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UCC2817D Datasheet(PDF) 11 Page - Texas Instruments |
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UCC2817D Datasheet(HTML) 11 Page - Texas Instruments |
11 / 31 page UCC2817, UCC2818, UCC3817, UCC3818 BiCMOS POWER FACTOR PREREGULATOR SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009 11 www.ti.com APPLICATION INFORMATION multiplier (continued) IMOUT(max) for this design is approximately 315 µA. The RMOUT resistor can then be determined by: R MOUT + V RSENSE I MOUT(max) In this example VRSENSE was selected to give a dynamic operating range of 1.25 V, which gives an RMOUT of roughly 3.91 k Ω. voltage loop The second major source of harmonic distortion is the ripple on the output capacitor at the second harmonic of the line frequency. This ripple is fed back through the error amplifier and appears as a 3rd harmonic ripple at the input to the multiplier. The voltage loop must be compensated not just for stability but also to attenuate the contribution of this ripple to the total harmonic distortion of the system. (refer to Figure 2). RIN RD + − Rf Cf VREF VOUT CZ Figure 2. Voltage Amplifier Configuration The gain of the voltage amplifier, GVA, can be determined by first calculating the amount of ripple present on the output capacitor. The peak value of the second harmonic voltage is given by the equation: V OPK + P IN 2 p f R C OUT V OUT In this example VOPK is equal to 3.91 V. Assuming an allowable contribution of 0.75% (1.5% peak to peak) from the voltage loop to the total harmonic distortion budget we set the gain equal to: G VA + DV VAOUT (0.015) 2 V OPK where ∆VVAOUT is the effective output voltage range of the error amplifier (5 V for the UCC3817). The network needed to realize this filter is comprised of an input resistor, RIN, and feedback components Cf, CZ, and Rf. The value of RIN is already determined because of its function as one half of a resistor divider from VOUT feeding back to the voltage amplifier for output voltage regulation. In this case the value was chosen to be 1 M Ω. This high value was chosen to reduce power dissipation in the resistor. In practice, the resistor value would be realized by the use of two 500-k Ω resistors in series because of the voltage rating constraints of most standard 1/4-W resistors. The value of Cf is determined by the equation: |
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