Datasheet

OUTE
(CCM Mode)
OUTF
(CCM Mode)
OUTE
OUTF
Transformer
Winding
Magnetizing
Current
OUTE and OUTF are disabled if V
CS
< V
DCM
OUTE and OUTF are disabled if V
CS
< V
DCM
Burst Mode at light load with T
MIN
maintaining ZVS
(The time scale is different versus above diagram)
UCC28950
www.ti.com SLUSA16B –MARCH 2010–REVISED OCTOBER 2011
Major waveforms of the phase-shifted converter during nominal operation mode are shown in Figure 48. Upper
six waveforms in the Figure 48 show the output drive signals of the controller. At nominal mode, the outputs
OUTE and OUTF overlap during the part of the switching cycle when the both rectifier MOSFETs are conducting
and the windings of power transformer are shorted. Current, I
PR
, is the current flowing through the primary
winding of power transformer. The bottom four waveforms show the drain-source voltages of rectifier MOSFETs,
V
DS_QE
and V
DS_QF
, the voltage at the output inductor, V L
OUT
, and the current through the output inductor, I L
OUT
.
Proper timing between the primary switches and synchronous rectifier MOSFETs is critical to achieve highest
efficiency and reliable operation in this mode. The controller device adjusts the turn OFF timing of rectifier
MOSFETs as function of load current to ensure the minimum conduction time and reverse recovery losses of
their internal body diodes.
ZVS is an important feature of relatively high input voltage converters to reduce switching losses associated with
the internal parasitic capacitances of power switches and transformers. The controller ensures ZVS conditions
over the entire load current range by adjusting the delay time between the primary MOSFETs switching in the
same leg in accordance to the load variation. Controller also limits the minimum ON-time pulse applied to the
power transformer at light load, allowing the storage of sufficient energy in the inductive components of power
stage for the ZVS transition.
As soon as the load current keeps reducing from the mid load current down to no-load condition, the controller
selects the most efficient power saving mode by moving the converter from the nominal operation mode to the
discontinuous-current diode-rectification mode and, eventually, at very light-load and at no-load condition, to the
burst mode. These modes and related output signals, OUTE, OUTF, driving the rectifier MOSFETs, are shown in
Figure 49.
Figure 49. Major Waveforms During Transitions Between Different Operating Modes
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