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.
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)》
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").