Three-level IGBT power module

To fully exploit the design advantages at the system level, the three-level neutral point clamped (NPC) topology circuit, which was previously mainly concentrated in high-power applications, has recently begun to appear in medium and low-power applications. Products that require filters, such as UPS systems or solar inverters, benefit from the improved spectral performance and lower switching losses of low-voltage devices. Until now, in order to realize three-level circuits, it was only possible to use discrete devices or at least combine three modules together. Now, using chip technology for higher breakdown voltages, by integrating the three-level bridge arms into separate modules and then matching the drive circuit, this topology can be made more attractive in new applications.

Working principle of three-level NPC topology

In the three-level NPC topology, each bridge arm is connected in series by four IGBTs with anti-parallel diodes , and two diodes DH and DL are used to connect their intermediate nodes to the neutral point of the DC bus. All power semiconductors used have the same breakdown voltage. According to the characteristics of output voltage and current, one cycle of base frequency output has four different freewheeling working states.

Figure 1. Commutation loop of a bridge arm in a three-level NPC. a) Short commutation loop; b) Long commutation loop

As can be seen from Figure 1a, when the voltage and current are in the positive direction, T1 and DH form the working mode of the BUCK circuit, and T2 outputs current in a normally on manner. When the voltage and current are in the negative direction, T4 and DB form the working mode of the BOOST circuit, and T3 outputs current in a normally on manner. In the above two cases, the commutation only occurs in two devices, which we call short freewheeling. However, when the output current is negative and the voltage is positive, the current flowing through T3 and DB must be commutated to D2 and D1 as shown in Figure 1b). This commutation involves four devices, so it is called a long commutation loop. In other cases, there will be another long commutation path. When designing a three-level converter, how to control the stray inductance and overvoltage problems of the long commutation loop is another challenge faced by designers.

Figure 2 EasyPACK 2B package

Latest IGBT modules for 3-level NPC topology

Although the IGBT module integrating a total of 4 IGBTs and 6 diodes is not suitable for high-power products, it can be applied to medium and small power products as long as the power range is certain and the number of control pins allows the use of standard packaging.

Figure 3 EconoPACK 4 package

For low-power products, the EasyPACK 2B package shown in Figure 3 has enough DBC area to integrate a complete 150A three-level module bridge arm. Since the pins can be arranged arbitrarily within a given grid, these pins can be used as power terminals or control terminals, so this package provides an ideal connection method. This package can provide auxiliary emitter terminals to ensure high-speed switching of IGBTs. For power terminals, up to 8 terminals can be connected in parallel to ensure the required rated current and reduce stray inductance and PCB heat.

For medium-power products, the newly launched EconoPACK 4 package provides an ideal choice, which can integrate all power devices in the three-level. The three power terminals on the right are used to separate the DC bus, bringing extremely low parasitic inductance to the three-level inverter, and the two power terminals opposite it are connected in parallel as the output terminals of each bridge arm. On both sides of the module package are control pins, and the PCB driver board can be directly connected through these terminals. The maximum current of the bridge arm in this packaged three-level module is up to 300A.

Integrating all devices of a three-level phase leg into one module is a promising solution in terms of reducing stray inductance. However, it is obvious that the device voltage of only 600V makes it difficult to meet typical applications because the bus voltage is not ideally balanced and the switching speed of 600V devices is too fast.

To make design easier and ensure that the devices have a higher margin in the application, these modules use enhanced IGBT and diode chips with a withstand voltage of 650V. These new chips have the same turn-on and switching characteristics as the well-known 600V IGBT3 devices; and the reliability has not changed (such as SOA, RBSOA, SCSOA). This has been achieved through the development of the latest IGBT and diode terminal structures, ensuring that the ultra-thin 70?m chip thickness has not changed. As a result, the collector-emitter saturation voltage VCE_SAT of the 650V IGBT remains at an extremely low level of 1.45V at 25°C (1.70V at 150°C). The switching losses of the device are low, and when the switching frequency is 16kHz, the losses account for only one third of the total losses of the inverter. In addition, the IGBT also has a very smooth current tailing characteristic, even under harsh conditions, without voltage overshoot. The diode's VF-Qrr relationship has also been optimized, with a forward voltage drop of 1.55V at 25°C (1.45V at 150°C) while maintaining soft turn-off characteristics.

Challenges in designing IGBT drivers for three-level topologies

In medium and low power three-level NPC topology applications, in order to optimize the system performance, some specific requirements are put forward for the IGBT drive.

Higher switching frequency Since the switching frequency ranges from 16kHz to 30kHz, the driver must provide consistent and small transmission delay time for each IGBT in order to reduce the dead time. Due to the fast switching speed of 650V devices, the dead time mainly depends on the variation of the transmission delay time of the driver. If the dead time is too long relative to the switching period, it will cause the output nonlinearity of the inverter, which will bring more challenges to the control algorithm.

Topological circuit structure Although the withstand voltage of these devices is only 600V or 650V, the isolation requirements of the driver are the same as 1200V. Since the number of driver circuits has doubled, a design suitable for the driver must be adopted, and its power supply requires a small number of components and a small PCB space. The protection features of the driver circuit, such as short-circuit detection and undervoltage lockout, must match the three-level NPC topology. First, turning off an internal IGBT (T2, T3 in Figure 1) will cause the bus voltage to be fully applied to this device, which will cause the device to fail immediately because it exceeds the device SCSOA or RBSOA area.

These requirements can be easily met using the new integrated IGBT driver technology of the EiceDRIVER family:

* Integrated micro-transformer technology provides basic insulation function with insulation voltage up to 1420 Vpeak.

* 集成的有源米勒箝位功能可以采用单电源来实现，这种驱动器在即便在较高开关速度条件下也不会有寄生导通风险[8]。

* Compared with the traditional driver technology using optocoupler, this micro transformer technology can significantly reduce the transmission delay time and mutual deviation.

* The integrated Vcesat protection function can also be used for the outer switch, but it needs to be shielded for the inner IGBT.

Experimental test results

This section will introduce the switching waveforms using the EasyPACK 2B three-level module. In this circuit, the IGBT gate of the IGBT drives the 1ED020I12-F driver chip. A current transformer is used to measure the current at the positive end DC+ or DC- of the DC bus.

Figure 4. Switching waveforms for short commutation (peak voltage is 550 V, which is still within the allowable voltage range.)

Short Commutation Loop Figure 4 shows the switching waveforms for the short commutation case at nominal current, 400V DC link voltage and 25°C junction temperature.

Figure 5 Switching waveforms for long commutation (peak voltage is 580 V. This voltage is only 30 V higher than the peak voltage for short commutation and still does not exceed the breakdown voltage of 650 V.)

Figure 5 shows the switching waveforms of the long commutation loop under the same conditions.

The first test results show that long commutation can achieve almost the same switching performance as short commutation, thanks to the integration of a complete three-level bridge arm in one module. However, in order to obtain sufficient margin under higher current conditions, the stray inductance of the circuit still needs to be further reduced. The parasitic inductance can be effectively reduced by connecting multiple capacitors in parallel and using multi-layer circuit boards to reduce the current loop between the module and the capacitor. In addition, it must be taken into account that current transformers are not used on the DC bus in actual applications. The use of current transformers here would generate a stray inductance of 15nH, which would lead to an overvoltage of 45V.

in conclusion

By integrating a complete three-level bridge arm into a module, increasing the device withstand voltage from 600V to 650V, and then matching it with a highly integrated drive solution, this three-level NPC topology provides a very attractive solution for medium and small power inverters such as high-efficiency UPS, PV and other applications that need to operate at higher switching frequencies and are equipped with filters.