Abesamis, Laguna 1. IDENTIFY THE TYPES OFAbesamis, Laguna 1. IDENTIFY THE TYPES OF

Abesamis, Beatrice A.                         Prelim                                 November 30, 2017

EC42FB1/ECE 003                      Homework No. 1                       Engr. Laguna

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1. IDENTIFY THE TYPES OF DIODES.

Fig. 1.   
Diode

A diode is a two-terminal device, having
two active electrodes, between which it allows the transfer of current in one
direction only. Technically, diodes are used for the purpose of rectifying
waveforms. They can also be used in circuits where ‘one way’ effect of diode is
needed.

Usually, diodes are made from
semiconductors as silicon. Diodes carries electric currents in one direction,
but, the manner in which they do so can vary. The different types of diodes are
presented below.

a.      
Light Emitting Diode

 

Fig. 2.   
Light Emitting Diode

. It is a semiconductor device that emits
visible light when an electric current pass through it.

b.      
Avalanche Diode

Fig. 3.   
Avalanche Diode

This operates in the reverse bias, and
used avalanche effect for its operation. When the voltage drop is constant and
is independent of current, the avalanche breakdown occurs across the entire PN
junction. Basically, this is used for photo-detection, wherein high levels of
sensitivity can be acquired by the avalanche process.

c.       
 Laser
Diode

Fig. 4.   
Laser Diode

The term LASER stands for Light Amplification
by Stimulated Emission of Radiation. Laser light is monochromatic, which means
that it consists of only a single color. Laser light is also called coherent
light which means it is only a single wavelength.

d.      
Schottky Diode

 

Fig. 5.   
Schottky Diode

This is also known as hot-carrier diodes.
These are high-current diodes used primarily in high-frequency and
fast-switching applications. The term hot-carrier is derived from the higher
energy level of electrons in the n region compared to those in the metal
region.

e.       
Zener Diode

Fig. 6.   
Zener Diode

It is a silicon p-n junction device that
is designed for operation in the reverse-breakdown region. The breakdown
voltage of a Zener diode is set by carefully controlling the doping level
during manufacture.

f.       
Photodiode

Fig. 7.   
Photodiode

It is a is a device that operates in
reverse bias and has a small transparent window that allows light to strike the
p-n junction.

g.      
Varicap/Varactor Diode

Fig. 8.   
Varactor

Varactor, short for variable
capacitance diode and also known as varicap, variable reactance diode, or
tuning diode. It is a diode that always operates in reverse bias and is doped
to maximize the inherent capacitance of the depletion region.

h.      
Rectifier Diode

Fig. 9.   
Rectifier Diode

      In power supplies, this is used to rectify
alternating power inputs. This can rectify current levels that range from an
amp upwards.

Diodes are used widely in the electronics industry, right from
electronics design to production, to repair. Besides the above-mentioned types
of diodes, the other diodes are PIN diode (It consists of heavily doped p and n
regions separated by an intrinsic region. The pin diode acts like a nearly
constant capacitance when reverse-biased and acts like a current-controlled
variable resistance when forward-biased.), point contact diode, signal diode,
step recovery diode (It uses graded doping where the doping level of the
semi-conductive materials are reduced as the p-n junction is approached. This
diode is used in very high frequency (VHF) and fast-switching applications.),
tunnel diode (exhibits a special characteristic known as the negative
resistance), IR LEDs, quantum dot display (These dots are semiconductor
nanocrystals which can produce pure monochromatic red, green, and blue light
and are made from semiconductor material such as silicon, germanium, cadmium
sulfide, cadmium selenide, and indium phosphide.), OLED (It is short for
Organic Light Emitting Diode. It is a flat light emitting technology, made by
placing a series of organic thin films between two conductors. When electrical
current is applied, a bright light is emitted.) transient voltage suppression
diodes, constant current diode, current regulator diode (It is often referred
to as a constant-current diode. Unlike the Zener diode, this diode maintains a
constant current.)  Shockley diode (It is
also known as “pnpn” diode. It is a four-layer semiconductor diode which is
equivalent to a thyristor with a disconnected gate), super barrier diode,
vacuum diode, peltier diode, and gold doped diodes. The type of diode to
transfer electric current depends on the type and amount of transmission, as
well as on specific applications.

