WO2010015999A1

WO2010015999A1 - Converter with controlled output current

Info

Publication number: WO2010015999A1
Application number: PCT/IB2009/053380
Authority: WO
Inventors: Gert-Jan Koolen; Araldo Van De Kraats
Original assignee: Nxp B.V.
Priority date: 2008-08-06
Filing date: 2009-08-04
Publication date: 2010-02-11

Abstract

In an isolated flyback converter for LED drivers usually the switching circuitry at the primary side needs to sense the output current at the secondary side in order to keep the current constant. This is usually done by an optocoupler. A flyback converter arrangement is introduced in which two alternative ways can be employed to control the current at the secondary side, without having the need for an optocoupler. The new flyback converter is especially suitable for driving LED’s since they need current control instead of the usual voltage control, and price is very important in this field.

Description

Converter with controlled output current

FIELD OF THE INVENTION

The present invention relates in general to the field of circuits for providing a controlled output current, in particular to power converter circuits and more particular to flyback converter circuits providing a controlled output current. Further, the present invention relates to a respective current control method.

Furthermore, the present invention relates to a converter circuit providing a controlled output current for driving a light emitting diode device. More particular, the present invention relates to a system comprised of such a converter circuit and light emitting diode device as well as to a respective replacement system for conventional candescent lamps.

BACKGROUND OF THE INVENTION

Light-emitting diodes (LED) are semiconductor diodes that by means of electroluminescence emit incoherent narrow-spectrum light when electrically biased in the forward direction of their pn-junction, as in the common LED circuit. The range of use and application for LED's starts from small area light sources, often with added optics to shape the radiation pattern and/or assist in reflection, up to high power applications such as flashlights and area lighting. The color of the emitted light goes from infrared via the visible range up to ultraviolet and depends on the composition and condition of the semi- conducting material used. More and more, LED's are entering the consumer market as a regular household light source. Besides new developed applications based thereon, LED's are entering in the huge replacement market for incandescent lamps. Accordingly any suitable electronics enabling LED application have to comply with demanding critical requirements for size, in particular the "must-fit" requirement in respect to existing sockets, and of course the aspect of cost when thinking about a high volume "off-the-shelf consumable.

For energy supply of LED applications, inter alia, flyback converters are one suitable driver topology due to their simplicity, efficiency and ability to provide electric isolation by means of a flyback-transformer. Since LED's require a specific current to operate, there is the need to control the current through the isolation. A usual solution employs an optocoupler device to provide feedback from the secondary side to the primary side of the flyback-transformer where the switching controller usually is located. However, optocouplers are both expensive and space consuming, while both aspects are critical design parameters, as mentioned above, and thus, are desired to be minimized.

Fig. 1 shows a typical arrangement 100 for supply of a regulated current to a LED device 110 via a stage 120 for rectifying and smoothing the current supplied by the flyback converter 100, which stage is comprised of an known arrangement of a diode D in the current path and a energy storing capacitor C connected in parallel to the LED device 110. The desired afore-mentioned isolation is provided by means of a flyback-transformer 102 having a primary winding P at its primary/input side and a secondary winding S at its secondary/output side. A current detection circuit 104 measures the instantaneous secondary current Is in the LED device 110 by means of a sensing resistor Rs. An optocoupler 106 implements a feedback path for providing the measurement information on the secondary current to a fly back controller 108. The flyback controller 108 controls a switching transistor 109, wherein by means of the applied switching frequency and duty cycle of the switching operation controls the energy supplied into the primary side by the primary current Ip of the fly back transformer and thus to the secondary side thereof.

The arrangement of Fig. 1 suffers from two major disadvantages. Firstly, the employed optocoupler is expensive and bulky resulting in increased cost as well as size of such a converter. Secondly, for sensing the secondary current Is a measurement element such as a resistor is used in the secondary high current path of the transformer. This reduces efficiency due to energy losses. For the application area these factors are extremely important.

