 # Chapter 3 – Diodes Introduction Textbook CD

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Chapter 3 – Diodes Introduction http://jas2.eng.buffalo.edu/applets/education/pn/biasedPN/index.html Textbook CD http://jas2.eng.buffalo.edu/applets/education/pn/cv/index.html

Introduction

The ideal diode: (a) diode circuit symbol; (b) i-v characteristic; (c) equivalent circuit in the reverse direction; (d) equivalent circuit in the forward direction. The Ideal Diode

Rectifier circuit Rectifier Circuit Output waveform. Input waveform. Equivalent circuit when v 1  0 Equivalent circuit when v 1 > 0

Rectifier Circuit Example 3.1

Rectifier Circuit Example 3.2

Terminal Characteristics of Junction Diodes – Forward Region Example 3.3

Terminal Characteristics of Junction Diodes – Reverse-Bias Region Exercise 3.9

Rectifier Circuit Exercises 3.4 and 3.5

The i-v characteristic of a silicon junction diode. Diode – i-v Characteristic

The diode i-v relationship with some scales expanded and others compressed in order to reveal details. Diode – i-v Characteristic Thermal Voltage 25mV at room temp. ln = 2.3 log

Diode – i-v Characteristic Exercise 3.6 Consider a silicon diode with n=1.5. Find the change in voltage if current changes from 0.1 mA to 10 mA.

Diode – i-v Characteristic A diode for which the forward voltage drop is 0.7 V at 1 mA and for which n=1 is operated at 0.5 V. What is the value of the current?

Simplified physical structure of the junction diode. (Actual geometries are given on Appendix A.) Diode – Simplified Physical Structure

Diode – Semiconductor Physics The semiconductor diode is what is called a pn junction and is shown in the figure on the right Both the p and the n sections are part of the same crystal of silicon. At room temp., some of the covalent bonds in silicon break and electrons are attracted to other atoms. These moving electrons leave a hole behind that is filled by another electron, thus continuing the cycle. In thermal equilibrium the concentration of holes (p) and the concentration of free electrons (n) are equal to each other and to ni which is the number of holes or free electrons in silicon at a given temp. Study of semiconductor physics yields the following equation for the free electrons.

Diode – Semiconductor Physics

Diode –Physical Structure

Minority-carrier distribution in a forward-biased pn junction. It is assumed that the p region is more heavily doped than the n region; N A  N D.

Diode –Physical Structure

Lessons In Electric Circuits copyright (C) 2000-2002 Tony R. Kuphaldt Diode – Characteristic

Lessons In Electric Circuits copyright (C) 2000-2002 Tony R. Kuphaldt Diode – Characteristic

Lessons In Electric Circuits copyright (C) 2000-2002 Tony R. Kuphaldt Diode – Characteristic

Lessons In Electric Circuits copyright (C) 2000-2002 Tony R. Kuphaldt Diode – Characteristic

Lessons In Electric Circuits copyright (C) 2000-2002 Tony R. Kuphaldt Diode – Applications

Lessons In Electric Circuits copyright (C) 2000-2002 Tony R. Kuphaldt Diode – Applications

A simple diode circuit. Analysis of Diode Circuits

Graphical analysis of the circuit above Graphical Analysis

Iterative Analysis Example 3.4

Approximating the diode forward characteristic with two straight lines. Simplified Diode Models

Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation. Simplified Diode Models Example 3.5

Development of the constant-voltage-drop model of the diode forward characteristics. A vertical straight line (b) is used to approximate the fast-rising exponential. The Constant-Voltage Drop Model

The constant-voltage-drop model of the diode forward characteristic and its equivalent circuit representation. The Constant-Voltage Drop Model

Development of the diode small-signal model. Note that the numerical values shown are for a diode with n = 2. The Small-Signal Model

