Hysteresis reduction in an ASIC boost converter with fixed levels

Abstract

The invention relates to a driver for driving a lighting load. The driver comprises an inductive element, a semiconductor switching element, an unidirectional switching element, an output couplable to the lighting load, a controller to control the semiconductor switching element, and a current sensor arranged to sense a current to the output and the semiconductor switching element, wherein the inductive element, the semiconductor switching element and the unidirectional switching element are coupled to each other and the semiconductor switching element is arranged to charge the inductive element when the semiconductor switching element is closed and wherein the unidirectional switching element is arranged to discharge the inductive element to the output when the semiconductor switching element is open, wherein the current sensor comprises a first sensing element and a second sensing element, wherein the first sensing element and the second sensing element are coupled in series with the semiconductor switching element, wherein the lighting load is couplable between the output and a node between the first sensing element and the second sensing element, wherein a series combination of the first sensing element and the second sensing element is used for sensing a current in the inductive element when the semiconductor switching element is closed, and wherein the second sensing element is used for sensing the current in the inductive element when the semiconductor switching element is open, wherein the controller is arranged to: open the semiconductor switching element when the current in the inductive element, when the semiconductor switching element is closed, exceeds a first threshold, and close the semiconductor switching element when the current through the inductive element, when the semiconductor switching element is open, falls below a second threshold, wherein the second threshold is lower than the first threshold.

Description

[0001] Hysteresis reduction in an ASIC BOOST converter with fixed levels

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a driver. The invention further relates to a lighting system.

[0004] BACKGROUND OF THE INVENTION

[0005] An electronic transformer is conventionally used to convert a mains voltage to a lower voltage level for powering low voltage halogen lamps. As an example, 12 V MR16 lamps are powered by electronic transformers. Halogen lamps are replaced by LED lamps. LED lamps require significant less power than a halogen lamp. An electronic transformer, and specifically a self-oscillating electronic transformer, require a minimum output current for proper operation. If the output current is too low, the electronic transformer will not work properly and may even be damaged. Because the LED lamp requires a low amount of power, the LED lamp may draw a current that is below the minimum required current for the electronic transformer. Therefore, the electronic transformer may not work properly, and the LED lamp will also not be powered properly. Additionally, this may result in flicker in the light output. It is a strong desire to provide a driver that can draw a current from the electronic transformer that remains above the minimum required current for the electronic transformer.

[0006] SUMMARY OF THE INVENTION

[0007] It is an objective of the invention to provide a driver that can draw a continuous current from an electronic transformer over its entire dimming range.

[0008] To overcome this concern, in a first aspect of the invention, a driver for driving a lighting load is provided. The driver comprises: an inductive element; a semiconductor switching element; an unidirectional switching element, an output couplable to the lighting load; a controller to control the semiconductor switching element, and a current sensor arranged to sense a current to the output and the semiconductor switching element, wherein the inductive element, the semiconductor switching element and the unidirectional switching element are coupled to each other and the semiconductor switching element is arranged to charge the inductive element when the semiconductor switching element is closed and wherein the unidirectional switching element is arranged to discharge the inductive element to the output when the semiconductor switching element is open, wherein the current sensor comprises a first sensing element and a second sensing element, wherein the first sensing element and the second sensing element are coupled in series with the semiconductor switching element, wherein the lighting load is couplable between the output and a node between the first sensing element and the second sensing element, wherein a series combination of the first sensing element and the second sensing element is used for sensing a current in the inductive element when the semiconductor switching element is closed, and wherein the second sensing element is used for sensing the current in the inductive element when the semiconductor switching element is open, wherein the controller is arranged to: open the semiconductor switching element when the current in the inductive element, when the semiconductor switching element is closed, exceeds a first threshold, and close the semiconductor switching element when the current through the inductive element, when the semiconductor switching element is open, falls below a second threshold, wherein the second threshold is lower than the first threshold.