US 2007/0121349 Al discloses a transformer-isolated flyback converter for delivering regulated power and current to an output load. The flyback- transformer has a primary winding and a secondary winding for delivering stored energy to the output load. An oscillator circuit is provided for generating a periodical signal. A switching circuit is coupled to the flyback-transformer and the oscillator circuit for energizing the primary winding to a reference current level each cycle of the oscillator circuit. The oscillator circuit has an integrator for deriving a time integral of a voltage at the primary winding. The oscillator circuit further comprises a peak detector coupled to the integrator for holding a peak value of the time integral. The oscillator circuit furthermore has a ramp generator for producing a ramp signal. A comparator is provided for comparing the peak value with the ramp signal and generating the periodical signal whenever the ramp signal exceeds the peak value. Hence, even that an isolated flyback converter for a LED load is provided, which does not need an optocoupler to adjust the clock frequency from an oscillator, the overall circuitry required is quite complex.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide a less complex flyback-converter arrangement, which does not require a costly and bulky optocoupler.

The object is achieved by switching power converter circuit for supply of a controlled output current to a load, in particular a light emitting diode device, according to claim 1.

Accordingly, the circuit comprises: a transformer element having a first primary winding for receiving electric energy from an input power source, at least one secondary winding for delivering said energy to the load coupled to thereto, and a second primary winding for sensing of the magnetization of the transformer; at least one switching element for switching a primary current path comprising the first primary winding, wherein the switching is controlled by a control unit of the circuit; and at least one current sensing element for sensing the current in the primary current path; wherein the control unit is configured to adjust a duty cycle of the switching element, which duty cycle is comprised of a first duration, in which the switching element is closed, to supply electrical energy from the input source to the first primary winding, and a second duration, in which the switching element is open, based on at least the current level and on the magnetization of the transformer. The controller unit may be configured to start a next first duration consecutive after termination of the second duration.

In one embodiment the converter further comprises a comparator unit for comparing a signal produced by the at least one current sensing element with a predetermined reference value corresponding to an amount of energy to be supplied into the first primary winding. An output of the comparator unit is provided to the controller unit, which is further configured to set the first duration in accordance to the output. The second primary winding is coupled to the control unit, which is further configured to define the second duration so that the magnetization of the transformer can decrease to a predetermined magnetization value. In certain embodiments the predetermined magnetization value is substantially zero. In other words, the converter waits until the transformer is out of energy, which happens sooner if more current is drawn. Thus, the output current is controlled. The converter circuit is fairly simple to implement. It is to be noted that in this embodiment the converter operates at a variable switching frequency, which is determined by first and second durations.

In a further development, the converter is operated with a constant switching cycle time and the controller unit is configured to control the second duration to the difference between the switching cycle time and the first duration. The converter further comprises a measurement unit, to which the second primary winding is coupled. Thus, the measurement unit can be configured to measure the demagnetization time, which starts at the end of the first duration and lasts until the magnetization of the transformer has decrease to a predetermined magnetization value. The measurement unit, or any other suitable part of the circuit such as the controller unit, can be configured to produce a duty cycle adjustment factor based on the switching cycle time and the measured demagnetization time. The signal produced by the at least one current sensing element can then be adjusted or compensated by means of the produced duty cycle adjustment factor; alternatively, the predetermined reference value corresponding to the amount of energy to be supplied into the first primary winding can be adjusted by means of the produced duty cycle adjustment factor.

In certain embodiments, the duty cycle adjustment factor corresponds the product of the switching frequency and the measured second duration. Thus, the measurement unit or any other suitable part of the circuit such as the controller unit, can be configured to adjust or compensate the signal produced by the at least one current sensing element by multiplying it with the duty cycle adjustment factor; again, the predetermined reference value corresponding to the amount of energy to be supplied into the first primary winding can alternatively be adjusted by means of the produced duty cycle adjustment factor. The object is further achieved by a system, according to claim 8.

Accordingly, the system comprises a converter circuit according to the invention and at least one light emitting diode device coupled to the output of said converter circuit for receiving the controlled output current from said converter.

The object is further achieved by a replacement system, according to claim 9.

Accordingly, the replacement system for a incandescent lamp device configured for fitting in a predetermined socket for said incandescent lamp device, which system comprises: a plug portion configured for fitting to the predetermined socket and for connecting to power supply contacts provided in said socket, and a system comprising a converter circuit according to the invention and at least one light emitting diode device coupled to the output of said converter circuit for receiving the controlled output current from said converter.

