WO2011008426A1

WO2011008426A1 - Switching power converter with current sensing transformer auxiliary power supply

Info

Publication number: WO2011008426A1
Application number: PCT/US2010/039405
Authority: WO
Inventors: John L. Melanson
Original assignee: Cirrus Logic, Inc.
Priority date: 2009-06-30
Filing date: 2010-06-22
Publication date: 2011-01-20
Also published as: EP2449662A1; US20100327838A1; CN102498652A; CN102498652B; EP2449662B1; TWI499175B; US8198874B2; TW201110515A

Abstract

A switching power converter (8) having a current sensing transformer (T1) providing input to an auxiliary power supply (12) provides efficient current sensing, while reducing the cost of the magnetic coupling element (L1). The auxiliary power supply (12) and current sense circuit (14) both receive input from a secondary winding (sec) of a current sensing transformer (T1) having a primary winding (pri) coupled in series with the converter's main magnetic coupling element (L1). To provide accurate sensing, the magnetization the current sensing transformer is accounted for. The magnetization is compensated for in the current sensing result, current sensing is performed during a part of the cycle in which charging of the auxiliary power supply is disabled, or the core of the current sensing transformer is made large, raising its mutual inductance. In another alternative technique, a circuit node can be pre-charged to a value that cancels the offset due to the magnetization current.

Description

SWITCHING POWER CONVERTER WITH CURRENT SENSING TRANSFORMER

AUXILIARY POWER SUPPLY

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates generally to switching power converter circuits, and more specifically, to a

switching power converter in which an auxiliary winding power supply and a current sensing circuit receive input from the same current sensing transformer secondary winding.

2. Background of the Invention

[0002] Operation of switching power converters in

continuous conduction mode (CCM) is desirable for efficiency and to reduce the peak current levels in the converter for reliability and reduction of component costs . In CCM

operation, the current through the magnetic coupling element never changes polarity and typically never fully drops to zero. Since the current is never permitted to reach zero, saturation of the magnetic coupling element can occur if an increasing magnetic flux progressively accumulates in the magnetic coupling element at the end of each switching cycle, a phenomenon sometimes referred to as "flux walking."

Eventually the core of the magnetic coupling element will saturate, causing the magnetic coupling element to appear as a short circuit, resulting in failure of the switching circuits. Therefore, in CCM operation, detection of the peak current, or another reference current level, through the series circuit formed by the magnetic storage element and the switching circuit is needed. The peak current value determines the off cycle period needed to discharge the inductor. In

discontinuous conduction mode (DCM) , there is always a period of zero current flowing in the magnetic coupling element, and therefore the core of the magnetic coupling element always resets to zero at each cycle.

[0003] In order to sense the current through the magnetic coupling element, a sense resistor may be inserted into the series switching circuit, or an additional current sensing transformer can be provided. The current sensing transformer approach is typically more efficient than the approach of adding a current sense resistor, as the transformer and current sensing circuit are designed to produce minimal disturbance and loss in the power switching circuit. A current sensing transformer primary winding typically includes only a few turns, so the voltage across the primary winding is small. The secondary winding of the current sensing transformer generally produces a voltage on the order of a volt or less, so that the magnetization current of the current sensing transformer (also referred to as the magnetizing current) can be disregarded. Therefore, the secondary current of the typical current sense transformer is substantially

proportional to the primary current, so that a voltage generated across a resistor connected across the secondary of the current sensing transformer is linearly related to the primary current and provides a measure of the current passing through the primary winding of the main magnetic coupling element of the converter.

[0004] In order to supply power to control circuits of a switching power converter, a low voltage power supply is needed. When the only voltage available or convenient is a high-voltage power supply, or when isolation of the auxiliary power supply is required, an auxiliary winding on the main magnetic coupling element of the switching converter is commonly used to supply power to the control circuits .

