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INDUSTRIAL CONTROL HANDBOOK - 3.2 SILICON CONTROLLED RECTIFIERS (SCRs)

Silicon controlled rectifiers (SCRs) were first used in control circuits in the early 1960s. Fig. 3-1 shows a symbol for the SCR and identifies the anode, cathode, and gate terminals. (Notice that the cathode is identified by the letter C or the letter K.) This figure also shows pictures of several types of SCRs. The SCR acts like a solid-state switch in that current will pass through its anode-cathode circuit to a load if a signal is received at its gate. The SCR is different from a traditional switch in that the SCR will change ac voltage to dc voltage (rectify) if ac voltage is used as the power supply. The SCR is also different from a switch in that the amount of time the SCR conducts can be varied so that the amount of current provided to the load will be varied from near zero to maximum of the power supply. Fig. 3-2 shows an SCR connected to a resistive load.

The SCR will operate differently with various types of power supplies. If the power supply in Fig. 3-2 is dc voltage with no ripple, the SCR will stay on once a positive voltage signal is applied to its gate. The only way to turn power off to the SCR is to turn off the power supply. You can see that this is not very useful. If the power supply to the SCR is pulsing dc or ac, the SCR will turn off (commutate) naturally at the end of each half-cycle when the voltage goes to zero. If the power supply is ac voltage, the SCR will only conduct during the positive half Chapter 3. Industrial Electronics cycle of the waveform.

FIGURE 3-1 Examples of SCRs and the electronic symbol of an SCR that identifies the anode, cathode, and gate.FIGURE 3-1 Examples of SCRs and the electronic symbol of an SCR that identifies the anode, cathode, and gate. (Copyright of Motorola, Used by Permission.)

FIGURE 3-2 SCR connected to a resistive load and power supply.FIGURE 3-2 SCR connected to a resistive load and power supply.

The SCR can vary the amount of current that is allowed to flow to the resistive load by varying the point in the positive half-cycle where the gate signal is applied. If the SCR is turned on immediately, it will conduct full voltage and current for the half-cycle (180°). If the turn-on point is delayed to the 90° point in the half-cycle waveform, the SCR will conduct approximately half of the voltage and current to the load.

If the turn-on point is delayed to the 175° point in the half-cycle, the SCR will conduct less than 10% of the power supply voltage and current to the load, since the half-cycle will automatically turn off the SCR at the 180° point. This means that the gate of the SCR can be used to control the amount of voltage and current the SCR will conduct from near zero to maximum.

 

3.2.1 Two-Transistor Model of an SCR

The proper name for the SCR is the reverse blocking triode thyristor. The name is derived from the fact that the SCR is a four-layer thyristor made of PNPN material. Fig. 3-3a shows the four-layer PNPN material. Fig. 3-3b shows the PNPN material split apart as two transistors, a PNP and an NPN. Fig. 3-3c shows the SCR as two transistors. These figures will help you understand how the operation of the SCR can be explained by the four-layer (two-transistor) model.

The anode is at the emitter of the PNP transistor (T2), and the cathode is at the emitter of the NPN transistor (T1). The gate is connected to the base of the NPN transistor. Since the anode is the emitter of the PNP, it must have a positive voltage to operate, and since the cathode is the emitter of the NPN transistor, it must be negative to operate.

When a positive pulse is applied to the gate, it will cause collector current Ic to flow through the NPN transistor (T1). This current will provide bias voltage to the base of the PNP transistor (T2). When the bias voltage is applied to the base of the PNP transistor, it will begin to conduct Ic which will replace the bias voltage on the base that the gate signal originally supplied. This allows the gate signal to be a pulse, which is then removed since the current through the SCR anode to cathode will flow and replace the base bias on transistor T1.

FIGURE 3-3 (a) Symbols of SCR. (b) SCR as a four-layer PNPN device, (c) shows the PNPN layers split apart as a PNP transistor and NPN transistors, (d) shows the diode operation using the two transistors.FIGURE 3-3 (a) Symbols of SCR. (b) SCR as a four-layer PNPN device, (c) shows the PNPN layers split apart as a PNP transistor and NPN transistors, (d) shows the diode operation using the two transistors. (Copyright of Motorola, Used by Permission.)

 

3.2.2 Static Characteristic Curve and Waveforms for the SCR

Fig. 3-4 shows the static characteristic curve for the SCR. From this figure you can see that the reverse voltage and forward voltage characteristics are similar to a junction diode. The main difference is that the SCR must be set into the conduction mode before it will begin to conduct. The main operating area of the SCR is the upper right quadrant of the graph where the SCR is conducting in the forward-bias mode. You can see that if reverse voltage is applied, the current flow will be blocked just like a junction diode. In the forward-bias mode you should notice that the current will be zero until the SCR is set into conduction. The point where the SCR goes into conduction is identified by the knee.

FIGURE 3-4 Static characteristic curve for the SCR.FIGURE 3-4 Static characteristic curve for the SCR. (Courtesy of Philips Semiconductors.)

