1 Introduction

Due to the high switching frequency, low conduction power loss, and convenient gate control characteristics of IGBTs, they are widely used in high-power conversion systems. In the application of IGBTs, in addition to their own technical level, another important factor to consider is whether the design of their drivers is reasonable and reliable. As the interface circuit between the power circuit and the controller, the IGBT driver has a significant correlation with the system's power consumption and reliability. An optimized driver is indispensable in power conversion systems, and the choice of an appropriate driver circuit is closely related to the reliability of the overall converter scheme.

The driver mainly performs the following three functions: first, the driving function, which provides sufficient driving current for the IGBT switch to ensure that the IGBT can reliably turn on and off under its control; second, the driver must have a protection function, which can turn off the IGBT in the shortest time when a short circuit or overcurrent occurs, protecting the power device. Additionally, in high-voltage and high-power applications, the driver must have electrical isolation to ensure that the control circuit is not affected by the power circuit. Under the premise of satisfying the above three functions, the driver must also consider the relationship between flexibility, performance, and price.

Due to the differences in current capacity and voltage levels of IGBTs, there are also differences in the technical requirements for their drivers. In low-power applications, due to the relatively small driving current, integrated drivers are mostly used, while in high-power and high-voltage applications, such as high-power UPS power supplies and high-voltage inverters, the driver is required to provide larger driving currents, higher isolation voltages, and more complete protection functions. This article analyzes some common technical characteristics, design points, and future development trends of high-power IGBT driver modules commonly used in the market, such as Semikron's SKHI22 and Concept's 2SD315A.

2 Technical Characteristics Analysis

2.1 Complete Signal Processing Functions

In high-voltage and high-power applications, considering the strong interference generated by switching and the high cost of IGBTs, ensuring the reliability of the IGBT driving signal is very important. Therefore, high-power IGBT driver modules usually have complete driving pulse signal preprocessing functions, aimed at ensuring the reliability of the pulse signal at the IGBT gate. Common driving signal processing functions are as follows:

(1) Dual Pulse Interlock Function

When the driver module outputs two pulse signals to control the upper and lower IGBTs on the same bridge arm, if the driving signals simultaneously control both IGBTs to turn on, a direct short circuit may occur, potentially damaging the IGBT or other devices. To prevent this situation, a signal interlock circuit is designed internally in the driver module to ensure that when both input pulse signals are high, both outputs are low, preventing a direct conduction phenomenon. When independent control of the dual driving signals is needed, the interlock function can also be shielded through external terminals.

(2) Narrow Pulse Suppression Function

Narrow pulse signals caused by control circuits or interference, when applied to the gate of the IGBT through the driver, may cause the IGBT to complete a switching process in a short time. If the pulse signal is too short, the IGBT may not be fully turned on before turning off, adversely affecting the output of the converter and increasing the switching loss of the IGBT, thereby reducing the system's efficiency. A filtering circuit is designed in the driver to remove narrow pulse signals, which helps improve the reliability of the IGBT.

(3) Dead Time Setting Function

In half-bridge operating mode, the two IGBTs must turn on alternately. To prevent both IGBTs from being in the on state during the switching alternation, a certain dead time must be added during the alternation of the two IGBTs. The dead time varies according to the characteristics of different IGBTs. In dual high-power driver modules, a dead time control circuit is designed internally, and the dead time can be adjusted through different connections of external terminals, such as by connecting external capacitors of different capacities (2SD106) or high and low levels (SKHI22A/B).

Figure 1 shows the signal processing block diagram of the high-power IGBT driver from Semikron, which includes various signal processing function modules aimed at ensuring the reliability of the IGBT driving signal.

2.2 Isolation Transmission Method of Driving Signals

Considering that high-voltage high-power IGBT drivers operate in high-voltage environments, to ensure that the controller is not affected by the high-voltage side, the driving pulse signal must be isolated before being transmitted to the IGBT gate. Common isolation methods include optical isolation and magnetic isolation. Optical isolation includes optocoupler isolation and fiber optic isolation. The optocoupler isolation method has relatively low isolation voltage and issues such as transmission delay, aging, and reliability, making it rarely used in high-voltage applications where the DC bus voltage exceeds 800V. In contrast, using pulse transformer isolation (magnetic isolation) can achieve relatively high isolation voltage, and the transformer has high reliability and low transmission delay, allowing for higher switching frequencies without aging issues. Therefore, pulse transformers are mostly used as isolation components in high-voltage IGBT drivers to complete the isolation transmission of driving signals.

Traditional pulse transformers for driving directly drive the IGBT or power MOSFET after isolating the amplified pulse signal. The basic circuit principle is shown in Figure 2. The role of the series capacitor on the primary side is to remove the DC component of the driving pulse. The parallel voltage regulator on the secondary side is used to prevent the output voltage from exceeding a certain level and damaging the power switch. This operating method does not require a separate driving power supply, and the circuit design is simple and cost-effective. However, when the duty cycle of the driving pulse varies widely, especially when the duty cycle is large, the output waveform of the transformer must maintain equal volt-second area over one cycle, which may reduce the amplitude of the output positive pulse, making it unable to drive the IGBT normally. Typically, the control pulse duty cycle is required to be less than 50%. Additionally, the saturation issue of the pulse transformer core also limits the conduction time of the control pulse. Another drawback is that the driving waveform may be distorted, especially when driving high-power IGBTs, as the input capacitance of the IGBT is relatively large, making it difficult for the secondary output of the pulse transformer to meet the driving requirements. Therefore, this driving method is mainly applied in low-power switching power supplies.