However, including an auxiliary winding increases the cost of the magnetic coupling element. In contrast to the

characteristics of the secondary winding in the above- described current sensing transformer, as the auxiliary power supply typically must provide a voltage on the order of 10V and also generate substantial current, on the order of tens or hundreds of milliamps. Therefore, an auxiliary winding generally has substantial magnetization current, and the voltage across the typical auxiliary winding does not provide an accurate indication of the current flowing through the primary winding of the main magnetic coupling element of the switching power converter. [0005] Therefore, it would be desirable to provide an auxiliary power supply in a switching power converter having a current sense transformer without requiring an auxiliary winding on the main magnetic coupling element of the switching power converter.

SUMMARY OF THE INVENTION

[0006] The above stated objective of providing an auxiliary power supply circuit and a current sensing circuit without requiring a sense resistor or an auxiliary winding on the main magnetic coupling element of a switching converter, is provided in a switching converter and a method of operation of the switching converter.

[0007] The switching converter has a magnetic coupling element coupled in series with a switching circuit and a current sensing transformer. The current sensing transformer secondary winding is provided to an auxiliary power supply circuit and also provides input to a current sensing circuit. In order to provide current sensing functionality in the face of a significant magnetization current due to the higher voltage required for auxiliary power supply charging, the current through the auxiliary winding is either corrected for the magnetization current, the current sensing is performed while charging of the auxiliary power supply is disabled, the inductance of the current sensing transformer is made large, or the offset due to the magnetization current is compensated for by pre-charging a circuit node to an opposing offset.

[0008] The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a block diagram depicting a switching converter in accordance with an embodiment of the present invention .

[0010] Figure 2 is a schematic diagram depicting details of a a current sensing circuit 14A and of an auxiliary power supply 12A that can be used to implement current sensing circuit 14 and auxiliary power supply 12 of Figure 1,

respectively, in accordance with an embodiment of the present invention .

[0011] Figure 3 is a schematic diagram depicting details of a a current sensing circuit 14B and of an auxiliary power supply 12B that can be used to implement current sensing circuit 14 and auxiliary power supply 12 of Figure 1,

respectively, in accordance with another embodiment of the present invention.

[0012] Figure 4 is a schematic diagram depicting details of a current sensing circuit 14C and of an auxiliary power supply 12C that can be used to implement current sensing circuit 14 and auxiliary power supply 12 of Figure 1, respectively, in accordance with yet another embodiment of the present

invention .

[0013] Figure 5 is a signal waveform diagram depicting details of operation of the switching converter of Figure 1 in accordance with an embodiment of the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

[0014] The present invention encompasses switching power converters having an auxiliary power supply operated from a winding of a current sensing transformer. The current sensing transformer is coupled in series with the switching circuit and main magnetic coupling element of the converter. The present invention also includes methods for providing power to control and/or other circuits internal to a switching power converter from a current sensing transformer.

[0015] Referring now to Figure 1, a switching power converter 8 in accordance with an embodiment of the present invention is shown. A switching controller 10 provides a switching control signal CS that controls a switching circuit implemented by a transistor Nl. When transistor Nl is active, a magnetic coupling element implemented by inductor Ll is charged by imposing input voltage V_1N across inductor Ll, causing a current I_L through inductor Ll to linearly increase. When transistor Nl is deactivated, charge is pushed through inductor Ll and diode Dl into capacitor Cl, raising the voltage at output terminal OUT. A current-sensing transformer Tl is connected in series with transistor Nl and inductor Ll to provide a signal indicating the magnitude and polarity of current I_L through inductor Ll without introducing significant losses, as would otherwise be present if a series resistor were used to produce such a signal.

[0016] Switching power converter 8 is a boost converter circuit that can control the voltage provided to output terminal OUT in conformity with a feedback voltage provided from terminal OUT. Alternatively, switching power converter 8 may be controlled entirely by current-mode feedback. In the illustrated application, the boost converter circuit is a power-factor corrector (PFC) that provides a high voltage DC output from alternating current (AC) power line input voltage V_1N. Switching controller 10 operates transistor Nl to maintain a phase relationship of zero degrees between input voltage V_ΪN and the input current. A current sensing circuit 14 provides an indication of the magnitude of current I_L derived from an input voltage received from secondary winding sec of current sensing transformer Tl. An auxiliary power supply 12, supplies a voltage V_DDH to switching controller 10, and is generally integrated in the same integrated circuit (IC) with switching controller 10 and current sensing circuit 14. The inputs of auxiliary power supply 12 are also connected to secondary winding sec of current sensing transformer Tl. Unlike typical current sensing transformers, current sensing transformer Tl has a higher turns ratio, so that a voltage V_DDH can be generated by auxiliary power supply 12 sufficient to operate control circuits in a controller IC, including switching controller 10, which contains the gate drive circuit needed to control switching of transistor Nl. Voltage V_DDH is therefore generally in the range of 10-15VDC.

