The New Protector Keeping High-Voltage Automotive Drives Safe from Harm
By Reggie Phillips, High Voltage Product Manager, KEMET Corporation
Protecting the sensitive but essential capacitors in hybrid and electric-vehicle power converters is a job for ArcShield™, a new technology from KEMET with super powers to stop the evil creepage and its vicious sidekick arc-over in their tracks.
The inverters and charging systems in hybrid or fully electric vehicles typically operate at voltages up to about 400V or more, but extreme space constraints mean components have to be small and separation distances for safety isolation are under pressure.
Multilayer ceramic capacitors (MLCCs) are needed as filters across high-voltage lines, to help stabilize the voltage and balance the flows of energy when charging or driving. Faced with demands to squeeze electronics into smaller and smaller spaces, designers often have to select MLCCs in the smallest available case sizes, such as 0603. An 0603 chip-size device saves 75% of the board space occupied by a 1206-size MLCC, for example.
Due to their small size, these devices are vulnerable to a serious threat. The shorter distance between the device terminals increases the risk that creepage – the natural tendency of an electric field to spread out over a dielectric surface – can create conditions for arcing between the terminals (figure 1) when the full working voltage is applied. This can cause capacitor failure, and can also burn nearby components. What’s more, this unwanted evil thrives on factors such as high atmospheric humidity or contamination on the component surface, which increase the likelihood of attack.
Figure 1 : Surface arcing between MLCC terminations.
Understanding the Enemy – How Arcing Occurs
When a high-voltage DC bias is applied to a high voltage MLCC, a potential difference is established between the opposing terminations and the opposing electrode structure. Simultaneously, an electric field concentration is localized in the termination area and respective first counter electrode within the MLCC, as illustrated in figure 2. This difference in potential begins to build along the surface of the chip, ionizing the air above it once the electrical breakdown of air is reached.
Figure 2 : Electrical conditions around the capacitor surface that can allow arcing to occur.
Once the inception voltage of the ionized air is reached, a conductive path is created allowing the energy in the concentrated electric field of the termination area to discharge. This discharge of energy travels through the air, along the surface of the capacitor and onto an area of lower potential, rather than through the capacitor. During discharge, there is a visible and audible electric arc across the surface of the chip.
This type of arcing can occur at applied voltages of about 300V or more. For some high-voltage capacitors, this may be lower than the rated voltage of the device.
If the arcing occurs between a termination surface and through the dielectric material of the ceramic body to the first internal counter electrode, this usually causes dielectric breakdown of the capacitor resulting in a short-circuit condition that leads to catastrophic failure.
Where Previous Heroes have Failed
Capacitor vendors have tried a number of approaches to prevent arcing. One of these is to apply a polymer or glass coating along the surface of the chip to fill any voids and provide a smooth surface that has naturally lower susceptibility to creepage.
Filling these voids with insulating material also helps exclude contaminates and improves the dielectric stability across the surface of the chip. Improving this stability reduces the ionisation of the air and increases the inception voltage along the surface, thereby reducing the potential for arcing and improving the voltage performance of the capacitor.
Designers have used surface coatings on PCBs in high-voltage applications for decades. This technology has been proven to increase performance, but its primary disadvantage is the cost of applying the coating. Many designers choose to avoid such cost unless it is deemed absolutely necessary to meet specific electrical safety standards. There are also concerns with air gaps under mounted components, and voids in and under the epoxy coating, which can allow equivalent opportunity for arcing as an uncoated device.
Surface coatings can be damaged during handling and assembly processes. A breach in the coating effectively reduces the creepage distance capability along the surface, leaving the capacitor susceptible to contamination and arc-over concerns (figure 3). Finally, designers should ensure that the coating material is compatible with all applicable assembly materials, processes and conditions. Incompatibility could result in damage or premature failure of the surface coating.
Figure 3 : Imperfections in the coating can leave the device vulnerable to arcing.
Series Electrode
An alternative technique, illustrated in figure 4, is “series electrode” construction. The first part of the diagram illustrates how five individual 1000 V 1000 µF capacitors can be connected in series to form an array that effectively raises the breakdown capability to 5000 volts, even though the total electric field experienced is the same as that for a single capacitor. One disadvantage, however, is that the total capacitance is reduced to 200 µF. The second part of the diagram shows the entire block of capacitors placed into a single monolithic structure with the same characteristics as the five series devices.
Figure 4 : Top: Five individual capacitors in series. Bottom: Monolithic series-electrode construction raises the breakdown voltage but reduces capacitance.
The New Protector
ArcShield prevents creepage from causing arc-over by introducing an additional internal shield electrode, as shown in figure 5. This shield opposes the effects that can cause surface arcing. But unlike a coating or serial electrode, this is one superhero that offers no weakness for its adversary to exploit.
The upper and lower shields seen in the diagram form a super-strong barrier preventing the terminal-to-terminal arcing that can occur in standard designs. In a standard design the electric field at the surface is very close to the terminal, which reduces the energy barrier for arcing to occur across the surface. The ArcShield™ design has a larger energy barrier because of the presence of the shield electrode of similar polarity to the termination.
When a high-voltage bias is applied to an ArcShield MLCC, a potential difference is established between the opposing terminations and the opposing electrode structure, but the electric field concentration is localised in the shield electrodes rather than the termination surface and respective first counter electrode. This minimizes the difference in potential along the surface of the chip and drastically improves the creepage distance capability even in smaller case size devices and when there is high porosity in the dielectric surface.
Figure 5 : The shield electrode reduces field strength in the region of the capacitor surface and first counter electrode.
ArcShield in Action
A standard overlap X7R MLCC is vulnerable to three basic high-voltage failure mechanisms. These are arcing between a terminal and the nearest electrode of opposite polarity, arcing between terminals, and internal breakdown.
KEMET ArcShield MLCCs address these failure mechanisms by introducing a shield electrode prevents arcing between terminals and any nearby opposing electrode. The devices also incorporate thicker active areas that effectively increase the breakdown voltage.
Figure 6 : Voltage breakdown in air (50pcs), comparing standard 1206 MLCC to ArcShield.
Surface arcing can occur at voltages as low as 300 V, especially with small case sizes. Applying ArcShield technology to smaller case sizes such as 1206 (figure 6) and 0805 or 0603 (table 1) increases voltage-withstand capability and enhances life test performance.
Table 1 : Performance data for smaller case size ArcShield MLCC.
As the results show, X7R capacitors in case sizes as small as 0603 can now withstand exposure to voltages much higher than typical hybrid/EV inverter or battery-charging voltages. All in a day’s work for a superhero. More information about capacitors with ArcShield can be found at http://www.mouser.com/kemetArcShieldcaps.
