Don’t Let the DC Link Capacitor be the Weak Link in Your Power Converter Design
Direct current (DC) link capacitors are a critical component in many applications, including three-phase inverters for electric vehicle (EV) motor drives, photovoltaic and wind power inverters, industrial motor drives, automotive onboard chargers, and power supplies for medical or industrial equipment. Keeping up with the latest developments is important. If not properly implemented, DC link capacitors can be a “weak link” that reduces energy density and reliability.
Unfortunately for designers, unlike semiconductor technology that’s advancing at a rapid pace, advances in capacitor technology are slow and can be overlooked. Adding to the challenge, various capacitor technologies are advancing at different paces: aluminum electrolytics are a more mature and slower evolving technology, while film capacitors and multilayer ceramic capacitors (MLCCs) are advancing more rapidly. Aluminum electrolytic capacitors typically offer greater capacitance per unit volume and higher energy densities compared with film capacitors and MLCCs; but the tradeoffs are not fixed.
For example, upgrading power switches with higher frequency devices—such as replacing IGBTs with MOSFETs or replacing silicon devices with wide bandgap (WBG) power switches—can be a good time to reconsider past choices for DC link capacitors. Each DC link capacitor technology offers a unique set of capabilities (Figure 1).
Figure 1: DC link capacitor comparison showing voltage vs capacitance for the major technologies. CeraLink capacitors from TDK are MLCCs optimized for DC link applications. (Image source: TDK Corporation)
Aluminum electrolytics (‘lytics) are the most common DC link capacitors. They offer a combination of high energy density and low cost. They are often used for industrial motor drives, uninterruptible power supplies (UPSs), and a variety of consumer, commercial and industrial applications. However, their relatively short lifespan and low-frequency operation can exclude ‘lytics from consideration in more demanding applications.
Film capacitors are often found as the DC link element in more demanding applications such as EV traction drives. Film capacitors have higher reliability, high current conduction capability, lower equivalent series resistance (ESR), and can be used at higher frequencies compared with ‘lytics. But, like ‘lytics, film capacitors have relatively low operating temperatures of about 105 degrees Celsius (°C).
MLCCs present a third possibility. These capacitors have a higher root mean square (rms) current rating and can withstand higher temperatures than other capacitors. The downside is that it can take a relatively large number of MLCCs for a given energy density, making it challenging to implement a capacitor layout that ensures equal current distribution. In addition, there can be reliability issues associated with MLCCs; the ceramic dielectric material is rigid and can crack due to mechanical or thermal stresses, creating a short-circuit between the terminals.
It’s apparent that the “perfect” capacitor technology for all DC link applications doesn’t exist. To arrive at the best design solution for a given project, you need to review the latest technological advancements and product developments. So, let’s consider some of the tradeoffs and capabilities of representative device types, including aluminum electrolytic from Cornell Dubilier Electronics, film from KEMET, and MLCCs from TDK Corporation.
Electrolytics for high ripple designs
For applications with high ripple currents, you can use the 381LR series from Cornell Dubilier Electronics that are rated for 200 to 450 Vdc and 56 to 2,200 microfarads (µF) and can handle at least 25% more ripple current compared with standard 105 °C snap-in ‘lytics (Figure 2). Recent advances in electrolyte formulations are key to the low ESR that gives these capacitors their ripple current capability. This means that fewer capacitors are needed in motor drives, uninterruptible power supplies (UPSs), and other high ripple current applications.
Figure 2: The 381LR aluminum electrolytic capacitors are rated for 200 to 450 volts DC and 56 to 2,200 µF. (Image source: Jeff Shepard, based on source material from Cornell Dubilier Electronics)
Film capacitors for automotive traction drives
If you’re designing systems for harsh environments such as automotive traction drives, the KEMET DC link C4AK film capacitors with a lifetime of 4,000 hours at 125°C and 1000 hours at 135°C are a good option (Figure 3). Designed for compact system designs, these devices have a radial box format for pc board mounting with a low profile and enable the use of fewer capacitors in parallel to handle peak and ripple currents.
Figure 3: The KEMET DC link C4AK film capacitor series have a lifetime of 4,000 hours at 125°C and 1000 hours at 135°C. (Image source: KEMET)
The C4AK DC link capacitors are designed for use in high-frequency, high-current EV system power converters, photovoltaic and fuel cell inverters, energy storage systems, wireless power transfer, and other industrial applications.
MLCCs for fast WBG semiconductors
When using WBGs, the CeraLink FA (flex assembly) family from TDK Corporation may provide an appropriate solution. The family includes capacitance values from 0.25 µF up to 10 µF and rated voltages between 500 and 900 volts DC. For example, the B58035U9255M001 is rated at 2.5 µF and 900 volts (Figure 4). The various devices in the CeraLink family are optimized for use as DC link capacitors, with features that include:
- Capacitance densities of 2 to 5 µF per centimeter cubed (cm³)
- Low self-inductance of 2.5 to 4 nanohenries (nH)
- The ability to be placed very close to the semiconductor power device with operation up to 150°C permissible (for a limited time)
- No limitation on voltage slew rate (dV/dt)
Figure 4: The B58035U9255M001 is part of TDK Corporation’s CeraLink FA family, a 2.5 µF, 900 volt MLCC stack. (Image source: TDK Corporation)
FA family capacitors are 9.1 millimeters (mm) wide by 7.4 mm high and are available in lengths of 6.3 mm, 9.3 mm, and 30.3 mm. They feature a ripple current capability of up to 47 amperes (A) rms.
Specifying a DC link capacitor is an important part of designing power converters. As shown, there’s a wide range of possible options, which are subject to change. Making a poor selection can result in a power converter that doesn’t meet expectations or one that’s too expensive. To avoid making a bad choice, you need to keep up to date on the latest developments in DC link capacitor technologies and products.