Electrolytic Capacitor Use and Design
What is an electrolytic capacitor and how does it differ from a standard capacitor?
The electrochemistry powering electrolytic capacitors and how to prevent failure and extend service life.
A few specific circuit use cases for electrolytic capacitors.
One of the peculiarities of electronics for those who don’t study or work with them is that a large enough capacitor unplugged for an extended period can still discharge and deliver a shock of varying danger. For tinkerers who dive into a piece of electronics to make simple repairs, this can be a painful lesson that seems to fly in the face of how most people understand electricity. While very much considered an active phenomenon, electrical energy can be stored in capacitors for a variety of reasons, and given a sufficient time constant, will still discharge if the proper avenue is provided. For most consumer-level electronics, care should be taken around electrolytic capacitors, which fittingly look a bit like a water reservoir tank rising above the board.
Electrolytic capacitor use is of course not to deter would-be engineers investigating their curiosity; instead, they act as a high-volume version of more standard ceramic capacitors and are primarily associated with power storage and transformation.
What Differentiates an Electrolytic Capacitor?
Electrolytic capacitors differ from the more common SMD and ceramic capacitors in a few key areas. Their primary function is the improved volumetric capacitance relative to other capacitor models and packages; for circuits that require mass capacitance in a reasonably sized package/footprint, electrolytic capacitors are ideal. This performance is achieved in a relatively (but certainly not absolutely) compact footprint by winding the anode, cathode, and dielectric layers in a cylindrical package. Importantly, electrolytic capacitors can only withstand current flow in one direction, meaning circuits require additional safety elements to prevent the establishment of a reverse bias mode. The polarity configuration traces back to the material construction; while nonpolarized capacitors feature two electrode layers that do not distinguish between anode and cathode, electrolytic capacitors feature a metal oxide layer that only operates as a dielectric when the electrode closest to the anode pin is, in fact, the anode.
The metal oxide layer is formed out of a handful of metals, but the most common are aluminum, tantalum, and niobium. Aluminum electrolytics dominate usage due to the wide range of voltage and capacitance ratings available as well as their relatively inexpensive cost. However, aluminum electrolytic caps tend to suffer in key performance metrics – loss, leakage current, and temperature susceptibility – when compared to tantalum or hybrid material electrolytic capacitors.
Electrolytic Capacitor Construction and Safe Operation
Owing in part to their bulk, electrolytic capacitors are more likely to encounter early application failures than nonpolarized capacitors. All PCB elements loathe heat, and electrolytic capacitors are no exception. While heat generation is unavoidable (thank you, Second Law of Thermodynamics), designers can effectively mitigate the deleterious effects of temperature at both the schematic and board level.
Interestingly, electrolytic capacitors have the ability to resist some forms of aging due to electrochemical reactions produced at the oxide layer. In a series of redox reactions, the elemental metal is oxidized (for example, aluminum into Al2O3) while environmental moisture is reduced to H2 gas. Under standard operating conditions, this reaction is spontaneous but has a very low rate of reaction. It is only when the electrolytic capacitor is stressed above standard ratings or operated in reverse polarity that the reaction rate rapidly accelerates – the rapid swelling of the capacitor can occasionally lead to explosive venting most students have either experienced or (hopefully instead) have demonstrated in an introductory electronics course.
There are multiple installing and operating factors to take into consideration to avoid damaging an electrolytic capacitor. A few of the stress factors are listed below (as well as techniques to reduce or eliminate their incidence):
Reverse voltage - The quickest way to damage, disable, or destroy an electrolytic capacitor is through backward installation or incurring reverse voltage during operation. Unlike the anode, the cathode does not have an oxide layer attached to it. When voltage is biased from the cathode to the anode, the redox reaction described above occurs, but because there is no oxide film present (which chemically acts as a buffer mechanism due to Le Chatelier’s principle), the reaction greatly favors the products and can rapidly lead to explosive venting or electrical breakdown conditions.
Ripple current - AC sources contain some periodic signal that is characterized by sharp spikes of narrow bandwidth and high amplitude. Ripple currents can rapidly evolve with changing impedance during operation, and electrolytic capacitors (and other circuit elements) will conduct based on their modeled equivalent series resistance (ESR). Due to Ohmic heating, a greater resistance dissipates more heat during operation. Additionally, because impedance is a frequency-dependent value, ripple current conduction can also fluctuate. For comparison purposes, this value is often scaled to a common frequency of operation (e.g., 120 Hz) and provided by the manufacturer. Designers need to ensure that electrolytic capacitors are rated above the operational ripple current, and may also want to select low-ESR models.
Series/parallel configurations - To counter ripple current or share voltage, electrolytic capacitors can be placed in parallel or series, respectively. This also has the effect of increasing or decreasing the capacitance. These configurations can extend to parallel of series (and conversely, series of parallels), which offer improved performance at the cost of reliability in the former and cost and complexity in the latter.
A Short List of Electrolytic Capacitor Uses
Power supply usage is a common mode of electrolytic capacitor usage, but like standard capacitors, they fill a number of roles within a circuit:
High voltage I/O smoothing - Capacitors condition signals by filling in during the gaps of the signal by converting stored electric fields into voltage. For high-voltage use cases, these capacitors need to be able to resist dielectric breakdown and exhibit low losses due to ESR.
Inverters - Inverters output AC voltage with a DC input and are able to provide a variable voltage range of operation that sees extensive usage in motor speed controllers. Due to their collective wide and high voltage input range, electrolytic capacitors are especially well-suited for this task.
Automotive - Electrolytic capacitors can deliver the bulk capacitance needed for many critical operations, including safety mechanisms, in a relatively small package compared to other available capacitor technologies.
Cadence Provides Comprehensive Simulation Tools for Electrolytic Capacitors in Design
Electrolytic capacitor use reflects its particular role as a high-capacitance solution in a minimized package, and like standard capacitors, it can perform a variety of functions with a circuit that makes it essential to modern electronics. Selecting the best capacitor for the job involves a reasonable amount of electrical and thermal modeling using data provided by the manufacturer as well as simulations to evaluate its in-circuit performance.
Cadence’s suite of PCB Design and Analysis Software is well-suited to provide rigorous simulation with an easy-to-use interface. Designers can intelligently incorporate design rules to accentuate electrolytic capacitor performance using OrCAD PCB Designer and its customizable Constraint Manager.
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