Author:
Moaz Elghazali, Ph.D. Product Engineer – Murata
Date
10/01/2023
PDF
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Figure 1: The application of safety capacitors in an electric vehicle's onboard charging circuit
The automotive industry is currently undergoing one of the most significant changes it has seen. Whereas development within the sector has been more evolutionary for the past few decades, the transition from internal combustion engine (ICE) to hybrid and battery electric vehicles (EVs) is far more revolutionary. Besides the significant task of replacing internal combustion engines with electric motors, the rapid advancement of EV technology poses yet another obstacle for the industry. The swift pace of this transition affects the entire supply chain, encompassing automotive brands and Tier 1 suppliers alike. Even minor components, like capacitors, are vital for ensuring the effective transition to EVs.
A significant recent development has been the increase in the operating voltage of EV battery packs. Typical EV packs operate in the 350-400V range, but these are now starting to be superseded by 800V DC packs, like those seen in the Porsche Taycan. By increasing the voltage, the I2R losses are reduced within the vehicle's wiring harnesses, providing multiple benefits. If the pack voltage is doubled from 400V to 800V, the current required to deliver the same amount of power will also be halved and the energy lost due to resistive heating will be quartered. By reducing the current level, wiring looms can be made lighter, which in turn reduces the vehicle's mass and enhances its range. Higher battery pack voltages can also simplify the construction of rapid chargers. Fluid coolants are used in the charging cables of many of today's most powerful chargers to combat resistive heating at very high currents. By moving to a higher voltage, which reduces the current, this requirement could be removed.
High Voltage Charging Challenges
Battery packs consist of hundreds of individual cells, with each producing roughly 4V when fully charged, with the cells connected in a series to generate the necessary operating voltage. A pack's design and the battery management system (BMS) dictate how each cell is charged and discharged, which enables faster charging. Another gating factor for faster charging is the availability of key components, such as safety capacitors, which have high enough operating-voltage ratings to implement it and meet automotive safety requirements.
The semiconductor industry has made significant strides in the evolution of necessary components, such as introducing silicon carbide MOSFETs and the development of 1200V silicon IGBTs. Nevertheless, if these parts reach their breakdown limits, it can cause the device to fail unpredictably. Breakdown voltages are usually specified at 25°C operating temperatures but may decrease at lower temperatures. Additionally, component manufacturers should consider that their components' safety will be tested at voltages much higher than their rated operating voltages for brief periods, and they should consider measurement-error margins.
Passive component manufacturers must minimize component aging during use, prevent moisture intrusion in harsh automotive operating conditions, and prevent 'creep,' which is the tendency for current to stray across component casings and printed circuit boards (PCBs) in the presence of very high voltages.
Capacitor Evolution
Manufacturers of capacitors are already facing these challenges. EV powertrains require a range of safety features, such as safety capacitors that serve as common-mode filters and circuit isolators. Devices, like the EVA series from Murata, are utilized either across power lines (referred to as class X applications), where a defect would not cause an electric shock, or between the line and the chassis of the vehicle (class Y), which could cause a fire or electric shock in the event of a short circuit.
Figure 1 shows the implementation of safety capacitors in an onboard charger, with two devices (CY1 and CY2) acting as Y capacitors in the primary side of the circuit, two more (CY4 and CY5) taking the same role in the secondary, a two more pairs (CY6 and CY7) performing the same function for the battery (CY6 and CY7) and the traction inverter (CY8 and CY9). Capacitors CX1 – 4 perform smoothing functions throughout the powertrain.
In order to handle the high operating voltages of modern electric vehicles, EV manufacturers currently use multiple lower-voltage capacitors. While these bulky through-hole mounted film capacitors work, they need special handling during manufacture. The adoption of multiple components to simulate the properties of a single high-voltage capacitor results in greater equivalent series resistance and series inductance for the collective device than for a monolithic solution, undermining its effectiveness in reducing electrical noise.
Murata has addressed these issues by creating a selection of surface-mountable multilayer ceramic capacitors (MLCCs) that can handle peak DC operating voltages of 1500V and peak AC operating voltages of up to 305Vrms. The EVA capacitors feature a creep distance of 6 and 10mm, which decreases the probability of arcing compared to typical solutions with a creep distance of 4mm. Incorporating Murata's EVA parts not only minimizes the PCB's size and assembly expenses, but can help to optimize manufacturing yields. The parts' capability to operate at high DC and AC voltages makes them perfect for both typical EV drivetrains and the growing number of vehicles that leverage 800V DC and above battery packs.
Two different packages of the EVA type are shown in Figure 2, one with outside bending type (10mm creepage) and the other with inside bending type (6mm creepage).
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Figure 2: EVA packages with external and internal bending
The EVA devices join Murata's range of safety capacitors, which includes the DE6 parts offered in a through-hole mounting package and the surface-mountable KCA parts, which have a creepage distance of 4mm.Despite the lengthening of EVA parts to meet the requirements of increased creepage distance, their mechanical strength remains unaffected and is similar to KCA parts, owing to their metal terminations and an internal design that focuses on electrical strength. The result is a range of components that will withstand cracks within the device due to PCB flexure and will resist solder cracking between the device and the board more effectively than devices without metal terminations (See Figure 3 and Figure 4).
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Figure 3: Common forms of mechanical failure for MLCCs due to PCB flexure
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Figure 4: A comparison of the robustness of ceramic capacitors with and without metal terminations
Adhering to standards
Components designated for automotive applications must conform to the AEC-Q200 quality standards set by the industry. They must also satisfy the RoHS regulations, exhibit an ability to function at temperatures up to 125C, and demonstrate an aptitude for effectively handling moisture over an extended period. The JEDEC standards body defines Moisture Sensitivity Levels as an indication of the extent to which a component could be damaged during reflow soldering because of moisture trapped within the part. Parts that need to have the lowest risk of this happening must be baked for a fixed period to dry them out, and then supplied on a tape reel in an aluminum pack with a desiccant. Murata's EVA range complies with MSL3, which means that they have been tested to show that they will not crack under MSL2 conditions following the JEDEC standard. Murata will also provide recommendations on the temperature and humidity required for parts storage.
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Figure 5: These cross-sections show how the EVA series capacitors reduce the effects of thermal cycling on critical solder joints
Summary
The rapid development of EVs is changing the way we think about designing vehicles. In order to keep up with the astonishing pace of EV progress, car makers have modified existing technologies, techniques, and components to suit their needs, but it is the responsibility of dedicated suppliers to produce new components that are optimized for the latest applications. Murata has already made considerable strides in creating products for the EV market, with the introduction of EVA MLCCs as proof. With solutions like the Murata EVA MLCCs, designers now have access to purpose-designed components, which can help them to further speed up the development of EV design.
