DC-Link Capacitors Cater to SiC Power Conversion

A DC-link capacitor is a key building block of a power converter placed between the inverter and rectifier stages, acting as a local energy reservior.

These passives play two primary roles: decoupling parasitic inductance between the input to the output sides of the power supply and providing a low-impedance path for high-frequency current. They can also serve as buffers that smooth out voltage ripples and switching voltage transients as well as filter out electromagnetic interference (EMI).

Given all that, they play a key role in the power electronics piling into the electric grid, including three-phase inverters for solar and wind and grid-tied battery energy storage systems (BESS). They're also finding their way into new solid-state transformers (SSTs) rapidly being developed for both AI data centers and DC fast chargers for electric vehicles. In the drive for higher power density and efficiency, these systems are all heavy adopters of silicon carbide (SiC), which provides high blocking margins and fast switching speeds.

While SiC MOSFETs can switch much faster than the IGBTs they replace, this drives both higher voltage transitions (dv/dt) and higher current ripple into the capacitor. As a result, companies are rolling out more advanced DC-link capacitors that counteract all that without forcing engineers to derate or oversize the components or add cooling.

Of particular importance for these parts are high DC voltage ratings, large capacitance values, wide operating temperature ranges, and low parasitic resistance and inductance.

TDK’s B25696H series is one such family of high-performance film capacitors that can fill the DC-link role in SiC-based power electronics. The new capacitors feature a metallized polypropylene (MKP) dielectric film, and come with capacitance values ranging from 47 to 1,280 µF and voltage ratings ranging from 900 to 2,000 V DC.

The passives, which can handle internal temperatures as high as 85°C, also have ultra-low self-inductance (ESL) and low equivalent series resistance (ESR). ESL and ESR are both key to improving efficiency and unlocking faster switching frequencies.

DC-Link Capacitors Bridge the Power Gap

In a BESS, for instance, DC-link capacitors are deeply embedded in the power converter system (PCS) that connects to the electric grid, a local microgrid, or a data center or other site. The PCS acts as the "brain" of the system, converting AC to DC to charge the lithium-ion battery cells and vice versa to discharge them. The BESS feeds stored energy (often solar, wind, and other renewables) into the grid when needed (particularly when the sun stops shining and wind dies down).

A DC-link capacitor bridges the gap between both sides of the bidirectional power supply. The AC-DC converter stage — often based on totem-pole power-factor correction (PFC) or other topologies such as active neutral point clamped (ANPC) or neutral point clamped (NPC) — convert AC from the grid into a DC bus voltage, using SiC in many cases. On the other side, the isolated DC-DC converter stage — the dual active bridge (DAB) topology is the industry standard — adjusts the DC bus voltage to provide correct DC levels to the batteries.

The DC-link capacitor acts as a buffer, handling high-frequency current ripple generated by the pulse-width-modulation (PWM) switching process. The DC link also decouples parasitic inductance within the system, reducing voltage overshoots that can occur during switching. Without a well-sized DC-link battery capacitor in these situations, switching events cause voltage spikes that can stress every component on the DC bus, including the SiC MOSFETs themselves.

In a smaller backup battery system, DC bus voltages commonly range from 200 to 800 V, while a large-scale, grid-tied BESS often ranges from 1,000 to 1,500 V DC to leverage the advantages of high-voltage operation. When a switching device turns off, the current flowing through the parasitic inductance of the DC bus attempts to continue flowing. That induces a voltage spike. The higher dv/dt of SiC MOSFETs and the higher voltages on the DC bus can make these voltage spikes a lot more severe than usual.

To suppress these voltage spikes, the DC-link capacitance (C_bulk) typically ranges from several hundred to thousands of microfarads. Due to the high demands on capacitance, small MLCCs are often paired with film capacitors in the DC link to fill in the gaps. The MLCCs can be integrated closer to the power switches to limit loop inductance on the PCB and get rid of more high-frequency current ripple.

Unique Design for the Unique Challenges of SiC

TDK said its new DC-link capacitor is differentiated by its unique internal busbar configuration that ensures uniform current distribution across the capacitor’s windings. This is the case even at the higher switching frequencies of SiC-based power electronics, typically 10 to 100 kHz. That drives down the ESL as low as 30 nH and the ESR to 0.8 mΩ at 10 kHz, while maintaining excellent ESR stability up to 100 kHz. Together, these characteristics help reduce ESL-related parasitic inductance and ESR-related power losses.

By minimizing both of those metrics, TDK said the B25696H can help reduce voltage overshoots and electrical stress on SiC power devices, which helps remove limits on operating frequencies.

And by reducing switching-related stress, the cylindrical, aluminum-encased devices can enhance thermal margins as well as overall system reliability and robustness. Thermally, the series is specified for operation within a hotspot temperature range of -40°C out to +85°C.

TDK said the cap can also handle high ripple current, which is critical because it's constantly consuming and releasing energy at the fast switching speeds of SiC. In turn, high ripple currents heat up the capacitor, causing degradation over time.

As a result, ripple current is a primary life-limiting factor for both the capacitor and the overall system’s reliability and longevity. At 10 kHz, TDK said the device can deal with ripple currents of 91 A at 60°C ambient temperatures.

The expected operating life of the capacitor is 100,000 hours at a +75°C hotspot temperature and rated voltage. But its lifespan could be extended up to 200,000 hours when voltage and temperature derating is done. The components comes in 85- or 100-mm diameters.

Engineers frequently use advanced thermal simulation tools such as TDK's CapThermal to accurately estimate component lifetime based on application-specific operating conditions.