[0009] A driver is provided to power a lighting load via an output. The driver has an inductive element, a semiconductor switching element and an unidirectional switching element. They may be formed as a switched mode power supply. The components are arranged such that when the semiconductor switching element is closed, the inductor will get charged. This means that a current through the inductor is increasing. In addition, when the semiconductor switching element is open, the stored energy in the inductor will be released to the output and the current through the inductor is decreasing. A controller is provided to control the semiconductor switching element. The controller can turn on and off the semiconductor switching element. Preferably, the controller is used for controlling the semiconductor switching element using a hysteretic control. This is done by using a sensed signal and compare it to an upper threshold and a lower threshold. In this example, a current sensor is provided. The current sensor has a series combination of a first sensing element and a second sensing element to sense the current through the inductive element when the semiconductor switching element is closed. The second sensing element is used for sensing the current in the inductive element when the semiconductor switching element is open. Therefore, when the status of the semiconductor switching element determines which sensing elements are used for sensing the current through the inductive element. The controller is arranged to open, if the semiconductor switching element is closed, the semiconductor switching element when the current through the inductive element exceeds a first threshold. This first threshold determines the peak current though the inductive element. By opening the semiconductor switching element, the current will start to decrease since the unidirectional switching element will provide the current through the inductive element to the output and therefore eventually to the lighting load. The current through the inductive element keeps on decreasing until it drops below the second threshold. At this moment, the controller closes the semiconductor switching element. This way of control is called a hysteretic control, using a first to threshold and a second bottom threshold. Sensing the current when the semiconductor switching element is closed using both the first sensing element and the second sensing element provides an accurate determination of the current reaching the first threshold. By coupling the lighting load between the output and the node between the first sensing element and the sensing element, the current being sensed when the semiconductor switching element is open is done by only the second sensing element. Sensing the current using the second sensing element, provides a lower sensed voltage by a similar current amplitude. Therefore, the lower threshold is faster reached than when sensing with both the first sensing element and the second sensing element. This allows a smaller hysteresis to be reached without having to change any of the hysteresis parameters in the controller. This means that the current ripple in the inductive element can be further reduced without any modifications to the controller.

[0010] In a further example, the inductive element, the semiconductor switching element and the unidirectional switching element form a boost converter.

[0011] Preferably, the inductive element, the semiconductor switching element and the unidirectional switching element form a switched mode power converter. Even more preferred, the switched mode power converter is a boost converter.

[0012] In a further example, the unidirectional switching element is a diode. In its simplest form, the unidirectional switching element is a diode. The unidirectional switching element may also be a MOSFET operated in a synchronous way. This means that the MOSFET will behave similarly as a diode.

[0013] In a further example, the first sensing element comprises a first resistance and the second sensing element comprises a second resistance.

[0014] In a very simple form, the current sensor has a first sensing element having a first resistance for sensing a current and a second sensing element having a second resistance for sensing a current. Preferably, the resistances are resistors.

[0015] In a further example, a ratio between the first threshold and the second threshold is smaller than a ratio between the first resistance and the second resistance.

[0016] Preferably, the ratio between the first threshold and the second threshold is smaller than the ratio between the first resistance and the second resistance. If this is not true anymore, the hysteresis of the controller will not work anymore and therefore the driver will not operate properly.

[0017] In a further example, the first sensing element comprises: a series combination of a first resistor and a second resistor; a first switch adapted to shunt the first resistor, and a second switch adapted to shunt the second resistor, wherein the controller is arranged to control the first switch and the second switch based on an amount of current required for the lighting load.

[0018] The driver may be adapted to provide different output currents to the lighting load for different light outputs. This means that the driver provides a dimmable current to the lighting load. At each dimming level, or current level, the hysteresis band will vary. The number of resistors forming the first sensing element can switched in dependency of the dimming level.

[0019] In a further example, the controller is arranged to close the first switch or the second switch when the current required for the lighting load exceeds a current threshold.

[0020] At higher output powers, the total resistance of the first sensing element is kept as low as possible. Preferably, at maximum power to be provided to the output, the lowest resistance possible for the first sensing element is provided.

