Chapter 2. Diodes & Applications

Chapter 2 Diodes & Applications

The Diode A diode is made from a small piece of semiconductor material, usually silicon, in which half is doped as a p region and half is doped as an n region with a pn junction and depletion region in between.

Diode Packages

Forward Bias Forward bias is the condition that allows current through the pn junction.

Requirements Forward Bias Negative side of VBIAS is connected to the n region of the diode and the positive side is connected to the p region. VBIAS >>> Barrier Potential

Effect of Forward Bias on the Depletion Region As more electrons flow into the depletion region, the number of positive ions is reduced. As more holes effectively flow into the depletion region on the other side of the pn junction, the number of negative ions is reduced. This reduction in positive and negative ions during forward bias causes the depletion region to narrow

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Barrier Potential When forward bias is applied, the free electrons are provided with enough energy from the bias-voltage source to overcome the barrier potential and cross the depletion region. The energy that the electrons require in order to pass through the depletion region is equal to the barrier potential. This energy loss results in a voltage drop across the pn junction equal to the barrier potential (0.7 V)

Reverse Bias Reverse bias is the condition that essentially prevents current through the diode.

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Reverse Current The extremely small current that exists in reverse bias after the transition current dies out is caused by the minority carriers in the n and p regions that are produced by thermally generated electron-hole pairs.

V-I Characteristic of a Diode

V-I Characteristic of a Diode Point A corresponds to a zero-bias condition. Point B corresponds to where the forward voltage is less than the barrier potential of 0.7 V. Point C corresponds to where the forward voltage approximately equals the barrier potential.

V-I Characteristic for Reverse Bias

The Complete V-I Characteristic Curve

Diode Approximations The Ideal Diode Model

Diode Approximations Since the barrier potential and the forward dynamic resistance are neglected, the diode is assumed to have a zero voltage across it when forward-biased V F = 0 V Forward Current Reverse Current Neglected

Diode Approximation Practical Diode Model

Diode Approximation Practical Diode Model Applying KVL

Diode Approximation Complete Diode Model

Diode Approximation Complete Diode Model Formulas

Example 2-1

Solution

Half-wave Rectifiers Because of their ability to conduct current in one direction and block current in the other direction, diodes are used in circuits called rectifiers that convert ac voltage into dc voltage. Rectifiers are found in all dc power supplies that operate from an ac voltage source.

Half-wave Rectifiers

Half-wave Rectifiers The rectifier converts the ac input voltage to a pulsating dc voltage. The rectifier converts the ac input voltage to a pulsating dc voltage. The regulator is a circuit that maintains a constant dc voltage for variations in the input line voltage or in the load.

Half-Wave Rectifier Operation

Half-wave Rectifier Average Value of the Half-Wave Output Voltage Example 2-2

Effect of Potential Barrier

Transformer Coupling It allows the source voltage to be stepped down as needed. The ac source is electrically isolated from the rectifier, thus preventing a shock hazard in the secondary circuit.

Transformer Coupling Turns Ratio = N sec / N pri Turns Ratio > 1 Step Up Turns Ratio < 1 Step Down Secondary Voltage of Transformer

Full-wave Rectifiers A full-wave rectifier allows unidirectional (one-way) current through the load during the entire 360 o of the input cycle.

Example 2-5 Full-wave Rectifier

Center-Tapped Full-Wave Rectifier Operation

Operation

Effect of the Turns Ratio on the Output Voltage

Bridge Full-wave Rectification

Peak Inverse Voltage For rectifier applications, peak inverse voltage (PIV) or peak reverse voltage (PRV) is the maximum value of reverse voltage which occurs at the peak of the input cycle when the diode is reverse-biased.

Power Supply Filters & Regulators A power supply filter ideally eliminates the fluctuations in the output voltage of a half-wave or full-wave rectifier and produces a constantlevel dc voltage. A voltage regulator prevents changes in the filtered dc voltage due to variations in input voltage or load.

Power Supply Filters & Regulators The small amount of fluctuation in the filter output voltage is called Ripple.

Capacitor-Input Filter

Ripple Voltage

Example 2-8

Solution

Voltage Regulators

Percent Regulation The line regulation specifies how much change occurs in the output voltage for a given change in the input voltage. It is typically defined as a ratio of a change in output voltage for a corresponding change in the input voltage expressed as a percentage.

Percent Regulation The load regulation specifies how much change occurs in the output voltage over a certain range of load current values, usually from minimum current (no load, NL) to maximum current (full load, FL).

Example 2-9

Diode Limiters & Clampers Diode circuits, called limiters or clippers, are sometimes used to clip off portions of signal voltages above or below certain levels. Another type of diode circuit, called a clamper, is used to add or restore a dc level to an electrical signal.

Diode Limiters

Biased Limiters

Diode Clampers A Clamper adds a dc level to an ac voltage. Operation During the negative half cycle of the input signal, the diode conducts and acts like a short circuit. The output voltage V o 0 volts. The capacitor is charged to the peak value of input voltage V m. and it behaves like a battery. During the positive half of the input signal, the diode does not conduct and acts as an open circuit. Hence the output voltage V o V m + V m This gives a positively clamped voltage.

Diode Clampers If the RC time constant is 100 times the period, the clamping action is excellent. An RC time constant of ten times the period will have a small amount of distortion at the ground level due to the charging current.

The Diode Datasheet