I. Field of the Invention
The present invention relates generally to a common mode noise filter for an electrical system.
II. Description of Related Art
There are many previously known electrical systems which have a source of electrical noise. Unless the electrical noise is eliminated, or at least reduced, the electrical noise can adversely affect not only the operation of the electrical system itself, but also associated or nearby electrical systems.
For example, in an electric or hybrid automotive vehicle, a switching power supply or inverter is typically employed to generate the voltages needed to power the electric motors for the vehicle. Conventionally, switching field effect transistors (FET), GTO, or IGBT are used to create the power signal from the power supply. These FETs, however, generate a great deal of electrical noise each time the FETs switch between an on and off condition.
Such systems typically involve two different types of electrical noise which vary from each other in terms of their propagation path. First, there is differential mode current in which the current flows through two or more cables in the opposite direction to each other relative to the source and the load. Secondly, there is common mode current which flows through two or more cables in the same direction. This common mode current is typically high frequency and, of the two different types of electrical noise, is more troublesome than differential mode noise.
One previously known method of reducing common mode noise is to utilize a common mode filter electrically connected to the electrical system, preferably near the noise source. These previously known filters typically comprise a common mode choke coil L and a capacitor connected in between each power line of the electrical system and ground. The capacitors thus block any DC current from flowing through the capacitors and the common mode choke coil does not block any DC current, but act as a conductance path for high frequency common mode noise and as a block for high frequency common mode noise.
However, no capacitor exhibits the perfect properties of a capacitor. Instead, real life capacitors exhibit stray inductance (ESL) which decreases the overall efficiency of the common mode noise filter.
For example, see
There have been a number of previously known ways to decrease the ESL of the filter, and thus reduce the amount of common mode noise passing through the filter. For example, a decrease in the cable length of the cabling used to connect the filter to ground will decrease the amount of any stray inductance in the capacitor cables and thus reduce the ESL. For example, graph 14 in
A still further method to reduce ESL is to use ESL cancelation. In ESL cancelation, each filter inductor of common mode filter is divided to two and those two inductors are coupled with each other and a negative inductance is shown up at the intermediate point of the two connected filter inductors. A capacitance can then be added between the intermediate point and ground. The inductor is ideally tuned to zero with a sum of ESL and the negative inductance. However, a conventional common mode LC filter with ESL cancelation uses a big common mode choke coil with much L. Then the negative inductance is also too much and need to add another coil in series with ESL of capacitance in order to tune the cancellation. Such cancellation circuits necessitate an increase in the overall number of parts, size, and weight of the noise filter. Furthermore, even if the inductance of the capacitor is a highly accurate inductor, the error of inductance can cause a large decrease in filter efficiency.
The present invention provides a common mode noise filter which overcomes the above mentioned disadvantages of the previously known noise filters.
In brief, the common mode noise filter of the present invention includes a first and a second inductor which are connected in series respectively between the noise source and the load. The inductance of the first inductor is much greater than the inductance of the second inductor.
A third inductor then has a first end connected to a junction between the first and second inductors. A second end of the third inductor is connected to one side of a capacitor which exhibits stray inductance (like all real capacitors) and the other side of the capacitor is connected to ground.
The first, second, and third inductors are selected and positioned relative to each other so that the mutual inductance M13 between the first and third inductors prevents the negative inductance from becoming too small. The inductance L′=ESL+L3−M12−M23+M13. In effect, by minimizing the inductance L′ and ESL from the connection between the first and second inductors and ground, the overall efficiency of the common mode noise filter is enhanced thus more closely approximating the characteristics of an ideal filter illustrated by graph 10 in
In one configuration, a wire is wound around a magnetic structure, such as a ring made up of a ferromagnetic material. A second wire is electrically connected to the first wire by a tap adjacent the load end of the wire and the second wire is then also wound around the magnetic structure and is then electrically connected to the load end. A third wire starts from the tap and goes around the magnetic structure along with the second wire. Then the third wire is connected to ground through a capacitor.