Equivalent circuit model for the diode for small changes around bias point Q. The incremental resistance r d is the inverse of the slope of the tangent at Q, and V D0 is the intercept of the tangent on the v D axis. The Small-Signal Model Example 3.6

The analysis of the circuit in (a), which contains both dc and signal quantities, can be performed by replacing the diode with the model of previous figure, as shown in (b). This allows separating the dc analysis [the circuit in (c)] from the signal analysis [the circuit in (d)]. The Small-Signal Model

Circuit symbol for a zener diode. Zener Diode - Characteristics The diode i-v characteristic with the breakdown region shown in some detail. Model for the zener diode. 6.8 –V, 10mA 0.5W, 6.8-V, 70mA Vz = Vzo + r2Iz Vz > Vzo

Block diagram of a dc power supply. Rectifier Circuits

(a) Half-wave rectifier. (b) Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus- resistance model. (c) transfer characteristic of the rectifier circuit. (d) Input and output waveforms, assuming that r D  R. Rectifier Circuits

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. Rectifier Circuits

The bridge rectifier: (a) circuit and (b) input and output waveforms. Rectifier Circuits PIV V s V DO 

Voltage and current waveforms in the peak rectifier circuit with CR  T. The diode is assumed ideal. Rectifier Circuits With A Filter Capacitor

Rectifier Circuits With A Filter Capacitor

Rectifier Circuits With A Filter Capacitor

Rectifier Circuits With A Filter Capacitor If Vp = 100 V R = 10 K Calculate the value of the capacitance C that will result in a peak-to-peak ripple Vr of 5 V, the conduction angle and the average and peak values of the diode current.

The Spice Diode Model and Simulation Examples The dc characteristics of the diode are determined by the parameters IS and N. An ohmic resistance, RS, is included. Charge storage effects are modeled by a transit time, TT, and a nonlinear depletion layer capacitance which is determined by the parameters CJO, VJ, and M. The temperature dependence of the saturation current is defined by the parameters EG, the energy and XTI, the saturation current temperature exponent. Reverse breakdown is modeled by an exponential increase in the reverse diode current and is determined by the parameters BV and IBV (both of which are positive numbers). nameparameterunitsdefaultexample ------------------------- 1ISsaturation currentA1.0E-141.0E-14* 2RSohmic resistanceOhm010 * 3 N emission coefficient-11.0 4 TT transit-time sec00.1Ns 5CJOzero-bias junction capacitanceF02PF* 6VJjunction potentialV10.6 7Mgrading coefficient -0.50.5 8EGactivation energyeV1.111.11 Si 0.69 Sbd 0.67 Ge

The Spice Diode Model and Simulation Examples The dc characteristics of the diode are determined by the parameters IS and N. An ohmic resistance, RS, is included. Charge storage effects are modeled by a transit time, TT, and a nonlinear depletion layer capacitance which is determined by the parameters CJO, VJ, and M. The temperature dependence of the saturation current is defined by the parameters EG, the energy and XTI, the saturation current temperature exponent. Reverse breakdown is modeled by an exponential increase in the reverse diode current and is determined by the parameters BV and IBV (both of which are positive numbers). nameparameterunitsdefaultexample a --------------------------- ----- 9XTIsaturation-current temp. exp-3.03.0 jn 2.0 Sbd 10KFflicker noise coefficient-0 11AFflicker noise exponent-1 12FCcoefficient for forward-bias-0.5 depletion capacitance formula 13BVreverse breakdown voltageVinfinite 40.0 14IBVcurrent at breakdown voltageA1.0E-3

PN Junction Diodes NameParameterUnitsDefault ISsaturation currentA1.0E-14 Nemission coefficient-1 BVreverse breakdown voltageVinfinite RSdiode series resistance  0 CJOzero-bias junction capacitanceF0 VJjunction potentialV1 Mgrading coefficient-0.5 The Spice Diode Model and Simulation Examples

A variety of basic limiting circuits. Limiting and Clamping Circuits