The object is further achieved by a method for driving a load, in particular a light emitting diode device with a controlled output current, in accordance with claim 10.

Accordingly, in the method electric energy is received from an input power source by a first primary winding of a transformer element, electric energy is delivered by at least one secondary winding of the transformer to said load coupled to thereto, and a primary current path comprising the first primary winding is switched. The method further comprises: sensing the current in the primary current path; sensing the magnetization of the transformer by a second primary winding of said transformer; adjusting a duty cycle of the switching of the primary current path so that the duty cycle is comprised of a first duration, in which the switching element is closed and electrical energy is supplied from the input source to the first primary winding, and of a second duration, in which the switching element is opened, based on at least the sensed current level and on the sensed magnetization of the transformer.

In particular embodiments the method comprises comparing the sensed current level with a predetermined reference value corresponding to an amount of energy to be supplied into the first primary winding, controlling the first duration based on the result of the comparing step, and defining the second duration so that the magnetization of the transformer can decrease to a predetermined magnetization value.

In a further development the method further comprises: performing the switching of the primary current path with a constant switching cycle time, defining the second duration to be the difference between the switching cycle time and the first duration, measuring the demagnetization time, starting at the end of the first duration and lasting until the magnetization of the transformer has decreased to a predetermined magnetization value, and adjusting the sensed current level with a duty cycle adjustment factor based on the switching cycle time and the measured demagnetization time. The adjusting of the sensed current level may in particular comprise multiplying the switching frequency and the measured demagnetization time to produce the duty cycle adjustment factor, and compensating the sensed current level with the duty cycle adjustment factor. As mentioned above, alternatively, the method may comprise adjusting the predetermined reference value corresponding to the amount of energy to be supplied into the first primary winding by means of the produced duty cycle adjustment factor.

The basic concept of the invention resides in the idea to control the current at the secondary side of the transformer without need for information on the secondary current. Accordingly, the actual duty cycle can be adjusted firstly, by waiting for transformer demagnetization during the time in which the switching element of the flyback converter is open, and secondly, by using the duty cycle of the secondary side to adjust the switching peak current. The invention employs a transformer primary side measurement to substitute for the secondary side current measurement, as e.g. used in the prior art arrangement of Fig. 1.

Thus, energy quantization can be provided by a switching means driven with a duty cycle adjusted based on the primary side current measurement. The variable switching frequency can be implemented based on transformer demagnetization, wherein a first duration or stroke, where the switching means are closed, has fixed time and a second duration or stroke, where the switching means are open, is varied. In a further development, the switching frequency is kept fixed, but the duty cycle is adjusted based on a switching peak current in the primary current path. Thus, the end of the first duration is modified depending on the duty cycle of the secondary side, which may be defined as the time as long as current flows through the secondary winding divided by the total cycle time; note, substantial no current flows through the secondary winding, when the second primary winding senses demagnetization of the transformer.

The present invention has particular use in LED drivers, which must be size and cost effective and need electric isolation. Preferred embodiments and further developments of the invention are defined in the dependent claims of the independent claims. It shall be understood that the apparatus and the method of the invention have similar and/or identical preferred embodiments and advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings, the figures are schematically drawn and not true to scale, and identical reference numerals in different figures, if any, may refer to corresponding elements. It will be clear for those skilled in the art that alternative but equivalent embodiments of the invention are possible without deviating from the true inventive concept, and that the scope of the invention is limited by the claims only.

Fig. 1 illustrates prior art flyback converter arrangement with secondary current measurement and optocoupler in a feedback path;

Fig. 2 shows a flyback converter arrangement according to the present invention;

Fig. 3 shows a further development of the flyback converter arrangement according to the present invention;

Fig. 4 illustrates a switching cycle period of the switching control signal of the converter of the invention, in particular parameters for adjusting the duty cycle; and

Fig. 5 depicts a flow chart illustrating the steps of a method for load current control in accordance to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In accordance with the present invention, there are two ways to control the current at the secondary side without need for information on the secondary current. The actual duty cycle can be adjusted firstly, by waiting for transformer demagnetization during the time in which the switching element of the flyback converter is closed, and secondly, by using the duty cycle of the secondary side to adjust the switching peak current.

