Abstract: This paper analyzes the main interference sources and coupling paths of the IGBT drive circuit in a three-level inverter system. Based on this, some EMC design issues of the IGBT drive circuit are studied, focusing on fiber optic signal transmission, auxiliary power supply design, transient noise suppression, and PCB anti-interference design. Design solutions are also presented. Keywords: Three-level inverter; IGBT drive circuit; electromagnetic interference; electromagnetic compatibility 0 Introduction In recent years, diode-clamped three-level inverters have been widely studied in high-voltage, high-power applications. Compared with ordinary two-level inverters, three-level inverters improve the output voltage waveform, reduce system electromagnetic interference, and can achieve high-voltage output using devices with lower voltage ratings. The circuit topology is shown in Figure 1. The three-level inverter system structure is shown in Figure 2, mainly consisting of an uncontrolled rectifier circuit, a three-level inverter, a filter, a drive circuit, a sampling circuit, and a DSP digital control circuit. Six IGBT drive circuits with two drive signal outputs were used in the design. As can be seen from the system structure diagram, the IGBT drive circuit connects the digital control circuit and the inverter's main power circuit, which is crucial for the inverter's normal operation. Because the drive circuit is close to the IGBT device, and strong and weak electrical signals coexist within it, it is more susceptible to electromagnetic interference. Therefore, the EMC design of the IGBT drive circuit is a key issue affecting the overall performance of the inverter system. This paper analyzes the main interference sources and coupling paths that affect the IGBT drive circuit in a three-level inverter system, and focuses on the EMC design of the IGBT drive circuit. 1 Interference Sources and Coupling Paths To design the EMC of the IGBT drive circuit, it is essential to first consider the potential interference sources and coupling paths of interference noise in the entire three-level inverter system. 1.1 Switching Noise of Power Semiconductor Devices As shown in Figure 2, the inverter system structure diagram shows that the grid voltage is input to the three-level inverter after passing through the three-phase uncontrolled rectifier circuit, and then supplies power to the load after passing through the inverter circuit and filter circuit. The power diodes in the uncontrolled rectifier circuit and the devices (IGBTs) in the inverter circuit have high di/dt during switching, which may cause transient electromagnetic noise through the parasitic inductance of the lines or components. Due to the large power capacity of the devices, the switching noise is the most significant source of interference in the entire system, significantly impacting the stability of the IGBT drive circuit. 1.1.1 Switching Noise of Power Diodes: When a power diode turns on, the current increases rapidly, and the voltage also experiences a rapid surge, resulting in broadband electromagnetic noise. When the diode turns off, there is a reverse recovery current pulse. Due to its large amplitude and di/dt, a high induced voltage is generated under the parasitic inductance of the circuit, causing strong transient electromagnetic noise. Since power diodes are used in three-phase uncontrolled rectifier circuits with high input voltages, the electromagnetic noise during switching has a more severe impact on other parts of the system. The auxiliary power supply in the IGBT drive circuit and DSP control circuit is a high-frequency switching power supply, which uses many fast recovery diodes to form the rectifier circuit. The reverse recovery time of fast recovery diodes is typically on the order of nanoseconds; therefore, the transient electromagnetic noise they generate through lead inductance is also significant. 1.1.2 Switching Noise of IGBTs IGBTs are hybrid devices with both majority and minority carriers, and their switching speed is relatively fast. Therefore, the transient electromagnetic noise caused by current changes during switching is more severe. The main power circuit of a three-level inverter uses 12 IGBT devices and operates in high-voltage, high-frequency, and high-current environments. The electromagnetic noise generated during switching is also a major source of interference for the entire system. 