[0017] Referring now to Figure 2, details of an auxiliary power supply circuit 12A that may be used to implement auxiliary power supply 12 of Figure 1 and details of a current sensing circuit 14A that can be used to implement current sensing circuit 14 of Figure 1 are shown in accordance with an embodiment of the invention. The input of auxiliary power supply 12A is connected to the secondary winding of current sensing transformer Tl. A diode DlO provides a half- wave rectifier that conducts only when the current provided from secondary winding sec is in the positive direction, i.e., when inductor Ll is being charged and transistor Nl is conducting. A resistor RlO ensures that when transistor Nl is de-activated and inductor Ll is being discharged, that the magnetization of current-sensing transformer Tl is reset at the end of each cycle. Current sensing circuit 14A includes a resistor RIl connected in series with secondary winding sec to provide a voltage V_Rn proportional to current I_sec to an amplifier Al having a gain determined by a pair of resistors R12 and R13. The output of amplifier Al is provided to switching controller 10, which in digital implementations will generally measure the output of amplifier Al with an analog- to-digital converter (ADC) . However, in other analog

embodiments of a switching control circuit in accordance with the invention, a peak detector and/or comparator may be used to detect a time at which current I_sec reaches a predetermined threshold for indicating to switching controller 10 the appropriate switching period to maintain CCM operation and avoid flux walking. A Zener diode Zl is provided to ensure that output voltage V_DDH does not exceed a maximum level, but Zener diode Zl is not activated normally in auxiliary power supply circuit 12A. The turns ratio of current sensing transformer Tl is generally determined such that voltage V_DDH is maintained below a maximum voltage level without activating Zener diode Zl except under abnormal operating conditions .

[0018] Current I_sec is measured by current sensing circuit 14A to provide an indication of the magnitude of current I_L through inductor Ll of Figure 1. However, the magnetization of current sensing transformer Tl produces an error in the direct proportionality of current I_sec to current I_L, which will be explained in further detail below, along with solutions provided in accordance with various embodiments of the invention, to reduce or correct for the error. In auxiliary power supply 12A, as mentioned above, transformer Tl has a higher secondary to primary turns ratio than ordinary current transformers, so that the higher voltage required to operate switching controller 10 is generated at the output of

auxiliary power supply 12A as output voltage V_DDH. Since the product of voltage V_SEC generated across secondary winding sec and the duration of the period for which transistor Nl is active determines the magnetization of the transformer at the end of the active portion of the switching cycle. The

magnetization causes an error that reduces the value of current I_sec through secondary winding sec below that of an ideal current level as computed from the turns ratio of transformer Tl.

[0019] The above-described error also reduces the magnitude of the indication of current I_sec provided by secondary winding sec to amplifier Al, since the current through resistor RIl will be less than an expected current calculated from the turns ratio of transformer Tl. The reduction in current is due to the magnetization current of transformer Tl, which is the integral of the voltage across secondary winding sec

multiplied by the mutual inductance of transformer Tl

(sometimes referred to as the magnetizing inductance) . Since voltage v^" _SEc is substantially constant over the switching period, the integral reduces to the product of the mutual inductance, the voltage and the duration of the pulse: Isec = N_sec/N_prl * I_prl - I_mag ~ N_sec/N_prl * I_L - d * V_sec/L_M, where I_sec is the secondary current, N_sec/N_pri is the secondary to primary turns ratio, I_L is the primary winding current (i.e., the switching current under measurement) and I_mag is the magnetization current. Magnetization current I_mag can be approximated by d * V_sec/LM, where d is the time period for which transistor Nl is on, V_sec is the voltage generated at secondary winding sec and L_M is the mutual inductance of transformer Tl. In ordinary current sensing transformer circuits, the term d * V_sec/L_M can be ignored, because V_sec is small, due to the smaller secondary to primary turns ratio. In the current sensing transformer circuit of the present

invention, the error cannot generally be ignored, unless the mutual inductance is made large by increasing the number of turns and providing a sufficiently-sized core, requiring more space and adding cost. The alternative is to provide

techniques for eliminating the error, one of which is provided in auxiliary power supply circuit 12A.