 

2. DISCUSS, ILLUSTRATE AND DERIVE THE RELATED
EQUATIONS OF VARIOUS RECTIFIER CIRCUITS.

a.      
Half-Wave Rectifier

Fig. 10.
Half-Wave Rectification

      A half-wave rectifier is made
by connecting a diode to an AC source and to a load resistor, RL.
When the sinusoidal input voltage (Vin) is positive, the diode
becomes forward-biased and conducts current through the load resistor. Then,
the current produces an output voltage across the load RL, which has
the same shape as the positive half-cycle of the input voltage. Fig. 11 shows
this operation.

Fig. 11.
Positive Alternation of Half-Wave Rectification

On the other hand, when the input
voltage goes negative during the second half of its cycle, the diode becomes
reverse-biased. There is no current, so the voltage across the load resistor is
0 V, as shown in Fig. 12.

Fig. 12.
Negative Alternation of Half-Wave Rectification

The net result is that only the
positive half-cycles of the ac input voltage shows across the load. It is a
pulsating dc voltage since the output does not change polarity with a frequency
of 60 Hz, illustrated in Fig. 13.

Fig. 13.
Half-Wave Output Voltage

The value you would measure on a dc
voltmeter is the average value of the half-wave rectified output voltage.
Mathematically, it is determined by solving for the area under the curve over a
full cycle and then dividing by the number of radians in a full cycle (by 2?). Fig.
14 shows this.

Fig. 14.
Average Value of the Half-Wave Rectified Signal

The result of this is expressed in (1),
in which Vp is the peak value of the voltage. This equation shows
that VAVG is approximately 31.8% of Vp for a half-wave
rectified voltage.

           
                       (1)

In the previous presentation, the
diode was considered ideal so the output voltage Vout is equal to
the input voltage Vin. However, when the practical diode model is
used with the barrier potential of 0.7 V taken into account, this results in a
half-wave output with a peak value that is 0.7 V less than the peak value of
the input, as shown in Fig. 15. The reason for this is that during the positive
half-cycle, the input voltage must overcome the barrier potential before the
diode becomes forward-biased.

Fig. 15.
Effect of Barrier Potential on the Half-Wave Rectified Output
Voltage

Hence, the expression for the peak
output voltage is:

                        (2)

The 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. For the diode in Fig. 16,
the maximum value of reverse voltage, designated as PIV, occurs at the peak of
each negative alternation of the input voltage when the diode is
reverse-biased. A diode should be rated at least 20% higher than the PIV.

Fig. 16.
Peak Inverse Voltage

Hence, the PIV for half-wave
rectification is:

                                   (3)

A transformer is often used to couple
the ac input voltage from the source to the rectifier, as shown in Fig. 7.
Transformer coupling provides two advantages. First, it allows the source
voltage to be stepped down as needed. Second, the ac source is electrically
isolated from the rectifier, thus preventing a shock hazard in the secondary
circuit.

Fig. 17.
Half-Wave Rectifier with Transformer-Coupled Input Voltage

The amount that the voltage is stepped
down is determined by the turns ratio of the transformer. According to IEEE,
turns ratio is the number of turns in the secondary (Nsec) divided
by the number of turns in the primary (Npri). A transformer with a
turns ratio less than 1 is a step-down type and one with a turns ratio greater
than 1 is a step-up type. It is common practice to show the numerical ratio
directly above the windings to show the turn ratio.

