UCC2817L Datasheet, PDF (11/31 Pages) Texas Instruments – BCMOS POWER FACTOR PREREGULATOR

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UCC2817, UCC2818, UCC3817, UCC3818
BiCMOS POWER FACTOR PREREGULATOR
SLUS395J - FEBRUARY 2000 - REVISED MARCH 2009
APPLICATION INFORMATION
multiplier (continued)
I_MOUT(max) for this design is approximately 315 µA. The R_MOUT resistor can then be determined by:
RMOUT
+
VRSENSE
IMOUT(max)
In this example V_RSENSE was selected to give a dynamic operating range of 1.25 V, which gives an R_MOUT of
roughly 3.91 kΩ.
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).
VOUT
Cf
Rf
CZ
RIN
−
RD
+
VREF
Figure 2. Voltage Amplifier Configuration
The gain of the voltage amplifier, G_VA, 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:
ǒ Ǔ ^{VOPK +} _{2 p}
fR
PIN
COUT
VOUT
In this example V_OPK 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:
ǒ Ǔ _{GVA +}
DVVAOUT (0.015)
2 VOPK
where ∆V_VAOUT 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, R_IN, and feedback components C_f, C_Z, and R_f. The
value of R_IN is already determined because of its function as one half of a resistor divider from V_OUT feeding
back to the voltage amplifier for output voltage regulation. In this case the value was chosen to be 1 MΩ. 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-kΩ resistors in series because of the voltage rating constraints of most standard
1/4-W resistors. The value of C_f is determined by the equation:
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