[0021] In a further example, the controller is arranged to open the first switch and the second switch when the current required for the lighting load drops below the current threshold. The lower the output power will be, the more resistances are to be provided for the first sensing element. Preferably at the lowest power possible to be provided to the output, all resistances are utilized.

[0022] In a further example, the controller is arranged to open and close the first switch and the second switch such that the current through the inductive element remains continuous independent of the amount of current required for the lighting load.

[0023] Preferably, the controller controls the first switch and the second switch such that over the entire dimming range, the current through the inductive element remains continuous i.e., it does not drop to zero.

[0024] In a further example, the first sensing element comprises a variable resistor.

[0025] To provide a homogeneous hysteresis reduction over a dimming range, a variable resistor can be used to continuously change the resistance over the dimming range. As an example, the resistance may be increased when the peak current is reduced. An example of a variable resistor can be a transistor, operated in its linear regime.

[0026] In a further example, the driver further comprises a buck converter coupled between an output of the unidirectional switching element and the output, wherein the unidirectional switching element is arranged to discharge the inductive element to the lighting load via the buck converter.

[0027] An additional buck converter can be placed between the output of the unidirectional switching element and the output. As an example, a boost converter may be used as a first stage and the buck converter may be coupled to the output of the buck converter. The boost converter may be used for drawing a continuous current from an electronic transformer. The boost converter may generate a bus voltage at its output e.g., the output of the unidirectional switching element, which is provided to the buck converter. The buck converter is used for generating a regulated current to the lighting load. The driver allows a good current control at the input and a good current regulation at the output.

[0028] In a further example, a lighting system is provided. The lighting system comprises a driver and the lighting load.

[0029] Preferably, a lighting system is provided. The driver, together with the lighting load have an optimized performance when connected to an electronic transformer.

[0030] In a further example, the lighting load comprises an LED.

[0031] Preferably, the lighting load has an LED. LEDs provide light in an energy efficient way and the light output can be controlled by providing a regulated current to the LED. In a further example, the lighting system is connectable to an electronic transformer, wherein the controller is arranged to maintain a current drawn from the electronic transformer continuously above a threshold.

[0032] An electronic transformer requires a minimum amount of current in order to operate stable. It is therefore desired that the driver draws enough current. By reducing the hysteresis in accordance to the invention, the current drawn by the driver can be controlled to remain continuous over a larger dimming range and therefore, the compatibility with the electronic transformer is improved.

[0033] In a further example, the threshold is defined above a current level such that the electronic transformer operates continuously in a stable mode.

[0034] Preferably, a threshold is defined for the driver to regulate the current to remain above this threshold to ensure the stable operation of the electronic transformer.

[0035] BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Examples of the invention will now be described with reference to the accompanying drawings, in which:

[0037] Fig. 1 shows an example of a commonly used boost converter.

[0038] Fig. 2 shows another example of a commonly used boost converter.

[0039] Fig. 3 shows an example of a graph showing the inductor current and sensor voltage in common used boost converters.

[0040] Fig. 4 shows an example of a driver.

[0041] Fig. 5 shows an example of a graph showing the inductor current and sensor voltage in a boost converter.

[0042] Fig. 6 shows a graph of a reduction of hysteresis over a peak current range.

[0043] Fig. 7 shows a detailed configuration of the first sensing element.

[0044] Fig. 8 shows a graph of a reduction of hysteresis over a peak current range.

[0045] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] The invention will be described with reference to the Figures.

[0047] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should also be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

[0048] Figure 1 shows a circuit diagram of a boost converter that is commonly used. The boost converter has an inductor LI, a MOSFET Ml and a diode DI. A capacitor Cl is placed in parallel with a lighting load LED for filtering out high frequency current ripples for the LED. A current sensor R1 is used for sensing the current flowing through the inductor LI and MOSFET ML The controller 1 uses the sensed current to control the MOSFET. In this example, the controller may keep the MOSFET Ml closed until the current sensor senses a current that exceeds a threshold. This control is also referred to a threshold control. The time that the MOSFET Ml remains off is fixed. The controller therefore provides an on-time that is based on the threshold level, more power to the load results in an increased threshold and vice versa, and an off-time that is fixed. A drawback of such a control is that the fixed off time may result in a moment in time that the current through the inductor LI drops to zero. This is also referred to as a discontinuous current control mode. When such a boost converter is connected to an electronic transformer, the discontinuity of the current causes the electronic transformer to cease operating and causes the electronic transformers to continuously try to restart. Especially for electronic transformers that are self-oscillating, the current to the driver is also used for maintaining a proper operation of the semiconductor switching elements in the electronic transformer.