 

3.2.3 Waveforms of the SCR and the Load

Confusion with waveforms may also arise when you use an oscilloscope to display the waveforms across the SCR and the load resistor. Since the SCR will exhibit characteristics like a switch, the voltage will be measured across the SCR when it is off, and the voltage will be measured across the load when the SCR is in conduction. This means that the oscilloscope will show the waveform of the firing angle when it is across the SCR, and it will show the conduction angle when it is across the load. Fig. 3-6 shows these two waveforms.

FIGURE 3-5 Waveforms of an ac sine wave applied to an SCR circuit. The top diagram shows the waveform of voltage that is meaured across the anode and cathode of the SCR. The bottom waveform showsFIGURE 3-5 Waveforms of an ac sine wave applied to an SCR circuit. The top diagram shows the waveform of voltage that is meaured across the anode and cathode of the SCR. The bottom waveform shows the voltage measured across the load. Notice the SCR is fired at 135°.

FIGURE 3-6 Diagram of an SCR controlling voltage to a resistive load. The diagrams below the SCR circuit show the waveforms that you would see if you place an oscilloscope across the AC source, across the SCR, and across the load resistor.FIGURE 3-6 Diagram of an SCR controlling voltage to a resistive load. The diagrams below the SCR circuit show the waveforms that you would see if you place an oscilloscope across the AC source, across the SCR, and across the load resistor.

 

3.2.4 Methods of Turning on an SCR

The SCR is normally turned on by a pulse to its gate. It can also be turned on by three alternative methods that include exceeding the forward breakover voltage, by excessive heat that allows leakage current, or by exceeding the dv/dt level (allowable voltage change per time change) across the junction. The three alternative methods of turning on an SCR generally cause conditions which should be controlled to prevent the SCR from being turned on when this is not wanted.

 

3.2.5 Turning on the SCR by Gate Triggering

When a positive pulse is applied to the gate of the SCR, it must be large enough to provide sufficient current to the first junction (the base terminal of transistor T1 in the two-transistor model in Fig. 3-3). If the current level of the pulse is sufficient, the first junction will go into conduction and the current flow through it will cause the second junction (transistor T2) to go into conduction. The current through the second junction will be sufficient to latch up the SCR by supplying an alternative source for the gate current. This means that the current to the gate can be removed and the SCR will remain in conduction. The SCR will commutate when the power supply it is connected to returns to the zero voltage level at 180° or when ac voltage is in reverse polarity (181° to 360°). If the pulse of current to the gate is too small or is not long enough in duration, the SCR will not turn on.

If you look at the SCR as a three-part device (anode, cathode, and gate), the positive pulse of gate current (IG) is applied to the gate terminal and it will flow through the cathode where it leaves the device. The timing of the pulse is very critical if the SCR is being used to control the current proportionally. Since the current is being controlled from zero to maximum, the amount of resolution will be determined by the accuracy of the gate pulse timing.

 

3.2.6 Basic Gate Circuit

Fig. 3-7 shows two sets of diagrams of the ac sine wave, the gate signal, the waveform across the SCR, and the waveform across the load. The minimum gate current IGT is shown as a dotted line in the diagram of the gate signal. In Fig. 3-7a the gate current becomes strong enough at the peak of the ac cycle at the 90° point. The waveform for the SCR and the load shows the SCR turning on at the 90° point and staying on to the 180° point where the ac reverses its polarity. In Fig. 3-7b, the variable resistor has been adjusted so that the amount of voltage for the gate signal has increased significantly. This increase in voltage provides an increase in gate current so that the minimum gate current IGT is exceeded at the 30° point. This means that the SCR is in conduction for 150° (30° to 180°). This method of gate control is rather simplistic since it depends on the gate current exceeding the minimum current requirement to turn on the SCR.

FIGURE 3-8 (a) Electronic symbol of a UJT. (b) Electronic symbol of a diac. These devices are used to fire an SCR.

Figure 3-7 (a) The top diagram shows the waveform of ac supply voltage measured across the A-K circuit of an SCR when the SCR is turned on at the 90° point. The waveform in the bottom diagram shows the amount of gate current required to cause the SCR to go into conduction at this point, (b) The top diagram shows the waveform of an SCR that goes into conduction at the 30° point. Notice in the bottom diagram that the level of gate current is increased, which causes it to reach the gate current threshold at the 30° point instead of the 90° point.

Fig. 3-8b shows a diac used to trigger an SCR. The UJT and diac are solid-state devices that provide a sharp pulse with sufficient current to cause the SCR to go into conduction. The pulse has a very sharp rise in current over a short time duration. The resistor and capacitor in each circuit provide an RC time constant that causes the time delay for the pulse. Since the resistor is variable, the larger the resistance is, the later in the sine wave the UJT or diac fires to provide the gate pulse. In some circuits, the SCR is so large that a smaller separate SCR is used to provide the gate signal for the larger one. These smaller SCRs are called gaters. Later we will cover UJTs, diacs, and other triggering devices in detail.