[0020] Auxiliary power supply circuit 12A includes a control circuit 16 that provides output signals for

controlling transistors NlO and PlO. Transistor NlO is

activated when current sensing circuit 14A is measuring the voltage across resistor RIl to generate the indication of current I_L in inductor Ll, which is provided to switching controller 10. Logical-AND gate ANDl controls the gate of transistor NlO so that when switching controller is in CCM mode and signal ccm is asserted and when the sense signal generated by switching controller 10 is active, transistor NlO is turned on. In general, transistor NlO can be activated during DCM operation as well, and there is no requirement that any selection of sensing and charging operations be dependent on whether switching controller 10 is operating in CCM or DCM mode. When activated, transistor NlO substantially reduces the voltage across secondary winding sec to near zero, since only the total voltage of the forward voltage drop across diode DlO, the voltage generated across resistor RIl and the drain- source voltage of transistor NlO will be present across secondary winding sec. Resistor RIl can be selected as a low resistance to reduce its voltage drop, and it is desirable to do so in general, since in the depicted circuit, resistor RIl reduces the voltage provided for charging capacitor C2 and therefore reduces the maximum possible value of output voltage V_DDH.

[0021] Transistor NlO is de-activated after a time period determined by signal sense provided from switching controller 10. Signal sense may have a fixed timing determined from the switching period of switching controller 10, or may be controlled in conformity with a determination that the current measurement has been completed, e.g., when current I_L has reached some threshold value as determined by circuits within switching controller 10 in response to the signal provided from current sensing circuit 14A. After signal sense is de- asserted, auxiliary power supply 12A is permitted to charge capacitor C2 from secondary winding sec to provide output voltage V_DDH. Transistor PlO is activated to permit the charging of capacitor C2 according to a charge signal

generated by logical-NAND gate NANDl. Logical-NAND gate NANDl combines an inverted version of the output signal from logical-AND gate ANDl provided from an inverter II, with the output of a comparator Kl, that is activated only when output voltage V_DDH is less than a threshold voltage V_TH, thereby providing regulation of output voltage V_DDH by the action of a comparator Kl. As long as output voltage V_DDH is at or above threshold voltage V_TH for some portion of the duty cycle of switching converter 8 while switching transistor Nl is active and while signal sense is inactive, capacitor C2 will be charged in that duty cycle. Charging of capacitor C2 will generally happen during all of or the later portion of each duty cycle if the loading by controller 10 and other circuits connected to V_DDH is sufficient and capacitor C2 is relatively small. Resistor RIl will have a voltage indicative of current I_L in inductor Ll at the end of the sensing portion of the duty cycle before auxiliary power supply 12A starts charging capacitor C2. The indication from the output of current sensing circuit 14A can be sampled at a predetermined time or peak-detected by switching controller 10. Alternatively, multiple sampling points can be gathered from the output of current sensing circuit 14A and used to better indicate to switching controller 10 the shape of a current waveform describing current I_L.

[0022] While current sensing circuit 14A first measures current I_L, and then permits auxiliary power supply circuit 12A to charge the capacitor C2 to maintain output voltage V_DDH, the order of current sensing and auxiliary power supply charging is generally not restricted. In other embodiments of the present invention, the sensing can be performed after

auxiliary power supply output voltage V_DDH is replenished, as long as the reference point for the measurement of current I_L is not required to be the peak current or some value in the later portion of the duty cycle. However, performing sensing before substantial magnetization of current transformer Tl has occurred generally requires that sensing be performed first, unless other measures are taken to correct for the

