Driver circuit for a light-emitting diode and lighting device with the same

Abstract

Light-emitting diodes, LED, driver circuit (130) configured to sequentially drive a plurality of series-coupled LED groups (LED1, ..., LED4) which have at least one LED, wherein the LED driver circuit (130) comprises: a plurality of middle nodes (210) each connected to an output of the plurality of LED groups (LED1, ..., LED4); a plurality of switches, each configured to form a current path between a corresponding middle node (210) and a common node (220), the common node (220) being connected to ground (GND); a detection resistor (Rcs) having a first terminal directly connected to the common node (220) and ground; a current sensing unit (240) connected to a second terminal of the detection resistor (Rcs) and configured to detect current flow through the common node (220) based on a voltage drop across the detection resistor (Rcs); and a current control unit (250) configured to generate a control signal to control each of the plurality of switches based on the detected current flow.

Description

BACKGROUND 1. Area The present invention relates to a method for driving a light-emitting diode (LED) and to an LED driver circuit and a lighting device which uses an AC power supply to drive a plurality of LED groups sequentially or one after the other. 2. Description of the state of the art Light-emitting diodes (LEDs) are photoelectric conversion semiconductor devices with a PN junction structure, formed by joining an n-type semiconductor region and a p-type semiconductor region. LEDs emit light by combining electrons and positive holes at the PN junction. Compared to a conventional incandescent bulb and fluorescent light, an LED exhibits reduced power consumption and increased lifespan. Therefore, LEDs can be used instead of conventional incandescent bulbs and fluorescent light for general lighting applications. An LED driver circuit generally uses a DC voltage, which is converted by a converter in a conventional AC power supply, to drive an LED. However, such an LED driver circuit creates a phase difference between the driver voltage and the driver current required for an LED device. This means that the conventional LED driver circuit cannot meet the required standards in a product such as an LED light in an environment with electrical characteristics such as power factor and total harmonic distortion. US Patent No. 6,989,807 (January 24, 2006) relates to an LED driver circuit, comprising a plurality of LEDs, a voltage sensing circuit, and a current switching circuit, and rearranges LEDs by the current switching circuit to improve power factor and efficiency in response to the detection of a voltage from a power source in the voltage sensing circuit. US Patent No. 7,081,722 (July 25, 2006) relates to an LED multiphase driver circuit and a method, comprising an LED group coupled to ground via separate conductive paths, and a phase switch forming each of the paths, and switching off a phase switch of an upper LED group to reduce power loss in response to a phase switch of a lower LED group being switched on. DE 10 2012 109 722 A1 discloses a light-emitting device and an LED driver method using the same. The light-emitting device comprises: a light source with first to nth LED groups connected in series and operated by DC voltage; and a driver controller for detecting a current flowing into an output terminal of the light source and for changing the number of LED groups operated in the light source if the detected current is outside a predetermined current range. US patent 2012 / 0194088A1 discloses an LED driver circuit comprising a rectification unit, an LED string, several electrical switches, a current sensing unit, and a current setting unit. The current setting unit turns the electrical switches on and off to create different circuits, thereby determining the number of activated LEDs and ensuring consistent brightness of the LED string. US Patent 2012 / 0299484A1 discloses a constant-current LED driver device comprising: a power supply, an LED module with one or more series-connected LEDs, a current sensing module for measuring the LED current, and an error amplification module for comparison and signal output. Several sequential control modules regulate the brightness and the switching on / off of the LEDs based on the amplified error and reference signals. WO 2013 / 100 736 A1 discloses an LED lighting device comprising: a rectification unit for converting alternating current, a series of LEDs connected in series, and several switches connected to the cathodes of the LEDs. A control unit measures the current across resistors, compares it to a reference value, and controls the switches accordingly to regulate the LED brightness. However, such methods for driving LEDs impose a spatial limitation (i.e., an integration limitation) on a module, since conventional designs either detect a level from an AC power supply or use multiple detection resistors to detect a phase voltage in each of the LED groups. Likewise, such conventional designs require a logic circuit to determine a current path according to a level from an AC power supply in each of the LED groups in order to switch on the LED groups. SUMMARY This summary is intended to introduce, in a simplified form, a selection of concepts that are subsequently described in detail. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. An LED driver circuit according to the invention is defined in claim 1. A lighting device according to the invention is defined in claim 11. A method according to the invention for driving a plurality of series-connected LEDs is defined in claim 12. Further developments of the invention are the subject of the dependent claims. In general terms, a light-emitting diode (LED) driver circuit is provided, which is configured to sequentially drive a plurality of series-coupled LED groups, each containing at least one LED. The LED driver circuit comprises a plurality of intermediate nodes coupled to terminals of the plurality of LED groups, a common node with a reference voltage, a switching unit configured to form a plurality of current paths between the common node and the plurality of intermediate nodes, and configured to select a current path based on a control signal, a current sensing unit configured to detect a current flow through the common node, and a current control unit configured to generate the control signal based on the detected current flow. The switching unit can have a plurality of switches, with the plurality of switches being connected to a corresponding middle node and the common node to form a current path. A current flow through the common node can correspond to a sum of currents flowing through the majority of the current paths. The current sensing unit can include a detection resistor, which is coupled to the common node to form a feedback loop. The current sensing unit can be configured to detect the magnitude or quantity of current flowing from the common node based on a voltage across both sides of the detection resistor. The detection resistor can be placed outside the LED driver circuit. The current control unit can be configured to differentially amplify a reference voltage, which is set for each of the majority of switches and the detected current flow, in order to control a corresponding switch. The set reference voltage can increase in response to an increase in the distance between an AC power supply and a central node to which a corresponding switch is coupled. The power control unit can be configured to turn off a switch in the selected power path in response to an increase in power flow, in order to refresh an actual or current power path. The current flow can increase in response to an increase in distance between the AC power supply and the selected current path. The current control unit can have a linear block configured to measure a level of the AC power supply and to control an amount of current flowing into each of the majority of the switches, so that the detected current flow responds to a change in the AC power supply. The current control unit may have an output control unit configured to measure a maximum level of the AC power supply in order to reduce the amount of current flowing into each of the majority of the switches to a ratio higher than a reference level. In another general aspect, a lighting device or illumination device is provided which includes a rectifier unit configured to rectify an AC voltage half-wave or half-wave, a light-emitting unit comprising a plurality of series-coupled LED groups, each comprising at least one LED, and an LED driver circuit configured to drive the plurality of LED groups sequentially.The LED driver circuit can include a plurality of middle nodes coupled to each of the terminals of the plurality of LED groups, a common node with a reference voltage, a switching unit configured to form a plurality of current paths between the common node and the plurality of middle nodes and configured to select a current path based on a control signal, a current sensing unit configured to detect a current flow through the common node, and a current control unit configured to generate the control signal based on the detected current flow. In another general aspect, a method for driving a plurality of series-coupled light-emitting diode (LED) groups is provided, each group comprising at least one LED, wherein the method involves detecting a current flow through a common node of a driver circuit, the driver circuit comprising the common node, a plurality of intermediate nodes coupled to terminals of the plurality of LED groups, a common node with a reference voltage, and a switching unit configured to form a plurality of current paths between the common node and the plurality of intermediate nodes; generating a control signal based on the detected current flow; and selecting a current path from the plurality of current paths based on the control signal. Other features and aspects will become apparent from the following detailed description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram illustrating an example of a light-emitting diode (LED) device. Fig. 2 is a block diagram illustrating an example of an LED driver circuit in the LED device shown in Fig. 1. Fig. 3 is a circuit diagram illustrating an example of a switching unit in the LED driver circuit of Fig. 2. Fig. 4 is a circuit diagram illustrating an example of a current control unit in the LED driver circuit of Fig. 2. Fig. 5 is a waveform diagram illustrating an example of the operation of an LED driver circuit from Fig. 1. Fig. 6 is a waveform diagram illustrating an example of the operation of an LED driver circuit featuring a linear block. Fig. 7 is a waveform diagram illustrating an example of the operation of an LED driver circuit featuring an output control unit. Throughout the drawings and detailed descriptions, unless otherwise specified or provided, the same reference symbols are understood to refer to the same elements, features, and structures. The drawings need not be to scale, and the relative size, proportions, and representation of elements in the drawings may be exaggerated for clarity, illustration, and convenience. DETAILED DESCRIPTION The following detailed description is intended to assist the reader in gaining a comprehensive understanding of the methods, devices, and / or systems described herein. However, various modifications, variations, and equivalents of the systems, devices, and / or methods described herein will be obvious to those skilled in the art. The sequence of processing steps and / or operations described is an example. The sequence of these steps and / or operations is not limited to that presented herein and may be modified as is known in the prior art, with the exception of steps and / or operations that necessarily occur in a specific order. Likewise, descriptions of functions and constructions that are well known