Design of H-bridge low-inductance laminated busbar for high-power converter system

1 Introduction

High- power converters are being used more and more widely. The shorter switching time of the IGBTs used in them leads to excessively high dv/dt and di/dt, which results in the distributed stray inductance having a more important impact on the turn-off characteristics of power devices. Laminated busbar technology can effectively suppress the overvoltage spikes of IGBTs.

In recent years, the research on DC busbars has mainly focused on two directions:

(1) As the switching frequency increases, the high-frequency model of the busbar becomes very important. The high-frequency model of the DC busbar is proposed in the literature [3]. However, these articles all use the PEEC method applicable to smaller devices to obtain the high-frequency model of the busbar by establishing an equivalent circuit. The application scope of the obtained busbar model is relatively narrow and lacks engineering practicality.

(2) Changing the shape of the busbar to achieve low inductance. Some literature has adopted the method of opening narrow and long gaps in the existing busbar to change the current flow direction, but the reliability of reducing the busbar inductance is questionable because the eddy current loss and uneven current distribution caused by the holes in the busbar may increase the inductance of the busbar.

Based on the actual situation, this paper proposes a new optimization design scheme for the H-bridge busbar of the 80kva/400a converter system. It ensures that the busbar inductance parameters are optimized from all aspects, including IGBT layout, busbar structure design, and buffer absorption circuit selection, and has good feasibility and reliability in practical applications.

2 IGBT parallel current sharing design

With the surge in market demand for megawatt-class high-power converters, IGBT paralleling has become a trend. This is because IGBT paralleling can provide higher current density, uniform heat distribution, flexible layout and high cost performance (depending on the device and type). By combining low-power IGBT modules (including discrete IGBTs) and high-power IGBT modules in parallel, equivalent modules with different rated currents can be obtained, and the connection methods for paralleling are also flexible and diverse. Taking the H-bridge topology power unit widely used in high-voltage inverters as an example, its parallel implementation can use IGBT modules with different circuit structures, such as half-bridge "FF", single "FZ", four-unit "F4" and six-unit "FS", as shown in Figure 1. Parallel connection can reduce module heat concentration, so that it can obtain a more uniform temperature gradient distribution and a lower average heat sink temperature, which is beneficial to increase the number of thermal cycles. Therefore, IGBT paralleling is one of the best solutions for high- power design applications.

However, the difference in static and dynamic performance between parallel IGBTs will affect current sharing, so that the output current has to be reduced. In addition, uneven current distribution will increase the stray inductance parameters. Due to the stray inductance of the DC link, overvoltage will occur when the IGBT is turned off, which may cause module damage. From the perspective of current sharing, the quality of parallel design plays a key role in derating, and it is far greater than the problems caused by the differences in IGBT parameters. Therefore, parallel connection should focus on how to ensure current sharing through design. There are documents that explain the five important factors that affect IGBT current sharing. The parallel design should focus on these factors to optimize the drive circuit, power conversion circuit, module layout and cooling conditions, etc., with the aim of ensuring that each parallel branch is as symmetrical as possible. Many documents provide IGBT current sharing measures, and IGBT manufacturers will also provide corresponding technical support, which will not be repeated here.

3 Busbar low inductance structure design

3.1 Laminated busbar structure

According to the principle of proximity effect, the high-frequency current of a conductor will form a radiation interference current in the adjacent conductor layer. For a double-layer copper busbar, when the current source path and the ground plane are stacked on each other and the spacing satisfies that the thickness of the insulation layer is much smaller than the width of the busbar, the high-frequency current will be mainly distributed on the two adjacent internal planes of the two copper bars, and some high-frequency magnetic fields can offset each other, which is equivalent to reducing the loop inductance. The comparison of the inductance of the laminated busbar and the parallel busbar is shown in Figure 2.

3.2 Current Path Design

If the connection lines and devices form a "loop", that is, the loop shown in the upper part of Figure 3. The inductive voltages superimposed on the commutation loop will be added to the power device together with the DC bus voltage, generating a turn-off voltage spike. Too high a spike may cause overvoltage breakdown of the device, increase switching losses, aggravate common-mode interference, and even bring the risk of partial discharge. Therefore, in the design of the busbar structure, loops should be avoided as much as possible or the current loops should be kept crossed.

3.3 Capacitor Arrangement Design

The suppression of distributed stray inductance in high-power devices is inseparable from buffer capacitors and electrolytic capacitors. For cost considerations, aluminum electrolytic capacitors are generally used to support the busbar DC voltage. Due to its low withstand voltage level, a large number of series and parallel connections are required. The stray inductance on the connecting line will cause uneven distribution of high-frequency currents between the parallel capacitors. The capacitors closer to the power devices will withstand currents higher than the rated value and heat up rapidly. Therefore, these two are the main problems in industrial engineering applications. In the design of capacitor structure, the main influencing factors are: capacitor terminal design direction and capacitor series structure design. Figure 4 measures the values ​​of inductance of different capacitor terminal designs, and Figure 5 shows the differences in capacitor series connection methods in typical design schemes and mainstream low-inductance design schemes. From Figure 5, it can be concluded that the direction of the capacitor terminal has a great influence on the inductance, and the use of a loop-free series connection method for capacitors can greatly reduce the busbar inductance. Figure 6 shows the finite element analysis of the busbar current when the capacitor terminal is well designed. It can be seen from Figure 6 that the current distribution on the busbar surface is very uniform, and the equivalent effect reduces the inductance.

4 Optimized busbar simulation and experimental results

4.1 Finite element software analysis results

Figure 7 shows the H-bridge busbar structure used after actual optimization. After considering the current path, capacitor terminal direction and other optimization methods, the surface current distribution was simulated using the finite element analysis software ANSOFT. The simulation results are shown in Figure 8. As can be seen from Figure 8, except for the uneven current distribution at the busbar opening due to the eddy current effect, the current distribution of the entire busbar is relatively uniform. The resulting stray inductance is directly extracted by the finite element software and the result is 21nh, which can meet industrial requirements.

4.2 Experimental Verification Results

This section mainly compares the busbar IGBT overvoltage waveform under the typical layout with the optimized layout through experiments, as shown in Figure 9. The experiment uses an 80kva/400v converter system. Figures 10 and 11 are the experimentally obtained IGBT overvoltage waveforms, of which Figure 11 is the IGBT overvoltage waveform measured in the H-bridge busbar with a typical layout, and Figure 11 is the IGBT overvoltage waveform measured in the busbar with an optimized layout. It can be seen from the figure that the new busbar layout has a good effect in suppressing overvoltage.

5 Conclusion

This paper takes the 80kva/400a converter system as an example, designs a new type of laminated busbar, and obtains the following conclusions:

(1) The application of the new laminated busbar enables each device to have good shutdown characteristics, which can reduce the number of absorption capacitors used, reduce the system volume, and have good electromagnetic compatibility characteristics;

(2) When laying out the laminated busbar, attention should be paid to issues such as the current path and capacitor terminal layout, which can effectively reduce stray inductance and improve the system's ability to suppress overvoltage.