Datasheet

IN ON
PEAK
V (t) T
I (t)
L
´
=
IN ON
AVG
V (t) T
I (t)
2 L
´
=
´
UCC28063
www.ti.com SLUSAO7 –SEPTEMBER 2011
APPLICATION INFORMATION
Principles of Operation
The UCC28063 contains the control circuits for two parallel-connected boost pulse-width modulated (PWM)
power converters. The boost PWM power converters ramp current in the boost inductors for a time period
proportional to the voltage on the error amplifier output. Each power converter then turns off the power MOSFET
until current in the boost inductor decays to zero, as sensed on the zero current detection inputs (ZCDA and
ZCDB). Once the inductor is demagnetized, the power converter starts another cycle. This on/off cycling
produces a triangle wave of current, with peak current set by the on-time and instantaneous power mains input
voltage, V
IN
(t), as shown in Equation 1.
(1)
The average line current is exactly equal to half of the peak line current, as shown in Equation 2.
(2)
With T
ON
and L being essentially constant during an AC-line period, the resulting triangular current waveform
during each switching cycle will have an average value proportional to the instantaneous value of the rectified
AC-line voltage. This architecture results in a resistive input impedance characteristic at the line frequency and a
near-unity power factor.
Natural Interleaving
Under normal operating conditions, the UCC28063 regulates the relative phasing of the channel A and channel B
inductor currents to be very close to 180°. This greatly reduces the switching-frequency ripple currents seen at
the line-filter and output capacitors, compared to the ripple current of each individual converter. This design
allows a reduction in the size and cost of input and output filtering. The phase-control function differentially
modulates the on-times of the A and B channels based on their phase and frequency relationship. The Natural
Interleaving method allows the converter to achieve 180° phase-shift and transition-mode operation for both
phases without tight requirements on boost inductor tolerance.
Ideally, the best current-sharing is achieved when both inductors are exactly the same value. Typically the
inductances are not the same, so the current-sharing of the A and B channels is proportional to the inductor
tolerance. Also, switching delays and resonances of each channel typically differ slightly, and the controller
allows some necessary phase-error deviation from 180° to maintain equal switching frequencies. Optimal phase
balance occurs if the individual power stages and the on-times are well matched. Mismatches in inductor values
do not affect the phase relationship.
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