Benefits of Rugged DC-Link Capacitors in Harsh Applications
The DC-link capacitor has a key role to play in several application sectors that are rapidly evolving such as electrically-propelled vehicles and renewable energy. However, as designers seek ever greater performance and reliability, DC-Link capacitors must work in increasingly challenging environments.
According to a Precedence Research report covering the electric vehicle market, the global electric vehicle market is expected to reach a compound annual growth rate (CAGR) of 40.7% from 2020 to 2027. As a result of this market expansion, the demand for high-performance, rugged DC-Link capacitors that are suited to automotive use will increase at a similar level.
This article, will look at where DC-Link capacitors are commonly used and examine the critical considerations for choosing the correct device for an application.
DC-Link capacitors – an overview. Image used courtesy of Bodo’s Power Systems
DC-Link capacitors – an overview
DC-Link capacitors are an essential element within power conversion systems, especially in modern applications such as electricallypropelled vehicles where they are used in inverters that drive the motors and onboard chargers (OBC). Other applications include renewable energy systems, including photovoltaic and wind power inverters and industrial motor drives. Acting as a charge reservoir to smooth out the ripples inevitably introduced by switching power conversion solutions, DC-Link capacitors play a vital role in enhancing energy density and reducing ripple and noise.
However, components for use in power systems increasingly have to contend with harsh environments. In almost every application, space is at a premium meaning that designs are compact and, therefore, cooling is more challenging, leading to higher temperatures. Additionally, the advent of wide-bandgap semiconductors (SiC & GaN) allows designers to operate power conversion designs at higher temperatures, reducing active cooling needs and increasing efficiency – albeit by exposing components to additional thermal stress.
The automotive arena is particularly challenging as long-term vibration is an additional challenge while the vehicle is in motion alongside the thermal challenges and extreme space and weight constraints. With battery voltages increasing to reduce losses and extend range, the voltage across the DC-Link capacitor increases, putting a greater strain on the dielectric material.
Strong and rapidly increasing demand for electric vehicles, and within the renewable sector, requires high-performance components that are compact, lightweight, and have high dielectric capacitance coupled with high heat resistance for extended operational profiles. Additionally, capacitors used in converter and motor drive inverters require greater assurance of high-temperature performance than ever before, further adding to the challenges designers face in these demanding sectors.
Selecting a DC-Link capacitor
Within electrically-propelled vehicles, one or more traction inverters is/are required to convert the high-voltage DC battery voltage into a three-phase drive for the electric traction motor(s). The DC-Link capacitor is an essential element of this system, ensuring a smooth and stable DC voltage on the DC-bus.
Figure 1. The DC-Link capacitor is an essential element of electric vehicle motor drive systems. [Source: https://ec.kemet.com/blog/dc-link-design-tips/]. Image used courtesy of Bodo’s Power Systems
The sole role of the DC-Link capacitor is to balance the fluctuating instantaneous power on the DC-bus created by switching activity within the battery controller (stage 1) and especially reflected ripple and noise from the high-frequency power switching devices within the three-phase drive (stage 3). To achieve this, it acts as a charge reservoir to buffer the sporadic demands for heavy current from the inverter in stage 3.
This formula gives the minimum capacitance needed to achieve a states ripple voltage:
$$C_{min}\frac{I_{out}\times\, dc\times\,(1 - dc)\times 1000}{f_{SW}\times V_{P(MAX)}}$$
Cmin = required minimum capacitance
Iout = output current
dc = duty cycle
fSW = switching frequency
VP(MAX) = peak-to-peak ripple voltage
It is possible to use different technologies such as aluminum electrolytic, film, or ceramic type devices for the DC-Link capacitor with the (sometimes difficult) choice depending upon the application it is to be used in. The first step in choosing a DC-Link capacitor is to consider the voltage ratings and nominal capacitance values of potentially suitable devices, ensuring high ripple current ratings. All DC-Link capacitors are required to regulate voltage and absorb the ripples that occur in the current waveform. As designs move to faster switching MOSFETs and IGBTs, the increasingly fast ripple affects performance as every ‘real’ capacitor contains some impedance and self-inductance. As the operating frequency of the inverter increases, some capacitor technologies are precluded – such as not using film capacitors above 1MHz.
Fully understanding the application is crucial when selecting DCLink capacitors. As well as the DC bus voltage and operating frequency, designers must take the anticipated lifetime of the application into account and understanding the maximum possible ripple current and whether it is steady-state or variable. Ideally, the capacitor selected for a DC-Link application will have low values of self-inductance as well as low Equivalent Series Resistance (ESR) and high ripple current tolerance. The operating frequencies and operating temperature need to be compatible with the application. In general, film capacitors can cope with high ripple current and offer a longer operational life while offering substantially higher values than ceramic types.
Meeting the challenge
Meeting all the parametric needs for a DC-Link capacitor will be challenging, requiring designers to review many candidates before finding the optimum device.
One device that is a strong contender is KEMET’s new high temperature, power film DC-Link capacitor. Denoted the C4AK series, it is specifically designed for continuous operation in rugged applications with elevated temperature profiles – such as those found in automotive applications.
C4AK devices offer a range of capacitance values from 1.5uF to 60uF and are rated for DC voltages up to 900V, making them suited to a wide range of EV powertrain uses. Even in harsh environmental conditions, they achieve a lifetime of 4,000 hours at 125°C and 1000 hours at 135°C, ensuring the long-term reliability of vehicles. The devices have been tested to the automotive AEC-Q200 standard, providing further reassurance to designers of their suitability.
A real-world example that has a current of 12A, operating voltage of 700V, switching frequency of 10kHz, and requires a DC-Link capacitor of 20uF. The application profile requires an operating temperature of 50°C for two-thirds of the time and 100°C for the remainder. KEMET’s new C4AKOBW5200A3LJ DC-Link capacitor achieves this application profile easily, handling both the current and voltage while delivering a predicted lifetime of 66,400 hours.
Conclusion
DC-Link capacitors are essential for many modern systems including electric vehicles, renewable energy, and industrial applications. However, market pressure is forcing designers to deliver ever more compact solutions which require high-performance components that can cope with elevated temperatures and the need to operate over long lifetimes. KEMET’s C4AK series addresses many of the challenges experienced in selecting a DC-Link capacitor for automotive applications. It combines long-term reliability with 900V operation at ambient temperatures as high as 135°C, making it ideal for demanding automotive applications.
This article originally appeared in Bodo’s Power Systems magazine.