1.2 Harmonic Interference Caused by Rectifier Circuit The grid voltage is input to the inverter section after passing through the uncontrolled rectifier circuit. Due to the switching on and off of the power diodes, the three-phase uncontrolled rectifier circuit will generate significant harmonic interference and electromagnetic noise during operation, as analyzed in the previous section. In addition, the uncontrolled rectifier circuit generates harmonic interference. Since the rectifier circuit is directly connected to the grid, the interference generated by the rectifier circuit itself and subsequent circuits will be conducted into the grid through the rectifier circuit, interfering with other equipment connected to the same grid. 1.3 Interference Caused by Potential Fluctuation During inverter operation, the emitter potential of the IGBTs fluctuates, and the potentials of different switching transistors differ significantly. Taking phase A of the circuit in Figure 1 as an example: when the upper bridge arm IGBTs Sa1 and Sa2 are turned on, the output of phase A is +Vdc, and the emitter potential of the IGBTs is also +Vdc. Similarly, when the lower bridge arm IGBTs Sa3 and Sa4 are turned on, the emitter potential of the IGBTs is 0, and when the two middle transistors Sa2 and Sa3 are turned on, the emitter potential is +Vdc/2. The reference potential of the IGBT gate drive signal is taken at the emitter (E) terminal of the IGBT. This requires the drive circuit to be directly connected to the power circuit, so the power supply potential of the drive circuit will also change with the IGBT potential. During inverter operation, such frequent and large-amplitude potential changes will generate significant electromagnetic interference to the drive circuit, especially between the two drive signals on the same drive board, which will interfere with each other and affect the normal operation of the circuit. 1.4 Coupling Paths of Electromagnetic Noise Electromagnetic noise can be coupled through conduction and radiation. In this inverter system, conduction coupling is the primary method. This means that the energy of electromagnetic noise is coupled to the affected circuit in the form of voltage or current through wires and other components (such as transformers). This paper mainly considers conducted coupling, which can affect the IGBT drive circuit. 1.4.1 Direct Conducted Coupling Direct conducted coupling is the most prevalent coupling method of electromagnetic noise in this system. Due to leakage resistance, parasitic inductance, and parasitic capacitance in the circuit wires, the effect of the equivalent impedance of the wires must be considered during EMC design. In this system, the IGBT devices operate at high frequencies, and the transient voltage generated by the parasitic inductance of the wires may damage the IGBTs and severely affect the drive circuit. Furthermore, the auxiliary power supply for the IGBT drive circuit uses a high-frequency switching power supply, and the electromagnetic noise generated by the power supply will also affect the IGBT drive signal through direct conducted coupling. 1.4.2 Common Impedance Coupling A three-level inverter is a complex system. There may be common impedances between the main power circuit, IGBT drive circuit, control circuit and auxiliary power supply. Electromagnetic noise generated by interference sources will affect the interfered circuit through common ground impedance coupling or common power supply impedance coupling. As for the IGBT drive circuit, this system uses 6 two-way output drive circuits, and each drive circuit board has an onboard auxiliary power supply. If the isolation and grounding of the auxiliary power supply are not properly designed, electromagnetic noise may be conducted through the common impedance of the auxiliary power supply; the interference signals between the drive circuits, especially the two paths on the same drive board, will affect each other and destroy the normal drive signal of the circuit. 2 EMC Design of Drive Circuit The IGBT drive circuit adopts the integrated drive module M57962L. In terms of anti-interference, it has the following advantages: (1) It has a high-speed optocoupler inside, which isolates the pulse control signal from the drive circuit. In this way, the digital control circuit and the drive circuit are electrically isolated, which can prevent interference caused by electrical coupling; (2) The gate drive adopts bipolar control voltage. Using negative gate voltage can obtain higher anti-interference performance. Figure 3 is a schematic diagram of the IGBT drive circuit. The control and feedback signals of the drive circuit are transmitted via optical fiber. HFBR-1522/2522 are optical signal transmitters and receivers; Von/Voff are onboard auxiliary power supplies. This paper will further analyze the electromagnetic compatibility design of the drive circuit from the following aspects. 2.1 Optical Fiber Transmission of Signals During the transmission of PWM signals, if the transmission line is long, strong electrical pulses will interfere with the PWM signal through the distributed capacitance and inductance of the transmission line. If the signal is interfered with or the delay is too large, the IGBT in the main circuit cannot be correctly turned on or off, which may cause a short circuit and damage the device. Anti-interference design for signal transmission is a key issue to consider in IGBT drive circuits. In this system, optical fiber is used to transmit signals between the digital control system and the drive circuit, which can effectively solve the anti-interference problem of PWM signal transmission. The principle of optical fiber signal transmission is shown in Figure 4. As can be seen, the transmitting and receiving circuits connected by optical fiber transmit signals through optical signals, without direct electrical connection, enabling precise transmission of PWM control signals. Fiber optic signal transmission not only solves the problem of strong and weak electrical isolation between power circuits and control circuits, minimizing electromagnetic interference, but also reduces delay and enables long-distance signal transmission. 