Diodes (non-linear devices)

Transcription

1 C H A P T E R 4 Diodes (non-linear devices)

2 Diode structure

3 Ideal Diode

4 Figure 4.2 The two modes of operation of ideal diodes and the use of an external circuit to limit (a) the forward current and (b) the reverse voltage.

5 Simple diode application: rectifier T=1/f T = period (second, 16.7ms) f = frequency (Hertz, 60Hz)

6 Simple diode application: the rectifier

7 Example The diode conducts when Vs exceeds 12V - The diode stops conducting when Vs falls below 12V - The current i D flows in the conduction angle 2-12V=24V*cos( ) then =60 O or conduction angle 2 = 120 O - Maximum current (peak current) I=(24-12)/100=120mA

8 Simple diode application: the diode logic gates Figure 4.5 Diode logic gates: (a) OR gate; (b) AND gate (in a positive-logic system).

9 Example 4.2.

10 Exercise 4.4 Figure E4.4

11 Terminal Characteristics of Junction Diodes Wed Sep 24 i D I S v V T ( e 1) vd V T ln i I S Thermal voltage: V T kt q I D = diode current v D = voltage across the diode Is= Diode saturation current k= Boltzmann s constant 1.38x10-23 J/K T= absolute temp. (273+t O C) q= electronic charge (1.60x10-19 coulomb)

12 Terminal Characteristics of Junction Diodes 1. Forward bias region: v>0 2. Reverse bias region: v<0 3. Break down region: v<-v zk i=-i S

13 Effect of Temperature V T kt q Figure 4.9 Temperature dependence of the diode forward characteristic. At a constant current, the voltage drop decreases by approximately 2 mv for every 1 C increase in temperature.

14 Diode Exponential Model Diode is shorted Diode is opened Figure 4.11 Graphical analysis of the circuit in Fig using the exponential diode model.

15 Diode Constant-Voltage-Drop Model Figure 4.12 Development of the diode constant-voltage-drop model: (a) the exponential characteristic; (b) approximating the exponential characteristic by a constant voltage, usually about 0.7 V i ; (c) the resulting model of the forward conducting diodes.

16 Example: Output 2.4V, current 1mA, diode voltage drop 0.7V, find R

17 Figure 4.13 Development of the diode small-signal model. i D I D V D d D T d v I r I V r / 1

18 Example

19 Use Diode Forward Drop in Voltage Regulation.

20 Operation in the Reverse Breakdown Region Zener Diodes

21 Zener Diode Model V Z VZ 0 r Z I Z

22 Example: Shunt regulator zener diode

23 Use of Zener Diode -Shunt Regulator -The diode is in parallel with the load -Temperature Sensing - Using temperature coefficient (temco) --2mV/ 0 C

24 Rectifier Circuits Friday Sept 26 ac ripple DC Figure 4.20 Block diagram of a dc power supply.

25 The half-wave rectifier PIV = v S Figure 4.21 (a) Half-wave rectifier. (b) Transfer characteristic of the rectifier circuit. (c) Input and output waveforms.

26 The full-wave rectifier Center tap transformer PIV = 2v S -V D Figure 4.22 Full-wave rectifier utilizing a transformer with a center-tapped secondary winding: (a) circuit; (b) transfer characteristic assuming a constant-voltage-drop model for the diodes; (c) input and output waveforms.

27 The bridge rectifier PIV = v S -V D Figure 4.23 The bridge rectifier: (a) circuit; (b) input and output waveforms.

28 The peak rectifier filtering with capacitor Figure 4.24 (a) A simple circuit used to illustrate the effect of a filter capacitor. (b) Input and output waveforms assuming an ideal diode. Note that the circuit provides a dc voltage equal to the peak of the input sine wave. The circuit is therefore known as a peak rectifier or a peak detector.

29 i D i C i L i C C dv dt I v i L 0 R

30 V I r L I L 2 fc V p R Figure 4.26 Waveforms in the full-wave peak rectifier.

31 Precision half-wave rectifier

32 Limiter Circuit Figure 4.28 General transfer characteristic for a limiter circuit. Figure 4.30 Soft limiting.

33 A variety of basic limiting circuits.

34 A variety of basic limiting circuits.

35 Example v O v I D1 on Vo=-5+1/2(Vi+5) Vo=-2.5-1/2Vi Both diodes cut-off D2 on Vo=+5 +1/2(Vi-5) Vo=2.5+1/2Vi

36 The clamped capacitor or dc restorer with a square-wave input and no load. The clamped capacitor with load resistance.

37 Voltage doubler: (a) circuit; (b) waveform of the voltage across D 1.

38 Other Diode devices: 1. Schottky-Barrier diode (SBD): - Metal anode, semiconductor cathode - Fast switching ON/OFF. - Low forward voltage drop ( V) 2. Varactors: - Capacitance between PN junction - Changing reverse voltage, changing capacitance 3. Photodiodes: - Reverse-biased PN junction illuminates - Converting light signal to electrical signal 4. LEDs: - Inverse function of photodiodes (electrical to light)

39 Summary (page 215, 216)

40 Summary (page 215, 216) 9/26/2014