Leakage current characteristics in capacitors: A case study

Capacitors, just like other electronic components, are constructed with imperfect materials. The imperfections and defects in these materials have significant effects on the electrical performance of capacitors.  Some of the parameters determined by these defects and imperfections include impedance, dissipation factor, inductive reactance, equivalent series resistance, and leakage current. When designing an electronic circuit, it is necessary to consider the leakage current characteristics of capacitors.

DC leakage current is one of the key characteristics to consider when selecting a capacitor for your design. Other important parameters include working voltage, nominal capacitance, polarization, tolerance, and working temperature. Basic leakage current definitions and their reciprocal value – insulation resistance can be found in the following article here.

A dielectric material separates the conductive plates of a capacitor. This material does not provide perfect insulation and allows a small current to flow when voltage is applied. This is referred to as DC leakage, and its value depends on the applied voltage, capacitor temperature, and charging period.

The amount of leakage varies between capacitor types due to differences in dielectric material and construction. Aluminium electrolytic capacitors typically allow more current to pass compared to ceramic, foil, and plastic film capacitors, which exhibit much smaller leakage.

In electronic circuits, capacitors are used for decoupling, filtering, and coupling. Applications like power supply systems and amplifier coupling demand capacitors with very low leakage. While aluminium electrolytic and tantalum capacitors are less suitable due to higher leakage levels, plastic and ceramic capacitors are commonly used for their lower leakage and reliability in coupling and storage applications.

The dielectric materials used in capacitors are not ideal insulators. A small DC current can flow or “leak” through the dielectric material for various reasons specific to each dielectric. As a result, when a capacitor is charged to a certain voltage, it will slowly lose its charge. As it loses its charge, the voltage between the capacitor’s electrodes will drop.

The leakage current (DCL) and the insulation resistance (IR) are in simple mathematical relation to each other:

R (IR) = V / I (DCL) or I (DCL) = V / R (IR)

Since the values are related, the terms leakage current and insulation resistance will vary depending on the dielectric type. Aluminum electrolytic capacitors have a relatively large leakage, thus called leakage current. Alternatively, plastic film or ceramic capacitors have a very small leakage current, so the effect is quantified as insulation resistance. See figure 1. overview of IR on most common capacitor dielectric types.

Generally, insulation resistance tends to decrease with higher values of capacitance. For practical reasons, the insulation resistance may be expressed in Megaohms at low capacitance values and in Ohm-Farads (equals seconds) at higher capacitances. The Ohm-Farad expression allows a single figure to being used to describe the insulation performance of a given component family over a wide range of capacitance values. The leakage current is also dependent on the temperature. As the temperature increases, so does the leakage current.

Figure 1. Values of capacitor types relative to dielectric Insulation Resistance (IR)

DCL leakage currents in electrolytic capacitors are also mentioned in the article here.

Figure 2. Capacitor charging and discharging curves

The leakage currents of some capacitors are dependent on time. The current peaks when the voltage is applied to a capacitor. The occurrence of this peak current depends on the construction of the capacitor. In the case of aluminum electrolytic capacitor, it is the forming characteristics of a capacitor and the internal resistance of the voltage source. When a capacitor is charged, its leakage current drops with time to a nearly constant value called operational leakage current. This small leakage current is dependent on both temperature and applied voltage.

Some capacitor technologies such as aluminium, tantalum, and film capacitors have self-healing properties. The self-healing process may have a significant effect on the leakage currents of the capacitors, while the exact mechanisms may be specific to the capacitor technology type. The time dependence of leakage currents is also caused by dielectric material type and structure. Other parameters that determine the value of this small current include the type of electrolyte, capacitance, and forming voltage of the anode. The leakage current of a ceramic capacitor does not change with time.

The leakage current of a capacitor depends on temperature, with the level of dependency varying among capacitor types. For aluminium electrolytic capacitors, an increase in temperature speeds up chemical reactions, leading to higher leakage.

Tantalum capacitors exhibit higher leakage than ceramic capacitors, and their DC leakage rises with increased temperature. Additionally, when stored in high-temperature or high-humidity environments, tantalum capacitors experience a temporary rise in leakage, which can be reversed by applying the rated voltage for a few minutes.

