Develop the basic principle of operation of a diode. Classify the different types of diodes and analyze their applications.

Transcription

1 Key educational goals: Develop the basic principle of operation of a diode. Classify the different types of diodes and analyze their applications. Reading/Preparatory activities for class i)textbook: Chapter 10: Sections ii) Power-point file:diodes 1

2 Questions to guide your reading and to think about ahead of time. 1. In what direction does current normally flow in an ordinary diode? 2. How is an ideal diode different from an actual diode? 3. How is a zener diode different from an ordinary diode? 4. Which type of diodes is used for voltage regulation? 5. Which type of diodes is used for rectification (converting AC to DC)? 6. Which types of diodes are used from clipping and clamping circuits? 7. What circuits elements are used for high voltage dc supply? Introduction Chapter 1: Introduction and Chapter 2:Resistive circuits The main concepts for the module 1. Analyze how an ordinary diode conducts. 2. Evaluate the different models of an ordinary diode. 3. Develop the concept of a load line to determine an operating point in a diode ciruit. 4. Analyze a diode circuit based on assumption/contradiction. 5. Analyze how a zener diode based voltage regulator works. 6. Analyze rectifier circuit operations. 7. Analyze clipping, clamping and voltage multiplier cirucits. 2

3 Summary The knowledge gained from this module will be useful for designing voltage regulators, rectifiers, clamping circuits for protection as may be required to interface transducer outputs to digital electronics and computers. For next time We will next look into the Bipolar Junction Transistors (BJT) whose base-emiiter characteristics closely follow that of a diode but unlike a diode works like a current controlled current source. Sample test/exam questions/problems to help you study: 1. Find the currents in diodes D 3 and D 4 (Fig.1) in the next slide. 2. In Fig.2 (next slide) V ss varies between 15-20V. R L varies between ohms. Find the value of R and its power rating. 3

4 Fig.1 Fig.2 AC Machines 4

5 Semiconductor Diodes Diode structure Diode model approximations Operating points and load lines Multiple diode circuits Rectifier circuits and design Zener diodes Voltage regulators using zener diodes Clipper and clamp circuits using zener diodes Voltage multipliers Other types of diodes Shockley equation Diodes 5

6 Semiconductor diode Diodes 6

7 First approximation of a semiconductor diode (Ideal-Diode) i D i D V s = 10V R=10 V s = 10V R=10 i D = 1A i D = 0A Diodes 7

8 Second approximation of a semiconductor diode i D Diode on Diode off v D Vf 0.7V i D i D V s = 10V R=10 V s = 10V R=10 i D = 0.93 A i D = 0A Diodes 8

9 Third approximation of a semiconductor diode i D Diode on v D /i D =Diode body (bulk) resistance R f Actual Characteristics Diode off v D V f 0.7V i D i D V s = 10V R=10 V s = 10V R=10 i D = 0.62 A (say R f = 5 ) i D = 0A Diodes 9

10 How to Find Out the Exact Diode Operating point? Diodes 10

11 Example of Diode Load Line If R = 200, Vss = 3V in the what is i D and v D? Operating point Diodes 11

12 Answer to the Diode Load Line Problem i D = 9.5 ma, v D = 1.1 V Diodes 12

13 Multiple Ideal-Diode Circuit Analysis Assume either a on (short) or off (open)state for a diode Determine the current direction in an on diode. Determine the voltage polarity of an off diode. If the assumption is correct, current flows from anode to cathode in all on diodes; cathode voltage should be positive with respect to anode in all off diodes. In a n diode circuit 2 n such combinations are possible Diodes 13

14 Example on Diode Circuit Diodes 14

15 What are the currents in D 1,D 2,D 3,D 4? Assume ideal diodes Diodes 15

16 Answer and Another Question i D1 = 2 ma, i D2 = 0 ma, i D3 = 0 ma, i D4 = 5 ma What should be the minimum value of the current source in the previous figure 10.17(c) in order to make D 3 conduct? Diodes 16

17 Rectifier Circuits Rectifier circuits convert AC voltage to DC voltage The output DC voltage magnitude is controlled by controlling the input AC voltage Usually classified by two types: Half-wave and Full-wave Has many applications such as battery charger, power supply. Diodes 17