The secondary voltage of a transformer
equals the turns ratio, n, times the primary voltage:

                                   (4)

If n > 1,
the secondary voltage is greater than the primary voltage. If n < 1, the secondary voltage is less than the primary voltage. If n = 1, then Vsec = Vpri. The peak secondary voltage, Vp(sec), in a transformer-coupled half-wave rectifier is the same as Vp(in) (2). Therefore, (2) written in terms of Vp(sec) is:                           (5) In addition, (3) in terms of Vp(sec) is:                                 (6) b.       Full-Wave Rectifier Fig. 18. Full-Wave Rectification The difference between half-wave and full-wave is that a full-wave rectifier allows unidirectional (one-way) current through the load during the entire 360° of the input cycle, whereas a half-wave rectifier allows current through the load only during one-half of the 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, as shown in Fig. 8. The number of positive alternations that make up the full-wave rectified voltage is twice that of the half-wave voltage for the same time interval. 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, as shown in (7). In short, VAVG is approximately 63.7% of Vp for a full-wave rectified voltage.                                      (7) There are two types of Full-Wave Rectifier: Center-tapped and Bridge. Fig. 19. Center-Tapped Full-Wave Rectifier A center-tapped rectifier is a type of full-wave rectifier that uses two diodes connected to the secondary of a center-tapped transformer, as shown in Fig. 19. The input voltage is coupled through the transformer to the center-tapped secondary. Half of the total secondary voltage appears between the center tap and each end of the secondary winding as shown. Fig. 20. Center-Tapped Positive Half-Cycle For a positive half-cycle of the input voltage, the polarities of the secondary voltages are as shown in Fig. 20. This condition forward-biases diode D1 and reverse-biases diode D2. The current path is through D1 and the load resistor RL, as indicated. Fig. 21. Center-Tapped Negative Half-Cycle For a negative half-cycle of the input voltage, the voltage polarities on the secondary are as shown in Fig. 21. This condition reverse-biases D1 and forward-biases D2. The current path is through D2 and RL, as indicated. Because the output current during both the positive and negative portions of the input cycle is in the same direction through the load, the output voltage developed across the load resistor is a full-wave rectified dc voltage, as shown. Fig. 22. Center-Tapped Negative Half-Cycle 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, as illustrated in Fig. 22. Half of the primary voltage appears across each half of the secondary winding (Vp(sec) = Vp(pri)). We will begin referring to the forward voltage due to the barrier potential as the diode drop. Fig. 23. Center-Tapped Full-Wave Rectifier with a Transformer Turns Ratio of 2 In order to obtain an output voltage with a peak equal to the input peak (less the diode drop), a step-up transformer with a turns ratio of n = 2 must be used, as shown in Fig. 23. In this case, the total secondary voltage (Vsec) is twice the primary voltage (2Vpri), so the voltage across each half of the secondary is equal to Vpri. 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. (8) shows this.                           (8) Fig. 24. Diode Reverse Voltage Each diode in the full-wave rectifier is alternately forward-biased and then reverse-biased. The maximum reverse voltage that each diode must withstand is the peak secondary voltage Vp(sec). This is shown in Fig. 24 where D2 is assumed to be reverse-biased (red) and D1 is assumed to be forward-biased (green) to illustrate the concept. When the total secondary voltage Vsec has the polarity shown, the maximum anode voltage of D1 is +Vp(sec)/2 and the maximum anode voltage of D2 is -Vp(sec)/2. Since D1 is assumed to be forward-biased, its cathode is at the same voltage as its anode minus the diode drop; this is also the voltage on the cathode of D2. The peak inverse voltage across D2 is: Since Vp(out) = Vp(sec)/2 - 0.7 V, then by multiplying each term by 2 and transposing,                       (9) Therefore, by substitution, the peak inverse voltage across either diode in a full-wave center-tapped rectifier is:                        (10) Fig. 25. During the positive half-cycle