12072

Part Number: PMP12072
Other Parts Discussed in Thread: UCC28086


With regard to the circuit shown in PMP12072 and the VDD line of the UCC28086: The only voltage I have available to apply to VDD is 24VDC (±10% or thereabouts).  What would be the best attenuation method to install before the VDD line so that I do not exceed the Absolute Maximum Rating of 15V and not limit the input current so the device still turns on?  Thank you.

  • I would recommend keeping the voltage at the VDD pin at 12V since that voltage creates the gate drive voltage on the transformer secondary side. Since you have 24V available, I suggest using a 12V zener diode (MMSZ4699T1) to clamp VDD to 12V. The value of R1 would stay the same at 10K.

    A new value for R3 would be required. This value is mainly determined by the current draw feeding the circuit, which consists of IVDD (UCC28086D bias current = 2.5mA), ICTRL (~100uA), Izener (50uA), the transformer drive current, and the secondary load (100K ohm or 120uA). The transformer drive current will be one of the larger current loads and would be related to the transformer magnetizing current.The Pulse transformer has a 980uH inductance and the it switches at 200KHz. Peak to peak magnetizing ripple current can be calculated from dI = V/L*dT = (12V / 980uH *2.5uS) = 30mApp (The on time is 50%, so dT is 2.5uS). The RMS of this would be dI / sqrt(12) = 30mA / 3.46 = 8.67mA.

    From this R3 can be calculates as R3 = (Vin_min - Vz) / (IVDD + ICNTL + Izener + I_drive + Isec_load) = (21V - 12V) / (2.5mA + 100uA + 50uA + 8.67mA+120uA) = 787 ohms (max). Power in R3 would be about 183mW at 24Vin.  The value of C3 could also be reduced to 0.1uF so the turn on delay is short.

    This should be a reasonable starting point. I hope this helps.

  • Thank you John for the very thorough explanation.  I will give your recommendation a try.