Large-capacity capacitors for electric vehicle inverters: Made from films below 3μm (Part 2)

Punching into a flat shape

This production process is actually very difficult. First, the drum must be rotated at a high speed to produce about 30km of film per hour on average, and at the same time, the metal must be deposited under conditions that do not damage the film and make the metal thickness uniform. If the metal is uneven, it will affect the heat resistance. If the metal is not cooled properly, the film will also be damaged. It can only be produced by appropriately controlling the rotation speed of the drum while monitoring the pressure, temperature and surface conditions of the film attached to the drum through various sensors

. Next, the produced electrode film is cut according to the width of the target component and then wound thousands of times to make a capacitor component. At this stage, the component is cylindrical. It is pressed to make it flat (Figure 4). The product developed this time uses 15 components as a module. After being made into a flat shape, the components are arranged without gaps, which makes it easy to achieve miniaturization.

Figure 4: Making capacitor elements flat The product developed this time uses an average of 15 capacitor elements per module. In order to increase the packing density when making the module, the cylindrical elements are punched to make them flat.

The module is assembled by drawing leads from the electrodes of the components, connecting them to a flat copper plate, and placing them in a resin shell. The shell size is about 240mm wide and 170mm deep, with a volume of 1.25L.

Reduce the heat generated by the components themselves

Regarding the heat resistance mentioned in item (2), the heat resistance of the newly developed product reaches about 110°C, which is much higher than the 85°C of the previous product. In addition to using a recently developed film with higher heat resistance, the film is also protected from damage due to heat during the manufacturing process. The film deteriorates when heated, and the original heat resistance of the film cannot be obtained.

The vapor deposition process is the one that generates the most heat load. This is because high-temperature metal vapor is directly applied to the film. During vapor deposition, the adhesion between the cooling roller and the film is improved so that cooling can be performed from the inside, and the temperature of the vapor deposition surface can be kept as low as possible.

To improve heat resistance, it is important to reduce the heat generation of the capacitor itself. If self-heating occurs, the heat resistance will be substantially reduced. For example, if a component with a heat resistance of 110°C has 5°C of self-heating, the allowable ambient temperature will become 105°C. To reduce the heat generation of the component itself, the metal must be evenly vapor-deposited on the film. The thinner the metal thickness, the greater the resistance and the greater the heat generation. If a part of the electrode is thin, the overall heat resistance depends on this thin part.

Preventing short circuits and overvoltage

In terms of safety mentioned in item (3), the adverse effects of short circuits are reduced by working on the metal vapor deposition pattern. Specifically, instead of vapor-depositing metal on the entire film surface, a grid pattern is used (Figure 5). The white lines in Figure 5 are the areas where metal is not vapor-deposited. The grids are connected to each other, and the entire film surface acts as an electrode.

Figure 5: Improving safety (a) A checkered pattern is formed when the thin film is evaporated to form a mechanism that causes a short circuit when a large current flows. (b) The white lines are the areas where no metal is evaporated.

If there is a part of the grid with a low withstand voltage, a large current will flow when a short circuit occurs, causing heat and evaporation of the metal connecting the grids. Because this part of the grid is insulated from other grids, no more current will flow and the damage will not spread further.

In order to improve safety, in addition to countermeasures for the capacitor itself, it is also important not to adversely affect switching elements such as IGBTs connected to the capacitor. For this reason, it is very important to prevent overvoltage (surge voltage) (Figure 6). If the voltage from the capacitor is too high and exceeds the allowable value of the IGBT, in the worst case, the element will be damaged.

Figure 6: Preventing voltage damage to IGBTs It is important to reduce the surge voltage of the capacitor. To do this, the inductance is reduced.

As a countermeasure, theoretically, we can consider using IGBTs with higher allowable voltages, reducing switching speeds, and adding countermeasure components. However, if we consider the overall cost, it is very difficult to choose these countermeasures. In terms of IGBTs, generally speaking, the higher the allowable voltage, the higher the on-resistance and the greater the loss. In addition, since the resistance of the coil and capacitor depends on the switching speed, if the speed is reduced, the volume of the two must be increased to increase the resistance. After adding countermeasure components, the cost will also increase.

Therefore, the only way to suppress overvoltage without taking the above countermeasures is to reduce inductance. Since overvoltage depends on the product of inductance and current change, the smaller the two are, the smaller the overvoltage is. To reduce the current change, the switching speed must be reduced, which, as mentioned above, will increase the volume. The only remaining solution is to reduce inductance. The inductance of the product developed this time is about 5nH, which is only 1/4 of that of the previous product. To reduce inductance

, it is possible to reduce the width of the wound film and increase the number of turns. The film width of the developed product is about 20% smaller than that of the previous product, and the number of turns has increased by several thousand. If the film width is reduced to a smaller value than this, the proportion of the non-deposited parts remaining at both ends of the film will increase, and the capacitance density will decrease.

Shizuki Electric Manufacturing Co., Ltd. will continue to improve film capacitors in the future. For this purpose, a roadmap has been drawn up (Figure 7). In addition to miniaturization, thinner films will be used. We plan to use films that are 20% thinner than the current ones in 2013. We will also focus on improving heat resistance and reducing environmental load materials.

Figure 7: Further miniaturization by 2013 (a) This shows the film capacitor development goal of Yue Electric. (b) By using thinner films than existing products, the volume will be reduced by about 25% by 2013.