EEG 215 Electronics (2) Lecture 9 PDF

# Electronics (2) ## Course Code: EEG 215 Second Year, Electrical Engineering Department | Fall | 2 | 02 | 3 | 1st Semester | ## Lecture 9: ### Thyristors: Four-layer pnpn Devices (II) **Lecturer:** Dr. Ayad Shohdy EE Dept. Sohag University Wednesday, December 06, 2023 ## Contents - Four-la...

# Electronics (2) ## Course Code: EEG 215 Second Year, Electrical Engineering Department | Fall | 2 | 02 | 3 | 1st Semester | ## Lecture 9: ### Thyristors: Four-layer pnpn Devices (II) **Lecturer:** Dr. Ayad Shohdy EE Dept. Sohag University Wednesday, December 06, 2023 ## Contents - Four-layer Devices - SCR (silicon-controlled rectifier), - SCS (silicon-controlled switch), - GTO (gate turn-off switch), - LASCR (light-activated SCR), - Shockley diode, - Diac, - Triac # 2. Silicon-Controlled Switch (SCS) - The silicon-controlled switch (SCS), like the silicon-controlled rectifier, is a four-layer pnpn device. All four semiconductor layers of the SCS are available due to the addition of an anode gate, as shown in the Figure. - The graphic symbol and transistor equivalent circuit are shown in the same Figure. The characteristics of the device are essentially the same as those for the SCR. The effect of an anode gate current is very similar to that demonstrated by the gate current in the Figure of the SCR characteristics. - The higher the anode gate current (magnitude), the lower is the required anode-to-cathode voltage to turn the device on. The anode gate connection can be used to turn the device either on or off. - To turn on the device, a negative pulse must be applied to the anode gate terminal, whereas a positive pulse is required to turn off the device. - The need for the type of pulse indicated above can be demostrated using the circuit transistor equivalent of the SCS. - A negative pulse at the anode gate will forward-bias the base-to-emitter junction of Q₁, turning it on. The resulting heavy collector current IC₁ will turn on Q₂, resulting in a regenerative action and the “on” state for the SCS device. - A positive pulse at the anode gate will reverse-bias the base-to-emitter junction of Q₁, turning it off, resulting in the open-circuit “off” state of the device. - In general, the triggering (turn-on) anode gate current is larger in magnitude than the required cathode gate current. - For one representative SCS device, the triggering anode gate current is 1.5 mA, whereas the required cathode gate current is 1 mA. - The required turn-on gate current at either terminal is affected by many factors, including the operating temperature, the anode-to-cathode voltage, the load placement, and the type of cathode, gate-to-cathode, and anode gate-to-anode connection (short-circuit, open-circuit, bias, load, etc.). Tables, graphs, and curves are normally available for each device to provide the type of information indicated above. - An advantage of the SCS over a corresponding SCR is: - The reduced turn-off time, typically within the range 1 µs to 10 µs for the SCS and 5 µs to 30 µs for the SCR. - Increased control and triggering sensitivity and a more predictable firing situation. - At present, however, the SCS is limited to low power, current, and voltage ratings. Typical maximum anode currents range from 100 mA to 300 mA with dissipation (power) ratings of 100 mW to 500 mW. # 2.1. SCS Turned Off - Three of the more fundamental types of turn-off circuits for the SCS are presented. - (i) Similarly, the positive pulse at the anode gate of as shown in the Figure will turn the SCS off by the mechanism described earlier in this section. - (ii) In the given Figure, when a pulse is applied to the transformer, the transistor conducts heavily (saturated), resulting in a low-impedance (short-circuit) characteristic between collector and emitter. This low-impedance branch diverts anode current away from the SCS, dropping it below the holding value and consequently turning it off. - (iii) The circuit of the following Figure can be turned either off or on by a pulse of the proper magnitude at the cathode gate. The turn-off characteristic is possible only if the correct value of Rᴀ is employed. It will control the amount of regenerative feedback, the magnitude of which is critical for this type of operation. Note the variety of positions in which the load resistor R₁ can be placed. - There are a number of other possibilities, which can be found in any comprehensive semiconductor handbook or manual. # 2.2. SCS Applications - Some of the more common areas of application include a wide variety of computer circuits (counters, registers, and timing circuits), pulse generators, voltage sensors, and oscillators - (i) Voltage Sensor - One simple application for an SCS as a voltage-sensing device is shown the Figure, an alarm system with n inputs from various stations. Any single input will turn that particular SCS on, resulting in an energized alarm relay and light in the anode gate circuit to indicate the location of the input (disturbance). - (ii) Alarm Circuit - One additional application of the SCS is in the alarm circuit of the Figure. Rₛ represents a temperature-, light-, or radiation-sensitive