to a person skilled in the art may be omitted for the sake of clarity and conciseness. The features described herein can be implemented in various forms and should not be considered limited to the examples described herein. Rather, the examples described herein have been provided so that this disclosure will be conscientious and complete and will convey the full scope of the disclosure to a person skilled in the art. Terms described in this revelation can be understood as follows. While terms such as "first" and "second" may be used to describe different components or constituents, such components should not be understood as being limited to the terms above. The terms above are used only to distinguish one component from another. For example, a first component may be referred to as a second component without altering the scope of rights of this disclosure, and similarly, a second component may be referred to as a first component. It will be understood that when an element is referred to as "connected to" another element, it may be directly connected to that other element, or intervening elements may also be present. Conversely, when an element is referred to as "directly connected to" another element, no intervening elements are present. Additionally, unless explicitly stated otherwise, the word "show" and variations such as "shows" or "showing" will be understood to imply the inclusion of the named elements, but not the exclusion of any other elements. Meanwhile, other expressions that describe relationships between components, such as "between," "immediately between," or "adjacent to" and "directly adjacent to," may be interpreted similarly. The singular forms "einer / eine / eines" and "der / die / das" in this disclosure are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, it will be understood that terms such as "exhibiting" or "having," etc., are intended to indicate the existence of the features, numbers, operations, actions, components, parts, or combinations thereof disclosed in the description, and are not intended to exclude the possibility that one or more other features, numbers, operations, actions, components, parts, or combinations thereof may exist or be added. The terms used in this application are used solely to describe various examples and are not intended to limit the present disclosure. Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meanings as those generally understood by those skilled in the art in the field to which this disclosure belongs, with respect to this disclosure. Such terms must be interpreted as those defined in a commonly used dictionary so that they have the same meanings as their contextual meanings in the relevant field, and they must not be interpreted to have ideal or excessively formal meanings unless explicitly defined in this disclosure. Fig. 1 illustrates an example of a light-emitting diode (LED) device. Referring to Fig. 1, a light-emitting diode (LED) device 100 comprises a power supply unit 110, a light-emitting unit 120 and an LED driver circuit 130. The power supply unit 110 can be configured to perform half-wave rectification of an AC voltage. For example, the power supply unit 110 can half-wave rectify an AC voltage applied to the LED device to create a pulsed voltage and can supply this pulsed voltage to the light-emitting unit 120 and the LED driver circuit 130. The power supply unit 110 can include a rectifier circuit for half-wave rectification of the AC voltage. The rectifier circuit can, for example, be implemented as a bridge diode. The power supply unit 110 does not require a separate converter that converts the AC voltage into a relatively uniform DC voltage. The light-emitting unit 120 can have a plurality of LED groups coupled in series, and each of the LED groups can have at least one LED. In this context, if an LED group comprises multiple LEDs, the majority of the LEDs can be connected in series, parallel, or in combination, depending on the product application. Similarly, each of the multiple LEDs can include a resistor component. This resistor component can be connected in series or parallel with the multiple LEDs. The LED driver circuit 130 is coupled to a terminal of the light-emitting unit 120 and the power supply unit 110 to form a plurality of current paths for the light-emitting unit 120 in order to determine a specific current path based on a current flow of the LED driver circuit 130 (for example, a total amount of current). Fig. 2 illustrates an example of a light-emitting diode (LED) driver circuit of an LED device according to Fig. 1. Referring to Fig. 2, the LED driver circuit 130 has a middle node 210, a common node 220, a switching unit 230, a current measuring unit 240 and a current control unit 250. The central nodes 210 are coupled to a terminal in each of the majority of the LED groups. For example, the central nodes 210 are coupled to a rear terminal in each of the LED groups, and the rear terminal corresponds to a cathode through which a current flows. For example, the light-emitting unit 120 can have a first to fourth LED group in series, and the LED driver circuit 130 can have a first to fourth middle node 211 to 214. In this example, the first middle node 211 corresponds to a node coupled to the first and second LED groups (i.e., a node located in the back terminal of the first LED group). Similarly, the second to fourth middle nodes 212 to 214 correspond to nodes located in the back terminals of the second to fourth LED groups. The common node 220 corresponds to a node that has a reference voltage. For example, the common node 220 is connected to an external reference voltage, and the common node 220 is connected to ground (GND) to cause the reference voltage to have a value of 0 [V]. The switching unit 230 couples the common node 220 with each of the plurality of middle nodes or disconnects the common node 220 from each of the plurality of middle nodes. In one example, the switching unit 230 has a plurality of switches