Now with respect to Fig. 2, which shows a first embodiment of the present invention wherein the duty cycle is adjusted by waiting for transformer demagnetization. Accordingly, the flyback converter arrangement 200 is configured to supply a regulated current to a LED device 210 via a stage 220 for rectifying and smoothing the current supplied by the flyback converter 200. The rectifying and smoothing stage 220 can be of any known kind and is by way of example depicted as comprised of a diode D in the current path and an energy storing capacitor C connected in parallel to the LED device 210. Isolation is provided by means of the flyback-transformer 202 having a primary winding Pl at its primary/input side and a secondary winding S at its secondary/output side.

In comparison to the arrangement of Fig. 1 there is no current detection circuit for measuring the instantaneous secondary current Is in the LED device 210, in particular no sensing resistor Rs. Further, there is no optocoupler as feedback path for providing measurement information on the secondary current to the flyback controller 208.

The flyback controller 208 controls a switching means 209 for control of the primary or input current of the transformer 202. The switching means can be implemented as a semiconductor switch such as a switching transistor T. As input parameters for control of the secondary current the flyback controller 208 gets information on the current in the primary side/winding Pl of the transformer 202 and information on the magnetization of the transformer by means of a auxiliary, e.g. a second primary, winding P2.

At every cycle of the flyback converter 200 a distinct amount of energy is transferred to the transformer by means of conduction of the switching means 209 in duration tl (cf. Fig. 4) and ending duration tl when the current in the primary current path, i.e. the current through the switching means equals a predetermined peak reference value. When this also happens at a fixed frequency then the amount of power transferred by the transformer 202 is constant. However if the converter waits until the transformer is out of energy before starting a new or the next cycle, which also depends on the amount of current drawn, then the current can be controlled. As can be seen from Fig. 2, the required arrangement is fairly simple to implement. It is worth noting that in this embodiment by the duty cycle adjustment, the converter 200 operates at a variable frequency, which is determined by inversion of the resultant cycle time T_totai comprised of the duration tl of stroke Sl and the duration t2 of stroke S2 (cf. Fig. 4). At every cycle of the flyback converter 200 a distinct amount of energy is transferred to the transformer 202 during duration tl of stroke Sl, and from the transformer to the LED device 210 during duration t2 of stroke S2.

Accordingly, in accordance with the present invention in the first stroke SI a precise/predetermined amount of energy is transferred into the transformer 202. Then, in the second stroke S2 it is waited for demagnetization of the transformer 202 before proceeding to or starting the next cycle. For determining the predetermined amount of energy supplied during stroke Sl into the transformer 202, the voltage drop at a measurement resistor Rl in the primary current path of the transformer can be used. Accordingly, when the voltage at the resistor Rl is compared by suitable comparison means, e.g. a comparator with a suitable reference voltage Vthr, the first stroke Sl is stopped when the reference voltage level Vthr is reached, i.e. the primary current reaches Ip = Vthr/Rl .

Then for determining the duration t2 of the second stroke S2, demagnetization of the transformer is monitored by means of the auxiliary or second primary winding P2. Thus, by means of voltage at the auxiliary winding P2, demagnetization can be detected by waiting for a zero crossing, i.e. the voltage becoming zero. Only then, it is preceded to the next first stroke.

Now with respect to Fig. 3, which shows a flyback converter arrangement 300 in accordance to a further development of the invention. In the following only the differences in comparison to the arrangement of Fig. 2 are described.

The signal of the second primary winding or auxiliary winding P2 of the transformer 202 is supplied to a duty cycle measurement unit 230, which is configured to determine the duty cycle of the secondary side and to generate a correction or adjustment factor for adjusting of the duty cycle of the switching means 209, in particular by adjusting the switching peak current in the primary side of the flyback transformer 202.

Thus, again the duty cycle of the controller is adjusted, but further the overall switching frequency is kept constant, i.e. the converter 300 operates at a fixed frequency. In the embodiment shown in Fig. 3, the voltage level Vthr at which stroke Sl ends is modified depending on the duty cycle of the secondary side, which is defined as the time during which current flows through the secondary winding divided by the total cycle time T_totai- In other words, the peak current at which the switching transistor 209 as switching means stops conducting is modulated by the duty cycle of the secondary side.