resistor, that is, an element whose resistance will decrease with the application of any of the three energy sources listed above. The cathode gate potential is determined by the divider relationship established by Rₛ and the variable resistor R'. Note that, the gate potential is at approximately 0 V if Rₛ equals the value set by the variable resistor since both resistors will have 12 V across them. However, if Rₛ decreases, the potential of the junction will increase until the SCS is forward-biased, causing the SCS to turn on and energize the alarm relay. The 100-kΩ resistor is included to reduce the possibility of an accidental triggering of the device through a phenomenon known as the rate effect. It is caused by the stray capacitance levels between gates. A high-frequency transient can establish sufficient base current to turn the SCS on accidentally. The device is reset by pressing the reset button, which opens the conduction path of the SCS and reduces the anode current to zero. # 3. Gate Turn-off Switch (GTO) - The gate turn-off switch (GTO) is the third pnpn device to be introduced in this topic. Like the SCR, however, it has only three external terminals. Its structure and graphical symbol is shown in the front Figure. - Although the graphical symbol is different from that of either the SCR or the SCS, the transistor equivalent is exactly the same and the characteristics are similar. - The most obvious advantage of the GTO over the SCR or SCS is the fact that: - Firstly, it can be turned on or off by applying the proper pulse to the cathode gate (without the anode gate and associated circuitry required for the SCS). A consequence of this turn-off capability is an increase in the magnitude of the required gate current for triggering. - For an SCR and GTO of similar maximum rms current ratings, the gate-triggering current of a particular SCR is 30 μA, whereas the triggering current of the GTO is 20 mA. The turn-off current of a GTO is slightly larger than the required triggering current. - The maximum rms current and dissipation ratings of GTOs manufactured today are limited to about 3 A and 20 W, respectively. - A second very important characteristic of the GTO is improved switching characteristics. The turn-on time is similar to that of the SCR (typically 1 µs), but the turn-off time of about the same duration (1 µs) is much smaller than the typical turn-off time of an SCR (5 µs to 30 μς). - The fact that the turn-off time is similar to the turn-on time rather than considerably larger permits the use of this device in high-speed applications. - A typical GTO and its terminal identification are shown in the Figure. - The GTO gate input characteristics and turn-off circuits can be found in a comprehensive manual or specification sheet. - The majority of the SCR turn-off circuits can also be used for GTOs. - Some of the areas of application for the GTO include counters, pulse generators, multivibrators, and voltage regulators. # Simple Sawtooth Generator Employing a GTO - The Figure is an illustration of a simple sawtooth generator employing a GTO and a Zener diode. - When the supply is energized, the GTO will turn on, resulting in the short-circuit equivalent from anode to cathode. - The capacitor C₁ will then begin to charge toward the supply voltage as shown in the Figure. - As the voltage across the capacitor C₁ charges above the Zener potential, a reversal in gate-to-cathode voltage will result, establishing a reversal in gate current. - Eventually, the negative gate current will be large enough to turn the GTO off. - Once the GTO turns off, resulting in the open-circuit representation, the capacitor C₁ will discharge through the resistor R₃ . - The discharge time will be determined by the circuit time constant τ=R₃C₁. The proper choice of R₃ and C₁ will result in the sawtooth waveform as shown in the Figure. Once the output potential V₀ drops below V₂, the GTO will turn on and the process will repeat. # 4. Light Activated SCR (LASCR) - The next in the series of pnpn devices is the light-activated SCR (LASCR). As indicated by the terminology, it is an SCR whose state is controlled by the light falling on a silicon semiconductor layer of the device. - The basic construction of an LASCR is shown in the Figure. As indicated, a gate lead is also provided to permit triggering the device using typical SCR methods. - Note also in the figure that the mounting surface for the silicon pellet is the anode connection for the device. - The graphical symbols most commonly employed for the LASCR are provided in the Figure. - The maximum current (rms) and power (gate) ratings for commercially available LASCRs are about 3A and 0.1W, respectively. - The characteristics (light triggering) of a representative LASCR are provided in the Figure. Note in this figure that an increase in junction temperature results in a reduction in light energy required to activate the device. # LASCR Applications - Some of the areas of application for the LASCR include optical light controls, relays, phase control, motor control, and a variety of computer applications. # (i) Light Switching Circuits - One interesting