[0049] Figure 2 shows an example of an improved boost converter that provides a different way of controlling the driver of Figure 1. The boost converter has the inductor LI, diode DI and MOSFET Ml coupled similar as the boost converter of Figure 1. The sense resistor R1 is placed in a different position. In this position, the inductor current can be continuously measured. When the MOSFET Ml is closed, the current through the inductor LI flows through the MOSFET Ml and through the resistor Rl. When the MOSFET Ml is open, the current through the inductor LI flows through the diode DI, load LED and capacitor Cl and sense resistor RL Placing the sense resistor Rl at this location therefore allows the maximum and minimum current of the inductor current to be measured and therefore, this allows hysteretic current control to be implemented. For hysteretic current control, two thresholds are determined. A maximum current threshold is determined and a minimum current threshold is determined. The controller 1 uses the maximum current threshold for opening the MOSFET Ml and the minimum current threshold is used for closing the MOSFET Ml. The controller 1 is normally a dedicated IC for controlling the driver as e.g. a boost converter. This specific IC may also be referred to as an Application Specific Integrated Circuit, ASIC. Such an IC normally has a built in fixed minimum hysteresis timing. As an example, an IC provides a regulation of the peak inductor current and a hysteresis of 40 mV which is to be provided to a voltage provided by the current sensor Rl. This means that the second threshold is set 40 mV below the first threshold. As an example, the sense resistor Rl has a resistance of 100 mQ. A maximum peak current may be set at 1.45 A, resulting in a sensed voltage of 145 mV. The first threshold is therefore set at 145 mV. The second threshold is then set at 145 mV - 40 mV = 105 mV. The current to which the second threshold corresponds is 100 mi Therefore, the hysteresis of the inductor current is 0.4 A. This hysteresis is the same over the entire dimming range of the driver. Lowering the output power is done by lowering the first threshold level. This therefore also lowers the second threshold level. Lowering the output power will eventually result in the inductor current to drop below a current value that allows the electronic transformer to operate in a stable way.

[0050] Figure 3 shows an exemplary waveform showing a sensed current V(s) and the actual current through inductor LI I(L1). The first threshold is set at a voltage of approximately 145 mV and the second threshold is set at 105 mV. This reflects back by controlling the MOSFET Ml such that the peak current, corresponding to the first threshold, through the inductor LI is at approximately 1.45 A and the current at the bottom, corresponding to the second threshold, is 1.05 A.

[0051] It is an insight of the inventors that lowering the hysteresis will increase the second threshold, allowing a deeper dimming to be achieved with a similar driver. The hysteresis can not be adjusted by the controller itself because it is a fixed level. Therefore, a modification to the circuit needs to be made. The inventors have found out that providing a different way of sensing the current through the inductor allows the hysteresis to be adjusted without having to alter the control and feedback circuitry.

[0052] Figure 4 shows a circuit diagram of a driver with an improved current sensor. The driver has an inductive element LI shown as an inductor, a unidirectional switching element DI shown as a diode, and a semiconductor switching element Ml shown as a MOSFET. In this example, these components form a boost converter. The output of the boost converter is buffered with a capacitor CL A current sensor R2, R3 is provided to sense the current through the inductive element LI. The current sensor is split up in two sensing elements. A first sensing element R2 has a first sense resistor. A second sensing element R3 has a second sense resistor. When the controller 1 closes the semiconductor switching element Ml, the current through the inductive element LI flows through the semiconductor switching element Ml. In this example, the current flows through the resistors R2 and R3. Therefore, the current sensor uses both the first sensing element R2 and the second sensing element R3. When the controller 1 opens the semiconductor switching element Ml, the current through the inductive element LI flows through the unidirectional switching element DI and therefore through the load LED and capacitor CL The current sensor only uses the second sensing element R3 since no current flows through the first sensing element R2. By sensing the current with different sensing elements when the semiconductor switching element Ml is open and when the semiconductor switching element Ml is closed, the current sensor will provide a voltage to the controller 1 that has a voltage step going to a lower voltage level. As an example, for the same current amplitude, the voltage provided by the current sensor will be larger when the semiconductor switching element Ml is closed than when the semiconductor switching element Ml is open. Therefore, closing or opening the semiconductor switching element Ml will result in a stepwise change in the voltage generated by the current sensor. This stepwise change will provide a reduction in hysteresis as the stepwise change will allow the second threshold to be reached much faster. The bigger this voltage step, the earlier the second threshold will be reached and therefore the lower the hysteresis will be. An example of how this stepwise change will lower the hysteresis will be provided with Figure 5.

[0053] Figure 5 shows an example of a graph showing the inductor current and sensor voltage in a boost converter. The inductor current is the current that flows through the inductive element LI. The sensor voltage is the voltage that is generated by the current sensor. When the semiconductor switching element Ml is closed, the current through the inductive element LI will ramp up. The controller 1 may be configured to compare the voltage provided by the current sensor with a reference voltage. In this example, the reference voltage may be set at 145 mV. In this example, the reference voltage may be equal to the first threshold. The current is sensed by the first sensing element R2 and the second sensing element R3. Resistor R2 has a resistance of 25 mfl and the resistor R3 has a resistance of 75 mfl. When the current reaches an amplitude of 1.45 A, the controller 1 receives a voltage of 145 mV from the current sensor R2, R3. The voltage of 145 mV corresponds to the first threshold and therefore, the controller 1 will turn off the semiconductor switching element ML In Figure 5, this opening of the semiconductor switching element Ml can be seen where the current through the inductive element LI reaches its peak. After opening the semiconductor switching element Ml, the current through the inductive element LI starts to decline. Since the declining current flows through the unidirectional switching element DI, the current sensor R2, R3 only uses the second sensing element R3 for sensing the current through the inductive element. The total sensing resistance of the current sensor drops from 100 mQ to 75 mQ. Therefore, directly after opening the semiconductor switching element Ml, the voltage provided by the current sensor R2, R3, drops by approximately 36 mV to approximately 109 mV. This is not an exact translation in reality since during the opening of the semiconductor switching element Ml, the inductor starts to commutate, which requires some time. After this voltage step to approximately 109 mV, the current through the inductive element LI keeps on falling until the current sensor R2, R3 provides a voltage to the controller 1 that corresponds to the second threshold level. This determines the second threshold to be 40 mV lower than the first threshold. This results in a voltage of 105 mV for the second threshold. This corresponds to a current through the inductive element LI of 1.4 A. Without any modifications, the current through the inductive element LI would be 1.05 A as shown in Figure 3. Therefore, the hysteresis has been reduced by 0.35 A. The current through the inductive element LI remains higher. Therefore, the current provided by the electronic ballast will remain higher, resulting in an improved compatibility.

[0054] Figure 6 shows of a reduction of hysteresis over a peak current range in a theoretical example. As an example, the first threshold is set at 145 mV and the second threshold is set at 105 mV. The hysteresis is therefore set at 40 mV. The first sensing element R2 has a resistance of 25 mQ and the second sensing element R3 has a resistance of 75 mQ. In the example, the peak current is varied and the effect on the hysteresis is projected. Changing the peak current amplitude results in a change in the sudden voltage drop after opening the semiconductor switching element Ml. A larger voltage drop results in a larger reduction of hysteresis. Therefore, a higher peak current will result in a higher voltage drop and therefore in a higher reduction of the hysteresis. This can be clearly seen in Figure 6, where the reduction of the peak current results in a relative reduction. Where e.g. a peak current of 1.5 A would result in a reduction of hysteresis of 93.75 %, a peak current of e.g. 0.75 A would result in a reduction of hysteresis of 46.9 %. It can be seen that in this configuration, at a peak current of 1.6 A, the hysteresis is reduced by 100 %. This is considered the physical boundary of the reduction of the hysteresis. At a 100 % hysteresis, the semiconductor switching element Ml will be switching at an infinite high speed. It is therefore desired to not achieve 100 % hysteresis reduction. A current higher than 1.6 A destroys the hysteresis, and the driver will not work anymore. It is therefore desired that a ratio between the first threshold and the second threshold is smaller than the ratio between the resistance of the first sensing element and the second sensing element.

