LED driver and lighting system using the same

Abstract

An LED driver includes a current sensing resistor arrangement having two parallel current sensing branches, each including at least two series resistors connected at a respective sensing node. The voltages at the two sensing nodes are processed to detect component failures. Normal current control is based on the voltage across the resistor arrangement.

Description

Technical Field

[0001] The present invention relates to an LED driver. Background Art

[0002] Controllable LED drivers can be used to vary the voltage and / or current supplied to a load in the form of an LED device. Control of such LED drivers is based, for example, on a feedback loop that reports the current or voltage at the LED device. The present invention particularly relates to LED drivers that utilize a current feedback signal representative of the current flowing through the LED device.

[0003] Figure 1A The LED driver is shown in schematic form and comprises a power converter 10 which receives a power supply Vsup and delivers power to a load in the form of an LED device 12. For simplicity, the LED device is shown as a single diode.

[0004] The current flowing through the LED device 12 is sensed by the current sensing circuit 13. A voltage representing the current flow is generated, and this voltage is provided as a feedback signal to the controller 14. The controller 14 controls the power converter 10 to deliver the target current to the LED device 12. The power converter 10 and the controller 14 can be considered together as a "driver unit."

[0005] The controller 14 may be implemented as a control IC, a control circuit composed of discrete components or a digital microprocessor, or a combination thereof.

[0006] Typically, for reasons of cost and simplicity, current sensing circuit 13 is a resistor connected in series with the LED string, preferably between the LED string and reference potential GND. Often, multiple small resistors are connected in series or parallel to distribute sensing losses and enable the use of inexpensive components. A layout using multiple small components is sometimes easier than using one large one.

[0007] In different markets, LED drivers must comply with different regulations, such as Underwriters Laboratories (UL) rules in the U.S. These include single-failure safety requirements, which means that if a single component fails (open or short), the driver remains safe.

[0008] A shorted current-sense resistor can cause the LED driver to lose control because the sense voltage, representing the current magnitude, is no longer proportional to the current and is instead permanently zero. Consequently, without further protection, the LED driver will deliver very high currents, which can easily exceed defined limits.

[0009] It is known to detect a faulty sensing resistor by adding another resistor between the reference potential GND and the negative power rail. This results in two low-side current sensing circuits connected to the positive and negative sides of the GND potential. In this way, the current through the LEDs of the LED arrangement is sensed twice. This allows detecting a short circuit in one current sensing resistor by comparing the two signals.

[0010] The disadvantage of this approach is that the LED current flows through both sense resistor combinations, and assuming that both sense resistor combinations have the same value, the losses are therefore doubled. One of the sense signals is negative with respect to GND, and therefore additional effort is required to process the signal.

[0011] Therefore, there is a need for an improved current sensing apparatus that detects individual component failures. Summary of the Invention

[0012] The invention is defined by the claims.

[0013] According to an example of one aspect of the present invention, there is provided an LED driver, comprising:

[0014] a driver unit for delivering current to the LED device; and

[0015] a resistor arrangement for sensing the delivered current and providing a feedback signal to the driver unit, wherein the feedback signal comprises a voltage across the resistor arrangement,

[0016] The resistor device includes:

[0017] a first current sensing branch comprising at least two series resistors connected at a first sensing node; and

[0018] a second current sensing branch, connected in parallel with the first current sensing branch, comprising at least two series resistors connected at a second sensing node;

[0019] The LED driver further includes a fault detection unit configured to process voltages at the first sensing node and the second sensing node to determine component failure.

[0020] This LED driver uses a resistor arrangement with two current sensing branches in parallel (serving as a current sensing circuit). Each branch has an output node (sense node) at which a voltage representing the current flow is delivered. By monitoring both branches, a current sensing resistor failure (open or short) can be detected by comparing the two voltages. The voltage difference between the two sense node voltages will only be significant (i.e., above a threshold) if a component fails. In normal operation, the voltage difference is small (there will always be a small deviation depending on component tolerances). The total resistance of the two parallel branches can be the same as that of a single resistor (if they both have the same value), and therefore does not increase losses compared to a solution with a single sense resistor.

