The UCC3961 advanced primary-side startup controller is a unique solution that provides all the primary-side functions required for a single-ended, isolated, switch-mode power converter incorporating secondary-side PWM control. It is usable with a wide range of secondary control circuits and is especially well suited for systems where sophisticated handling of overload conditions is required.

Secondary-side control assumes that output voltage and current measurements are interfaced directly to an output ground-referenced PWM stage that develops the power switch command for the supply. This digital PWM command can then be transmitted to the primary-side power switch through a simple and low-cost isolating pulse transformer. With secondary-side control, it is much easier to monitor and control the system load with tightly coupled analog control loops. Load-oriented features such as output current sharing and synchronous rectification are implemented more easily.

The UCC3961 provides all the circuitry required on the primary side of a secondary-side controlled power supply. It features a free running 60-kHz to 360-kHz oscillator which is synchronizable to the secondary-side PWM signal and also has the ability to accept start/stop PWM commands from the isolating pulse-edge transformer (PET). The use of an extremely small and low-cost pulse transformer allows for higher converter bandwidth. This also eliminates the loop-gain variations due to initial accuracy and aging of an opto-coupler feedback element or the size penalty of a gate transformer. It also includes an undervoltage lockout circuit with 2-V hysteresis, a low-current startup with active low during UVLO, a soft-start capability, a 5-V reference and a high-current power output.Read more