A controllable inductor

Abstract

A controllable inductor with a varying inductance, comprising a ferrite core having three ferrite legs that are connected by two ferrite bars above and below, wherein one of the legs comprises an air gap; a winding on the ferrite core leg with the gap to form an AC power inductor, that may include a DC current component; a DC winding on one the non-gapped legs; a capacitor of low impedance, at the AC operating frequency, being across the terminals of the DC winding. A DC current excitation is applied to the terminals of the DC winding for obtaining a desired level of magnetization of the non-gapped legs to reduce their magnetic permeability and thereby reduce the total inductance of the power inductor. The magnetic fluxes generated by the AC winding flows via a closed magnetic loop formed by the other remaining ferrite core leg and not via the leg with the DC winding and hence the DC winding has practically zero AC current.

Description

[0001] 1 -

[0002] A CONTROLLABLE INDUCTOR

[0003] Field of the Invention

[0004] The present invention relates to the field of controllable inductors. More particularly, the present invention relates to a system and method for a controlled inductor design and varying its inductance.

[0005] Background of the Invention

[0006] Power electronic circuits, such as PWM converters and power resonant converters, use an inductor as a means to supply power to the load by releasing its stored energy to the output. By switching between on-state and off-state constantly, the converter is able to decrease or increase the voltage from the input to the output. In traditional applications, the value of the inductor is fixed and selected according to the application requirements, while being aware that this inductor will not operate optimally over the envelope of the operating conditions. For example, there are cases in which the efficiency of the converter decreases at lower power level, if the inductor is fixed. It has been documented that a variable inductor can increase the efficiency at low power level by tuning the inductance value to optimally match the new operating conditions.

[0007] Fig. 1 (prior art) illustrates a prior art implementation of a variable inductor. The variable inductor 10 consists of a ferrite toroid core 11 with an air-gap 13, which reduces the permeability and by that allowing energy storage as required by a power inductors. The variable inductor 10 comprises two windings 14 and 19 on the ferrite core 11. The winding 14 allow a DC current to flow through it, in order to reduce the permeability (as a result of the magnetization induced by the DC current) of the ferrite core 11 and thereby, reduces the equivalent AC inductance seen over the inductor's terminals 16. However, since the core includes an air gap, the DC current needed to magnetize the core is relatively high due to the high reluctance of the gap. This will increase the power loss in 14. Furthermore, since windings 14 and 19 are wound on the same core, there is a magnetic coupling between 14 and 19 and consequently, there will be an AC voltage imposed on 14. To reduce the magnitude of the DC current, 142 -

[0008] will include a high number of turns and as a result, the voltage on 14 will be very high. This will necessitate high insulation on the wires, increasing the size and cost of the variable inductor. In addition, if the DC source of 14 is of low impedance, the imposed voltage on 14 will generate a high AC current in 14 and will effectively short the inductance seen at the terminals 16. To overcome this, a high impedance, preferably a high inductance inductor, will have to be placed in series with the terminals of 14, hence increasing the size and cost of the variable inductor assembly.

[0009] It is therefore an object of the present invention to provide a system and method for a variable inductor design with reduced induced AC voltage on the DC magnetization winding.

[0010] It is another object of the present invention to provide a system and method for rapidly varying the inductance of a controllable inductor, without requiring an external DC power supply.

[0011] Other objects and advantages of the invention will become apparent as the description proceeds.

[0012] Summary of the Invention

[0013] A method for varying the inductance of a controllable inductor, comprising:

[0014] a) providing a ferrite core having three ferrite legs that are connected by two ferrite bars above and below, wherein one of the legs comprises an air gap; b) providing a winding on the ferrite core leg with the gap to form an AC power inductor, that may include a DC current component;

[0015] c) providing a DC winding on one the non-gapped legs;

[0016] d) providing a capacitor of low impedance, at the AC operating frequency, across the terminals of the DC winding;

[0017] e) applying a DC voltage excitation to the terminals of the DC winding for obtaining a desired level of magnetization of the non-gapped legs to reduce3 -

[0018] their magnetic permeability and thereby reduce the total inductance of the power inductor,

[0019] wherein the magnetic fluxes generated by the AC winding flows via a closed magnetic loop formed by the other remaining ferrite core leg and not via the leg with the DC winding and hence the DC winding has practically zero AC current.

[0020] The ferrite core may form a double-loop magnetic path.

[0021] The shunt capacitor may be sufficiently large to reflect a short-circuit for AC input voltages.

[0022] The AC current that is generated in the AC feed windings may be used to generate the required DC feed current by directing it to flow in different alternative paths.

