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UCC2817APWRG4 Datasheet(PDF) 11 Page - Texas Instruments |
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UCC2817APWRG4 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 33 page UCC2817A, UCC2818A UCC3817A, UCC3818A SLUS577B − SEPTEMBER, 2003 − REVISED FEBRUARY 2006 11 www.ti.com APPLICATION INFORMATION Multiplier The output of the multiplier of the UCC3817A is a signal representing the desired input line current. It is an input to the current amplifier, which programs the current loop to control the input current to give high power factor operation. As such, the proper functioning of the multiplier is key to the success of the design. The inputs to the multiplier are VAOUT, the voltage amplifier error signal, IIAC, a representation of the input rectified ac line voltage, and an input voltage feedforward signal, VVFF. The output of the multiplier, IMOUT, can be expressed as: I MOUT + IIAC V VAOUT * 1 K V VFF 2 where K is a constant typically equal to 1 V . The electrical characteristics table covers all the required operating conditions for designing with the multiplier. Additionally, curves in Figures 10, 11, and 12 provide typical multiplier characteristics over its entire operating range. The IIAC signal is obtained through a high-value resistor connected between the rectified ac line and the IAC pin of the UCC3817A/18A. This resistor (RIAC) is sized to give the maximum IIAC current at high line. For the UCC3817A/18A the maximum IIAC current is about 500 µA. A higher current than this can drive the multiplier out of its linear range. A smaller current level is functional, but noise can become an issue, especially at low input line. Assuming a universal line operation of 85 VRMS to 265 VRMS gives a RIAC value of 750 kΩ. Because of voltage rating constraints of standard 1/4-W resistor, use a combination of lower value resistors connected in series to give the required resistance and distribute the high voltage amongst the resistors. For this design example two 383-k Ω resistors were used in series. The current into the IAC pin is mirrored internally to the VFF pin where it is filtered to produce a voltage feed forward signal proportional to line voltage. The VFF voltage is used to keep the power stage gain constant; and to provid input power limiting. Please refer to Texas Instruments application note SLUA196 for detailed explanation on how the VFF pin provides power limiting. The following equation can be used to size the VFF resistor (RVFF) to provide power limiting where VIN(min) is the minimum RMS input voltage and RIAC is the total resistance connected between the IAC pin and the rectified line voltage. R VFF + 1.4 V V IN(min) 0.9 2 R IAC [ 30 kW (9) (10) |
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