Vol.3 Application to EV Plug-in Power Charging Systems Overview

Solution Guides

Thanks to increased capacitance, increased high withstand voltage, and other advancements in MLCCs (multilayer ceramic chip capacitors), replacement with MLCCs is now possible even in fields where film capacitors are used. For usage which requires a high level of precision and reliability, C0G MLCCs used in temperature compensation (class 1) with particularly outstanding temperature characteristics offer a variety of replacement merits including significant space savings.
In a temperature range of -55 to +125°C, C0G characteristics meet the very strict standards of a temperature coefficient of 0 ppm/°C and a tolerance of ±30 ppm/°C. Through C0G characteristics, TDK's high voltage MLCCs with C0G characteristics achieve withstand voltage of 1000V at the broadest capacitance range (1nF to 33nF) in the industry. The solution guide "Guide for Replacing Film Capacitors with MLCCs (Vol. 2)" explained EV wireless power transfer systems. However, for the time being, it is certain that the plug-in method will play a leading role in the popularization of EVs. This method charges the battery of EVs (BEVs/PHVs) from household AC power supplies.
Therefore, this section mainly explains the advantages of using MLCCs to replace film capacitors in the onboard chargers (OBC) of plug-in charging systems.

Onboard chargers are required for plug-in charging

The differences between HEVs and EVs (BEVs) are summarized in Figure 1.

Figure 1: Comparison of HEVs and EVs (BEVs)

The biggest difference is that while HEVs run using the combination of a fuel engine and an electric motor, EVs use only an electric motor. Consequently, a system for charging the vehicle battery from an external power supply is essential for EVs.
Cruising distance increases concurrently with increases in battery capacity. For that reason, the trend is for EV battery size to increase. Furthermore, as a result of desire for a shorter charging time, there is a trend of increasing battery voltage.

《Characteristics of EVs (BEVs)》

Battery size is larger than in HEVs. To increase cruising distance, battery size tends to increase.
EV batteries have a voltage of about 400V to 600V or higher, compared to a voltage of about 150V to 300V in HEVs.
Onboard chargers are required for charging from commercial AC current.
A BMS (battery management system) for processing several kW of power is required.

Basic structure and function of onboard chargers

There are two types of EV plug-in power charging systems: rapid charging and normal charging. Rapid charging systems are installed as charging stands at highway service areas / parking areas, large commercial facilities, etc. Rapid charge systems use three-phase AC current sent from high-voltage charging facilities. These systems have the advantage of a short charging time. However, the systems require a dedicated infrastructure and are expensive.
Normal charging systems use commercial AC current. Charging is performed via a cable connecting the EV to external outlets of homes, etc. Although this method takes longer than rapid charging, it eliminates the inconvenience of going to a charging stand and has the advantage of enabling inexpensive charging at home at any time. However, when using plug-in charging, since it is not possible to charge the battery using AC current, it is necessary to use the onboard charger to convert to DC current. The basic principles of onboard chargers are shown in Figure 2.

Figure 2: Basic structure of onboard charger (OBC)

First, commercial AC current is rectified and smoothed in the AC block of the onboard device. Next, the power is sent to the DC-DC converter via the PFC (Power Factor Correction / harmonic current suppression circuit) block. The DC-DC converter changes the input voltage to an appropriate output voltage and then charges the battery.
Onboard charger DC-DC converters are used at a higher voltage than DC-DC converters installed in general electronic devices, and they also require a high conversion rate in order to increase cruising distance. Consequently, more and more manufacturers are using LLC resonance DC-DC converters (hereinafter referred to as "LLC converters").

Example of replacing with MLCC: resonance capacitor of LLC converter

Figure 3 shows an example circuit (full bridge type) for a current resonance LLC converter used in an onboard charger.

Figure 3: Example circuit (full bridge type) for current resonance LLC converter

Lr and Lm are the leakage inductance and excitation inductance of the transformer respectively, and compose the resonance circuit along with the capacitor Cr. The name "LLC converter" is used because the converter is composed of two inductances (L, L) and a capacitor (C). Since a resonance capacitor is connected in series to the transformer in this circuit, this type is known as a serial resonance converter or a current resonance converter.
Normal DC-DC converters employ the PWM (pulse width modulation) method, in which they obtain the required output voltage by controlling the width of pulse current sent to the transformer at a certain switching frequency. On the other hand, the LLC converter uses the PFM (pulse frequency modulation) method, which controls the switching frequency while maintaining a fixed pulse width. Therefore, the resonance capacitor requires superior characteristics.

Little variation in capacitance and tanδ ; optimal as a resonance capacitor

Since LLC converters have a PFM power supply which uses LC resonance, transformers and resonance capacitors are both extremely important components. The following types of characteristics are required in resonance capacitors which are used in the LLC capacitors of onboard chargers.

《Characteristics required in resonance capacitors of LLC converters》

Superior temperature characteristics
Since the resonance capacitors are used in resonance circuits, it is extremely important that the capacitance change caused by temperature fluctuations is small.
Superior withstand voltage characteristics
LLC converters are power supplies appropriate for use with relatively high power. However, since larger voltage rectangular waves than those in general electronic devices are applied, high withstand voltage (rated voltage) is required.
Superior ESR characteristics
Since a large current flows in resonance circuits, superior ESR is required.

In the past, film capacitors were normally used as resonance capacitors in the LLC converters of onboard chargers. This was because film capacitors have a good balance of withstand voltage and relatively high capacitance. However, in recent years, MLCCs have been developed with characteristics that approach the region of film capacitors, and there is an increasing need for a replacement for film capacitors in automotive electronics.
MLCCs are divided into two major categories according to their type of dielectric, namely class 1 (temperature compensating) and class 2 (high dielectric constant). Since class 1 MLCCs have a small capacitance change caused by temperature, and possess outstanding frequency characteristics, they are used in circuits, etc. which require high precision. Among class 1 MLCCs, C0G MLCCs have extremely superior temperature characteristics, making them optimal as resonance capacitors. Another advantage of C0G MLCCs is that they are smaller than film capacitors.

Automotive Grade MLCC (multilayer ceramic chip capacitor) CGA series C0G characteristics

TDK has mid voltage MLCCs (rated voltage 100 to 630V), high voltage MLCCs (rated voltage of 1000V and higher), and other MLCCs in our automotive grade CGA series. In this series, we offer the following as products with a rated voltage of 1000V, C0G temperature characteristics, and capacitances of 1nF to 33nF. In addition to resonance capacitors for magnetic resonance wireless power transfer, these MLCCs can also be used to replace film capacitors in applications which require high accuracy, such as time constraint circuits, filter circuits, oscillation circuits, etc. for downsizing and surface mount technology (SMT). Furthermore, for even greater reliability, TDK offers a MEGACAP type and a soft termination series which are highly resistant to external environmental stresses such as board bending that causes body cracks, heat shock which causes solder cracks, and vibration.
Wireless power transfer technology for efficient charging of batteries is the key to automotive evolution such as EVs and autonomous driving. In magnetic resonance wireless power transfer, resonance capacitor characteristics are closely related to power transmission efficiency. TDK's high voltage MLCCs with C0G characteristics that achieve withstand voltage of 1000V are temperature compensation (class 1) MLCCs. They possess optimal characteristics as resonance capacitors in EV wireless power transfer.
Another important element of high voltage MLCCs with C0G characteristics is extremely low ESR. TDK will continue to work at further enhancing our product lineup by improving withstand voltage, capacitance range, and other characteristics.