Zener Diode & Special Purpose Diodes

Transcription

1 Zener Diode & Special Purpose Diodes Zener Diode: The diodes designed to work in breakdown region are called zener diode. If the reverse voltage exceeds the breakdown voltage, the zener diode will normally not be destroyed as long as the current does not exceed maximum value and the device closes not over load. When a thermally generated carrier (part of the reverse saturation current) falls down the junction and acquires energy of the applied potential, the carrier collides with crystal ions and imparts sufficient energy to disrupt a covalent bond. In addition to the original carrier, a new electron-hole pair is generated. This pair may pick up sufficient energy from the applied field to collide with another crystal ion and create still another electron-hole pair. This action continues and thereby disrupts the covalent bonds. The process is referred to as impact ionization, avalanche multiplication or avalanche breakdown. There is a second mechanism that disrupts the covalent bonds. The use of a sufficiently strong electric field at the junction can cause a direct rupture of the bond. If the electric field exerts a strong force on a bound electron, the electron can be torn from the covalent bond thus causing the number of electronhole pair combinations to multiply. This mechanism is called high field emission or Zener breakdown. The value of reverse voltage at which this occurs is controlled by the amount of doping of the diode. A heavily doped diode has a low Zener breakdown voltage, while a lightly doped diode has a high Zener breakdown voltage. At voltages above approximately 8V, the predominant mechanism is the avalanche breakdown. Since the Zener effect (avalanche) occurs at a predictable point, the diode can be used as a voltage reference. The reverse voltage at which the avalanche occurs is called the breakdown or Zener voltage. A typical Zener diode characteristic is shown in fig. 1. The circuit symbol for the Zener diode is different from that of a regular diode, and is illustrated in the figure. The maximum reverse current, IZ(max), which the Zener diode can withstand is dependent on the design and construction of the diode. A design guideline that the minimum Zener current, where the characteristic curve remains at VZ (near the knee of the curve), is 0.1/ IZ(max). 1

2 Fig. 1 - Zener diode characteristic The power handling capacity of these diodes is better. The power dissipation of a zener diode equals the product of its voltage and current. PZ= VZ IZ The amount of power which the zener diode can withstand (VZ.IZ(max)) is a limiting factor in power supply design. Zener Regulator: When zener diode is forward biased it works as a diode and drop across it is 0.7 V. When it works in breakdown region the voltage across it is constant (VZ) and the current through diode is decided by the external resistance. Thus, zener diode can be used as a voltage regulator in the configuration shown in fig. 2 for regulating the dc voltage. It maintains the output voltage constant even through the current through it changes. 2

3 Fig. 2 Fig. 3 The load line of the circuit is given by Vs= Is Rs + Vz. The load line is plotted along with zener characteristic in fig. 3. The intersection point of the load line and the zener characteristic gives the output voltage and zener current. To operate the zener in breakdown region Vs should always be greater than Vz. Rs is used to limit the current. If the Vs voltage changes, operating point also changes simultaneously but voltage across zener is almost constant. The first approximation of zener diode is a voltage source of Vz magnitude and second approximation includes the resistance also. The two approximate equivalent circuits are shown in fig. 4. Fig. 5 Fig. 4 3

4 If second approximation of zener diode is considered, the output voltage varies slightly as shown in fig. 5. The zener ON state resistance produces more I * R drop as the current increases. As the voltage varies from V1 to V2 the operating point shifts from Q1 to Q2. The voltage at Q1 is V1 = I1 RZ +VZ and at Q2 V2 = I2 RZ +VZ Thus, change in voltage is V2 V1 = ( I2 I1 ) RZ Δ VZ =Δ IZ RZ Design of Zener regulator circuit: A zenere regulator circuit is shown in fig. 6. The varying load current is represented by a variable load resistance RL. The zener will work in the breakdown region only if the Thevenin voltage across zener is more than VZ. If zener is operating in breakdown region, the current through RS is given by Fig. 6 and load current Is= Iz + IL 4