The length of wire between the noise source and the electrical tap on the wound, first wire, forms a first inductor L1 while the length of wire from the tap around the magnetic structure to the load forms the second inductor L2. The third inductor L3 caused by the third wire, furthermore, is positioned closely adjacent the second inductor so that the second and third inductors are well coupled by mutual inductance.
All three inductors exhibit mutual inductance relative to each other. For example, M12 is the mutual inductance between the first and second inductors, M23 is the mutual inductance between the second and third inductors, and M13 is the mutual inductance between the first and the third inductors. Each inductance is coupled with each other and the polarity is also shown in
Consequently, the inductance of the path from the junction of the first and second inductors to the capacitor is equal to L3−M12−M23+M13 where L3 is the magnitude of the third inductor. Ideally, this inductance is equal to the minus ESL for the capacitor thus more closely approximating an ideal filter. In general, L1 and L2 should be large in order to block high frequency current going between source and load and it also causes large M12 and M23. ESL usually is much smaller value than M12 and M23. However, L3 and M13 can prevent L3−M12−M23+M13 from being too small for ESL.
Although the filter of the present invention is particularly useful for use as a common mode noise filter, it may also be used as a differential mode noise filter with ESL cancellation if the one side of the common mode filter is just ground line.
A better understanding of the present invention will be had upon reference to the following detailed description when read in conjunction with the accompanying drawing, wherein like reference characters refer to like parts throughout the several views, and in which:
With reference first to
The noise filter 20 includes a magnetic structure 26, such as a ferrite bead. Wires 28 and 30 carry the common mode noise current from the noise source 22 to the load 24. A ground 32, such as a vehicle frame, is also associated with the noise filter 20.
Both the wires 28 and 30 between the noise source 22 and the load are wound around the magnetic structure 26. Furthermore, the two wires 28 and 30 are wound around the magnetic structure 26 in an identical fashion, and have identical components associated with them. Consequently, only the design and components associated with the wire 28 will be described in detail. An identical description shall also apply to the winding of the wire 30.
Still referring to
The tap 34 effectively divides the winding of the first wire 28 around the magnetic structure 26 into two inductors L1 and L2. The inductors L1 and L2 are connected in series with each other respectively between the noise source 22 and the load 24. Furthermore, the magnitude of the inductance of L1 exceeds the inductance of the inductor L2.
A second wire 36 is electrically connected to the junction point, tap 34, between the inductors L1 and L2. This wire 36 is wound around the magnetic structure 26 and forms a third inductor L3. The third inductor L3, furthermore, is positioned closely adjacent the second inductor L2 thus increasing the mutual inductance between the inductors L2 and L3. Thus, the inductor L3 is wound so that it is cancelled and causes a negative inductance relative to the inductance of the inductors L1 and L2.
The wire 26 is then connected to one side of a capacitor 40 and the other side of the capacitor 40 is electrically connected to ground 32. The capacitor 40, together with its attached wires 36 and connection to ground 32, exhibits stray inductance ESL. Ideally, the inductance L′−ESL=0 so that series inductance of the capacitance of the equivalent circuit from the tap 34 between the first inductor L1 and second inductor L2 and ground 32 and capacitor 40, is equal to zero in order to approximate an ideal noise filter.
Each of the three inductors L1, L2, and L3, however, is affected by the mutual inductance from the other two inductors. Consequently, assuming an equivalent circuit, three branches of equivalent inductance Lx, Ly, and Lz in
L
x
=L
1
+M
12
+M
23
+M
13,
L
y
=L
2
+M
12
−M
23
−M
13,
L
z
=L
3
−M
12
−M
23
+M
13,
where M is mutual inductance.
Ideally, Lz−ESL equals 0. In actual applications, the filter may be tuned by adjusting the number of windings of the inductors L1, L2, and/or L3 as well as by adjusting the mutual inductance M12 between the inductors L1 and L2 by adjusting the length and width of the magnetic path of the inductors L1 and L2 on the magnetic structure 26.
With reference now to
From the foregoing, it can be seen that the present invention provides a simple yet highly effective common mode noise filter. Although the concept of the common mode noise filter may be used to decrease noise in differential mode electronic currents, it has proven highly effective in reducing common mode noise.
Having described our invention, however, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims.