Current Driver and Driving Method

Abstract

An LED circuit includes a current drive circuit for driving current through a parallel combination of an LED device and an output capacitor. A rewrite device rewrites the current level setting to a default current level during startup, and the rewrite device is disabled when a sensor senses current flow. The current setting of the driver is ignored until a threshold current through the LED device is sensed. Thereby, the delay associated with the initial startup charging of the output capacitor is avoided.

Description

Technical Field

[0001] The present invention relates to a current driver circuit for driving an LED lighting load, for example. Background Art

[0002] To minimize cost, it is desirable to use a single-stage high power factor LED driver. High power factor relies on the presence of an output capacitor to reduce the amount of mains frequency current ripple.

[0003] To control the average output current for driving an LED, a feedback circuit is typically used that measures the combined output capacitor and LED current. This applies to both single-stage switch-mode power supplies and linear drivers.

[0004] A switch-mode power supply incorporates a switch regulator to efficiently convert the electrical power for transfer to a DC load. The switch regulator continuously switches between a fully-on state and a fully-off state, which minimizes wasted energy. Voltage or current regulation is achieved by changing the duty cycle of the switch regulator. In contrast, a linear driver regulates the output voltage or current by continuously dissipating power. Thus, a linear driver has lower power efficiency, but unlike a switch-mode power supply, a linear driver does not contain high-frequency switching elements that degrade the electromagnetic interference (EMI) performance of the driver.

[0005] The value of the output capacitor (usually an electrolytic capacitor) depends on the required amount of ripple reduction and the dynamic resistance of the LED. The more reduction required, the larger the capacitor needs to be.

[0006] A large output capacitor takes a long time to charge when powered on. Especially if the required output current is set to a low dimming value (e.g., 2% of full current), the charging time of the output capacitor may take up to several seconds before any light is produced.

[0007] It would be desirable to reduce this time delay without significantly increasing the complexity of the driver circuit. Summary of the Invention

[0008] The present invention is defined by the claims.

[0009] According to an example of one aspect of the present invention, there is provided an LED circuit comprising:

[0010] A current drive circuit;

[0011] An LED device;

[0012] An output capacitor in parallel with the LED device, wherein the current drive circuit is adapted to drive current through the parallel combination of the LED device and the output capacitor;

[0013] A control signal for setting the current level delivered by a current drive circuit;

[0014] A sensor for sensing when current flows through an LED device; and

[0015] A rewrite device for rewriting the current level setting of the control signal to a default current level,

[0016] wherein the LED circuit is adapted to deactivate the rewrite device in response to sensing by the sensor.

[0017] The LED circuit has a rewrite device that ignores the current setting of the driver until a threshold current through the LED device is sensed. In this way, while the parallel output capacitor is charging, the current level of the current drive circuit can be set to a high level. As soon as a small current flows through the LED device itself, the current drive circuit can return to the desired current level, thus avoiding over-driving the LED device or producing a flash. However, the delay associated with the initial startup charging of the output capacitor (no light output, or light output below the minimum level corresponding to maximum dimming) is avoided.

[0018] The sensing of when current is flowing can be based on sensing the actual current, or on sensing the light output from an optocoupler or from the LED device itself that is produced by the flowing current.

[0019] Sensing when current is flowing can be based on the detection of a threshold current or a corresponding amount of light output.

[0020] The default current level is, for example, the maximum current level setting. Thus, the output capacitor is charged as quickly as possible.

[0021] For example, the sensor includes a current sensing resistor in series with the LED device. This provides a simple way to separately monitor the LED current. The resistor can be external to the main driver IC, but equally, it can be incorporated within the driver IC.

[0022] A deactivation switch can be provided for deactivating the rewrite device, wherein the control terminal voltage of the switch is set by the sensor (such as the voltage across the current sensing resistor). Thus, the deactivation switch conducts and turns off according to the flowing LED current. In one example, the deactivation switch is off during startup. When sufficient current flows so that the deactivation switch conducts, the rewrite function is deactivated, and normal current control resumes.

[0023] The capacitor is preferably in parallel with the current sensing resistor. This stores the gate terminal voltage.

[0024] The control signal preferably has a pulse-width modulation profile, where the duty cycle of the pulse-width modulation profile defines the current level, and the rewriting device is used to implement an "OR" function between the control signal and the rewriting signal before being applied to the current drive circuit.