[0055] Figure 7 shows an example a detailed configuration of the first sensing element R2. In this example, the sensing element is provided with multiple resistors coupled in series. The resistors R2_a, R2_b and R2_c form the first sensing element R2 resistance. In this example, each resistor is provided with a shunt switch. Switch SW1 is placed in parallel with resistor R2_a, switch SW2 is placed in parallel with resistor R2_b and switch SW3 is placed in parallel with resistor R2_c. Since at any moment at least one resistor is needed to be placed between the semiconductor switching element Ml and the second sensing element R3, one of the switches SW1, SW2 or SW3 may be omitted. As can be seen in Figure 6, the reduction of the hysteresis is lowered when the peak current is reduced. The main reason to reduce the peak current is to provide a lower power to the lighting load, by reducing the peak current, the overall current is also reduced. This causes the current to approach the minimum operating current of the electronic transformer. An increase in hysteresis may cause the current to drop below this minimum operating current, causing the electronic transformer to malfunction. It is therefore further desired to optimize the hysteresis reduction at lower current levels.

[0056] As has been stated before, the current peak determines the amount of voltage drop when the semiconductor switching element Ml opens. Therefore, a reduction is peak current will result in a reduction in voltage drop and therefore in a reduction of the hysteresis reduction. By increasing the resistance of the first sensing element R2, the voltage drop is increased again. As an example and for simplicity, resistors R2_a, R2_b and R2_c are identical and have a resistance of 25 mQ. Preferably, at a high peak current, only one resistor is active. This can be done by closing two switches and leaving one open. In this situation, the hysteresis reduction will be the same as the one shown in Figure 6. If the reduction of the hysteresis is not sufficient anymore e.g., the peak current has been reduced to a first peak current threshold, one of the closed switches will be opened. Therefore, the resistance of the first sensing element R2 will be increased to 50 mQ, resulting in an increase in voltage drop and an increase in hysteresis reduction. Similarly, further reducing the peak current until a second peak current threshold has been reached, will cause all switches to be opened. The resistance of the first sensing element R2 will be increased to 75 mQ, resulting again in an increase in voltage drop and an increase in hysteresis reduction. Figure 8 shows a graph of a reduction of hysteresis over a dimming range. The reduction of the peak current determines the dimming level. In this example, the values of hysteresis reduction are based on the values that can be derived when using a circuit as described in Figure 7. At a peak current of 1.5 A, the hysteresis is reduced by approximately 94 %. Reducing the peak current results in a reduction of hysteresis reduction. At a peak current of 0.75 A, one of the switched opens and the first sensing element R2 has a total resistance of 50 mQ. Therefore, the hysteresis reduction changes from 47 % to 94 % at the peak current of 0.75 A when opening one additional switch. The peak current can be reduced even further. At a peak current of 0.25 A, the hysteresis reduction is 31 %. At this peak current amplitude, all of the switches are opened. This results in a total resistance of 75 mQ for the first sensing element R2. The hysteresis reduction changes from 31% to 47 %.

[0057] As an additional improvement, the driver can be provided with a buck converter for driving the lighting load LED. The output of e.g. the boost converter, can be coupled to the input of the buck converter. The boost converter can therefore be optimized to draw a well-regulated current from the electronic transformer and the buck converter can then be configured to provide a well-regulated current possible to the lighting load.

[0058] Preferably, the driver is integrated in a lighting system that also comprises the lighting load LED. Preferably, the lighting load is a low power load such as an LED or laser diode.

