ThyristorsThyristorsPart IIIPart III

Prepared by

Dr. Mohammad Abdul Mannan

Assistant Professor, Department of EEE

American International University - BangladeshAmerican International University - Bangladesh

Commutation CircuitCommutation Circuit of Thyristorsof Thyristors

Thyristor Turn-OFFA Thyristor which is in the ON-state can be turn OFF by reducing theforward current to a level below the holding current.The junctions J1 and J3 are forward when thyristor turn ON. If forwardcurrent is reduced to zero, J1 and J3 continue to conduct due to minority

i hi h i t d i th j ti d th b lk i d tcarriers which remain stored in the pn-junction and the bulk semiconductormaterial.The minority carries require a certain time to recombine with oppositecharges and to be neutralized This time is called reverse recovery time t ofcharges and to be neutralized. This time is called reverse recovery time, trr, ofthe diode.The junction J2 is reverse biased whenthyristor turn ON. If forward current isyreduced to zero, J2 will require a timeknown as recombination time trc torecombine the excess carriers.The turn off time tq (=trr+trc) is the minimumvalue of time interval between the instantwhen on-state current has decreased to zeroand the instant when the thyristor iscapable of withstanding forward voltagewithout turning on.

Thyristor CommutationCommutation is the process of turning-off a thyristor, and it normallycauses transfer of current flow to other parts of circuit.A commutation circuit normally uses additional components to accomplishthe turn-off.Th t ti t h i LC ( d d d RLCThe commutation techniques use LC resonance (or an underdamped RLCcircuit) to force the current and/or voltage of the thyristor to zero, therebyturning off a power device.

Classification of Commutation1. Natural or line commutation, and2. Forced commutation

Natural commutation techniques takes place in: AC voltage controller,

Natural or Line Commutationq p g ,

Phase controlled rectifiers, and Cycloconverters.

If the source or input voltage is ac, the thyristor current goes through anatural zero and a reverse voltage appears across the thyristor The devicenatural zero, and a reverse voltage appears across the thyristor. The deviceis then automatically turned off due to the natural behavior of the sourcevoltage. This is known as natural commutation or line commutation.

Natural or Line CommutationThe thyristor is triggered synchronously with the zero crossing of theThe thyristor is triggered synchronously with the zero crossing of thepositive input voltage in every cycle in order to provide a continuous controlof power.

The following figures show the circuit arrangements for naturalThe following figures show the circuit arrangements for naturalcommutation and the voltage and current waveforms with a delay angle.

The delay angle α is defined as the angle between the zero-crossing of theinput voltage and the instant the thyristor is fired.

Forced CommutationIf the input voltage is dc and the forward current of the thyristor ise pu vo ge s dc d e o w d cu e o e y s o sforced to zero by an additional circuitry called commutation circuit toturn off the thyristor. This technique is called forced commutation.

Forced commutation techniques takes place in: DC voltage controller, DC -DC converters (choppers), and DC – AC converters (inverters).

Classification of Forced Commutation1. Self-commutation,1. Self commutation,2. Impulse commutation,3. Resonant pulse commutation,4. Complementary commutation,p y ,5. External pulse commutation,6. Load-side commutation, and7. Line-side commutation

Self CommutationSelf Commutation with Initially Capacitor UnchargedA thyristor is turned off due to the natural characteristic of the circuit.Let us consider the circuit in Fig. 7.2(a) with the assumption that thecapacitor is initially uncharged.When thyristor T1 is fired, the current starts to rise and capacitor starts tocharge up.When capacitor charge voltage equal to source voltage, the current starts tof ll d it h t t f ll t Fi ll itfall and capacitor charges up to current falls to zero. Finally, capacitorcharge voltage becomes double of source voltage and current falls to zero,and thyristor T1 goes to turn off.

The duration of timebetween T1 turn-on andturn off is calledcommutation time ofcircuit.This method of turningoff a thyristor is calledself-commutation andthyristor T1 is said to beself-commuted.