[0023] In one aspect, at a first phase, the shunt capacitor is charged by the DC component of the inductor current and feeds the required DC current, while reducing its charge and at a second phase, the current is directed to a different path, for recharging the shunt capacitor.

[0024] Switching between paths may be done in cycles by a control circuit that comprises two switches, such that in each cycle, the current is averaged by the capacitor C.

[0025] The switches may be realized by MOS-FETs, such that one of the switches is implemented by a pair of back-to-back MOS-FETs, to prevent discharging of the shunt capacitor as a result of current leakage via the diode of a MOS-FET being in cutoff.

[0026] The DC current may be controlled using bidirectional drive, by:

[0027] a) using a bridge to charge the shunt capacitor by controlling a first pair of MOS- FETs to conduct; and4 -

[0028] b) whenever fast current drop is required, using the bridge to turn over the current direction and thereby, rapidly discharging the shunt capacitor.

[0029] The DC current may be controlled by:

[0030] a) allowing a charging current to flows into an inductor, which is being charged and controlled by a circuit that functions as a buck converter;

[0031] b) adding a serial connection of a rectifier diode and a Zener, which are inactive as long as the inductor is being charged; and

[0032] c) whenever current reduction is required, forcing the inductor current to flow via the serial connection, such that the voltage at the inductor's terminal becomes negative to thereby generate a negative gradient which discharges the shunt capacitor at a controlled desired rate.

[0033] The method may further comprise the steps of:

[0034] a) using the magnetic flux at the ferrite core to generates an AC voltage; and b) rectifying the AC voltage by a rectifier circuit to obtain the required DC voltage for feeding the control circuit.

[0035] The DC excitation applied to the terminals of the DC winding may be voltage excitation.

[0036] The DC excitation applied to the terminals of the DC winding may be current excitation.

[0037] A controllable inductor with a varying inductance, comprising:

[0038] a) a ferrite core having three ferrite legs that are connected by two ferrite bars above and below, wherein one of the legs comprises an air gap;

[0039] a) a winding on the ferrite core leg with the gap to form an AC power inductor, that may include a DC current component;

[0040] b) a DC winding on one the non-gapped legs; and

[0041] c) a capacitor of low impedance, at the AC operating frequency, being across the terminals of the DC winding;5 -

[0042] wherein a DC current excitation is applied to the terminals of the DC winding for obtaining a desired level of magnetization of the non-gapped legs to reduce their magnetic permeability and thereby reduce the total inductance of the power inductor, wherein the magnetic fluxes generated by the AC winding flows via a closed magnetic loop formed by the other remaining ferrite core leg and not via the leg with the DC winding and hence the DC winding has practically zero AC current.

[0043] Brief Description of the Drawings

[0044] The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative detailed description of preferred embodiments thereof, with reference to the appended drawings, wherein:

[0045] Fig. 1 (prior art) illustrates a conventional implementation of a variable inductor;

[0046] Fig. 2 illustrates a possible implementation of a controlled inductor, using the double-loop ferrite core, according to an embodiment of the invention.

[0047] Fig. 3 illustrates reducing the permeability of the second leg and of the entire loop 25 reduced due to saturation;

[0048] Fig. 4 shows an equivalent magnetic circuit of a single leg of a ferrite;

[0049] Fig. 5 illustrates an equivalent magnetic circuit of the coil of Fig. 2;

[0050] Fig. 6 illustrates another possible implementation of a controlled inductor using the double-loop ferrite core, according to another embodiment of the invention;

[0051] Figs. 7A-7B illustrate an implementation of a DC power supply for inputting the required DC current, according to an embodiment of the invention; and Fig. 8 illustrates another way of controlling the DC current and discharging the capacitor C, according to an embodiment of the invention.

[0052] Detailed Description of the Present Invention

[0053] The present invention proposes a controllable inductor system and method for efficiently varying the inductance of an inductor by a DC bias winding while substantially reducing the AC voltage across the DC winding.6 -

[0054] The controllable inductor with a varying inductance comprises a ferrite core with three ferrite legs that are connected by two ferrite bars above and below, such that one of the legs comprises an air gap. A winding is implemented on the ferrite core leg with the gap to form an AC power inductor that may include a DC current component. A DC winding is implemented on one the non-gapped legs. A capacitor of low impedance, at the AC operating frequency, is connected across the terminals of the DC winding. A DC current (or voltage) excitation is applied to the terminals of the DC winding to obtain a desired level of magnetization of the non-gapped legs to reduce their magnetic permeability as well as the total inductance of the power inductor. The magnetic fluxes generated by the AC winding flows via a closed magnetic loop formed by the other remaining ferrite core leg and not via the leg with the DC winding and hence the DC winding has practica I ly / esse ntia lly zero AC current.