2.2 Anti-interference Design of Auxiliary Power Supply Each driver circuit board requires an auxiliary power supply to provide two different level signals: +15.8V/-8.2V required by the driver chip M57962L, providing positive and negative bias signals to the IGBTs to be driven, respectively. Additionally, it needs to provide +5V power to other auxiliary circuits on the driver board, such as fiber optic receivers/transmitters, latches, and optocouplers. The auxiliary power supply is designed to use a 24V DC input, which outputs a high-frequency square wave voltage after passing through the half-bridge inverter circuit module. This voltage is then isolated, boosted, and filtered by a transformer before outputting the required voltage signal. As analyzed in Sections 1.1 and 1.4 of this paper, the onboard auxiliary power supply of the driver circuit not only generates interference signals, but improper design can also become a major coupling path for electromagnetic noise, affecting the accuracy of the drive signal and leading to the failure of the entire inverter system. Therefore, the anti-interference design of the auxiliary power supply is also an important issue for IGBT driver circuits. Based on the above analysis, the design of the onboard auxiliary power supply mainly considers the following points. 2.2.1 The isolation power supply drive circuit should be connected to the main power section of the inverter, as shown in Figure 3. Point G is the IGBT gate drive signal input terminal; point E is the IGBT emitter, connected to the ground potential of the auxiliary power supply, providing a reference potential for the drive signal. As analyzed in Section 1.3 of this article, since the IGBT emitter potential (i.e., the potential at point E) floats during the operation of the inverter, if an isolation power supply is not used, the large change in the potential at point E will inevitably cause a change in the potential at the power input terminal through the auxiliary power supply circuit, leading to power supply damage. Therefore, in high-voltage, high-power applications, the isolation of the auxiliary power supply in the drive circuit is particularly important. In the design of the auxiliary power supply of the drive circuit in this system, the 24V DC input voltage is isolated by a transformer after passing through the half-bridge inverter circuit, which can solve this problem. 2.2.2 Multiple independent power supplies As mentioned above, the drive circuit board needs to provide two different level signals for the drive chip M57962L and other circuits. The power supply for the driver chip will be connected to the main power circuit of the inverter; while another 5V power supply powers the low-voltage parts of the driver circuit, such as latches and gate circuits. To prevent interference between strong and weak signals, these two signals must be provided by two independent isolated power supplies, and cannot use the same reference point as the ground point. In addition, each driver board has two drive signal outputs, and the driver chips for these two signals must also be powered by two different isolated power supplies. As analyzed above, the potential of the driver circuit will change with the emitter potential of the IGBT, and the emitter voltage change during turn-on and turn-off can be over kilovolts; the potential difference between different IGBTs can also be several hundred to several thousand volts, so the power supply for the driver circuit must be provided by isolated power supplies. Therefore, each driver circuit board will have three independent isolated power supplies, providing the corresponding voltage signals for the two IGBT driver chips and other circuit parts. 2.2.3 The stability of the auxiliary power supply using a common-mode choke is also crucial to the accuracy of the driver circuit signal. In this system, the reference potentials of each auxiliary power supply undergo significant and frequent changes. Although each power supply is an independent isolated power supply, the influence of parasitic parameters can cause electromagnetic interference between them. Furthermore, the high-frequency common-mode noise generated by the switching transistors also affects the auxiliary power supplies. In the design, a common-mode choke (T in Figure 5) is added to each isolated power supply. The common-mode choke has high impedance characteristics for high-frequency common-mode noise signals, thus effectively suppressing the influence of common-mode noise on the circuit and ensuring the stability of the power supply output voltage. 