Switched mode power supplies are widely used in a variety of applications due to their efficiency over alternative electric power regulators such as linear regulators. One technique for increasing the efficiency of a switched mode power supply (SMPS) is synchronous rectification. Rectifiers (e.g. diodes) are conventionally used on the secondary side to convert the alternating current (AC) waveform propagated through the transformer in a direct current (DC) at a regulated level, either voltage or current. The voltage drop across a diode, however, is a source of loss.
To reduce the loss due to rectification a switching transistor such as a metallic oxide semiconductor field effect transistor (MOSFET) or an enhancement mode gallium arsenide transistor can be used instead. The switching transistor, although it has an inherent body diode, is switched on when current flows in the appropriate direction, and then switched off otherwise to operate as a rectifier. Since a switching transistor can have on the order of milliohms or a few tenths of a milliohm of on-resistance, much less of a voltage drop results typically, compared to use of even a Schottky diode for most power converter applications.
Control of the switching transistor rectifier is typically based on current through the switching transistor. On a normal conduction phase, voltage build across the switching transistor, and some leakage current may pass through the body diode of the switching transistor. When this voltage drop is sensed, the switching transistor can be switched on. When the voltage across the switching transistor then drops back to zero, or sufficiently low to indicate the end of the conduction phase, the switching transistor can be turned back off to provide proper rectifier operation.
However, noise resulting from switching transients can result in false triggering of the switching transistor, leading to inefficient operation. False triggering can be alleviated by the use of minimum on and off times, which allow transients to dissipate before sensing. Thus, transients that occur during these “blanking” times are ignored and have no effect on the switch control of the switching transistor rectifier. A problem still remains, though, after turn on, or after enabling the synchronous controller, since those events can occur just prior to the occurrence of a transient, which can be falsely detected as a proper triggering event and rectifier control will not be properly synchronized.
Accordingly, there is a need for a method and apparatus for self-synchronous rectifier control in response to turn-on and enabling events.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. The details of well-known elements, structure, or processes that would be necessary to practice the embodiments, and that would be well known to those of skill in the art, are not necessarily shown and should be assumed to be present unless otherwise indicated.
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