FS Capacitor - A Space Saving New Electrolytic Capacitor For Demanding Applications
Today, one can hardly find a consumer, industrial or high-reliability electronic device that does not make use of a voltage regulator. Such electronic devices often contain several sub-circuits with different voltage requirements (sometimes higher and sometimes lower than the supply voltage). A simple buck converter can only produce voltages lower than the input voltage and a boost converter, only voltages higher than the input. To provide voltages over the complete range a circuit known as a buck-boost converter is required.
A typical buck-boost DC-DC converter circuit is illustrated in figure 1. Such circuits converter from one voltage level to another level, by storing the input energy temporarily in inductors or capacitors when switch sw1 is ON and sw2 is OFF, and then releasing that energy to the output at a different voltage value when sw2 is ON and sw1 is OFF. The efficiency of conversion ranges from 75% to 98%. This high efficiency is typically achieved by using power MOSFET’s (metal oxide semiconductor field-effect transistor).
It has been reported in the literature that the capacitors and transistors degrade faster than any other components in the DC-DC converters (Imam, et al., 2005). Degradation in these components leads to decrease in the operating efficiency of the converters and eventually failure in the system. The selection of a suitable capacitor plays an important part in the design of switching voltage converters. “Some 99 percent of so-called ‘design’ problems associated with linear and switching regulators can be traced directly back to the improper use of capacitors” states the National Semiconductor IC Power Handbook.
The aluminium electrolytic capacitor is a passive component that has kept pace with advancements in technology. Aluminum electrolytic capacitors are used in various applications because they can achieve high capacitance and high voltage ratings in small, cost-efficient case sizes and have furthermore proven to be relatively insensitive to voltage spikes. This type of capacitor has traditionally been used for filtering, timing networks, bypassing, coupling, and other applications requiring a cost-effective, volumetrically efficient, and highly reliable component.
However, today’s buck regulators typically use just one type of output capacitor because it becomes too difficult to design with different capacitance and ESRs. This forces many designers to use more expensive capacitor types like polymer or tantalum that provide lower ESR than average aluminium electrolytic capacitors but has other disadvantages such as a closed failure mode.
Electrolytic advantages and basic properties of capacitors:
A capacitor is made of two parallel plates, the electrodes, with a dielectric between them. The amount of capacitance is directly proportional to the surface area of the electrode, but inversely proportional to the dielectric thickness. If the thickness is reduced by one-half, the capacitance is doubled. The high volumetric efficiency of an electrolytic capacitor is to its enhanced plate surface area, which is enhanced by an etching process. A large internal surface can be created on the aluminium electrodes by electromechanical etching. The dielectric is typically an oxide with a high dielectric strength, which is electromechanically deposited in thin layers. The combination produces a high capacitance in small volume. The aluminium electrolytic capacitor consists of an anode foil, a cathode foil, and a separator paper that are wound together and impregnated with an electrolyte. The anode foil has an aluminium oxide layer acting as the dielectric. After the thin aluminium foil (65 to 100 microns) is electrochemically etched to increase the plate’s surface area, the dielectric is produced by anodic oxidation on its surface. The cathode foil, in general, utilizes no oxidation process. An illustration of a typical aluminium electrolytic capacitor is shown in figure 3 below.
Figure 2 & Figure 3
In a power supply application, when the capacitor is used as an input buffer, end of life is when the capacitance drops below the value needed to supply energy to the DC-DC converter when the output voltage is too low. For the output buffer, life ends when the impedance increases past the point where the ripple voltage on the output voltage causes problems in the circuit driven by the power supply.
The most common cause of failure is the electrolyte itself, which can evaporate over time, altering the electrical properties as it goes. The electrolyte is soaked into paper either side of which are the aluminium foils. Typically the lifetime increases by a factor of two for every 10°C fall in temperature over the normal operating range. Increasing the voltage rating to reduce the chance of a dielectric failure can have the opposite effect as this increases the ESR, which increase the capacitor’s temperature and thus the chance of failure. ESR also increases as temperature falls, which can increase the output voltage ripple and loop gain. In a power supply, this can cause oscillations and thus instability.
FS Series' Properties
A good example of a long life electrolytic capacitor is the Panasonic EEU-FS series. These radial leaded aluminium have quoted lifetime of up to 10.000 hours at 105°C (valid for all case-sizes ≥ 10 mm), more than double that of the previous generations and with a capacitance about a third higher. Panasonic EEU-FS therefore combines long life, high ripple and low impedance properties. They are suitable for switching mode adapters, line noise removal LCD backlighting, LED systems and similar applications. These would be typically used in industrial controls, LED lighting, alternative energy equipment and home appliances.
The capacitance of the devices ranges from 27 to 10,000µF and they have a full operating temperature range from -40 to +105°C. The full range of case size is from 5 to 16mm. The Panasonic EEC-FS series are halogen free, RoHS compliant and REACH certified. Also production complies with international ISO standards.
Due to the latest innovation in design and performance increase, case-size can be reduced by around 20%, compared to state-of-the-art technology (FR-series). Additionally to this miniaturization, the spectrum of rated voltages is between 6.3 V.DC up to 63 V.DC, which makes them suitable for a variety of applications.