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E3065/06/1ELECTRONIC CONTROL DEVICES (PART II)

ELECTRONIC CONTROL DEVICES (PART II)

OBJECTIVES

General objective : To understand the concept of electronic control devices.

Specific objectives : At the end of the unit you should be able to:

Identify the Phase-Control Thyristors Identify the Gate-Turn-Off Thyristors Define the symbol of GTO Identify the Programmable Unijunction Transistor (PUT) Define the symbol of PUT Identify the Uni-junction Transistor (UJT)

UNIT 6


6.1 PHASE-CONTROL THYRISTORS (SCRs)

This type of thyristors generally operates at the line frequency and is turned off by natural commutation. The turn-off time,tq, is of the order of 50 to 100 µs. This is most suited for low-speed switching applications and is also known as converter thyristor. Since a thyristor is basically a silicon-made controlled device, it is also known as silicon-controlled rectifier (SCR).

The on-state voltage , VT, varies typically from about 1.15 V for 600 V to 2.5 V for 4000 V devices; and for a 5500-A 1200-V thyristor it is typically 1.25 V. The modern thyristors use an amplifying gate, where an auxiliary thyristor TA is gated on by a gate signal and then the amplified output of TA is applied as a gate signal to the main thyristor TM. This shown in Fig. 6.1. The amplifying gate permits high dynamic characteristics with typical dv/dt of 1000 V/µs and di/dt of 500 A/µs and simplifies the circuit design by reducing or minimizing di/dt limiting inductor and dv/dt protection circuits.

INPUT


Figure 6.1: Amplifying gate thyristor

6.2 THYRISTOR TURN-OFF

A thyristor which is in the on-state can be turned off by reducing the forward current to a level below the holding current IH. There are various techniques for turning off a thyristor. In all the commutation techniques, the anode current is maintained below the holding current for a sufficiently long time, so that all the excess carriers in the four layers are swept out or recombined.

6.3 GATE-TURN-OFF THYRISTOR

A gate-turn-off thyristor (GTO) like an SCR can be turned on by applying a positive gate signal. However, it can be turned off by a negative gate signal. A GTO is a latching device and can be built with current and voltage ratings similar to those of an SCR. A GTO is turned on by applying a short positive pulse and turned off by a short negative pulse to its gate. The GTOs have advantages over SCRs: (1) elimination of commutating components in forced commutation, resulting in reduction in cost, weight, and volume; (2) reduction in acoustic and electromagnetic noise due to the elimination of commutation chokes; (3) faster turn-off, permitting high switching frequencies; and (4) improved efficiency of converters.

In low-power applications, GTOs have the following advantages over bipolar transistors: (1) a higher blocking voltage capability; (2) a high ratio of peak controllable


current to average current; (3) a high ratio of peak surge current to average current, typically 10:1; (4) a high on-state gain (anode current/gate current), typically 600; and (5) a pulsed gate signal of short duration. Under surge conditions, a GTO goes into deeper saturation due to regenerative action. On the other hand, a bipolar transistor tends to come out of saturation.

A GTO has low gain during turn-off, typically 6, and requires a relatively high negative current pulse to turn off. It has higher on-state voltage than that of SCRs. The on-state voltage of a typical 550-A 1200-V GTO is typically 3.4 V. A 160-A 200-V GTO of type 160PFT is shown in Fig. 6.3.

Figure 6.3: Fast-switching thyristors


Activity 6A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

6.1 What is a SCR?

6.2 What are the advantages of GTOs over SCRs?

Hii !!!!!…..Good Luck and Try your best ….


Feedback To Activity 6A

6.1 Silicon-controlled rectifier (SCR) is basically a silicon-made controlled device.

6.2 The GTOs have advantages over SCRs:

i) elimination of commutating components in forced commutation, resulting in reduction in cost, weight, and volume

ii) reduction in acoustic and electromagnetic noise due to the elimination of commutation chokes

iii) faster turn-off, permitting high switching frequencies; and (4) improved efficiency of converters.


