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the first element of an electronics power supply that any incoming power will meet is the transformer and AC rectifier circuits. This element of any electronics power supply converts the incoming power to a form which can be accepted by the smoothing and regulator circuits.
When running from an AC source, a transformer is used to transform the incoming mains voltage to the correct value required for the power supply electronics circuitry. The resulting voltage waveform is an alternating current. This must be rectified to enable the power to be smoothed and regulated for use by electronics circuitry. To achieve this an AC rectifier circuit is used. While the rectifier circuit may appear to be very simple at first sight, there are several different forms of AC rectifier circuit that can be used. The choice of the actual AC rectifier circuit chosen will depend upon a number of factors, and it could impact upon the type of transformer used as well.
Half and full wave rectifier circuits
AC rectifier circuits can use diodes in a variety of circuit configurations. By utilising the diodes in different ways it is possible to achieve different levels of performance. There are two basic types of AC rectifier circuits:
- Half wave rectifier circuits
- Full wave rectifier circuits
Of the two forms of AC rectifier circuit, the full wave rectifier circuit is more commonly used, especially in applications where performance is needed. A half wave rectifier is more normally used for applications where power is required for a small ancillary circuit and where less current is drawn.
Half wave rectifier circuits
As the name implies, half wave AC rectifier circuits only use half of the AC waveform in the rectification process. In other words they allow through one half of the cycle and block the other half. This means that power is supplied, to the output of the rectifier circuit - often the smoothing circuit only over half the cycle and this leaves half a cycle when no power is supplied. Accordingly the voltage across any smoothing capacitor drops over this period as charge is removed from the smoothing capacitor by the load circuit. Accordingly levels of ripple are higher than those experienced with full wave rectification as will be seen below.
The circuits for half wave rectifiers are relatively simple. The rectification process can be achieved using a single diode. It is the simplicity of the circuit that makes the half wave rectifier circuit attractive in many applications. It uses a minimum of components and it is able to provide a voltage quite adequately for many uses.
When choosing diodes for use in AC rectifier circuits, one parameter which is of importance is the reverse voltage rating. This is called the Peak Inverse Voltage, PIV. For a half wave rectifier the PIV for the diode must be at least twice the peak voltage of the AC waveform. The reason for this is that it must be assumed that the smoothing capacitor will hold the peak voltage of the AC waveform. Then as the diode is in the non conducting part of the waveform the AC waveform reaches its peak, the diode rectifier will see this peak on top of the peak voltage held by the capacitor, i.e. twice the peak value of the waveform. It is worth noting that the peak value of a sine wave is 1.414 times the RMS value. Thus the PIV rating for the diode must be 2 x 1.414 times the RMS value of the AC waveform. On top of this it is worth leaving a healthy margin to accommodate any spikes that may appear on the supply line.
Full wave rectifier circuits
Full wave rectifier circuits are able to utilise both halves of the incoming waveform, and in this way they are more effective than the half wave varieties. However to achieve this, these rectifier circuits require the use of more diodes.
The full wave AC rectifier circuit provides two different paths, one for each half of the cycle. In this way one diode of sets of diodes conducts on one half of the cycle, whereas another diode of set of diodes conducts on the other half of the cycle.
The bridge rectifier circuit is used in many full wave rectifier circuits. Comprising four diodes, it is an effective form of rectification. In view of this many manufacturers make bridge rectifier blocks containing four diodes. Often as these bridge rectifiers pass significant levels of current, they dissipate some power and become hot. To prevent them overheating, these bridge rectifiers are often made on a format that enables them to be bolted to a heat-sink of some form.
AC rectifier circuits are widely sued in all forms of electronic equipment. Wherever an AC power supply is used, a rectifier circuit will be incorporated because electronics circuits use DC to supplies for their operation. Although power supplies may not form a direct part of the function of the equipment, they are essential because without some form of power supply, the whole circuit would not operate. As mains supplies use AC because of the transmission characteristics, and it is also necessary to use AC to enable transformers to be employed, AC rectifier circuits will always be found. Within these circuits bridge rectifiers are also very common because they form a cheap and effective form of component to be used in these circuits.
By Ian Poole
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