[0055] Fig. 2 illustrates a possible implementation of a controlled inductor 20, using the double-E ferrite core that forms two loops, according to an embodiment of the invention. The first leg 21 of the ferrite core (that belongs to the first loop) has an airgap 13, where the AC winding 18 (that spreads over the leg) has the input terminals 16a and 16b. The second leg 22 of the ferrite core comprises a DC winding 23 with terminal 17a and 17b. Upon inputting a DC current to DC winding 23, via terminals 17a and 17b, the permeability of the second leg 22 and of the entire loop 25 that includes leg 26 is reduced due to saturation, as illustrated in Fig. 3. A large shunt capacitor C, over the terminals 17a and 17b reflects a short-circuit for AC voltages.

[0056] In this implementation, according to this invention, the AC voltage across the DC winding 23 is practically zero, independent of the turn ratio between the DC winding 23 and the AC winding 21. This unique behavior is due to Lenz's Law which states: "direction of the electric current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes changes in the initial magnetic field. Therefore, as depicted in Fig. 3b, if the flux 27 generated by the AC winding 21 is attempting to penetrate into the DC winding leg 22 with an AC short,7 -

[0057] it will invoke a back MMF current in the winding 23 to cancel the flux. Consequently the flux 27 will close the path via leg 26 which has no back MMF and the AC voltage across the DC winding 23 will approach zero. This process can be described by a reluctance model as follows.

[0058] Fig. 4 shows an equivalent magnetic circuit of a single leg of a ferrite. The leg 40 comprises an input coil 41 with n windings. The length of leg 40 is le and the area is Ae. Current I that flows via input coil 41 generates a Magneto Motive Force (MMF) which is given by:

[0059] MMF = n - 1

[0060] of the ferrite is given by:

[0061] >

[0062]

[0063] (1)

[0064] Where0is the permeability of air and

[0065]

[0066] is the relative permeability.

[0067] Fig. 5 illustrates an equivalent magnetic circuit of the variable inductor Fig. 2 according to one embodiment of this invention. The AC input current is represented by the voltage source nlac. The reluctance of the first ferrite leg 21 is represented by a resistor RL. The reluctance of the air-gap in the first leg is represented by a resistor Rg. The reluctance of the middle ferrite core leg 22 is represented by a resistor RM. The reluctance of the third ferrite leg 23 is represented by a resistor RR. There are two ferrite bars above and below. The reluctance of the ferrite bar that connects between leg 21 and leg 22 is represented by a resistor RLM. The reluctance of the ferrite bar that connects between leg 22 and leg 23 is represented by a resistor RMR. The DC input current is represented by the voltage source MMFDC. This MMF generates a DC flux that will primarily flow via the right leg since the reluctance Rg in the left leg is very high. The magnetic fluxACgenerated by MMFACflows via reluctances RL, Rgand RLMand is split toACMandACR. However, due to the generation of the MMFac according8 -

[0068] to Lenz's Law, the AC magnetic flux OACL, will flow mainly via the third ferrite core leg 26.

[0069] The advantage of this configuration is that on one hand, the AC magnetic flux will mainly flow via the third ferrite leg 23 and on the other hand, the magnetic flux generated by the DC current will see a closed un-gapped magnetic loop consisting of the second and third ferrite legs 22 and 23. This closed magnetic loop has a relatively low reluctance, so it will be easy to obtain a desired saturation by a rather low DC current that will decrease the relative permeability

[0070]

[0071] and hence the inductance of the controlled inductor.

[0072] Fig. 6 illustrates another possible implementation of a controlled inductor 20, using the double-loop ferrite core that forms a double-loop magnetic path, according to another embodiment of the invention. In this embodiment, the air gap is in the second ferrite leg 22. The AC input voltage is applied to the windings around the second ferrite leg 22, while the DC input voltage is applied to the windings around the third ferrite leg 26. In this case, the closed magnetic loop will be via the first ferrite leg 21, which will have the lowest reluctance to the generated magnetic fluxesAc and magnetic fluxDc-

[0073] Figs. 7A-7B illustrate the generation of the DC current for driving the magnetization winding without an external, low DC voltage (about 0.1-0.2 volts due to the low resistance f the DC winding) power supply, according to an embodiment of the invention. This method is applicable to cases in which the AC inductor is carrying a DC current with some ripple superimposed, like in a Buck converter.

