3rd thyristor

7/30/2019 3rd thyristor

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3rd slide 1

Thyristors


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3rd slide 2

A thyristor is a four-layersemiconductor device of pnpn-structure with three pn-junction.

When the anode voltage is madepositive with respect to thecathode, the junctions J1 and J3are forward biased.

The junction J2 is reverse biasedand only a small leakage currentflows from anode to cathode.

The thyristor is then said to be inthe forward blocking or off-statecondition and the leakagecurrent is known as off-statecurrent, ID.


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3rd slide 3

If the anode to cathode voltage, VAK, isincreased to a sufficiently large value, the

reverse biased junction J2 will break. Thisis knows as avalanche breakdownandthe corresponding voltage is calledforward b reakdown vol tage, V

BO.

Since the other junctions J1 and J3 arealready forward biased, there will be free

movement of carriers across al threejunction, resulting in a large forward anodecurrent. The device will the be in a

conduction state or on state.


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3rd slide 4

In the on-state , the anode current is limited by an

external impedance or a resistance, RL. The

anode current must be more than a value knowsas latch ing cu rrentIL, in order to maintain the

required amount of carrier flow across the

junction, otherwise the device will revert to the

blocking condition as the anode to cathodevoltage is reduced.

Latching current, IL

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.


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3rd slide 5


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3rd slide 7

The holding current is in the order of mili-

amperes and is less than the latching

current IL. That is IL > IH.

Holding current IH is minimum current to

maintain the thyristor in the on-state. Theholding current is less than the latching

current.


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3rd slide 8

A thyristor can be turned on by increasing theforward voltage VAK beyond VBO, but such aturn-on could be destructive.

In practice, the forward voltage is maintainedbelow VBO and the thyristor is turned on byapplying a positive voltage between its gate and

cathode.

Once a thyristor is turned on by a gating signal andits anode current is greater than the holding

current, the device continues to conduct due topositive feedback, even if the gating signal isremoved. A thyristor is a latching device


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3rd slide 9

Thyristor Turn-On

A thyristor is turned on by increasing the anode

current. This can be accomplished in one of

the following ways.

- Themals

- Lights

- High voltage

- dv/dt- Gate current


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3rd slide 10

Thermal

Temperature then number of electron-hole

pairs this would increase the leakagecurrent.

LightsIf light is allowed to strike the junction of

thyristor, the electron-hole pairs will

increase and thyristor may be turned on.The light-activated thyristors are turned onby allowing the light to strike the siliconwafers.


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3rd slide 11

High-voltage

If the forward anode to cathode voltage VAK is increasedbeyond the forward breakdown voltage VBO , highenough leakage currents will flow, causing regenerativeturn-on. This type of turn-on is destructive and should beavoided.

dv/dtIf the rate or rise of the anode to cathode voltage is high,

(for example, when there is a voltage spike), thecharging current of the capacitive junctions may be highenough to turn on the thyristor. A high value of charging

current may cause damage to the thyristor and must beavoided. Hence, thyristors must be protected againsthigh dv/dt and must be operated within themanufacturer's dv/dt specifications.


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3rd slide 12

Gate Current

The injection of gate current into a forward

biased thyristor would turn-on the device.As the gate current is increased, the

forward voltage required to turn-on the

device decreases.


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3rd slide 13


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3rd slide 14


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3rd slide 15

Turn-on Time, tonThe turn-on time ton is defined as the time interval

between 10% of steady-state gate current and90% of steady-state thyristor on-state current.

Delay Time, tdThe delay time t

dis defined as the time interval

between 10% of gate current and 10% ofthyristor on-state current.

Rise Time, trThe rise time, tr,,, is defined as the time required

for the anode current to rise from 10% of the on-state current to 90% of the on-state current.


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3rd slide 16

Gate Control Circuit DesignConsideration must be given to the following points when

designing gate control circuits.

- The gate signal should be removed after the thyristor hasbeen turned on. A continuous gate signal will increasethe power loss in the gate junction.

- No gate signal should be applied when the thyristor isreversed biased. If a gate signal is applied under theseconditions, the thyristor may fail due to an increasedleakage current.

- The width of the gate pulse must be greater than the timerequired for the anode current to rise to the holdingcurrent. In practice, the gate pulse width is made wider

than the turn-on time of the thyristor.


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3rd slide 17

di/dt Protection

A minimum time is required for the thyristor tospread the current conduction uniformlythroughout the junctions. If this time is notallotted and the rate of rise of anode current isvery high compared to the spreading velocity atturn-on, then this could lead to localised "hot-spot" heating and the device may fail as a resultof excessive heating.

Protection against di/dt is necessary and anexample is shown in figure below. The circuitanalysis is as follows:


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3rd slide 18

For an inductive load, when thyristor T1

is turned

off, free-wheeling diode Dm conducts load

current. If thyristor T1 is fired when diode Dm is

still conducting, di/dt can be very high. In order

to reduce the high di/dt a series inductor Ls isadded to the circuit as shown. The forward di/dt

is given as


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3rd slide 19

dv/dt Protection

As seen earlier, a high dv/dt may cause

damage to a thyristor. In order to protect athyristor from high dv/dt, the circuits shown

below could be used.


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3rd slide 20

If the switch S1 in figure (a) isclosed at time t = 0, a stepvoltage will be applied

across thyristor T1 anddv/dt may be high enoughto turn on the thyristor.

dv/dt can be limited byconnecting capacitor CSacross the thyristor asshown in figure (b). Since

then


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3rd slide 21

and the rate of rise of voltage is limited by thevalue of the capacitor used. In order tolimit the capacitor discharge current whenthe thyristor is turned on, a resistor RS isinserted in series with the capacitor asshown in figure (c).

This resistor capacitor arrangement is knownas a snubber circuit. For figure (c), whenswitch S1 is closed at time t = 0, thevoltage across the capacitor is given by

and this charging capacitor voltage is seen bythe thyristor anode to cathode terminals asVAK. This is depicted by the waveform offigure (d). The rate of rise of voltageacross the thyristor can be represented by

where 0.632VS is one time constant.


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3rd slide 22

The value of the snubber time constantRSCS can be found for a known dv/dt.

And for a known discharge current ITD ,the value of resistor RS can be foundusing

It is sometimes necessary to use oneresistor for dv/dt and another for

limiting the discharge current of thesnubber capacitor. This arrangement isshown in figure (e). In this circuit, R1and CS are used for dv/dt protection,while R1 + R2 is used for limiting thecapacitor discharge current.

The load can also be placed in series withthe snubber components as shown infigure (f).

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3rd thyristor