[0059] 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. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A driver for driving a lighting load (LED), the driver comprising: an inductive element (LI); a semiconductor switching element (Ml); an unidirectional switching element (DI); an output couplable to the lighting load (LED); a controller (1) to control the semiconductor switching element (Ml), and a current sensor arranged to sense a current to the output and the semiconductor switching element (Ml), wherein the inductive element (LI), the semiconductor switching element (Ml) and the unidirectional switching element (DI) are coupled to each other and the semiconductor switching element (Ml) is arranged to charge the inductive element (LI) when the semiconductor switching element (Ml) is closed and wherein the unidirectional switching element (DI) is arranged to discharge the inductive element (LI) to the output when the semiconductor switching element (Ml) is open, wherein the current sensor comprises a first sensing element (R2) and a second sensing element (R3), wherein the first sensing element (R2) and the second sensing element (R3) are coupled in series with the semiconductor switching element (Ml), wherein the lighting load (LED) is couplable between the output and a node between the first sensing element (R2) and the second sensing element (R3), wherein a series combination of the first sensing element (R2) and the second sensing element (R3) is used for sensing a current in the inductive element (LI) when the semiconductor switching element (Ml) is closed, and wherein the second sensing element (R3) is used for sensing the current in the inductive element (LI) when the semiconductor switching element (Ml) is open, wherein the controller (1) is arranged to: open the semiconductor switching element (Ml) when the current in the inductive element (LI), when the semiconductor switching element (Ml) is closed, exceeds a first threshold, andclose the semiconductor switching element (Ml) when the current through the inductive element (LI), when the semiconductor switching element (Ml) is open, falls below a second threshold, wherein the second threshold is lower than the first threshold.

2. The driver of claim 1, wherein the inductive element (LI), the semiconductor switching element (Ml) and the unidirectional switching element (DI) form a boost converter.

3. The driver according to any of the preceding claims, wherein the unidirectional switching element (DI) is a diode.

4. The driver according to any of the preceding claims, wherein the first sensing element (R2) comprises a first resistance and the second sensing element (R3) comprises a second resistance.

5. The driver according to claim 4 wherein a ratio between the first threshold and the second threshold is smaller than a ratio between the first resistance (R2) and the second resistance (R3).

6. The driver according to any of the preceding claims, wherein the first sensing element (R2) comprises: a series combination of a first resistor (R2_a) and a second resistor (R2_b); a first switch (SW1) adapted to shunt the first resistor, and a second switch (SW2) adapted to shunt the second resistor, wherein the controller (1) is arranged to control the first switch (SW1) and the second switch (SW2) based on an amount of current required for the lighting load (LED).

7. The driver according to claim 6, wherein the controller (1) is arranged to close the first switch (SW1) or the second switch (SW2) when the current required for the lighting load (LED) exceeds a current threshold.

8. The driver according to claim 7, wherein the controller (1) is arranged to open the first switch (SW1) and the second switch (SW2) when the current required for the lighting load (LED) drops below the current threshold.

9. The driver according to claim 7, wherein the controller is arranged to open and close the first switch (SW1) and the second switch (SW2) such that the current through the inductive element (LI) remains continuous independent of the amount of current required for the lighting load (LI).

10. The driver according to claim 1, wherein the first sensing element (R2) comprises a variable resistor.

11. The driver according to any of the preceding claims, further comprising a buck converter coupled between an output of the unidirectional switching element (DI) and the output, wherein the unidirectional switching element (DI) is arranged to discharge the inductive element (LI) to the lighting load (LED) via the buck converter.

12. A lighting system comprising a driver according to any of the preceding claims and the lighting load.

13. The lighting system according to claim 12, wherein the lighting load comprises an LED.

14. The lighting system according to claim 13, wherein the lighting system is connectable to an electronic transformer, wherein the controller (1) is arranged to maintain a current drawn from the electronic transformer continuously above a threshold.

15. The lighting system according to claim 14, wherein the threshold is defined above a current level such that the electronic transformer operates continuously in a stable mode.

Images