Lightweight electromagnetic interference filter

This invention relates generally to circuits for filtering electromagnetic noise. More specifically, the invention relates to a low pass filter with reduced weight for use in aircraft.

Electromagnetic interference, or “noise,” is a common problem in electrical circuit design. Noise may originate from natural sources, such as background radiation or lightning strikes, but the more common and more problematic noise is electromagnetic noise generated by components in electrical systems. Modern aircraft include numerous electrical systems, so filtering of electromagnetic noise is a significant concern.

Filtering circuits are used to reduce the effect of electromagnetic noise in the electrical systems of aircraft. Electromagnetic noise signals are high frequency signals, so filtering circuits are usually designed as low pass filters, which allow the desirable low frequency signals to pass through a circuit, while filtering out the undesirable high frequency electromagnetic noise signals. Because the weight of components is of critical importance in aircraft design, it is beneficial to reduce the weight of components that are used in electrical circuits, including filtering circuits.

Therefore, there is a need in the art for an electrical filtering circuit that is able to provide sufficient filtering of electromagnetic noise in electrical circuits used in aircraft, while reducing the size and weight of the filtering circuit in order to optimize its use in aircraft electronics.

The invention is a low pass filter that replaces inductors with capacitors in the damping portion of the circuit. The resulting low pass filter significantly reduces the size and weight of the circuit, making it particularly useful in applications in which size and weight are important parameters, such as aircraft.

FIG. 1A is a schematic diagram showing a low pass filter circuit according to an embodiment of the invention.

FIG. 1B is a schematic diagram showing a configuration of capacitors for a common mode filter according to an embodiment of the invention.

FIG. 2 is a schematic diagram showing a low pass filter according to the prior art.

FIG. 1A is a schematic diagram that shows low pass filter circuit 100 according to an embodiment of the invention. Low pass filter circuit 100 includes three inputs 110 , pull-down resistors R 1 , R 2 and R 3 , differential mode filter 120 , common mode filter 130 and three outputs 140 . There are three signal lines (or “rails”) in low pass filter circuit 100 , identified in FIG. 1 as line 1 , line 2 and line 3 . Low pass filter circuit 100 filters out electromagnetic noise in electrical circuits, particularly noise affecting electrical circuits in aircraft.

Electromagnetic noise includes two components: differential mode noise and common mode noise. Differential mode noise results from current flowing from a positive voltage terminal, through a load and returning through a negative voltage terminal. Common mode noise results from current flowing from a lead, through a load and returning through the ground terminal. Noise filtering circuits must eliminate both common mode noise and differential mode noise. The invention relates particularly to filtering common mode noise signals.

Electromagnetic noise enters circuit 100 at inputs 110 . While electromagnetic noise generally consists of high frequency signals, these signals may also include a direct current component. Resistors R 1 , R 2 and R 3 are pull-down resistors to control common mode transient signals. Transients are high voltage bursts that may be caused by, for example, starting or switching of large electrical motors. The resistors R 1 , R 2 and R 3 provide a low resistance path to ground to filter out the direct current component of the electromagnetic noise. In this way, the system is able to start with minimal overshoot.

Differential mode filter 120 includes inductor L 1 , inductor L 2 , inductor L 3 , capacitor C 1 , capacitor C 2 and capacitor C 3 . Inductor L 1 , inductor L 2 and inductor L 3 are positioned in line 1 , line 2 and line 3 , respectively. Capacitor C 1 is positioned between line 1 and line 2 , capacitor C 2 is positioned between line 2 and line 3 , and capacitor C 3 is positioned between line 1 and line 3 . Differential mode filter 120 works as a conventional differential mode filter, in which inductor L 1 , inductor L 2 and inductor L 3 , capacitor C 1 , capacitor C 2 and capacitor C 3 work together to filter out differential mode noise signals.

Common mode filter 130 includes inductors 132 (including inductor L 4 , inductor L 5 and inductor L 6 ), capacitors 134 (including capacitor C 4 , capacitor C 5 and capacitor C 6 ) and damping circuit 136 (including capacitor C 7 , capacitor C 8 , capacitor C 9 and resistor R 4 ). Inductors 132 are connected in each of the three signal lines: inductor L 4 is connected in line 1 , inductor L 5 is connected in line 2 and inductor L 6 is connected in line 3 . Capacitors 134 are connected between the signal lines and ground: capacitor C 4 is connected between line 1 and ground, capacitor C 5 is connected between line 2 and ground and capacitor C 6 is connected between line 3 and ground. For ease of explanation, capacitors capacitor C 4 , capacitor C 5 and capacitor C 6 are shown and discussed as individual capacitors. In some embodiments, each of these capacitors may be constructed by connecting two or more capacitors together. This is illustrated in FIG. 1B , which shows capacitor C 4 consisting of capacitors C 4 a and C 4 b , capacitor C 5 consisting of capacitors C 5 a and C 5 b , and capacitor C 6 consisting of capacitors C 6 a and C 6 b.

