Power Diodes and Rectifiers

The Power Diode

In the previous tutorials we saw that a diode will only conduct current in one direction (from Anode to Cathode) but not in the reverse direction acting a bit like a one way valve. A widely used application of this feature is in the conversion of alternating voltages (AC) to direct voltages (DC), in other words, Rectification, especially used in power supply applications. Small signal diodes can be used as rectifiers in low-power, low current rectifiers or applications but were larger currents and higher voltages are involved the PN-junction of a signal diode would melt so larger Power Diodes are used instead.

Power Diodes have much larger PN-junctions resulting in a high forward current capability, typically up to several hundred amps. Since the power diode has a large PN-junction, it is not suitable for high frequency applications but special and expensive high frequency, high current diodes are available. Power diodes are designed to have a forward "ON" resistance of fractions of an ohm while their reverse blocking resistance is in the megaohms range. They are mainly used in power conversion, such as power supplies, rectifiers and inverters. Some of the larger value diodes are designed to be "stud mounted" onto heatsinks reducing their thermal resistance to between 0.1 to 1^oC/Watt.

Power diodes can be used individually or connected together to produce a variety of rectifier circuits such as "Half-Wave", "Full-Wave" or as "Bridge Rectifiers". The most commonly used individual power diode for basic electronics applications is the general purpose 1N400X Series Glass Passivated type rectifying diode with standard ratings of forward current from 1amp and reverse voltage ratings from 50v (1N4001) up to 1000v (1N4007) with the small 1N4007GP being the most popular.

Half-wave Rectification

A rectifier is a circuit which converts Alternating Current (AC) into a Direct Current (DC) form and the simplest of all the rectifier circuits is that of the Half-Wave Rectifier. A half-wave rectifier circuit uses just one half of each complete sine wave or cycle of the AC supply in order to convert it to a DC supply. Then this type of circuit is called a "half-wave" rectifier because it passes only half of the incoming AC power signal as shown below.

Half-wave Rectifier Circuit

During each "positive" half cycle of the AC sine wave, the diode is Forward Biased (Anode is positive with respect to the Cathode) and current flows through it. Since the DC load is resistive (resistor R), the current flowing in the load resistor is therefore proportional to the voltage (Ohm´s Law), and the voltage across the load resistor is the same as the supply voltage, Vs (minus Vf), that is the "DC" voltage across the load is sinusoidal for the first half cycle only. Then Vout = Vs.

During each "negative" half cycle of the AC sine wave, the diode is Reverse Biased (Anode is negative with respect to the Cathode) and No current flows through it. Therefore, in the negative half cycle of the supply, no current flows in the load resistor as no voltage appears across it. Then Vout = 0.

The current on the DC side of the circuit flows in one direction only making the circuit Unidirectional and the value of the DC voltage V_DC across the load resistor is calculated as follows.

Where V_max is the maximum voltage value of the AC supply, and V_S is the r.m.s. value of the supply.

Example No1.

Calculate the current (I_DC) flowing through a 100Ω resistor connected to a 240v single phase half-wave rectifier as shown above, and also the power consumed by the load.

As the voltage across the load resistor is only present during the positive half of the cycle, the resultant voltage is "ON" and "OFF" during every cycle resulting in a low average DC value. This variation on the rectified waveform is called "Ripple" and is an undesirable feature. The resultant DC ripple has a frequency that is equal to that of the AC supply frequency. Very often when rectifying an alternating voltage we wish to produce a "steady" direct voltage free from any voltage variations or ripple. One way of doing this is to connect a Capacitor across the output voltage terminals in parallel with the load resistor as shown below. This type of capacitor is known commonly as a "Reservoir" or Smoothing Capacitor.

Half-wave Rectifier with Smoothing Capacitor

When rectification is used to provide a direct voltage power supply from an alternating source, the amount of ripple can be reduced by using larger value capacitors but there are limits both on cost and size. For a given capacitor value, a greater load current (smaller load resistor) will discharge the capacitor more quickly (RC Time Constant) and so increases the ripple obtained. Then for single phase, half-wave rectifier circuits it is not very practical to try and reduce the ripple voltage by capacitor smoothing alone so it is more practical to use "Full-wave Rectification" instead as discussed in the next tutorial.