Design and Application Analysis of Infineon Triac

In 1958, starting from the first industrial thyristor developed by General Electric Company of the United States, the conversion and control of electric energy entered the era of converters composed of power semiconductor devices from rotating converter units and stationary ion converters. Thyristor unidirectional thyristor and Infineon triac. One-way thyristor is generally used for overcurrent and overvoltage protection circuits of color TVs. Infineon triacs are commonly used in AC regulation circuits such as dimming table lamps and AC power control in fully automatic washing machines.

Infineon two-way thyristor features

Infineon triacs can be thought of as a pair of anti-parallel connected common thyristors that work in the same way as ordinary unidirectional thyristors. Figure 1 shows the basic structure of Infineon triac and its equivalent circuit. It has two main electrodes T1 and T2, one gate G, and the gate enables the device to trigger conduction in both the positive and negative directions of the main electrode. Therefore, the Infineon triac has symmetrical volt-ampere characteristics in the first and third quadrants. Infineon's triac gate plus positive and negative trigger pulses can cause the tube to trigger conduction, so there are four trigger modes.

Infineon triac applications

In order to use Infineon triacs normally, it is necessary to quantitatively master the main parameters, and properly select Infineon triacs and take corresponding measures to meet the requirements of each parameter.

Introduction

Infineon triac is developed on the basis of ordinary thyristor. It can replace not only two thyristors with reverse polarity and parallel, but only one trigger circuit. It is an ideal AC switch device. Has been the main power control device in the home appliance industry. In recent years, with the development of semiconductor technology, high-power Infineon triacs have emerged and are widely used in the field of variable current and frequency conversion, and the application technology of thyristors is becoming more and more mature. This article focuses on the design and application of Infineon two-way thyristors, which are widely used in the home appliance industry.

• Selection of withstand voltage level: The smaller of VDRM (off-state repeated peak voltage) and VRRM (reverse repeat peak voltage) is usually labeled as the rated voltage of the device. When selected, the rated voltage should be 2~3 times of the normal working peak voltage as the allowable operating overvoltage margin.

• Determination of current: Since Infineon triac is commonly used in AC circuits, its rms value is represented by an rms value without an average value. Since the overload capability of the thyristor is smaller than that of the general electromagnetic device, the current value of the thyristor used in the general household appliance is 2 to 3 times the actual working current value. At the same time, the peak current when the thyristor is subjected to the off-state repeating peak voltage VDRM and the reverse repeating peak voltage VRRM should be less than the IDRM and IRRM specified by the device.

• On-state (peak) voltage VTM selection: It is the transient peak voltage drop at which the thyristor passes a specified multiple of rated current. In order to reduce the heat loss of the thyristor, VTM small thyristor should be selected as much as possible.

• Maintain current: IH is the minimum main current necessary to maintain the thyristor to maintain the on-state. It is related to the junction temperature. The higher the junction temperature, the smaller the IH.

Resistance against 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. Exceeding this value may cause the thyristor to be misdirected. Since the thyristor manufacturing process determines the parasitic capacitance between A2 and G, as shown in Figure 2. We know that the dv/dt change will have an equivalent current at both ends of the capacitor, and this current will become Ig, that is, the trigger current will appear, resulting in false triggering.

Switching voltage rise rate dVCOM/dt. When driving a highly reactive load, substantial phase shifts typically occur between the waveforms of the load voltage and current. The Infineon triac switches when the load current crosses zero because the phase difference voltage is not zero. The Infineon triac must immediately block this voltage. If the generated switching voltage rise rate (dVCOM/dt) exceeds the allowable value, the Infineon triac will be forced to return to the conducting state because the carrier does not have sufficient time to withdraw from the junction.

The high dVCOM/dt withstand capability is affected by two conditions:

dICOM/dt—Load current drop rate at switching. When dICOM/dt is high, the dVCOM/dt capacity is reduced.

The higher the junction temperature Tj, the lower the dVCOM/dt withstand capability. If the allowable value of dVCOM/dt of Infineon triac may be exceeded, to avoid false triggering, an RC snubber circuit can be placed between T1 and T2 to limit the rate of voltage rise. Generally, 47~100Ω carbon film resistors capable of withstanding inrush current, 0.01μF~0.47μF capacitors, the main current zero-crossing in the thyristor turn-off process will quickly return to zero current from the reverse peak. This process can be A peak voltage of 5-6 times the normal operating peak voltage is generated across the device. It is generally recommended to connect the RC tank as close as possible to the component itself.

The voltage change rate dvD/dt in the off state. If the cut-off Infineon triac (or gate-sensitive thyristor) has a high voltage change rate, although not exceeding VDRM, the capacitive internal current can generate a sufficiently large gate current and trigger the device to conduct. . The gate sensitivity increases with temperature. If such a problem occurs, an RC snubber circuit should be added between T1 and T2 (or between the anode and cathode) to limit dvD/dt.

