Rectifiers receive an alternating current (AC) with an average value of zero volts and supply a direct current (DC), single polarity signal with a net value greater than zero volts, a process otherwise known as rectification. The essential component in a rectifier is the diode. The diode is an electronic component that allows current to flow in only one direction, from an anode (+) to a cathode (-). A single rectifier diode will only allow half of an AC signal to propagate while blocking the reverse polarity so long as it does not exceed the breakdown voltage. Several circuits are available that allow for half-wave and full-wave rectification.
Half-Wave / Full-Wave Rectifiers
There are two types of rectifiers, the half-wave rectifier (HWR) and the full-wave rectifier (FWR). They both receive an AC input signal with an average dc voltage of zero, and return a DC signal with a non-zero average voltage.
Half-wave rectifier diodes only allow for a single polarity of the AC input signal to be transmitted. The reverse polarity is blocked and the transmitted signal is of a single polarity with an average value greater than 0 volts.
Full-wave rectifier diodes produce a unidirectional DC current by rectifying both the positive and negative half-cycles of the AC input.
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Rectifiers are available in many different configurations and most diode junctions in the circuit are either a PN junction or Schottky diode.
PN Junction Diode
PN junction diodes are used for general-purpose applications. They are also known as PN diodes, PN junctions, or small-signal diodes. The junction is composed of two semiconductors, an anode with a positive charge or P-region, and a cathode with negative charge or N-region. Applying a forward bias voltage to the PN junction causes current to flow in only one direction as electrons from the N-region fill “holes” in the P-region. The capacitance, acquired from the forward bias, requires a reverse recover period before the junction can switch states.
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Schottky diodes are also known as Schottky barrier diodes or hot-carrier diodes. They consist of a junction between a metal layer and a semiconductor element. The metal layer, cathode, is heavily occupied with conduction-band electrons. The semiconductor element, anode, is a lightly doped N-type semiconductor. When forward-biased, the higher-energy electrons in the N region are injected into the metal region, allowing the junction to operate in the ON state. Schottky diodes achieve high switching speeds as they give up their excess energy very rapidly as they fluctuate between ON and OFF states.
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Rectifiers use one of several general circuitry designs in order to rectify an AC supply or signal. Common rectifier configurations include bridge rectifiers, center-tap rectifiers, and single diodes.
Bridge rectifiers are full wave rectifiers consisting of four diodes assembled in a bridge configuration. The AC input has two paths at each terminal. During the first period of a sine wave the polarity allows a direct path for current to flow to each corresponding terminal in the DC output. During the second period of the sine wave, when the polarity is reversed at the AC input, the path reverses so that the polarity across the DC output remains constant.
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The basic circuitry for a center-tap rectifier uses a center tap transformer wired through two diodes. The center-tap is a reference point or ground, and the potential above or below the reference point is directed through one of two diodes. The output signal is a DC source or signal that utilizes the full-wave of the AC supply.
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A single diode is capable of rectifying an AC supply. It allows the forward voltage of the first half-wave of the AC signal to propagate through the diode. The reverse voltage is then blocked during the second half-wave of the AC input providing a single polarity non-zero voltage output. While the single diode configuration is capable of rectifying an AC signal it is not as efficient as half of the input signal is lost.
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Advantages / Disadvantages
Depending on your application you may benefit from any of the aforementioned circuitries. Single diodes are commonly used with smaller voltages where power loss is not a concern. Center-tap rectifiers pre-date bridge rectifiers and are popular outside of semiconductors where space and heat loss is feasible. Bridge rectifiers have become the preferred choice for many electronic applications.
|Bridge Rectifier||Efficient (Space and Power); Smoother Output Signal||Requires Input Signal Isolation (Commonly Used with a Transformer)|
|Center-tap||Full-Wave Rectifier||Increased Heat Loss (Center-tap Transformer); Larger Package|
|Single Diode||Simplistic Design; Economical||Choppy Signal (Ripple); Inefficient (Power Loss)|
Performance specifications for rectifier diodes include average rectified current, reverse current, forward voltage, peak forward surge current, reverse recovery time, and junction operating temperature.
