A new series of high-rated X2 capacitors with up to 45 μF allows the construction of space-saving and low-inductance EMI filters.

The number of nonlinear loads in industrial and consumer electronics is increasing continuously. Whether it’s a frequency converter, PC or energy-saving lamp − hardly any appliance now draws sinusoidal current from the power network. The consequence is considerable pollution of the power system with harmonics and interference. Figure 1 shows a typical sinusoidal power voltage distorted by harmonics.

	FIG. 1: SINUSOIDAL POWER VOLTAGE
The sinusoidal power voltage is greatly distorted by a high proportion of harmonics.

Harmonics and interference represent a hazard to electronic equipment. They may cause sporadic malfunctions, failures or even permanent damage. These risks can be eliminated only by suitably dimensioned EMI capacitor filters. They not only protect the equipment from external interference, but also prevent harmonics generated within the equipment from being emitted to the outside.

Ever-higher capacitances required

X2 capacitors are typically used for filtering. The rising proportion of harmonics as well as their high amplitudes require higher capacitances to assure a sufficient filter effect. To obtain these values, several capacitors are usually connected in parallel. However, parallel circuits have their own drawbacks, such as parasitic inductances and losses in the connecting leads. They also involve higher labor and material costs, as well as taking up more space on the circuit board.

Single-capacitor solution

To solve this problem, EPCOS has developed the new B3292*E/F series of X2 capacitors rated for up to 45 μF. The following example shows the potential for space saving that it offers: a filter with the required capacitance of 20 μF designed with five conventional 4 μF-X2 capacitors each with dimensions of 18.0 x 41.5 x 32.5 mm³ (W x L x H) has a volume of 121.39 cm³. In contrast, a 20-μF capacitor of the new B3292*E/F series with dimensions of 30 x 42 x 45 mm³ takes up only 56.7 cm³ – meaning a volume reduction of more than 50 percent. The footprint saving is even more impressive: the new solution from EPCOS occupies only 12.6 cm² instead of 37.37 cm² for the discrete capacitor solution, corresponding to a space saving of about two thirds (Fig. 2). Figure 3 clearly shows the greatly increased packing density of the new series.

	FIG. 2: SAVING SPACE THANKS TO THE B3292*E/F SERIES
The new B3292*E/F series (left) reduces the volume by more than 50 percent and the space requirement by more than 60 percent compared to a solution consisting of several capacitors.

	FIG. 3: INCREASED PACKING DENSITY OF B3292*E/F
The new B3292*E/F series offers a much higher packing density at all capacitance values.

In order to develop filters it is necessary to know the precise impedance curve of the capacitor elements as a function of the frequency. This curve yields the typical resonant frequency f_r at which the inductive and capacitive components each have the same value with a different algebraic sign, so that the reactive part is equal to zero. The total impedance consequently has a minimum value at this frequency. The impedance curves in Figure 4 show clearly that the capacitors of the new series have a higher resonant frequency than a design that consists of discrete capacitors. The newly developed capacitors exhibit significantly lower impedance and thus better attenuation, especially in the range above the resonant frequency.

	FIG. 4: IMPEDANCE CURVES COMPARED
The capacitors of the new series show a much higher resonant frequency than types of the existing series.

Just like the existing X2 series, the new B3292*E/F series is based on EPCOS’ winding technology and polypropylene for the dielectric. These capacitors are self-healing and satisfy all relevant safety standards. The table shows a comparison of the old and new series.