[0021] The voltage across the entire resistor arrangement (ie, the combination of the two branches) is used as the current sensing feedback signal for current regulation in the absence of component failures.

[0022] The fault detection unit comprises, for example, an operational amplifier circuit, which is a low-cost and low-power solution.

[0023] The operational amplifier circuit comprises, for example, a comparator circuit.Thus, the difference in the sense node voltage is processed so as to be able to indicate a fault.

[0024] The operational amplifier circuit includes, for example, an amplifier having an inverting input and a non-inverting input, which are connected to the first sensing node and the second sensing node through input resistors, respectively.

[0025] The operational amplifier circuit, for example, includes a negative feedback resistor (to the inverting input) and a biasing device connected to the non-inverting input for providing a positive voltage bias. The negative feedback resistor (in combination with the inverting input resistor) provides appropriate gain for amplifying the difference to generate a fault detection signal.

[0026] The fault detection unit may also include a window comparator, to which the output of the operational amplifier circuit is supplied. Thus, a component fault may be detected simply by determining whether the output signal of the operational amplifier circuit falls within or outside a signal range. The window comparator may be implemented as a transistor circuit.

[0027] A processor may alternatively be provided, to which the output of the operational amplifier circuit is provided.The processor and operational amplifier circuit may, for example, be integrated into a controller IC.

[0028] The LED driver is preferably adapted to enter a safe mode when a component failure is detected. For example, this involves stopping the supply of current to the LED device.

[0029] The present invention also provides a lighting circuit, comprising:

[0030] An LED driver as defined above; and

[0031] An LED device (e.g., an LED string) is connected to the output of an LED driver.

[0032] The present invention also provides a method for controlling an LED driver, comprising:

[0033] delivering current to the LED device;

[0034] sensing the delivered current using the resistor arrangement and providing a feedback signal comprising a voltage across the resistor arrangement;

[0035] processing voltages at a first sensing node and a second sensing node of the resistor arrangement to determine a component failure, wherein the first sensing node is a connection between at least two series resistors of a first current sensing branch and the second sensing node is a connection between at least two series resistors of a parallel second current sensing branch; and

[0036] When a component failure is detected, the LED driver is placed in a safe mode.

[0037] Safe mode involves the LED driver stopping delivering current to the LED device to prevent further damage to the driver and, for example, eliminate the risk of fire.If the LED driver is part of a connected system, the fault condition may also be reported to a higher level control system.

[0038] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which:

[0040] Figure 1A An exemplary prior art topology of a known LED driver is shown;

[0041] Figure 1B An example of an LED driver according to the present invention is shown;

[0042] Figure 2 shows the basic configuration of a current sensing resistor;

[0043] Figure 3 It is known to detect a faulty sensing resistor by adding a second resistor between the reference potential and the negative supply rail;

[0044] Figure 4 shows an example of a current sensing circuit according to the present invention; and

[0045] Figure 5 An example of a comparator unit for processing the output of a current sensing circuit is shown. DETAILED DESCRIPTION

[0046] The present invention will be described with reference to the accompanying drawings.

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

[0048] The present invention provides an LED driver having a current sensing resistor arrangement with two parallel current sensing branches, each comprising at least two series resistors connected at a respective sensing node. The voltages at the two sensing nodes are processed to detect component failures. Normal current control is based on the voltage across the entire resistor arrangement.

[0049] Figure 1B An example of an LED driver according to the present invention is shown. The current sensing circuit 13 provides a signal to a fault detection unit 15 which then enables a fault detection signal "Fault" to be generated. This signal indicates a fault in a resistor of the current sensing circuit.

[0050] Figure 2 A basic known configuration for a current sensing resistor is shown. In this example, the current sensor resistor comprises a bank 30 of four resistors connected in parallel, wherein the bank is connected in series with an LED device 32. The voltage across the bank is provided to the current sense ("CS") input of the controller 14, and the bank is connected between the current sense input CS and ground GND. VCC is the low voltage supply of the controller 14, such as 5V or 3.3V, and GND is ground, although any reference potential may be used.

