# Design and Verification of Multiphase Multilevel Traction Inverter
## 2 Materials and Methods
### Hardware Design Procedure  
The development stage was divided into multiple individual groups which followed each other:  
* _Power module design stage:_ In this stage, the power module of the TNPC inverter was designed and developed. The main target was good heat dissipation, a small form factor, and placing all heat-generating semiconductors in one place to maximize heat transfer. The proposed design has all one-phase semiconductors located in one Printed Circuit Board (PCB). This includes top and bottom 1200 V SiC MOSFETs and midpoint 650 V MOSFETs from OnSEMI (Phoenix, AZ, USA). Figure 1 shows diagram of the power transistors configuration with their respective labelling. Due to the power output of the converter, all MOSFETs must be doubled to ensure current handling capability. In total, 8 MOSFETs must be placed on this PCB. Additionally, gate connectors with protections are located on this board. For heat monitoring, the NTC thermistor is located near the power semiconductors to measure module temperature and implement overtemperature protection. Power connections are made using Wurth Elektronik (Waldenburg, Germany) stand-off with M4 internal thread. There are four terminals for DC+, DC\(-\), MID potential, and OUTPUT. Six power PCBs are required to construct a 6-phase TNPC inverter. The 3D model of the PCB is shown in the Figure 2:  
Figure 1: Power transistors configuration on the IMS board. Figure 2: Power IMS board with semiconductors. Due to the high-power dissipation from the power MOSFETs, the PCB must transfer this heat to the heatsink. At the power output of 100 kW, the expected power dissipation of one module is around 144 W. This heat must be transferred from the semiconductors to the heatsink. Despite this, the Insulated Metal Substrate (IMS) based PCB was selected. This type of board has a copper or aluminum core, which quickly transfers heat through the PCB. In this application, the aluminum core PCB was selected due to the lower price and sufficient thermal conduction. This IMS comes with a SiO\({}_{2}\) insulation layer and 4 oz copper weight to handle the high output currents of the module. Manufactured PCB can be seen in Figure 3:  
Designed board provides superior thermal cooling of the transistors. Additionally, symmetrical design provides equal current paths which improves EMC performance of the power part of the inverter. The main advantage of the module is that it contains all switches for one phase in one compact PCB. * _Driver design stage:_ The second design stage was the insulated driver design to properly drive all transistors in the TNPC configuration. Because the drivers must be insulated, three isolated power supplies are required for this design--one for the bottom transistors, one for the top transistor, and one for the middle transistors. The used power supplies are RS3K from RECOM (Gmunden, Austria), with an output voltage of 20 V and an output power of 3 W. With the help of additional circuits, this voltage is further separated to the +16 V and \(-\)4 V to close SiC MOSFETs properly. These voltages are fed to the isolated driver from Texas Instruments UCC21755 (Dallas, TX, USA). This driver is designed for SiC MOSFET driving and has multiple built-in protection circuits, such as desaturation protection or miller clamp. All these circuits, power supplies, and auxiliary components are designed on a 4-layer PCB with dimensions of 123 \(\times\) 49 mm. The designed PCB can be seen in Figure 4.  
Figure 4: Single phase driver board. Figure 3: Manufactured unpopulated power board. The long pin headers connect the transistors' gate to the driver's output. Since the bus-bar PCB is between the driver and the power PCB, the pin headers must be higher. * _Driver interconnection PCB design stage:_ Due to the high component count on the driver PCB, it is impossible to populate the connector to the control board right on the driver PCB. Another PCB must be designed to distribute power rails and driving signals to the right places on the driver PCB.