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EE301 GB04 Page 1

Lecture 4. Silicon Controlled Rectifiers Part 1

In this lecture:

4.1. SCR construction and operation

4.2. Controlled rectification – half-wave

4.3. Full-wave SCR rectification

• The silicon-controlled rectifier , or thyristor , is a multi-junction device withlatching switch behaviour.

SCR

 

Similar to a diode, but with one importantdifference. Two conditions have to be met forforward current to flow:

1. Forward-biased voltage

2. A pulse of current on the gate terminal.

This allows the thyristor to control very large currents very easily.

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4.1 SCR Construction

A thyristor (or silicon-controlled rectifier ) is a pnpn device with three terminals:

1. Anode (A)

2. Cathode (K)

3. Gate (G)

p1

n1

p2

n2

Anode, A

Gate, G

Cathode, K

 

Figure 4. 1 Thyristor pnpn construction 

• When the device is reverse-biased (V AK<0), no current flows.

• When the device is forward-biased (V AK>0), no current flows until a currentpulse is applied to the gate.

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To see how it works, we can visualise the thyristor like two bipolar junctiontransistors (BJTs), back-to-back (Figure 4. 2):

(a)

(b)

Figure 4. 2 A thyristor can be regarded as a pnp BJT and an npn BJT, connected back-to- back 

1. The base of the pnp device is connected to the collector of the npn device

2. The base of the npn device is connected to the collector of the pnp device.

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Latching switch behaviour

1. When I G=0, both BJTs are switched off , and no current flows no matterwhat V AK is.

2. If a pulse of current I G is applied to the gate, then

• The npn device turns on, drawing collector current from the base ofthe pnp down to the cathode, K.

• The pnp turns on, forcing collector current down from the anode tothe base of the npn

3. After the current pulse I G has ended:

• The large current flowing from the pnp device’s collector to the npndevice’s base is enough to keep the npn turned on

• Then the large current drawn from the pnp’s base is enough to keepthe pnp turned on.

• Hence, the thyristor stays on. This behaviour is called latching switch behaviour .

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EE301 GB04 Page 5

Example 

10+50 V

D.C.- I G 

I R 

+V R 

-

 

Table 4. 1Although forward-biased, the thyristor conducts only after a current pulse is applied to the gate 

The thyristor is switched off initially, so

•  I R = 0, V R = 0, V AK  ≈ 50 V.

• So the thyristor is forward biased, but no current flows.

Some time later, apply a pulse of I G : thyristor switches on.

•  V AK  → ~ 0 V, V R  → ~ 50 V, I R  → ~ 5 AEE301 GB04 Page 6

The gate current pulse

The current pulses in the gate, I G , must deliver enough charge to overcomecarrier depletion in the two BJTs and switch them on. Thus:

Thyristors are reliable and flexible, but not very fast.

To turn the device off:

1. The anode voltage must fall below the minimum required to keep thetransistors on.

OR 

2. Apply a negative current to the gate, momentarily sucking carriers out ofthe npn device’s base region and turning the BJTs off.

• Depending on the value of βpnp and βnpn, this I G usually needs tobe much larger than the turn-on I G .

• This is the basis for the operation of the gate turn-off thyristor , orGTO .

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4.2. Half-Wave Rectification

• SCRs are used to rectify A.C. voltages more flexibly than diodes, sinceSCRs can be switched on whenever we want .

• For example, a common application is the light dimmer switch .

• SCR rectifiers can vary the DC voltage by changing the turn-on time ofthe SCRs in the circuit.

Figure 4. 3 A SCR half-wave rectifier 

Consider the half-wave SCR rectifier in Figure 4. 3 above, supplied by anA.C. voltage source.

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We can apply the turn-on I G pulse at any stage in the supply’s cycle.

Let’s delay SCR’s the turn-on by, for example, 45° (=π /4). We would get thefollowing graph for the load voltage, v (t ).

Figure 4. 4 Half-wave SCR rectifier, with a turn-on delay of π  /4. Supply voltage (broken line) and Load voltage (steady line)

As you can guess, the average output voltage is quite high, almost the same

as the diode case.

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EE301 GB04 Page 9

If we increase the delay of the current pulse, we get different-shaped loadvoltages…

φ =π /2

(a quarter-period)

φ =3π /4:

(3/8 of a period)

Depending on the delay (generally called the phase delay, φ ), we can easilycontrol the average output voltage.

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The average output voltage

Because the output SCR voltage is entirely positive, we can use thetraditional averaging formula instead of RMS.

Some quick simplifications: 

1. Between π and 2π the load voltage is zero.

2. At a phase delay of φ radians the load voltage is zero from 0 to φ .

So the average voltage can be found, as before, by

[ ]

[ ]

[ ]φ π 

φ π π 

ω π 

ω ω π 

ω ω π 

π 

φ 

π 

φ 

π 

cos12

coscos2

cos2

sin2

1

)(2

12

0

+=

+−=

−=

=

=

∫

∫

m 

m 

m 

m 

avg 

V 

V t 

V 

t d t V 

t d t v V 

 

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• This is a wonderful result! It allows us to control the average loadvoltage, just by changing the phase-delay on the SCR.

• So as φ increases, the average voltage decreases:

o When φ =0 rad, the SCR performs just like a diode rectifier.

o As the phase delay increases, the average voltage decreasesuntil it reached zero at φ =π rad (that’s 180°, or T  /2 seconds).

o Beyond φ = π rad the SCR is reverse-biased, and so the rectifierstays off.

Figure 4. 5 V AVG vs delay angle φ for a controlled half-wave rectifier 

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4.3. Full-wave rectification

(a) Resistive load

A more efficient circuit is the SCR bridge rectifier . This is identical to thefamiliar diode bridge rectifier, except that each diode is now replaced by athyristor.

R L

V in

V LoadA

C 

B 

D 

 

Figure 4. 6 An SCR bridge rectifier 

As before, by delaying the switching time we can control the average outputvoltage. But this time the available power is doubled .

With a resistive load this circuit performs in a similar way to the diode circuit:

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1. v in >0 :

• one pair of SCRs (A,D) can turn on while the other pair stay off.

• Positive current load flows left-to-right.

2. v in >0 :

• SCRs A and D become reverse biased (turn off), while the other pair,B and C, become forward-biased.

• A pulse of I G into B and C then turns them on, and positive loadcurrent flows right-to-right again !

Again, by delaying the devices’ turn-on times, the load voltage goes to zerowhile the switches are off, giving the waveform below.

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120 140 160

 

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The average voltage can be easily derived as before:

• In the half-wave case we only had non-zero load voltage for the positivehalf-period of the supply voltage.

• Now we have non-zero load voltage for both the positive and thenegative half-periods of the supply.

• So the average load voltage V avg is simply twice the half-wave average.

( )

( ) Bridge)SCR(Full cos1

rectifier)SCR(Single cos12

φ π 

φ π 

+=

+=

m avg 

m avg 

V V 

V V 

 

Next lecture: we will see how the SCR bridge rectifier behaves supplying aninductive load, and also a three-phase SCR rectifier.

END OF LECTURE