[0051] The use of a resistor bank spreads the sensing losses and enables the use of low-cost components.As mentioned above, the controller 14 may be implemented as a control IC, a control circuit composed of discrete components or a digital microprocessor, or a combination thereof.

[0052] like Figure 3As shown in FIG, it is known to detect a faulty sensing resistor (within resistor group 30a) by adding a second resistor (in this example, a second resistor group 30b) between the reference potential GND and the negative power supply Vsup-. There are then two low-side current sensing circuits 30a, 30b connected to the positive and negative sides of the GND potential. In this arrangement, the current through the LEDs of the LED device is sensed twice. The first current sense signal is provided to the positive current sense input CS, and the second current sense signal is provided to the negative current sense input NCS. This allows a short circuit of a current sense resistor to be detected by comparing the two signals.

[0053] The disadvantages of this approach are first that the LED current flows through both sense resistors combined (as shown) and therefore the losses are doubled (assuming the resistor values are the same). Second, the NCS signal is negative with respect to ground (or other reference used), which requires additional effort to process the signal.

[0054] Figure 4 An example of a current sensing circuit 42 and a fault detection unit 15 according to the present invention is shown. The fault detection unit 15 includes an amplification circuit 44 and a comparator unit 45.

[0055] As explained above, the current sensing circuit is intended for use within an LED driver, where a power converter delivers current to an LED device 32. The LED device can be any combination of multiple LEDs (both in series and in parallel). For simplicity, only a single LED is shown. Controller 14 is used to control power converter 10, and controller 14 and the power converter can be considered a driver unit. Resistor device 42 senses the delivered current and provides a feedback signal CS to the driver unit, specifically controller 14. This feedback signal CS is based on the voltage across the resistor device.

[0056] A first current sensing branch of the resistor arrangement includes two series resistors R1, R2 connected at a first sensing node X. A second current sensing branch is connected in parallel with the first current sensing branch and includes two series resistors R3, R4 connected at a second sensing node Y. The two branches together form a resistor arrangement 42. The fault detection unit 15 processes the voltages at the first and second sensing nodes X, Y to detect component faults.

[0057] Figure 4 The current sensing circuit 42 in the device thus uses four resistors, two in series and two of these series connections in parallel. With this arrangement, the total resistance is equal to the value of a single resistor, such as 50 mΩ. Therefore, the same current sensing feedback signal CS is generated as with a single current sensing resistor, with the same losses.

[0058] However, there may be more than four resistors, as each branch may have more than two resistors in series, or there may be sub-branches in parallel within each branch.

[0059] As explained above, the sense voltage used for current control in the LED driver is the voltage across the entire current sensing circuit, ie, the voltage to the current sensing node CS.

[0060] The amplifier unit 44 of the fault detection unit 15 amplifies the voltage difference between the sense nodes X and Y to detect whether one of the resistance values deviates significantly from its expected value (in the extreme case, an open circuit or a short circuit). The component tolerance of the sense resistor will also result in a small voltage difference, but this difference is small compared to the difference caused by the fault condition.

[0061] If all of the resistors R1 to R4 are of similar values (and much smaller than the values of the resistors used in amplifier unit 44), the voltage difference between sense nodes X and Y is very small. In particular, amplifier unit 44 draws almost no current (relative to the LED current through resistors R1 to R4) and therefore has a negligible effect on the voltages at sense nodes X and Y.

[0062] If one of the resistors fails due to an open circuit or short circuit, the voltage difference between the sense nodes X and Y will increase. This is used to detect component failure and the LED driver can be shut down to enter a safe operating mode without exceeding current and / or voltage limits or risking fire.

[0063] exist Figure 4 , the sense nodes X and Y are connected to an amplifier unit 44 in the form of a simple operational amplifier circuit. This is advantageous because the total current sense voltage (measured at CS) is small (e.g. 100 mV) and the maximum voltage difference in the event of a single faulty component (open or short) is even smaller (such as half the sense voltage and thus a maximum of about 50 mV).

