Rectification synchronous

First, the designer should choose the type of rectification technology that is most appropriate for the application. The choice is whether to use passive rectification in which semiconductor rectifiers are used or synchronous recification in which power MOSFE B are placed in parallel with a smaller passive rectifier. Synchronous rectifiers are typically used in battery operated portable products where the added efficiency, usually an added two to eight percent, is important to extend the operating life of the battery or in applications where heat is important. In today s switching power supplies, passive rectifiers can dissipate 40 to 60 percent of the total losses within the power supply. Synchronous rectifiers affect only the conduction loss, which can be reduced by as much as 90 percent. 

For those applications where high efficiency is important, synchronous rectification may be used on the higher current (power) outputs. Synchronous rectifier circuits are much more complicated than the passive 2-leaded rectifier circuits. These are power MOSFE B, which are utilized in the reverse conduction direction where the anti-parallel intrinsic diode conducts. The MOSFET is turned on whenever the rectifier is required to conduct, thus reducing the forward voltage drop to less than O.f V. Synchronous rectifiers can be used only when the diode current flows in the forward direction, that is in continuousmode forward converters. 

Power Supply Cookbook, Second Edition has been updated with the latest advances in the field of efficient power conversion. Efficiencies of between 80 to 95 percent are now possible using these new techniques. The major losses within the switching power supply and the modern techniques to reduce them are discussed at length. These include synchronous rectification, lossless snubbers, and active clamps. The information on methods of control, noise control, and optimum printed circuit board layout has also been updated. 

To measure impedance, an external current must be applied it is an exogenous method. Thus, impedance measurements are ideally suited for synchronous rectification (SR) because the external excitation signal is available. The SR acts as a sort of sharp tracking filter with a bandwidth dependent on the time constant of the output of the low-pass filter. The result is a very noisy robust system. 

The original signal frequencies are recovered by demodulation of the amplified signal. In the simplest case the demodulation consists of rectification and subsequent low-pass filtering. A better method uses a synchronous rectifier where the a.c. signal is multiplied by a sine or a square wave of constant amplitude derived from the chopper action. The phase of this reference frequency must be properly adjusted the... 

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