1 Diodes. 1.1 Diode Models Ideal Diode. ELEN 236 Diodes

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ELEN 236 Diodes 1 Diodes 1.1 Diode Models 1.1.1 Ideal Diode Current through diode is zero for any voltage less than zero i.e. reverse biased case Current through diode is not limited by diode if voltage across diode is forward biased (i.

1 ELEN 236 Diodes 1 Diodes 1.1 Diode Models Ideal Diode Current through diode is zero for any voltage less than zero i.e. reverse biased case Current through diode is not limited by diode if voltage across diode is forward biased (i.e., diode acts like a short V-I diagram: Equivalent circuit forward biased (looks like a short): Equivalent circuit reverse biased: 1

2 1.1.2 Second Approximation Similar to ideal diode, but takes into account the barrier voltage If diode voltage is less than barrier voltage no current If diode voltage is at barrier voltage current in system is not limited by diode (i.e. diode acts as a short) V-I Diagram: Equivalent forward biased circuit: Equivalent reverse biased circuit: Third Approximation Takes into account the barrier voltage, and the finite resistance of the diode when it is forward biased 2

3 When reverse biased no current When forward biased, and V D is less than barrier voltage no current When forward biased and V D is greater than barrier voltage current begins to flow as if it is flowing through resistor V-I diagram: Equivalent forward biased circuit: R diode is also known as r f, ΔV It is equal to the change in voltage divided by the change in current ΔI, or another way to look at it is it is equal to the slope of the line Actually quite a good approximation of actual behaviour of diode in the conduction region as the slope of the line there is approximately linear Equivalent reverse biased circuit: 3

4 1.1.4 Diode Characteristic Curve Two main regions, each with two sub regions Forward Bias o When low voltage (less than V barrier ), high resistance (but not infinite) o Knee voltage is point where V-I curve really starts to curve upwards o Above knee voltage is conduction region acts as low resistance where R = slope Reverse Bias o High resistance below PIV o Low resistance above PIV breakdown region Resistance for any I-V curve is always equal to slope of line o Resistor slope is constant o Diode is non-linear therefore slope changes o From calculus, slope of line is equal to derivative of equation describing line v / nvt o Equation of diode: i = I x ( e 1) o o Derivative of that will give you slope and therefore diode resistance at any point Don t need to know this only need to know from looking at graph above, which regions are high, and which regions are low, and if I give you a ΔV and ΔI you should be able to calculate the resistance. 4

1.2 Examples and Applications 1.2.1 Limiting and Clamping Very often in

voltage from going too high, or too low.

5 1.2 Examples and Applications Limiting and Clamping Very often in circuit and IC design we want to protect part of the circuit by preventing the voltage from going too high, or too low. Diodes can be used for limiting the voltage to certain parts of the circuit. The following diagrams show some examples: 5

1.2.2 Power Supply Filtering Diodes have an important role in power supply circuits Help convert AC to DC Main role is as a rectifier AC Signal Rectifier Filter Regulator DC Signal Input from wall

6 1.2.2 Power Supply Filtering Diodes have an important role in power supply circuits Help convert AC to DC Main role is as a rectifier AC Signal Rectifier Filter Regulator DC Signal Input from wall Removes Negative Values Smooth out signal Maintain solid steady voltage To your electronics Half Wave Rectification A half wave rectification takes an input AC signal, and cuts off the negative portion of the signal. The half wave rectifier circuit and signal diagram look like this: 6

Note that there is a voltage drop between the input and the rectified signal. This voltage drop is equal to the barrier voltage. 1.2.

The following diagram shows a full wave rectified signal. Note the voltage drop between input and output. One common diode configuration for full wave rectification is called the bridge rectifier.

7 Note that there is a voltage drop between the input and the rectified signal. This voltage drop is equal to the barrier voltage Full Wave Rectification A full wave rectifier takes an input AC signal, modifies it so that every half cycle of the input signal is output as a positive half cycle. The following diagram shows a full wave rectified signal. Note the voltage drop between input and output. One common diode configuration for full wave rectification is called the bridge rectifier. It consists of 4 diodes, and each set of 2 diodes (one set is D2, D3 and the other is D1, D4) allows conduction only during one of the half cycles of the input signal. When in the positive phase of the input signal, only D2 and D3 conduct. When in the negative phase of the input signal, only D1 and D4 conduct. The result is that the measurement for V L will always give a positive value. See the circuit diagram for more details: 7

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Lecture (03) Diodes and Diode Applications I

Lecture (03) Diodes and Diode Applications I By: Dr. Ahmed ElShafee ١ Agenda VOLTAGE CURRENT CHARACTERISTIC OF A DIODE Forward bias Reverse Bias V I Characteristic for Forward Bias V I Characteristic for