Diodes & Rectifiers Nafees Ahamad

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1 Diodes & Rectifiers Nafees Ahamad Asstt. Prof., EECE Deptt, DIT University, Dehradun Website: 1

2 Diodes Electronic devices created by bringing together a p-type and n-type region within the same semiconductor lattice. Used for rectifiers, LED etc 2

3 Diodes It is represented by the following symbol, where the arrow indicates the direction of positive current flow. 3

4 Actual Diode Picture 4

5 Forward Bias Forward Bias : Connect positive of the Diode to positive of supply & negative of Diode to negative of supply 5

6 Reverse Bias Reverse Bias: Connect positive of the Diode to negative of supply & negative of diode to positive of supply. 6

7 Characteristics of Diode Diode always conducts in one direction. Diodes always conduct current when Forward Biased ( Zero resistance) Diodes do not conduct when Reverse Biased (Infinite resistance) 7

8 V-I characteristics of Ideal diode 8

9 V-I Characteristics of Practical Diode 9 Note: The 1N4007 can sustain a peak reverse voltage of 1000 volts.

10 Diode Equivalent circuits S.No Type Model Characteristic 1 Practical Diode 2 Simplified Model 3 Ideal Model 10 V D = Diode knee voltage in forward biased = 0.7 V for Si & 0.3 V for Ge diode r f = Diode resistance when forward biased

11 Ideal Diode models, V D = 0 & r f = 0 11

12 Practical Diode models with r f =0 V D = 0.7V 12

13 Rectification Converting ac to dc is accomplished by the process of rectification. Two types: Half-wave rectifier; Full-wave rectifier. Centre-tap rectifier Bridge rectifier 13

14 Half-wave Rectification Simplest process used to convert ac to dc. V D = Diode Voltage 14

15 Half-wave Rectification Average value of the output voltage, V dc 15

16 Half-wave Rectification Where T = 2π 16

17 Half-wave Rectification and and 17

18 Half-wave Rectification Determine the rms Voltage, V rms 18

19 Half-wave Rectification Output dc power, P dc = V dc I dc 19

20 Output ac power, P ac = V rms I rms 20

21 Half-wave Rectification Efficiency, η = P dc /P ac 21

22 If diode forward biased resistance r f 0 22

23 If diode forward biased resistance r f 0 Efficiency, η = P dc /P ac Note: η will be maximum when r f =0 & η max =40.6% 23

24 Half-wave Rectification Effective (rms) value of the ac component of the output voltage(ripple Voltage), V ac = V rms2 V dc 2 Where V rms = RMS component of overall output waveform V ac = RMS value of the AC component (ripple component) (Please see the filtered output waveform) 24

25 Half-wave Rectification Form factor, FF = V rms /V dc 25

26 Half-wave Rectification Please see waveform of capacitor as filter in half wave rectifier 26

27 Half-wave Rectification PIV is the maximum (peak) voltage that appears across the diode when reverse biased. Here, PIV = V m. 27

28 Half-wave Rectification - - PIV

29 Full-Wave Center-Tapped Rectifier PIV = 2V m 29

30 Full-Wave Bridge Rectifier Conduction pattern D 1 D 2 D 3 D 4 30 PIV = V m

31 Comparison S.No Parameter Type of rectifier Half wave Centre tap Bridge 1 No of diodes O/P DC voltage 3 O/P RMS voltage 4 O/P DC power 5 O/P AC power 6 Efficiency 40.6% 81.2% 81.2% 7 Form Factor Ripple Factor PIV 10 TUF O/P frequency f 2f 2f TUF By: Nafees = Ahamad, Transformer EECE Deptt, DITUutilization factor = P dc /V s I s P dc = DC power output, Vs & Is = Transformer Secondary current & Voltage respectively 31

32 Filters The DC signal obtained at the output still contains some ripples. To reduce these ripples at the output, we use a filter. The filter is an electronic device that converts the pulsating Direct Current into pure Direct Current. The filter is made up of a combination of electronic components such as resistors, capacitors, and inductors. A single capacitor in parallel with the load will also act a filter in half wave and full wave rectifiers as shown in next slides. 32

33 Capacitor as filter in half wave rectifier 33

34 Capacitor as filter in half wave rectifier In +ve half cycle, the capacitor C starts charging to the maximum value of the supply voltage Vm(See charging period). When the capacitor is fully charged, it holds the charge until the input ac supply to the rectifier reaches the negative half cycle. As soon as the negative half supply is reached, the diode gets reverse biased and thus stops conducting. During the non-conducting period, the capacitor C discharges all the stored charges through the output load resistance R. Capacitor discharges exponentially with a time constant (C*R) discharging period(see discharging period). 34

35 Capacitor as filter in half wave rectifier The value of the discharge time constant (C*RLoad) being very large, the capacitor C will not have enough time to discharge properly. As soon as the capacitor starts discharging, the time becomes over. If the value of load resistance is large, the discharge time constant will be of a high value, and thus the capacitors time to discharge will get over soon. This lowers the amount of ripples in the output and increases the output voltage. If the load resistance is small, the discharge time constant will be less, and the ripples will be more with decrease in output voltage. 35

36 Capacitor as filter in half wave rectifier Ripple factor RF Where f=frequency of pulsating DC which in this case is same as that of AC mains 36

37 Capacitor as filter in fullwave rectifier 37

38 Capacitor as filter in fullwave rectifier Ripple factor RF Where f=frequency of pulsating DC which in this case is same as that of AC mains 38

39 Questions Book: Principles of electronics by V.K Mehta S. Chand & Company Pvt Ltd. 39

40 Questions 40

41 Questions 41

42 Questions 42

43 Questions 43

44 Thank you? 44

Electronic I Lecture 3 Diode Rectifiers. By Asst. Prof Dr. Jassim K. Hmood

Electronic I Lecture 3 Diode Rectifiers By Asst. Prof Dr. Jassim K. Hmood Diode Approximations 1- The Ideal Model When forward biased, act as a closed (on) switch When reverse biased, act as open (off)