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Analysis of High Power IGBT
2021/11/11 03:11:06

NTC thermistor chip for IGBT produced by EXSENSE Electronics Technology Co., Ltd. plays the role of temperature monitoring and temperature controlling in high power IGBT module, which is widely used in rail transit, battery management system (BMS) of new energy vehicle, smart home and other fields due to its conversion characteristics between AC and DC.

In order to ensure that the controller is not affected when the high-power IGBT driver works in the high-voltage environment, the drive pulse signal must be isolated before transmitted to the grid of IGBT. The general isolation methods are optical isolation and electromagnetic isolation, optical isolation includes optical coupling isolation and optical fiber isolation. Due to the relatively low isolation voltage, optical coupling isolation has problems in transmission delay, aging and reliability, which is rarely used in high-voltage applications where the DC bus voltage exceeds 800V. With pulse transformer isolation (electromagnetic isolation) can achieve relatively high isolation voltage, and high reliability of the transformer, transmission delay is small, which can achieve high switching frequency, there is no problem of aging. So in most high power IGBT drivers, pulse transformer is used as the isolating element to complete the isolated transmission of the driving signal.

The traditional pulse transformer used for driving is to directly drive IGBT or power MOS tube after isolating the amplified pulse signal. This working mode does not need a separate driving power supply, it is simple circuit design and low cost. However, when the duty cycle of the driving pulse varies widely, especially when the duty cycle is large, due to the transformer output waveform must have equal voltage-second product in one period, the output positive pulse amplitude may be reduced, so that IGBT can not be driven normally. Usually, the control pulse duty cycle is less than 50%. At the same time, the saturation of pulse transformer core also limits the conduction time of control pulse. In addition, there is distortion in the drive waveform, especially in the drive high power IGBT, due to the input capacitance of IGBT is relatively large, the secondary output of the pulse transformer drive pulse waveform is difficult to meet the drive requirements. Therefore, this drive mode is mainly used in low power switching power supply.

In order to facilitate the user to design the driving power supply, high power IGBT drive module usually has its own DC/DC converter. The DC/DC converter with high isolation voltage level does not require the user to design isolation power supply separately. The integrated isolation converter usually adopts half-bridge or push-pull structure. In order to increase the isolation voltage and simplify the control circuit of the converter, the closed-loop control is generally not taken, and the linear voltage stabilizer is added to the output end of the individual driver to realize the stability of the driving voltage. In order to reduce the volume of the transformer, the working frequency is more than 100khz. In high-voltage and high-power applications, the high isolation voltage tolerance must be required between the primary and secondary drivers according to different bus voltages, the bus voltage of 900V DC must have at least 4KV AC isolation voltage. Another factor that must be considered is DV/DT tolerance. When IGBT switches at high speed, it can produce very high DV/DT tolerance. This signal can be coupled to the primary control circuit through isolation transformer or pulse transformer, causing interference to the control circuit. Therefore, in the isolation transformer design also requires that it has a very small primary-secondary coupling capacitor, according to the specific requirements of DV/DT tolerance to determine the capacity of transformer coupling capacitor, usually less than 20PF.

Production process of transformer is the key to achieve the high isolation voltage, in order to increase the isolation voltage tolerance, reduce the coupling capacitance of primary-secondary or secondary, usually is to wind the winding severally, separated by insulating baffle in middle. Sometimes the core is coated with a thick insulating material or winded with three layers of insulating wire.

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