Active rectification: Difference between revisions

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Active rectification is a technique for improving the efficiency of rectification by replacing diodes with actively-controlled transistors, usually power MOSFETs.

Applications

The constant voltage drop of a standard p-n junction diode is typically between 0.7V and 1.7V and affects the power lost in the diode. Electric power depends on current and voltage, so the power loss rises linearly with current, at a fixed voltage.

A simple solution is replacing the p-n diode with a Schottky diode, which exhibits a much lower voltage drop, some as low as 0.3V. Compared to the typical diode drop of 0.7V, at equal currents, the power loss can be reduced by two thirds. In some applications this may still be too much power loss and active rectification becomes necessary.

Replacing a diode with a MOSFET is the heart of active rectification. MOSFETs have a constant very low resistance when conducting, known as on-resistance ( R_DS(on)), usually around 10 milliohms. The voltage drop across the transistor is then much lower, meaning 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 is usually dealt with by placing several transistors in parallel, reducing the current through each individual one.

The control circuitry for active rectification usually uses comparators 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 capacitors present in passive examples.

Using active rectification to implement AC/DC conversion can allow a design to undergo further improvements (with more complexity) to achieve 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.

Construction

See H-Bridge.

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 __NOTOC__
-'''Active rectification''' is a technique for improving the efficiency of rectification by replacing a [[diode]] with a [[transistor]] (usually a [[power MOSFET]]).
+'''Active rectification''' is a technique for improving the efficiency of rectification by replacing [[diode|diodes]] with actively-controlled [[transistor|transistors]], usually [[power MOSFET|power MOSFETs]].
 == Applications ==
 [[Image:FET_Diode_Comparison_Chart.JPG|thumb|250px|'''Graph 1:'''<br />Plot of power dissipated vs. current in 4 devices.]]
-The constant voltage drop of a "standard" [[diode]] is typically around 0.7V and affects the [[electric power|power]] lost across the [[diode]].  Since [[electric power|power]] is volts-times-amps, 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.
+The constant voltage drop of a standard p-n junction diode is typically between 0.7V and 1.7V and affects the power lost in the diode. [[power|Electric power]] depends on current and voltage, so the power loss rises linearly with current, at a fixed voltage.
-A "standard" [[diode]] can be replaced with a [[Schottky diode]], which exhibits a much lower voltage drop, some as low as 0.3V.  Compared to the typical diode drop of 0.7V, at equal currents, that's a 57% reduction in the power lost.  In some applications there may still be too much power lost in even a [[Schottky diode]] and a better solution does exist.
+A simple solution is replacing the p-n diode with a [[Schottky diode]], which exhibits a much lower voltage drop, some as low as 0.3V.  Compared to the typical diode drop of 0.7V, at equal currents, the power loss can be reduced by two thirds.  In some applications this may still be too much power loss and active rectification becomes necessary.
-Replacing a diode with a [[MOSFET]] is the heart of Active Rectification.  [[MOSFET]]s have a constant resistance when conducting, known as on-resistance and often abbreviated R<sub>DS(on)</sub>.  [[Ohm's Law]] then governs the voltage drop across the [[MOSFET]].  For any given R<sub>DS(on)</sub> there will be a current at which the voltage drop will approach that of typical [[Schottky diode]]s or even "standard" diodes.  A [[MOSFET]] is only a valid replacement up to this current (see '''Graph 1'''), e.g. 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 [[MOSFET]] and below this there will be more power lost in the diode.  However, one could place more [[MOSFET]]s in parallel, lowering the total, effective R<sub>DS(on)</sub>, and the power dissipated.  Paralleling more diodes will not lower total power dissipation, but will help a design allow more total current because the diodes can share the load.
+Replacing a diode with 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>), usually around 10 milliohms. The voltage drop across the transistor is then much lower, meaning 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 is usually dealt with by placing several transistors in parallel, reducing the current through each individual one.
+The control circuitry for active rectification usually uses [[comparator|comparators]] 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|capacitors]] present in passive examples.
-A [[diode bridge|full bridge rectifier]] can be built using an [[H-bridge]] arrangement of [[MOSFET]]s.  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 an Active Rectifier is even more difficult since the [[MOSFET]] switches have to be timed so the output always sees current in the same direction.  This requires complex circuitry that turns on the appropriate pair of [[MOSFET]]s just after the absolute value of the AC voltage rises above the DC bus voltage, and turns them off when the absolute value of the AC voltage drops below the DC bus voltage.
 Using active rectification to implement [[AC/DC conversion]] can allow a design to undergo further improvements (with more complexity) to achieve [[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.

Active rectification: Difference between revisions

Revision as of 09:34, 20 August 2009

Applications

Construction

References

Further reading