PN Junction Diode as a Rectifier

PN JUNCTION DIODE AS A RECTIFIER 

A PN junction diode can act as a rectifier that is it can convert A.C. supply into D.C. supply. The reason is that it acts as a switch. 

When a diode is forward biased (during positive cycle) it conducts and behaves as a closed switch. When it is reversed biased (during negative cycle) it does not conduct and behaves as an open switch.

A diode, therefore, is a unidirectional device i.e., it conducts only in one direction, this property of the diode makes it suitable as rectifier.

TYPES OF SEMICONDUCTOR DIODE RECTIFIERS 

We know that the generation, transmission and distribution of A.C. is economical but at few places, we need D.C. supply, e.g. in electronic devices and circuits. Here, we convert the available A.C. supply into D.C. supply. The process is called 'rectification' and the devices used for rectification are called 'rectifiers'. 

The diode rectifiers can be classified as: 

(1) Half wave rectifiers 

(2) Full-wave rectifiers 

(a) Centre tap rectifiers 

(b) Bridge rectifiers 

It may be recalled that a diode acts only in one direction, ie., when it is forward biased. 

HALF-WAVE (H.W.) RECTIFIERS

These circuits rectify only the half (positive) cycle of the A.C. supply and hence the name. The circuit uses only one diode. 

Circuit 

The A.C. supply to the diodes is given through a stepdown transformer which steps down the voltage to be supplied to the diode. The output D.C. valsobtained across a load

Operation

The stepdown voltage appears across secondary MN of the becomes the A.C. transformer. This input to the diode

(a) When positive cycle of the A.C. end M input appears across the diode, i.e., the is positive and end N is negative, the diode becomes biased and forward is short circuited. As a result, the whole A.C. input positive cycle appears across the load.

(b) When negative cycle of the supply input appears, i.e., end M becomes negative acros and N positive. The diode is reverse biased and is open circuited. As a re sult, the whole A.C. input of the negative cycle appears across the diode and output output across the load is zero.

FULL-WAVE (F.W.) RECTIFIERS

In these circuits more than one diode is used. This enables the circuit to process both cycles of the A.C. supply. During both the cycles, current flows through the load in the same direction.

CENTRE TAP F.W. RECTIFIER

Circuit: The circuit uses a stepdown transformer with the centre tapped (C.T:) secondary.

One diode processes the positive half cycle and the other processes the negative half cycle.

Operation 

(a) During positive half cycle of the A.C. supply, diode D, is forward biased as the end A is positive and B negative. This makes diode D, forward biased and D, reverse biased. As a result current lows through the diode D, and through the load R, as shown by arrows. The diode D, does not conduct during this period.

(b) During negative cycle, end B becomes positive and A negative. This makes D2 v. forward biased and D, reverse biased.

The D2 conducts and current flows through D2 and load R; as shown by arrows. The diode D, does not conduct during this period. It can be seen current in both the cases flows through R, from S to K. Hence we get a unidirectional (direct) current.

F.W. BRIDGE RECTIFIER

As already mentioned, in case of centre tap (C.T.) rectifier circuit, it is difficult to locate correctly the centre point on the secondary of the transformer, this may give distortion in the rectified D.C. output. Moreover, PIV value is also high in C.T. rectifier circuit. These drawbacks have been eliminated in bridge rectifier which employs 4 diodes. By using 4 diodes, its output is twice that of the C.T. circuit for the same secondary voltage. The load R, bridges the two ends P and R, the arrangement gives it the name of a bridge rectifier.

Operation

During positive half cycle of the secondary voltage, the end A of the secondary becomes positive and B becomes negative. As a result the diodes D, and D3 become forward biased which conduct. The direction of current is shown it.

During the negative cycle, the end A becomes negative and B becomes positive.

It can be seen that current in both cases flows through R, from P to R.

Hence we get a unidirectional (direct) current.

Note:

(a) Each of the cycle is rectified by two diodes simultaneously; hence, the output is more than the C.T. rectifier circuit.

(b) The D.C. output in this case is also not pure, i.e., we get pulsating D.C., however, in this circuit, A.C. components are further reduced.

Wave Shapes

(c) The diodes required are of lesser PIV value than in C.T. circuit. The value of PIV across each diode in this case is the max. voltage across the secondary. Remember that here the value of P.I.V. is same as in the case of HW circuit.

(d) The two diodes simultaneously conduct in series, hence voltage drop in them increases.

RIPPLE FACTOR

The output of a EW. rectifier is not pure but contains D.C. as well as A.C. components. The A.C. components are responsible for pulsations in the wave. These A.C. components are called Ripples.

The ratio of RMS value of A.C. components to D.C. components in the rectifier output is called ripple factor (R.F.)

Ripple factor = A.C. components Ac Ripple factor p.C. components If Ac. is more than Ipc, clearly ripple factor is more than 1; in other words, the output is more of A.C. nature than D.C. Inversely, the lesser the Ihc, the more pure is the D.C. output.

For proper functioning. electronic devices require pure D.C. The ripples are undesirable, they badly effect the performance.

The frequency of ripples in D.C. output is as follows:

(a) In case of H.W. rectifier output, this is the same as the frequency of supply mains.

(b) In case of EW. rectifier output, this is double that of the frequency of supply mains.

i.e., if supply frequency is f. the frequency of H.W. rectifier output is also of, whereas the frequency of F.W. rectifier output is 2f.