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1. Diodes

2. P-n junction Diode The p-n junction forms a popular semiconductor device called p-n junction diode. There are two terminals are called electrodes. One is from p region and other n region. It can conduct current only in one direction. Applying external voltage to diode is called biassing . Depending on the direction of DC voltage applied to diode is classified as A. Forword Biasing B. Reverse Biasing.

3. The  anode  which is the  positive terminal  of a diode is represented with  A  and the  cathode , which is the  negative terminal  is represented with  K . To know the anode and cathode of a practical diode, a fine line is drawn on the diode which means cathode, while the other end represents anode. Direction of current Anode Cathode

4. Biasing of a Diode Forward Biased Condition When a diode is connected in a circuit, with its  anode to the positive  terminal and  cathode to the negative  terminal of the supply. such a connection is said to be  forward biased  condition. This kind of connection makes the circuit more and more forward biased and helps in more conduction. A diode conducts well in forward biased condition .

5. Reverse Biased Condition When a diode is connected in a circuit, with its  anode to the negative terminal and  cathode to the positive terminal of the supply, then such a connection is said to be Reverse biased condition . This kind of connection makes the circuit more and more reverse biased and helps in minimizing and preventing the conduction. A diode cannot conduct in reverse biased condition.

6. Working under Forward Biased When an external voltage is applied to a diode such that it cancels the potential barrier and permits the flow of current is called as  forward bias . When anode and cathode are connected to positive and negative terminals respectively, the holes in P-type and electrons in N-type tend to move across the junction, breaking the barrier. There exists a free flow of current with this, almost eliminating the barrier .

7. When an external voltage is applied to a diode such that it increases the potential barrier and restricts the flow of current is called as  Reverse bias . Anode and cathode are connected to negative and positive terminals respectively, the electrons are attracted towards the positive terminal and holes are attracted towards the negative terminal. Hence both will be away from the potential barrier  increasing the junction resistance and preventing any electron to cross the junction . Working under Reverse Biased

8. Turms Peak Inverse Voltage Peak Inverse Voltage is shortly called as  PIV . It states the maximum voltage applied in reverse bias. Voltage can be defined as “ The maximum reverse voltage that a diode can withstand without being destroyed ”. This voltage is considered during reverse biased condition . It denotes how a diode can be safely operated in reverse bias.

9. V - I Characteristics of a Diode An ammeter is connected in series and voltmeter in parallel while the supply is controlled through a variable resistor . when the diode is in forward biased condition, at some particular voltage, the potential barrier gets eliminated. Such a voltage is called as  Cut-off Voltage  or  Knee Voltage . If the forward voltage exceeds beyond the limit, the forward current rises up exponentially and if this is done further, the device is damaged due to overheating.

10. Types There are many types of diodes depending upon many factors such as the frequency used, their working and construction, their applications etc. Let us go through few of them. Junction diodes The junction diodes are the normal PN junction diodes but differ in construction. There are three types of junction diodes as:

11. Rectifier Diode These diodes are the normal PN junction diodes, which allow current to flow through them in only one direction and stop in the other direction . These diodes are used in rectifier circuits to convert alternating current into direct current . minimize the heat distribution. metal projection is called as  Heat sink Improvement of the diode performance diodes will be able to withstand high powers, without getting affected. Used in   Half wave rectifier  and  Full wave rectifier  circuits

12. Zener Diode This is a special kind of diode P ermits current flow not only in forward direction, but also in reverse direction. A normal diode, when operated in reverse bias, gets damaged if the reverse current above a certain value is passed through it. This “certain value” is called as the  Breakdown voltage . The breakdown voltage of a Zener diode is very low . But   this diode allows the reverse current to pass through it, once this breakdown voltage is exceeded . That breakdown voltage is called as  Zener Voltage . Hence there is a controlled breakdown which does not damage the diode when a reverse current above the Zener voltage passes through a Zener diode.

13. A Zener diode in its reverse bias, exhibits a controlled breakdown voltage and it allows the current flow to keep the value of voltage across that Zener diode close to the Zener breakdown voltage value. This value of Zener breakdown voltage makes any Zener diode to be chosen for certain applications.

14. V-I Characteristics of a Zener diode The V-I Characteristics of a Zener diode are common for any diode when operated in forward bias . But the reverse bias operation of a Zener diode makes it very important to consider.

