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.


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Diode — Figure 1: Closeup of a diode, showing the square shaped semiconductor crystal (black object on left) …   Wikipedia

  • Diode bridge — Three bridge rectifiers. The size is generally related to the current handling capability …   Wikipedia

  • DC-to-DC converter — An assortment of 78xx series ICs, linear DC DC converters A DC to DC converter is an electronic circuit which converts a source of direct current (DC) from one voltage level to another. It is a class of power converter. Contents …   Wikipedia

  • Taiwanization — (Chinese: 臺灣本土化運動; pinyin: Táiwān běntǔhuà yùndòng; Pe̍h oē jī: Tâi oân pún thó͘ hòa ūn tōng), also known as the Taiwanese localization movement, is a political term used in Taiwan to emphasize the importance of a separate Taiwanese culture… …   Wikipedia

  • Rectifier — A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers …   Wikipedia

  • Vacuum tube — This article is about the electronic device. For experiments in an evacuated pipe, see free fall. For the transport system, see pneumatic tube. Modern vacuum tubes, mostly miniature style In electronics, a vacuum tube, electron tube (in North… …   Wikipedia

  • china — /chuy neuh/, n. 1. a translucent ceramic material, biscuit fired at a high temperature, its glaze fired at a low temperature. 2. any porcelain ware. 3. plates, cups, saucers, etc., collectively. 4. figurines made of porcelain or ceramic material …   Universalium

  • China — /chuy neuh/, n. 1. People s Republic of, a country in E Asia. 1,221,591,778; 3,691,502 sq. mi. (9,560,990 sq. km). Cap.: Beijing. 2. Republic of. Also called Nationalist China. a republic consisting mainly of the island of Taiwan off the SE coast …   Universalium

  • Nanofluidic circuitry — is a nanotechnology aiming for control of fluids in nanometer scale. Due to the effect of an electrical double layer within the fluid channel, the behavior of nanofluid is observed to be significantly different compared with its microfluidic… …   Wikipedia

  • Tohu and Tikun — For other Jewish, and wider meanings of Tohu and Tikun, see Tohu and Tikkun disambiguation pages Part of a series on …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.