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By:
Dr. Ahmed ElShafee
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١
Lecture (05)PN Diode
applications III, Special Purpose
diodes
Agenda
• Diode limiters
• Diode Clampers
• Zener Diode
• Zener Diode Applications
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٢
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
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٣
• When the input voltage goes back below 0.7 V, the diode is reverse‐biased and appears as an open.
• If R1 is small compared to R1, then Vout=Vin ٤
Example
• What would you expect to see displayed on an oscilloscope connected across RL in the limiter shown in Figure
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٥
•
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٦
• Biased Limiters
• The voltage at point A must equal VBIAS + 0.7 V before the diode will become forward‐biased and conduct.
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٧
•
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٨
• the positive limiter can be modified to limit the output voltage to the portion of the input voltage waveform above
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٩
• the negative limiter can be modified to limit the output voltage to the portion of the input voltage waveform
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٠
Example
• shows a circuit combining a positive limiter with a negative limiter. Determine the output voltage waveform
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١١
•
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٢
• Voltage‐Divider Bias
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٣
• 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.
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٤
Example
• Describe the output voltage waveform for the diode limiter in Figure
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٥
•
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٦
Voltage multiplier
• Half‐wave voltage doubler
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٧
• Full‐Wave Voltage Doubler
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٨
• Voltage Tripler
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I١٩
• Voltage Quadrupler
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٢٠
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.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢١
• This volt‐ampere characteristic is shown again in Figure with the normal operating region for zener diodes shown as a shaded area.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢٢
• 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.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢٣
Breakdown Characteristics
• A minimum value of reverse current, IZK, must be maintained in order to keep the diode in breakdown for voltage regulation
• maximum current, IZM, above which the diode may be damaged due to excessive power dissipation.
• A nominal zener voltage, VZ, is usually specified on a datasheet at a value of reverse current called the zener test current
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢٤
Zener Equivalent Circuits
• Ideal model
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢٥
Practical Model
• zener impedance (resistance), ZZ
• It is best to avoid operating a zenerdiode near the knee of the curve because the impedance changes dramatically in that area. Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢٦
Example 01
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢٧
Example 01
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢٨
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%/0c) of will exhibit a 1.2 mV increase in VZ
when the junction temperature increases one degree Celsius.
• VZ is the nominal zener voltage at the reference temperature of 250C
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٢٩
• 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
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٠
• In some cases, the temperature coefficient is expressed in V/0C rather than as %V/0C
• For these cases, ΔVZ is calculated as mV/°C
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣١
Example 02
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٢
Example 02
Solution
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٣
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.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٤
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/0C
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٥
Example 03
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٦
Example 03
Solution
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٧
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Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٨
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Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٣٩
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Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٤٠
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Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٤١
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Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٤٢
Example 04
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Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٤٣
Example 04
Solution
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@50
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٤٤
Example 04
Solution
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@25
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits٤٥
Thanks,..
See you next week (ISA),…
Dr. Ahmed ElShafee, ACU : Fall 2017, Electronic Circuits I٤٦