Average rectified current (Io) is the maximum allowable continuous average current in the forward direction under specified conditions.
Reverse current or leakage current (IR), the current at which the specified reverse voltage is applied, measures the current that flows when reverse bias is applied to a semiconductor junction.
Forward voltage (VF) is the voltage across the diode terminals resulting from the flow of current in the forward direction.
Peak forward surge current (IFSM) is the maximum allowable surge value of forward current without repetition.
Reverse recovery time (trr) is the time taken for the reverse current (IR) to reach a specified level when the reverse voltage is applied while the device is conducting in the forward direction.
Junction operating temperature (Tj) is the range of temperatures at which diode are designed to operate.
IC Package Types
IC package types for rectifier diodes include transistor outline (TO), diode outline (DO), small outline transistor (SOT), and small outline diode (SOD). Other rectifier diodes are available in a discrete package (DPAK) or in D2PAK, a large surface-mounted package that includes a heat sink. SC-59, SC-74, and SC-76 are plastic, surface-mounted IC packages with three leads. Metal electrode leadless face (MELF) diodes have metallized terminals at each end of a cylindrical body and are designed to fit the same footprints as flat components. QuadroMELF diodes have a square cross section to provide better on board stability and greater "pick and place" accuracy. MiniMELF is a miniature version of MELF and MicroMELF has the same footprint as the SOD110 and SOD323 packages.
Product Lifecycle Stages
Rectifier diodes follow product lifecycle stages that are defined by the Electronic Industries Alliance (EIA) in EIA-724. Numbered stages range from zero to eight and cover product introduction, growth, maturity, market saturation, market decline, phase out, last shipment, and removal. The first stage, Life Cycle Stage Code 0, describes rectifier diodes that are in the planning or early design stages. The last stage, Life Cycle Stage Code 8, describes rectifier diodes that are no longer stocked in inventory or available for sale.
RoHS / WEEE Compliance
Restriction of Hazardous Substances (RoHS) is a European Union (EU) directive that requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of the following hazardous substances: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether. RoHS became effective on July 1, 2006.
Waste Electrical and Electronics Equipment (WEEE) is a European Union (EU) directive designed to encourage the reuse, recycling and recovery of electrical and electronic equipment. WEEE is also designed to improve the environmental impact and performance of this equipment. The WEEE directive establishes requirements and criteria for the collection, treatment, recycling and recovery of electrical and electronic equipment. It also makes producers responsible for financing these activities. Retailers and distributors must provide a way for consumers to return used or obsolete equipment without charge.
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Current Limiting Diodes
Current limiting diodes (CLD) regulate current over a wide voltage range. There are several types of current limiting diodes (CLD). Examples include a current regulator diode, constant current diode, and current limit diodes.
General Purpose Diodes
General-purpose diodes are electric components that conduct electric current in only one direction, functioning similarly to a one-way valve.
Gunn and IMPATT Diodes
Gunn diodes or transfer electron devices (TED) exhibit a negative resistance region. They are used in high-frequency applications, often for building RF oscillators. Impact ionization avalanche transit-time (IMPATT) diodes are designed to operate at very high frequency and power. They are used as elements in RF and microwave devices.
Power diodes are used mainly in high-power applications. They are built with large P-N junctions in order to pass large amounts of current and dissipate large amounts of heat.
Schottky diodes in their simplest form consist of a metal layer that contacts a semiconductor element. The metal / semiconductor junctions exhibit rectifying behavior (i.e., the current passes through the structure more readily with one polarity than the other).
Step Recovery Diodes
Step recovery diodes produce an abrupt turn-off (step) time by allowing a very fast release of stored charge when switching from forward to reverse bias, and from reverse to forward bias.
Zener diodes are PN junction devices that are designed to operate in the reverse-breakdown region.