Active rectification

Active rectification is a technique for improving efficiency of Diode Bridge rectifiers. It consists of replacing a diode with a transistor (usually a power MOSFET).

Applications

In Full Bridge Rectifiers, the voltage drop of a diode (typically around 0.7V for a silicon diode at its rated current) affects the power efficiency because the power lost across the diode is a function of the drop across it times the current through it. The more current that passes through, the more power lost. The power lost goes up linearly with the current since the voltage stays (fairly) constant for any amount of current.

A classic solution to this power loss problem consists of using Schottky diodes, which exhibit much lower voltage drops than traditional diodes, some as low as 0.3 volts. Compared to the typical diode drop of 0.7V, at equal currents, that's a 57% reduction in the power lost in the bridge. But, in some circumstances there is still too much power lost and better solutions do exist.

Replacing the diodes with MOSFETs is the heart of Active Rectification. This topology is virtually identical to the H-Bridge arrangement. The difference? An active rectifier inputs AC power at the center of the bridge and outputs DC power to the load at the top and bottom of the bridge. The MOSFETs are very carefully timed to turn on and off in a fashion that allows the AC current to always flow the same way through the DC load. An H-Bridge's inputs and outputs are reversed: it inputs DC power at the top and bottom of the bridge and outputs power at the center in one of two discrete directions, typically to a DC motor, to control the motor's direction of spin. The MOSFETs are turned on and off purely to control the direction of spin, and timing is not of near as great importance.

Further differences include the way in which these two devices are driven. Control circuitry for an H-Bridge is complex enough, a destructive condition known as "shoot-through" occurs when two FETs on either the left or the right turn on together and short the input power, typically letting out the "magic smoke." Control circuitry for the Active Rectifier is even more difficult to time the MOSFET switches so the output always sees current in the same direction. This requires complex circuitry that turns off one set of FETs just before the AC waveform approaches 0V from either the positive or negative direction, and waits to turn the other set on until the AC waveform gets equally as far from 0V in the opposite direction.

It can be noted that MOSFETs may not always be the more efficient device. Diodes have a set voltage drop across them which is constant for the entire rated current capability of the diode in an ideal diode and varies very little in a realized diode. As current through the diode increases, so too does the power lost in the diode. Since the voltage drop stays the same as the current increases then the diode acts like a current-dependent resistor whose resistance lowers as the current increases. While diodes have a fairly constant voltage drop across them no matter the amount of current passing through them the power dissipated (lost) in the diode is a function linearly increasing with the current because power equals the current through a device times the voltage across that device (P=I*V). MOSFETs act more like true resistors, and so the more current passing through them the higher the voltage drop across them becomes. A more convenient way to think of the power loss is the equation I^2*R (I-squared times R). Since the resistance is constant this time every increase in current is a power of two increase in power loss. At extremely high currents it may be possible that a diode would be more efficient than a MOSFET; however, a MOSFET can always be paralleled with more MOSFETs to decrease the on-state resistance and lower the power lost. (Example: at 70Amps it transitions to becoming more beneficial to use a 0.7V diode instead of a 10milliohm MOSFET because the power lost in each is equal, above this there will be more power lost in the FET and below this there will be more power lost in the diode).

Construction

See H-Bridge.

Further reading

*T. Grossen, E. Menzel, J.J.R. Enslin. (1999) Three-phase buck active rectifier with power factor correction and low EMI. "IEE Proceedings - Electric Power Applications, Vol. 146, Iss. 6, Nov. 1999, pp. 591-596." Digital Object Identifier:10.1049/ip-epa:19990523.

*W. Santiago, A. Birchenough. (2005). [http://gltrs.grc.nasa.gov/reports/2006/TM-2006-214045.pdf Single Phase Passive Rectification versus Active Rectification Applied to High Power Stirling Engines] . AIAA 2005-5687.