**4. Design of a High-Power-Density Converter Prototype**

In our research, a 5L-HANPC and 3L-NPC (5L-3L) asymmetrical voltage-level multilevel back-to-back converter prototype was built to evaluate the power density of the power converter. The schematic of this prototype is demonstrated in Figure 6. The specification of the prototype, and the selection results of all main components are given in Table 4. The design of the volume of the passive components followed the criterion, which is for selecting the minimum volume while satisfying requirements of voltage ripples or current ripples.

From the selection results, it can be found that, compared to the DC link capacitor, the flying capacitor required a much larger capacitance. It is because that each flying capacitor was connected in a single phase. A large AC current went through each capacitor directly. Film-type capacitors were applied to both the DC link capacitor and the flying capacitor because of their high reliability, especially because of their open-circuit failure mode. The three-phase AC inductor was designed to have an electrical inductance of 66 μH and a rated input current of 150 A. The inductance was small compared to the traditional voltage source inverter (VSI) design, because the rectifier stage used a 5L converter.

**Figure 6.** Schematic of a 5L-HANPC and 3L-NPC converter prototype.

**Table 4.** Specifications and components selection results of the 5L-HANPC and 3L-NPC prototype.


The design of a heatsink for the air-cooling system is one of the most challenging parts of the prototype design. According to Table 3, there were 18 IGBT modules; the 3L-NPC was composed of six IGBT modules, and the 5L-HANPC was composed of the other 12 modules. Each converter had a different loss performance. Furthermore, each IGBT module had its own loss. This led to different temperature increases if we used a common heatsink design, where all IGBT modules are mounted on one surface and have an almost identical thermal resistance from the junction to the ambient. On the contrary, an ideal converter thermal system design is to let all IGBTs have the same temperature rise, and thus, the converter can make full use of the cooling system capacity.

In order to design a heatsink structure that can fully exploit the capability of the cooling system, the loss distributions among all IGBT modules under the rated conditions were calculated. The results are given in Table 5. The numbers of modules and switches are kept the same as what are illustrated in Figure 6. It can be found that the high-frequency switches of IGBTs S5–S8 in the 5L-HANPC and those of IGBTs in the 3L-NPC had higher power losses. The low-frequency switches of IGBTs S1–S4 in the 5L-HANPC had lower power losses. Based on this loss distribution, an innovative heatsink structure was investigated. IGBT modules with higher power loss were placed on the two side surfaces. The IGBT modules with less power loss were placed on the top surface. All fins of the heatsink were arranged to be parallel with the top surface, and thus the two side surfaces could have a better convection area. It is named as triple-surface-mounted heatsink structure, as shown in Figure 7.


**Table 5.** Theoretical power losses on all IGBT modules.

**Figure 7.** Heatsink design: (**a**) traditional planar structure; and (**b**) proposed triple-surface structure.

The thermal performance of the proposed heatsink structure was verified by the finite element analysis (FEA). Two important questions were answered: (1) How large were the optimized areas of the side and the top surfaces? (2) Did the temperature rises on each IGBT module become even? Figure 8 shows the FEA results of the heatsink version I, where the IGBT modules were placed side by side on three surfaces and the width of one surface was equal to the length of one IGBT module. According to the results, the average temperature rises on the heatsink surface did not exceed the upper limit. It meant that heat dissipation capability was strong enough even if the heatsink surface area was just equal to the total bottom side area of the IGBT modules. However, the hottest part was the top surface area near the outlet of the cooling air flow. The temperature difference between the top surface and the side surface was higher than 15 ◦C. It meant that the top surface area without any fin was hard to dissipate the power loss even if the IGBT modules on the top surface had much less loss.

**Figure 8.** (**a**) Heatsink version I; and (**b**) its finite element analysis (FEA) results (maximum temperature on the heatsink surface: 89 ◦C).

An improved heatsink version II is demonstrated in Figure 9. A thick plate was soldered to the center of the top surface. In this way, part of thermal flow conducted from the top surface to the center plate and was brought out by the airflow. In addition, the bottom side surface, which had a minimum effect on cooling, was removed to further save the total weight. According to the FEA results for version II, the temperature rise on each surface became almost even.

**Figure 9.** (**a**) Improved heatsink version II; and (**b**) its FEA results (maximum temperature on the heatsink surface: 78 ◦C).

The final structure design and the picture of this back-to-back prototype are demonstrated in Figure 10. The switches of IGBTs S5–S8 in the 5L-HANPC and those of IGBTs in the 3L-NPC were placed on the two side surfaces of the heatsink version II. The switches of IGBTs S1–S4 in the 5L-HANPC were placed on the top surface. The IGBT modules were connected by a copper bar. On the top of the IGBT modules at the two side surfaces, the DC link capacitors and the flying capacitors were fixed by using aluminum stand-offs. The top area was the total surface area of the PCBs for gating, sampling, display, and auxiliary power supply. This structure not only fully made use of the space, but also saved a large amount of weight by decreasing the size of the heatsink and the length of the connecting copper bar compared to structures, where all IGBTs are placed on one surface.

**Figure 10.** (**a**) Schematic, (**b**) three-dimensional (3D) structure, and (**c**) photograph demonstration of the converter prototype design.

Table 6 shows the weight distribution of the prototype. The heatsink, IGBT modules, and the AC inductor (not shown in the converter structure) were the main heavy parts. It should be noticed that the flying capacitors with their PCBs used by the 5L-HANPC also contributed to 9.77% of the total weight. This somehow affected the benefit of the 5L-HANPC applied to aerospace.


**Table 6.** Weight distribution analysis for the converter prototype.
