November 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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Did you know that General
Electric introduced the first
silicon-controlled
rectifier (SCR) for commercial use? It was in 1957 when this article
appeared in Popular Electronics magazine. The SCR was one of the first
really high power devices in the semiconductor industry, not for its voltage and
power gain, but for its ability to switch very large currents on and off while having
a relatively low "on" voltage drop. This article gets into the basic theory, operation,
and application of the SCR. The S-band airport surveillance radar that I worked
on in the USAF originally used a vacuum tube thyratron to trigger the magnetron's
pulse forming network, but that tube was replaced with a solid state SCR circuit
that plugged directly into the original thyratron tube socket.
The How and Why of the SCR
Cutaway view above of an SCR courtesy of International Rectifier
Corporation.
Principles of Operation and Applications of the Silicon Controlled Rectifier
By Joseph H. Wujek
When the semiconductor industry began to expand in the 1950's, transistors and
solid-state diodes and rectifiers quickly replaced their vacuum-tube counterparts
in many applications. Then as now, the complete transition from tubes to semiconductors
was not possible because of the limitations of the latter. In 1957, however, an
important step toward the goal of total replacement by semiconductors was taken
when General Electric Co. introduced the silicon controlled rectifier, or SCR.
Briefly, the thyratron permits the control of power in switching applications
with only a small energy loss in the control circuit. By applying a signal to a
control grid, the thyratron is made to conduct between a pair of electrodes (anode
and cathode) and remains conducting with no further excitation at the control grid.
In fact, in normal operation, the grid ceases to control the thyratron once conduction
begins. To stop conduction, the anode must go from a high positive potential to
near zero as in the phase reversal of a 60-Hz power line.
Fig. 1 - The transistor circuit at left is equivalent to
actual SCR at right.
The SCR performs in an analogous manner; and, in addition to the inherent improvements
in reliability and simplicity afforded by semiconductors, some of the kindred devices
of the SCR can function as turn-on/off systems to control bidirectional currents,
an impossible task for the thyratron and other vacuum tubes.
How It Works. The operation of the SCR is perhaps best understood
by examining the device's pnpn junction, shown in equivalent form by the two transistors
in Fig. 1. Assume that the control (gate) electrode is connected so that its
voltage is the same as, or slightly negative with respect to, the voltage on the
cathode. Transistor Q2 is cut off and only leakage current flows in the circuit.
If the gate voltage is made positive with respect to ground, the base-emitter junction
of Q2 becomes forward biased and Q2 begins to conduct. Moreover, Q1 also becomes
forward biased and conducts. As Q1 starts conducting, its collector current aids
in turning on Q2, just as collector current from Q2 assists in turning on Q1.
This mutual aid is a form of regeneration, or positive feedback. A point is reached
at which the switching action "runs away" from the control input and becomes self-sustaining.
In regeneration, Q1 and Q2 are operated at saturation, and the voltage drop from
the collector of Q2 to ground is the sum of the 0.7-volt base-emitter drop of Q1
and the 0.2-volt collector-emitter drop of Q2. (The voltages are for silicon transistors
only.) Thus, the switch exhibits a low voltage drop and requires no control input
power to sustain conduction.
To turn off the circuit, the current in the transistor bases must be internally
reduced to a level at which the current gain of Q1 and Q2 is insufficient to supply
the required currents. Since it is not practical to get into the transistor junctions,
the current in the emitter-collector branch is reduced. This is accomplished automatically
if the supply voltage is derived from an ac source. (The SCR is primarily an ac
device, although in dc applications it will serve as a "latch," or memory switch,
and remain conducting until the anode current is reduced or interrupted.)
The point at which the anode current of an SCR is sufficient to keep the device
conducting is called the holding current. The peak voltage (anode positive with
respect to cathode) at which the SCR does not undergo breakdown for given conditions
of bias between the gate and cathode is the the peak forward blocking voltage; this
is usually specified with the gate connected to the cathode through a low resistance.
The peak reverse voltage with the anode negative with respect to the cathode
is also specified with the gate connected to the cathode through a low resistance.
These diagrams show the steps in the fabrication of a silicon
controlled rectifier as made by General Electric.
Leakage currents increase with temperature increases and roughly double for every
10° C rise. In Fig. 1, the transistors cannot distinguish between currents
caused by leakage or from a triggering pulse. Hence, care must be exercised in determining
the temperature environment and external circuit conditions to prevent thermal turn-on.
Other unwanted turn-on mechanisms are the device's built-in junction capacitances
which provide paths for current when the anode-cathode voltage is changing. Current
through a capacitor is proportional to the voltage rate of change with time. A fast
changing voltage can introduce sufficient current to trigger the SCR. This parameter
is specified as the "critical time rise" and usually is given in V/μS.
The forward and reverse breakdown volt-ages have already been mentioned. Unless
some means of externally limiting the current is used, these breakdown voltages
will destroy an SCR. Except where severe transient voltages are present, the breakdown
voltages will present no problems if the specified ratings are not exceeded.