which are coupled to each of the middle nodes 210 and the common node 220 to form a current path. Here, each of the plurality of switches is turned on or off based on a control signal to form a current path between the middle nodes 210 and the common node 220. In one example, each of the majority of switches can be implemented as a metal oxide silicon field effect transistor (MOSFET). For instance, each of the majority of switches can be implemented as a high-voltage NMOS to provide AC voltage stability. Fig. 3 is a circuit diagram illustrating an example of a switching unit of an LED driver circuit according to Fig. 2. Referring to Fig. 3, the switching unit 230 has four MOSFETs which are coupled in parallel with the middle nodes 210 and the common node 220. In this, a drain and a source of each of the MOSFETs are coupled to a corresponding one of the middle nodes 210 and the common node 220, and each of the MOSFETs operates based on the control signal which is received through a corresponding gate. A voltage applied between a gate and a source in the MOSFET (i.e., the saturation voltage flowing through the MOSFET according to the control signal) can increase. Conversely, as the voltage applied between a drain and a source of the MOSFET increases, the current flowing through the MOSFET within a saturation voltage range can also increase. The switching unit 230 can control the amount or quantity of current flowing through the majority of LED groups in response to the control signal. Referring back to Fig. 2, the current measuring unit 240 is configured to detect a current flow of the common node 220. For example, the current measuring unit 240 can determine a total amount of current flowing from the LED driver circuit 130, and the total amount of current can correspond to a sum or summation of a current flowing into at least one of the majority of the LED groups and a current consumed to drive the LED driver circuit 130. In one example, the current measuring unit 240 can have a feedback loop. The feedback loop has a voltage measuring terminal VCS and a detection resistor or measuring resistor RCS. The voltage measuring terminal VCS is coupled to a terminal of the power supply unit 110. The detection resistor RCS is located externally and is coupled between the voltage measuring terminal VCS and the common node 220. A voltage at the voltage sensing terminal VCS (i.e., a voltage across the detection resistor RCS) is represented as the product of the magnitude of a common current IC and the magnitude of the detection resistor RCS (i.e., VCS = -IC × RCS). The common current IC flows through the common node 220 in boundary regions or outer areas. The current sensing unit 240 detects the magnitude of a current in the common node 220 based on the voltage at the voltage sensing terminal VCS and can estimate the status of an AC input voltage. The current control unit 250 generates the control signal to control the switching unit 230 based on the detected current flow. In one example, the current control unit 250 can detect a change in a current, which is detected by the current measuring unit 240, in order to select a current path inside the switching unit 230. The following section describes in detail an example of the operation of the 250 power control unit. The LED driver circuit 130 receives a full-wave rectified power supply voltage, and a driver current increases due to an internal component of the LED driver circuit 130. In response to the power supply voltage being sufficiently high, the LED driver circuit 130 operates by setting an internal bias voltage. The current flowing through an LED is generally small, due to LED characteristics, when the voltage applied to the LED is less than or equal to a threshold voltage. However, the current increases rapidly when the voltage applied to the LED exceeds the threshold voltage. A threshold voltage in each of the plurality of LED groups can be determined based on at least one LED contained in that group and a corresponding topology configuration. If the voltage applied to each of the plurality of LED groups is greater than the corresponding threshold voltage, a current can flow in that LED group. In the current control unit 250, a power supply voltage flows sequentially through the majority of LED groups in response to the fact that the power supply voltage is greater than the threshold voltages of the majority of the LED groups and a current is varied incrementally. The current control unit 250 can control the switching unit 230 so that it forms an optimal current path. Fig. 4 is a circuit diagram illustrating an example of a current control unit of an LED driver circuit according to Fig. 2. Referring to Fig. 4, the light-emitting unit 120 has four LEDs, each of which is contained in four LED groups (i.e., LED 1 to LED 4). In one example of the light-emitting unit 120, the switching unit 230 has four switches, each corresponding to one of the four LEDs. Each of the four switches can be implemented with an NMOSFET and can include a resistive component. The current control unit 250 has four amplifiers, each corresponding to one of the four switches. An input terminal in each amplifier is coupled to an output terminal of the current measuring unit 240 and to each of the reference voltages Vref1 to Vref4. In this example, the current measuring unit 240 can be implemented as a combination of a current source, an amplifier, and a resistor, and an output voltage of the current measuring unit 240 can be proportional to a detected current. The reference voltages Vref1 to Vref4 can be adjusted during a manufacturing process. In response to an increase in the distance between a middle node connected to a corresponding switch (i.e., one of middle nodes 211 to 214) and an AC power supply, the corresponding reference voltage can increase relative to the source voltage. For example, each of the reference voltages Vref1 to Vref4 can be adjusted incrementally, such that reference voltage Vref1 can be