Resonant MLCC for High-Power Wireless Charging
AEI by Dempa Publications, Inc., September 2019
The newly developed multilayer ceramic capacitors from Murata Manufacturing Co., Ltd. have achieved rated voltages of 630V and 1,000V by employing a low-loss material. They are designed for application in resonant circuits incorporated in wireless power transfer (WPT) for vehicles, on-board chargers for electric vehicles (EVs) and plug-in hybrid vehicles (PHVs), LLC power supplies, and other high-power applications.
Capacitors for these resonant circuits require at least 10nF and stable capacitance, and, conventionally, fi lm capacitors were the only option. However, today, multilayer ceramic capacitors are the mainstream as they have the following advantages compared with film capacitors: 1) small volume; 2) low heat generation [low equivalent series resistance (ESR)]; 3) low equivalent series inductance (ESL); 4) excellent long term reliability, and 5) high maximum operating temperature.
Capacitors for resonant circuits are divided into two product types: the common chip capacitor and the capacitor with a metal terminal attached to chip capacitor (Table 1).
The metal terminal type enables double stacking of large-size (5750M size) chips, leading to the reduction of mounting area. It also reduces the risk of solder cracks, which is an issue in the automotive market.
LLC Resonant Circuit in Power Supply
More products adopt WPT
Products that incorporate WPT are not limited to small products, such as smartphones, watches, and tablets, but the adoption has also been expanding to large products, such as automobiles, transfer robots for use in the manufacturing process, and drones.
In WPT, large power is exchanged using LC resonant circuits, and therefore, capacitors for low-loss resonant circuits are needed. The multilayer ceramic capacitors with rated voltage of 630V and 1,000V developed this time adopt a product design suitable for use in high-power applications.
Large power supplies with LLC resonant circuits
The adoption of high-efficiency LLC resonant circuits has been expanding in large power supplies that exceed 100W, such as on-board chargers for EVs and PHVs, power supplies for servers, power supplies for infrastructure equipment, and power supplies for large facilities. In particular, LLC resonant circuits are adopted in more than 90 percent of onboard chargers for EVs and PHVs according to Murata.
Characteristics of Capacitors for Resonant Circuits
High applied voltage
A high voltage V(p-p) of several hundred volts V(p-p) to ten thousand volts V(p-p) is applied to capacitors for resonant circuits. In particular, WPT for automobiles uses large power, and in some cases, the voltage reaches 10,000V(p-p). The developed multilayer ceramic capacitors are rated at 630V and 1,000V, and therefore, when the operating voltage is high, it is necessary to connect the capacitors in series. As the combined capacitance is reduced when the capacitors are connected in series, it is necessary to secure the necessary capacitance using parallel connection.
As a result, in most resonant circuits, multilayer ceramic capacitors are used by adopting multiple series and multiple parallel connections
High resonant frequency, high current
For example, in the automotive market, the resonant frequency of WPT for automobiles is fixed at 85kHz, as an international standard. However, the resonant frequency of on-board chargers for EVs and PHVs is in the range of 60 to 400kHz depending on the maker. The application of high voltage with high frequency to the capacitor makes it susceptible to self-heating.
Therefore, capacitors for resonant circuits are required to feature low loss and temperature rise suppression due to self-heating during long term use.
Long period of use
Many large products that incorporate resonant circuits are used in automobiles, industrial electric vehicles, and infrastructure, with long period of use, and therefore, capacitors also need to have reliability in the long term. The developed products are designed to have a target life of 10 years with continuous use.
Important Points in Selecting Capacitors
Choosing the wrong capacitors for high-power application can lead to issues, like smoking and ignition of the equipment, and therefore, capacitors should be selected based on their characteristics.
According to Murata, the capacitors’ self-heating and allowable voltage curve are especially important characteristics.
In capacitors used for high-power application, temperature rise due to self-heating is observed after initial heat generation immediately after the application of voltage. In high-power application, temperature rise due to self-heating is unavoidable. However, voltage and frequency conditions under which the maximum operating temperature exceeds 125°C during the target life (for example, 10 years) should be avoided (Fig. 1). With the developed products, the allowable voltage is set at the voltage when the surface temperature of the capacitors reaches the maximum operating temperature of 125°C becomes the target life. Therefore, when selecting capacitors, operating voltage V(p-p) needs to be within the allowable voltage.
Murata sets the allowable voltage curve, which indicates the allowable voltage in accordance with the frequency for each item (Fig. 2). It is available in the specifications of products or in the specification sheet on the company’s homepage.
The relationship between the allowable voltage and frequency, according to Murata can be described below. The allowable voltage curve shown in Fig. 2 has been created by generalizing the allowable voltage graph set for each item, and it can be divided into three regions in accordance with frequency band.
Region : Region limited on the basis of rated voltage (~several 10kHz)
As the frequency is low at 10kHz or lower, capacitor’s self-heating is small, and therefore, rated voltage becomes the allowable voltage.
However, few capacitors for medium and high-voltage resonant circuits are used in such low frequency region.
Region : Region limited on the basis of continuous temperature rise (several 10 to several 100kHz)
Self-heating immediately after the application of voltage is within ΔT20°C*1. However, in this area, temperature rise due to self-heating is observed by the application of high voltage of several 10kHz to several 100kHz.
In this region, the voltage when the capacitor’s surface temperature reaches the maximum operating temperature of 125°C becomes the target life is defined as the allowable voltage. (Target life is set at 10 years in the case of products introduced this time.)
Most of capacitors for medium- and high-voltage resonant circuits belong to this region.
Region : Region limited on the basis of initial heating immediately after the application of voltage (Several 100kHz~)
With higher frequency, self-heating of capacitor immediately after the application voltage exceeds ΔT20°C. As described above, Murata Manufacturing sets the conditions for the use of capacitors with self-heating within ΔT20°C regardless if it is low-loss-type or high dielectric constant-type chip capacitors, the voltage at which self-heating of capacitor becomes ΔT20°C is the allowable voltage. However, few capacitors for medium- and high-voltage resonant circuits are used in such high-frequency region.
Part Number Selection Tool
Selecting capacitors for resonant circuits is difficult due to the following reasons: 1) As the use voltage tends to become high, in many cases, capacitors are used by adopting multiple series and multiple parallel connection, necessitating the calculation of combined capacitance; and 2) The applied voltage V(p-p) of the capacitor itself needs to be set at the allowable voltage or lower.
Murata Manufacturing has been preparing a tool, which displays optimum product and the number of series and parallel connections based on the customer’s use conditions. It can be done by simply inputting operating voltage, operating temperature, or necessary capacitance, thus reducing customers’ burden in selecting part number and in designing. This tool will be made available at Murata’s homepage in the spring of 2020.
*1 Murata Manufacturing sets the conditions for the use of capacitors that self-heating of capacitor is within ΔT20°C regardless if it is low-loss or high dielectric constant-type chip capacitors.
About This Article:
The author, Makoto Kashiwai, is from the Product Planning Sec. 2, Product Engineering Dept., Capacitor Div., Murata Manufacturing Co., Ltd.