Parameters & Characteristics. If the SCR is to be intelligently
employed, it is essential that the user be familiar with the device's various parameters
and characteristics. These specifications are given in the manufacturer's data sheets.
In choosing an SCR, first check the maximum allowable ratings, including the maximum
current handling capacity which may be stated as average current or rms current
or both. To use either specification, the current waveform through the SCR must
be known.
The peak surge current, usually specified for a 60-Hz half-wave excursion, is
the current the SCR can handle on a low duty-cycle basis, permitting the SCR to
cool off between surges. These currents can be as much as 10 times greater than
the rms current. Such ratings are useful when the SCR is employed in "crowbar" operation
to discharge a capacitor bank.
Power ratings for the entire SCR, as well as for the gate circuit are often stated.
These ratings depend on ambient and case temperatures. Maximum voltage and current
in the gate circuit are sometimes specified.
Finally, temperature limits for storage and operation are given. The low-temperature
limit is dictated primarily by the differences in thermal expansion between the
chip and surrounding materials. The upper limit is set by considerations of damage
to the crystal substrate.
Typical SCR packages for International Rectifier Corp. units
which have current ratings from 50 to 100 amps.
When using the SCR as part of a circuit, the peak reverse and peak forward blocking
figures specified are the currents that flow at given sets of bias conditions when
the SCR is not conducting. These currents can be viewed as leakage and must be stated
for a given temperature or temperature range. An SCRs leakage is on the order of
0.1 percent of its forward current. Hence, an SCR rated at 100 amperes forward current
cannot be used to control a 50-mA load since the leakage current will be about the
same as the current being controlled.
The gate trigger voltage and current are specified for given anode-to-cathode
voltages and gate-to-cathode resistances. They are temperature-dependent and often
graphically plotted for SCR's not to trigger. The minimum values for firing at given
temperatures also appear on the plots. This information specifies the voltage and
current required for triggering the SCR, as well as the bias conditions to be maintained
in the blocking state.
The peak on voltage is the drop between the anode and cathode for a given load
current and temperature. It is generally in the range of 1 to 2 volts. The holding
current specifies the level to maintain to prevent the SCR from turning off.
The turn-on and turn-off times are stated for SCR's intended for high-speed switching.
The operating conditions must be specified if these parameters are to be useful.
Some fast SCR's have low-current switching times in tens of nanoseconds.
Design Considerations. Once the SCR is inserted between the
power source and the load, a means must he provided for triggering it. When used
to control ac, one of the simplest ways of triggering is to use the phase control
method. The negative alternation takes care of the turn-off. Then all that is necessary
to drive the SCR into conduction is application of a pulse to the gate when the
anode is positive with respect to the cathode. A phase control triggering scheme
in its simplest form is shown in Fig. 2. By choosing the appropriate resistance
and capacitance values for the network, the time, or phase, relationship of the
gate with respect to the anode-to-cathode voltage can be determined. Household lamp
dimmers often are designed this way and may employ two SCR's back-to-back to control
both ac alternations.
Because the phase between the gate and anode-to-cathode voltages determines the
time the SCR conducts, the average current through the SCR is dependent upon this
relationship. The firing angle can also be derived from an isolated source like
an error signal in a feedback system. When more current is needed, the error signal
"tells" the trigger circuit to advance the gate voltage to turn on the SCR earlier
in the cycle. This results in an increase in average current flow since the SCR
conducts for a longer period of time.
Fig. 2 - Schematic of a typical pulse triggering circuit
to turn on SCR. Waveforms below show voltages and current and indicate the firing
angle.
A transformer provides good isolation between the trigger circuit and the load.
The control signal might be a dc voltage, such as the on/off conditions of a switch
or logic circuit. A simple oscillator can be used to furnish the gate pulses, controlled
by a simple AND gate.
If moderate or high currents are to be controlled, the fast turn-on of the SCR
can generate high-frequency noise that will be radiated into space and passed along
ac power lines. These noise spikes may interfere with radio and TV reception and
cause malfunctions in interference-sensitive equipment. Filters can be used in the
power line to reduce this noise, but a different means exists for drastically reducing
or eliminating the noise.
If the time at which the anode voltage crosses through zero and begins its swing
toward positive (with respect to the cathode) can be sensed, a trigger pulse can
be provided at that instant. The SCR then starts conducting early in the positive
alternation and the current (in a resistive load) follows the sine wave of voltage
rather than suddenly jumping from leakage level to a high forward level (see Fig. 2).
Several manufacturers offer IC's designed specifically as zero-voltage detectors
to use in this application.
Applications. Apart from the familiar lamp dimmer switch and
speed controls for certain types of ac motors, the SCR is used in the home to provide
continuous (as opposed to stepped) control of heat in electric kitchen ranges. In
industry, the SCR is used to control power in battery chargers, power supplies,
and machine tools. Welders, power regulators, and temperature control systems have
been designed using the SCR as a power control element. Among the most popular of
automotive electronic ignition systems available is the SCR-fired system and its
variations. And new applications for the SCR are continuously being discovered.
Posted February 13, 2024 (updated from original
post on 10/19/2017)
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