This invention related to a passive low pass filter and more particularly to a passive low pass filter of a surface mount device configuration.
Low pass filters are used in computer, communications, and automotive industries. Currently, discrete components such as ferrite block and surface mounted device capacitors are used as low pass filters to reduce noise generated by nearby electrical components. The filters reduce noise by passing electrical signals having frequencies within a specified filter pass band and attenuating noise induced frequencies that lie outside the pass band.
The passive low pass filter is a circuit essentially behaving as a frequency sensitive voltage divider. At high frequencies the output behaves as if it is shorted while at low frequencies the output appears as an open circuit. Passive low pass filters comprise one or more inductances and compacitences tuned to suppress or attenuate the unwanted signals while allowing passage of a band of wanted signals. A typical low pass filter has an inductor, a capacitor and a dielectric encapsulant. The encapsulant commonly encases the filter and engages to and separates a plurality of plates which comprise the capacitor. A variety of low pass filters of a surface mount configuration exist, they include a T-filter, a LC-filter and a xcfx80-filter.
The consumer markets for low pass filters are requiring greater frequency attenuation utilizing smaller components. As attenuation increases, noise becomes a greater problem. The magnetic field of the inductor, otherwise known as a coil, may produce a form of noise known as eddy currents across the capacitor plates.
The invention provides a passive low pass filter of a surface mounted device configuration. The filter comprises two coils, each wound about their own parallel centerlines. The coils are preferably counter-wound to enhance the flux linkage between the windings of the coil, thereby, increasing the inductance of the coil. Each coil is in electrical contact with at least one capacitor. Each capacitor has a plurality of hot plates and at least one ground plate. Each one of the ground plates alternate between each one of the hot plates. The ground and hot plates are generally perpendicular to the centerlines. Separating the ground plates from the hotplates is a dielectric encapsulant which also encases the coils. Preferably, the coils generate a magnetic field parallel to ground conductor plates and end conductor plates mounted externally to the encapsulant and generally perpendicular to the capacitor plates. This orientation eliminates the problem of induced eddy currents at the ground and end conductor plate locations.
Preferably, the low pass filter is of a xcfx80-type configuration wherein the coils are disposed between a first and a second capacitor. The plurality of hotplates of each capacitor have a feed plate connecting to a bottom winding of each coil. Top windings for the first and second coils of the xcfx80-type configuration are directly connected.
Thus, an advantage of the present invention is a reduced size.
Another advantage of the present invention is an inductance increase.
Yet another advantage of the present invention is the elimination of eddy currents.