Capacitor manufacturers are reacting to changing demand for lower ESR
KEY TAKEAWAYS:
- Prolonged incremental gains are moving ESR in the right direction
- Emerging markets and their demands are directing efforts
- Research shows signs of a potential breakthrough
If a switched-mode power supply fails after years of service, there’s a good chance a capacitor could be the cause, and the failure may be from self-heating due to the capacitor’s equivalent series resistance (ESR).
Over the last five years or so, capacitor manufacturers have made progress toward reducing ESR in capacitors. The real goal is to keep ESR low as the operating voltage increases. This is a parallel market demand driven by newer applications, such as electric vehicles and the more general shift toward 48V systems.
ESR represents the combined and unwanted resistances that accrue inside a capacitor due to its construction and materials used. As a resistance, it interplays with the current flowing through and voltages developed across the capacitor. These combine under Ohm’s Law to produce unwanted power, manifesting as heat. That heat is difficult to dissipate, thereby raising the capacitor’s temperature during operation and shortening its useful lifetime.
The gains include tweaks to the materials used and advances in capacitor construction. It’s a story of continuous, incremental improvement, but some research indicates a bigger breakthrough may be close.
How does capacitor chemistry affect ESR?
Wet aluminum is the baseline technology and the electrolytic capacitor technology most engineers encounter first. Aluminum is used for both the anode and cathode, with a paper spacer soaked in liquid electrolyte. This electrolyte is the main source of the capacitor’s ESR. The EEU-FR series from Panasonic is a favorite for SMPS designers, as is the EEU-FS series, which has a smaller case size but similar ESR and ripple current.
Solid aluminum electrolytics and some tantalum capacitors use manganese dioxide (MnO2) as the dielectric. This material behaves contrary to wet aluminum, as its conductivity improves at low temperatures. Both types have long-term ESR stability, and MnO2 tantalum capacitors are widely deployed in all verticals, including aerospace, so their reliability isn’t in question. However, they have a unique failure mode in the form of thermal runaway. If a MnO2 tantalum capacitor has a dielectric defect, it can suffer from self-heating caused by leakage current and its negative temperature coefficient. The failure mode is well understood, and it is standard practice to derate operating voltage by 50% and specify a minimum ESR.
Electrolytic manufacturers have largely moved away from MnO2 and wet aluminum cathodes in favor of tantalum-polymers and aluminum-polymers, primarily to achieve lower ESR. The chemical name for the preferred material is poly(3,4-ethylenedioxythiophene), but it is normally referred to simply as PEDOT. This is typically doped with poly(4-styrenesulfonate) to produce PEDOT:PSS. Using PEDOT:PSS instead of MnO2 can reduce ESR to about one-tenth of that of an equivalent MnO2-based tantalum capacitor.
Research as recently as 2025 shows that fully-dissolved self-doped PEDOT, or S-PEDOT, achieves lower ESR than PEDOT:PSS. Separate studies show secondary doping and partially removing PSS also lowers ESR.
What are polymer hybrid capacitors?
Conductive-polymer hybrid aluminum capacitor technology occupies a widening niche. A hybrid capacitor has a solid conductive polymer and a liquid electrolyte, tackling both ESR and dielectric defects.
It addresses the tradeoff, choosing between a polymer for low ESR, or wet aluminum for its self-healing characteristic. This shift is driven almost entirely by the move to 48V power in the automotive industry and in data center architectures. Both of these applications now demand higher voltage ratings than hybrids could previously offer.
The main gain with the shift toward polymer hybrids is keeping ESR low while pushing voltage levels and operating temperatures up. Those figures, voltage and temperature, are trending upward, following the curve of wider electrification. The same need will likely be felt in other market sectors in the future.
The market for hybrid capacitors is expected to grow faster than the broader market, making it a standout yet established technology. As EV makers move toward 800 V battery platforms, they put more stress on DC-link capacitors, while 48 V mild hybrid vehicles are expected to account for over half the market’s growth.
Together, the wider use of pure aluminum and hybrid capacitors represent the most significant capacitor technology shift the industry is likely to see in the next five years.
How capacitor architecture can lower effective resistance
Chemistry can lower the specific resistance, but effective resistance can be lowered by the structure of the capacitor. Multi-anode devices use several anodes in parallel inside a single tantalum-polymer case. This can push ESR down to single digits in the milliohm range. As many as 40 anodes can be placed in parallel to exploit this technique. The Kyocera AVX TPM multi-anode series features a ‘mirror’ mode to cut ESR in half.
Stacking thin polymer aluminum is another technique to lower ESR. Looking at MLCCs, TDK has developed multi-terminal and reverse/lateral electrode geometries to push both ESR and ESL downward.
The combination of incremental gains in hybrid polymers, multi-anode MLCC architectures and developments in materials are converging to give power supply designers greater choice and the options they need, as the markets they address continue to demand more.
