10
higher ESR causes more ripple, influencing stability of the control loop.
1 22µF 25V 0.1 100 1k 10k Frequency - Hz
Figure 3: Typical relationship between ESR and frequency for tantalum capacitor (AVX)
decreases inversely with frequency, eventually becoming small compared with the contact material resistance (Figure 3).
why is esr important?
Electrolytic capacitors are used as input buffers to supply energy when the mains input voltage is too low, store energy while an AC/ DC converter adapts to a new power level, and prevent switching noise from the converter reaching the power source. On the output of a converter, they act as a filter and current sink for inductive elements, and, in DC/DC conversion, function as an energy buffer when the power output demand changes.
In both cases, losses due to ESR will inhibit a capacitor’s ability to quickly source or sink charge. At the input, increasing ESR increases high frequency noise across the capacitor, decreasing filtering effectiveness. At the output,
100k 1M
ESR is particularly important in applications with low duty-cycle, high-frequency current pulses. Here, the ripple voltage due to the ESR will be greater than expected based on capacitance alone, although the negative correlation of ESR with temperature means that ripple decreases as the assembly warms up. Also, the introduction of a resistive element into what designers may assume is a purely reactive circuit can lead to unexpected shifts in phase response, again affecting stability.
what Can be done?
Some capacitors are designed specifically for low-ESR, but manufacturers of aluminium electrolytic capacitors do not specify ESR consistently. The value at 25°
C and 100kHz is
commonly quoted, with a formula provided to calculate the value at the operating frequency. Some suppliers specify at 120Hz; others leave the designer to calculate the figure at the frequency of interest from the dissipation factor (tan∂) and specified maximum ripple current. Furthermore, for capacitors of comparable size and CV, a device with higher capacitance and lower voltage rating will have lower ESR and ESR tends to be lower for aluminium electrolytic devices with long, thin cases because the resistance of the foil is reduced. Larger overall case sizes can cut ESR too.
Also, several smaller-value components can be used in parallel to achieve lower high- frequency ESR, at the expense of board space. An analysis of some of the most popular low-ESR components can be found in Figure 4,
Figure 4: Some of the most popular low-ESR capacitors manufactured by suppliers of Avnet Abacus Manufacturer Series
AVX AVX
Kemet Kemet Murata
Nichicon Nichicon Nichicon Panasonic Panasonic Panasonic Panasonic Rubycon Rubycon Vishay
TCM TPS
A700 T510 ECAS CD HE CK FT FR
SP-Cap Style
Chip Chip Chip Chip Chip SMD
Radial SMD SMD
Radial Chip
ZA/ZC SMD ZLH
PC-CON Chip 146 RTI
Radial Radial Technology
Polymer Tantalum Multi-Anode Tantalum
Polymer Aluminium Tantalum Multi-Anode Polymer Aluminium Aluminium Electrolytic Aluminium Electrolytic Polymer Aluminium Aluminium Electrolytic Aluminium Electrolytic Polymer Aluminium Polymer Hybrid
Aluminium Electrolytic Polymer Aluminium Aluminium Electrolytic
Low ESR down to Data link [m‡ @ 20C/100khz]
10 30 6
18 6
35 12 8
60 12 6
20 12
4.5 18
http://tinyurl.com/AVX-TCM http://tinyurl.com/AVX-TPS http://tinyurl.com/Kemet-A700 http://tinyurl.com/Kemet-T510 http://tinyurl.com/Murata-ECAS http://tinyurl.com/Nichion-CD http://tinyurl.com/Nichion-HE http://tinyurl.com/Nichion-CK http://tinyurl.com/Panasonic-FT http://tinyurl.com/Panasonic-FR http://tinyurl.com/Panasonic-SP-Cap http://tinyurl.com/Panasonic-ZA-ZC http://tinyurl.com/Rubycon-ZLH http://tinyurl.com/Rubycon-PC-CON http://tinyurl.com/Vishay-146-RTI
which includes URLs from which data sheets can be downloaded. The choice is varied, requiring detailed analysis of data sheets, or advice from a specialist, such as Avnet Abacus, to make an informed selection.
other advanCes
Several manufacturers have been combining solid and liquid electrolytes to produce hybrid components with ultra-low ESR (low tens of milliohms), with minimal temperature variation, while increasing withstand voltage and accommodating high ripple current. As ESR varies with external surface area tantalum capacitor manufacturers produce multi-anode devices for high-reliability markets that may point the way to developments for more mainstream applications. But low ESR is not always the primary consideration and, in a development outside of electrolytic capacitors, Murata has actually increased ESR of some of its multilayer ceramic chip capacitors, in order to control unwanted resonances that can reduce the decoupling performance at peak frequencies.
ConClusion There has never been a more diverse range of electrolytic technologies for engineers to choose from. For power applications in particular, the data printed on the component is rarely sufficient to make an informed choice. Talking to an independent specialist that can offer components from a selection of the world’s largest capacitor manufacturers is the surest and quickest route to finding the most appropriate device for any given application.
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focus magazine - issue 14
ESR Ω
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