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UCC28950 Datasheet(PDF) 37 Page - Texas Instruments |
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UCC28950 Datasheet(HTML) 37 Page - Texas Instruments |
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37 / 45 page  OUTE (CCM Mode) OUTF (CCM Mode) OUTE OUTF Transformer Winding Magnetizing Current OUTE and OUTF are disabled if V CS < VDCM OUTE and OUTF are disabled if V CS < VDCM Burst Mode at light load with T MIN maintaining ZVS (The time scale is different versus above diagram) UCC28950 www.ti.com SLUSA16A – MARCH 2010 – REVISED JULY 2010 Major waveforms of the phase-shifted converter during nominal operation mode are shown in Figure 47. Upper six waveforms in the Figure 47 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, IPR, is the current flowing through the primary winding of power transformer. The bottom four waveforms show the drain-source voltages of rectifier MOSFETs, VDS_QE and VDS_QF, the voltage at the output inductor, V LOUT, and the current through the output inductor, I LOUT. 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 48. Figure 48. Major Waveforms During Transitions Between Different Operating Modes Copyright © 2010, Texas Instruments Incorporated Submit Documentation Feedback 37 Product Folder Link(s): UCC28950 |
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