Thyristors Why Thyristors?

Many industrial electronics application require the control of large voltage and current. The control of such high voltage and current is possible with the use of variable resistor and a transformer but due to their large sizes and expensive maintenance, they are not used.

Solid state devices such as thyristors are used in the industry to control the voltage and current as they are cheap, require minimum maintenance and have long life. SCR, triac, diac, UJT and PUT are some of the examples of thyristor. Comparison between a thyristor and mechanical switch

a) Thyristors have long life as compared to the mechanical switch b) Thyristors have fast turn on and turn of times. c) Thyristor are simple devices.

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Figure 5–2 Circuit relationship among the voltage supply, an SCR, and the load.

Silicon-controlled rectifier (SCR) “ or thyristor” is a three-terminal solid state device used to control large current to a load in one direction. It acts like a solid state switch because load current flows through its anode-cathode circuit. As shown in the figure, when a trigger current is applied at the gate terminal, the SCR is turned on thus allowing the current to flow from anode to cathode terminal.

SCR is on for a certain percentage of AC cycle and is off state for the remainder of the time. An SCR varies load current by varying the point in the positive half cycle of AC input when the current is applied at the gate terminal.

If the SCR is turned on immediately, then the load will receive current for nearly half of the cycle(180˚C).

SCR can be turned on in only positive half cycle.

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SCR ELECTRICL CHARACTERISTICS

vt

iT

On-state characteristic (Forward conduction)

IG3 > IG2 > IG1 > IG0

Forward blockingstate characteristic IDRM at VDRM

Off-state characteristic (Reverse blocking)

Latching current IL

Holding current IH

On state voltage drop VTM

Reverse breakdown voltage Forward

breakdown voltageIT, IH, IGT, VTM , IL ? See the

definitions in your textbook on page#219

An SCR acts very much like a switch. When it is turned ON acts like a closed switch. When it is turned OFF, then it acts like an open switch. Because it is a Solid state device, the switching action is very fast.Elecetrical characteristics of SCR are shown in the figure below

TURN-ON CHARACTERISTICS

During the off state very current IDRM flows through the SCR but forward voltage VDRM is very high.

But when the gate current is applied, the forward voltage drops quickly to VTM and the current rises quickly to the holding current, IH . SCR is now in the on state.

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Requirements to turn-on a thyristor:SCR’s breakdown voltage should never be exceeded because the SCR goes into conduction even when a gate current is not applied. It remain turn on even if the breakdown voltage is reduced.

1The thyristor must be forward biased (VAK > 0) AND2There must be sufficient trigger current into the gate through the gate-cathode junction

Requirements to turn-off a thyristor:

1he main terminal IT current must be below the holding current IH (IT < IH)

2. The thyristor is reversed biased

.

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Figure 5–3Ideal waveforms of SCR main terminal voltage (VAK) and load voltage: (a) for a firing delay angle of about 60, conduction angle of 120, (b) for a firing delay angle of about 135, conduction angle of 45.Firing angle delay is defined as number of degrees that the SCR is off before it turns on whereas the conduction angle is number of degrees that the SCR is on.

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Figure 5–6Very simple triggering circuit for an SCR.

TYPICAL GATE TRIGGERING CIRCUITS1. Resistive Circuit

• Supply is AC (Sinusoidal) in Figure 5-6.

• When SW is OPEN, the SCR can never turn on. The load therefore is deenergized.

• When SW is CLOSED, there will be a current into the gate G of the SCR.

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• The variable Resistance R2 will control the firing delay angle.

– If R2 is low, IG will be sufficient to fire the SCR when the supply voltage is low (early firing, or delay angle is small). And Vice versa.

– R1 is to maintain a fixed value of gate resistance when R2 is set to Zero.

• Once the SCR is fired, all the load current will pass through it since it is a closed switch and there will be no gate current. The waveform of the load voltage will be similar to that in Figure 5-3.

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The disadvantage of this circuit, is that the maximum firing delay angle is 90o only. This fact is shown in figure 5-7.

If the supply voltage is DC, the SCR will remain ON until the supply is removed.

One of the application of this simple circuit is in alarm systems.

Figure 5–7Ideal waveforms of SCR main terminal voltage and gate current. The dashed line represents the gate current necessary to fire the SCR (IGT). (a) Gate current is low, resulting in a firing delay angle of about 90. (b) Gate current is greater, resulting in a firing delay angle of nearly 0.

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Figure 5–8SCR gate control circuit which is an improvement on the circuit of Fig. 5–6. The capacitor provides a greater range of adjustment of the firing delay angle.

