A Closer Look at Resonant Capacitors in Electric Vehicles

One of the top priorities for electric vehicle (EV) applications is to improve the efficiency of its power conversion devices. The more efficient the power conversion, the longer the EV can travel on a single charge.

For example, if the losses in a DC/DC converter can be reduced, the heat generated by the associated components will be reduced, and their design will be more efficient, which will also improve energy efficiency and benefit the converter (and the vehicle).

There are many types of DC/DC converters on the market, but this article will focus on resonant DC/DC converters. A resonant converter is a type of DC/DC converter topology that controls the switching frequency to achieve a resonant circuit with a constant output voltage.

Resonant converters are often used in high voltage applications to smooth the waveform, improve the power factor, and reduce switching losses caused by high frequency power switches such as MOSFETs and IGBTs.

It is important to note that resonant converters often use LLC circuits because they can achieve zero voltage switching (ZVS) and zero current switching (ZCS) within the operating range, support higher switching frequencies, reduce component footprint, and reduce electromagnetic interference (EMI).

Figure 1. Schematic diagram of a resonant converter (Source: STMicroelectronics)

The resonant converter is built on a resonant inverter, which uses a switching network to convert the DC input voltage into a square wave, which is then applied to the resonant circuit.

As shown in FIG. 2 , the resonant circuit is composed of a resonant capacitor Cr, a resonant inductor Lr and a magnetizing inductance Lm of a transformer connected in series.

The LLC circuit works by selectively absorbing the maximum power at a specific resonant frequency of the square wave and releasing the sinusoidal voltage through magnetic resonance to filter out any higher-order harmonics.

This AC waveform is amplified or reduced by a transformer, rectified, and then filtered to produce a converted DC output voltage.

Figure 2. Simplified LLC resonant DC/DC converter.

When selecting a suitable resonant capacitor Cr for a DC/DC converter, the capacitor's root mean square (RMS) current is one of the important parameters to consider.

It affects capacitor reliability, voltage ripple and overall converter performance (depending on the resonant circuit topology). Heat dissipation is also affected by the RMS current and other internal losses.

In fact, self-heating of capacitors is a common problem in electric vehicle converter systems and other high-power applications.

For resonant capacitors, we usually recommend multilayer ceramic capacitors (MLCCs) with stable capacitance, high precision, and low power consumption to prevent overheating - our 500V-1kV, 4.7nF-39nF C0G capacitors are ideal for such applications.