For high-voltage high-power IGBTs, the aforementioned driving method is clearly not applicable. The usual method is to modulate the driving pulse signal, converting its rising and falling edges into two inverted narrow pulse signals. The pulse transformer only couples these two pulse signals to the secondary side, and then the driving pulse signal is restored through secondary reconstruction. Its working principle is shown in Figure 3.

This method can be referred to as the pulse edge coupling transmission method. The advantage of this method is that the pulse transformer only transmits narrow pulse signals with a fixed pulse width, which can adapt to a wide range of duty cycle variations in the driving pulse signal. Since the transformer transmits narrow pulse signals, the magnetic core and winding of the transformer can be relatively small, and the corresponding leakage inductance and distributed capacitance are also relatively small, which is beneficial for the design of pulse transformers and the transmission of signals. The drawback is that it increases the conversion and reconstruction circuits, making the circuit relatively more complex. Figure 4 shows the primary experimental waveform of the pulse transformer after conversion.

2.3 Built-in DC/DC Isolated Converter

High-power IGBT driver modules usually come with a built-in DC/DC converter for the convenience of users in designing the driving power supply. The DC/DC converter with a high isolation voltage level does not require users to design an isolated power supply separately. The integrated isolation converter typically adopts a half-bridge or push-pull structure. To increase the isolation voltage and simplify the control circuit of the converter, it generally does not have closed-loop control. Some drivers add a linear voltage regulator at the output to stabilize the driving voltage. To reduce the size of the transformer, the operating frequency is often above 100 kHz. In high-voltage and high-power applications, depending on the different bus voltages, the driver must have a very high isolation voltage tolerance between the primary and secondary sides, with a bus voltage of 900 VDC requiring at least 4 kVAC of isolation voltage. Another factor that must be considered is the dv/dt tolerance. When the IGBT switches at high speed, it may generate a very high dv/dt. This signal can couple to the primary control circuit through the isolation transformer or pulse transformer, causing interference to the control circuit. Therefore, the design of the isolation transformer also requires a very small primary-to-secondary coupling capacitance, with the capacity of the transformer coupling capacitance usually being less than 20 pF, depending on the specific requirements for dv/dt tolerance.

The manufacturing process of the transformer is key to achieving the aforementioned high isolation voltage. To increase the isolation voltage tolerance and reduce the coupling capacitance between the primary, secondary, or between secondaries, the windings are usually wound separately and separated by insulating spacers. Sometimes, it is also necessary to coat the surface of the magnetic core with thick insulating material or use triple-insulated wire for winding. Figure 5 shows a schematic diagram of the transformer structure of the Eupec IGBT driver module 2ED300C17.

 

2.4 Short Circuit Protection and Threshold Adjustment

The commonly used method for IGBT short circuit or overcurrent protection is achieved by detecting the voltage value of Vce. When a short circuit or overcurrent occurs in the IGBT, its operating region will exit the saturation region, causing the Vce voltage to rise. The specific principle of the protection circuit is shown in Figure 6. The IGBT's undersaturation detection is achieved by connecting a diode D to the collector of the IGBT. The rise in Vce voltage will correspondingly raise the anode potential of the series diode. When it exceeds the set short circuit threshold, the protection circuit activates and turns off the IGBT. Since the collector voltage of the IGBT is relatively high at the initial turn-on stage, if the protection circuit operates at this time, it may cause a false action. Therefore, a blind zone time must be set during which the short circuit protection circuit does not operate. This function is achieved through switch S and an external parallel resistor Rce and capacitor Cce. When the IGBT is turned off, S is turned on, and capacitor Cce is charged to 15V. When the IGBT is turned on, S is turned off, and capacitor Cce discharges through Rce, with the discharge termination voltage being:

This allows the reference voltage to be higher than the detection voltage at the initial turn-on stage of the IGBT, preventing false actions of the protection circuit. The waveform during normal operation is shown in Figure 7(a), and the waveform during a short circuit or overcurrent fault is shown in Figure 7(b).

2.5 User Interface Method

To accommodate IGBT modules packaged by different manufacturers, IGBT drivers must have a user-friendly interface. They should also possess broad flexibility and economical costs. Currently, common driver modules in the market mainly use soldering on PCB boards to connect with IGBTs, such as SKHI22, 2SD315A, and 2ED300C17. For ease of installation, there are also direct insertion connection methods. Figure 8 shows the appearance of the driver module Skyper developed by Semikron. It connects to the driver interface board through a direct insertion method.