of the input, D1 and D2 are forward biased. Meanwhile, D3 and D4 are reverse-biased. The bridge rectifier uses four diodes connected as shown in Fig. 25. When the input cycle is positive, diodes D1 and D2 are forward-biased and conduct current in the direction shown. A voltage is developed across RL that looks like the positive half of the input cycle. During this time, diodes D3 and D4 are reverse-biased. Fig. 26. During the negative half-cycle of the input, D3 and D4 are forward biased. Meanwhile, D1 and D2 are reverse-biased. When the input cycle is negative as in Fig. 26, diodes D3 and D4 are forward-biased and conduct current in the same direction through RL as during the positive half-cycle. During the negative half-cycle, D1 and D2 are reverse-biased. A full-wave rectified output voltage appears across RL as a result of this action. Fig. 27. Ideal Diodes A bridge rectifier with a transformer-coupled input is shown in Fig. 27. Assuming that the diodes are ideal, during the positive half-cycle of the total secondary voltage, diodes D1 and D2 are forward-biased. Neglecting the diode drops, the secondary voltage appears across the load resistor. The same is true when D3 and D4 are forward-biased during the negative half-cycle.                               (11) Fig. 28. Practical Diodes As can be seen in Fig. 28, two diodes are always in series with the load resistor during both the positive and negative half-cycles considering that the diodes have a drop. If these diode drops are taken into account, the output voltage is:                              (12) Fig. 29. Peak Inverse Voltage for Ideal Diode Model Assuming that D1 and D2 are forward-biased, the reverse voltage across D3 and D4 is examined. Visualizing D1 and D2 as shorts (ideal model), as in Fig. 29, you can see that D3 and D4 have a peak inverse voltage equal to the peak secondary voltage. Since the output voltage is ideally equal to the secondary voltage,                                 (13) If the diode drops of the forward-biased diodes are included as shown in Fig. 30, the peak inverse voltage across each reverse-biased diode in terms of Vp(out) is shown in (14). Fig. 30. Peak Inverse Voltage for Practical Diode Model                     (14)   3. DISCUSS THE BLOCK DIAGRAM OF A POWER SUPPLY. YOU MAY ILLUSTRATE IT AND THE CORRESPONDING WAVEFORMS.       The dc power supply converts the standard 120 V, 60 Hz ac voltage available at wall outlets into a constant dc voltage. The voltage produced is used to power all types of electronic circuits including consumer electronics (televisions, DVDs, etc.), computers, industrial controllers, and most laboratory instrumentation systems and equipment. Figure 21 shows the block diagram for a basic DC power supply.   Fig. 31. Block Diagram of Power Supply First, the ac input line voltage is stepped down to a lower ac voltage with a transformer. A transformer changes ac voltage based on the turns ratio between the primary and secondary. If the secondary has more turns than the primary, the output voltage across the secondary will be higher and the current will be smaller. If the secondary has fewer turns than the primary, the output voltage across the secondary will be lower and the current will be higher. Second, the rectifier converts the ac input voltage to a pulsating dc voltage, called a half-wave rectified voltage. The rectifier can be either a half-wave rectifier or a full-wave rectifier. Next, the filter eliminates the fluctuations in the rectified voltage and produces a relatively smooth dc voltage. Then, the regulator maintains a constant dc voltage for variations in the input line voltage or in the load. Regulators vary from a single semiconductor device to more complex integrated circuits. Finally, the load is a circuit or device connected to the output of the power supply and operates from the power supply voltage and current.   4. STATE OTHER APPLICATIONS OF A DIODE. a. Rectifying a voltage, such as turning AC into DC voltages b. Isolating signals from a supply c. Voltage reference d. Controlling the size of a signal e. Mixing signals f. Detection signals g. Lighting h. Laser applications i. Power conversion j. Signal demodulation k. Over-voltage protection / Voltage spike suppression l. Current steering m. Clipping circuits n. Clamping circuits o. Logic gates p. Voltage multiplier circuits q. Reverse current protection r. Solar panels