Lecture (05) PN Diode. Special Purpose diodes

Transcription

1 Lecture (05) PN Diode applications III, Special Purpose diodes By: Dr. Ahmed ElShafee ١ Agenda Diode limiters Diode Clampers Zener Diode Zener Diode Applications ٢

2 Diode limiters Figure shows a diode positive limiter (also called clipper) that limits or clips the positive part of the input voltage. As the input voltage goes positive, the diode becomes forward biased and conducts current. Point A is limited to +0.7 V when the input voltage exceeds this value ٣ When the input voltage goes back below 0.7 V, the diode is reverse biased and appears as an open. ٤ If R1 is small compared to R1, then Vout=Vin

3 Example What would you expect to see displayed on an oscilloscope connected across RL in the limiter shown in Figure ٥ ٦

4 Biased Limiters The voltage at point A must equal VBIAS V before the diode will become forward biased and conduct. ٧ ٨

5 the positive limiter can be modified to limit the output voltage to the portion of the input voltage waveform above ٩ the negative limiter can be modified to limit the output voltage to the portion of the input voltage waveform ١٠

6 Example shows a circuit combining a positive limiter with a negative limiter. Determine the output voltage waveform ١١ ١٢

7 Voltage Divider Bias ١٣ A Limiter Application almost all digital circuits should not have an input level that exceeds the power supply voltage. An input of a few volts more than this could damage the circuit. To prevent the input from exceeding a specific level, you may see a diode limiter across the input signal path in many digital circuits. ١٤

8 Example Describe the output voltage waveform for the diode limiter in Figure ١٥ ١٦

9 Voltage multiplier Half wave voltage doubler ١٧ Full Wave Voltage Doubler ١٨

10 Voltage Tripler ١٩ Voltage Quadrupler ٢٠

11 Zener Diode A zener diode is a silicon pn junction device that is designed for operation in the reverse breakdown region. when a diode reaches reverse breakdown, its voltage remains almost constant even though the current changes drastically, and this is the key to zener diode operation. ٢١ This volt ampere characteristic is shown again in Figure with the normal operating region for zener diodes shown as a shaded area. ٢٢

12 Zener Regulation The ability to keep the reverse voltage across its terminals essentially constant is the key feature of the zener diode. A zener diode operating in breakdown acts as a voltage regulator because it maintains a nearly constant voltage across its terminals over a specified range of reverse current values. ٢٣ Breakdown Characteristics A minimum value of reverse current, I ZK, must be maintained in order to keep the diode in breakdown for voltage regulation maximum current, I ZM, above which the diode may be damaged due to excessive power dissipation. A nominal zener voltage, V Z, is usually specified on a datasheet at a value of reverse current called the zener test current ٢٤

13 Zener Equivalent Circuits Ideal model ٢٥ Practical Model zener impedance (resistance), Z Z It is best to avoid operating a zener diode near the knee of the curve because the impedance changes dramatically in that ٢٦ area.

14 Example 01 ٢٧ Example 01 ٢٨

15 Temperature Coefficient specifies the percent change in zener voltage for each degree Celsius change in temperature example, a 12 V zener diode with a positive temperature coefficient (+0.01%/ 0 c) of will exhibit a 1.2 mv increase in V Z when the junction temperature increases one degree Celsius. V Z is the nominal zener voltage at the reference temperature of 25 0 C ٢٩ A positive TC means that the zener voltage increases with an increase in temperature or decreases with a decrease in temperature. A negative TC means that the zener voltage decreases with an increase in temperature or increases with a decrease in temperature ٣٠

16 In some cases, the temperature coefficient is expressed in V/ 0 C rather than as %V/ 0 C For these cases, ΔVZ is calculated as mv/ C ٣١ Example 02 ٣٢

17 Example 02 Solution ٣٣ Zener Power Dissipation Zener diodes are specified to operate at a maximum power called the maximum dc power dissipation, PD(max). 1N746 zener is rated at a PD(max) of 500 mw and 1N3305A is rated at a PD(max) of 50 W. ٣٤

18 Power Derating The maximum power dissipation of a zener diode is typically specified for temperatures at or below a certain value ( for example). Above the specified temperature, the maximum power dissipation is reduced according to a derating factor. The derating factor is expressed in mw/ 0 C ٣٥ Example 03 ٣٦

19 Example 03 Solution ٣٧ ٣٨

20 ٣٩ ٤٠

21 ٤١ ٤٢

22 Example 04 ٤٣ Example 04 ٤٤

23 Example 04 ٤٥ Thanks,.. See you next week (ISA), ٤٦