Low ESR Capacitors

What is ESR?

ESR is an abbreviation for Equivalent Series Resistance, the characteristic representing the sum of resistive (ohmic) losses within a capacitor. While ESR is undesirable, all capacitors exhibit ESR to some degree. Materials and construction techniques used to produce the capacitor all contribute to the component's ESR value. ESR is a frequency dependent characteristic, so comparison between component types should be referenced to the same frequency. Industry standard reference for ESR is 100KHz, +25°C. ESR is an important characteristic, as the power dissipation (watts) within the capacitor, and the effectiveness of the capacitor's noise suppression characteristics, will be related directly to the ESR value.

What's driving demand for Low ESR?

An industry wide trend towards lower voltage - higher current circuit design, fueled by lower voltage silicon devices is causing designers to specify capacitors with minimal ESR. Higher levels of functionality in today's designs means that despite voltage level falling, circuit power levels have not dropped accordingly. Ohms law tell us, in every simple fashion, that at the same power dissipation level, lower voltage operation will mean higher current levels. This greatly increases the demands on the power management circuit (power supply or DC/DC converter) to deliver energy during periods of high current load stepping. Lower voltage circuit operation also imposes greater restrictions upon the output voltage variation level as well. The output capacitors or capacitor bank, used in the power management circuit, need to exhibit low ESR characteristics. Ripple voltage (noise) on the output supply voltage will be directly proportional to the ESR of the capacitor used. By considering the formula: Vr = I x R, where Vr is the ripple voltage and R is the ESR, we can see that if the current (I) increases from say 4A to 20A then the ripple voltage will also increase by a factor of five. Increased ripple voltage (Vr) cannot be toleranted in todays and next generation designs. This is fueling demand for very lower ESR capacitors.

What types of Low ESR Capacitors are available?

Capacitance values greater than 10ìF are often required to supply energy to today's electronic circuits, during load current stepping (low to high current stepping). This requirement is met through the use of single or multiple surface mount (SMT) electrolytic capacitors or combination electrolytic and high capacitance MLCC (ceramic chip) capacitor. Surface mount configurations are preferred as it allows closer component placement, reduces performance robbing series inductance and can reduce total PCB assembly costs. Recent low ESR electrolytic capacitor development has focused on techniques and materials designed to reduced the resistance of the cathode connection, either with a lower resistivity solid electrolyte. The cathode connection of electrolytic capacitors is the largest contributor to the electrolytic capacitors total ESR figure.

Low ESR SMT capacitors available today chiefly fall into 7 types.

ESR = Equivalent Series Resistance (ohm)

RCR = Ripple Current Rating (Amp)

1. LIQUID ELECTROLYTE, VERTICAL CAN CHIP ALUMINIUM ELECTROLYTIC CAPACITORS

Lowest cost solution.

Pros: High capacitance values, high voltage ratings, moderate to low ESR, moderate RCR and lowest cost.

Cons: Liquid electrolyte exhibits dry-out under high temperature, medium to large sizes.

2.HYBRID ELECTROLYTE, VERTICAL CAN CHIP ALUMINIUM ELECTROLYTIC CAPACITORS

Provides solid electrolyte performance (very low ESR) at much lower cost than solid electrolyte types.

Pros: Very Low ESR, High RCR, moderate capacitance values and moderate cost.

Cons: Liquid electrolyte component exhibits dry-out under high temperature, low voltage ratings and medium sizes.

3. SOLID POLYMER ELECTROLYTE, VERTICAL CAN CHIP ALUMINIUM ELECTROLYTIC CAPACITORS

Lowest ESR and highest RCR of the vertical can chip types, but at highest cost.

Pros: Very Low ESR, High RCR, moderate capacitance values and solid electrolyte for good long-term performance at high temperature.

Cons: High cost, low voltage ratings and medium sizes.

4. SOLID POLYMER ELECTROLYTE RESIN ENCAPSULATED FLAT CHIP ALUMINIUM ELECTROLYTIC CAPACITORS

Low ESR and high RCR, but at highest cost of all aluminium electrolytic types.

Pros: Very Low ESR, High RCR, Smallest size aluminium electrolytic type, moderate capacitance values and solid electrolyte for good long-term performance at high temperature.

Cons: High cost and low voltage ratings.

5. SOLID ELECTROLYTE, RESIN ENCAPSULATED FLAT CHIP MnO2 CATHODE TANTALUM ELECTROLYTIC CAPACITORS.

Standard tantalum chip capacitor construction processed for low ESR.

Pros: Moderate to Low ESR, Moderate RCR, Small size, Low ESR versions produced with manganese dioxide cathode (MnO2) construction and solid electrolyte for good long-term performance at high temperature.

Cons: Low voltage rating and limited transient (reverse or surge conditions) tolerance, could combust upon failure.

6. SOLID ELECTROLYTE, RESIN ENCAPSULATED FLAT CHIP POLYMER CATHODE TANTALUM ELECTROLYTIC CAPACITORS.

Standard manganese dioxide cathode (MnO2) is replaced by a highly conductive polymer (polypyrrole) cathode that considerably reduce ESR. The conductivity of polypyrrole is more than 100 times that of manganese dioxide.

Pros: Very Low ESR, High RCR, Small size, polymer cathode construction suppresses combustion = increased safety factor and solid electrolyte for good long-term performance at high temperature.

Cons: High Cost and low voltage ratings.

7. MLCC - SURFACE MOUNT CERAMIC CHIP CAPACITORS.

Capacitance values up to 100uF are available today in ultra-small sizes.

Pros: Ultra Low ESR, Moderate RCR, Smallest size, Non-polarized for applications where reverse operation or transient conditions occur, High temperature rating and good soldering heat exposure characteristics.

Cons: Low voltage ratings, Large effective capacitance loss under VDC operation, Capacitance decrease over time (aging), Piezoelectric effects.

Summary

Circuit designs incorporating lower voltage semiconductors and IC's are driving increasing demand for better and lower ESR capacitors. SMT low ESR type electrolytic capacitors offer the combined solution of high capacitance, to supply energy during high-speed load stepping, and low ESR to reduce the output filter ripple (noise) voltage to meet the needs of today's and tomorrow's power management design challenges.