[0025] The rewriting signal then means that the result of the "OR" function is such that the current drive circuit is driven to its maximum level. The rewriting signal is, for example, a DC signal that is at or above the high voltage level of the PWM signal.

[0026] In one example, the rewriting device includes a circuit having a pull-up transistor that is used to pull up the control signal to a default voltage (e.g., equal to or higher than the PWM high voltage) when turned on, and to isolate the default voltage from the control signal when turned off.

[0027] Thus, the default voltage rewrites the normal current control signal.

[0028] The disabling switch turns off the pull-up transistor when turned on. This allows the control signal to operate without being rewritten.

[0029] The current drive circuit includes, for example, a linear current source. This provides a low-cost implementation.

[0030] The circuit preferably has a mains input and a rectifier, where the rectifier output is provided to the current drive circuit, the LED device, and the output capacitor.

[0031] The present invention also provides an LED driving method, which includes:

[0032] Receiving a current level setting;

[0033] During startup, rewriting the current level setting to a default current level;

[0034] Driving the default current level through a parallel combination of the LED device and the output capacitor;

[0035] Sensing when current is flowing through the LED device;

[0036] Responsive to the sensing, disabling the rewriting function; and

[0037] After disabling, driving the received current level setting through a parallel combination of the LED device and the output capacitor.

[0038] This method avoids the initial delay associated with the charging of the output capacitor during circuit startup.

[0039] For example, the default current level is the maximum current level setting.

[0040] For example, the sense current includes: deriving a voltage across a current sense resistor in series with the LED device.

[0041] For example, the rewrite includes: implementing an "OR" function between the current level setting and the rewrite signal. The rewrite signal causes the default current level to be set.

[0042] The rewrite may include: pulling up the control signal, which limits the current level setting to the default voltage. For example, when the rewrite is disabled, the control signal has a pulse width modulation profile, where the duty cycle of the pulse width modulation profile defines the current level.

[0043] These and other aspects of the invention will be apparent and elucidated by reference to the (one or more) embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] To better understand the present invention and to more clearly show how the present invention may be implemented, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0045] Figure 1 An example of an LED circuit according to the present invention is shown; and

[0046] Figure 2 A flowchart of an LED driving method is shown. DETAILED DESCRIPTION

[0047] The present invention will be described with reference to the accompanying drawings.

[0048] It should be understood that while exemplary embodiments of the apparatus, system, and method are indicated, the detailed description and specific examples are for illustrative purposes only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, system, and method of the present invention will become better understood from the following description, the appended claims, and the 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 components.

[0049] The present invention provides an LED circuit that includes a current drive circuit for driving current through a parallel combination of an LED device and an output capacitor. A rewrite device rewrites the current level setting to a default current level during startup and is disabled when current flow is sensed (directly or indirectly) by a sensor. The current setting of the driver is ignored until a threshold current through the LED device is sensed. Thereby, the delay associated with the initial startup charging of the output capacitor is avoided.

[0050] Figure 1 An example of an LED circuit 10 according to the present invention is shown.

[0051] Circuit 10 includes a mains input represented by voltage source V1. Resistor R1 is provided downstream of the input and is a surge current limiting resistor that also serves as a fuse.

[0052] The mains input is connected to a full bridge rectifier of diodes D1 to D4. The rectifier output is the bus (or line) voltage VBUS.

[0053] The rectifier output is also provided to a series circuit that includes a current drive circuit B1 and a parallel combination of an LED device D10 and an output capacitor C2. Capacitor C2 is a large (e.g., 100 uF) electrolytic capacitor for smoothing the rectified output. The current drive circuit is adapted to drive current through the parallel combination of the LED device D10 and the output capacitor C2. The LED device can be a series arrangement of LEDs or, in fact, multiple parallel branches of LEDs.

[0054] The control signal PWM is used to set the current level delivered by the current drive circuit B1.

[0055] The average current of the current source B1 is regulated by the PWM signal. Depending on the implementation of the current source, the current shape is a constant current or a shaped current waveform to limit the losses in the current source B1. A high voltage across B1 will result in a lower instantaneous current setting for B1 while maintaining the average value at a preset level.

