Lecture (03) Diodes and Diode Applications I

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1 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 Reverse Bias DIODE MODELS The Basic DC Power Supply Half wave rectifier ٢

V I Characteristic for Forward Bias forward bias voltage is applied across a

٣ With 0 V across the diode, there is no forward current increase the forward

increase, A portion of the forward bias voltage is dropped across the limiting

2 V I Characteristic for Forward Bias forward bias voltage is applied across a diode, current flow called forward current and is designated I F. ٣ 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 ٤

3 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) ٥ dynamic resistance resistance of the forward biased diode is not constant over the entire curve ٦

4 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. ٧ ٨

5 Temperature Effects as temperature is increased, the forward current increases for a given value of forward voltage ٩ DIODE MODELS Bias Connections ١٠

6 The Ideal Diode Model: Forward biased is represented by a simple closed switch. reverse biased, is presented as open switch ١١ 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) ١٢

7 ١٣ Applying KVL : Ohms law Substituting ١٤

8 The Complete Diode Model Add a small forward dynamic resistance (r d) and the large internal reverse resistance (r R). ١٥ ١٦

9 ١٧ Example 01 ١٨

10 Ideal model: Practical model: ١٩ Complete model ٢٠

11 ٢١ Ideal model Practical model: ٢٢

Complete model ٢٣ 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

12 Complete model ٢٣ 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 ٢٤ A basic block diagram of the complete power supply

13 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, ٢٥ 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. ٢٦

14 Half wave rectifier When the sinusoidal input voltage (Vin) goes positive, the diode is forward biased and conducts current through the load resistor ٢٧ 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, ٢٨

15 The resulted voltage drop across load is stimulating DC voltage ٢٩ ٣٠

16 ٣١ Example 01 What is the average value of the half wave rectified voltage in Figure ٣٢

17 ٣٣ Effect of the Barrier Potential on the Half Wave Rectifier Output ٣٤

18 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). ٣٥ 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. ٣٦

withstanding this amount of repetitive reverse voltage

19 ٣٧ 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. ٣٨

20 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. ٣٩ turns ratio If n < 1, then Vsec <Vpri. ٤٠

21 Example 03 Determine the peak value of the output voltage for Figure, if the turns ratio is 0.5. ٤١ ٤٢

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

22 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 ٤٣ 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 ٤٤

23 Example 04 Find the average value of the full wave rectified voltage in Figure ٤٥ ٤٦

24 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 ٤٧ For a positive half cycle of the input voltage; D1 is forwardbiases diode and D2 is reverse biases diode. ٤٨

25 For a negative half cycle of the input voltage; D2 is forwardbiases diode and D1 is reverse biases diode. ٤٩ 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, ٥٠

26 a step up transformer with a turns ratio of n = 2 must be used ٥١ 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. ٥٢

27 PIV calculation: Since x2 sub ٥٣ Example 5 ٥٤

28 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. ٥٥ ٥٦

29 Thanks,.. See you next week (ISA), ٥٧

Lecture (03) Diode applications

Lecture (03) Diode applications By: Dr. Ahmed ElShafee ١ Agenda The Basic DC Power Supply Half wave rectifier Full wave rectifier Filters Voltage Regulators ٢ The Basic DC Power Supply All active electronic