Even during production, there is a risk of humidity becoming trapped in the capacitor. High humidity during operation can attack the vaporized metallized layer on the film capacitors. If the tightness of the plastic case or compound is insufficient, humidity may penetrate the capacitor.
In the case of RFI (radio frequency interference) capacitors, humidity also causes, in addition to corrosion of the metallized layer, what is known as the corona effect when voltage is applied. The resulting ionization process damages the metallized layer of the film. The consequences are loss of capacitance and a faster rise in the loss factor.
With DC capacitors, humidity in the component also leads to corrosion of the metallized layer, which has a particularly negative impact on its reliability and service life. Therefore, it is imperative to prevent the ingress of humidity so as not to compromise the performance of the components.
Besides empirical studies, numerous analyses also show that the ingress of unwanted humidity into film capacitors is the number one cause of a shortened service life.
What is THB?
A recognized standard for accelerated service life testing is the temperature-humidity-bias (THB) test. This reliability test is aimed at accelerating the aging process of capacitors and measuring whether the capacitors maintain their capacitance, loss factor, and isolation resistance at a given temperature, relative humidity (RH), and nominal voltage over a defined period of time (Table 1).
Depending on the testing level (Table 2), the capacitors meet various climatic requirements. For example, in a Grade IIIB high robustness under high humidity test, the capacitors must withstand 85°C and 85% relative humidity for 1,000 hours of operation undamaged with an applied nominal voltage.
With capacitors that survive these tests, the most important electrical parameters remain significantly more stable over the entire service life than with a standard component. As a result, THB components offer a longer service life and greater reliability.
As shown in Table 2, THB Grade IIB and IIIB are among the toughest test conditions and differ only in terms of how long the voltage is applied.
Where does the trend toward THB components come from?
Demand for THB components is increasing. One reason for this is the IEC standard for RFI capacitors that has been in force since 2016. It stipulates that capacitors must meet at least THB Grade I.
A strong driver of the trend toward THB components is the automotive segment, but THB components are also increasingly being used in industry. The reasons for this are obvious when considering the climatic conditions under which solar inverters, for example, must operate as long and as reliably as possible.
Since a supplier of inverters, for example, usually has no prior knowledge of where its products are going to be used, it makes sense to design the inverter – and thus also the capacitors – for the worst-case conditions. This means they perform reliably even under the toughest operating conditions.
How do THB components differ from standard components?
The suppliers achieve an increased service life and reliability of THB capacitors through a special design. Five factors are crucial in this respect:
- Package: PPT package or PPS package with increased tightness
- Compound: High-degree epoxy resin with high resistance to humidity
- Plastic film: Thicker, i.e. multiple-layered, film for better self-healing
- Metallized layer: Mixture of different metals for better self-healing
- Production: Ensuring low humidity in production
Some suppliers also rely on internal series arrangement of the films to further reduce the ionization voltage on the film.
Where are THB capacitors used?
THB capacitors are predestined for applications exposed to high temperatures – or more importantly, large temperature fluctuations – and/or high humidity. Their use is particularly recommended in these conditions. Mainly due to the high temperature loads, this includes practically all automotive applications for the actual vehicle, but also applications for the vehicle infrastructure, e.g. charging stations or Wallboxes. In the industrial sector, this mainly encompasses inverters, solar inverters, frequency converters, smart meters, and wind turbines. In addition, THB models are ideal for all applications that place high requirements on the service life and reliability of a film capacitor.
The comparatively new type of film capacitors is characterized by increased reliability and longer service life. Thanks to these improved properties, new and sustainable uses are possible in trendsetting applications, e.g. in the field of electromobility or renewable energies.
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