What is Safety Film Capacitor
Safety Film Capacitor the dielectric films, depending on the desired dielectric strength, are drawn in a special process to an extremely thin thickness, and are then provided with electrodes. The electrodes of film capacitors may be metallized aluminum or zinc applied directly to the surface of the plastic film, or a separate metallic foil. Two of these conductive layers are wound into a cylinder-shaped winding, usually flattened to reduce mounting space requirements on a printed circuit board, or layered as multiple single layers stacked together, to form a capacitor body. Film capacitors, together with ceramic capacitors and electrolytic capacitors, are the most common capacitor types for use in electronic equipment, and are used in many AC and DC microelectronics and electronics circuits.
Advantages of Safety Film Capacitor
Wide temperature range
Film capacitors have a relatively broad temperature range, with minimal restrictions imposed by temperature variations.
Extended lifespan
These capacitors exhibit an exceptionally long lifespan; as long as they remain undamaged, there is virtually no limit to their usability, allowing for prolonged and continuous use.
Self-healing properties
Film capacitors, being metalized, possess self-healing capabilities. In the event of minor damage, they can automatically repair themselves.
High voltage tolerance
During operation, these devices can withstand high voltages without encountering issues or failures.
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Film capacitors are made of a thin dielectric film which may or may not be metallized on one side. The film is extremely thin, with the thickness being under 1 µm. After the film is drawn to the desired thickness, the film is cut into ribbons. The width of the ribbons depends on the capacity of the capacitor being produced. Two ribbons of film are wound together into a roll, which is often pressed into an oval shape so that it can fit into a rectangular case. This is important because rectangular components save precious space on the printed circuit board. Electrodes are added by connecting each of the two electrodes to one of the films. A voltage is applied to burn out any imperfections using the self-healing property of film capacitors. The case is then sealed using silicon oil to protect the film roll against moisture, and dipped in plastic to hermetically seal the interior.
Typical film capacitors have capacitances ranging from below 1nF to 30µF. They can be made in voltage ratings as low as 50V, up to above 2kV. They can be manufactured for use in high-vibration automotive environments, high temperature environments and high-power applications. Film capacitors offer low losses and high efficiency while providing a long service life.
Check the capacitor with a multimeter: Set the multimeter to the "capacitance" mode and touch the leads to the capacitor's terminals. The multimeter should display the capacitor's value in farads (F). If the value is significantly different from the value listed on the capacitor or if the meter displays "OL" (overload), the capacitor may be damaged.
Check the capacitor's physical condition: Look for any visible signs of damage, such as bulging, leaks, or burn marks.
Check the capacitor's performance: Discharge the capacitor by shorting the terminals with a wire, then measure the voltage across the terminals with a multimeter. The voltage should drop quickly to zero. If the voltage does not drop or drops very slowly, the capacitor may be damaged.
Check the capacitor's frequency response: Connect the capacitor to an AC signal generator and measure the capacitor's impedance at various frequencies. The impedance should decrease as the frequency increases. If the impedance does not follow this trend, the capacitor may be damaged.
Safety Film Capacitor Common Failure Mechanisms
Though film capacitors are generally quite durable, they are susceptible to a few long-term wear mechanisms. Over time, the dielectric materials used weaken, become brittle, and experience degradation in their voltage withstanding capability, which eventually leads to a dielectric breakdown failure. The process is accelerated by temperature and voltage stress, and reducing either can extend service life. Depending on the severity of the dielectric breakdown event, the failure modes exhibited can range from relatively benign to quite spectacular. A mild breakdown event that is arrested either by a film capacitor's self-healing properties will manifest as an incremental reduction in capacitance. As more such events occur over time, the cumulative effect causes a reduction in capacitance and increased ESR, until the point where the device's performance is no longer within specification and it is considered to have failed parametrically.
In a more extreme case, which can follow a parametric failure if parametrically-failed devices are not removed from service, a cascading failure can occur when the thermal energy released during self-healing prompts additional dielectric breakdowns nearby. Because self-healing events remove portions of the capacitor from the circuit, application stresses are re-distributed across an ever-shrinking portion of the device as self-healing progresses, causing an increase in stresses placed on the portions of the device that remain effectively in-circuit. The next weakest portion of the capacitor then fails, dumping its burden on what's left, prompting more breakdown events, more stress concentration, more breakdown events, etc. in an exponential fashion. If this process occurs rapidly enough, the gaseous byproducts from the self-healing process can build sufficient pressure to violently rupture the device's case. Larger devices often include a venting mechanism to limit/prevent collateral damage from flying debris when this happens, and may also include a fusing mechanism to remove the device from the circuit in the event of an internal overpressure condition. Note that parametric failures due to repeated self-healing can simply be a waypoint on the route to a more catastrophic, explosive failure, if devices that have failed parametrically are left in operation.