· Suppression of current rise rate: The effect of current rise rate is mainly reflected in the following two aspects:

1dIT/dt (current rise rate during turn-on)—When the Infineon triac or thyristor is turned on under the gate current, the gate is immediately turned on and then rapidly spreads over the entire effective area. This late time has a limit, which is the permissible value of the load current rise rate. Excessive dIT/dt may cause partial burnout and short-circuit T1-T2. Infineon triacs may survive if the process limits dIT/dt to a lower value. Therefore, if the DDI/dt of the Infineon triac VDRM may be exceeded or turned on during a severe, abnormal power supply transient, a few μH of unsaturation may be placed in series on the load (hollow )inductance.

2dICOM/dt (switching current rate of change) - factors that cause high dICOM/dt values are: high load current, high grid frequency (assumed sinusoidal current) or non-sinusoidal load current, which cause the switching current rate to exceed the maximum The allowable value makes the Infineon triac not even support the dV/dt with a 50Hz waveform rising from zero. Adding a few mH of inductance and load in series can limit dICOM/dt.

· In order to solve the problems caused by high dv/dt and di/dt, Hi-Com Infineon triac can also be used, which is different from the internal structure of the traditional Infineon triac. One of the differences is that the two internal "thyristors" are better separated, reducing the mutual influence. This brings the following benefits:

1 high dVCOM/dt. It can control the reactive load and does not need a buffer circuit in many cases to ensure no failover. This reduces component count, backplane size and cost, and eliminates the power dissipation of the snubber circuit.

2 high dICOM/dt. The performance of switching high frequency currents or non-sinusoidal currents is greatly improved without the need for series inductance on the load to limit dICOM/dt.

3 high dvD/dt (voltage change rate in the off state). Infineon triacs are more sensitive at high temperatures. At high temperatures, when turned off, it is easy to turn on due to false triggering at high dV/dt. Hi-ComInfineon triacs reduce this tendency. This can be used in high temperature appliances to control resistive loads such as kitchens and heating appliances, while conventional Infineon triacs cannot be used.

·Selection of gate parameters:

Gate trigger current—In order to make the thyristor reliably trigger, the trigger current Igt selects α times the max value at 25 degrees, α is the gate trigger current—the junction temperature characteristic coefficient, which can be obtained from the data sheet, and the lowest work in the characteristic curve is taken. The coefficient at temperature. If there is no special need 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 value of max at Vgt 25 degrees. β is the gate trigger voltage—the junction temperature characteristic coefficient. Check the data sheet for the coefficient at the lowest operating temperature in the characteristic curve. If there is no special need 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 Infineon triac), in addition to the gate current ≧IGT, the load current must be ≧IL (carry current) and considered at the lowest possible temperature. . Therefore, it is possible to reliably trigger the pulse width Tgw of the thyristor at twice or more at 25 degrees.

In an environment with electronic noise, if the interference voltage exceeds the trigger voltage VGT and there is enough gate current, a false trigger will occur, causing the Infineon triac to switch. The first line of defense is to reduce clutter in the adjacent space. The shorter the gate wiring, the better, and ensure that the common return line of the gate drive circuit is directly connected to the TI pin (the cathode to the thyristor). If the gate wiring is hard, you can use a spiral double wire, or simply use a shielded wire. These necessary measures are to reduce the absorption of clutter. To increase resistance to electronic noise, a 1kΩ or less resistor can be placed in series between the gate and T1 to reduce the sensitivity of the gate. If a high frequency bypass capacitor is used, it is recommended to add a resistor between the capacitor and the gate to reduce the peak value of the capacitive current through the gate and reduce the possibility of overcurrent burnout in the Infineon triac gate region.

• Control of junction temperature Tj: For long-term reliable operation, it should be ensured that Rth j-a is low enough to maintain Tj not higher than 80% Tjmax, the value corresponding to the highest possible ambient temperature.

Infineon two-way thyristor installation

Infineon triacs with low load or short current duration (less than 1 second) work in free space. However, in most cases, it needs to be mounted on a heat sink or a heat sink. In order to reduce the thermal resistance, thermal grease should be applied between the thyristor and the heat sink.

There are three main methods of fixing Infineon triacs to the heat sink, crimping, bolting and riveting. The installation tools for the first two methods are easy to obtain. Riveting is not a recommended method in many situations and is not covered in this article.

Clamp crimp

This is the recommended method with minimal thermal resistance. The clip applies pressure to the plastic seal of the device. The same applies to non-insulated packages (SOT82 and SOT78) and insulative packages (SOT186 F-pack and newer SOT186A X-pack). Note that the SOT78 is the TO220AB.

Bolted

The SOT78 assembly comes with M3 kits, including rectangular washers, which are placed between the bolt head and the tab. No force should be applied to the plastic body of the device.

The screwdriver must not exert any force on the plastic body of the device during installation.

The surface of the heat sink that is in contact with the tab should be treated to ensure flatness, and the allowable deviation of 0.02 mm on 10 mm.

The mounting torque (with washer) should be between 0.55Nm and 0.8Nm.

Self-tapping screws should be avoided as they can cause bulging around the mounting holes and affect the thermal contact between the device and the heat sink. The mounting torque cannot be controlled and is a disadvantage of this type of installation.

The device should be mechanically fixed first and then soldered. This can reduce the undue stress of the leads.

Conclusion

In the thyristor design, the appropriate parameters and the corresponding hardware and software design are selected. The converter device composed of thyristor has the characteristics of energy saving and low cost, and has been rapidly developed in the industry.

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