Diodes (non-linear devices)

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Lester White
5 years ago
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1 C H A P T E R 4 Diodes (non-linear devices)

2 Ideal Diode

3 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.

4 Simple diode application: rectifier

5 Simple diode application: rectifier

6 Example 4.1.

7 Diode logic gates Figure 4.5 Diode logic gates: (a) OR gate; (b) AND gate (in a positive-logic system).

8 Example 4.2.

9 Examples Figure E4.4

10 Terminal Characteristics of Junction Diodes i v = T V I S ( e 1) v = V T ln i I S Thermal voltage: V T = kt q k= Boltzmann s constant 91.38x10-23 J/K T: absolute temp. (273+xOC) q= electronic charge (1.60x10-19 coulomb)

11 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

12 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.

13 Diode Exponential Model Figure 4.11 Graphical analysis of the circuit in Fig using the exponential diode model.

14 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.

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

16 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

17 Example

18 Use Diode Forward Drop in Voltage Regulation.

19 Operation in the Reverse Breakdown Region Zener Diodes

20 Zener Diode Model V + Z= VZ 0 r Z I Z

21 Example: Shunt regulator zener diode

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

23 Rectifier Circuits ripple Figure 4.20 Block diagram of a dc power supply.

24 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.

25 The full-wave rectifier 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.

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

27 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.

28 i = i + i D C L i C = C dv dt I v 0 i L = R

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

30 Precision half-wave rectifier

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

32 A variety of basic limiting circuits.

33 A variety of basic limiting circuits.

34 Example

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

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

37 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, change capacitance 3. Photodiodes: - Reverse-biased PN junction illuminates - Converting light signal to electrical signal 4. LEDs: - Inverse function of photodiodes (electrical to light)

38 Summary (page 215, 216)

Zener Diodes. Specifying and modeling the zener diode. - Diodes operating in the breakdown region can be used in the design of voltage regulators.

Zener Diodes. Specifying and modeling the zener diode. - Diodes operating in the breakdown region can be used in the design of voltage regulators. Zener Diodes - Diodes operating in the breakdown region can be used in the design of voltage regulators. Specifying and modeling the zener diode Dynamic resistance, r Z a few ohms to a few tens of ohms