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| News > Industry News |
| Design and application of bidirectional thyristor |
| Time： 2018-06-20 |
The design and parameter selection method of bidirectional silicon controlled rectifier are analyzed, and the installation method of bidirectional silicon controlled rectifier is introduced.
Characteristics of bidirectional thyristor
Bidirectional thyristor can be considered as a pair of anti parallel connection of common SCR integration, working principle is the same as ordinary one way SCR. Figure 1 is the basic structure of bidirectional thyristor and its equivalent circuit. It has two main electrodes T1 and T2, a gate G, and the gate enables the device to be triggered in the positive and negative two directions of the main electrode, so the bidirectional thyristor has symmetrical volt ampere characteristics in the first and third quadrants. Bidirectional thyristor plus positive and negative trigger pulses can trigger the tube to turn on, so there are four triggering modes.
Application of bidirectional thyristor
For the normal use of bidirectional thyristor, the main parameters should be mastered quantitatively. The appropriate selection of bidirectional thyristor and the corresponding measures should be taken to meet the requirements of the parameters.
Introduction
In 1958, the first industrial silicon controlled silicon control (SCR) was developed from the Ge Corp in the United States. The conversion and control of electrical energy from the rotating converter and static ion converter into the converter age of power semiconductor devices. The SCR is divided into unidirectional silicon controlled silicon and bidirectional thyristor. One way SCR is commonly used for overcurrent and overvoltage protection circuits of color TV. Bidirectional thyristors are commonly used in AC regulating circuits, such as dimmer lamps and AC power controls in fully automatic washing machines.
Bidirectional thyristor is developed on the basis of normal silicon controlled silicon. It can not only replace two thyristor, but also a trigger circuit. It is an ideal AC switch device at present. It has always been the main power control device in the home appliance industry. In recent years, with the development of semiconductor technology, high-power bidirectional thyristor has been constantly emerging and widely used in the field of variable flow and frequency conversion, and the application technology of SCR is becoming more and more mature. This paper mainly discusses the design and application of bidirectional thyristors widely used in household appliances industry.
Pressure level selection: the smaller values in the VDRM (off state repetition peak voltage) and the VRRM (reverse repetitive peak voltage) are usually rated as the rated voltage of the device. When selecting, the rated voltage should be 2~3 times the peak value of the normal working voltage as an allowable operating overvoltage margin.
Determination of current: since bidirectional thyristors are commonly used in AC circuits, they do not use the average value and use the RMS to represent their rated current values. Because the overload capacity of thyristor is smaller than that of general electromagnetic devices, the current value of silicon controlled rectifier used in household appliances is 2~3 times of the actual working current value. At the same time, the peak current of thyristor withstand repetitive peak voltage VDRM and reverse repetitive peak voltage VRRM should be less than the IDRM and IRRM specified by the device.
The choice of on state (peak) voltage VTM: it is the transient peak voltage drop of thyristor when multiplying the rated current. In order to reduce the heat loss of SCR, VTM SCR should be chosen as far as possible.
Maintain current: IH is the minimum main current necessary to maintain thyristor state. It is related to junction temperature. The higher the junction temperature, the smaller the IH.
• resistance to voltage rise rate: dv/dt refers to the rising slope of the voltage at the off state, which is a key parameter to prevent false triggering. This value overrun will cause misalignment of SCR. Because of the manufacturing process of thyristor, parasitic capacitance between A2 and G is determined, as shown in Figure 2. We know that the change of dv/dt will produce an equivalent current at the two ends of the capacitor, and this current will become Ig, that is, the trigger current, which leads to false triggering.
The rise rate of switching voltage is dVCOM/dt. When driving high resistance load, the phase shift between load voltage and current waveform usually occurs. When the load current is zero, the bidirectional thyristor switches, because the phase difference voltage is not zero. The voltage is immediately blocked by the bidirectional thyristor. The generated switching voltage rise rate (dVCOM/dt), if more than the allowed value, will force the bidirectional thyristor to return to the state because the carrier has no sufficient time to withdraw from the junction.
The bearing capacity of high dVCOM/dt is influenced by two conditions:
DICOM/dt - load current drop rate when switching. When dICOM/dt is high, the capacity of dVCOM/dt is decreased.
The higher the junction temperature Tj, the lower the dVCOM/dt bearing capacity. If the dVCOM/dt permissible value of bidirectional thyristor may be exceeded, to avoid false triggering, the RC buffer circuit can be installed between T1 and T2 to limit the voltage rise rate. The capacitor of 47~100 Omega, which can withstand the carbon film resistance of the surge current, and the capacitance of 0.01 mu F~0.47 F, will quickly recover from the reverse peak to zero current after the main current is over zero in the process of switching off the thyristor. This process can produce a peak voltage of 5-6 times the normal working peak voltage at both ends of the component. It is generally recommended to connect the resistance capacitance absorption loop as close as possible to the component itself.
The voltage change rate is dvD/dt in the disconnected state. If the cut-off bi-directional thyristor (or gate sensitive thyristor) has a high voltage change rate, although it does not exceed VDRM, the internal current of capacitance can produce enough gate current and trigger the device to pass. The gate sensitivity increases with the temperature. In case of such a problem, a RC buffer circuit should be added between T1 and T2 (or between anode and cathode) to limit dvD/dt.
The inhibition of current rise rate: the influence of current rise rate mainly lies in the following two aspects:
(1) dIT/dt (current rising rate during conduction) - when the bidirectional thyristor or gate current is triggered by a gate current triggered, the gate near the gate immediately passes through, and then rapidly expands to the entire effective area. There is a limit in this late time.
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