15. The point where the bent is shown in the reverse bias operation, is the  Zener breakdown voltage , after which the diode allows high reverse currents through it. This Zener voltage is indicated by  V Z . This incredible quality of Zener diode made it the most reliable one and have got many applications too.

16. Applications of Zener diode This diode has many applications such as − It is mostly used as a Voltage Regulator. Provides fixed reference voltage in transistor biasing circuits. For peak clipping or limiting in wave shaping circuits. As a Surge protector in many circuits. For meter protection against damage from accidental applications.

17. Switching Diode This is a normal single PN junction diode which is especially designed for switching purposes. This diode can exhibit two states of high and low resistance clearly which can be used alternatively. The junction capacitance of this diode is made very low so as to minimize other effects. The switching speed is made quite high. When the diode has high resistance it works as an open switch and it acts as a closed switch during low resistance. This transition occurs at a faster rate in switching diode, than in any ordinary one.

18. Applications of switching diode Used in high-speed rectifying circuits Used in ring modulators Used in radio frequency receivers Used as reverse polarity protectors Used for both General purpose and high speed switching applications

19. Rectifier Which converts A.C. voltage to pulsating D.C voltage using using one or more p-n junction diodes. Half wave rectifier Full wave rectifier

20. Transformer decides peak value of secondary voltage. N1 – primary number of turns N2 secondary number of turns Ideal V c R f v i v i V i = V M sin ( t) Half wave rectifier equivalent circuit Input volta-ge to the half wave rectifier circuit sinusoidal ac voltage having 50Hz frequency =  = 2 π f N2∕N1 = V M ∕ V i A B

21. Positive Half Cycle: In the positive half cycles when the input AC power is given to the primary winding of the step down transformer, we will get the decreased voltage at the secondary winding which is given to the diode. The diode will allow current flowing in clock wise direction from anode to cathode in the forward bias (diode conduction will take place in forward bias) which will generate only the positive half cycle of the AC. The diode will eliminate the variations in the supply and give the pulsating DC voltage to the load resistance RL. We can get the pulsating DC at the Load resistance. Negative Half Cycle: In the negative half cycle the current will flow in the anti-clockwise direction and the diode will go in to the reverse bias. In the reverse bias the diode will not conduct so, no current in flown from anode to cathode, and we cannot get any power at the load resistance. Only small amount of reverse current is flown from the diode but this current is almost negligible. And voltage across the load resistance is also zero.

22. Half Wave Rectifier We initially consider the diode to be ideal, such that V C =0 and R f =0 The (ideal) diode conducts for v i >0 and since R f =0 v  v i • For v i < 0 the (ideal) diode is an open circuit (it doesn’t conduct) and v  0.

23. Half Wave Rectifier In this simplified (ideal diode) case the input and output waveforms are as shown The diode must withstand a peak inverse voltage

24. Half Wave Rectifier The average d.c . value of this half-wave-rectified sine wave is

25. 1. Efficiency:  The efficiency is defined as the ratio of input AC to the output DC. Efficiency, Ƞ = P  dc  / P ac DC power delivered to the load,  P dc  = I 2 dc  R L  = ( I max/pi  )  2  R L = I 2 m   / π 2 * R L ----------------1 AC power input to the transformer, P ac  = Power dissipated in junction of diode + Power dissipated in load resistance R L   P ac =  I 2 rms (V+ RL+Rs) I rms = I m   / 2 Half wave P ac = {I 2 max /4}[RF + RL + Rs]------------2

26. Rectification Efficiency, Ƞ = P dc / P ac   = {4/ π 2 }RL/ (RF + RL+ Rs) = 0.406/{1+ (RF+RS/RL) } If R F  is neglected, the efficiency of half wave rectifier is 40.6%. %Ƞ = 0.406 X 100 = 40.6% 

27. Ripple factor: Output contains pulsating component called Ripple Measures of ripple present in o/p with help of factor is: It is defined as the amount of AC content in the output DC. It nothing but amount of AC noise in the output DC. Less the ripple factor, performance of the rectifier is more. The ripple factor of half wave rectifier is about 1.21 Full wave rectifier has about 0.48.  It can be calculated as follows: ripple factor, γ = RMS value of a.c . Component of output Average of d.c . component of output I ac = RMS value of AC Component I dc = value DC Component I rms – RMS value of total output current  Ripple factor = I ac / I dc γ  = (I 2 rms   – I 2 dc ) /   I dc   = {( I  rms   / I dc 2 )-1} = K f 2  – 1) Where Kf    is the form factor of the input voltage. Form factor is given as K f   =  I rms  / I dc = (I max / 2 )/ (I max /pi) = pi/2 = 1.57 So, ripple factor, γ = (1.57 2  – 1) = 1.21