18 Half-Wave Rectifier Circuits During the positive half-cycle of the source the diode conducts During the negative half-cycle the source diode blocks Current through the diode looks similar to the voltage across R L Diodes 18

19 A simple Battery charger-example of a Rectifier Can be used to charge a car battery from the alternator Diodes 19

20 Smoothening the Output Voltage of a Rectifier-Add a Capacitor across Load Diodes 20

21 Design of Filter Capacitor Assume capacitor takes negligible time to charge Assuming constant load current, the charge lost by the capacitor is Q I L T (L21) The charge lost in the capacitor is reflected in the load voltage as ripple voltage, V r and V r Q/C (L22) Eliminating Q from (L21) and (L22) one gets C = I L T / V r (L23) where T = 1/f (f= frequency of the input sinusoidal source) Diodes 21

22 Design of Filter Capacitor(Contd..) Load Voltage V L = V m - V r /2; where V m = peak of the sinusoidal source Diodes 22

23 Full-wave rectifier Normally used with center-tap transformer Rectifies both the positive and negative half cycle Capacitor size is half that of the half-wave case for the same load current and ripple voltage V out V in V in C.T. V out Diodes 23

24 Diode-bridge Full-wave rectifier Can be used without a transformer Rectifies both the positive and negative half cycle Capacitor size is half that of the half-wave case for the same load current and ripple voltage (why?) V in V out V in V out Diodes 24

25 Comparison of Rectifier Circuits Half-wave Full-wave Full-wave Bridge # of diodes DC output voltage(**) V in (peak) V in (peak) (*) V in (peak) DC diode current I L 0.5I L 0.5I L PIV (***) 2V in (peak) 2V in (peak) V in (peak) Ripple Frequency f 2f 2f * With 1:2 transformer turns ratio; ** With Large C *** Peak Inverse Voltage (also with Large C) Diodes 25

26 Example on Rectifier Circuits Design the capacitor (value and voltage rating) and the diode (average current and (PIV) peak inverse voltage) for a rectifier which has to supply 15 VDC and 2 ADC current. Maximum ripple voltage, V r, (peak-to peak) is 1V. The input AC supply frequency is 60 Hz. i) For a half-wave rectifier C = I L V r. T = I L V r. 1 f = = 33,333 μf V m = V dc + V r 2 = = 15.5 V. Therefore the capacitor should be 33,333 μf minimum and rated for minimum 15.5 V. The diode should have a minimum PIV of V = 31V and current rating 2 ADC. Diodes 26

27 Example on Rectifier Circuits(2) ii) For a full-wave rectifier C = I L V r. T 2 = I L V r. 1 2f = = 16,666 μf V m = V dc + V r 2 = = 15.5 V. Therefore the capacitor should be 16, 666 μf minimum and rated for minimum 15.5 V. The diode should have a minimum PIV of V = 31V and current rating 2/2 =1 ADC. Diodes 27

28 Example on Rectifier Circuits(3) iii) For a full-wave bridge rectifier C = I L V r. T 2 = I L V r. 1 2f = = 16,666 μf V m = V dc + V r 2 = = 15.5 V. Therefore the capacitor should be 16, 666 μf minimum and rated for minimum 15.5 V. The diode should have a minimum PIV of 15.5V and current rating 2/2 =1 ADC. Diodes 28

29 SPICE simulation of Full-wave Bridge Rectifier Circuit Diodes 29

30 Unlike ordinary diodes they are used in the breakdown region Zener diodes are used to get reasonably regulated dc voltage when the input voltage and load resistance vary This is achieved by passing controlled current through the zener diode in the breakdown region The nominal breakdown voltage V Z is specified as the zener voltage (example 4.7 V zener, 8.2 V zener) Symbol looks like Zener Diodes Diodes 30

31 Example1 on Zener Diode Voltage Regulator V SS = 24V R = 1.2 k R L = 6 k ; 1.2 k Find I S and v L using the zener characteristics Diodes 31

32 Example1 on Zener Diode Voltage Thevenin voltage Regulator(contd..) Compute the Thevenin equivalent of the previous circuit with the zener diode as the load V T V ss R L R R RR L Thevenin Resistance R T 1;0. 6 R R L L 20V;12V k We can then write V T +R T i D +v D = 0 and find out v D,, i D using the zener diode characteristics v L = v D and I S = v L /R L + i D Diodes 32

