The capacitor is the most common component in electronics and used in almost every electronics application. There are many types of capacitor
available in the market for serving different purposes in any
electronic circuit. They are available in many different values from 1
Pico-Farad to 1 Farad capacitor and Supercapacitor.
Capacitor also have a different types of ratings, such as working
voltage, working temperature, tolerance of the rated value and leakage
current.
The leakage current of capacitor is a crucial factor for the application, especially if used in Power electronics or Audio Electronics. Different types of capacitors provide different leakage current ratings.
Apart from selecting the perfect capacitor with proper leakage, circuit
should also have the ability to control the leakage current. So first
we should have a clear understanding of capacitor leakage current.
Relation with Dielectric Layer
The leakage current of a capacitor has a direct relationship with the dielectric of the capacitor. Let's see the below image -
The above image is an internal construction of the Aluminum Electrolytic Capacitor. An Aluminum Electrolytic Capacitor has few parts which are encapsulated in a compact tight packaging. The parts are Anode, Cathode, Electrolyte, Dielectric layer Insulator, etc.
The dielectric insulator provides insulation of
the conductive plate inside the capacitor. But as there is nothing
perfect in this world, the insulator is not an ideal insulator and has
an insulation tolerance. Due to this, a very low amount of current will
flow through the insulator. This current is called as Leakage current.
The Insulator and the flow of current can be demonstrated by using a simple capacitor and resistor.
The resistor has a very high value of resistance, which can be identified as an insulator resistance and
the capacitor is used to replicate the actual capacitor. Since the
resistor has a very high value of resistance, the current flowing
through the resistor is very low, typically in a number of nano-amperes.
Insulation resistance is dependent on the type of dielectric insulator
as different type of materials changes the leakage current. The low
dielectric constant provides very good insulation resistance, resulting
in a very low leakage current. For example, polypropylene, plastic or
teflon type capacitors are the example of low dielectric constant. But
for those capacitors, the capacitance is very less. Increasing the
capacitance also increases the dielectric constant. Electrolytic
capacitors typically have very high capacitance, and the leakage current
is also high.
Dependent Factors for Capacitor Leakage Current
Capacitor Leakage Current generally depends on below four factors:
- Dielectric Layer
- Ambient Temperature
- Storing Temperature
- Applied Voltage
1. The Dielectric layer is not working properly
Capacitor construction requires a chemical
process. The dielectric material is the main separation between the
conductive plates. As the dielectric is the main insulator, the leakage
current has major dependencies with it. Therefore, if the dielectric is
tempered during the manufacturing process, it will directly contribute
to the increase of leakage current. Sometimes, the dielectric layers
have impurities, resulting in a weakness in the layer. A weaker
dielectric decreases the flow of current which is further contributed to
the slow oxidation process. Not only this, but improper mechanical
stress also contribute to the dielectric weakness in a capacitor.
2. Ambient Temperature
The capacitor has a rating of the working
temperature. The working temperature can be ranged from 85 degree
Celsius to the 125 degree Celsius or even more. As the capacitor is a
chemically composed device, the temperature has a direct relationship
with the chemical process inside the capacitor. The leakage current
generally increases when the ambient temperature is high enough.
3. Storage of the Capacitor
Storing a capacitor for a long time without voltage is not good for the capacitor. The storing temperature is also a important factor for leakage current.
When the capacitors are stored, the oxide layer is attacked by the
electrolyte material. The oxide layer starts to dissolve in the
electrolyte material. The chemical process is different for different
type of electrolyte material. The water-based electrolyte is not stable
whereas inert solvent-based electrolyte contributes less leakage current
due to the reduction of the oxidation layer.
However, this leakage current is temporary as the
capacitor has self-healing properties when applied to a voltage. During
the exposure to a voltage, the oxidation layer starts to regenerate.
4. Applied Voltage
Each capacitor has a voltage rating. Therefore,
using a capacitor above the rated voltage is a bad thing. If the voltage
increases, the leakage current also increases. If the voltage across
the capacitor is higher than the rated voltage, the chemical reaction
inside a capacitor creates Gases and degrade the Electrolyte.
If the capacitor is stored for a long time such as
for years, the capacitor is needed to be restored into the working
state by providing rated voltage for a few minutes. During this stage,
the oxidation layer built up again and restores the capacitor in a
functional stage.
How to reduce Capacitor Leakage Current to improve the Capacitor Life
As discussed above a capacitor has dependencies
with many factors. The first question is how the capacitor life is
calculated? The answer is by calculating the time until the electrolyte
is run out. The electrolyte is consumed by the oxidation layer. Leakage
current is the primary component for the measurement of how much the
oxidation layer is hampered.
Therefore, the reduction of leakage current in the capacitor is a major key component for the life of a capacitor.
1. Manufacturing or the production plant is the first place of a capacitor life cycle where capacitors are carefully manufactured for low leakage current. The precaution needs to be taken that the dielectric layer is not damaged or hampered.
2. The second stage is the storage. Capacitors need to be stored in proper temperature.
Improper temperature affects the capacitor electrolyte which further
downgrades the oxidation layer quality. Make sure to operate the
capacitors in proper ambient temperature, less than the maximum value.
3. In the third stage, when the capacitor is
soldered on the board, the soldering temperature is a key factor.
Because for the electrolytic capacitors, the soldering temperature can
become high enough, more than the boiling point of the capacitor. The
soldering temperature affects the dielectric layers across the lead
pins and weakens the oxidation layer resulting in high leakage current.
To overcome this, each capacitor comes with a data sheet where the
manufacturer provides a safe soldering temperature rating and maximum
exposure time. One needs to be careful about those ratings for the safe
operation of the respective capacitor. This is also applicable for the
Surface Mount Device (SMD) capacitors too, the peak temperature of
reflow soldering or wave soldering should not exceed than the maximum
allowable rating.
4. As the voltage of the capacitor is an important factor, the capacitor voltage should not exceed the rated voltage.
5. Balancing the capacitor in Series connection. The capacitor series connection is a bit complex job to balance the leakage current.
This is due to the imbalance of leakage current divide the voltage and
split between the capacitors. The split voltage can be different for
each capacitor and there can be a chance that the voltage across a
particular capacitor could be excess than the rated voltage and the
capacitor start to malfunction.
To overcome this situation, two high-value resistors are added across the individual capacitor to reduce the leakage current.
In the below image, the balancing technique is shown where two capacitors in series are balanced using high-value resistors.
By using the balancing technique, the voltage difference influenced by leakage current can be controlled.
https://circuitdigest.com/tutorial/what-is-capacitor-leakage-current-and-how-to-reduce-it
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