Basic DC Power Supply-Diode Applications

Electronics 1

1. Basic DC Power Supply

2. Basic DC Power Supply
•The DC power supply converts the standard
120 V, 60 Hz ac voltage available at wall
outlets into a constant dc voltage

3. Basic DC Power Supply
•A transformer changes ac voltages based on the turns
ratio between the primary and secondary
•The rectifier converts the ac input voltage to a
pulsating dc voltage
•The filter eliminates the fluctuations in the rectified
voltage and produces a relatively smooth dc voltage.
•The regulator is a circuit that maintains a constant dc
voltage for variations in the input line voltage or in the
load
•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. Half-Wave Rectifier

5. Half-Wave Rectifier

6. Peak Inverse Voltage
•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.
•Occurs at the peak of each half-cycle of the input voltage when the diode is
reverse-biased. In this circuit, the PIV occurs at the peak of each negative
half-cycle.

7. Transformer Coupling
•Turns Ratio is “the number of turns in the secondary (Nsec) divided by the
number of turns in the primary (Npri).”

8. Full Wave Rectifier
A full-wave rectifier allows unidirectional (one-way) current through the load
during the entire 360
o
of the input cycle, whereas a half-wave rectifier allows
current through the load only during one-half of the cycle.

9. 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

10. Center-Tapped Full Wave Rectifier

11. Full Wave Bridge Rectifier

12. Clippers
• Clipping circuit: A wave shaping circuit which controls the shape of the
output waveform by removing or clipping a portion of the applied wave.
• Half wave rectifier is the simplest example. (It clips negative half cycle).
• Also referred as voltage limiters/ amplitude selectors/ slicers.
• Applications:
- In radio receivers for communication circuits.
- In radars, digital computers and other electronic systems.
- Generation for different waveforms such as trapezoidal, or square
waves.
- Helps in processing the picture signals in television transmitters.
- In television receivers for separating the synchronizing signals from
composite picture signals

13. Types of clippers
• According to non- linear devices used:
- Diode clippers and Transistor clippers
•According to biasing
- Biased clippers and Unbiased clippers.
• According to level of clipping
- Positive clippers, Negative clippers and combination clippers

14. THUMB RULE
Action of biasing on diode
• When diode is forward biased, it acts as a
closed switch ( ON state).
• When diode is reverse biased, it acts as a
open switch ( OFF state).

15. Diode Clippers
•
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The diode in a series
clipper “clips” any
voltage that does not
forward bias it:
•A reverse-biasing
polarity
•A forward-biasing
polarity less than 0.7 V
(for a silicon diode)

16. Biased Clippers
16
Adding a DC source in
series with the
clipping diode
changes the effective
forward bias of the
diode.

17. Parallel Clippers
17
The diode in a
parallel clipper
circuit “clips” any
voltage that forward
bias it.

DC biasing can be
added in series with
the diode to change
the clipping level.

18. Summary of Clipper Circuits
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more…

19. Summary of Clipper Circuits
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20. Drawbacks
• Series Diode Clipper
When diode is “OFF”, there should be no transmission of
input signal to output. But in case of high frequency, signal
transmission occurs through diode capacitance which is
undesirable.
• Shunt Diode clippers
When diode is “OFF”, transmission of input signal to output
should take place. But in case of high frequency input signals,
diode capacitance affects the circuit operation and signal gets
attenuated.

21. Clampers
21
A diode and capacitor can be
combined to “clamp” an AC
signal to a specific DC level.

22. Note:

•Start the analysis of clamping network, by considering that part of
the input signal that will forward bias the diode.
•During the period that the diode is in the “ON” state, assume that
capacitor will charge up instantaneously to a voltage level
determined by the network.
• Assume that during the period when the diode is in “OFF” state,
capacitor will hold on its established voltage level.
•Keep in mind the general rule, that
Total swing of total output = Swing of input signal

23. Biased Clamper Circuits
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The input signal can be any type of
waveform such as sine, square,
and triangle waves.

The DC source lets you adjust the
DC clamping level.