33 Example1 on Zener Diode Voltage Regulator(contd..) Load Line : v D = -V T -R T i D Diodes 33

34 Answer to Example1 on Zener Diode Voltage Regulator v L = 10V I S = v L /R L + i D = 10/6 +10 ma = 11.67mA v L = 9.5 V I S = v L /R L + i D = 9.5/ ma = 12.92mA Note that load resistance change has caused the output voltage to change by 0.5V due to non-ideal reverse characteristics of the zener diode Diodes 34

35 Example 2 on Zener Diode Voltage Regulator (ideal-characteristics) Find v 0 for i L = i) 0 ii) 30 iii) 80 ma Diodes 35

36 Example 2 on Zener Diode Voltage Regulator The characteristics shown for the zener diode is the ideal characteristics- one without any zener bulk resistance. Hence the zener will be able to provide ideally regulated voltage unless the load current demand exceeds (15- v 0 )/100 = 50 ma. When i L 50 ma the zener absorbs the differential (excess) current to maintain the load voltage at 10 volts. When i L 50 ma the zener loses its regulating capacity(since it cannot generate current!) and the output voltage starts drooping. Then v 0 = i L. Diodes 36

37 Answer to Example 2 on Zener Diode Voltage Regulator i) 10V, ii) 10V, iii) 7 V Another question What is the maximum possible value i L? What is the load resistance under this condition? Diodes 37

38 Wave-shaping Circuits Necessary to shape waveforms Many examples can be found in transmitters and receivers in TV or radar or equipment control and protection circuits Uses diodes and zener diodes for clipping and clamping (shaping) input Diodes 38

39 A Clipper Circuit (Assume ideal diode) Diodes 39

40 A Clipper Circuit (Practical Implementation) Diodes 40

41 A Clamp (adds dc offset to ac) Circuit Assumption: RC (2 )/ Diodes 41

42 How Does A Clamp Circuit Work? In the positive half cycle C gets charged through D to 10V (peak of sinewave + 5 V) with the straight plate of C at a higher potential. D Clips the output to a maximum of -5V. In the negative half cycle D is reverse biased. The output can reach a minimum of 15V (-V C + negative peak of sinewave). Diodes 42

43 Application of Clamp Circuit A Voltage-Doubler Assumption: R1C1 or R1C2 (2 )/ Diodes 43

44 How does a Voltage-Doubler works In the negative half cycle C1 gets charged through D2 to peak of sinewave (say V m ) with the right plate of C1at a higher potential. D1is reverse biased since D2 conducts. In the positive half cycle D1 is conducting and D2 is reverse biased. Thus C2 gets charged to the maximum of V m + V m sin( t) or 2V m with the top plate of C2 at a higher potential Diodes 44

45 Voltage-Doubler waveforms Diodes 45

46 Voltage-Doubler waveforms different start point Diodes 46

47 Other types of Diodes LED (Light Emitting Diodes) They emit light when forward biased. Typical forward voltage drop for these diodes are V for currents between ma. Application areas: Instrument/Equipment panel indicators, Seven Segment Displays. Photo Diodes Get forward biased when light falls on them. Application areas: Optocouplers for electrical isolation. Schottky diodes, Ultrfast recovery diodes Application areas:power Electronics Diodes 47

48 Shockley Equation An equation named after the inventor of Transistor Describes approximately the Diode curves like the one shown below Is an important equation to be used in Transistor Analysis Diodes 48

49 Shockley Equation(2) Shockley equation: i D v D I S exp 1 nvt V T kt q Where I S is the saturation current,v T is the thermal voltage,n is the emission coefficient that takes values between 1 and 2, k is Boltzmann s constant ( k = 1.38*10-23 J/K),q is the magnitude of electrical charge on an electron (q = 1.60*10-19 J/K), T is p-n junction temperature in degree Kelvin. For example, at T =300 0 K, I S = A, V T V Diodes 49

50 Plus and minuses of Shockley Equation(2) Fairly accurate with diode near the forward biased region Not a good predictor of I S which happens to be much larger in magnitude Does not predict reverse breakdown Usually simpler models for diodes are useful Diodes 50