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POWER DEVICE
GROUP MEMBERS:
MOHD NASRULLAH
MOHD HAFZAN
NURUL AZRA
RAMIZAH RAIHAN
ELECTRONIC0124
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1- Introduction of Power Electronics Devices.
2- Introduction, symbol, constructions,
characteristic, basic operation and application of :
1) Thyristor
2) Shockley diode
3) SCR(Silicon Control Rectifier)
4) DIAC5) TRIAC
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Power Electronics is power conversion and control from
one form of power (energy) source to a desired from by
using electronic means.
Power electronics is power processing circuits and control.
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The thyristor family of semiconductors
consists of several very useful devices.
The most widely used of this family aresilicon controlled rectifiers (SCRs), triacs,
and diacs.
In the presence of forward current it will not
turn off even after the gate voltage has been
removed.The thyristor will only turn off when the
forward current drops to zero.
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Latching Current ILThis is the minimum anode current required to maintain the thyristor in the on-state
immediately after a thyristor has been turned on and the gate signal has been
removed.
If a gate current greater than the threshold gate current is applied until the anode
current is greater than the latching current IL then the thyristor will be turned on ortriggered.
Holding Current IHThis is the minimum anode current required to maintain the thyristor in the on-state.
To turn off a thyristor, the forward anode current must be reduced below its holding
current for a sufficient time for mobile charge carriers to vacate the junction.
If the anode current is not maintained below IH for long enough, the thyristor will not
have returned to the fully blocking state by the time the anode-to-cathode voltage risesagain.
It might then return to the conducting state without an externally-applied gate current.
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Reverse Current IRWhen the cathode voltage is positive with respect to the anode, the
junction J2
is forward biased but junctions J1 and J3 are reverse
biased.
The thyristor is said to be in the reverse blocking state and a reverseleakage current known as reverse current IR will flow through the
device.
Forward Breakover Voltage VBOIf the forward voltage V
AKis increased beyond VBO , the thyristor can
be turned on.
But such a turn-on could be destructive.In practice the forward voltage is maintained below VBO and the
thyristor is turned on by applying a positive gate signal between gate
and cathode.
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Thyristors are mainly used where high
currents and voltages are involved.
Often used to control alternating currents,
where the change of polarity of the current
causes the device to automatically switch off
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Shockley diode is a metal-semiconductor
(MS) diode.
Contains four-layer PNPN semiconductordevices.
These behave as a pair of interconnected
PNP and NPN transistors.
Tend to stay on once turned on (latched),
and stay off once turned off.
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CONSTRUCTION & SYMBOL 2-TRANSISTOR VERSION
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Ametal-semiconductor junction is formed
between a metal and a semiconductor, creating
a Shottky barrier.Typical metals used are platinum, chromium or
tungsten.
The metal side acts as the anode and N-type
semiconductor acts as the cathode.
This Shottky barrier results in both very fastswitching and low forward voltage drop.
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Shottky diodes differ from PN-junction devices.
Rectification occurs because of differ in work function
between the metal contact and the semiconductor.
Conduction is controlled by thermionic emission of
majority carriers over the barrier created by the unequal
work functions.
Switching speed is not limited by minority carrier
effects.Many metals can create a Shottky barrier on either
silicon or platinum, titanium and gold.
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TURN ON:
To latch a Shockley diode exceed the anode-to-
cathode break-overvoltage
Exceed the anode-to-cathode critical rate of voltage
rise.
TURN OFF:
Reduce the current going through it to a level below
its low-current dropoutthreshold.
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Non-conducting state (off-state):
It operates on lower line with negligible current and a
voltage less than switching voltage or break-over voltage.When the voltage tries to ex-ceed the break-over voltage,
the device breaks down and switches along the dotted line
to the conducting or on-state.
The dotted line indicates an unstable or a temporary
condition.
The device can have current and voltage values on thisdotted line only briefly as it switches between the two stable
operating states.
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In conducting state (on-state):
The device operates on the upper line.
As long as the current through the device isgreater than the holding current IH, then the voltage
across it is slightly greater than knee voltage, VK.
When the current falls below the level of the
holding current IH, the device switches back along
the dotted line to the non-conducting or off-state.
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One common application of the Shockley
diode is as a trigger switch for an SCR.
The circuit is shown in figure.
When the circuit is energized, the capacitor
will start getting charged and eventually, the
voltage across the capacitor will be sufficiently
high to first turn-on Shockley diode and then the
SCR.
Another application of this diode is as a
relaxation oscillator.
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controlled rectifier constructed of a silicon
semiconductor material with a third terminal for
control purposes.
It is widely used as a switching device in power
control applications.
It can control loads by switching on and off up to
many thousand times a second.
It can switch on for a variable lengths of timeduration.
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Anode - taken from the outer p-type material
Cathode- taken from the outer n-typematerial
Gate - taken from the inner p-type material
SCR is a three terminal device.
Anode (A)Cathode (K)
Gate (G)
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TURN ON:
Apply positive current to the gate
Apply a voltage greater than forward break-overvoltage
TURN OFF:
Anode current interuption
Force commutation
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Both transistor are ON when the pulse
applied
The anode must be positive than thecathode
SCR will stay ON once it is triggered
Very low resistance between the anode and
cathode can be approximated by closed
switch
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Relay Controls
Time Delay Circuits
Regulated Power
Suppliers
Motor Controls
Choppers
Inverters
CycloconvertersProtective Circuits
Static Switches
Phase Controls
Battery Chargers
Heater Controls
Emergency Lighting
System
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Two-electrode bidirectional avalanche diode which can
be switched from off-state to the on-state for either
polarity of the applied voltage
P-N-P-N structured four-layer, two-terminal
semiconductor device
The switching from off-state to on-state is achieved by
simply exceeding the avalanche break down voltage in
either direction.
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MT1 and MT2 are the two main terminals of the device.
There is no control terminal in this de vice.
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The diac acts like an open-circuit until its switching or
break-over voltage is exceeded.
At that point the diac conducts until its current reduces
toward zero (below the level of the holding current of thedevice).
Because of its peculiar construction, diac does not
switch sharply into a low voltage condition at a low current
once it goes into conduction, the diac maintains an
almost continuous negative resistance characteristic.
Voltage decreases with the increase in current.
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Lamp dimmer circuit
Heat triggered circuit
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Triac is an abbreviation for three terminal ac switch.
1) Tri-indicates that the device has three terminals
2
) ac indicates that the device controls alternatingcurrent or can conduct in either direction.
Four layer bidirectional semiconductor device
Triac have three terminal
Two SCRs connected in parallel in oppositedirections
Diac with gate
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The gate terminal (G) makes ohmic contacts
with both the N and P materials.
This permits trigger pulse of either polarity to
start conduction
Terminals are designated as main terminal 1
(MT1), main terminal 2 (MT2) and gate (G).
It has become common practice to specify all
voltages and currents using MT1 as the
reference.
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The break-over potential decrease as
the gate current increase
Triac conduct when anode currentdrops below holding current
Turn off:
Reduce the current to a sufficiently
low level
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AC phase control circuit
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THANK YOU
Q & A
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