[0074] At a first phase shown in Fig. 7A, the capacitor C (which is charged) feeds the required DC current, while its charge is being reduced. At a second phase shown in Fig. 7B, the current is directed to the path that recharges capacitor C. The switching between paths is done in cycles by a control circuit 70 that comprises two switches Sxand S2, such that in each cycle, the current is averaged by the capacitor C. The switches Sxand S2may be realized by MOS-FETs, as shown in Fig. 7C. In this case, switch S is implemented by a9 -

[0075] MOS-FET Q5, and switch S2is implemented by a pair of back-to-back MOS-FETs Q6and Q7, in order to prevent current leakage via the diode of a MOS-FET in cutoff, which may discharge the capacitor C.

[0076] Figs. 7D-7E illustrate another way of controlling the DC current, using bidirectional drive, according to an embodiment of the invention. Whenever required, it is possible to use a bridge to charge the capacitor C by controlling a first pair of MOS-FETs Q4and Q6-7to conduct and thereby, direct the current to charge the capacitor C, as shown in Fig. 7D. Whenever fast current drop is required, it is possible to use the bridge to turn over the current direction by controlling a second pair of MOS-FETs Q5and Qsto conduct (as shown in Fig. 7E) and thereby, rapidly discharging the capacitor C, rather than waiting that capacitor C will discharge via the winding's resistance (which takes longer time). This also allows to better control the discharge.

[0077] Fig. 8 illustrates another way of controlling the DC current and discharging the capacitor C, according to an embodiment of the invention. When Q conducts and Q2is in cutoff, the current flows into inductor L, which is being charged. If Q3conducts, the combination of inductor L and Qxand Q2function as a buck converter, which controls the charging current. The circuit also comprises a serial connection of a rectifier diode D and a Zener DZ4, which are inactive as long as the inductor L is being charged.

[0078] Whenever it is required to reduce the current, QltQ2and Q3are controlled to be in cutoff. As a result, the inductor current will have to flow via diode D and Zener DZ4, and the voltage at the inductor's terminal becomes negative. This causes a negative gradient A / to be developed, which discharges the DC capacitor at a controlled desired rate. This requires measuring the current.

[0079] Since the AC current generates magnetic flux at the ferrite core, this magnetic flux generates an AC voltage which is rectified by a rectifier circuit 80, in order to obtain the required DC voltage for feeding the control transistors

[0080]

[0081] Q3. This way, the DC voltage1G

[0082] is taken from the controlled inductor 20 itself, rather than requiring an external DC power supply.

[0083] The above examples and description have of course been provided only for the purpose of illustrations, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the invention.

Claims

11CLAIMS1. A method for varying the inductance of a controllable inductor, comprising:a) providing a ferrite core having three ferrite legs that are connected by two ferrite bars above and below, wherein one of the legs comprises an air gap; b) providing a winding on the ferrite core leg with the gap to form an AC power inductor, that may include a DC current component;c) providing a DC winding on one the non-gapped legs;d) applying a DC voltage excitation to the terminals of the DC winding for obtaining a desired level of magnetization of the non-gapped legs to reduce their magnetic permeability and thereby reduce the total inductance of said power inductor,wherein the magnetic fluxes generated by said AC winding flow via a closed magnetic loop formed by the other remaining ferrite core legs and not via the leg with the DC winding and hence, the DC winding has practically zero AC current.

2. A method according to claim 1, further comprisinga) providing a capacitor of low impedance, at the AC operating frequency, across the terminals of the DC winding;b) applying a DC current to the terminals of the DC winding for obtaining a desired level of magnetization of the non-gapped legs to reduce their magnetic permeability and thereby, reducing the total inductance of said power inductor.

3. A method according to claim 1, wherein the ferrite core forms a double-loop magnetic path.

4. A method according to claim 1, wherein the shunt capacitor is sufficiently large to reflect a short-circuit for AC input voltages.

125. A method according to claim 1, wherein the AC current that is generated in the AC feed windings is used to generate the required DC feed current by directing it to flow in different alternative paths.

6. A method according to claim 1, wherein at a first phase, the shunt capacitor is charged by the DC component of the inductor current and feeds the required DC current, while reducing its charge and at a second phase, the current is directed to a different path, for recharging said shunt capacitor.

7. A method according to claim 5, wherein switching between paths is done in cycles by a control circuit that comprises two switches, such that in each cycle, the current is averaged by the capacitor C.

