7.1 Overview

The UCC28780 is a transition-mode (TM) active clamp flyback (ACF) controller, equipped with advanced control schemes to enable significant size reduction of passive components for higher power density and higher average efficiency. The control law is optimized for Silicon (Si) and Gallium Nitride (GaN) power FETs in a half-bridge configuration and is capable of driving high-frequency AC/DC converters up to 1 MHz. The zero voltage switching (ZVS) control of the UCC28780 is capable of auto-tuning the on-time of a high-side clamp switch (Q_H) by using a unique lossless ZVS sensing network connected between the switch-node voltage (V_SW) and SWS pin. The ACF controller is designed to adaptively achieve targeted full-ZVS or partial-ZVS conditions for the low-side main switch (Q_L) with minimum circulating energy over wide operating conditions. Auto-tuning eliminates the risk of losing ZVS due to component tolerance, input/output voltage changes, and temperature variations, since the Q_H on-time is corrected cycle-by-cycle.

Dead-times between PWML (controls Q_L) and PWMH (controls Q_H) are optimally adjusted to help minimize the circulating energy required for ZVS. Therefore, the overall system efficiency can be significantly improved and more consistent efficiency can be obtained in mass production of the soft-switching topology. The programming features of the RTZ, RDM, BUR, and SET pins provide rich flexibility to optimize the power stage efficiency across a range of output power and operating frequency levels.

The UCC28780 uses four different operating modes to maximize efficiency over wide load and line ranges. Adaptive amplitude modulation (AAM) adjusts the peak primary current at the higher load levels. Adaptive burst mode (ABM) modulates the pulse count of each burst packet in the medium to light load range. Low power mode (LPM) reduces the peak primary current of each two-pulse burst packet in the very light load range. Standby power (SBP) mode minimizes the loss during no load conditions.

The unique burst mode control of the UCC28780 maximizes the light load efficiency of the ACF power stage, while avoiding the concerns of conventional burst operation - such as output ripple and audible noise. The burst control provides an enable signal through the RUN pin to dynamically manage the static current of the half-bridge driver and also adaptively disables the on-time of Q_H. These functions can be used to manage the quiescent power consumed by the half-bridge driver, further improving the converter’s light-load efficiency and reducing its standby power.

Instead of using a conventional high-voltage resistor, the UCC28780 starts up the VDD supply voltage with an external high-voltage depletion-mode MOSFET between the SWS pin and the switch node. Fast startup is achieved with low standby power overhead. Moreover, the HVG pin controls the gate of the depletion-mode FET to also allow this MOSFET to be used in a lossless ZVS sensing. This arrangement avoids additional sensing devices.

The UCC28780 also integrates a robust set of protection features tailored to maximize the reliability. These features include internal soft start, brown in/out, output over-voltage, output over-power, system over-temperature, switch over-current, output short-circuit protection, and pin open/short.