SLUSC60B December 2017 – October 2019 UCC28064A
Under normal operating conditions, the UCC28064A device regulates the relative phasing of the channel A and channel B inductor currents to be approximately 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.
Interleaving may not be ideal under all conditions. In particular a loss of interleaving may be experienced at light loads near the zero crossings. In some cases there may be insufficient current to trigger a large enough signal to trip the zero crossing detectors. In addition the turn off delay in the MOSFET may dominate the overall on-time at very light loads. This creates a very limited ability for the controller to correct for phase errors in the system.