8. A method according to claim 6, wherein the switches are realized by MOS-FETs, such that one of the switches is implemented by a pair of back-to-back MOS-FETs, to prevent discharging of the shunt capacitor as a result of current leakage via the diode of a MOS-FET being in cutoff.

9. A method according to claim 1, wherein the DC current is controlled using bidirectional drive, by:a) using a bridge to charge the shunt capacitor by controlling a first pair of MOS- FETs to conduct; andb) whenever fast current drop is required, using said bridge to turn over the current direction and thereby, rapidly discharging the shunt capacitor.

10. A method according to claim 1, wherein the DC current is controlled by:a) allowing a charging current to flow into an inductor, which is being charged and controlled by a circuit that functions as a buck converter;b) adding a serial connection of a rectifier diode and a Zener, which are inactive as long as said inductor is being charged; and13c) whenever current reduction is required, forcing the inductor current to flow via said serial connection, such that the voltage at the inductor's terminal becomes negative to thereby generate a negative gradient which discharges the shunt capacitor at a controlled desired rate.

11. A method according to claim 1, further comprising:a) using the magnetic flux at the ferrite core to generate an AC voltage; and b) rectifying said AC voltage by a rectifier circuit to obtain the required DC voltage for feeding the control circuit.

12. A method according to claim 1, wherein the DC excitation applied to the terminals of the DC winding is voltage excitation.

13. A method according to claim 1, wherein the DC excitation applied to the terminals of the DC winding is current excitation.

14. A controllable inductor with a varying inductance, comprising:a) a ferrite core having three ferrite legs that are connected by two ferrite bars above and below, wherein one of the legs comprises an air gap;b) a winding on the ferrite core leg with the gap to form an AC power inductor, that may include a DC current component;c) a DC winding on one the non-gapped legs; andd) a capacitor of low impedance, at the AC operating frequency, being across the terminals of the DC winding;wherein a DC current excitation is applied to the terminals of the DC winding for obtaining a desired level of magnetization of the non-gapped legs to reduce their magnetic permeability and thereby reduce the total inductance of said power inductor, wherein the magnetic fluxes generated by said AC winding flows via a closed magnetic loop formed by the other remaining ferrite core leg and not via the leg with the DC winding and hence the DC winding has practically zero AC current.1415. A controllable inductor according to claim 14, further comprising a capacitor of low impedance, at the AC operating frequency, across the terminals of the DC winding, wherein a DC current is applied to the terminals of the DC winding for obtaining a desired level of magnetization of the non-gapped legs to reduce their magnetic permeability and the total inductance of said inductor.

16. A controllable inductor according to claim 14, wherein the ferrite core forms a double-loop magnetic path.

17. A controllable inductor according to claim 14, wherein the shunt capacitor is sufficiently large to reflect a short-circuit for AC input voltages.

18. A controllable inductor according to claim 14, wherein the AC current that is generated in the AC feed windings is used to generate the required DC feed current by directing it to flow in different alternative paths.

19. A controllable inductor according to claim 14, wherein at a first phase, the shunt capacitor is charged by the DC component of the inductor current and feeds the required DC current, while reducing its charge and at a second phase, the current is directed to a different path, for recharging said shunt capacitor.

20. A controllable inductor according to claim 18, wherein switching between paths is done in cycles by a control circuit that comprises two switches, such that in each cycle, the current is averaged by the capacitor C.

21. A controllable inductor according to claim 14, wherein the switches are realized by MOS-FETs, such that one of the switches is implemented by a pair of back-to-back MOS-FETs, to prevent discharging of the shunt capacitor as a result of current leakage via the diode of a MOS-FET being in cutoff.IS22. A controllable inductor according to claim 14, wherein the DC current is controlled using bidirectional drive, by:a) using a bridge to charge the shunt capacitor by controlling a first pair of MOS- FETs to conduct; andb) whenever fast current drop is required, using said bridge to turn over the current direction and thereby, rapidly discharging the shunt capacitor.c) A controllable inductor according to claim 14, wherein the DC current is controlled by:d) allowing a charging current to flows into an inductor, which is being charged and controlled by a circuit that functions as a buck converter;e) adding a serial connection of a rectifier diode and a Zener, which are inactive as long as said inductor is being charged; andf) whenever current reduction is required, forcing the inductor current to flow via said serial connection, such that the voltage at the inductor's terminal becomes negative to thereby generate a negative gradient which discharges the shunt capacitor at a controlled desired rate.

23. A controllable inductor according to claim 14, wheein the DC excitation applied to the terminals of the DC winding is voltage or current excitation.

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