[0064] Figure 4The operational amplifier circuit shown in FIG4 includes an operational amplifier 46 having an inverting input and a non-inverting input connected to a first sense node X and a second sense node Y, respectively, via input resistors R5 and R6. Negative feedback resistor R7 sets the gain of the operational amplifier circuit. Specifically, the operational amplifier circuit has a gain in the range of, for example, 20 to 100. The appropriate gain depends on the design of the sense resistor and the current flowing through the LED (and the sense resistor). The tolerance of the sense resistor also plays a role. For example, assuming a 2A LED current and a 50mΩ sense resistor with a 2% tolerance, X might be 49mV and Y might be 51mV (this is the worst-case difference). Amplifying this 2mV difference by a gain of 100 will result in a 200mV change at point Z. A fault condition will result in 50mV being amplified 100 times, or 5V. Therefore, a fault condition can be easily distinguished from tolerance variations in the circuit.

[0065] Biasing devices R8 and R9 are connected to the non-inverting input of op amp 46 to provide a positive voltage bias. The biasing device comprises a resistor divider between the controller's (low voltage) supply, VCC, and ground, GND. This boosts the output voltage by, for example, VCC / 2, thus avoiding the need for a dual-supply solution (with its attendant additional cost and components).

[0066] The amplified output Z may then be provided to a window comparator, forming the comparator unit 45 of the fault detection unit 15. Once the amplified voltage leaves a predefined allowed voltage window around VCC / 2 (offset voltage), the comparator in the window comparator triggers the LED driver to enter a safe state.

[0067] In an alternative arrangement, the amplifier output, Z, can be compared to the low-voltage supply rails (VCC and GND) to trigger the LED driver into a safe state.

[0068] Figure 5 An example of a comparator cell 45 based on a transistor circuit for realizing a window comparator is shown.

[0069] The output Z from the amplifier unit 44 of the fault detection unit 15 is supplied to a transistor circuit, which functions as a simple window comparator.

[0070] The first circuit portion 50 is used to generate a high output "Fault" when the signal Z rises above the upper limit of the window, and the second circuit portion 60 is used to generate a high output "Fault" when the signal Z falls below the lower limit of the window. When the signal Z is within the window, the output "Fault" is pulled low by the pull-down resistor R13.

[0071] There are two fault conditions, depending on which sense resistor fails and whether it fails due to a short circuit or an open circuit. In one fault condition, the voltage difference between X and Y is large and positive, or the voltage difference between X and Y is large and negative. The first case results in a large negative voltage at Z (for example, the inverting op amp 46 will saturate and its output will be almost GND), and the second case results in a large positive voltage at Z (for example, the inverting op amp 46 will saturate and its output will be almost VCC).

[0072] If the voltage at Z approaches VCC (greater than the base-emitter voltage of T2), T2 stops conducting and no base current flows to transistor T3 through Rb3. The combination of R12 and D2 pulls the output "Fault" high, indicating a fault condition. In this condition, T1 remains on and no current flows through D1.

[0073] Likewise, if the voltage at Z approaches GND level (below the base-emitter voltage of T1), T1 stops conducting and current can flow through R10 and D1, pulling the output "fault" high. In this case, T2 and T3 remain on, and no current flows through D2.

[0074] In normal operation, the voltage difference between X and Y is small, and therefore the output Z of the amplifier unit 44 of the fault detection unit 15 is also equal to the offset (VCC / 2) plus the small amplified difference voltage. Then both transistors T2 and T1 remain on, and the fault output is held low by the pull-down resistor R13 at the output.

[0075] The circuit can therefore be implemented using low cost transistors, forming a transistor window comparator circuit.The gain of amplifier unit 44 can be selected to trigger the pull-up and pull-down transistor circuits at an appropriate voltage difference between sense nodes X and Y.

