“In 1958, starting from the successful development of the first industrial thyristor by General Electric in the United States, the conversion and control of electric energy has entered the era of converters composed of power semiconductor devices from rotating converter units and stationary ion converters. SCR is divided into one-way SCR and two-way SCR. One-way thyristors are generally used in the overcurrent and overvoltage protection circuits of color TVs.
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In 1958, starting from the successful development of the first industrial thyristor by General Electric in the United States, the conversion and control of electric energy has entered the era of converters composed of power semiconductor devices from rotating converter units and stationary ion converters. SCR is divided into one-way SCR and two-way SCR. One-way thyristors are generally used in the overcurrent and overvoltage protection circuits of color TVs. Triacs are generally used in AC adjustment circuits, such as dimming desk lamps and AC power control in fully automatic washing machines.
Triac is developed on the basis of ordinary thyristor. It can not only replace two thyristors connected in parallel with reverse polarity, but also only needs one trigger circuit. It is currently an ideal AC switching device. It is the main power control device in the home appliance industry. In recent years, with the development of semiconductor technology, high-power triacs have continuously emerged and are widely used in the fields of current conversion and frequency conversion, and the application technology of triacs has become increasingly mature. This article mainly discusses the design and application of triacs widely used in the home appliance industry.
Features of Triac
The triac can be considered as the integration of a pair of anti-parallel connection of ordinary thyristors, and the working principle is the same as that of ordinary one-way thyristors. Figure 1 shows the basic structure of the triac and its equivalent circuit. It has two main electrodes T1 and T2, and a gate G. The gate makes the device trigger conduction in both the positive and negative directions of the main electrode. So the triac has symmetrical volt-ampere characteristics in the first and third quadrants. Triac gate plus positive and negative trigger pulses can trigger the tube to turn on, so there are four trigger modes.
Figure 1 Triac structure and equivalent circuit
Triac applications
In order to use bidirectional thyristor normally, it is necessary to quantitatively grasp its main parameters, make appropriate selection of bidirectional thyristor and take corresponding measures to meet the requirements of each parameter.
Selection of withstand voltage level: usually the smaller value of VDRM (off-state repetitive peak voltage) and VRRM (reverse repetitive peak voltage) is marked as the rated voltage of the device. When selecting, the rated voltage should be 2~3 times the normal working peak voltage as the allowable operating overvoltage margin.
Determination of current: Since the triac is usually used in AC circuits, it does not use the average value but the effective value to express its rated current value. Since the overload capacity of the SCR is smaller than that of general electromagnetic devices, the current value of the SCR used in general household appliances is 2 to 3 times the actual working current value. At the same time, the peak current when the SCR is subjected to the off-state repetitive peak voltage VDRM and the reverse repetitive peak voltage VRRM should be less than the IDRM and IRRM specified by the device.
The selection of the on-state (peak) voltage VTM: it is the transient peak voltage drop of the SCR at a specified multiple of the rated current. In order to reduce the heat loss of the thyristor, a thyristor with a small VTM should be selected as much as possible.
Maintenance current: IH is the minimum main current necessary to maintain the on-state of the thyristor. It is related to the junction temperature. The higher the junction temperature, the smaller the IH.
Resistance of voltage rise rate: dv/dt refers to the rising slope of the voltage in the off state, which is a key parameter to prevent false triggering. If this value exceeds the limit, it may lead to misconduction of the thyristor. Because the manufacturing process of the thyristor determines that there will be a parasitic capacitance between A2 and G, as shown in Figure 2. We know that the change of dv/dt will have an equivalent current at both ends of the capacitor, and this current will become Ig, that is, a trigger current appears, leading to false triggering.
Figure 2 Equivalent schematic diagram of triac
Switching voltage rise rate dVCOM/dt. When driving a highly reactive load, a substantial phase shift usually occurs between the waveforms of the load voltage and current. When the load current crosses zero, the triac switches, because the phase difference voltage is not zero. At this time, the triac must immediately block the voltage. If the resulting switching voltage rise rate (dVCOM/dt) exceeds the allowable value, it will force the triac to return to the conducting state, because the carriers do not have sufficient time to withdraw from the junction, as shown in Figure 3.
