GuruIntroduction
The activation of a thyristor is an essential part of how it functions, deciding when the device transitions from not conducting to conducting. It is crucial to comprehend the different types and methods of thyristor triggering in order to design and manage power electronic circuits efficiently. In this detailed blog post, we will examine the complexities of thyristor triggering, discussing its various types, operational principles, and widespread uses in different sectors.
Understanding Thyristor Triggering
A thyristor is a three-terminal semiconductor device consisting of anode, cathode, and gate. It functions in two primary states: the forward block state (not conducting) and the forward conduct state (conducting). Triggering involves starting the conduction of the thyristor by introducing a particular signal or voltage to its gate terminal.
Types of Thyristor Triggering
A thyristor can be switched from a nonconducting state to a conducting state in several ways described as follows.
1.Forward Voltage Triggering
When anode-to-cathode forward voltage is increased with gate circuit open, the reverse biased junction `J_2` will have an avalanche breakdown at a voltage called forward breakover voltage `V_{BO}`. At this voltage, a thyristor changes from OFF state (high voltage with low leakage current) to ON-state characterized by a low voltage across it with large forward current. The forward voltage-drop across the SCR during the ON state is of the order of 1 to 1.5 V and increases slightly with load current.
2.Thermal Triggering (Temperature Triggering)
Like any other semiconductor, the width of the depletion layer of a thyristor decreases on increasing the junction temperature. Thus, in a thyristor when the voltage applied between the anode and cathode is very near to its breakdown voltage, the device can be triggered by increasing its junction temperature.By increasing the temperature to a certain value (within the specified-limits), a situation comes when the reverse biased junction collapses making the device conduct.This method of triggering the device by heating is known as the thermal triggering process.
3.Radiation Triggering (Light Triggering)
In this method, as the name suggests, the energy is imparted by radiation. Thyristor is bombarded by energy particles such as neutrons or photons. With the help of this external energy, electron-hole pairs are generated in the device, thus increasing the number of charge carriers. This leads to instantaneous flow of current within the device and the triggering of the device. For radiation triggering to occur, the device must have high value of rate of change of voltage `({dv}/dt)`. Light activated silicon controlled rectifier (LASCR) and light activated silicon controlled switch (LASCS) are the examples of this type of triggering.
4.`left(frac{dv}{dt}right)` Triggering
We know that with forward voltage across the anode and cathode of a device,the junctions `J_1` and `J_3` are forward biased, whereas the junction `J_2` becomes reverse biased. This reverse biased junction `J_2` has the characteristics of a capacitor due to charges existing across the junction. If a forward voltage is suddenly applied, a charging current will flow tending to turn the device ON. If the voltage impressed across the device is denoted by V, the charge by Q and the capacitance by `C_j` , then
`i_c=left(frac{dQ}{dt}right)=frac d{dt}left(C_jVright)=C_jleft(frac{dV}{dt}right)+Vleft(frac{dC_j}{dt}right)`
The rate of change of junction capacitance may be negligible as the junction capacitance is almost constant. The contribution to charging current by the later term is negligible.Hence above equation reduce to,
`i_c=C_jleft(frac{dV}{dt}right)`
Therefore, if the rate of change of voltage across the device is large, the device may turn-on even though the voltage appearing across the device is small.
5.Gate Triggering
This is the most commonly used method for triggering SCRs. In laboratories,almost all the SCR devices are triggered by this process. By applying a positive signal at the gate terminal of the device, it can be triggered much before the specified breakover voltage. The conduction period of the SCR can be controlled by varying the gate signal within the specified values of the maximum and minimum gate currents.For gate triggering, a signal is applied between the gate and the cathode of the device. Three types of signals can be used for this purpose. They are either d.c. signals, pulse signals or ac signals.
1.D.C. Gate Triggering
- In this type of triggering, a d.c. voltage of proper magnitude and polarity is applied between the gate and the cathode of the device in such a way that the gate becomes positive with respect to the cathode.When the applied voltage is sufficient to produce the required gate current, the device starts conducting.
- One drawback of this scheme is that both the power and control circuits are d.c. and there is no isolation between the two.Another disadvantage of this process is that a continuous d.c. signal has to be applied, at the gate causing more gate power loss.
2.A.C. Gate Triggering
- A.C. source is most commonly used for the gate signal in all application of thyristor control adopted for A.C. applications. This scheme provides the proper isolation between the power and the control circuits.The firing angle control is obtained very conveniently by changing the phase angle of the control signal.
- However, the gate drive is maintained for one half cycle after the device is turned ON, and a reverse voltage is applied between the gate and the cathode during the negative half cycle. The drawback of this scheme is that a separate transformer is required to step down the a.c. supply, which adds to the cost.
3.Pulse Gate Triggering
- This is the most popular method for triggering the device. In this method, the gate drive consists of a single pulse appearing periodically or a sequence of high frequency pulses. This is known as carrier frequency gating. A pulse transformer is used for isolation.
