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**CHAPTER SEVEN (New textbook)**

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

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CONTENT Introduction, Classification &Applications, Thryristor Circuits Triac Circuits Diac Circuits Practical Firing ( Triggering) Circuits Thyristor Commutation (turning-off)

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**Chapter 7-A 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 A P P K A K 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)

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**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)

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**Thyristor Main Parameters:**

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 .

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**Thyristor Dynamic Performances**

V-source V-source V-gate V-gate P-load P-load V-thyris (Math Modeling) (Gate Circuits)

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**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)

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**Mathematical . 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)

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I-V curve

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Conclusion 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 > V at least; VBR > Vm ; ILmin > IL at 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 . yyy

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**Chapter 7-B Triac Circuits 2- Symbol: 3- I-V Curve:**

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:

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**Triac Firing Circuits 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 0.000 V V V V Load current Load current V 0.000ms 15.00ms 30.00ms 45.00ms V 3.000 A 35.00ms 50.00ms 65.00ms 80.00ms 2.000 A A: r2[i] 2.500 A 1.000 A 0.000 A 1.500 A A 0.500 A A A A A 0.000ms 15.00ms 30.00ms 45.00ms A Gate voltage Gate voltage 35.00ms 50.00ms 65.00ms 80.00ms 1.500 V 1.000 V 0.500 V 2.500 V 1.500 V A: d1_k 0.000 V V 0.500 V V V V 0.000ms 15.00ms 30.00ms 45.00ms (Math Modelation) V 35.00ms 50.00ms 65.00ms 80.00ms

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**3-Triac firing circuits using UJT**

Source voltage 125.0 V A: v3_1 75.00 V 25.00 V V V V 0.000ms 10.00ms 20.00ms 30.00ms Pulse generator 25.00 V A: tr_3 15.00 V 5.000 V V V 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 V V V Capacitor voltage 0.000ms 10.00ms 20.00ms 30.00ms UJT needles 5.000 V A: tr_3 25.00 V 3.000 V A: c1_2 15.00 V 1.000 V 5.000 V V V V V V V 0.000ms 10.00ms 20.00ms 30.00ms 5.000ms 15.00ms 25.00ms 35.00ms Load voltage 250.0 V A: tr_2 150.0 V 50.00 V (Math Modeling) V V V 0.000ms 10.00ms 20.00ms 30.00ms

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**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) .

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**Chapter 7C 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:

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**5- Time-varying performances:**

Phase control circuit with Diac & Triac: (Math Modeling) (Add. circuits)

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

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**Additional Firing circuits**

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**1- Practical circuit using UJT:**

Source voltage A: d1_3 65.00 V 45.00 V 25.00 V 5.000 V V V 0.000ms 15.00ms 30.00ms 45.00ms Zener voltage A: r4_3 65.00 V 45.00 V 25.00 V 5.000 V V V 0.000ms 15.00ms 30.00ms 45.00ms Capacitor voltage A: r4_1 40.00 V 30.00 V 20.00 V Gate needles 10.00 V 0.000 V V A: scr2_2 1.250 V 0.000ms 15.00ms 30.00ms 45.00ms 0.750 V Gate needles 0.250 V A: scr2_2 3.500 V V 2.500 V V 1.500 V V 0.500 V 0.000ms 15.00ms 30.00ms 45.00ms V Thyristor voltage V Thyristor voltage 0.000ms 15.00ms 30.00ms 45.00ms 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 V 1.000 V V V 0.000ms 15.00ms 30.00ms 45.00ms Load power V 0.000ms 15.00ms 30.00ms 45.00ms Load power 60.00 W A: r5[p] 40.00 W A: r5[p] 71.00 W 20.00 W 51.00 W 0.000 W 31.00 W W 11.00 W W W 0.000ms 2- High =R4C1 15.00ms 30.00ms 45.00ms 1- Low =R4C1 W 0.000ms 15.00ms 30.00ms 45.00ms

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**2- Practical circuits using UJT and Isolation Transformer:**

Capacitor voltage 66.50 V A: c2_2 16.50 V V UJT Signal at B2 0.000ms 15.00ms 30.00ms 45.00ms 26.50 V B1 A: q2_2 6.500 V B2 V 15.00ms 0.000ms 30.00ms 45.00ms Gate needles 2.000 V Capacitor voltage A: scr1_2 A: c2_2 7.500 V 2.500 V 1.000 V V 5.000ms 20.00ms 35.00ms 50.00ms Gate needles 0.000 V 0.000ms 15.00ms 30.00ms 45.00ms 1.000 V Thyristor voltage A: scr1_2 50.00 V A: scr1_1 0.500 V Thyristor voltage 0.000 V 0.000 V 5.000ms 20.00ms 35.00ms 50.00ms V A: scr1_1 50.50 V 0.000ms 15.00ms 30.00ms 45.00ms Load power 0.500 V 100.0 W A: r10[p] Load power V 5.000ms 20.00ms 35.00ms 50.00ms 0.000 W A: r10[p] 100.5 W 0.500 W W 5.000ms 20.00ms 35.00ms 50.00ms W 5.000ms 20.00ms 35.00ms 50.00ms

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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 A: r6_2 250.1 V 150.1 V 50.10 V V V V 0.000ms 30.00ms 60.00ms 90.00ms Load Vg-th1 250.1 V A: r8_2 150.1 V 50.10 V V V V 0.000ms 30.00ms 60.00ms 90.00ms Vg-th2 250.1 V A: r5_1 150.1 V 50.10 V V V 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] V 100.0 W V 50.00 W V 0.000 W V W 0.000ms 15.00ms 30.00ms 45.00ms W 0.000ms 15.00ms 30.00ms 45.00ms Ic1 I-load 1.250 A A: r6[i] 12.49 W 0.750 A A: c1[p] 7.490 W 0.250 A 2.490 W A W A W A W (Zero-circuit) 0.000ms 15.00ms 30.00ms 45.00ms 0.000ms 15.00ms 30.00ms 45.00ms

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**Zero-Voltage switching**

S=Off S=ON S V-source Vg-th1 Vth1 Load power

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**Thyristor Commutation**

Chapter 7D 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.

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2.1 Natural Commutation: Because of 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:

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

2.1 Forced Commutation: 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

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*- Self 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:

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***- Complementary Commutation: **

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

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

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***- Impulse Commutation: **

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

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

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***- Resonant Commutation:**

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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