science

Active rectification

Active rectification is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Active rectification rather than just read about it. In short: 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 to 1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop.

Key takeaways

Active rectification belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Active rectification to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Active rectification from memory before moving on to harder problems.

Reference excerpt

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 to 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).

Motivation

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.

Description

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)). A typical modern MOSFET has an on-resistance of less than an ohm, with many high-performance devices having specifications of on the order of 10 mΩ or less. 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 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 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 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

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 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. 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

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). Single Phase Passive Rectification versus Active Rectification Applied to High Power Stirling Engines. AIAA 2005-5687.

Illustrations

Worked examples

Example 1 — a first encounter with Active rectification

Start with the simplest possible case. Write down what Active rectification claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Active rectification before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Active rectification ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Active rectification

In research: Active rectification appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Active rectification in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Active rectification is common in secondary-school and first-year university syllabi. It links to neighbouring topics Rectifiers, so understanding it makes those chapters shorter.
In everyday life: Look for Active rectification outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Active rectification in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Active rectification means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Active rectification out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Active rectification in simple terms?

Why does Active rectification matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Active rectification?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Active rectification.