Since the driver module (driver chip) only provides the most important general functions in the driver, its connection with different modules in various applications relies on the interface board. The entire module-driver unit includes a power module with a spring interface, a standard or enhanced driver chip, and an interface board connecting the driver chip to the specified module. A customizable interface board has a significant advantage: users can adjust and determine the switching characteristics of the IGBT themselves, such as changing the turn-on or turn-off speed by adjusting Rgon or Rgoff; adjusting the dead time or disabling the interlock function; adjusting the Vce protection point and window time, etc. Compared to the current market's intelligent power modules (IPM), the interface board makes the entire system more flexible and easier to adapt to different applications. Once the system parameters are set, the entire system can be used as conveniently as an IPM. The electrical connection between the Semix module and the interface board is achieved through the built-in spring in the Semix module and the contacts at the bottom of the interface board. After assembly, the contacts of the interface board press against the spring contacts of the module, completing the electrical connection through pressure contact. Compared to soldering technology, pressure contact improves the reliability of the power module. Similarly, the plug-in connection between the driver chip and the interface board is also to avoid soldering.

2.6 High Integration

The trend in the development of drivers is high integration, which can reduce the size of the driver and allow for closer integration with the IGBT, making installation more convenient, reducing the length of connecting wires between the driver and IGBT modules, and decreasing lead inductance. To achieve this goal, some companies abroad have developed IGBT driver modules that use self-developed dedicated integrated circuits (ASIC), such as Semikron's SKIC2001A and Concept's LDI001 and LGD001. Through the application of ASIC, most control and protection functions can be implemented with ICs, greatly reducing the size of the driver and increasing the reliability of the IGBT driver.

3 Development Trends of High Voltage High Power IGBT Driver Modules

As a composite power semiconductor, IGBT is being increasingly widely used in high-power converters due to its low power consumption, high switching frequency, and large current capacity. The requirements for its driving circuits will also become higher, with the main technical development directions reflected in the following aspects.

(1) Higher Integration

Currently, the size of high-power IGBT driver modules is still relatively large. To increase the isolation voltage tolerance, transformers are usually used for isolation. The size and weight of transformers are relatively large, and integration is quite difficult. Therefore, future drivers will adopt smaller and more easily integrated isolation devices, such as piezoelectric transformers or advanced magnetic integration technology to reduce the size and weight of isolation components and increase integration. It is foreseeable that in the future, high-power IGBTs will definitely be integrated within the same module as their driver circuits, allowing users to directly introduce control signals into the power module to control the IGBT.

(2) Higher isolation voltage

Current drivers use optocouplers and transformers for isolation. The advantage of optocouplers is their small size, but they have drawbacks such as low isolation voltage, susceptibility to aging, and significant delay. Transformer isolation has a higher isolation voltage and lower delay, but is larger in size. Therefore, in situations requiring high voltage isolation, transformers are still mostly used for isolation. Currently, the highest isolation voltage of transformer-isolated driver modules is about 3300V. The maximum voltage level of IGBTs has reached 6500V, so to adapt to higher voltage applications, drivers with higher isolation voltages must be used.

(3) Greater driving power

The capacity of IGBT modules is continuously increasing, with a single module's current capacity reaching 3600A. Sometimes, to increase capacity, parallel operation is usually adopted, which raises the requirements for the driving power of the driver. The maximum output current of the driver must correspondingly increase, especially when multiple modules are used in parallel. The average output power requirement of the driver must reach 5W to 10W, and the instantaneous maximum output current requirement must exceed 30A.

(4) Higher switching frequency

To adapt to applications in induction heating power supplies and other areas, the switching frequency of IGBTs is continuously increasing. With the development of manufacturing technology, the maximum switching frequency of IGBTs has reached over 100kHz, which can partially replace power MOSFETs. For drivers, this means that they must provide greater driving power, and the driver must have shorter driving pulse delay times and rise and fall times, providing greater instantaneous maximum driving current, etc.

(5) More complete functions

The gate drive technology widely used today cannot control the di/dt and dv/dt caused by the IGBT switching process, thus controlling the EMI of the conversion circuit. Active gate drive technology can effectively control the high di/dt and dv/dt caused by IGBT switching, allowing the IGBT to operate in a safer working area, reducing the EMI generated during the switching process, and correspondingly reducing the IGBT's snubber absorption circuit. Among them, the three-segment active gate drive technology is a widely applicable active gate drive technology. Additionally, to meet the needs of series and parallel IGBT applications, the driver must also have dynamic voltage and current sharing functions.

4 Conclusion

As a key power semiconductor device in power electronic systems, IGBTs have been continuously growing for several years. They enable power electronic devices and equipment to achieve higher efficiency, higher switching frequencies, and miniaturized designs of power conversion devices. With continuous performance improvements, the application fields of IGBT devices have expanded to a wider range, not only in industry but also in many other power conversion systems. They have replaced high-power bipolar transistors (GTRs), power MOSFETs, and even show a trend of replacing gate turn-off thyristors (GTOs). The technology of high-power IGBT driver modules will continue to improve, and the degree of integration will also increase, thereby reducing IGBT power consumption and EMI, and improving system reliability. With the development of IGBT manufacturing technology, the application fields will further increase, and the performance requirements for their drivers are also continuously rising. Various driver manufacturers are developing IGBT driver products with more refined performance to adapt to the performance of the new generation of IGBTs.

 

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