set to a value of 1 [V] and reference voltages Vref2 to Vref4 can be adjusted incrementally to a value of 10 [mV] relative to reference voltage Vref1. Each amplifier differentially amplifies one of the reference voltages Vref1 to Vref4 and an output of the current measuring unit 240 to generate the control signal. This control signal is fed to a gate of the switches. When an output voltage of the current measuring unit 240 is greater than a corresponding reference voltage (i.e., one of the reference voltages Vref1 to Vref4), the corresponding switch is turned off. Conversely, when the output voltage is lower than the corresponding voltage, the corresponding switch remains turned on. Below, another example of operating the LED driver circuit based on a power supply voltage Vin will be described. Firstly, when the power supply voltage Vinan is applied to the LED driver circuit 130, and the power supply voltage Vingeringer is a threshold voltage of the first LED (LED1), there is essentially no current flowing through the common node 22 via the switches. Consequently, the output voltage of the output measuring unit 240 has a value of essentially 0, and all of the switches are maintained in an on state. Secondly, in response to the fact that the power supply voltage Vin increases and the power supply voltage Vin is more than a threshold voltage of the first LED (LED1), the voltage of the first middle node is higher than the first reference voltage. In such a case, a small current I1 flows through the first switch according to a voltage between the terminals of the first switch. A current IC flowing through the common node 220 (hereinafter referred to as a common current) can correspond to a current I1 flowing through the first switch. Herein, the common current IC is essentially equal to a current flowing through the light-emitting unit 120. It is assumed below that the common current IC is the same as the current flowing through the light-emitting unit 120. This is the case because the magnitude of a driver current according to driving the LED driver circuit 130 is relatively small compared to the current flowing through the light-emitting unit 120. Meanwhile, the current measuring unit 240 is configured to detect a current from the common node 220 in order to provide a corresponding voltage to the voltage control unit 250. As described above, the current measuring unit 240 can detect the current from the common node 220 via a feedback loop. The current IC detected in the current measuring unit 240 is essentially equal to the current I1 flowing through the first switch (i.e., IC = I1 if the driver current for the LED driver circuit 130 is ignored), and the current measuring unit 240 outputs a voltage having a value of IC×k (constant) to provide the voltage for the current control unit 250. The current control unit 250 amplifies a voltage difference between the first reference voltage Vref1 and an output voltage of the current measuring unit 240 by means of an amplifier, in order to provide the voltage difference for the first switch. If the output voltage of the current measuring unit 240 is lower than the first reference voltage Vref1 (i.e., Ic×k) <Vref1), erhält der erste Schalter einen Anschaltzustand aufrecht. Hierin wird ein Zeitvorgabepunkt beziehungsweise Zeitpunkt, in welchem der erste Schalter einen Anschaltzustand aufrechterhält (d. h. ein Zeitvorgabepunkt, in welchem der erste Schalter abschaltet) basierend auf Werten von k und Vref1bestimmt. The second through fourth switches maintain their on states in a similar manner to the first switch, since their reference voltages Vref2 to Vref4 are higher than the first reference voltage Vref1. However, if the power supply voltage Vin is not greater than the threshold voltage of the second through fourth LEDs (LED2...LED4), current cannot flow through the second through fourth switches. Instead, current can flow through a current path formed by the first switch. Thirdly, in response to the fact that the power supply voltage Vin increases and the power supply voltage Vin is greater than a summation or sum of the threshold voltages in the first and second LEDs (LED1, LED2), a small current I2 flows through the second switch. The current control unit 250 is configured to amplify the voltage difference between the second reference voltage Vref2 and an output voltage of the current measuring unit 240 using an amplifier, in order to provide the voltage difference for the second switch. This is because the output voltage of the current measuring unit 240 is less than the second reference voltage Vref2 (i.e., I1×I2×k). <Vref2), erhält der zweite Schalter einen Anschaltzustand aufrecht. Meanwhile, the current control unit 250 is configured to amplify the voltage difference between the first reference voltage Vref1 and an output voltage of the current measuring unit 240 using an amplifier, in order to provide the voltage difference for the first switch. If the output voltage of the current measuring unit 240 is greater than the first reference voltage Vref1 (i.e., I1×I2×k > Vref1), the first switch is turned off. If the power supply voltage Vin is not more than a threshold voltage of the third and fourth LEDs (LED3, LED4), a current cannot flow through the third and fourth switches and the current flows through a current path formed by the second switch. Fourthly, in response to the fact that the power supply voltage Vin increases and the power supply voltage Vin is greater than a sum of the threshold voltages in the first to third LED (LED1... LED3) or the first to fourth LED (LED1... LED4), the current control unit 250 calculates a differential voltage between each of the reference voltages Vref1 to Vref4 in the majority of the switches and an output voltage of the current measuring unit 240 in order to control an operation in each of the first to fourth switches. Fifthly, the LED driver circuit responds to the decrease in the power supply voltage Vinab by operating in the other way