magnetization or to remove it. [0023] Referring now to Figure 3, details of an auxiliary power supply circuit 12B that may be used to implement auxiliary power supply 12 of Figure 1 and details of a current sensing circuit 14B that can be used to implement current sensing circuit 14 of Figure 1 are shown in accordance with an alternative embodiment of the invention. Auxiliary power supply circuit 12B and current sensing circuit 14B of Figure 3 are similar to auxiliary power supply circuit 12A and current sensing circuit 14A of Figure 2, and therefore only

differences between them will be described below. In auxiliary power supply 12B, control signal sense is provided from switching controller 10, which controls transistors NlO and PlO. Control signal sense may or may not be qualified by switching controller 10 being operated in CCM mode, so that sensing is not performed when switching controller 10 is operating in DCM mode. Control signal sense can be asserted first in either polarity, although as mentioned above, sensing is generally performed first with control signal sense being asserted for the first portion of the duty cycle, and the timing can be determined from switching signals within switching controller 10. Alternatively, a result of measuring the output of current sensing circuit 14B can be used to determine when measurement is complete and auxiliary power supply 12B can begin to charge capacitor C2. In such an embodiment, signal sense is asserted at the beginning of a switching duty cycle, reducing the voltage across secondary winding sec and then transistor NlO is subsequently

deactivated and transistor PlO is activated to charge

capacitor C2. Since switching controller 10 controls the switching cycle of transistor Nl, the timing of operation of signal sense can be completely determined. Therefore, current sensing and auxiliary power supply charging could be performed in either order, depending on the relative priority of sensing accuracy and sensing the current every cycle versus

maintaining low voltage variation in auxiliary power supply output voltage V_DDH.

[0024] Current sensing circuit 14B differs from current sensing circuit 14A of Figure 2, in that resistor RIl is omitted, and the voltage generated across transistor NlO by current I_sec

is used to generate the indication of current I_L. The voltage drop of additional resistor RIl is eliminated, reducing the magnetization of current transformer Tl during the entire duty cycle and therefore reducing error due to the magnetization. Eliminating the voltage drop across resistor RIl also provides a higher voltage for charging capacitor C2 during the charging portion of the duty cycle.

[0025] Referring now to Figure 4, details of an auxiliary power supply circuit 12C that may be used to implement auxiliary power supply 12 of Figure 1 and details of a current sensing circuit 14C that can be used to implement current sensing circuit 14 of Figure 1 are shown in accordance with an alternative embodiments of the invention. Auxiliary power supply circuit 12C can be used to implement auxiliary power supply 12 of Figure 1. Auxiliary power supply circuit 12C is illustrative of multiple alternative embodiments of the present invention depending on the inclusion or omission of optional elements as described in further detail below.

Auxiliary power supply circuit 12C and current sensing circuit 14C of Figure 4 are similar to auxiliary power supply circuit 12B and current sensing circuit 14B of Figure 3, and therefore only differences between them will be described below. In one embodiment of auxiliary power supply 12C, an optional pre- charge circuit 32, illustrated as a dashed block, is

controlled by a switching signal /CS provided from switching controller 10, which is the complement of control signal CS that activates transistor Nl in switching converter 8 of Figure 1. At the beginning of a duty cycle, i.e., before control signal CS is asserted, pre-charge circuit 32 charges secondary winding sec to a magnetization that is equal to and has an opposite polarity to the magnetization caused by voltage V_SEC produced across secondary winding sec during the duty cycle when transistor Nl is active. No switches or transistors are required in auxiliary power supply 12C, as current sensing circuit 14C receives the correct value of the indication of the current I_L directly from current I_SEC as provided to current sensing circuit 14C, and capacitor C2 can be charged for the full duration of the active pulse provided from secondary winding sec.

[0026] In an alternative embodiment also illustrated in Figure 4, pre-charge circuit 32 can be omitted and a optional correction block 30 may be provided in switching controller 10, as indicated by the dashed line block. Correction block 30 computes the error in the indication of current I_L from either a predetermined known value of the mutual inductance of transformer Tl or from a mutual inductance measurement made at start-up or during operation. In another embodiment,

correction block 30 may determine the error by comparing the different slopes of I_sec during the sensing and charging portions of the duty cycle to estimate an actual magnetization current I_MAG- The mutual inductance can be measured by driving secondary winding sec with a predetermined voltage and determining the slope of current I_sec by observing the rate of decay of current I_sec after transistor Nl is deactivated.