Electromagnetic noise, whether it is differential mode noise or common mode noise, is composed of high frequency signals. An inductor's impedance is directly proportional to frequency, while a capacitor's impedance is inversely proportional to frequency. Thus, inductors present relatively high impedance to high frequency noise signals, while capacitors present relatively low impedance to high frequency noise signals. Therefore, inductors 132 present high impedance to high frequency signals, blocking these high frequency signals and thereby filtering out a significant portion of the high frequency noise signals which are sought to be eliminated. In addition, capacitors 134 present low impedance to high frequency noise signals. Any high frequency noise signals that get through inductors 132 will find that capacitors 134 present an extremely low impedance path to ground. The high frequency signals that are not filtered out by inductors 132 will be shunted to ground by capacitors 134 . Therefore, high frequency signals are prevented from reaching output 140 .

Damping circuit 136 , which is part of common mode filter 130 , includes capacitor C 7 , capacitor C 8 , capacitor C 9 and resistor R 4 . The combination of inductors 132 and capacitors 134 have a certain resonant frequency that will depend upon the particular inductance and capacitance of all of those components. If the electromagnetic noise is at that particular resonant frequency, inductors 132 and capacitors 134 may enter a resonant state, which can lead to overheating and eventual failure of the circuit. Damping circuit 136 prevents low pass filter circuit 100 from entering a resonant state.

In one embodiment of the invention, common mode filter 130 is designed as a low pass filter that filters out noise signals at 150 kiloHertz (kHz) and above. In this embodiment of the invention, inductor 132 of common mode filter 130 is made up of 5400 microHenry (μH) nanocrystalline common mode inductors, capacitor C 4 , capacitor C 5 and capacitor C 6 are each two 47 nanoFarad (nF) common mode capacitors, capacitor C 7 , capacitor C 8 and capacitor C 9 are each single one microFarad (μF) capacitors, and resistor R 4 is a 100 Ohm (Ω) resistor. In this configuration, common mode filter 130 exhibits essentially no impedance to signals of 1450 Hertz (Hz) and lower, while filtering out essentially all signals of 150 kiloHertz (kHz) and higher.

For purposes of comparison, FIG. 2 shows a prior art low pass filter circuit 200 , which includes inputs 210 , differential mode filter 220 and common mode filter 230 . Differential mode filter 220 includes inductor L 11 , inductor L 12 , inductor L 13 , capacitor C 11 , capacitor C 12 , and capacitor C 13 . Differential mode filter 120 in low pass filter circuit 100 and differential mode filter 220 in low pass filter circuit 200 function in essentially the same way.

Low pass filter circuit 200 also includes common mode filter 230 , which includes inductors 232 (including inductor L 14 , inductor L 15 and inductor L 16 ), capacitors 234 (including capacitor C 14 , capacitor C 15 and capacitor C 16 ) and damping circuit 236 (including inductor L 17 , inductor L 18 , inductor L 19 and resistor R 4 ). In this particular prior art circuit, the inductors 232 are 500 microHenry (μH) common mode inductors, capacitors 234 are fifteen 1 microFarad (μF) common mode capacitors (i.e., capacitors C 14 , C 15 and C 16 are each five 1 microfarad (μF) common mode capacitors) and damping circuit 236 has three 100 milliHenry (mH) inductors and a 6 Ohm (Ω) resistor.

Comparing inductors 132 of circuit 100 with inductors 232 of circuit 200 , the invention embodied in circuit 100 includes inductors with higher inductance. The performance of low pass filter 130 is proportional to the product of the inductance of inductors 132 and the capacitance of capacitors 134 . Increasing the inductance of inductors 132 over the prior art allows the capacitance of capacitors 134 to be reduced over the prior art. As a result, circuit 100 only uses six capacitors for capacitors 134 , in comparison to the fifteen capacitors used as capacitors 234 in circuit 200 . This reduces the size and weight of circuit 100 by reducing the number of capacitors and the space needed for them in the circuit.

Similarly, comparing damping circuit 136 of circuit 100 with damping circuit 236 of circuit 200 , the invention embodied in circuit 100 includes capacitors instead of inductors in the damping circuit. Damping circuit 236 in FIG. 2 includes three inductors and a resistor. When starting large motors during zero and low speeds, or at transient fault conditions, inductors L 17 , L 18 and L 19 of damping circuit 236 are subjected to DC and low frequency voltages. At such frequencies, inductors L 17 , L 18 and L 19 exhibit low impedance, leading to high current flow in damping circuit 236 to a point of possible destruction of damping circuit 236 . By contrast, damping circuit 136 has high impedance to DC and low frequency voltages during low speed motor starting, or at transient fault conditions, which renders damping circuit 136 immune to destruction due to long low speed starting and fault situations.

In addition, as a result of replacing the inductors of damping circuit 236 with capacitors in damping circuit 136 , the weight of low pass filter 100 is significantly lower than the weight of low pass filter 200 . For example, in a comparison of circuits designed according to one embodiment of the invention and the prior art, the inductors in damping circuit 236 weighed about 2 pounds (0.9 kilograms), while the capacitors used in damping circuit 136 weighed about 0.2 pounds (0.09 kilograms). Because weight is a significant factor in aerospace and other applications, low pass filter 100 presents significant advantages over the prior art.

The invention is a low pass filter that filters EMI noise. The low pass filter includes a damping circuit that employs three capacitors and a resistor. Using capacitors in the damping circuit significantly reduces the overall weight of the low pass filter circuit, which presents significant advantages in certain applications, such as aerospace.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.