as described above. If the maximum voltage of the power supply is less than the sum of the threshold voltages in the first to fourth LEDs (LED1...LED4), the current I4 flowing through the fourth LED (LED4) can be 0 [A]. If the common current Ic decreases rapidly (i.e., Ic×k), <vref3), wird der dritte schalter angeschaltet und strom i3fließt durch die led (led3).In response to a decrease in the level of the power supply voltage Vinab, the current control unit 250 can control the operation of the first to fourth switches as illustrated above. Accordingly, the LED driver circuit 130 can set an optimal current path without a separate logic circuit to determine a current path according to a level of AC power. Fig. 5 is a curve diagram illustrating the operation of an LED driver circuit according to Fig. 1. In Fig. 5 (a) the power supply voltage Vin corresponds to a pulsation voltage which is generated by half-wave rectification or half-wave rectification of an AC voltage. In Fig. 5(b), the common current Ic corresponds to a current flowing out of the LED driver circuit 130 through the common node 220. The common current Ic exhibits a stepped curve that changes step by step as the power supply voltage Vin increases or decreases to correspond to a specific voltage Vth1, Vth2, Vth3, or Vth4. The common current is not changed until the power supply voltage Vin is greater than a first defined voltage Vth1. Here, the first defined voltage Vth1 can correspond to a threshold voltage of the first LED group. Before the power supply voltage Vin exceeds the threshold voltage of the first LED group, no current flows through the common node 220 via the intermediate nodes 211, 212, 213, 214, and therefore the switches maintain an on state. If the power supply voltage Vin is greater than the threshold voltage of the first LED group, a small current passing through the first LED group can be applied to the common node 220 via the first middle node 211 and the first switch. The current control unit 250 can detect a change in the small current to determine a current path, such that a current from the light-emitting unit 120 flows into the first switch. The common current IC is saturated to maintain a constant value before the power supply voltage Vin exceeds a second specific voltage Vth2. Here, the second specific voltage Vth2 can be a summation, or sum, of each of the threshold voltages in the first and second LED groups. As described above, when the power supply voltage Vin exceeds the second specific voltage Vth2, a small current entering the second LED group is applied to the common node 220 via the second intermediate node 210 and the second switch. The current control unit 250 can detect a change in the small current in order to refresh the current path, so that current from the light-emitting unit 120 flows through the second switch. This means that the current control unit 250 can switch off the first switch via the control signal. As described above, the common current ICin is modified in response to the increase in the power supply voltage Vin to exceed each of the third and fourth specified voltages Vth3 and Vth4. The current control unit 250 can detect such a change to refresh the current path. The common current IC can change in the other way in the case that the power supply voltage Vin decreases, rather than that the power supply voltage Vin increases. In the event that the power supply voltage Vin under the fourth specific voltage Vth4 drops below a maximum voltage, the LED current can decrease rapidly, since the voltage applied to the fourth LED group is no more than a corresponding threshold voltage. The current control unit 250 can refresh the current path based on a change in current. This means that the current control unit 250 can switch on the third switch. Figure 5(c) illustrates the waveforms of currents I1 to I4 flowing through the first to fourth switches. A current In flowing through an nth switch has a specific value in response to the fact that the power supply voltage Vin corresponds to a value between an nth threshold voltage and an (n+1)th threshold voltage. A current I1 flowing through the first switch has a specific value in response to the fact that the power supply voltage Vin corresponds to a value between the first threshold voltage Vth1 and the second threshold voltage Vth2. Therefore, as the power supply voltage Vin increases, the current path is sequentially changed from the first switch to the fourth switch. Conversely, as the power supply voltage Vin decreases from a maximum voltage, the current path is sequentially changed from the fourth switch to the first switch. In one example, the current control unit 250 can further include a line shape block. The line shape block detects a level of the power supply voltage Vin and controls the amount of current flowing into each of the plurality of switches, so that the detected current flow responds to a change in the power supply voltage Vin. For example, the line shape block can detect a level of the power supply voltage Vin. The line shape block can calculate a difference in voltage between the power supply voltage Vin and a signal output by the current measuring unit 240, and can add this difference to the control signal generated by the current control unit 250 to control the amount of current flowing into each of the plurality of switches.For example, if the majority of the switches are implemented as MOSFETs, and the linear block is controlled such that the control signal applied to the MOSFETs increases in accordance with a level of the power supply voltage Vin, a maximum value of the current flowing into the MOSFET is reached, and the current measuring unit 240 can detect the common current IC which changes in response to a change in the power supply voltage Vin. Fig. 6 is a curve diagram illustrating the operation of an example LED driver circuit which has a linear shape block. Figure 6(a) shows a curve from an LED driver circuit without a line shape