Silicon-Controlled Rectifier (SCR)
Shockley diodes are curious devices, but rather limited in application. Their usefulness may be
expanded, however, by equipping them with another means of latching. In doing so, each
becomes true amplifying devices (if only in an on/off mode), and we refer to these as silicon-
controlled rectifiers, or SCRs.
The progression from Shockley diode to SCR is achieved with one small addition, actually
nothing more than a third wire connection to the existing PNPN structure: (Figure below)
The Silicon-Controlled Rectifier (SCR)
If an SCR’s gate is left floating (disconnected), it behaves exactly as a Shockley diode. It may be
latched by breakover voltage or by exceeding the critical rate of voltage rise between anode and
cathode, just as with the Shockley diode. Dropout is accomplished by reducing current until one
or both internal transistors fall into cutoff mode, also like the Shockley diode. However, because
the gate terminal connects directly to the base of the lower transistor, it may be used as an
alternative means to latch the SCR. By applying a small voltage between gate and cathode, the
lower transistor will be forced on by the resulting base current, which will cause the upper
transistor to conduct, which then supplies the lower transistor’s base with current so that it no
longer needs to be activated by a gate voltage. The necessary gate current to initiate latch-up, of
course, will be much lower than the current through the SCR from cathode to anode, so the SCR
does achieve a measure of amplification.
This method of securing SCR conduction is called triggering, and it is by far the most common
way that SCRs are latched in actual practice. In fact, SCRs are usually chosen so that their
breakover voltage is far beyond the greatest voltage expected to be experienced from the power
source, so that it can be turned on only by an intentional voltage pulse applied to the gate.
It should be mentioned that SCRs may sometimes be turned off by directly shorting their gate
and cathode terminals together, or by “reverse-triggering” the gate with a negative voltage (in
reference to the cathode), so that the lower transistor is forced into cutoff. I say this is
“sometimes” possible because it involves shunting all of the upper transistor’s collector current
past the lower transistor’s base. This current may be substantial, making triggered shut-off of an
SCR difficult at best. A variation of the SCR, called a Gate-Turn-Off thyristor, or GTO, makes
this task easier. But even with a GTO, the gate current required to turn it off may be as much as
20% of the anode (load) current! The schematic symbol for a GTO is shown in the following
illustration: (Figure below)
Phase-shifted signal triggers SCR into conduction.
Because the capacitor waveform is still rising after the main AC power waveform has reached its
peak, it becomes possible to trigger the SCR at a threshold level beyond that peak, thus chopping
the load current wave further than it was possible with the simpler circuit. In reality, the
capacitor voltage waveform is a bit more complex that what is shown here, its sinusoidal shape
distorted every time the SCR latches on. However, what I’m trying to illustrate here is the
delayed triggering action gained with the phase-shifting RC network; thus, a simplified,
undistorted waveform serves the purpose well.
SCRs may also be triggered, or “fired,” by more complex circuits. While the circuit previously
shown is sufficient for a simple application like a lamp control, large industrial motor controls
often rely on more sophisticated triggering methods. Sometimes, pulse transformers are used to
couple a triggering circuit to the gate and cathode of an SCR to provide electrical isolation
between the triggering and power circuits: (Figure below)
SCR Applications
1. Power Control.
2. Switching.
3. Zero Voltage Switching.
4. Over-Voltage Protection.
5. Pulse Circuits.
6. Battery Charging Regulator.
Function of the gate terminal
The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).
Layer diagram of thyristor.
When the anode is at a positive potential VAK with respect to the cathode with no voltage applied
at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is
reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the
breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor
starts conducting (On state).
If a positive potential VG is applied at the gate terminal with respect to the cathode, the
breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of
VG, the thyristor can be switched into the on state quickly.
Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the
gate voltage, until: (a) the potential VAK is removed or (b) the current through the device
(anode−cathode) is less than the holding current specified by the manufacturer. Hence VG can be
a voltage pulse, such as the voltage output from a UJT relaxation oscillator.
The gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current
(IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident
that there is a minimum gate charge required to trigger the thyristor.
