Synchronous rectifiers are also known as active rectifiers and they are used to improve the efficiency of diode rectifier circuits.
The semiconductor diodes are replaced with active switching elements: transistors that may be power MOSFETs or power bipolar transistors that are switched on and off at the required times to allow the rectification to occur.
As the switching obviously has to occur in synchronism with the incoming waveform, these rectifiers are often referred to as synchronous rectifiers or sometimes as active rectifiers.
Rationale for synchronous rectifiers
The need for synchronous rectifiers or active rectifiers results from the constant drop that occurs across a diode when it is conducting.
Although the turn on voltage for a silicon diode - the type most usually used for rectifiers is around 0.6 volts, the actual drop across the diode may rise to in excess of 1 volt at its rated current.
The use of Schottky diodes can reduce the voltage drop, but it can still be an issue, especially where the highest levels of efficiency are required. Synchronous rectifiers are able to provide improvements even over Schottky diode rectifiers.
The issue of efficiency is even more acute when using low voltage converters. With voltage levels of just a very few volts, and with the possibility of high current levels the voltage drops introduced by diodes become unacceptable and synchronous rectifier techniques become essential
Synchronous rectification basics
In a typical diode rectifier, the diode turns on when it is forward biased and off when it is reverse biased. It is possible to control an active element so that the effect same happens. The advantage with an active rectifier is that the conducting resistance and voltage drop are much less than they are with diodes.
As the switching of the active element has to be timed correctly it is actually in synchronism with the waveform being rectified. It is for this reason that these rectifiers are known as synchronous rectifiers.
Often power MOSFETs are ideal active elements for synchronous rectification, and they have a very low on resistance, RDS on that may be as low as a few tens of mΩ or less. The voltage drop across this level of resistance is likely to be very much less than that across a diode. Where the voltage drop across a power MOSFET does become an issue, then several devices can be placed in parallel.
The downside to synchronous or active rectifiers is that they require control circuitry to ensure the devices turn on synchronously, i.e. at the right time. Circuitry required for the control of the synchronous rectifier normally includes voltage level detectors and drive circuity for the active devices.
One key issue for the control circuitry is to ensure that two devices in opposing legs of the rectifier do not turn on together otherwise a short circuit would be presented to the input. The turn on and turn off of devices is normally controlled to ensure that even at the point where one turns on and another off, there is a short gap to prevent both devices being on together.
Active rectification or synchronous rectification is often employed in AC/DC converters where efficiency is a key issue. Using a synchronous rectifier enables power losses to be minimised and efficiency levels to be improved, although at the expense of additional complexity.