1. Field of the Invention
The present invention relates to a circuit to reduce ripple current in a first winding by employing a second steering winding magnetically coupled to that first winding.
2. Description of the Related Art
FIG. 1 is a schematic representation of an idealized winding 10 which comprises N.sub.1 turns, an inductance L.sub.c produced by those turns and a leakage inductance of L.sub.L1. The inductance L.sub.L1 is that produced by flux leakage associated with winding 10 and the inductance L.sub.c is the magnetizing inductance produced by the turns N.sub.1 wound on a ferromagnetic core. Leakage inductance L.sub.L1 changes as winding 10 becomes magnetically coupled to other windings. The circuit of FIG. 1 shows winding 10 connected in series with a total inductance L.sub.1 comprising L.sub.ext1 plus L.sub.L1, where L.sub.ext1 is the inductance of an external inductor 12. A voltage V.sub.1 is shown connected across the total inductance L.sub.1 and the N.sub.1 turns of winding 10. A voltage V.sub.L1 appears across L.sub.1 and a voltage V.sub.Lc appears across L.sub.c.
The change in current through winding 10 at any given time, di.sub.1 /dt, is referred to as the ripple current. The ripple current di.sub.1 /dt is defined in accordance with Farraday's Law as being equal to the voltage V.sub.1 divided by the total inductance L.sub.1 plus L.sub.c. The ripple current is also equal to V.sub.L1 /L.sub.1 and to (V.sub.1 -V.sub.Lc)/L.sub.1).
In many applications, the existence of a ripple current is undesirable and various prior art methods have been employed to reduce ripple current to zero. One such scheme is shown in FIG. 2 which comprises the utilization of a second steering winding 20 of N.sub.2 turns connected in series with a total inductance L.sub.2 across a second voltage V.sub.2. The total inductance L.sub.2 of FIG. 2 comprises the leakage inductance L.sub.L2 of winding 20 and the inductance L.sub.ext2 of any externally connected inductor 22. In FIG. 2 the inductance L.sub.c again represents that produced by the turns N.sub.1 of winding 10 as seen across that first winding 10.
According to conventional teachings, the ripple current di.sub.1 /dt found in winding 10 can be reduced to zero if the voltage V.sub.L1 across L.sub.1 can be reduced to zero. Since voltage source V.sub.1 is applied across the series combination of L.sub.1 and L.sub.c, if the coupling of windings 10 and 20 can be made to induce a voltage V.sub.1 across inductance L.sub.c alone, the resultant voltage across L.sub.1 will be zero and the ripple current di.sub.1 /dt will be zero. The prior art achieves this result by choosing the ratio of N.sub.1 to N.sub.2 and the value of L.sub.2 to satisfy the following equation: EQU L.sub.2 =(N.sub.2 /N.sub.1).sup.2 (L.sub.c) ((aN.sub.1 /N.sub.2)-1)(1.
When this relationship is achieved, the voltage across L.sub.1 is zero so that the ripple current di.sub.1 /dt is also reduced to zero. When the relationship is not maintained, ripple current di.sub.1 /dt results. If both windings 10 and 20 are excited by the same source, "a" equals 1 and, according to equation (1), either L.sub.2 must be zero or N.sub.1 /N.sub.2 must be greater than 1. As a practical matter, L.sub.2 can never be zero; therefore, N.sub.1 /N.sub.2 must be greater than 1. For a limited range of L.sub.2 and L.sub.c, the ratio N.sub.1 /N.sub.2 can be adjusted to meet the requirements of equation (1). To maximize the opportunities for the adjustment of the ratio N.sub.1 /N.sub.2 to meet equation (1), the geometry of winding 20 may be adjusted to affect the leakage inductance L.sub.L2 or additional external inductance may be added L.sub.ext2 to influence the final value of L.sub.2. Adjustment of the turns ratio may be impractical if there are only a few turns.
Although the value of L.sub.1 in such a prior art circuit is theoretically irrelevant to the reduction of the ripple current di.sub.1 /dt, L.sub.1 is conventionally set as close to zero as possible to minimize ac flux core loss due to changing current in the winding of L.sub.1.
This prior art technique is particularly effective in environments where the ratio between N.sub.1 and N.sub.2 can be varied sufficiently to permit L.sub.2 to fall within an achievable range. However, the prior art represented by FIG. 2 cannot be utilized when the winding with N.sub.1 turns is magnetically coupled with a third winding with N.sub.3 turns as shown in FIG. 3 to form a transformer 30 having a turns ratio N.sub.1 to N.sub.3. If the technique of FIG. 2 is applied to the circuit of FIG. 3, the additional turns N.sub.2 of winding 20 materially alters the effective operation of transformer 30. Specifically, the technique of FIG. 2 requires the ratio of N.sub.1 /N.sub.2 to be greater than 1, thereby causing the winding 20 to affect the overall operation of transformer 30. It should be noted that in FIG. 3, capacitor 32 is connected in series with winding 20 and any external additional inductance L.sub.ext2. Capacitor 32 in combination with first voltage source V.sub.1 replaces second voltage source V.sub.2. The technique of replacing second voltage source V.sub.2 using a capacitor 32 is known in the prior art.