block. The x-axis and y-axis of the curve each represent a time and a level of a power supply voltage Vin or a magnitude of the common current IC. As stated above, the power supply voltage Vin corresponds to a pulsation voltage and the common current Ic corresponds to a stepped curve, which changes when the power supply voltage Vin is more than a certain voltage (for example, a threshold voltage of LEDs). In Fig. 6 (b) an LED driver circuit with a linear shape block is represented and the common current Ic is varied with a slope per certain section in response to a change in the power supply voltage Vin. The LED driver circuit 130 can increase a current range during a single period (that is, an average current) to improve power efficiency and luminous efficiency. In one example, the current control unit 250 can also include an output control unit. The output control unit measures a maximum level of the power supply voltage Vin in order to reduce the amount of current flowing into each of the majority of the switches to a level above the reference level. For example, the output control unit can measure a maximum level of the power supply voltage Vinmessen, a ratio above a predetermined reference level, and reduce a control signal generated by the current control unit 250 to that ratio in order to control the amount of current flowing into the majority of the switches. In response to the fact that an LED driver circuit has a reference level of the power supply voltage Vinhat, which corresponds to a value of 220 [Vrms], and a maximum level of the power supply voltage Vinhat, which corresponds to a value of 242 [Vrms], the output control unit can measure a maximum level of the power supply voltage Vinvon of the power supply unit 110, calculate a ratio above 220 [V] (i.e. the reference level) as 10%, and reduce the amount of current flowing into each of the plurality of switches by the ratio of 10%, compared to the amount of a conventional current (a current that flows when a reference level of the power supply voltage Vinan is applied to the LED circuit). Fig. 7 is a curve diagram illustrating the operation of an example LED driver circuit which has an output control unit. In Fig. 7 (a), the power supply voltages Vin1 and Vin2 applied to the LED driver circuit are represented in a curve. An x-axis and a y-axis of the curve each indicate a time and a level of a power supply voltage Vin or a magnitude of the common current Ic. A reference power supply voltage Vin1 and a real power supply voltage Vin2 are represented in Fig. 7 (a). The level of the real power supply voltage Vin2 is higher than that of the reference power supply voltage Vin1. In Fig. 7 (b) a common reference current IC1 and a real common current IC2 are represented in response to the reference power supply voltage Vin1 and the real power supply voltage Vin2. In an LED driver circuit without an output control unit, the common reference current IC1 and the actual common current IC2 are equal. However, as described above, the actual power supply voltage Vin2 reaches a certain voltage (for example, a threshold voltage of LEDs) more quickly, and the actual common current IC2 flows for a longer time per section compared to the common reference current IC1. In an LED driver circuit with an output control unit, the magnitude of the actual common current IC2 can be reduced by a slope of the calculated ratio. The LED driver circuit 130 can maintain a constant current range (average current) during a single period, instead of varying the power supply voltage Vin, to maintain a constant LED brightness. In one example, the LED driver circuit 130 can still have a driver power unit. The driver power unit is coupled to the power supply unit 110 and provides a power supply voltage for the operation of the LED driver circuit 130. For example, the driver power can be implemented as a JFET (Junction Gate Field Effect Transistor). Several examples described above relate to an LED driver circuit that allows for easier integration into a lighting device. For example, the LED driver circuit may not require a logic circuit to determine a current path according to the level of an AC power supply. The described technology can have the following effects. However, this does not mean that a particular example should exhibit all or only the following effects, nor should it be interpreted as meaning that the scope of a claim for the described technology is not limited to the following effects. Rather, the scope of a claim is determined by the language or wording of the claim. Several examples, described above, can capture a current from a common node coupled to groups of LEDs in order to determine a current path of the LED groups, which can facilitate integration into a lighting device. Several examples, described above, can determine a current path based on the magnitude of a current variation at a common node, thereby eliminating the need for a logic circuit to detect voltage in groups of LEDs. While this disclosure provides certain examples, it will be obvious to those skilled in the art that various modifications in form and detail can be made to these examples without altering the intent and scope of the claims and their equivalents. The examples described herein must be regarded as merely descriptive and not for limitation purposes. Descriptions of features and aspects in each example must be regarded as applicable to similar features or aspects in other examples.Suitable results can be achieved if the described techniques are carried out in a different sequence and / or if components in a described system, architecture, device, or circuit are combined and / or replaced or supplemented in a different manner by other components or their equivalents. Accordingly, the scope of the disclosure is not defined by the detailed description but by the claims and their equivalents, and all variations within the scope of the claims and their equivalents are to be considered as included in the disclosure.