2.2.4 Filtering Circuit In addition to using common-mode chokes, the auxiliary power supply also employs other filtering measures. As shown in Figure 5, a 100μF electrolytic capacitor and a 0.1μF ceramic capacitor are added after the rectifier circuit and the common-mode choke, respectively, which can effectively absorb the high-order harmonics and voltage spikes generated by the rectifier circuit. The circuit's rear end is connected to a three-terminal voltage regulator module to ensure the stability of the output voltage. Electrolytic capacitors and ceramic capacitors are also connected in parallel after the voltage regulator circuit for filtering, reducing output voltage ripple and ensuring high-quality auxiliary power output. Additionally, 0.1μF ceramic capacitors are connected in parallel near the power input terminals of each chip as decoupling capacitors to suppress high-frequency coupling noise interference. 2.3 Transient Noise Suppression During IGBT switching, high di/dt noise may be generated, causing Ldi/dt voltage on the gate parasitic inductance, resulting in gate surge voltage. This transient voltage may be much greater than the gate drive voltage that the IGBT can withstand, damaging the device. To control gate surge voltage, the gate voltage is usually clamped at one end of the gate resistor. In this design, the circuit uses a pair of Zener diodes with a breakdown voltage of 17V connected back-to-back to the gate and emitter terminals of the IGBT, as shown in Figure 3; the connection position is as close as possible to the gate and emitter of the IGBT to achieve good suppression effect. Pin 1 of the driver chip is the overvoltage detection terminal, connected to the collector C of the IGBT. When an excessively high voltage is detected at terminal C, the circuit is protected. Pin 1 may also be affected by transient noise interference, leading to excessive voltage and damage to the chip. Similarly, a Zener diode with a breakdown voltage of 30V can be used for protection. 2.4 PCB Anti-interference Design The layout and routing of the PCB also have a significant impact on the anti-interference performance of the driver circuit. For this driver circuit, the following points should be noted. 2.4.1 Circuit Layout The two drive signals on each driver board are used to drive two IGBTs in the same bridge arm, and there is a large potential difference between them. This requires that there be sufficient insulation between these two signals. In addition, since there are both strong and weak signals in the driver circuit, the different signals should be separated and kept at a certain distance during PCB layout to avoid coupling between strong and weak signals and increased interference. After comprehensively considering factors such as the size of the driver board and anti-interference, the PCR layout is shown in Figure 6: the left half is mainly the low-voltage part, with various digital circuits; the right half is the two driver circuits A and B and their respective auxiliary power supplies, which are connected to the inverter's power circuit. It can be seen that the driver circuit part is a certain distance from the digital part, and there is also sufficient distance between driver circuits A and B to ensure that the required insulation level of the circuit is met. 2.4.2 Routing Issues Anti-interference design should also be considered when routing the PCB. For this driver circuit, firstly, the power lines and ground lines should have sufficient width to accommodate large current flows; copper can be laid where possible to enhance anti-interference; all connecting lines, especially high-frequency signal lines (such as control signal input terminals), should be as short as possible to reduce unidirectional inductance; in addition, attention should be paid to minimizing loop area during routing to suppress power supply radiation interference. 2.5 Driving Waveform The driving waveform generated by the IGBT under normal operating conditions is shown in Figure 7, where CH1 is the input control signal waveform and CH2 is the output driving signal waveform. When the control signal is low, the drive circuit generates a voltage signal of approximately 15V, which can effectively drive the gate of the IGBT to turn it on. When the drive signal is high, the drive circuit generates a negative voltage of approximately 8V, which can effectively turn off the IGBT. The overcurrent protection function of the drive circuit is shown in Figure 8. When the control signal is low, the drive circuit outputs a high level to turn on the IGBT; however, when the circuit experiences an overcurrent, the drive circuit detects the overvoltage of the IGBT's Vce and uses the internal circuitry of the M57962L to softly turn off the gate voltage, preventing damage to the device due to voltage overshoot. This effectively protects the entire inverter system by shutting down the switching device when an overcurrent occurs. 3 Conclusion This paper analyzes the main interference sources and coupling paths in a three-level inverter system, and focuses on the impact of these interferences on the IGBT drive circuit used in the inverter. Based on this analysis, it proposes some issues that should be considered when designing the EMC of the IGBT drive circuit. Specifically, it discusses issues such as fiber optic signal transmission, auxiliary power supply design, transient noise suppression, and PCB anti-interference design. After taking corresponding measures, the anti-interference performance of the drive circuit was significantly improved.