imported>LR.127: Adding short description: "Technique in electrical devices"

2025-02-18T18:57:46Z

Adding short description: "Technique in electrical devices"

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{{Short description|Technique in electrical devices}}
[[Image:Diode mosfet.svg|thumb|250px|Voltage drop across a diode and a MOSFET. The low on-resistance property of a MOSFET reduces ohmic losses compared to the diode rectifier (below 32 A in this case), which exhibits a significant voltage drop even at very low current levels. Paralleling two MOSFETs (pink curve) reduces the losses further, whereas paralleling several diodes won't significantly reduce the forward-voltage drop.]]

'''Active rectification''', or '''synchronous rectification''', is a technique for improving the efficiency of [[rectification (electricity)|rectification]] by replacing [[diode]]s with actively controlled switches, usually [[power MOSFET]]s or power [[bipolar junction transistor]]s (BJT).<ref name="emadi"/> Whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5 to 1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop.

Historically, [[vibrator (electronic)|vibrator]]-driven switches or motor-driven [[commutator (electric)|commutator]]s have also been used for [[mechanical rectifier]]s and synchronous rectification.<ref>
{{cite book
| title = Standard polyphase apparatus and systems
| edition = 5th
| author = Maurice Agnus Oudin
| publisher = Van Nostrand
| date = 1907
| page = [https://archive.org/details/standardpolypha00oudigoog/page/n248 236]
| url = https://archive.org/details/standardpolypha00oudigoog
| quote = synchronous rectifier commutator.
}}</ref>

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).<ref name="emadi"/>

== Motivation ==
[[Image:FET Diode Comparison Chart.JPG|thumb|250px|Plot of power dissipated vs. current in four devices.]]

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 [[DC to DC converter|converters]] (around 10 [[volt]]s 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 diode]]s, 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 [[ampere]]s of output current), Schottky rectification does not provide adequate efficiency. In such applications, active rectification becomes necessary.<ref name="emadi">
{{cite book
| title = Integrated power electronic converters and digital control
| edition =
| author = Ali Emadi
| publisher = CRC Press
| date = 2009
| isbn = 978-1-4398-0069-0
| pages = 145–146
| url = https://books.google.com/books?id=phX659AzaxUC&pg=PA145
}}</ref>

== Description ==
[[File:Synchron Gleichrichter.svg|thumb|250px|Active full-wave rectification with two MOSFETs and a center tap transformer.]]
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 (R<sub>DS(on)</sub>). They can be made with an on-resistance as low as 10 mΩ or even lower. The voltage drop across the transistor is then much lower, causing a reduction in power loss and a gain in efficiency. However, [[Ohm's law]] governs the voltage drop across the MOSFET, meaning that at high currents, the drop can exceed that of a diode. This limitation is usually dealt with either by placing several transistors in parallel, thereby reducing the current through each individual one, or by using a device with more active area (on FETs, a device-equivalent of parallel).

The control circuitry for active rectification usually uses [[comparator]]s to sense the voltage of the input AC and open the transistors at the correct times to allow current to flow in the correct direction. The timing is very important, as a short circuit across the input power must be avoided and can easily be caused by one transistor turning on before another has turned off. Active rectifiers also clearly still need the [[smoothing capacitor]]s present in passive examples to provide smoother power than rectification does alone.

Using active rectification to implement [[AC/DC conversion]] allows a design to undergo further improvements (with more complexity) to achieve an [[Power factor#Active PFC|active power factor correction]], which forces the current waveform of the AC source to follow the voltage waveform, eliminating reactive currents and allowing the total system to achieve greater efficiency.

== MOSFET-based ideal diode ==
{{Not to be confused with|Ideal diode equation}}
A MOSFET actively controlled to act as a [[rectifier]]—actively turned on to allow current in one direction but actively turned off to block current from flowing the other direction—is sometimes called an ''ideal diode''.

Using these ideal diodes rather than standard diodes for [[solar electric panel]] bypass, reverse-battery protection, or [[bridge rectifier|bridge rectifiers]] reduces the amount of power dissipated in the diodes, improving efficiency and reducing the size of the circuit board and the weight of the heat sink required to deal with the power dissipation.<ref>
[http://www.mouser.com/ds/2/277/MP6914_r1.0-526360.pdf "Ideal Diode for Solar Panel Bypass"].
</ref><ref>
[http://cds.linear.com/docs/en/datasheet/4320fb.pdf "Ideal Diode Bridge Controller"].
</ref><ref>
[http://www.eejournal.com/archives/news/20131022-05/ "Ideal Diode Bridge Controller Minimizes Power Loss & Heat in PoE Powered Devices"]
</ref><ref>[https://www.maximintegrated.com/en/app-notes/index.mvp/id/636 "Reverse-Current Circuitry Protection"] {{Webarchive|url=https://web.archive.org/web/20190813173244/https://www.maximintegrated.com/en/app-notes/index.mvp/id/636 |date=2019-08-13 }}.</ref><ref>
[http://www.ti.com/lit/an/slva139/slva139.pdf "Reverse Current/Battery Protection Circuits"].
</ref><ref>
[http://www.arrl.org/files/file/Technology/HandsOnRadio/Thoughts%20on%20Reverse%20Power%20Protection%20using%20Power%20MOSFETs%20-%20Wheeler%20N0GSG.pdf "Reverse Power Protection using Power MOSFETs"].
</ref>

Such a MOSFET-based ideal diode is not to be confused with an op-amp based [[super diode]], often called a precision rectifier.

== Construction ==
See [[H-bridge]].

== References ==
{{reflist}}

== 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, November 1999, pp. 591–596.'' Digital Object Identifier:10.1049/ip-epa:19990523.
* W. Santiago, A. Birchenough. (2005). [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060012149_2006012537.pdf Single Phase Passive Rectification versus Active Rectification Applied to High Power Stirling Engines]. AIAA 2005-5687.

[[Category:Rectifiers]]

[[de:Gleichrichter#Synchrongleichrichter]]

Active rectification - Revision history

imported>LR.127: Adding short description: "Technique in electrical devices"