Figure 3 Current and voltage changes during switching
High dVCOM/dt endurance is affected by two conditions:
dICOM/dt-load current drop rate during switching. If dICOM/dt is high, the dVCOM/dt endurance will decrease. The higher the junction temperature Tj, the lower the dVCOM/dt endurance. If the allowable value of dVCOM/dt of the triac may be exceeded, in order to avoid false triggering, an RC buffer circuit can be installed between T1 and T2 to limit the voltage rise rate. Usually choose the carbon film resistor of 47~100Ω that can withstand the inrush current, and the capacitor of 0.01μF~0.47μF. During the turn-off process of the thyristor, the main current will quickly recover from the reverse peak value to zero current after the main current crosses zero. A spike voltage that is 5-6 times the normal working peak voltage is generated at both ends of the component. It is generally recommended to connect a resistance-capacitance absorption circuit as close as possible to the component itself.
The voltage change rate dvD/dt in the disconnected state. If the cut-off triac (or sensitive gate thyristor) has a high rate of voltage change, even if it does not exceed the VDRM, the capacitive internal current can generate a large enough gate current and trigger the device to turn on. The gate sensitivity increases with temperature. If such a problem occurs, an RC snubber circuit should be added between T1 and T2 (or between anode and cathode) to limit dvD/dt.
Suppression of the current rise rate: The influence of the current rise rate is mainly manifested in the following two aspects:
①dIT/dt (current rise rate when conducting)-When the triac or thyristor is turned on under the trigger of the gate current, the vicinity of the gate is turned on immediately, and then rapidly expands to the entire effective area. This late time has a limit, that is, the allowable value of the load current rise rate. Too high dIT/dt may cause partial burnout and short-circuit T1-T2. If dIT/dt is restricted to a lower value during the process, the triac may survive. Therefore, if the VDRM of the triac may be exceeded during a serious and abnormal power supply transient or the dIT/dt when it is turned on may be exceeded, a few μH unsaturated (hollow) can be connected in series to the load. inductance.
②dICOM/dt (switching current rate of change) ―The factors that lead to high dICOM/dt values are: high load current, high grid frequency (assuming sine wave current) or non-sine wave load current, the switching current rate of change caused by them exceeds the maximum The allowable value makes the triac even unable to support the small dV/dt when the 50Hz waveform rises from zero. Adding a few mH of inductance in series with the load can limit dICOM/dt.
・In order to solve the problems caused by high dv/dt and di/dt, Hi-Com triacs can also be used, which is different from the internal structure of traditional triacs. One of the differences is that the two internal “thyratrons” are better separated, reducing the mutual influence. This brings the following benefits:
①High dVCOM/dt. It can control reactive loads, and no buffer circuit is needed in many occasions to ensure fault-free switching. This reduces the number of components, board size and cost, and also eliminates the power dissipation of the snubber circuit.
②High dICOM/dt. The performance of switching high-frequency current or non-sine wave current is greatly improved, without the need to connect inductance in series with the load to limit dICOM/dt.
③High dvD/dt (voltage change rate in disconnected state). Triacs are more sensitive at high temperatures. At high temperature, when it is in the cut-off state, it is likely to be turned on due to false triggering under high dV/dt. Hi-Com triacs reduce this tendency. Therefore, it can be used in high-temperature electrical appliances to control resistive loads, such as kitchens and heating appliances, while traditional triacs cannot be used.
Selection of gate parameters
Gate trigger current-In order to make the thyristor trigger reliably, the trigger current Igt is selected to be α times the max value at 25 degrees, and α is the gate trigger current-junction temperature characteristic coefficient, which can be obtained from the data manual, and the lowest operating characteristic in the characteristic curve The coefficient at temperature. If there is no special requirement for the working environment temperature of the device, usually α is greater than 1.5 times.
