power devices

8/3/2019 Power Devices

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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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