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CONTROLABLE SWITCHING DEVIES DESIGNED BY DR. SAMEER KHADER PPU “E-learning Project”

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CONTROLABLE SWITCHING DEVIES

DESIGNED

BY

DR. SAMEER KHADER

PPU

“E-learning Project”

Introduction, Classification &Applications,

Thryristor Circuits

Triac Circuits

Diac Circuits

Practical Firing ( Triggering) Circuits

Thyristor Commutation (turning-off)

Thyristor Circuits

1- Construction : Four PNPN layers with special doping

in each layer, with purpose to obtain different electron and

holes in these layers. Each one has different potential voltage

Th.

N

N

P

P

A

K

K

A

G

Principle of operation :

The thyristor construction

Presents three diodes

In series ( two forward

biased and the third reverse biased).

The thyristor will conduct only if D2 forward biased, therefore current will flow from A to K. This case could be achieved by different ways as follow :

G

A

K

D1

D3

D2

G

(1F)

Methods for Switching- on the thyristor

The switching process of the thyristor is called “ Firing”, because after Switching process is ceased, WHERE the firing signal may can removed with purpose to reduce the gate loss .There\'re several methodS Applied to realize this purpose :

1-Gate-firing method :by supplying the gate terminal with positive voltage ( this is the most applied method - major method).

2-by suddenly increasing the Anode voltage

3-by increasing the thyristor temperature over predetermined limit.

4- Photo effect method, which used in photo devices ( Photo thyristor)

Thyristor

I-V curve

Gate-firing method: the firing circuit is shown below:

(Expl.)

(Parameters)

(Performance)

(Conclusion)

There’re several parameters related to static & dynamic performance of the thyristor,

these parameters are as follow :

1-VAK- thyristor voltage at steady state 2 V;

2-VBO- -break over voltage , voltage after which thyristor will turning on at constant

gate current ;

3-VBR- break down voltage in reverse biasing state;

4-IH- thyristor holding current :this a minimized load current keeping the thyristor in

conducting state ( if the current goes down the thyristor will switch-off);

5- IL- thyristor latching current :this a minimized load current keeping the thyristor in

conducting state after removing the gate signal ;

6-VGT- minimum gate voltage required to firing the thyristor at given loadind condition

, VGT 0.8…12V;

7-IGT- minimum gate current., IGmax- maximum gate current ;

8-di/dt- speed of (increasing/decreasing) of thyristor current ;

9-dv/dt - speed of (increasing/decreasing) of thyristor voltage .

Thyristor Dynamic Performances

V-source

V-source

V-gate

V-gate

P-load

P-load

V-thyris

(Math Modeling)

(Gate Circuits)

2-Phase Control Gate Firing Circuits:1- RC relaxation oscillator

R-load

R-load

AC -circuit

DC -circuit

R1

R1

Th1

Th1

Th2

Th2

C

R2

C

R2

V-source

V-source

V-gate

V-gate

V-thyris

V-thyris

P-load

P-load

(Math Modeling)

1- Gate firing circuit using RC relaxation oscillator;

2- Gate firing circuits using RC circuit and called Phase control ;

These circuits may can use to fire thyristor in AC or DC circuit: in both sources the connected elements must be with the following relations with purpose to realized successful operation: R2<<R1; and R-load << R1;

* DC source VBOTh2 < Vs ; and IH2 < Vs/R1;

** AC source VBOTh2 < Vm; and IH2 < Vm/R1; Vs(t)=Vm.sin (t);

The thyristor Th2 will conduct when Vc=VBOTh2;

This could be occurred at t=tp ; this time called (firing instant)

The firing angle of previous firning circuits in AC circuit can

Determine as follow :

9<<90 ( without C)

- In DC source, tp- presents delay time , so by increasing Ig the thyristor allow more current to follow ; therefore increasing the load power ;
- In AC source, tp- presents delay angle which corresponds to =tp.360/T, so by increasing Ig, decreases, thus load power increases P()=Pmax . Cos(), where Pmax-maximum allowable power.
- may can change from 0 to 90 ( without C) or to 145 (with C) ;
- The thyristor gate voltage must be > + 0.85 V at least;
- VBR > Vm ; ILmin > ILat firing( remains conduct);and ILmin < IH ( swith off) .
- By increasing di/dt at given Ig the thyristor capable to carry additional current ILoad .
- By increasing Ig, VBO( ac circuits), which means that the thyristor
- is fired at earliest time , therefore increasing the load voltage and power .
- The gate pulse must removed after successfully firing the thyristor , with aim to reduce the gate losses .