Another overstress failure mode found in film capacitors occurs when peak current limits are exceeded, due to a fuse-like action at the region where the “plates” of the capacitor join to the external leads. This is particularly common with the metallized film types, due to their very small electrode thickness and the resulting delicacy of their connection to the outside world. Many film type capacitors will specify a maximum rate of voltage change (dV/dt) that is to be applied across the capacitor. This is tantamount to specifying a peak current through the device since I(t)=C*dV/dt, though voltages are typically more convenient to measure than currents.
Environmental conditions also play a role in the longevity of film capacitors. As with other devices, elevated temperatures reduce device lifetime considerably. More unique to film devices is a vulnerability to moisture; prolonged exposure to high humidity environments or post-assembly wash cycles can cause the ingress of moisture into a device, through imperfections in the epoxy-to-metal seals around the device leads or by diffusion through a device's polymer case. Moisture ingress is bad on several fronts; it both degrades the dielectric material, and promotes corrosion of the electrode materials. Particularly in metal-film type devices where the electrodes are only a few dozen nanometers thick to start with, it takes very little corrosion to cause problems. Additionally, high-vibration environments can also be troublesome, by causing mechanical failure of device leads, attachment between leads and electrodes, or by exacerbating moisture ingress problems.
The dominant factors in film capacitor reliability and longevity are applied voltage, followed by temperature. Suppliers' service life models vary, but generally are based on taking the ratio of rated and applied voltage to a large exponent (usually between 5 and 10) while the influence of temperature follows the Arrhenius relationship of a factor-of-2 change with each 10℃ temperature increment. Between the two effects, de-rating voltage by 30% and temperature by 20℃ adds nearly two decimal places to service life estimates.
Safety Film Capacitor Manufacturing Process
Film Selection: The first step in the production of film capacitors is the selection of the film material. The most commonly used film materials for capacitors are polyester (PET), polypropylene (PP), and polyethylene terephthalate (PET). These films are chosen based on their dielectric properties, temperature stability, and mechanical strength.
Film Coating: Once the film material is selected, it is coated with a thin layer of metal on both sides. The metal coating is usually made of aluminum, zinc, or a combination of both. The metal coating serves as the electrode of the capacitor and helps in the conduction of electric current.
Winding: After the film is coated with metal, it is wound into a cylindrical shape to form the capacitor's core. The winding process is crucial as it determines the capacitance and voltage rating of the capacitor. The winding is done using automated machines that ensure precision and consistency in the winding process.
Impregnation: Once the film is wound into a cylindrical shape, it is impregnated with a dielectric fluid to improve its electrical properties. The dielectric fluid helps in reducing the dielectric losses and increases the breakdown voltage of the capacitor. The impregnation process is done under controlled conditions to ensure uniform impregnation of the film.
Drying: After impregnation, the capacitor core is dried to remove any excess moisture from the dielectric fluid. The drying process is done in ovens at a specific temperature and time to ensure complete removal of moisture. Proper drying is essential to prevent any electrical breakdown or short circuits in the capacitor.
Encapsulation: Once the core is dried, it is encapsulated in a protective casing to shield it from external environmental factors. The casing is usually made of plastic or metal and provides mechanical support to the capacitor. The encapsulation process is done using automated machines that ensure proper sealing of the capacitor core.
Testing: The final step in the production process is testing the capacitor for its electrical properties and performance. The capacitor is subjected to various tests such as capacitance, voltage, and temperature tests to ensure it meets the required specifications. Any defective capacitors are rejected, and only the ones that pass the tests are approved for further processing.
Working Voltage
The selection of film capacitors depends on the applied high voltage and is affected by factors such as the applied voltage and voltage waveform, frequency, and ambient temperature. Before use, please check whether the superimposed voltage across the capacitor is within the rated value, and do not exceed the upper voltage limit of the capacitor at any time.
Working Current
Due to the loss of the capacitor, when used under high-frequency or high-pulse conditions, the pulse or AC passing through the capacitor will cause the capacitor itself to heat up and have a temperature rise, and there will be a risk of thermal breakdown (smoke, fire). Therefore, the use conditions of capacitors are not only limited by the rated voltage, but also limited by the rated current.
Discharge and Charge
Since the charging and discharging of a capacitor depends on the product of the capacitance and the rate of voltage change, even for low-voltage charging and discharging, excessive capacitance or too fast a rate of voltage change will also cause damage to the capacitor. Therefore, when multiple film capacitors are connected in parallel for voltage testing or life testing, please connect an appropriate current limiting resistor in series with each capacitor.
Flame Retardancy
Although a fire-resistant flame-retardant material --- flame-retardant epoxy resin or plastic case is used in the outer packaging of the film capacitor, the external continuous high temperature or flame can still deform the capacitor core and cause the outer packaging to rupture, causing the capacitor core to be damaged, melt or burnt.
Capacitors for Suppressing Electromagnetic Interference in Power Supplies
When a capacitor is used in the power line to suppress interference, not only the normal voltage, but also an abnormal pulse voltage occurs, which may cause the capacitor to emit smoke or catch fire. Therefore, the standards of cross-line capacitors are strictly regulated in different countries. Please use certified safety capacitors. It is not recommended to use DC capacitors in jumper circuits.
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