[0076] In another example, the output Z can be connected to an analog input of a microprocessor. The microprocessor can then use software to decide to put the driver into a safe operating mode based on the actual amplified differential voltage value.

[0077] In another example, the fault detection unit 15 (i.e., amplifier unit 44 and comparator unit 45) is embedded in the LED driver's IC controller. This significantly reduces cost, component count, and board space, while improving robustness. Only two additional pins are required to connect to the X and Y sense node signals. The current sense resistor can also be located within the IC controller.

[0078] The present invention can be applied to any driver architecture that uses current sensing to provide feedback control. The driver can use a switch-mode power converter. For example, the driver then has a switching stage (such as a two-transistor inverter stage) and the switching duty cycle and / or frequency are controlled depending on the feedback signal. The power converter can be a buck converter, a boost converter, or a flyback converter. The present invention can be applied to resonant converters or non-resonant converters. The present invention can be applied to isolated or non-isolated drivers. In addition, the present invention can be applied to linear drivers that do not use high-frequency switching.

[0079] Therefore, the present invention is applicable to various drivers using a resistive current sensing circuit.

[0080] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention by studying 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 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

1. An LED driver, comprising: a driver unit (10, 14) configured to deliver current to the LED device; as well as a resistor arrangement (42) configured to sense the delivered current and provide a feedback signal to the driver unit, wherein the feedback signal comprises a voltage across the resistor arrangement, wherein the resistor device comprises: a first current sensing branch comprising at least two series resistors (R1, R2) connected at a first sensing node; as well as a second current sensing branch connected in parallel with the first current sensing branch, the second current sensing branch comprising at least two series resistors (R3, R4) connected at a second sensing node, wherein the first sensing node is a connection between the at least two series resistors (R1, R2) of the first current sensing branch, and the second sensing node is a connection between the at least two series resistors (R3, R4) of the second current sensing branch, The LED driver further comprises a fault detection unit (15) configured to process the voltages at the first sensing node and the second sensing node to determine component failure.

2. The LED driver according to claim 1, wherein the fault detection unit (15) comprises an operational amplifier circuit (44).

3. The LED driver according to claim 2, wherein the operational amplifier circuit (44) includes an amplifier (46) having an inverting input and a non-inverting input, the inverting input and the non-inverting input being connected to the first sensing node and the second sensing node respectively through input resistors (R5, R6).

4. The LED driver according to claim 3, wherein the operational amplifier circuit (44) includes a negative feedback resistor (R7) and a biasing device (R8, R9), the biasing device (R8, R9) being connected to the non-inverting input for providing a positive voltage bias.

5. The LED driver according to any one of claims 3 to 4, wherein the fault detection unit (15) further comprises a window comparator (45), and the output of the operational amplifier circuit (44) is provided to the window comparator.

6. The LED driver according to claim 5, wherein the window comparator (45) comprises a transistor circuit (50, 60).

7. The LED driver according to any one of claims 3 to 4, wherein the fault detection unit (15) further comprises a processor, and the output of the operational amplifier circuit (44) is provided to the processor.

8. The LED driver according to claim 7, wherein the processor and the operational amplifier circuit are integrated into a controller IC.

9. The LED driver according to any one of claims 1 to 4, wherein the LED driver is adapted to enter a safe mode when a component failure is detected.

10. A lighting circuit comprising: The LED driver according to any one of claims 1 to 9; as well as An LED device is connected to an output of the LED driver.

11. A method for controlling an LED driver, comprising: delivering current to the LED device; sensing the delivered current by using a resistor arrangement and providing a feedback signal comprising a voltage across the resistor arrangement; processing voltages at a first sensing node and a second sensing node of the resistor arrangement to determine a component failure, wherein the first sensing node is a connection between at least two series resistors (R1, R2) of a first current sensing branch and the second sensing node is a connection between at least two series resistors (R3, R4) of a second current sensing branch, wherein the second current sensing branch is coupled in parallel with the first current sensing branch; as well as When a component failure is detected, the drive is placed into a safe mode.

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