thyristors cont

8/18/2019 Thyristors Cont.

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THYRISTORS

cont.)


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Outline:

I. Unijunction TransistorII. Programmable Unijunction

Transistor

III. Insulated Gate Bipolar TranIV. Gate Turn off Switches


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UNIJUNCTION TRANSISTO

a three-terminal device with a p – n

junction formed between an aluminum rodand an n- type silicon slab. Once the emitter

firing potential is reached, the emitter voltage

will drop with an increase in emitter current,

establishing a negative-resistance region

excellent for oscillator applications. Once the

valley point is reached, the characteristics of

the device take on those of a semiconductordiode . The higher the applied voltage across

the device, the higher is the emitter firing

potential

originally called a duo (double) base diode

due to the presence of two base contacts


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the emitter leg is drawn at anangle to the vertical line

representing the slab of n-

type material

The arrowhead is pointing in

the direction of conventionalcurrent (hole) flow when the

device is in the forward-biased,

active, or conducting state


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APPLICATION: SCR TRIGGERING


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Determine the value of this

circuit that will ensure proper

turn-on and turn-off of the UJT.

The characteristic of the UJT

exhibits the following values: η= 0.5, VV = 1V, IV = 10mA, IP =

20μA, and VP = 14V.


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PROGRAMMABLE UNIJUNCTIO

TRANSISTOR

a four-layer pnpn device with a gate connecteddirectly to the sandwiched

the term programmable is applied because RBB, η,

and VP as defined for the UJT can be controlled

through the resistors RB1, RB2, and the supply voltage

VBB


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PROGRAMMABLE UNIJUNCTIO

TRANSISTOR


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PROGRAMMABLE UNIJUNC

TRANSISTOR APPLICATION: RELAXATION OSCILLATOR


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PROGRAMMABLE UNIJUNC

TRANSISTOR APPLICATION: RELAXATION OSCILLATOR


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INSULATED GATE BIPOLA

TRANSISTOR

a hybrid device of power MOSFET and BJT high input impedance and high switching speeds of a MOSFE

low saturation voltage of a bipolar transistor, and comb

together to produce another type of transistor switching dev

capable of handling large collector-emitter currents with vir

gate current drive


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TRANSISTOR Power JT whose basecurrent is provided by

FET

IGBT combines the lowconduction loss of a BJTwith the high switchingspeed of a powerMOSFET an optimal

solid state switch existswhich is ideal for use inpower electronicsapplications An insulated gate bipolar transistor is simply turned “ON” or “O

and deactivating its Gate terminal. Applying a positive input vol

the Gate and the Emitter will keep the device in its “ON” state,

input gate signal zero or slightly negative will cause it to turn “O

same way as a bipolar transistor or eMOSFET. Another advantag


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TRANSISTOR

requires a small voltage on the Gate to maintain conduction through the devi

unidirectional device, meaning it can only switch current in the “forward direc

from Collector to Emitter

resistance offered by the main conducting channel when current flows throug

its “ON” state is very much smaller in the IGBT

the current ratings are much higher when compared with an equivalent powe


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TRANSISTOR

main advantages of using the Insulated Gate ipolar Transitypes of transistor devices are its high voltage capability, low

ease of drive, relatively fast switching speeds and combined

drive current makes it a good choice for moderate spee

applications such as in pulse-width modulated (PWM), variable

switch-mode power supplies or solar powered DC-AC inverter

converter applications operating in the hundreds of kilohertz ra


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TRANSISTOR

Device

Characteristic

Power

ipolar

Power

MOSFET

Voltage Rating High


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GATE TURN-OFF SWITCH

most obvious advantage of the GTO over the SCR or SCS is thecan be turned on or off by applying the proper pulse to the cat(without the anode gate and associated circuitry required for th


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A consequence of this turn-off capability is an increase in the m

the required gate current for triggering. For an SCR and GTO ofmaximum rms current ratings, the gate-triggering current of a pSCR is 30 mA, whereas the triggering current of the GTO is 2turn-off current of a GTO is slightly larger than the required trigcurrent. The maximum rms current and dissipation ratings of GTmanufactured today are limited to about 3 A and 20 W

second very important characteristic of the GTO is improved swcharacteristics. The turn-on time is similar to that of the SCR (tybut the turn-off time of about the same duration (1 ms) is mucthan the typical turn-off time of an SCR (5 ms to 30 ms). The facturn-off time is similar to the turn-on time rather than consider

permits the use of this device in high-speed applications


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thyristors cont

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