Mathematical Analysis

Wh th i t i it h d th it h i t i(t) iWhen thyristor is switched on, the capacitor charging current i(t) isgiven by

∫ =++=+= )0(1 tCvidtCdtdiLLvCvSV

Considering all initial voltages and currents are zero, then the solutionof i(t) is given as follows:

tmLC

SVti ωsin)( = LCm /1=ω

The capacitor voltage vc(t) is given by )cos1()( tmsVtCv ω−=

After time t=t0=π√(LC) , i(t 0) = 0 and vc(t 0) = 2VS and thyristor T1 is switched off.t0 is called commutation time.

Self Commutation with Initially Capacitor Charged

Let us consider the circuit in Fig. 7.2(a) with the assumptionthat the capacitor is initially charged -V0.

When thyristor T1 is fired, the current starts to rise andy 1 ,capacitor starts to discharge.When capacitor voltage becomes zero, the current starts to falland capacitor charges up to current falls to zero Finallyand capacitor charges up to current falls to zero. Finally,capacitor charge voltage becomes initial charged voltage andcurrent falls to zero, and thyristor T1 goes to turn off.

Mathematical Analysis

Wh th i t i it h d th it h i t i(t) iWhen thyristor is switched on, the capacitor charging current i(t) isgiven by

∫ −+=+= 010 VidtCdt

diLLvCv

Considering initial current is zero, then the solution of i(t) is given asfollows:

tmLCVti ωsin0)( = LCm /1=ω

The capacitor voltage vc(t) is given by tmVtCv ωcos0)( −=

After time t=tr=π√(LC) , i(t 0) = 0 and vc(t 0) = V0 and thyristor T1 is switched off.tr is called reverse time.

Impulse CommutationAn impulse-commutated circuit is shown in Fig. 7.6.It is assumed that the capacitor is initially charged to a voltageof –V0 with the polarity shown.

Let us assume that thyristor T1 is initially conducting andcarrying a load current of Im.When the auxiliary thyristor T is fired thyristor T is reversedWhen the auxiliary thyristor T2 is fired, thyristor T1 is reversedbiased by the capacitor voltage, and T1 is turned-off and thecapacitor would carry the load current as shown in Fig. 7.7(a).

The capacitor will discharge from –V0 to zero and then charge to the dcinput voltage VS [(as shown in Fig. 7.7(b)] when the capacitor current falls tozero and thyristor T2 turns off.zero and thyristor T2 turns off.The charge reversal of the capacitor from V0 ( = VS ) to –V0 is then done byfiring thyristor T3 is self-commutated similar to the circuit in Fig. 7.3.The time required for the capacitor to discharge from -V0 to zero is called the0circuit turn-off time toff and must be greater than the turn off time of thethyristor, tq.toff is also called the available turn-off time.The discharging time will depend on the load current and assuming aconstant load current of Im, toff is given by

offtmIdIV fft

∫1 CV

t 0CoffdtmICV offt =∫= 0

10 mIofft 0=

Since a reverse voltage of V0 is applied across T1 immediately after firing ofthyristor T2, this is known as voltage commutation.Due to the use of auxiliary thyristor T2, this type of commutation is also calledDue to the use of auxiliary thyristor T2, this type of commutation is also calledauxiliary commutation.Thyristor T1 is sometimes known as the main thyristor because it carries theload current.

It is seen from the previou equation that the toff is inversely proportional to theload current; and at a very light load (or low load current) the turn-off time willbe large. On the other hand, at high load current the turn-off time will be small.In an ideal commutation circuit, the turn off time should be independent of loadcurrent in order to guarantee the commutation of thyristor T1.

The discharge of the capacitor can beaccelerated by connecting a diode Daccelerated by connecting a diode D1and an inductor L1 across the mainthyristor as shown in Fig. 7.8.