- The main advantage of this method is that there is no need of applying continuous signals and hence,the gate losses are very much reduced. Electrical isolation is also provided between the main device supply and its gating signals.
Triggering Characteristics of thyristor
The static characteristics gives no indication as to the speed at which the SCR is capable of being switched from the forward blocking voltage to the conducting state and vice-versa. However, the transition from one state to the other does not take place instantaneously, it takes a finite period of time. This is illustrated in Fig. 1. As shown, the total turn-on time ton of the SCR is subdivided into three distinct periods, called the delay time, rise time and spread time. These time periods are defined in terms of the waveforms of the anode voltage and current obtained in a circuit in which the anode-load consists of a pure-resistance.
1.Delay time (`t_d`)
- This is the time between the instant at which the gate current reaches 90% of its final value and the instant at which the anode current reaches 10% of its final value.It can also be defined as the time during which anode voltage falls from `V_a` to 0.9 `V_a`, where `V_a` is the initial value of the anode voltage.
- The gate current has non-uniform distribution of current density over the cathode surface due to the p-layer. Its value is much higher near the gate but decreases rapidly as the distance from the gate increases. It shows that during `t_d`,anode current flows in a narrow region near the gate where gate current density is the highest.
2.Rise Time (`t_r`)
- This is the time required for the anode current to rise from 10 to 90% of its final value. It can also be defined as the time required for the forward blocking off-state voltage to fall from 0.9 to 0.1 of its initial value-OP.
- This time is inversely proportional to the magnitude of gate current and its build up rate. Thus, `t_r` can be minimized if high and steep current pulses are applied to the gate.
- For series RL circuit, the rate of rise of anode current is slow,therefore, tr is more and for the RC series circuit, `({di}/dt)` is high thus `t_r` is less.
During rise-time, turn-on losses are the highest due to high anode voltage `V_a` and large anode current `I_t` occuring together in the thyristor.
3.Spread-time (`t_s`)
- The spread time is the time required for the forward blocking voltage to fall from 0.1 to its value to the on-state voltage drop (1 to 1.5 V). After the spread time, anode current attains steady-state values and the voltage drop across SCR is equal to the on-state voltage drop of the order of 1 to 1.5 V.
Figure 1
4.Turn-on Time (`T_{on}`)
- This is the sum of the delay time, rise-time and spread time. This is typically of the order of 1 to 4 µs, depends upon the anode circuit parameters and the gate signal waveshapes.The width of the firing pulse should, therefore, be more than 10 µs, preferably in the range of 20 to 100 µs. The amplitude of the gate-pulse should be 3 to 5 times the minimum gate current required to trigger the SCR.
- From Fig. 1, it is noted that during rise-time, the SCR carries a large forward current and supports an appreciable forward voltage. This may result in high instantaneous power dissipation creating local internal hot spots which could destroy the device. It is, therefore, necessary to limit the rate of rise of current. Normally, a small inductor, called di/dt inductor is inserted in the anode circuit to limit the di/dt of the anode current.
The shadow area under the power-curve in Fig. 1 represents the switching loss of the device. This loss may be significant in high-frequency applications.
Applications of Thyristor Triggering
Thyristor triggering is integral to a wide range of applications across industries:
- Power Control Systems: The function of thyristors in power control systems includes voltage regulation, phase regulation, and power switching, in industrial equipment, motor drives, and heating systems.
- Inverters and Converters: Thyristors enable AC to DC conversion, DC to AC inversion, and frequency control inverter and converters for renewable energy sources, UPS systems, and motor drives.
- Electric Vehicles (EVs): Thyristor is utilized in EV powertrain for motor control, battery charging/discharging and regenerative braking and hence the EV powertrain becomes efficient and reliable.
- HVDC Transmission: HVDC transmission systems employ thyristor-controlled converters based on the triggering of pulse width modulation for the purpose of long-distance power transfer, grid interconnection as well as renewables integration.
- Welding Equipment: The thyristors are used in welding machines to ensure precision level current control, arc stability, and welding process modification among others.
- Light Dimmers and Controllers: Thyristor-based light dimmers and controllers which adaptable level of brightness, ambiance and energy saving provide solutions in the lighting systems for residential, commercial, and industrial use.
- Power Factor Correction: Thyristors made it possible to integrate power factor correction (PFC) circuits to reactive power compensation, thereby improving energy efficiency and grid stability.
- HVAC Systems: Thyristor triggering is used by HVAC(Heating, ventilation, and air conditioning) systems whereby it works for temperature control, fan speed modulation and energy efficient operation.
Conclusion
In summary, it is crucial to comprehend the varieties and processes of thyristor activation in order to create effective and dependable power electronic systems for a range of uses. Advancements in thyristor triggering technologies will continue to improve due to ongoing research and development efforts, resulting in enhanced performance.