UC3848

The UC3848 family of PWM control ICs makes primaryside average current mode control practical for isolated switching converters. Average current mode control insures that both cycle by cycle peak switch current and maximum average inductor current are well defined and will not run away in a short circuit situation. The UC3848 can be used to control a wide variety of converter topologies. In addition to the basic functions required for pulse width modulation, the UC3848 implements a patented technique of sensing secondary current in an isolated buck derived converter from the primary side. A current waveform synthesizer monitors switch current and simulates the inductor current down slope so that the complete current waveform can be constructed on the primary side without actual secondary side measurement. This information on the primary side allows for full DC control of output current.

The UC3848 circuitry includes a precision reference, a wide bandwidth error amplifier for average current control, an oscillator to generate the system clock, latching PWM comparator and logic circuits, and a high current output driver. The current error amplifier easily interfaces with an optoisolator from a secondary side voltage sensing circuit.

A full featured undervoltage lockout (UVLO) circuit is contained in the UC3848. UVLO monitors the supply voltage to the controller (VCC), the reference voltage (VREF), and the input line voltage (VIN). All three must be good before soft start commences. If either VCC or VIN is low, the supply current required by the chip is only 500mA and the output is actively held low.

Two on board protection features set controlled limits to prevent transformer core saturation. Input voltage is monitored and pulse width is constrained to limit the maximum volt-second product applied to the transformer. A unique patented technique limits maximum duty cycle within 3% of a user programmed value. These two features allow for more optimal use of transformers and switches, resulting in reduced system size and cost.

Patents embodied in the UC3848 belong to Lambda Electronics Incorporated and are licensed for use in applications employing these devices.