Overview
Diode rectifier circuit types include: half-wave rectifier, full-wave rectifier, dual-diode full-wave rectifier, bridge full-wave rectifier, and synchronous rectifier.
Diode rectifier circuits are fundamental in electronic equipment. They are used in switch-mode power supplies and linear power supplies, RF signal demodulation, RF power detection, and other applications.
There are several rectifier configurations, each with its own advantages and disadvantages. The choice of topology depends on the specific application requirements.
Diode symbol and basic concept
Any rectifier circuit relies on one or more diodes. A diode allows current to flow in one direction while blocking current in the opposite direction.
Historically, Ambrose Fleming invented the first form of diode—the thermionic diode—and called it a valve because of its one-way action. Semiconductor diodes now serve the same function but occupy much less space and are typically much lower in cost.
PN diode VI characteristics
Semiconductor diodes show the characteristic behavior illustrated above. In the forward direction, a small voltage is required across the diode before it conducts; this is the forward voltage.
The actual forward voltage depends on the diode type and the materials used. For a standard silicon diode the forward voltage is about 0.6 V. Germanium diodes have a forward voltage around 0.2–0.3 V. Silicon Schottky diodes also have forward voltages in the 0.2–0.3 V range.
In the reverse direction, a diode conducts only a very small leakage current. Leakage is usually negligible for most modern rectifier diodes. However, with increasing reverse voltage, the diode will eventually undergo breakdown. The breakdown voltage is typically much higher than the forward voltage.
For power rectification, power diodes or Schottky diodes are commonly used. For signal rectification, small junction diodes, signal diodes, or Schottky diodes can be used. The lower forward voltage of Schottky diodes is advantageous for RF detection because the detected signal levels can be small; the forward threshold is therefore important. Schottky diodes generally exhibit higher reverse leakage than standard silicon diodes.
Diode packaging
Semiconductor diodes are available in many packages. Leaded diodes are common for through-hole applications; some small signal diodes use glass leaded packages, while others use plastic leaded packages.
For higher-power applications, diodes are often supplied in bolt-mounted packages for attachment to heatsinks. These devices suit very high-current circuits where significant heat may be dissipated due to forward voltage drop and current.
Surface-mount diodes are widely available and increasingly used in automated production. While their power handling may be insufficient for the highest-power designs, they are suitable for many electronic circuits.
Another common package type is the bridge rectifier, which contains four diodes in a single package to provide full-wave rectification. Bridge rectifiers are convenient because a single component can replace four discrete diodes.
How diode rectifiers work
A diode permits current flow in one direction only. When an alternating waveform is applied to a diode, it conducts during the half-cycle aligned with its forward direction and blocks the opposite half-cycle. The result is that only half of the waveform is passed through.
Rectifier circuit configurations
Various rectifier configurations are used, each with specific trade-offs. The most common are described below.
Half-wave rectifier
The half-wave rectifier is the simplest form and typically uses a single diode to block one half of the AC cycle while allowing the other half to pass. The advantage is simplicity; the disadvantage is the long interval between consecutive peaks of the rectified waveform, which reduces smoothing effectiveness and makes ripple suppression more difficult.
Half-wave rectification is rarely used for power supplies and is more commonly found in signal detection and level-detection applications.
Bridge full-wave rectifier
The bridge full-wave rectifier uses four diodes in a bridge topology. It is widely used for power rectification and is available as a single packaged component that contains the four diodes.
This topology conducts through two diodes during each half-cycle, so two diode drops are present in the conduction path and some power is dissipated. The bridge topology eliminates the need for a center-tapped transformer, saving cost and complexity. Also, the diodes in a bridge do not need as high a reverse voltage rating as the diodes used in a two-diode, center-tapped configuration.
Because there are two diode drops, bridge rectifiers are less common in sensitive signal detection applications, but they are well suited to linear power supplies and are often used in switch-mode power supplies.
Synchronous rectifier
Synchronous or active rectifiers use active switches instead of diodes to perform the rectification. These switches can eliminate diode conduction losses and significantly improve efficiency.
Synchronous rectifiers are widely used in high-efficiency switch-mode power supplies. They are more complex than diode-based rectifiers, but the efficiency gains often justify the extra complexity and cost.
Choosing a rectifier topology
The choice of rectifier topology depends on the required performance. In many power-supply applications a form of full-wave rectification is preferred. Bridge rectifiers are commonly used because they are inexpensive and avoid the need for a center-tapped transformer.
As efficiency requirements for modern power supplies increase, many designers choose synchronous rectification despite the higher complexity and cost, because the efficiency improvements can be significant.
