Parasitics are a part of dealing with electronics: as much as engineers, designers, and manufacturers wish it were possible to work with ideal components, the real world has other plans. Mitigating these parasitic effects is the approach, and there’s a considerable effort at all stages of device DFM dedicated to tracking and compensating for parasitics where they do show up. While other forms of parasitics are present in capacitors, perhaps none is as crucial to device performance as the equivalent series resistance or ESR. By prioritizing low-ESR capacitors and design/manufacture techniques that delay the increases in ESR, device performance matches specifications for as long as possible.
| Step-by-Step Guide to Low-ESR Capacitors | ||
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Low-ESR Capacitors: Less Loss, Longer Life, Better Performance
Usually, reducing components’ resistance (and thus the resistive losses) is welcome as it improves power efficiency, and low-ESR capacitors are no exception. ESR can be succinctly summarized as the resistance contributions to the total capacitor package; in other words, it represents the real-valued resistance R in the vector-form impedance Z = R + jX (where j, X are the imaginary number and reactance, respectively). It’s possible to further break down the contributions as such:
- Lead resistance
- Plate resistance
- Connection resistance
- Leakage resistance
- Dielectric resistance
The value of a low-ESR capacitor is multidimensional; since the resistance from the ESR is parasitic from the perspective of the capacitor function, optimizing its reduction ensures performance meets specifications. Additionally, a higher resistance means more energy losses due to dissipation. This factor makes a circuit less efficient from an energy standpoint, and since the energy lost dissipates as heat, the component and surrounding materials experience accelerated heat-related aging. Generally, ESR tends to increase with higher voltage ratings and capacitance values, but the material and construction of the capacitor will significantly affect the range of ESR.
How To Select (And Maintain) Low-ESR Values
An acceptable ESR value will be highly dependent on the role in the circuit as well as the material and construction of the capacitor. Generally, an ESR of 500 milliohms is the upper limit of acceptability for electrolytic capacitors, and manufacturers can provide ceramic chip capacitors with ESR values magnitudes smaller. Optimization of switched-mode power supplies (SMPS) will also necessitate a sufficiently low ESR: low-ESR capacitors are beneficial for their superior signal-smoothing capabilities in pure circuit performance. A lower ESR means current flows with less resistance (and, therefore, less loss), such that charge/discharge cycles are more responsive to the peaks and troughs of the waveform. This action makes these capacitors suitable for high-AC noise filtering applications, like those generated by the currents from large inductors in SMPS.
Engineers can stipulate low-value ESR capacitors as part of the part buys, but the operation conditions of the device can influence how fast the ESR grows during the service life. Some of these facets are unavoidable: aluminum electrolytic capacitors that feature a wet electrolyte experience drying that ultimately increases the ESR of the component to the point of failure. However, designers and engineers can ensure a maximum lifespan of these components by controlling for these deleterious effects:
- Temperature – Minimize temperature cycling as much as possible. Excessive heat below maximum ratings can accelerate material aging.
- Overvoltage/overcurrent conditions – Failure to heed capacitor ratings for reasonable current/voltage spikes can cause damage that prematurely degrades performance.
- Corrosion – Atmospheric moisture and salts can react with exposed metals and significantly increase the ESR from the circuit’s perspective.
A Proper Layout Minimizes ESR
Engineers have a few options when filling out the capacitors for a design. The most straightforward option is selecting a manufacturer’s offering that fits design specifications within the stated tolerances. There are a few caveats: cost and availability may preclude this option, but overall, it’s a sound approach. When no such capacitor exists to order from the manufacturer (either due to inventory or lack of corresponding off-the-shelf parameters), engineers can instead design capacitor networks to add or divide capacitor values in parallel or series respectively. The downside of this method is the additional space required for capacitor placement – larger footprints may negatively affect placement and routing difficulty during the layout.
Furthermore, capacitor networks have an interesting and sometimes overlooked impact on the ESR: resistors add in series and divide in parallel – exactly opposite behavior to capacitors. Therefore, designers can reduce ESR beyond manufacturers’ values when combined in a network. This configuration is one of the central premises behind decoupling capacitor placement, which uses smaller capacitors with lower ESR as close as possible to the matching power pins of an IC, with larger capacitance values/higher ESR further away from the IC. The smaller capacitors can more quickly charge and discharge to meet near-instantaneous, low-power requirements; larger bulk capacitors with higher ESRs (though potentially low overall, depending on capacitor type) provide more extensive power solutions with greater charge storage capacity.
The layout will notably affect the inductance of low-ESR capacitors – the primary reason trace lengths from components to decoupling capacitors are kept short is to minimize the inductance built up over the length of the trace. Minimizing the trace length is paramount for high-frequency capacitor applications where inductive reactance dominates because excessive inductance can cause capacitors to act as inductors.
Your Component Manufacturer Improves Power Performance
Low-ESR capacitors help future-proof designs by maximizing service life: an elevated upfront cost for these capacitors bears out with reduced repairs and replacements. Moreover, troubleshooting designs during prototyping is much simpler with the complete confidence that capacitors function as intended. Here at VSE, we’re a team of engineers committed to building electronics for our customers, including a dedicated procurement team that deftly balances cost, availability, and performance to optimize your design. We’ve been realizing life-saving and life-changing products for over forty years with our valued manufacturing partners.
