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Concept
Active rectification
Summary
Active rectification, or synchronous rectification, is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches, usually power MOSFETs or power bipolar junction transistors (BJT). Whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5-1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop.
Historically, vibrator driven switches or motor-driven commutators have also been used for mechanical rectifiers and synchronous rectification.
Active rectification has many applications. It is frequently used for arrays of photovoltaic panels to avoid reverse current flow that can cause overheating with partial shading while giving minimum power loss. It is also used in switched-mode power supplies (SMPS).
The constant voltage drop of a standard p-n junction diode is typically between 0.7 V and 1.7 V, causing significant power loss in the diode. Electric power depends on current and voltage: the power loss rises proportional to both current and voltage.
In low voltage converters (around 10 volts and less), the voltage drop of a diode (typically around 0.7 to 1 volt for a silicon diode at its rated current) has an adverse effect on efficiency. One classic solution replaces standard silicon diodes with Schottky diodes, which exhibit very low voltage drops (as low as 0.3 volts). However, even Schottky rectifiers can be significantly more lossy than the synchronous type, notably at high currents and low voltages.
When addressing very low-voltage converters, such as a buck converter power supply for a computer CPU (with a voltage output around 1 volt, and many amperes of output current), Schottky rectification does not provide adequate efficiency. In such applications, active rectification becomes necessary.
Replacing a diode with an actively controlled switching element such as a MOSFET is the heart of active rectification. MOSFETs have a constant very low resistance when conducting, known as on-resistance (RDS(on)).
Official source
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