[0056] Zener diode D8 is placed in parallel with the current source to absorb the high voltage between two transistors Q1 and Q2 (discussed further below). This potentially allows the use of a lower rated voltage (Vce) for transistor Q1 and thus lower cost.

[0057] The current through the LED device D10 is measured using a current sensor, specifically a current sensing resistor R3. The current sensor R3 is placed in series with the LED device D10, and the series arrangement of the current sensor R3 and the LED device D10 is placed in parallel with the output capacitor C2. The current sensor R3 can be placed such that it senses only the current flowing through the LED device D10.

[0058] Based on the current flowing through the LED device D10, a user-defined current level can be achieved through the current drive circuit B1, for example, to achieve a user-selected dimming level setting, or otherwise the setting can be overridden to allow more current to flow and thus charge the output capacitor C2 faster. This override occurs during circuit startup.

[0059] For this purpose, there is a rewriting device 20 for rewriting the current level setting to a default current level. The rewriting device 20 forces the current drive circuit B1 to deliver a default (e.g., maximum) current (thereby rewriting the user setting of the current level), or else it allows the use of the user current setting.

[0060] A setting disable switch Q1 is provided to disable the rewriting device 20 when a threshold current is sensed by the current sensor R3. Thus, the rewriting device 20 is active until sufficient current flows through the LED device.

[0061] The LED circuit D10 thus has a rewriting device 20 that ignores the current setting of the driver until a threshold current through the LED device D10 is sensed. In this way, the current level of the current drive circuit B1 can be set to a high level while the parallel output capacitor C2 is charging. As soon as a small current flows through the LED device D10 itself, the current drive circuit B1 reverts to the user-selected current level, thus avoiding over-driving the LED device D10 or producing a flash. However, the delay associated with the initial start-up charging of the output capacitor C2 is minimized to approximately the same time as the delay in the start-up of full light output.

[0062] For example, the threshold current is one-fifth to one-twentieth of the minimum dimming level (2%). This means that the threshold current can be between 0.1% (= 0.02 / 20) and 0.4% (0.02 / 5) of the full output current.

[0063] This will result in a relatively high ohmic resistor R3, but once Q1 is fully conducting, the total voltage drop across the resistor R3 will never exceed the base-emitter voltage Vbe of Q1.

[0064] The expected voltage across the resistor R3 will drop by approximately 0.7V, and the current will mainly flow through the emitter-base diode of the transistor Q1. This will minimize the losses in the current measurement circuit R3.

[0065] More generally, the transistor Q1 is a disable switch. The control gate (base) terminal voltage is set by the voltage across the current sensing resistor R3. In the example shown, it is off during start-up. As the current increases, the base voltage is pulled down (by increasing the voltage drop across R3) until at a certain current, the pnp transistor conducts. Then the rewriting function is disabled in the manner explained below, and normal current control resumes. A capacitor C3 in parallel with the current sensing resistor R3 stores the base voltage.

[0066] For example, as shown in the figure, the rewriting device 20 includes a circuit that receives the control signal PWM as an input. The control signal PWM is generated by a microcontroller unit (MCU) (usually wirelessly controlled). For example, the MCU can be an RF MCU that uses Zigbee, or infrared, or WiFi communication.

[0067] The source of the control signal PWM is represented as the voltage source V3 in Figure 1 . Normally, the control signal would be directly provided to the current drive circuit B1. The present invention provides an additional rewriting device.

[0068] The rewriting device 20 in the example shown has a pull-up transistor Q3 that is used to pull up the control signal PWM to the default voltage V2 (through the resistor R9) when conducting, and isolate the default voltage V2 from the control signal when off.

[0069] Therefore, the default voltage V2 rewrites the normal current control signal.

[0070] The resistor R9 is part of the resistor divider R8, R9 between the pull-up transistor Q3 and the voltage source V3.

[0071] For example, the output of the voltage source V3 can be a PWM signal between 0V and 3.3V (i.e., the voltage rail of the controller IC). V2 can be a constant voltage of 16V.

[0072] Therefore, when Q3 is off, the control signal is a PWM signal from 0V to 3.3V. When Q3 is on, the voltage divider of R8 and R9 means the control signal PWM is 3.2V (when V3 = 0) or 5.8V (when V3 = 3.3V). When applied to the current drive circuit, both of these correspond to the maximum drive current.