28. Peak Inverse Voltage: PIV is the voltage across the diode in the reverse direction when diode is reverse bised .

29. Voltage Regulation This is a factor which tells about the change in the dc output voltage as a load changes from no load to full load condition. %Voltage regulation = ( V dc ) N L – ( V dc ) F L ( V dc ) F L Less the value of VR, rectifier performce better. X 100

30. Half Wave Circuit ( V dc ) N L = E sm / π ( V dc ) F L = I dc R L = ( I m / π )* R L = [ E sm /  π (R F  + R L + Rs)] * R L %R= (R F  + Rs)/ R L * 100 Rs ==0 = R F  /R L * 100

31. Transformer Utilization Factor (TUF):  The TUF is defined as the ratio of DC power is delivered to the load and the AC rating of the transformer secondary. Half wave rectifier has around 0.287 and full wave rectifier has around 0.693. TUF = P dc /P ac P dc  = I 2 dc  R L  = ( I max/pi  )  2  R L = I 2 m   / π 2 * R L P ac = E rms   I rms = E sm I sm / 2*(2   ) -2 

32. Half Wave Rectifier If we don’t consider the diode to be ideal then from the equivalent circuit we obtain, for v i >V c: v i – V c – i R f - iR =0 i.e. Giving

33. Non-Ideal Half Wave Rectifier V M

34. Non-Ideal Half Wave Rectifier A plot of v against v i is known as the transfer characteristic V C v i R/(R + R f )

35. Non-Ideal Half Wave Rectifier • W e usually have R>> R f so that R f can be neglected in comparison to R. • Often V M >> V c so V c can also be neglected. The transfer characteristic then reduces to v  v i

36. Full-Wave (Bridge) Rectifier vi

37. Full-Wave (Bridge) Rectifier We initially consider the diodes to be ideal, such that V C =0 and R f =0 The four-diode bridge can be bought as a package vi

38. Full-Wave (Bridge) Rectifier During positive half cycles v i is positive. Current is conducted through diodes D1, resistor R and diode D2 Meanwhile diodes D3 and D4 are reverse biased. vi

39. Full-Wave (Bridge) Rectifier During negative half cycles v i is negative. Current is conducted through diodes D3, resistor R and diode D4 Meanwhile diodes D1 and D2 are reverse biased. vi

40. The Full-Wave Rectifier

41. The Full-Wave Rectifier The full wave rectifier consists of two diodes and a resister as shown in Figure 1. The transformer has a centre-tapped secondary winding. This secondary winding has a lead attached to the centre of the winding. The voltage from the centre tap to either end terminal on this winding is equal to one half of the total voltage measured end-to-end. Circuit Operation shows the operation during the positive half cycle of the full wave rectifier. Note that diode D1 is forward biased and diode D2 is reverse biased. Note the direction of the current through the load.

42. During the negative half cycle, (figure 2) the polarity reverses. Diode D2 is forward biased and diode D1 is reverse biased. Note that the direction of current through the load has not changed even though the secondary voltage has changed polarity. Thus another positive half cycle is produced across the load.

43. Calculating Load Voltage and Currents Using the ideal diode model, the peak load voltage for the full wave rectifier is V m . The full wave rectifier produces twice as many output pulses as the half wave rectifier. This is the same as saying that the full wave rectifier has twice the output frequency of a half wave rectifier. For this reason, the average load voltage (i.e. DC output voltage) is found as Vave =2V m /pi……..1 Figure 3 below illustrates the average dc voltage for a full wave rectifier.