Alternatively, if transistors, such as transistors NlO and PlO of Figure 2 and Figure 3 are provided, the indication of current I_L during two equivalent cycles in DCM mode, one with transistor NlO active and one with transistor PlO active can be compared to determine the error. In such an implementation, transistors NlO and PlO are used only for calibration.

Transistor NlO would be deactivated and transistor PlO

activated during normal operation, and correction block 30 would directly compensate for the error due to magnetization of transformer Tl by subtracting the measured value of the error from the indication of current I_L produced by current sensing circuit 14.

[0027] Current sensing circuit 14C differs from control circuit 14B of Figure 3, in that a current mirror Ml is used to reflect current I_sec into a current-input amplifier formed by amplifier Al and resistor R13. Therefore, the voltage drop introduced by the sensing circuit is similar to that of transistor NlO in current sensing circuit 14B.

[0028] Referring now to Figure 5, signal waveforms within switching converter 8 of Figure 1 including an auxiliary power supply 12A of Figure 2 are illustrated. Other embodiments of the auxiliary power supply have similar output waveforms, but are not directly illustrated. Prior to time T₂, switching converter 8 is operated in CCM mode, and signal ccm is asserted. Inductor current I_L increases rapidly during charging phases and decreases more slowly as the energy in inductor Ll is released into capacitor Cl. Voltage V_sec consists of a positive pulse, a negative transition spike and a lower-voltage negative pulse during the discharge of inductor Ll. During the first portion of the positive pulse, current sensing is performed, as indicated by signal sense being in an active state. Transistor NlO clamps voltage V_sec to a relatively low value until signal sense is de-asserted. The second portion of the positive pulse, after signal sense is de-asserted and voltage V_sec increases to its full potential as determined by the turns ratio of current transformer Tl, charges capacitor C2 to restore output voltage V_DDH if output voltage V_DDH is below threshold voltage V_TH. After time Ti, switching converter 8 operates in DCM mode and the sensing action is suspended, but capacitor C2 is still charged if output voltage V_DDH is below threshold voltage V_TH. Secondary current I_sec illustrates a change in slope during the auxiliary supply charging portion of the duty cycle. A current error I_MAQ represents the difference between a value of current I_sec that would be produced with a very low magnetization current, i.e., if transistor NlO were active throughout the duty cycle, and the actual value of I_sec? which is reduced by the

magnetization of current transformer Tl.

[0028] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention .

Claims

WHAT IS CLAIMED IS:

1. A switched-power circuit, comprising:

a magnetic coupling element for coupling an input of the switched-power circuit to an output of the switched-power circuit and having at least one winding;

a switching circuit for controlling energizing of the primary winding of the magnetic coupling element from an input voltage source connected to the input of the switched-power circuit;

a current sensing transformer having a primary winding coupled in series with the at least one winding of the magnetic coupling element and the switching circuit, and having a secondary winding;

a control circuit for controlling switching of the switching circuit;

a current sensing circuit coupled to the secondary winding of the current sensing transformer for providing an indication of a magnitude of current conducted through the at least one winding of the magnetic coupling element; and

an auxiliary power supply having an input coupled to the secondary winding of the current sensing transformer and an output coupled to the control circuit for providing a supply voltage to the control circuit.

2. The switched-power circuit of Claim 1, wherein the current sensing circuit corrects the indication of the magnitude of the current to compensate for a mutual inductance of the current sensing transformer.

3. The switched-power circuit of Claim 2, wherein the current sensing circuit approximates the mutual inductance of the secondary winding of the current sensing transformer from an estimation determined from a measurement of a voltage across the secondary winding of the current sensing transformer.

4. The switched-power circuit of Claim 2, wherein the current sensing circuit approximates the mutual inductance of the current sensing transformer from a predetermined value of the mutual inductance of the current sensing transformer.