Triac Circuits

1- Triac( Triode Alternating Current Switch ) – presents two parallel connected thyristors with common gate, which energized with positive and negative voltage. The main purpose of the Triac is to control the RMS load voltage, therefore there\'re several applications such as : * Lighting control ( dimmer circuits); **- Temperature control ;

*** Torque –speed control of induction machines.

2- Symbol:

3- I-V Curve:

3- Circuit application:

1- Phase angle control without diode

2- Phase angle control with diode

Load

Triac

voltage

Triac

voltage

300.0 V

200.0 V

200.0 V

A: r2_2

100.0 V

100.0 V

0.000 V

-100.0 V

0.000 V

-200.0 V

-100.0 V

Load

current

Load

current

-300.0 V

0.000ms

15.00ms

30.00ms

45.00ms

-200.0 V

3.000 A

35.00ms

50.00ms

65.00ms

80.00ms

2.000 A

1.000 A

2.500 A

A: r2[i]

0.000 A

1.500 A

-1.000 A

0.500 A

-2.000 A

-0.500 A

-3.000 A

0.000ms

15.00ms

30.00ms

45.00ms

-1.500 A

-2.500 A

Gate

voltage

Gate

voltage

1.500 V

35.00ms

50.00ms

65.00ms

80.00ms

1.000 V

2.500 V

0.500 V

1.500 V

0.000 V

A: d1_k

-0.500 V

-1.000 V

0.500 V

-1.500 V

0.000ms

15.00ms

30.00ms

45.00ms

-0.500 V

(Math Modelation)

-1.500 V

35.00ms

50.00ms

65.00ms

80.00ms

3-Triac firing circuits using UJT

Source voltage

125.0 V

A: v3_1

75.00 V

25.00 V

-25.00 V

-75.00 V

-125.0 V

0.000ms

10.00ms

20.00ms

30.00ms

Pulse

generator

25.00 V

A: tr_3

15.00 V

5.000 V

-5.000 V

-15.00 V

-25.00 V

0.000ms

10.00ms

20.00ms

30.00ms

B1

Load

voltage

125.0 V

A: tr_2

75.00 V

25.00 V

B2

-25.00 V

-75.00 V

-125.0 V

Capacitor

voltage

0.000ms

10.00ms

20.00ms

30.00ms

UJT

needles

5.000 V

A: tr_3

3.000 V

25.00 V

A: c1_2

1.000 V

15.00 V

-1.000 V

5.000 V

-3.000 V

-5.000 V

-5.000 V

-15.00 V

0.000ms

10.00ms

20.00ms

30.00ms

-25.00 V

5.000ms

15.00ms

25.00ms

35.00ms

Load

voltage

250.0 V

A: tr_2

150.0 V

50.00 V

(Math Modeling)

-50.00 V

-150.0 V

-250.0 V

0.000ms

10.00ms

20.00ms

30.00ms

Mathematical Modeling of Triac Circuits

Three main circuits are introduced with purpose to fire the Triac device( Phase control with or without diode, with UJT and with Diac device). The presence of diode in the gate circuit remove one half cycle , therefore convert the Triac into Thyristor . In both circuits there are several relations characterized the application of such a device . These relations are as follow :

1-when 0<</2

0<Vrms<Vs;

2- Vdc=0 for

symmetrical firing

3- Vdc0 for

asymmetrical firing

4- the existing of

inductance , reduced

The control rang of

Prms=F().

UJT – circuit:

,VBB-base to base UJT’s voltage:

, ujt- UJT’s intrinsic factor <=1

,Vp- UJT’s peak voltage;

, tp-delay time ( firing instant) .