6.4 PROGRAMMABBLE UNIJUNCTION TRANSISTOR (PUT)

The programmable unijunction transistor (PUT) is small thyristor shown in Fig. 6.4(a). A PUT can be used as a relaxation oscillator as shown in Fig. 6.4(b). The gate voltage VG is maintained from the supply by the resistor divider R1 and R2, and determines the peak voltage Vp. In the case of the UJT, Vp is fixed for device by the dc supply voltage. But Vp of a PUT can be varied by varying the resistor divider R1 and R2. If the anode

voltage VA is less than the gate voltage VG, the device will remain in its off-state. If VA

exceeds the gate voltage by one diode forward voltage VD, the peak point is reached and the device turns on. The peak current Ip and the valley point current Iv both depend on the equivalent impedance on the gate RG = R1 R2 /( R1 + R2) and the dc supply voltage Vs. in general , Rk is limited to a value below 100 Ω.

Vp is given by

INPUT


Vp = Vs

which gives the intrinsic ratio as

η = =

Figure 6.4: PUT triggering circuit.

6.5 UNIJUNCTION TRANSISTOR (UJT)

The unijunction transistor (UJT) is commonly used for generating triggering signals for SCRs. A basic UJT-triggering circuit is shown in Fig. 6.5(a). A UJT has three terminals, called the emitter E, base-one B1, and base-two B2. Between B1 and B2 the unijunction has the characteristics of an ordinary resistance. This resistance is the interbase resistance RBB and has


values in the range 4.7 to 9.1 kΩ. The static characteristics of a UJT are shown in Fig. 6.5(b). When the dc supply voltage Vs is applied, the capacitor C is charged through resistor R since the emitter circuit of the UJT is in the open state. The time constant of the charging circuit is T1 = RC. When the emitter voltage VE, which is the same as the capacitor voltage vc , reaches the peak voltage Vp, the UJT turns on and capacitor C will discharge through RB1 at a determined by the time constant T2 = RB1C. T2 is much smaller than T1. When the emitter voltage VE decays to the valley point Vv, the emitter ceases to conduct, the UJT turns off, and the charging cycle is repeated. The waveforms of the emitter and triggering voltages are shown in Fig. 6.5 (c).


(b)

Figure 6.5: UJT triggering circuit


Activity 6B

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

6.3 What is PUT?

6.4 What are the advantages of a PUT?

6.5 What is a UJT?

6.6 What is the peak voltage of a UJT?


Feedback To Activity 6B

6.3 The programmable unijunction transistor (PUT) is small thyristor . A PUT can be used as a

relaxation oscillator.

6.4 The advantages of a PUT are Vp of a PUT can be varied by varying the resistor divider R1

and R2. If the anode voltage VA is less than the gate voltage VG, the device will remain in its off-state. If VA exceeds the gate voltage by one diode forward voltage VD, the peak point is reached and the device turns on.

6.5 The unijunction transistor (UJT) is commonly used for generating triggering signals for SCRs.


6.6 The peak voltage Vp, the UJT turns on and capacitor C will discharge through RB1 at a determined by the time constant T2 = RB1C. T2 is much smaller than T1.

SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section and check your answers with those given in the Feedback on Self-Assessment 5 given on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Question 6-1

a. What are Phase-Control Thyristors (SCRs)?

CONGRATULATIONS!!!!…..May success be with you always….


b. What are Gate-Turn-Off Thyristors?

Question 6-2

a. What are the advantages of PUT over a UJT?

b. What are the THREE terminals of a UJT?

c. What is the intrinsic stand-off ratio of a UJT?

Feedback To Self-Assessment

Answer 6-1:

a. Phase-Control Thyristors (SCRs) is type of thyristors generally operates at the line frequency and is turned off by natural commutation. The turn-off time,tq, is


of the order of 50 to 100 µs. This is most suited for low-speed switching applications and is also known as converter thyristor.

b. A gate-turn-off thyristor (GTO) like an SCR can be turned on by applying a positive gate signal. It can be turned off by a negative gate signal. A GTO is a latching device and can be built with current and voltage ratings similar to those of an SCR. A GTO is turned on by applying a short positive pulse and turned off by a short negative pulse to its gate.

Answer 6-2:

a. The UJT, Vp is fixed for device by the dc supply voltage. But Vp of a PUT can be varied by varying the resistor divider R1 and R2.

b. The THREE terminals for UJT:

i) the emitter Eii) base-one B1

iii) base-two B2

c. The intrinsic stand-off ratio of a UJT:

η = =

CONGRATULATIONS!!!!…..May success be with you always….

azmi bin ayup & zul mt lajin · web viewthe unijunction transistor (ujt) is commonly used...

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