STRUCTURE OF A PN JUNCTION
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A pn junction diode consists of a piece of p-type
semiconductor bonded to a piece of n-type
Leads are attached to both ends, usually by soldering
Lead attached to p-type end is the anode
Lead attached to n-type end is the cathode
In order for correct operation, both n-type and p-type
materials must be made of the same crystal (e.g.
silicon)
Schematic
p
n
Anode
Cathode
Circuit symbol
forward current flow
9. P-N Junction Diode
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GENERAL CHARACTERISTICS
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There are two important characteristics of a pn junction
diode
Forward bias A large current will flow if voltage is
applied in such a direction as to make the p-type
material more positive than the n-type
Reverse bias A minute current flows if the voltage is
reversed in polarity
9. P-N Junction Diode
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VOLTAGE – CURRENT
CHARACTERISTICS
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When forward biased, current iD flows in the direction
from anode (p) to cathode (n) as the voltage across
diode (vD) reaches 0.6V
When reversed biased, there is no current flow through
the diode
When a certain reverse biased voltage is reached,
breakdown occurs, producing sharp rise in current.
May lead to overheating and damage diode.
9. P-N Junction Diode
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DIODES AS RECTIFIERS
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Junction diodes are used in rectifier circuits, such as
those in mobile phone chargers, to convert AC into DC
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Above is circuit of half wave rectifier
As input increases positively from zero, diode becomes
forward biased. Current will flow whenever the
instantaneous value of input is greater than about 0.6V
When input voltage goes negative, diode becomes
reverse biased, and diode behaves like an open circuit
when input voltage drops below 0.6V.
Output voltage therefore consists of positive half cycles
of a sinusoid
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9. P-N Junction Diode
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PASSIVE SMOOTHING CIRCUITS
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For many applications, especially in DC power supplies
such as mobile phone rechargers, the output voltage
waveform is not smooth enough for acceptable
operation
Extra components must be added to smoothen the
output voltage
Passive components do not rely on power supplied
from a separate power supply in order to operate
Passive smoothing networks consist of combinations of
capacitors, inductors and resistors
Inductors are rarely used today due to their large size
and expense
The most commonly used passive technique is the
addition of a capacitor connected in parallel with the
load
9. P-N Junction Diode
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CAPACITOR INPUT FILTER
AC
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Here a rectifier feeds into initially uncharged capacitor
During negative half cycle of supply voltage, rectifier is
reverse biased, no current flows in circuit
During a positive half cycle, rectifier becomes forward
biased, allowing current to flow
Rectifier thus acts as a short circuit and the voltage
across the capacitor follows the supply voltage until it
reaches its peak
When the supply voltage starts to fall from its positive
peak, the capacitor voltage can’t follow it, as it would
have to discharge through the diode – which it can’t as
no current can flow in the reverse direction – i.e. back
through the diode
The capacitor will thus remained charged up to the
peak of the supply voltage
Circuit known as a peak detector
9. P-N Junction Diode
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CAPACITOR INPUT FILTER AND
RESISTIVE LOAD
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Works similarly to input filter on previous slide
However, here as the input voltage falls, the capacitor
can now discharge through load resistor RL, which
allows the output voltage to fall
Rectifier will only conduct when its anode is positive
with respect to its cathode which only occurs for a small
part of the cycle
9. P-N Junction Diode
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