Claims

Light-emitting diodes, LED, driver circuit (130) configured to sequentially drive a plurality of series-coupled LED groups (LED1, ..., LED4) which have at least one LED, wherein the LED driver circuit (130) comprises: a plurality of middle nodes (210) each connected to an output of the plurality of LED groups (LED1, ..., LED4); a plurality of switches, each configured to form a current path between a corresponding middle node (210) and a common node (220), the common node (220) being connected to ground (GND); a detection resistor (Rcs) having a first terminal directly connected to the common node (220) and ground; a current sensing unit (240) connected to a second terminal of the detection resistor (Rcs) and configured to detect current flow through the common node (220) based on a voltage drop across the detection resistor (Rcs); and a current control unit (250) configured to generate a control signal to control each of the plurality of switches based on the detected current flow. LED driver circuit according to claim 1, wherein the current flow of the common node (220) corresponds to a sum of currents flowing through the majority of the current paths. LED driver circuit according to claim 1, wherein the detection resistor (Rcs) is coupled to the common node (220) to form a feedback loop; and wherein the current measuring unit (240) is configured to detect an amount of current flowing out of the common node (220) based on a voltage across both sides of the detection resistor (Rcs). LED driver circuit according to claim 3, wherein the detection resistor (Rcs) is placed outside the LED driver circuit (130). LED driver circuit according to claim 1, wherein the current control unit (250) is configured to differentially amplify a reference voltage, which is set for each of the plurality of switches, and the detected current flow in order to control a corresponding switch. LED driver circuit according to claim 5, wherein the set reference voltage increases in response to an increase in a distance between an AC power supply and a central node (210) to which a corresponding switch is coupled. LED driver circuit according to claim 1, wherein the current control unit (250) is configured to turn off a switch in the selected current path in response to an increase in current flow in order to refresh a current path. LED driver circuit according to claim 1, wherein the current flow increases in response to an increase in distance between an AC power supply and the selected current path. LED driver circuit according to claim 1, wherein the current control unit (250) has a linear block configured to measure a level of an AC power supply and to control an amount of current flowing in each of the plurality of switches, such that the detected current flow responds to a change in the AC power supply. LED driver circuit according to claim 1, wherein the current control unit (250) has an output control unit configured to measure a maximum level of an AC power supply in order to reduce an amount of current flowing into each of the plurality of switches to a ratio above a reference level. Lighting device comprising: a rectifier unit configured to rectify an AC voltage half-wave; a light-emitting unit (120) comprising a plurality of series-coupled LED groups (LED1, ..., LED4), each of which comprises at least one LED; and an LED driver circuit (130) configured to sequentially drive the plurality of LED groups (LED1, ..., LED4), wherein the LED driver circuit (130) comprises: a plurality of middle nodes (210), each of which is connected to an output of the plurality of LED groups (LED1, ..., LED4); a plurality of switches, each configured to form a current path between a corresponding middle node (210) and a common node (220), the common node (220) being connected to ground (GND); a detection resistor (Rcs) having a first terminal directly connected to the common node (220) and ground; a current sensing unit (240) connected to a second terminal of the detection resistor (Rcs) and configured to detect current flow through the common node (220) based on a voltage drop across the detection resistor (Rcs); and a current control unit (250) configured to generate a control signal to control each of the plurality of switches based on the detected current flow. Method for driving a plurality of series-coupled light-emitting diodes, LED groups, each of which has at least one LED connected to a corresponding central node, the method comprising: measuring a current flow through a common node (220) of a driver circuit (130) using a current measuring unit (240), based on a voltage drop across a detection resistor (Rcs), wherein the driver circuit (130) comprises the common node (220), which is connected to a ground (GND) and to a first terminal of the detection resistor (Rcs), a plurality of switches, and the current measuring unit (240), which is connected to a second terminal of the detection resistor (Rcs); providing a control signal to each switch of the plurality of switches;and forming a current path between the corresponding middle node and the common node (220), which is directly connected to the first terminal of the detection resistor (Rcs), in response to the control signal.

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