Diac Circuits

1- Diac( Diode Alternating Current Switch ) – presents two anti-parallel connected diodes with special construction , aiming to maintain relatively high threshold voltage across its terminals . The main purpose of the Diac is to divide the source voltage between its terminals and the load terminals , therefore there\'re several applications such as :

* Firing device in Triac –gate circuit ; **- Over voltage protective device ;

2- Symbol:

4- I-V Curve:

3- Circuit modification:

The main equations are as follow , and can derives when Vdiac =Vc at given angle.

1- Practical circuit using UJT:

Source voltage

65.00 V

A: d1_3

45.00 V

25.00 V

5.000 V

-15.00 V

-35.00 V

0.000ms

15.00ms

30.00ms

45.00ms

Zener

voltage

65.00 V

A: r4_3

45.00 V

25.00 V

5.000 V

-15.00 V

-35.00 V

0.000ms

15.00ms

30.00ms

45.00ms

Capacitor voltage

40.00 V

A: r4_1

30.00 V

20.00 V

Gate needles

10.00 V

0.000 V

-10.00 V

1.250 V

0.000ms

15.00ms

30.00ms

45.00ms

A: scr2_2

0.750 V

Gate needles

0.250 V

3.500 V

A: scr2_2

-0.250 V

2.500 V

-0.750 V

1.500 V

0.500 V

-1.250 V

0.000ms

15.00ms

30.00ms

45.00ms

-0.500 V

Thyristor

voltage

-1.500 V

0.000ms

15.00ms

30.00ms

45.00ms

Thyristor

voltage

60.00 V

A: scr2_1

40.00 V

61.00 V

A: scr2_1

20.00 V

41.00 V

0.000 V

21.00 V

-20.00 V

1.000 V

-40.00 V

-19.00 V

0.000ms

15.00ms

30.00ms

45.00ms

-39.00 V

Load

power

0.000ms

15.00ms

30.00ms

45.00ms

Load

power

60.00 W

A: r5[p]

71.00 W

40.00 W

A: r5[p]

51.00 W

20.00 W

31.00 W

0.000 W

11.00 W

-20.00 W

-9.000 W

-40.00 W

2- High=R4C1

1- Low=R4C1

-29.00 W

0.000ms

15.00ms

30.00ms

45.00ms

0.000ms

15.00ms

30.00ms

45.00ms

2- Practical circuits using UJT and

Isolation Transformer:

Capacitor voltage

66.50 V

A: c2_2

16.50 V

-33.50 V

UJT

Signal at B2

0.000ms

15.00ms

30.00ms

45.00ms

26.50 V

A: q2_2

B1

6.500 V

B2

15.00ms

-13.50 V

0.000ms

30.00ms

45.00ms

Gate needles

2.000 V

A: scr1_2

Capacitor voltage

7.500 V

A: c2_2

1.000 V

2.500 V

-2.500 V

0.000 V

5.000ms

20.00ms

35.00ms

50.00ms

Gate needles

0.000ms

15.00ms

30.00ms

45.00ms

Thyristor

voltage

1.000 V

A: scr1_2

50.00 V

A: scr1_1

0.500 V

0.000 V

Thyristor

voltage

0.000 V

5.000ms

20.00ms

35.00ms

50.00ms

-50.00 V

50.50 V

A: scr1_1

0.000ms

15.00ms

30.00ms

45.00ms

Load

power

0.500 V

100.0 W

A: r10[p]

Load

power

-49.50 V

5.000ms

20.00ms

35.00ms

50.00ms

0.000 W

100.5 W

A: r10[p]

0.500 W

-100.0 W

5.000ms

20.00ms

35.00ms

50.00ms

-99.50 W

5.000ms

20.00ms

35.00ms

50.00ms

3: ON-OFF firing circuit :This circuit illustrates firing techniques used in AC Voltage controller based on so called ON-OFF method, where it’s necessary to fire the thyristor at the beginning of both half-cycles .