44. Peak Inverse Voltage When one of the diodes in a full-wave rectifier is reverse biased, the peak voltage across that diode will be approximately equal to V m . This point is illustrated in figure 2. With the polarities shown, D1 is conducting and D2 is reverse biased. Thus the cathode of D1 will be at V m . Since this point is connected directly to the cathode of D2, its cathode will also be V m . With – Vm applied to the anode of D2, the total voltage across the diode D2 is 2V m . Therefore, the maximum reverse voltage across either diode will be twice the peak load voltage. PIV =2V m (2)

45. Full-Wave Rectifier with Capacitor filter Similar to the half-wave rectifier, smoothing is performed by a large value capacitor connected across the load resistance (as shown in figure 4) to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The diagram below shows the unsmoothed varying DC (thin line) and the smoothed DC (thick line). The capacitor charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output.

46. Note that smoothing significantly increases the average DC voltage to almost the peak value. However, smoothing is not perfect due to the capacitor voltage falling a little as it discharges, giving a small ripple voltage. For many circuits a ripple which is 10% of the supply voltage is satisfactory and the equation below gives the required value for the smoothing capacitor.

47. In the full-wave circuit, the capacitor discharges for only a half-cycle before being recharged. Hence the capacitance required is only half as much in the full-wave circuit as for the half-wave circuit. C= IL T/ 2V p -p …………(3)

48. Positive cycle , D2 off, D1 conducts; Vo – Vs + V  = 0 Vo = Vs - V  Full-Wave Rectification – circuit with center-tapped transformer Since a rectified output voltage occurs during both positive and negative cycles of the input signal, this circuit is called a full-wave rectifier . Also notice that the polarity of the output voltage for both cycles is the same Negative cycle , D1 off, D2 conducts; Vo – Vs + V  = 0 Vo = Vs - V 

49. Vs = Vpsin  t V  -V  Notice again that the peak voltage of Vo is lower since Vo = Vs - V  V p Vs < V  , diode off, open circuit , no current flow, Vo = 0V

50. Positive cycle , D 1 and D 2 conducts, D 3 and D 4 off; + V  + Vo + V  – Vs = 0 Vo = Vs - 2 V  Full-Wave Rectification – Bridge Rectifier Negative cycle , D3 and D4 conducts, D1 and D2 off + V  + Vo + V  – Vs = 0 Vo = Vs - 2 V  Also notice that the polarity of the output voltage for both cycles is the same

51. A full-wave center-tapped rectifier circuit is shown in Fig. 3.1. Assume that for each diode, the cut-in voltage, V  = 0.6V and the diode forward resistance, r f is 15  . The load resistor, R = 95  . Determine: peak output voltage, V o across the load, R Sketch the output voltage, V o and label its peak value .   ( sine wave )

52. SOLUTION peak output voltage, V o V s (peak) = 125 / 25 = 5V V  +I D (15) + I D (95) - V s (peak) = 0 I D = (5 – 0.6) / 110 = 0.04 A V o (peak) = 95 x 0.04 = 3.8V   3.8V Vo t

53. EXAMPLE 3.2   Calculate the transformer turns ratio and the PIV voltages for each type of the full wave rectifier center-tapped bridge Assume the input voltage of the transformer is 220 V ( rms ), 50 Hz from ac main line source. The desired peak output voltage is 9 volt; also assume diodes cut-in voltage = 0.6 V.

54. Solution: For the centre-tapped transformer circuit the peak voltage of the transformer secondary is required The peak output voltage = 9V Output voltage, Vo = Vs - V  Hence, Vs = 9 + 0.6 = 9.6V Peak value = Vrms x 2 So, Vs (rms) = 9.6 / 2 = 6.79 V The turns ratio of the primary to each secondary winding is The PIV of each diode: 2V s(peak) - V  = 2(9.6) - 0.6 = 19.6 - 0.6 = 18.6 V

55. Solution: For the bridge transformer circuit the peak voltage of the transformer secondary is required The peak output voltage = 9V Output voltage, Vo = Vs - 2V  Hence, Vs = 9 + 1.2 = 10.2 V Peak value = Vrms x 2 So, Vs (rms) = 10.2 / 2 = 7.21 V The turns ratio of the primary to each secondary winding is The PIV of each diode: V s(peak) - V  = 10.2 - 0.6 = 9.6 V

56. Type of Rectifier PIV Half Wave Peak value of the input secondary voltage, V s (peak) Full Wave : Center-Tapped 2V s(peak) - V  Full Wave : Bridge V s (peak) - V 

57. Diode Clamper Circuits The following circuit acts as a d.c. restorer. see Q9, example sheet1.

58. Diode Clamper Circuits A bias voltage can be added to pin the output to a level other than zero.