Diode rectifier circuits are one of the key circuits used in electronic equipment. They can be used in power supplies, RF signal demodulation, RF power sensing and very much more.
There are several different types of diode rectifier circuit, each with its own advantages and disadvantages. Decisions about which type of diode circuit to use depend upon the given situation.
Diode rectifier circuit basics
The key component in any rectifier circuit is naturally the diode or diodes used. These devices are unique in only allowing current through in one direction. Interestingly Ambrose Fleming who invented the first form of diode called his version a valve because of its one way action. Semiconductor diodes now perform the same function, but occupy a fraction of the space and are normally only a small fraction of the cost.
The semiconductor diode has a characteristic something like that shown below. In the forward direction, a small voltage is required across the diode before it conducts - this is known as the turn on voltage. The actual voltage depends on the type of diode rectifier and the material used. For a standard silicon diode rectifier this turn on voltage is around 0.6 volts. Germanium diodes have a turn on voltage of around 0.2 - 0.3V, and silicon Schottky diodes have a similar turn on voltage in the region of 0.2 - 0.3V
In the reverse direction, the diode rectifier will ultimately break down. The breakdown voltage is normally well in excess of the turn on voltage - the scales on the diagram have been altered (compressed) in the reverse direction to illustrate that reverse breakdown occurs.
Note on the Diode Types:
Although the basic function of a diode remains the same, the there are many different types with slightly different characteristics. Some many be optimised for power rectification, others for signal rectification, others use the diode junction to emit light, or have a variable capacitance, etc.
Read more about the Types of Semiconductor Diode.
For power rectification applications, power diodes or Schottky diodes are normally used. For signal rectification small point contact diodes, signal diodes, or Schottky diodes may be used. The Schottky diode has the advantage that it only requires a forward voltage of around 0.2 - 0.3volts for forward conduction. This is particularly useful when detecting small radio signals, and when used as a power rectifier the power losses are reduced. However the reverse leakage characteristics are not as good as normal silicon diodes.
Diode symbol and packages
The diode circuit symbol is widely known. Diodes also come in a variety of packages, although some of the more usual formats are shown in the diagram below.
Diode rectifier action
The action of the diode is to allow current to flow in only one direction. Therefor is an alternating waveform is applied to a diode, then it will only allow conduction over half the waveform. The remaining half is blocked.
Diode rectifier circuit configurations
There are a number of different configurations of diode rectifier circuit that can be used. These different configurations each have their own advantages and disadvantages, and are therefore applicable to different applications.
- Half wave rectifier circuit: This is the simplest form of rectifier. Often using only a single diode is blocks half the cycle and allows through the other. As such only half of the waveform is used.
While the advantage of this circuit is its simplicity, the drawback is the fact that there is longer between successive peaks of the rectified signal. This makes smoothing less effective and more difficult to achieve high levels ripple rejection. . . . . Read more about the half wave rectifier .
- Full wave rectifier circuit: This form of rectifier circuit uses both halves of the waveform. This makes this form of rectifier more effective, and as there is conduction over both halves of the cycle, smoothing becomes much easier and more effective. There are two types of full ave rectifier.
- Two diode centre taped transformer full wave rectifier: The two diode version of the full wave rectifier circuit requires a centre tap in the transformer. When vacuum tubes / thermionic valves were used, this option was widely used in view of the cost of the valves. However with semiconductors, a four diode bridge circuit saves on the cost of the centre tapped transformer and is equally effective.
- Bridge full rectifier circuit: This is a specific form of full wave rectifier that utilises four diodes in a bridge topology. Bridge rectifiers are widely used, especially for power rectification, and they can be obtained as a single component contain the four diodes connected in the bridge format.
- Synchronous rectifier circuit: Synchronous or active rectifiers use active elements instead of diodes to provide the switching. This overcomes the diode losses and significantly improves the efficiency levels.
In view of the variety of different types of rectifier circuit, there is a good choice of which type to use. In many cases this is dictated by the level of performance required, and in most cases a form of full wave rectifier is needed. With the availability and low cost of bridge rectifiers, this is normally the cheapest option, rather than saving on the diodes and then requiring a centre taped transformer./p>
With modern power supplies requiring ever higher levels of efficiency, many designers are looking to the use of synchronous rectifiers. Although they are more complicated and therefore cost more, this cost is often worth the returns they give in increasing the efficiency level.
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