For that purpose, the duty cycle measurement unit 230 measures t2/(tl+t2), where Ttotal=tl+t2 is the total cycle time, which is kept constant. Then, the detected threshold voltage at resistor Rl is compensated with this value, e.g. by using a multiplication, performed by suitable multiplication means 236. As a result, the secondary current on the secondary side is kept constant. Moreover, even tolerances of the transformer inductance do not affect the output current.

Now with respect to Fig. 4, in which a switching cycle period T_totai of the switching control signal of the converter of the invention, in particular the parameters for adjusting the duty cycle thereof is illustrated. In accordance with the basic idea, the converter operates at a variable frequency, which is determined by the first and second duration tl, t2. At every cycle of the flyback converter a distinct amount of energy is transferred during the duration tl to the transformer, and from the transformer to the load during duration t2. That is to say, by monitoring the current in the primary winding of the transformer, e.g. by means of a voltage over a resistor in the primary current path (Rl in Fig. 2 and 3), duration tl ends, when the predetermined current/voltage level is reached, i.e. duration tl is constant as long as the distinct amount of energy to be transferred and/or the primary input voltage is/are not altered. Duration t2 last as long as the transformer requires for demagnetization. That is to say, it is then proceed to the next cycle, when no substantially no current flows in the secondary winding of the transformer. The control of the output current is basically achieved by adjusting the switching cycle time T_totai accordingly (cf. 0 in

Fig. 4). This makes sure that the current at the secondary side is constant.

According to the further development of the invention, the converter is operated at a fixed frequency, but the voltage level at which duration tl ends is modified depending on the duty cycle of the secondary side, which is defined as the time current flows through the secondary winding divided by the total cycle time (cf.

© in Fig. 4). Thus, by measuring the duty cycle t* and calculating a duty cycle correction factor as t*/T_totai, the duration tl can be adjusted by compensating the predetermined peak current level in the primary winding of the transformer, at which the supply of electrical energy to the transformer is stopped. This can be done by compensating the detected or sensed threshold for the current in the primary current path with this duty cycle correction factor, e.g. by using a multiplication. This results in that the current on the secondary side is constant, and that even the tolerance on the transformer inductance does not affect the output power.

Now with respect to Fig. 5, in which the method steps of the method for driving a load, in particular a light emitting diode (LED) device with a controlled output current, e.g. in a circuit as depicted in Fig. 2 or 3 is illustrated. In step SlOO, electric energy is received from an input power source by a first primary winding of a transformer element by closing a switch in the primary current path so that electrical current provided by the input power source can increase in the primary winding of the transformer. The switch is opened when the current in the primary winding reaches a predetermined level. Then, in step S200 the magnetization of the transformer is measured by means of an auxiliary winding, e.g. a second primary winding. In step S300 the method branches depending on whether the converter is operated with a constant switching cycle time or frequency or not.

Accordingly, if the switching cycle time or frequency of the converter is kept variable, the method proceeds to step S400, in which second duration of the cycle time lasts until the transformer is substantially demagnetized, i.e. no current flows in/through the secondary winding thereof. During the second duration, electric energy is delivered by the secondary winding of the transformer to a load coupled to thereto. Then a new switching cycle is started, i.e. the method returns to step SlOO.

Accordingly, if the switching cycle time or frequency of the converter is kept constant, the method proceeds to step S410, in which the demagnetization time is measured, which starts at the end of the first duration and lasts until the magnetization of the transformer has decreased to a predetermined magnetization value. Then in step S420, the second duration is set to the difference between the switching cycle time and the first duration. Next in step S430, a duty cycle adjustment factor is generated based on the switching cycle time and the measured demagnetization time of the transformer, which duty cycle adjustment factor is used to compensate the sensed current level in the primary current path in the next step SlOO. The adjustment of the sensed current level may comprise multiplying the switching frequency and the measured demagnetization time to produce the duty cycle adjustment factor, and compensating the sensed current level with the duty cycle adjustment factor. Alternatively, the reference value for the sensed current level may be adjusted instead.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single means or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A switching power converter for supply of a controlled output current to a load, in particular a light emitting diode device, the circuit comprising: a transformer element having a first primary winding for receiving electric energy from an input power source, at least one secondary winding for delivering said energy to the load coupled to thereto, and a second primary winding for sensing of the magnetization of the transformer; at least one switching element for switching a primary current path comprising the first primary winding, wherein the switching is controlled by a control unit of the circuit; and at least one current sensing element for sensing the current in the primary current path; wherein the control unit is configured to adjust a duty cycle of the switching element, which duty cycle is comprised of a first duration, in which the switching element is closed, to supply electrical energy from the input source to the first primary winding, and a second duration, in which the switching element is open, based on at least the current level sensed by the current sensing element and on the magnetization of the transformer sensed by means of the second primary winding.

