3 Chapter 7-A Thyristor Circuits 1- Construction : Four PNPN layers with special dopingin each layer, with purpose to obtain different electron andholes in these layers. Each one has different potential voltageTh.NNAPPKAKGPrinciple of operation :The thyristor constructionPresents three diodesIn series ( two forwardbiased 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 :GAKD1D3D2G(1F)
4 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 voltage3-by increasing the thyristor temperature over predetermined limit.4- Photo effect method, which used in photo devices ( Photo thyristor)ThyristorI-V curveGate-firing method: the firing circuit is shown below:(Expl.)(Parameters)(Performance)(Conclusion)
5 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 constantgate current ;3-VBR- break down voltage in reverse biasing state;4-IH- thyristor holding current :this a minimized load current keeping the thyristor inconducting state ( if the current goes down the thyristor will switch-off);5- IL- thyristor latching current :this a minimized load current keeping the thyristor inconducting 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 .
8 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 canDetermine as follow :9<<90 ( without C)
10 ConclusionIn 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 thyristoris 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
11 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:
14 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<</20<Vrms<Vs;2- Vdc=0 forsymmetrical firing3- Vdc0 forasymmetrical firing4- the existing ofinductance , reducedThe control rang ofPrms=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) .
15 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:
16 5- Time-varying performances: Phase control circuit with Diac & Triac:(Math Modeling)(Add. circuits)
17 The firing angleThe main equations are as follow , and can derives when Vdiac =Vc at given angle.
21 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 voltageA: r6_2250.1 V150.1 V50.10 VVVV0.000ms30.00ms60.00ms90.00msLoadVg-th1250.1 VA: r8_2150.1 V50.10 VVVV0.000ms30.00ms60.00ms90.00msVg-th2250.1 VA: r5_1150.1 V50.10 VVVV0.000ms30.00ms60.00ms90.00msP-loadV-triac15.00 VA: r6_15.000 V150.0 WA: r6[p]V100.0 WV50.00 WV0.000 WVW0.000ms15.00ms30.00ms45.00msW0.000ms15.00ms30.00ms45.00msIc1I-load1.250 AA: r6[i]12.49 W0.750 AA: c1[p]7.490 W0.250 A2.490 WAWAWAW(Zero-circuit)0.000ms15.00ms30.00ms45.00ms0.000ms15.00ms30.00ms45.00ms
22 Zero-Voltage switching S=OffS=ONSV-sourceVg-th1Vth1Load power
23 Thyristor Commutation Chapter 7DThyristor Commutation1. Objectives:1. to study the concept of thyristor commutation2. to illustrate some of commutation techniques3. to study how to express the required mathematical model4. To determine the turning-off time, and how could be affected5. Describing some examples2. 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.
24 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:
25 - 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 CommutationComplementary CommutationResonant CommutationImpulse CommutationLoad-side commutationLine-side commutation
26 *- 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:
27 *- 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=ONLet Vs=200V; R=5Ω; =10µFTherefore: toff=34.4µS
28 Waveforms:Hereinafter the circuit waveforms for both T1, T2, Vg1, Vg2, I1,I2, and VR1.
29 *- 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=ONLet Vs=200V; R=5Ω; =10µFTherefore: toff=34.6µS
30 Waveforms:Hereinafter the circuit waveforms for both T1, T2, Vg1, Vg2, I1, and Vload.
31 *- 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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