 

3.2.7 Methods of Commutating SCRs

Once an SCR is turned on, it will continue to conduct until it is commutated (turned off). Commutation will occur in an SCR only if the overall current gain drops below unity (1). This means that the current in the anode-cathode circuit must drop below the minimum (near zero) or a current of reverse polarity must be applied to the anode-cathode. Since the ac sine wave provides both of these conditions near the 180° point in the wave, the main method to commutate an SCR is to use ac voltage as the supply voltage. In an ac circuit, the voltage will Chapter 3. Industrial Electronics drop to zero and cross over to the reverse direction at the 180° point during each sine wave. This means that if the supply voltage is 60 Hz, this will happen every 16 msec. Each time the SCR is commutated, it can be triggered at a different point along the firing angle, which will provide the ability of the SCR to control the ac power between 0° to 180°. The main problem with using ac voltage to commutate the SCR arises when higher-frequency voltages are used as the supply voltage. You should keep in mind the SCR requires approximately 3-4msec to turn off; therefore, the maximum frequency is dependent on the turn-off time.

FIGURE 3-9 (a) A switch is used to commutate the SCR in a dc circuit by interrupting current flow. This type of circuit is used to provide control in alarms or emergency dc voltage lighting circuits, (b) A series RL resonant circuit use to commutate an SCR. (c) A parallel RL resonant circuit used to commutate an SCR.FIGURE 3-8 (a) Electronic symbol of a UJT. (b) Electronic symbol of a diac. These devices are used to fire an SCR.

FIGURE 3-10 Typical SCR packages. Notice that the terminals are larger for larger-sized wire required to carry the currents. If more than one SCR is mounted in a package, they are connected internally as pairs so the module is easy to install.FIGURE 3-9 (a) A switch is used to commutate the SCR in a dc circuit by interrupting current flow. This type of circuit is used to provide control in alarms or emergency dc voltage lighting circuits, (b) A series RL resonant circuit use to commutate an SCR. (c) A parallel RL resonant circuit used to commutate an SCR.

FIGURE 3-10 Typical SCR packages. Notice that the terminals are larger for larger-sized wire required to carry the currents. If more than one SCR is mounted in a package, they are connected internally as pairs so the module is easy to install.FIGURE 3-10 Typical SCR packages. Notice that the terminals are larger for larger-sized wire required to carry the currents. If more than one SCR is mounted in a package, they are connected internally as pairs so the module is easy to install. (Courtesy of Darrah Electric Company, Cleveland, Ohio.)

FIGURE 3-11 Diagrams of SCRs used in half-wave and full-wave rectifier applications.

FIGURE 3-11 Diagrams of SCRs used in half-wave and full-wave rectifier applications.FIGURE 3-11 Diagrams of SCRs used in half-wave and full-wave rectifier applications. (Courtesy of Darrah Electric Company, Cleveland, Ohio.)

FIGURE 3-12 Electronic symbols for the complementary SCR and the silicon controlled switch (SCS), and the gate turn-off device (GTO).FIGURE 3-12 Electronic symbols for the complementary SCR and the silicon controlled switch (SCS), and the gate turn-off device (GTO). (Copyright of Motorola. Used by Permission.)

SCRs are also used in this circuit in the inverter section where the dc voltage is turned back into ac voltage. Since the devices must provide both the positive and the negative half-cycles, a diode is connected in inverse parallel to a diode provide the hybrid ac switch. This combination of devices is not used as often newer drives because a variety of larger triacs and power transistors is available that can do the job better. This type of circuit was very popular.

 

3.2.8 Complementary SCRs, Silicon Controlled Switches, Gate Turn-Off Devices, and Other Thyristors

Several variations of the SCR have been produced to correct deficiencies in SCR. These solid-state devices provide functions that allow the thyristor to control current somewhat differently than the SCR. The devices include the complementry SCR, silicon controlled switch (SCS), gate turn-off devices (GTOs), and light-gated SCRs (LASCRs). Fig. 3-12 shows the electronic symbols for the complimentry SCR, SCS, and GTO.

The complementary SCR uses a negative gate pulse instead of a positive pulse. This feature is useful in circuits where the gate must be pulsed by the cathode side of the circuit. In some cases it is important to control the turn-off feature the device, so devices like the silicon controlled switch (SCS) and the gate turn-off device (GTO) are used. The SCS is a low-powered SCR with two gate terminals. If the anode-cathode current is less than 4 mA, the cathode gate (Gc) is used. If the current is greater than 4 mA, the anode gate (GA) is used. The gates in the device are used to switch the device off.

The gate turn-off (GTO) devices are useful in circuits that use higher frequencies up to 100 kHz. This is possible because the gate is used to turn the SCR as well as on. When the GTO is forced off, it interrupts current flow faster the normal SCR. This means that the GTO will be ready to turn on the next faster. GTOs are now commonly used in motor frequency drives where frequency up to 120 Hz are used. The GTO is used in place of a typical SCR and it provides a greater degree of control at the higher frequencies.

FIGURE 3-13 Electronic symbol of the triac and a diagram of its pn structure.FIGURE 3-13 Electronic symbol of the triac and a diagram of its pn structure. (Courtesy of Philips Semiconductors.)


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