Comparison of advantages of metallized film capacitors and disadvantages and solutions

Characteristics of metallized film capacitors

First, aluminum electrolytic capacitors have a low rated voltage (usually ≤450V). To obtain a higher withstand voltage rating, they usually need to be used in series, and the voltage balancing problem must be considered during the series connection process. The voltage of a single film capacitor can reach up to 20kV, so there is no need to consider the series connection problem in medium and high voltage frequency conversion applications. Of course, there is no need to consider connection problems such as voltage balancing and the corresponding costs and manpower.Secondly, the ripple current capability of metallized film capacitors can reach ten to dozens of times the rated ripple current of aluminum electrolytic capacitors of the same capacity. In order to achieve higher current resistance, aluminum electrolytic capacitors usually use larger capacity to meet the requirements, and larger capacity is an unnecessary waste of cost and installation space.

Furthermore, the ESR of film capacitors is usually very low, generally below 1mΩ, and the parasitic inductance is also very low, only a few dozen nH. These are beyond the reach of aluminum electrolytic capacitors. The extremely low ESR greatly reduces the voltage stress on the switch tube, which is beneficial to the reliability and stability of the switch tube.

In addition, aluminum electrolytic capacitors cannot precisely control the capacity error due to the inability to control the oxidation area of the aluminum plate. The general error is within 20%. Metallized film capacitors can precisely control the capacity tolerance by directly controlling the area of the wound metal film plate. If the customer needs it, the tolerance can be controlled within 1%.At present, conventional products generally use a manufacturing tolerance of 5% and 10%. In addition, the loss of aluminum electrolytic capacitors is very large, of which reactive power loss can account for 20% of the total reactive capacity, and the loss of aluminum electrolytic capacitors will increase with the increase of frequency. After the frequency exceeds 1000HZ, the loss change rate gradually increases. The increase in active power loss increases the heat generation. This is also the reason why aluminum electrolytic DC support capacitors have high heat generation, high failure rate and short life in medium and high frequency high power environments.The expected life under normal rated conditions is less than 5,000 hours, and damage to aluminum electrolytic capacitors often causes the entire device to cease operation. During the entire life of the high-voltage inverter, film capacitors can be maintenance-free, greatly saving the maintenance costs and manpower of end customers.

Advantages of Metallized Film Capacitors

Metallized polyester film winding, non-inductive structure

Epoxy resin encapsulation, CP line one-way lead-out

Good self-healing performance, high insulation resistance, stable capacitance

Applicable to DC and pulsating circuits, widely used in filtering, DC isolation, bypassing, coupling and noise reduction of various electronic and electrical equipment

Comparison of advantages of metallized film capacitors

Organic dielectric capacitors are also called metal film capacitors. They are mainly paper dielectric (MP), metal dielectric, polyester (Mylar also known as polyester), polystyrene (MKS), polypropylene (MKP), polycarbonate (MKC) and polyethylene terephthalate (MKT). Among them, paper dielectric, polyester and polypropylene capacitors are divided into two types: with and without metallized dielectric.Generally speaking, once the dielectric of a metallized dielectric capacitor is broken down, the arc generated at the breakdown point will cause the metal film to melt and evaporate, eliminating the short circuit and automatically resuming normal operation with self-healing function. This type of metal film capacitor has good performance and is generally used in power supplies.

Metallized film capacitors, such as metallized polyester capacitors (Mlylar) and metallized polypropylene capacitors (MKP), can eliminate the inductance caused by winding and make better non-inductive capacitors. For this type of capacitor, you should choose a capacitor with a nominal value of X2. The winding process is special and it is a non-inductive capacitor, so it is also a type of metal film capacitor. (Insert a collection diagram, the collection diagram is centered on the page)

Inorganic capacitors are mainly mica, porcelain, monolithic, glass glaze and other capacitors. Electrolytic capacitors are represented by aluminum electrolytic capacitors, which have the characteristics of small size and large capacity, ranging from tens of microfarads to 20,000 or 30,000 microfarads, but their impedance frequency characteristics are poor, temperature characteristics are not good, insulation resistance is low, leakage current is large, and they will deteriorate and fail if not used for a long time.

Due to the poor conductivity of the electrolyte, the large resistance, large loss, and the distributed capacitance and error are not very small. For various reasons, the performance of electrolytic capacitors is far inferior to that of metallized film capacitors. It is only suitable for power supply filtering and power supply decoupling, and is not suitable for use in audio coupling circuits.

Disadvantages of Metallized Film Capacitors and Solutions

The capacitor composed of double-sided metallized polypropylene film and non-metallized polypropylene film rolled or laminated is called metallized film capacitor. In principle, metallized film capacitors should not have short-circuit failure mode, while metal foil capacitors will have many short-circuit failure phenomena. Although metallized film capacitors have the above-mentioned huge advantages, they also have certain disadvantages compared with metal foil capacitors.

First, the capacitance stability is not as good as that of foil capacitors. This is because metallized capacitors are prone to capacity loss under long-term working conditions, and their capacity may decrease after self-healing. Therefore, if used in oscillation circuits that require high capacitance stability, metal foil capacitors should be used.

Another major disadvantage is the poor ability to withstand high currents. This is because the metallized film layer is much thinner than the metal foil and has a weaker ability to carry high currents. In order to improve this disadvantage of metallized film capacitors, there are currently improved high-current metallized film capacitor products in the manufacturing process.

To improve the shortcomings of metallized film capacitors, double-sided metallized films can be used as electrodes; or the thickness of the metallized coating can be increased; as well as the end metal welding process can be improved to reduce contact resistance.