application of an LASCR is in the light switching circuits as shown in the Figure. - Only when light falls on LASCR₁ and LASCR₂, will the short-circuit representation for each be applicable and the supply voltage appear across the load. For the parallel circuit, light energy applied to LASCR₁ or LASCR₂ will result in the supply voltage appearing across the load. - The LASCR is most sensitive to light when the gate terminal is open. Its sensitivity can be reduced and controlled somewhat by the insertion of a gate resistor, as shown in the Figure. # (ii) Latching Relay - A second application of the LASCR appears in the given Figure. - It is the semiconductor analog of an electromechanical relay. - Note that it offers complete isolation between the input and the switching element. The energizing current can be passed through a light-emitting diode or a lamp, as shown in the figure. - The incident light will cause the LASCR to turn on and permit a flow of charge (current) through the load as established by the dc supply. The LASCR can be turned off using the reset switch S₁. - This system offers the additional advantages over an electromechanical switch of long life, microsecond response, small size, and the elimination of contact bounce. # 5. Shockley Diode - The Shockley diode is a four-layer pnpn diode with only two external terminals, as shown in the Figure with its graphical symbol. - The characteristics of the device are exactly the same as those encountered for the SCR with Iɢ=0. - As indicated by the characteristics, the device is in the "off" state (open-circuit representation) until the breakover voltage Vʙʀ is reached, at which time avalanche conditions develop and the device turns on (short-circuit representation). - One common application of the Shockley diode is shown in the Figure, where it is employed as a trigger switch for an SCR. - When the circuit is energized, the voltage across the capacitor will begin to change toward the supply voltage. Eventually, the voltage across the capacitor will be sufficiently high to first turn on the Shockley diode and then the SCR. - The charge equation of the capacitor: - Vᴄ= Vᵥ+ (V – Vᵥ)(1 - e⁻ᵗ/(ᴿᶜ)) - Vᴄ=V–(V – Vᵥ)e⁻ᵗ/(ᴿᶜ) - Vᴄ=V–(V – Vᵥ)e⁻ᵗ/τ, τ=RC → time constant. # 6. Diac - The diac is basically a two-terminal parallel-inverse combination of semiconductor layers that permits triggering in either direction. - The characteristics of the device, presented in the Figure, clearly demonstrate that there is a breakover voltage in either direction. - This possibility of an on condition in either direction can be used to its fullest advantage in ac applications. - The basic arrangement of the semiconductor layers of the diac is shown in the Figure, along with its graphical symbol. - Note that neither terminal is referred to as the cathode. Instead, there is an anode 1 (or electrode 1) and an anode 2 (or electrode 2). When anode 1 is positive with respect to anode 2, the semiconductor layers of particular interest are p₁n₂p₂ and n₃. For anode 2 positive with respect to anode 1, the applicable layers are p₂n₂p₁ and n₁ . - For the unit appearing in the Figure, the breakdown voltages are very close in magnitude but may vary from a minimum of 28 V to a maximum of 42 V. They are related by the following equation provided in the specification sheet: - Vʙʀ₁=Vʙʀ₂ ± 0.1Vʙʀ₂ - The current levels (Iʙʀ₁=Iʙʀ₂) are also very close in magnitude for each device. For the unit in the Figure, both current levels are about 200 μA = 0.2 mA. # 7. Triac - The triac is fundamentally a diac with a gate terminal voltage Vɢᴛ for controlling the turn-on conditions of the bilateral device in either direction. In other words, for either direction the gate current can control the action of the device in a manner very similar to that demonstrated for an SCR. - The graphical symbol for the device and the distribution of the semiconductor layers are provided in the Figure with photographs of the device. For each possible direction of conduction, there is a combination of semiconductor layers whose state will be controlled by the signal applied to the gate terminal. - The characteristics, however, of the triac in the first and third quadrants are somewhat different from those of the diac, as shown in the following Figure. - Note the holding current in each direction doesn't present in the characteristics of the diac. - One fundamental application of the triac is presented in the given Figure. In this capacity, it is controlling the ac power to the load by switching on and off during the positive and negative regions of the input sinusoidal signal. - The action of this circuit during the positive portion of the input signal is very similar to that encountered for the Shockley diode. - The advantage of this configuration is that during the negative portion of the input signal, the same type of response will result since both the diac and the triac can fire in the reverse direction. The resulting waveform for the current through the load is provided in the Figure. By varying the resistor R, one can control the conduction angle. There are units available that can handle in excess of 10-kW loads.