2. Capacitor Triggering Circuit

Figure 5-8 is an improvement on the circuit of Fig. 5–6 where a capacitor is added at the bottom of the gate lead resistance.

The capacitor will guarantee the possibility of firing delay angle to be beyond 90O.

For the negative half-cycle of the supply, the capacitor will charge in the reverse direction (top plate negative and the bottom plate positive).

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• For the positive half-cycle, the capacitor takes time to fully discharge and then charge in the positive direction (top plate positive and the bottom plate negative).

• This will delay the current from flowing to the gate until the capacitor is charged higher than +0.6V.

• The delay will depend on the value of R2. The

bigger the value of R2 the slower the charging and the bigger the firing delay angle.

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Figure 5-9 (a) shows a triggering circuit similar in operation to the previous one. The insertion of R3 requires the capacitor to charge higher than 0.6V to trigger the SCR. So the triggering is further delayed.

In figure 5-9 (b), the firing is further delayed by the insertion of capacitor C2.

Instead of immediately fire the SCR once the VC1 is enough to overcome the R3 drop, it will be used to charge C2 and only after C2 is charged higher than 0.6V IG will flow into the gate.

Figure 5–9Improved SCR gate control circuits. Either one of these circuits provides a greater range of adjustment of the firing delay angle than the circuit of Fig. 5–8.

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• For given capacitor values (usually 0.01-1µF), the minimum firing delay angle is set by fixed resistor R1and R3 and the maximum delay angle is set by the size of the variable resistor R2

• Practically, (R1+R2)*C1 and R3*C2 should fall in the range of 1-30ms for a 60Hz supply.

• Two problems are associated with the previous triggering circuits:– Temperature dependency.– Inconsistent firing behavior between SCRs of

the same type.

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Figure 4–10SCR gate control circuit using a four-layer diode (or any breakover device). The four-layer diode provides consistency of triggering behavior and reduces the temperature dependence of the circuit.

3.Using a Breakover Device in the Gate Lead

To overcome these problems a breakover device (e.g. four layer diode) is used as shown in figure 5-10.

This device has certain breakover voltage point, if it is reached by VC it will act as a closed switch and allow the current to flow into the gate and hence firing the SCR, otherwise it will remain open.

This device is relatively temperature-independent, and the breakover voltage can be held constant from one SCR to another, therefore the imperfection of the SCRs are of no importance.

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SCRs in DC Circuits• When an SCR used in DC circuit the automatic turn-OFF does

not occur, because the supply voltage does not pass thru Zero.• To off the SCR either disconnect the DC supply (which is not

practical) or• Connect a temporary (for few µs) short circuit from the anode to

the cathode of the SCR as shown in Figure 5-14(a).or by• Reverse-biasing the SCR as shown in Figure 5-14 (b).

Figure 5–14SCR commutation circuits. (a) The transistor switch shorts out the SCR, thereby turning it OFF. (b) The transistor switch puts a charged capacitor in parallel with the SCR for reverse-bias turn-OFF. Often another SCR is used in place of the transistor.

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Application Circuits of SCRs• 1. Unidirectional Full-Wave Control

See page 228-229 for the description of the operation of this circuit (full-wave rectifier). This circuit usually used to design a dc power supply; as the output waveform is dc and the input supply is AC.

Figure 5–11(a) Full-wave rectified power control, using two SCRs and a center-tapped winding. (b) Supply voltage and load voltage waveforms. Both ac half cycles are being used to deliver power, but the load voltage has only one polarity (it is rectified).

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• 2. Bidirectional Full-Wave Control

Figure 5–12(a) Full-wave unrectified power control, using two SCRs. (b) The same circuit drawn another way. (c) Load voltage waveform. Both ac half cycles are being used to deliver power, and the load voltage is unrectified.

SCR1 fired on during the positive half-cycle of the ac supply, while SCR2 is fired ON during the negative half-cycle. Hence the output (load) voltage is uncertified. This circuit is called sometimes AC controller.

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3. Bridge Circuit containing an SCR

In the positive half-cycle of the AC supply, diodes A and C are forward biased; if the SCR is fired ON the ac voltage will be applied to the load.

For the negative half-cycle, diodes B and D are forward biased and if the SCR fires the ac voltage will be applied to the load. The load voltage waveform will be similar to that shown in Figure 5-12 (c)

Figure 4–13Full-wave bridge combined with an SCR to control both halves of the ac line. (a) With

the load inserted in one of the ac lines leading to the bridge, the load voltage is unrectified, as in Fig. 5–12(c). (b) With the load inserted in series with the SCR itself,

the load voltage is rectified, as in Fig. 5–11(b). 17