The UCC28521 and UCC28528 combination PFC/PWM controllers provide complete control functionality for any off-line power system requiring compliance with the IEC1000-3-2 harmonic reduction requirements. By combining the control and drive signals for the PFC and the PWM stages into a single device, significant performance and cost benefits are gained.
Based on the UCC28511, the new devices employ an average current mode control architecture with input voltage feedforward. The major difference of the UCC28521/28 is the trailing-edge / trailing-edge (TEM/TEM) modulation scheme for the PFC/PWM stages. The UCC28528 differs from the UCC28521 as its PWM stage does not turn off with the falling bulk voltage. The UCC28528 PWM was designed for low power auxiliary supplies.
These devices offer performance advantages over earlier generation combination controllers. A key PWM feature is programmable maximum duty cycle. For the PFC stage, the devices feature an improved multiplier and the use of a transconductance amplifier for enhanced transient response.
The core of the PFC section is in a three-input multiplier that generates the reference signal for the line current. The UCC28521/28 features a highly linearized multiplier circuit capable of producing a low distortion reference for the line current over the full range of line and load conditions. A low-offset, high-bandwidth current error amplifier ensures that the actual inductor current (sensed through a resistor in the return path) follows the multiplier output command signal. The output voltage error is processed through a transconductance voltage amplifier.
The transient response of the circuit is enhanced by allowing a much faster charge/discharge of the voltage amplifier output capacitance when the output voltage falls outside a certain regulation window. A number of additional features such as UVLO circuit with selectable hysteresis levels, an accurate reference voltage for the voltage amplifier, zero power detect, OVP/enable, peak current limit, power limiting, high-current output gate driver characterize the PFC section.
The PWM section features peak current mode control (with a ramp signal available to add slope compensation), programmable soft-start, accurate maximum duty cycle clamp, peak current limit and high-current output gate driver. The PWM stage is suppressed until the PFC output has reached 90% of its programmed value during startup. During line dropout and turn off, the UCC28521 allows the PWM stage to operate until the PFC output has dropped to 71% of its nominal bulk output value. The UCC28528 on the other hand allows the PWM stage to operate continuously until the bias falls below the UVLO turn-off threhold.
Both devices are available in 20-pin DW package.
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