The hottest IGBT and single bidirectional thyristo

2022-08-16
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IGBT and single bidirectional thyristor

insulated gate bipolar transistor IGBT is a device composed of MOSFET and bipolar transistor. Its input pole is MOSFET and output pole is PNP transistor. Therefore, it can be regarded as a Darlington tube of MOS input. It combines the advantages of these two devices. It not only has the advantages of simple and fast MOSFET device driving, but also has the advantages of large capacity of bipolar devices around the cylindrical part of the drill cone. Therefore, it has been more and more widely used in modern power electronics technology

in medium and high-power switching power supply devices, IGBT has gradually replaced thyristor or GTO because of its simple control and driving circuit, high working frequency and large capacity. However, in the switching power supply device, because it works under the conditions of high frequency, high voltage and large current, it is easy to be damaged. In addition, as the front stage of the system, the power supply bears greater stress due to the influence of power fluctuations, lightning strikes and other reasons. Therefore, the reliability of IGBT is directly related to the reliability of the power supply. Therefore, in addition to considering derating when selecting IGBT, the protection design of IGBT is also a key link to be considered in power supply design

1igbt working principle

igbt equivalent circuit is shown in Figure 1. It can be seen from Figure 1 that if a driving positive voltage is applied between the gate and emitter of the IGBT, the MOSFET will be turned on, so that the collector and base of the PNP transistor will be in a low resistance state, so that the transistor will be turned on if the electromechanical device has a load; If the voltage between the gate and emitter of IGBT is 0V, the MOSFET is cut off, cutting off the supply of PNP transistor base current, making the transistor cut off

therefore, the safety and reliability of IGBT are mainly determined by the following factors:

-- the voltage between IGBT grid and emitter

-- voltage between IGBT collector and emitter

-- current flowing through IGBT Collector Emitter

-- junction temperature of IGBT

if the voltage between the IGBT grid and the emitter, that is, the driving voltage is too low, the IGBT cannot work stably and normally. If it is too high and exceeds the withstand voltage between the grid and the emitter, the IGBT may be permanently damaged; Similarly, if the allowable voltage between the collector and emitter of IGBT self-developed medium and advanced vibration testing machine exceeds the withstand voltage between the collector and emitter, the current flowing through the IGBT Collector Emitter exceeds the maximum current allowed by the collector emitter, and the junction temperature of IGBT exceeds the allowable value of its junction temperature, IGBT may be permanently damaged

2 protective measures

during circuit design, corresponding protective measures should be taken targeted at the factors affecting the reliability of IGBT

2.1 IGBT grid protection

the guaranteed value of IGBT grid emitter driving voltage VGE is ± 20V. If a voltage exceeding the guaranteed value is added between its grid and emitter, IGBT may be damaged. Therefore, a grid voltage limiting circuit should be set in the IGBT driving circuit. In addition, if there is an open circuit between the grid and emitter of IGBT, and a voltage is applied between the collector and emitter, with the change of collector potential, due to the existence of parasitic capacitance between the grid and collector and emitter, the grid potential increases, and there is current flow between the collector and emitter. At this time, if the collector and emitter are in a high voltage state, IGBT may be heated or even damaged. If the grid circuit is disconnected during the transportation or vibration of the equipment, and the voltage is applied to the main circuit without being noticed, the IGBT may be damaged. In order to prevent this kind of situation, a resistance of tens of K Ω should be connected between the grid and emitter of IGBT, and this resistance should be as close to the grid and emitter as possible. As shown in Figure 2

since IGBT is a complex of power MOSFET and PNP bipolar transistor, especially its gate is MOS structure, in addition to the above due protection, like other MOS structure devices, IGBT is also very sensitive to static voltage, so the following matters must be paid attention to when assembling and welding IGBT:

-- before touching IGBT by hand, electrostatic discharge on human body should be carried out before operation, And try not to touch the drive terminal part of the module. When it is necessary to contact, make sure that all the static electricity carried by the human body has been discharged at this time

-- in order to prevent static electricity from damaging IGBT during welding, the welding machine must be reliably grounded

2.2 overvoltage protection between collector and emitter

there are mainly two kinds of overvoltage, one is the high DC voltage applied to the IGBT collector emitter, and the other is the high surge voltage on the collector emitter

2.2.1 DC overvoltage

the cause of DC overvoltage is due to the abnormal input of the previous stage of the input AC power supply or IGBT. The solution is to carry out derating design when selecting IGBT; In addition, the input of IGBT can be cut off when this overvoltage is detected to ensure the safety of IGBT

2.2.2 surge voltage protection

due to the existence of distributed inductance in the circuit and the high switching speed of IGBT, when IGBT is turned off and the reverse recovery diode connected with it is reverse recovered, a large surge voltage ldi/dt will be generated, threatening the safety of IGBT

generally, the surge voltage waveform of IGBT is shown in Figure 3

in the figure: VCE is IGBT? Voltage waveform between electrode and emitter

ic is the collector current of IGBT

ud is the DC voltage of the input IGBT

vcesp=ud + ldic/dt, which is the peak value of surge voltage

if vcesp exceeds the Collector Emitter withstand voltage value Vces of IGBT, IGBT may be damaged. The main solutions are:

-- consider the design margin when selecting IGBT

-- adjust the RG of IGBT drive circuit during circuit design to make di/dt as small as possible

-- install the electrolytic capacitor close to IGBT as far as possible to reduce the distributed inductance and implement garbage classification and utilization

-- add buffer protection circuit according to the situation to bypass high-frequency surge voltage

since the buffer protection circuit plays a very important role in the safe work of IGBT, the types and characteristics of the buffer protection circuit are introduced here

-- the C buffer circuit is shown in Figure 4 (a). It uses thin-film capacitors and is installed close to IGBT. Its characteristic is that the circuit is simple. Its disadvantage is that the LC resonant circuit is composed of distributed inductors and buffer capacitors, which is easy to produce voltage oscillation, and the collector current is large when IGBT is turned on

-- RC buffer circuit is shown in Figure 4 (b), which is characterized by being suitable for chopper circuit, but when using high-capacity IGBT, the buffer resistance must be increased, otherwise, when opening

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