SLUSD37E October 2017 – November 2019 UCC28056
Power drawn by the PFC stage from the Line supply may be represented by the following expression.
Assuming a typical application, with constant load power, for some parts of the Line cycle excess power is drawn from the supply and stored in the output capacitor. In other parts of the Line cycle load power exceeds input power and this deficit must be supplied from the output capacitor. This process of energy transfer to an from the output capacitor necessarily results in twice Line frequency output voltage ripple. The amplitude of this twice Line frequency ripple depends only upon the ratio POut/COut and the Line frequency.
Choose an output capacitor value by prioritizing one of a number of application requirements:
For this design example assume that the twice Line frequency output ripple voltage amplitude is less than 3% of its regulation level. The POutMax/COut ratio required to achieve this can be calculated using Equation 41
Use Equation 42 to calculate the required capacitance value for this 165-W example design.
For best Line current total harmonic distortion (THD), the maximum output voltage ripple amplitude must satisfy the condition presented in Equation 43. Satisfying this condition ensures that the error amplifier non-linear gain does not activate due to extremes of the output voltage ripple.
Use Equation 44 to calculate the maximum RMS ripple current flowing in the output capacitor.
This current flowing into the output capacitor includes a switching frequency component (ICOutRMSHF) and a twice Line frequency ripple component (ICOutRMSLF).
Electrolytic capacitors typically have a ripple current rating at twice Line frequency (120 Hz) and a different ripple current rating at switching frequency (100 kHz). These ratings reflect the fact that the capacitor ESR is higher at twice Line frequency and hence ripple current at this frequency leads to higher power loss than the same amplitude of switching frequency ripple. Consider the equivalent high-frequency ripple current flowing in the capacitor in order to select the correct capacitor.
The parameter KHLF is the ratio of high frequency to low frequency RMS ripple current rating for the particular capacitor series to be used.
In this example design, for reasons of size and rating, two 68-µF, 450 V capacitors are selected from Rubycon BXW series (450BXW68MEFC12.5X45), connected in parallel. In this way, both the capacitance value requirement and ripple current rating are met with some additional margin.