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By:
Dr. Ahmed ElShafee
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١
Lecture (03)Diodes and
Diode Applications I
Agenda
• VOLTAGE‐CURRENT CHARACTERISTIC OF A DIODE
– Forward bias
– Reverse Bias
– V‐I Characteristic for Forward Bias
– V‐I Characteristic for Reverse Bias
• DIODE MODELS
• The Basic DC Power Supply
• Half wave rectifier
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢
V‐I Characteristic for Forward Bias
٣
• forward‐bias voltage is applied across a diode, current flow called forward current and is designated IF.
• With 0 V across the diode, there is no forward current
• increase the forward‐bias voltage, the forward current and the voltage across the diode gradually increase,
• A portion of the forward‐bias voltage is dropped across the limiting resistor
• When Forward bias reach 0.7 volt ((barrier potential), forward current begins to increase
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤
• Increasing forward bias current continues to increase very rapidly, but the voltage across the diode increases only gradually above 0.7 V (voltage drop due to diode limiting resistor)
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥
• dynamic resistance
• resistance of the forward‐biased diode is not constant over the entire curve
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٦
V‐I Characteristic for Reverse Bias
• 0 V across the diode, there is no reverse current
• gradually increase the reverse‐bias voltage, there is a very small reverse current
• When reverse voltage across the diode (VR) reaches the breakdown value (VBR), the reverse current begins to increase rapidly.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٧
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٨
• Temperature Effects
• as temperature is increased, the forward current increases for a given value of forward voltage
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٩
DIODE MODELS
• Bias Connections
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٠
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١١
• The Ideal Diode Model:
• Forward biased is represented by a simple closed switch.
• reverse‐biased, is presented as open switch
• The Practical Diode Model
• If diode is forward‐biased, is equivalent to a closed switch in series with a small equivalent voltage source (VF) equal to the barrier potential (0.7 V)
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٢
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٣
• Applying KVL :
• Ohms law
• Substituting
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٤
• The Complete Diode Model
• Add a small forward dynamic resistance (r’d) and the large internal reverse resistance (r’R).
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٥
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٦
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٧
Example 01
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٨
• Ideal model:
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I١٩
• Practical model:
• Complete model
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢٠
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢١
• Ideal model
• Practical model:
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢٢
• Complete model
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢٣
The Basic DC Power Supply
• All active electronic devices require a source of constant dc that can be supplied by a battery or a dc power supply
• dc power supply converts the standard 220V, 50Hz ac voltage available at wall outlets into a constant dc voltage
A basic block diagram of the complete power supply٢٤
• Stage 1: transformer changes ac voltages based on the turns ratio between the primary and secondary.
• Stage 2: rectefire (half wave) The rectifier converts the ac input voltage to a pulsating dc voltage,
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢٥
• Stage 3:The filter eliminates the fluctuations in the rectified voltage and produces a relatively smooth dc voltage.
• Stage 4: The regulator is a circuit that maintains a constant dc voltage for variations in the input line voltage or in the load.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢٦
Half wave rectifier
• When the sinusoidal input voltage (Vin) goes positive, the diode is forward‐biased and conducts current through the load resistor
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢٧
• When the input voltage goes negative during the second half of its cycle, the diode is reverse‐biased.
• There is no current, so the voltage across the load resistor is 0 V,
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢٨
• The resulted voltage drop across load is stimulating DC voltage
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٢٩
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٠
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣١
•
Example 01
• What is the average value of the half‐wave rectified voltage in Figure
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٢
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٣
• Effect of the Barrier Potential on the Half‐Wave Rectifier Output
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٤
• It’s acceptable to neglect the effect of the barrier potential, when the peak value of the applied voltage is much greater than the barrier potential (at least 10 V, as a rule of thumb).
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٥
Example 02
• Draw the output voltages of each rectifier for the indicated input voltages, as shown in Figure. The 1N4001 and 1N4003 are specific rectifier diodes.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٦
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٧
• Peak Inverse Voltage (PIV)
• equals the peak value of the input voltage, and the diode must be capable of withstanding this amount of repetitive reverse voltage
• A diode should be rated at least 20% higher than the PIV.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٨
• Transformer Coupling
• transformer is used to couple the ac input voltage from the source to the rectifier,
• Advantages:
– Step down voltage
– electrically isolated rectifier, thus preventing a shock hazard in the secondary circuit.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٣٩
• turns ratio
• If n < 1, then Vsec <Vpri.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٠
Example 03
• Determine the peak value of the output voltage for Figure, if the turns ratio is 0.5.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤١
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٢
Full wave rectifier
• allows unidirectional (one‐way) current through the load during the entire 360 of the input cycle
• The result of full‐wave rectification is an output voltage with a frequency twice the input frequency and that pulsates every half‐cycle of the input
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٣
• The average value, which is the value measured on a dc voltmeter, for a full‐wave rectified sinusoidal voltage is twice that of the half‐wave
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٤
Example 04
• Find the average value of the full‐wave rectified voltage in Figure
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٥
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٦
Center‐Tapped Full‐Wave Rectifier Operation
• A center‐tapped rectifier uses two diodes connected to the secondary of a center‐tapped transformer
• Half of the total secondary voltage appears between the center tap and each end of the secondary winding
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٧
• For a positive half‐cycle of the input voltage; D1 is forward‐biases diode and D2 is reverse‐biases diode.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٨
• For a negative half‐cycle of the input voltage; D2 is forward‐biases diode and D1 is reverse‐biases diode.
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٤٩
• Effect of the Turns Ratio on the Output Voltage
• If the transformer’s turns ratio is 1, the peak value of the rectified output voltage equals half the peak value of the primary input voltage less the barrier potential,
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥٠
• a step‐up transformer with a turns ratio of n = 2 must be used
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥١
• General rule for center tapped rectifier:
• In any case, the output voltage of a center‐tapped full‐wave rectifier is always one‐half of the total secondary voltage less the diode drop, no matter what the turns ratio.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥٢
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥٣
• PIV calculation:
Since x2
sub
Example 5
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥٤
• There is a 25 V peak across each half of the secondary with respect to ground.
• The output load voltage has a peak value of 25 V, less the 0.7 V drop across the diode.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥٥
•
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥٦
Thanks,..
See you next week (ISA),…
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits I٥٧