5 The circuit is designed such that the diode always operates in the breakdown region and the voltage VZ across it remains fairly constant even though the current IZ through it vary considerably. If the load IL should increase, the current IZ should decrease by the same percentage in order to maintain load current constant Is. This keeps the voltage drop across Rs constant and hence the output voltage. If the input voltage should increase, the zener diode passes a larger current, that extra voltage is dropped across the resistance Rs. If input voltage falls, the current IZ falls such that VZ is constant. In the practical application the source voltage, vs, varies and the load current also varies. The design challenge is to choose a value of Rs which permits the diode to maintain a relatively constant output voltage, even when the input source voltage varies and the load current also varies. We now analyze the circuit to determine the proper choice of Rs. For the circuit shown in figure, (E-1) (E-2) The variable quantities in Equation (E-2) are vz and il. In order to assure that the diode remains in the constant voltage (breakdown) region, we examine the two extremes of input/output conditions, as follows: The current through the diode, iz, is a minimum (IZ min) when the load current, il is maximum (IL max) and the source voltage, vs is minimum (Vs min). The current through the diode, iz, is a maximum (IZ max) when the load current, il, is minmum (il min) and the source voltage vsis minimum(vs max). When these characteristics of the two extremes are inserted into Equation (E-1), we find (E-3) In a practical problem, we know the range of input voltages, the range of output load currents, and the desired Zener voltage. Equation (E-4) thus represents one equation in two unknowns, the maximum and (E-4) 5

6 minimum Zener current. A second equation is found from the characteristic of zener. To avoid the nonconstant portion of the characteristic curve, we use an accepted rule of thumb that the minimum Zener current should be 0.1 times the maximum (i.e., 10%), that is, (E-5) Solving the equations E-4 and E-5, we get, (E-6) Now that we can solve for the maximum Zener current, the value of Rs, is calculated from Equation (E- 3). Zener diodes are manufactured with breakdown voltages VZ in the range of a few volts to a few hundred volts. The manufacturer specifies the maximum power the diode can dissipate. For example, a 1W, 10 V zener can operate safely at currents up to 100mA. Special Purpose Diodes Example 1: Design a 10-volt Zener regulator as shown in fig. 1 for the following conditions: a. The load current ranges from 100 ma to 200 ma and the source voltage ranges from 14 V to 20 V. Verify your design using a computer simulation. b. Repeat the design problem for the following conditions: The load current ranges from 20 ma to 200 ma and the source voltage ranges from 10.2 V to 14 V. Use a 10-volt Zener diode in both cases Solution: Fig. 1 6

7 (a). The design consists of choosing the proper value of resistance, R i, and power rating for the Zener. We use the equations from the previous section to first calculate the maximum current in the zener diode and then to find the input resistor value. From the Equation (E-6), we have I Zmax = A Then, from Equation (E-3), we find R i as follows, It is not sufficient to specify only the resistance of R i. We must also select the proper resistor power rating. The maximum power in the resistor is given by the product of voltage with current, where we use the maximum for each value. P R = ( I Zmax + I Lmin ) (V smax V Z ) = 6.3 W Finally, we must determine the power rating of the Zener diode. The maximum power dissipated in the Zener diode is given by the product of voltage and current. P z = V z l zmax = 0.53 x 10 = 5.3 W Light Emitting Diode : In a forward biased diode free electrons cross the junction and enter into p-layer where they recombine with holes. Each recombination radiates energy as electron falls from higher energy level to a lower energy level. I n ordinary diodes this energy is in the form of heat. In light emitting diode, this energy is in the form of light. The symbol of LED is shown in fig. 2. Ordinary diodes are made of Ge or Si. This material blocks the passage of light. LEDs are made of different materials such as gallium, arsenic and phosphorus. LEDs can radiate red, green, yellow, blue, orange or infrared (invisible). The LED's forward voltage drop is more approximately 1.5V. Typical LED current is between 10 ma to 50 ma. 7

8 Fig. 2 Fig. 3 Seven Segment Display: Seven segment displays are used to display digits and few alphabets. It contains seven rectangular LEDs. Each LED is called a Segment. External resistors are used to limit the currents to safe Values. It can display any letters a, b, c, d, e, f, g. as shown in fig. 3. 8

9 Fig. 4 The LEDs of seven-segment display are connected in either in common anode configuration or in common cathode configuration as shown in fig. 4. Photo diode: When a diode is reversed biased as shown in fig. 5, a reverse current flows due to minority carriers. These carriers exist because thermal energy keeps on producing free electrons and holes. The lifetime of the minority carriers is short, but while they exist they can contribute to the reverse current. When light energy bombards a p-n junction, it too can produce free electrons. Fig. 5 In other words, the amount of light striking the junction can control the reverse current in a diode. A photo diode is made on the same principle. It is sensitive to the light. In this diode, through a window light falls to the junction. The stronger the light, the greater the minority carriers and larger the reverse current. Opto Coupler: It combines a LED and a photo diode in a single package as shown in fig. 6. LED radiates the light depending on the current through LED. This light fails on photo diode and this sets up a reverse current. The advantage of an opto coupler is the electrical isolation between the input and output circuits. The only contact between the input and output is a beam of light. Because of this, it is possible to have an insulation resistance between the two circuits of the order of thousands of mega ohms. They can be used to isolate two circuits of different voltage levels. Fig. 6 9