Polymer Hybrid Capacitors - Tapping into expertise at its source

09/26/2019 Know-How

Polymer hybrid capacitors resemble each other like two drops of water – at least when you look at the data sheets. To find the optimum component, it is therefore advisable to draw on the know-how of manufacturers and distributors. Because there definitely are differences.

Polymer hybrid capacitors are characterized by their stability in extreme conditions, long service life, low equivalent series resistance (ESR), a specification option of up to 165°C, and certification to AEC-Q200. Due to these properties, they are now used in numerous applications, including in cars, for instance in electric control units (ECU) for oil or water pumps, cooling fans and electric power steering (EPS) systems. Nonetheless, care must be taken when selecting the appropriate capacitor: The data sheets of manufacturers all look roughly the same and it is impossible to detect any subtle differences. They do exist however - but are only shown through testing.

Generally speaking, the production processes for polymer hybrid capacitors are patented. In addition to differences in production, manufacturers choose to use varying raw materials, for example different polymer compositions both in terms of their quantity and substances. The ERS behavior of capacitors may, therefore, vary in a range of 10kHz or 20kHz for automotive applications, although this does not differ at all according to the respective data sheet. There are differences between the components of various manufacturers also in the negative temperature range. It is therefore worthwhile to utilize the know-how of the manufacturer or a "neutral" distributor.

Arrhenius formula
A key aspect is, for instance, the life expectancy of the hybrid capacitor. To determine it, developers like to use the well-known Arrhenius formula. To do so, they require the service life specified by the manufacturer Lb, the maximum temperature Tmax, the temperature rise ΔT0 (6K, maximum permissible value, may vary according to the series and manufacturer) when the ripple current is applied, and the surface temperature of the capacitor Tc during application. The life expectancy is thus calculated as follows:

L = Lx  ((2 Tmax + ΔTo - Tc) x 10-1)

However, the formula does not do justice to the technology of polymer hybrid capacitors. This is because it roughly describes a quantitative temperature dependence and has the disadvantage that the impact of the ripple current on the capacitor is not taken into account sufficiently, since only the maximum scenario is assumed. Nonetheless, self-heating caused by the ripple current has a significant influence on the service life of the capacitor. In addition, ripple currents in the real application rarely remain constant at any temperature over the entire service life. Working as precisely as possible and using the manufacturer's or expert's know-how in the service life calculation are therefore key to delivering an effective design.

More precise data and certain specific values are not available online or in the data sheet but exclusively from the actual manufacturer. Based on this know-how, available formulas, and in-house measurement data, the manufacturer calculates the service life. In addition, the manufacturer analyzes the greatest possible load on the capacitor and passes this information on to customers for comprehensibility. This provides the customer with a list of which model is best suited for the respective application, which quantity is ideal - e.g. for a parallel circuit - and how long the capacitor will last under the given conditions. After all, this is also the manufacturer guarantee.

Lifetime table and mission profile
In the so-called lifetime tables, manufacturers list the varying values from the test results. This can be used to determine how the service life of the respective circuit can be maximized through the parameters package temperature and ripple current at 100kHz. If a temperature of 125°C at 2A is assumed on the basis of the fictitious lifetime table (Fig. 1), for example, the service life is 5,000 hours. At 145°C and 6A, the capacitor would achieve a service life of 850 hours. The rated area refers to the range determined by measurement results, while the extended area refers to extrapolations based on the measurement results.

The lifetime tables of the manufacturers show that in practice significantly higher values are possible than those stipulated in the data sheet and create confidence in the technology of polymer hybrid capacitors.

A mission profile (Fig. 2) describes the stresses and strains to which a capacitor is exposed in real life use. These include, for example, the changing ambient and operating temperatures, the load duration, and the measured ripple current at a specific frequency. The measurement of such a mission profile costs valuable time in development but is worthwhile if the circuit can be designed more efficiently and the manufacturer confirms, for instance, three instead of four capacitors in the parallel circuit. This obviously provides customers with precise information on the reliability of the capacitor in the respective application.

Overload test for components
In addition, manufacturers carry out overload tests and incorporate the respective findings into their calculations. Seeing as the technology is less than ten years old and thus still relatively new, these tests represent an important source of information for manufacturers regarding the quality and further development of capacitors.

For a test, for example, a 25V capacitor in a 10mmx10mm design, which is specified for 2A ripple current, 100kHz, 20mΩ ESR, and 4,000h at 125°C ambient temperature, was exposed to considerably higher ripple currents. This was carried out at two locations at a constant ambient temperature of 125°C using 200 components at each site. When testing with 6A, i.e. a triple overload, the capacitors achieved over 19,000 hours and operated for even longer. The capacitance drift stabilized at approx. -18%, while the end-of-life definition is -30% according to the data sheet. The ESR remained constant (start at 18mΩ, data sheet value of 20mΩ, leveled off at approx. 22mΩ). The experts at Rutronik came to a similar conclusion: The ESR did not change even when capacitors were frozen to -55°C. For this purpose, the product marketing engineers cooperated with the laboratory engineers at RUTRONIK to develop a portable demonstration tool that freezes a Low ESR SMD capacitor and a polymer hybrid capacitor within a few seconds while constantly measuring the ESR. It can be observed live how the ESR of the polymer hybrid capacitor remains absolutely stable while that of the electrolytic capacitor increases more than five times.

At the highest overload of 14A per capacitor, which corresponds to a core temperature of roughly 150°C in the capacitor, only one of the four lots failed in the test after 4,300 hours. However, the reason for this was not the technology itself: The heat led to the rubber stopper becoming porous. To eliminate this weak point, manufacturers are already looking for other sealing mechanisms and new designs.

Such tests show that the possibilities of hybrid technology are far from exhausted. All manufacturers are still working hard to further optimize their polymer hybrid capacitors, thereby maximizing performance. The aim is to achieve higher capacities, voltages, and temperatures over a longer service life as well as further SMD cap dimensions to deliver greater miniaturization under higher loads.

Replacing capacitors in a circuit
It is already often worth replacing other types of capacitor with a polymer hybrid capacitor. For example, if two or even three aluminum electrolytic capacitors in a circuit can be replaced with a hybrid model, it equates to significant savings in terms of size, installation height, and PCB space. In addition, due to its specific properties, the hybrid guarantees greater stability than the aluminum electrolytic capacitor with regard to increasing ESR, drift during the service life, frequency, and temperature as well as changes in capacitance.

In one specific application, for instance, axial capacitors could be replaced (Fig. 3). The actual choice was between a traditional axial aluminum electrolytic capacitor and the hybrid capacitor, both with a leaded design. The ripple current for each cap was similar for both, only the total capacitance of the hybrid was lower. This factor occurs in most polymer hybrid capacitor solutions but does not usually affect how they work in the circuit. Since the use of these capacitors is defined by the ESR and ripple current, a range in which even large axial or SolderStar capacitors have strengths but also demonstrate the typical weaknesses of aluminum electrolytic capacitors. Apart from this fact, the hybrid capacitor required much less installation space, had a significantly lower ESR, and provided stability during its entire service life. Besides space and weight savings in the circuit, the hybrid capacitor also ensured cost savings.

Customers who also want to benefit from this should utilize the know-how of manufacturers and, in particular, that of the experts at Rutronik, who are able to assess the technology from a neutral viewpoint. The FAE team supports developers on-site in their selection by providing manufacturer-independent advice on products and technology. To guarantee the ideal circuit design, Rutronik is the intermediary with direct contact to the various experts employed by the manufacturers.

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