24. For t1-t2 cycle

25. Summary of Clamper Circuits
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26. Practical Applications
•Rectifier Circuits
– Conversions of AC to DC for DC operated circuits
– Battery Charging Circuits

•Simple Diode Circuits
– Protective Circuits against
– Overcurrent
– Polarity Reversal
– Currents caused by an inductive kick in a relay circuit

•Zener Circuits
– Overvoltage Protection
– Setting Reference Voltages


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27. Transformer Coupled Example

28. Transformer Coupled Example

30. Transformer Coupled Example

32. Filter
•The filter is simply a capacitor connected from
the rectifier output to ground.
•R
L represents the equivalent resistance of a
load.
•The half-wave rectifier will be used to
illustrate the basic principle and then expand
the concept to full-wave rectification.

35. Ripple Voltage
•As you have seen, the capacitor quickly
charges at the beginning of a cycle and slowly
discharges through RL after the positive peak of
the input voltage (when the diode is reverse-
biased).
•The variation in the capacitor voltage due to
the charging and discharging is called the ripple
voltage

36. Ripple Factor
•Generally, ripple is undesirable; thus, the
smaller the ripple, the better the filtering
action

37. Ripple Factor
•The ripple factor (r) is an indication of the effectiveness of
the filter and is defined as






•The lower the ripple factor, the better the filter.
•The ripple factor can be lowered by increasing the value of
the filter capacitor or increasing the load resistance.


where Vr(pp) is the peak-to-peak ripple voltage and VDC is the dc (average) value of
the filter’s output voltage

38. Example

40. Voltage Regulators
•While filters can reduce the ripple from power
supplies to a low value, the most effective
approach is a combination of a capacitor-input
filter used with a voltage regulator.
•A voltage regulator is connected to the output
of a filtered rectifier and maintains a constant
output voltage (or current) despite changes in
the input, the load current, or the temperature

41. Voltage Regulators
•Most regulators are integrated circuits and have three
terminals—an input terminal, an output terminal, and a
reference (or adjust) terminal
•Three-terminal regulators designed for fixed output voltages
require only external capacitors to complete the regulation
portion of the power supply





•Filtering is accomplished by a large-value capacitor between the
input voltage and ground.
•An output capacitor (typically ) is connected from the output to
ground to improve the transient response.

42. Basic Power Supply

43. Percent Regulation

44. Example

45. Zener Diode
•A zener diode is a silicon pn
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

46. Varactor Diode
•A varactor is a diode that always operates in
reverse bias and is doped to maximize the
inherent capacitance of the depletion region
•The depletion region acts as a capacitor
dielectric because of its nonconductive
characteristic. The p and n regions are
conductive and act as the
capacitor plates

47. Light Emitting Diode
•The basic operation of the light-
emitting diode (LED) is as follows.
When the device is forward-
biased, electrons cross the pn
junction from the n-type material
and recombine with holes in the
p-type material.

48. LEDs
•When recombination takes place, the recombining
electrons release energy in the form of photons.
The emitted light tends to be monochromatic (one
color) that depends on the band gap (and other
factors). A large exposed surface area on one layer
of the semiconductive material permits the
photons to be emitted as visible light. This
process, called electroluminescence,

49. LEDs Semiconductive Materials
•The semiconductor gallium arsenide (GaAs)
was used in early LEDs and emits IR radiation,
which is invisible. The first visible red LEDs
were produced using gallium arsenide
phosphide (GaAsP) on a GaAs substrate. The
efficiency was increased using a gallium
phosphide (GaP) substrate, resulting in
brighter red LEDs and also allowing orange
LEDs.

50. LEDs Semiconductive Materials
•Later, GaP was used as the light-emitter to
achieve pale green light. By using a red and a
green chip, LEDs were able to produce yellow
light. The first super-bright red, yellow, and green
LEDs were produced using gallium aluminum
arsenide phosphide (GaAlAsP).
•By the early 1990s ultrabright LEDs using indium
gallium aluminum phosphide (InGaAlP) were
available in red, orange, yellow, and green.

51. LEDs Semiconductive Materials
•Blue LEDs using silicon carbide (SiC) and
ultrabright blue LEDs made of gallium nitride
(GaN) became available.
•High intensity LEDs that produce green and
blue are also made using indium gallium nitride
(InGaN). High-intensity white LEDs are formed
using ultrabright blue GaN coated with
fluorescent phosphors that absorb the blue
light and reemit it as white light.

52. Zener Diode
•V i and R Fixed
Determine the state of the Zener diode by removing it from
the network and calculating the voltage across the resulting
open circuit.

53. Example

54. Zener Diode
•Fixed Vi and Variable RL

55. Zener Diode
•Fixed RL, Variable Vi