Ceramic and film capacitors have much smaller leakage relative to electrolytic capacitors. In multilayer ceramic capacitors (MLCCs), intrinsic leakage increases exponentially with temperature. For film capacitors, the properties of the dielectric material affect insulation resistance, where higher temperatures lead to reduced resistance and slightly increased leakage.

A capacitor’s DC leakage current depends greatly on the applied voltage. For aluminium electrolytic capacitors, this current increases with operating voltage. As the voltage approaches the forming voltage, it rises exponentially. When the voltage exceeds the surge voltage, secondary reactions such as temperature rise, electrolyte degradation, and excess gas formation occur, making operation beyond the rated voltage intolerable. Reducing the applied voltage below the rated level sharply decreases leakage.

For aluminium electrolytic capacitors stored for long periods, their characteristics can be restored through reconditioning, a process that involves applying rated voltage for about half an hour.

In ceramic capacitors, intrinsic leakage is highly dependent on voltage, increasing superlinearly with higher voltage levels. However, their insulation resistance remains independent of voltage.

There are some common myths related to the DCL leakage current of capacitors that can still be heard today:

Myth 1: IR/DCL leakage current is due to the cracks in the dielectric.

This was one of the first imaginative theories of why dielectrics have a leakage current without a details understanding of the physical mechanisms inside the insulators. It is true that cracks and “imperfections” in dielectric structures can be a cause of leakage current increase and catastrophic failures on individual “faulty” components. On the other hand, it may not be the prime issue for the basic leakage current level – we have to understand the physical conductivity mechanisms that take place at the specific capacitor technology dielectric.

The details conductivity mechanisms description is beyond this lecture focus, but let’s simplify it that in a capacitor, the conductivity through the dielectric can be composed of three major mechanisms (all three are typical for electrolytic capacitors):

• Ohmic conductivity
• Poole Frankel mechanism – It can be imagined as electrons or holes “hopping” through a trap in the dielectric inner volume
• Tunneling mechanism – this dangerous zone should happen above the operational voltage. Under high electric field intensity, electrons/holes are accelerated to cross all the barriers with the risk of avalanche effect and catastrophic breakdown of the part resulting in a short circuit. So we can assume this is the main electrical breakdown mechanism

Another big influence on the value of DCL is the construction of the capacitor itself – namely, the electrical potential between the electrode systems and the insulating dielectric. Metal electrodes may have some sub-oxide layers that are semiconducting, and also electrolytes in electrolytic capacitors may exhibit rather semiconducting behavior – so, in-fact in many cases on capacitors, we are not faced with a simple Metal-Insulator-Metal structure, but more complex Metal-Insulator-Semiconductor systems, where interface barriers may play the leading role to the overall DCL leakage current values.

Myth 2: IR/DCL leakage current is a measure of the component reliability

This common myth is related to Myth 1 as the imagination was that the part with a higher leakage current also has a larger number of cracks, thus presenting a higher reliability risk.

As we learned in the Myth 1 mitigation above, the actual leakage current of a “standard” capacitor is due to its dielectric conductivity mechanisms and construction (electrical potential matching). DCL of statistically normally produced capacitors is not a measure of reliability. It was often confirmed that screening of DCL tail distribution is not improving the basic reliability numbers.

HOWEVER, the Change of DCL as the structural robustness to external load can be a measure of reliability. There is a number of proven screening methods that are part of specifications (MIL, ESA) or applied internally by manufacturers as their know-how where a certain (thermo)mechanical and electrical stress is applied with subsequent DCL screening to improve reliability level and sort-out suspicious parts.

It was common to hear that leakage current on tantalum solid electrolytic capacitors with MnO2 electrodes is due to the cracks in dielectrics. When a conductive polymer electrode was developed, it replaced MnO2 solid electrolyte, but DCL increased in 10fold time. This raises natural questions: Why DCL increased when the dielectric is identical, and we exchange electrolyte material only? Does it mean that polymer-type reliability is ten times worse? … of course, it is not true; the answer is that we replace one semiconducting electrolyte with the other and influence the electrical barriers that are “more open” now, and it drains more electrons/holes through it. But this is a natural stage of the capacitor construction without directly impacting its reliability. How many times do I still hear today that cracks are the leading cause of leakage current in capacitors … It is cited in the literature and still copied and pasted by many authors without any more profound insight.

Resource: Passive components Blog