2. Converter according to claim 1, wherein the converter further comprises a comparator unit for comparing a signal produced by the at least one current sensing element with a predetermined reference value corresponding to an amount of energy to be supplied into the first primary winding, and wherein an output of the comparator unit is provided to the controller unit, which is further configured to set the first duration in accordance to the output, wherein the second primary winding is coupled to the control unit, which is further configured to start the first duration when the magnetization of the transformer decreases to a predetermined magnetization value.

3. Converter according to claim 3, wherein the predetermined magnetization value is substantially zero.

4. Converter according to claim 2 or 3, wherein the controller unit is further configured to start a next first duration consecutive after termination of the second duration.

5. Converter according to claim 2, wherein the converter is operated with a constant switching cycle time, wherein the converter further comprises a measurement unit, to which the second primary winding is coupled, wherein the measurement unit is configured to measure the demagnetization time, starting at the end of the first duration and lasting until the magnetization of the transformer has decrease to a predetermined magnetization value, and to produce a duty cycle adjustment factor based on the switching cycle time and the measured demagnetization time, and wherein the signal produced by the at least one current sensing element or the predetermined reference value is adjusted by the duty cycle adjustment factor.

6. Converter according to claim 5, wherein the duty cycle adjustment factor corresponds the product of the switching frequency and the measured second duration.

7. Converter according to claim 5 or 6, wherein the measurement unit or the controller unit is configured to compensate the signal produced by the at least one current sensing element or the predetermined reference value by multiplying it with a value based on the duty cycle adjustment factor.

8. System comprised of a converter circuit according to one of the claims 1 to 7 and at least one light emitting diode device coupled to the output of said converter circuit for receiving the controlled output current from said converter.

9. Replacement system for a incandescent lamp device configured for fitting in a predetermined socket for said incandescent lamp device, which system comprises: a plug portion configured for fitting to the predetermined socket and for connecting to a power supply contacts provided in said socket, and a system according to claim 8.

10. Method for driving a load, in particular a light emitting diode device with a controlled output current, wherein in the method electric energy is received from an input power source by a first primary winding of a transformer element, electric energy is delivered by at least one secondary winding of the transformer to said load coupled to thereto, and a primary current path comprising the first primary winding is switched; and wherein the method further comprises: sensing the current in the primary current path; sensing the magnetization of the transformer by a second primary winding of said transformer; adjusting a duty cycle of the switching of the primary current path so that the duty cycle is comprised of a first duration, in which the switching element is closed and electrical energy is supplied from the input source to the first primary winding, and of a second duration, in which the switching element is opened, based on at least the sensed current level and on the sensed magnetization of the transformer.

11. Method according to claim 10, wherein the method comprises: comparing the sensed current level with a predetermined reference value corresponding to an amount of energy to be supplied into the first primary winding, controlling the first duration based on the result of the comparing step, defining the second duration such that the magnetization of the transformer is decreased to a predetermined magnetization value.

12. Method according to claim 10, wherein the method comprises: performing the switching of the primary current path with a constant switching cycle time, controlling the second duration to the difference between the switching cycle time and the first duration, measuring the demagnetization time, starting at the end of the first duration and lasting until the magnetization of the transformer has decreased to a predetermined magnetization value, and adjusting the sensed current level with a duty cycle adjustment factor based on the switching cycle time and the measured demagnetization time.

13. Method according to claim 10, wherein adjusting the sensed current level comprises: multiplying the switching frequency and the measured demagnetization time to produce the duty cycle adjustment factor, and compensating the sensed current level or the predetermined reference value with the duty cycle adjustment factor.