Source voltage

250.1 V

A: r6_2

150.1 V

50.10 V

-49.90 V

-149.9 V

-249.9 V

0.000ms

30.00ms

60.00ms

90.00ms

250.1 V

Load

Vg-th1

A: r8_2

150.1 V

50.10 V

-49.90 V

-149.9 V

-249.9 V

0.000ms

30.00ms

60.00ms

90.00ms

250.1 V

Vg-th2

A: r5_1

150.1 V

50.10 V

-49.90 V

-149.9 V

-249.9 V

0.000ms

30.00ms

60.00ms

90.00ms

P-load

V-triac

15.00 V

A: r6_1

5.000 V

150.0 W

A: r6[p]

-5.000 V

100.0 W

-15.00 V

50.00 W

-25.00 V

0.000 W

-35.00 V

-50.00 W

0.000ms

15.00ms

30.00ms

45.00ms

-100.0 W

1.250 A

0.000ms

15.00ms

30.00ms

45.00ms

Ic1

I-load

A: r6[i]

0.750 A

12.49 W

A: c1[p]

0.250 A

7.490 W

2.490 W

-0.250 A

-2.510 W

-0.750 A

-7.510 W

-1.250 A

(Zero-circuit)

-12.51 W

0.000ms

15.00ms

30.00ms

45.00ms

0.000ms

15.00ms

30.00ms

45.00ms

Thyristor Commutation

1. Objectives:

1. to study the concept of thyristor commutation

2. to illustrate some of commutation techniques

3. to study how to express the required mathematical model

4. To determine the turning-off time, and how could be affected

5. Describing some examples

2. The Concept of Commutation Process:

- This is a process of removing the circuit current by forcing it to flow in another loop with purpose to be ceased “eliminated”.

- Depending on the source voltage, there are two types of commutation strategies:

- Natural commutation : applied in AC circuits

- Forced commutation : Applied in DC circuits.

Becauseof the load current varies sinusoidally, the thyristor should be turned –off when the load current falls below the holding value: ILoad<IH . Furthermore, in the negative half cycle, the applied source voltage being negative with respect to anode-cathode terminals, causing reverse biasing of the device.

Principle electrical circuit is shown below:

In this case, because of no alternating character of the current “ DC “, therefore it must force decreases by applying the following approaches:

- the load current must reduced below the holding value: ILoad<IH

-by applying negative voltage across the thyristor, causing forced removing of internal charge, therefore the load current falls below the holding value IH .

Several techniques realized these approaches:

- Self Commutation
- Complementary Commutation
- Resonant Commutation
- Impulse Commutation
- Load-side commutation
- Line-side commutation

The thyristor is self turning-off due to resonant behavior of the current flows in RLC circuit as well shown on the figure below, where it is clearly shown that when the current becomes negative the thyristor turned-off.

Mathematical modeling:

In this case, second thyristor which called " Auxiliary" operates in complementary sequence ( turning-on first thyristor caused turning-off second device) .

The figure shown below illustrates the principle circuit, where it is clearly shown that each thyritor operates for predetermine time with complementary sequence. The connected capacitor play the role of applying negative voltage across T1 and T2.

Mathematical modeling:

T1=ON

Let Vs=200V; R=5Ω; =10µF

Therefore: toff=34.4µS

Hereinafter the circuit waveforms for both T1, T2, Vg1, Vg2, I1,I2, and VR1.

In this case, second thyristor T2 which called " Auxiliary" used to connect the capacitor across T1 with inverse voltage, therefore reducing the thyristor current below IH.

The figure shown below illustrates the principle circuit, where the circuit waveforms illustrates these behaviors.

Mathematical modeling:

T1=ON, after then T2=ON

Let Vs=200V; R=5Ω; =10µF

Therefore: toff=34.6µS

Hereinafter the circuit waveforms for both T1, T2, Vg1, Vg2, I1, and Vload.

In this case, second thyristor T2 used to connect the capacitor across T1 with inverse voltage, therefore reducing the thyristor current below IH, while third thyristor T3 is used to recharging the capacitor with polarity appropriate to turning-off T1.

The figure shown below illustrates the principle circuit, where the circuit waveforms illustrates these behaviors.

Waveforms:

Hereinafter the circuit waveforms for two cases: 1- C is recharged through resistance R2; 2- C is recharged throug inductance L2

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