The UCC3960 primary-side startup controller is a unique solution that provides all the primary-side functions required for a single-ended, isolated offline, switch-mode power converter that uses secondary-side PWM control. It is usable with a wide range of secondary circuits and is especially well suitable 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 UCC3960 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 that 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. 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.

In a non-typical use, the UCC3960 can accommodate an analog feedback signal through an opto-isolator where it can operate in voltage-mode control mode with primary-side peak current limiting.

The UCC3960 and the UCC2960 are available in the 8-pin SOIC (D) and PDIP (P) packages. A 14-pin package version is available as UCC2961 and UCC3961 that includes additional protection features such as multimode overcurrent protection, volt-second clamp, programmable overvoltage and undervoltage sense lines, and the self-bias regulator.

• Operates With Secondary-Side PWM Control
• Receives Isolated PWM Command Through a Pulse-Edge-Transformer
• Initial Free-Running Soft-Start Up with Duty-Cycle Clamping
• Up to 400-kHz Synchronizable Switching Frequency
• High-Current FET Drive (1.5-A Sink, 0.75-A Source)
• Overcurrent Protection
• Undervoltage Lockout with 2-V Hysteresis
• Low-Current Startup typical operating circuit