5. The switched-power circuit of Claim 1, further comprising a circuit for selectively reducing a voltage across the

secondary winding of the current sensing transformer during a measurement interval in which the indication of the magnitude of the current is determined, whereby an effect of a

magnetization current of the current sensing transformer on the indication of the magnitude of the current is

substantially reduced.

6. The switched-power circuit of Claim 5, further comprising a circuit for isolating the output of the auxiliary power supply when the voltage across the secondary winding is selectively reduced.

7. The switched-power circuit of Claim 5, wherein voltage across the secondary winding of the current sensing

transformer is only reduced during the measurement interval if the switched-power circuit is operating in continuous

conduction mode.

8. The switched-power circuit of Claim 1, further comprising a pre-charging circuit for magnetizing the secondary winding of the current sensing transformer prior to a measurement cycle to a value opposite the magnetization current caused by the voltage across the secondary winding, whereby a magnetization current of the current sensing transformer is compensated for by the magnetizing of the secondary winding.

9. A method of operating a switched-power circuit, comprising: switching an input voltage source across at least one winding of a magnetic coupling element to transfer power to an output of the switched-power circuit;

sensing a current flowing through the at least one winding of the magnetic coupling element using a sensing transformer having a primary winding coupled in series with the at least one winding of the magnetic coupling element; and generating an auxiliary power supply from the secondary winding of the current sensing transformer for providing a supply voltage to a control circuit of the switched-power circuit .

10. The method of Claim 9, further comprising correcting the indication of the magnitude of the current to compensate for a mutual inductance of the current sensing transformer.

11. The method of Claim 10, wherein the correcting comprises approximating the mutual inductance of the secondary winding of the current sensing transformer from an estimation

determined by measuring a voltage across the secondary winding of the current sensing transformer.

12. The method of Claim 10, wherein the correcting comprises approximating the mutual inductance of the current sensing transformer from a predetermined value of the mutual

inductance of the current sensing transformer.

13. The method of Claim 9, further comprising selectively reducing a voltage across the secondary winding of the current sensing transformer during a measurement interval in which the indication of the magnitude of the current is determined, whereby an effect of a magnetization current of the current sensing transformer on the indication of the magnitude of the current is substantially reduced.

14. The method of Claim 13, further comprising isolating the output of the auxiliary power supply during the selectively reducing.

15. The method of Claim 13, wherein the selectively reducing only reduces the voltage across the secondary winding of the current sensing transformer during the measurement interval if the switched-power circuit is operating in continuous

conduction mode.

16. The method of Claim 9, further comprising magnetizing the secondary winding of the current sensing transformer prior to a measurement cycle to a value opposite the magnetization current caused by the voltage across the secondary winding, whereby a magnetization current of the current sensing transformer is compensated for by the magnetizing of the secondary winding.

17. An integrated circuit, comprising:

a switching control circuit for controlling a switch for charging an external magnetic coupling element through at least one primary winding;

at least one input terminal for coupling to a secondary winding of a current sensing transformer;

a current sensing circuit having an input coupled to the at least one input terminal for providing an indication of a magnitude of current conducted through the at least one winding of the external magnetic coupling element; and

an auxiliary power supply having an input coupled to the at least one input terminal and an output coupled to the switching control circuit for providing a supply voltage to the control circuit.

18. The integrated circuit of Claim 17, wherein the current sensing circuit corrects the indication of the magnitude of the current to compensate for a mutual inductance of the current sensing transformer.

19. The integrated circuit of Claim 17, further comprising a circuit for selectively reducing a voltage across the

substantially reduced.

20. The integrated circuit of Claim 19, further comprising a circuit for isolating the output of the auxiliary power supply when the voltage across the secondary winding is selectively reduced.

21. The integrated circuit of Claim 19, wherein the voltage across the secondary winding of the current sensing

conduction mode.

22. The integrated circuit of Claim 17, further comprising a pre-charging circuit for magnetizing the secondary winding of the current sensing transformer prior to a measurement cycle to a value opposite the magnetization current caused by the voltage across the secondary winding, whereby a magnetization current of the current sensing transformer is compensated for by the magnetizing of the secondary winding.