Power Electronics Dr. Imtiaz Hussain Assistant Professor URL : Lecture-6 Thyristor Gate Control Circuits 1

Outline Introduction Voltage Divider Triggering RC Triggering Double RC Triggering

Introduction The popular terms used to describe how SCR is operating are conduction angle and firing delay angle. – Conduction angle is the number of degrees of an ac cycle during which the SCR is turned ON. – Firing delay angle is the number of degrees of an ac cycle that elapses before the SCR is turned ON. Of course, these terms are based on the notion of total cycle time (360 0 )

Introduction An SCR is fired by a short burst of current into the gate (I G ). The amount of gate current needed to a fire particular SCR is symbolized as I GT. Most SCRs require current between 0.1 and 50mA. Since there is a standard pn-junction between gate and cathode, voltage between these two terminals (V GK ) must be slightly greater than 0.7 volt.

Example-1 For the circuit shown in figure below, what voltage is required at point X to fire the SCR? The gate current needed to fire 2N3669 is 20mA under normal conditions. Solution The voltage between point X and cathode must be sufficient to forward bias the junction between X and K (0.7V). And also at least cause 20mA to flow from 150Ω resistor. For 20mA current to flow in XG branch we need Therefore,

Gate Control Circuits Gate Control Circuit Design Consideration must be given to the following points when designing gate control circuits. The gate signal should be removed after the thyristor has been turned on. A continuous gate signal will increase the power loss in the gate junction. No gate signal should be applied when the thyristor is reversed biased. If a gate signal is applied under these conditions, the thyristor may fail due to an increased leakage current. The width of the gate pulse must be greater than the time required for the anode current to rise to the holding current. In practice, the gate pulse width is made wider than the turn-on time of the thyristor. 6

A simple type of gate control circuit (triggering circuit) is shown in following figure. Gate Control Circuits When SW is closed, there will be current into the gate when supply voltage goes positive. Firing delay angle is determined by setting of R 2.

One disadvantage of this simple triggering circuit is that the firing delay angle is adjustable is only from about 0 0 to Gate Control Circuits This can be understood by referring to following figure.

Example-2 For following figure assume that the supply is 115V rms, I GT =15mA, and R 1 =3KΩ. The firing delay is desired to be 20 o. To what value should R 2 be adjusted? Solution At 20 o instantaneous supply voltage is 3KΩ 40Ω Voltage drop across Load

Example-2 Total resistance in the gate lead is given by 3KΩ 40Ω Therefore, R 2 is

Example-3 For following figure assume that the supply is 115V rms, I GT =15mA, and R 1 =3KΩ. The firing delay is desired to be 30 o. To what value should R 2 be adjusted? Solution At 30 o instantaneous supply voltage is 3KΩ 40Ω Voltage drop across Load

Example-3 Total resistance in the gate lead is given by 3KΩ 40Ω Therefore, R 2 is

Example-4 For following figure assume that the supply is 115V rms, I GT =15mA, and R 1 =3KΩ. The firing delay is desired to be 60 o. To what value should R 2 be adjusted? Solution At 30 o instantaneous supply voltage is 3KΩ 40Ω Voltage drop across Load

Example-4 Total resistance in the gate lead is given by 3KΩ 40Ω Therefore, R 2 is

Example-5 For following figure assume that the supply is 115V rms, I GT =15mA, and R 1 =3KΩ. The firing delay is desired to be 90 o. To what value should R 2 be adjusted? Solution At 90 o instantaneous supply voltage is 3KΩ 40Ω Voltage drop across Load

Example-5 Total resistance in the gate lead is given by 3KΩ 40Ω Therefore, R 2 is

Example-6 For following figure assume that the supply is 115V rms, I GT =15mA, and R 1 =3KΩ. The firing delay is desired to be 150 o. To what value should R 2 be adjusted? Solution 3KΩ 40Ω At 150 o instantaneous supply voltage is Voltage drop across Load

Example-6 Total resistance in the gate lead is given by 3KΩ 5Ω5Ω Therefore, R 2 is R 2 is same as it was for firing angle of 30 o. Therefore with this circuit arrangement it is not possible to fire SCR beyond 90 o.