Gate voltage drop-you can choose β times the max value when Vgt is 25 degrees. β is the gate trigger voltage-junction temperature characteristic coefficient, which can be obtained by checking the data sheet, and the coefficient at the lowest operating temperature in the characteristic curve is taken. If there is no special requirement for the working environment temperature of the device, usually β is 1~1.2 times.
Trigger resistance―Rg=(Vcc-Vgt)/Igt
Trigger pulse width-In order to turn on the thyristor (or triac), in addition to the gate current RIGT, the load current must reach RIL (holding current), and consider the lowest temperature that may be encountered. Therefore, it can be more than 2 times the pulse width Tgw for reliable triggering of the thyristor at 25 degrees.
In an environment flooded with Electronic noise, if the interference voltage exceeds the trigger voltage VGT and there is sufficient gate current, false triggering will occur, causing the triac to switch. The first line of defense is to reduce clutter in adjacent spaces. The shorter the gate wiring, the better, and make sure that the common return line of the gate drive circuit is directly connected to the TI pin (the thyristor is the cathode). If the gate wiring is a hard wire, you can use a spiral double wire, or simply use a shielded wire. These necessary measures are all to reduce the absorption of clutter. In order to increase the resistance to electronic noise, a resistor of 1kΩ or less can be connected in series between the gate and T1 to reduce the sensitivity of the gate. If a high-frequency bypass capacitor has been used, it is recommended to add a resistor between the capacitor and the gate to reduce the peak value of the capacitor current passing through the gate and reduce the possibility of overcurrent burnout in the triac gate area.
Control of junction temperature Tj: For long-term reliable operation, Rth ja should be kept low enough to maintain Tj not higher than 80% Tjmax, and its value corresponds to the highest possible ambient temperature.
Triac installation
For triacs with small load or short current duration (less than 1 second), it can work in free space. But in most cases, it needs to be installed on a radiator or a heat-dissipating bracket. In order to reduce the thermal resistance, the thyristor and the radiator should be coated with thermal grease.
There are three main methods for fixing the triac to the radiator, clamp crimping, bolting and riveting. The installation tools for the first two methods are easy to obtain. In many cases, riveting is not a recommended method, so this article will not introduce it.
Clip crimping
This is the recommended method, and the thermal resistance is minimal. The clamp exerts pressure on the plastic encapsulation of the device. The same applies to non-insulated packages (SOT82 and SOT78) and insulated packages (SOT186 F-pack and newer SOT186A X-pack). Note that SOT78 is TO220AB.
Bolted
The SOT78 assembly comes with a M3 set of installation parts, including a rectangular washer, which is placed between the bolt head and the joint piece. No force should be applied to the plastic body of the device.
During the installation process, the screwdriver must not exert any force on the plastic body of the device.
The surface of the radiator in contact with the tabs should be treated to ensure that it is flat, and the allowable deviation is 0.02mm on 10mm.
The installation torque (with washer) should be between 0.55Nm and 0.8Nm.
Avoid using self-tapping screws, because extrusion may cause bulges around the mounting holes and affect the thermal contact between the device and the heat sink. The installation torque cannot be controlled, which is also the disadvantage of this installation method. The device should be mechanically fixed first, and then the leads should be soldered. This can reduce undue stress on the leads.
In the thyristor design, suitable parameters and corresponding software and hardware designs are selected. The converter device composed of thyristors has the characteristics of energy saving and low cost, and it is currently developing rapidly in the industry.
In addition, the application market for thyristors is quite broad. For example, there are thyristors in the field of automation control, electromechanical fields, industrial electrical appliances, and home appliances. In the consumer market, U-shaped motors and electric resistance wires are also more applications for SCRs. Under normal circumstances, a triac (TRIAC) or combined with an AC power supply is generally used to achieve motor control, constant temperature and constant power control circuits.
Regardless of whether it is a thermal resistance wire or a U motor, due to the addition of a triac device such as TRIAC and a PID control algorithm, the related circuit design becomes very complicated, in order to make it easier for everyone to implement For the design of related products, I specially set up this live class to help everyone systematically understand the control circuit design of SCR in actual specific products.
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