Example-7 For following figure assume that the supply is 115V rms, I GT =15mA, and R 1 =3KΩ. The firing delay is desired to be 10 o. To what value should R 2 be adjusted? Solution At 10 o instantaneous supply voltage is 3KΩ 40Ω Voltage drop across Load

Example-7 Total resistance in the gate lead is given by 3KΩ 40Ω Therefore, R 2 is Cannot have firing angle of 10 o. For extended firing angle R 3 can be made smaller.

Example-8 For following figure assume that the supply is 115V rms, I GT =15mA, and R 1 =3KΩ. The firing delay is desired to be 18 o. To what value should R 2 be adjusted? Solution At 15 o instantaneous supply voltage is 3KΩ 40Ω Voltage drop across Load

Example-8 Total resistance in the gate lead is given by 3KΩ 40Ω Therefore, R 2 is

Conclusion The value of resistor R 2 is increasing as firing angle is further delayed. S. NoFiring AngleR2R2 110 o -1.21KΩ Ω 320 o 600Ω 430 o 2.3KΩ 560 o 9.3KΩ 690 o 7.7KΩ 7150 o KΩ Range of Firing Angles

RC Triggering Circuits The simplest method of improving gate control is to add a capacitor at the bottom of the gate lead resistance as shown in following figure. Advantage of this circuit is that the firing delay angle can be adjusted past 90 o.

RC Triggering Circuits This can be understood by focusing on the voltage across Capacitor C. When the ac supply is –ve, the reverse voltage across SCR is applied to RC triggering circuit, charging the capacitor –ve on top plate and +ve on bottom plate. When the supply enters its positive half cycle, the forward voltage drop across SCR tends to charge C in opposite direction. However, voltage buildup in new direction is delayed until the –ve charge is removed.

RC Triggering Circuits The idea can be extended to achieve even extended firing angles by modifying the circuit slightly. A resistor has been inserted into the gate lead, requiring the capacitor to charge higher than 0.7 V to trigger the SCR. With the resistor in place, capacitor voltage must reach a value large enough to force sufficient current (I GT ) through the resistor.

RC Triggering Circuits The firing delay angle can further be extended by the use of double RC network as shown in following figure. The delayed voltage across C 1 is used to charge C 2 resulting in even further delay in building up the gate voltage.

Triggering 50Hz sine wave takes 1/50 seconds to complete one cycle.

RC Triggering Circuits Capacitors in RC triggering circuits usually fall in the range from 0.01µF to 1µF. For the given capacitor sizes minimum firing delay angle (maximum load current) is set by fixed resistors R 1 and R 3. The maximum firing angle (minimum load current) is set mostly by variable resistor R 2. When these gate control circuits are used with 50Hz AC supply, the time constant of the RC circuit should fall in the range of 1- 20ms.

RC Triggering Circuits For single RC circuit of fig (a) the product (R 1 +R 2 )C 1 should fall in the range 1ms to 20ms. For double RC circuit of fig(b) (R 1 +R 2 )C 1 should fall in that range and R 3 C 2 should also fall in that range. Fig(a) Fig(b)

Example-9 For the circuit shown in following figure approximate the R 1, R 2 and R 3 to give wide range of firing adjustment.

Example-9 Minimum time constant occurs in RC network-1 when R 2 is set to minimum.

Example-9 Maximum time constant occurs in RC netwrok-1 when R 2 is set to maximum.

Example-9 Time constant of RC netwrok-2 is 2ms.

Example-9 Minimum and maximum firing angles are (18 o =1ms)

Example-10 For the circuit shown in following figure, to what value the potentiometer be set to obtain a firing delay angle of 120 o.

Use of Break Over Devices The firing circuits discussed so far share two disadvantages. 1.Temperature dependence 2.Inconsistent firing behaviour between SCRs of same type These problems can be eliminated by introducing a break over device at gate terminal

Use of Break Over Devices Four layer diode (Shockley Diode) has certain break over voltage.

Shockley Diode

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