[0073] The current drive circuit reacts to a 3.2V input in the same way as it does to a 5.8V input.

[0074] Therefore, when the pull-up transistor is off, the control signal PWM has a pulse-width modulation distribution, and the duty cycle of the pulse-width modulation distribution defines the current level.

[0075] Initially, the deactivation switch Q1 is off. The transistor Q2 is off, and the base of Q3 is pulled high through the base resistor R7.

[0076] The deactivation switch Q1 turns off the pull-up transistor Q3 when conducting. This allows the control signal to operate without being rewritten. In particular, when Q1 is on, since current passes through Q1 and is delivered to the base through the Zener diode D11 and the resistor R5, Q2 conducts. Q2 then pulls down the base of Q3, thus turning it off.

[0077] It can be seen that the function of the rewriting device is to implement an "OR" function between the control signal and the rewriting signal (i.e., the voltage source V2 when fed through the transistor Q3). This "OR" function occurs before the current setting signal is applied to the current drive circuit.

[0078] The PWM signal can be used, for example, to set a very low current corresponding to a low dimming level of 2 - 5%. However, initially, the current drive circuit can deliver a 100% current level until a small threshold current starts to flow through the LED device.

[0079] Note that Figure 1 This is just an example of the implementation method. Some or all of the circuits in the circuit can be integrated into the current drive circuit. Current sensing can be performed inside or outside the drive IC. In the IC implementation, current sensing can also be done in different ways. For example, the analog circuit with Q2 and Q3 can be replaced by a logic circuit in which the rewriting signal forces the PWM to logic 1 in a manner similar to that explained above.

[0080] Q1 needs to have a sufficient rated voltage (in combination with D11), which involves a certain amount of cost. The object of the present invention is to stop the fast charging as soon as the current starts to flow through the LED.

[0081] Directly sensing the LED current by means of the resistor R3 is just one option. An alternative is to place the LED side of the optocoupler in series with the LED, and then the detected current can directly activate the output transistor of the optocoupler, which functions the same as Q2. This is an alternative implementation of the sensing circuit. This option uses the generation and detection of light. Another alternative is to use a photodiode or a phototransistor to implement the detection of the light from the LED device.

[0082] In these cases, the sensor for sensing the current flowing through the LED uses an optical sensor that senses the light caused by the current flow rather than directly detecting the current.

[0083] Figure 1 Some component values are shown. These values are only examples providing an order of magnitude and are not intended to be limiting in any way.

[0084] Figure 2 An LED driving method is shown, which includes:

[0085] In step 30, a current level setting is received;

[0086] In step 32, during startup, the current level setting is rewritten as a default current level;

[0087] In step 34, drive a default current level through a parallel combination of the LED device D10 and the output capacitor C2;

[0088] In step 36, sense when current flows through the LED device;

[0089] In step 38, deactivate the rewrite function in response to the sensing; and

[0090] Step 40, after deactivation, drive the received current level setting through a parallel combination of the LED device D10 and the output capacitor C2.

[0091] Sensing when current flows can involve: sensing when a particular threshold current flows, either directly or based on optical sensing of the corresponding light output.

[0092] This method avoids the initial delay associated with charging the output capacitor during circuit startup.

[0093] The above example is based on a linear current driver. However, the present invention can also be applied to drivers utilizing a switched-mode power supply.

[0094] The above example is based on an analog rewrite circuit and a deactivation switch. However, the LED current (as the voltage across a current sensing resistor) can be sensed and then the signal can be provided to a signal processor instead of the deactivation switch, and the signal processor then digitally implements all the functions explained above.

[0095] Generally, the present invention is relevant for linear drivers (IC-based or with discrete components) or SMPS drivers (such as IC driver circuits) for LED lamps, regardless of topology.

[0096] By studying the drawings, the disclosure, and the appended claims, those skilled in the art can understand and realize variations of the disclosed embodiments when practicing the claimed invention. 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 advantageously. If the term "adapted to" is used in the claims or the specification, it should be noted that the term "adapted to" is intended to be equivalent to the term "configured to". Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An LED circuit (10), comprising: Current drive circuit (B1); LED device (D10); Current sensor (R3) for sensing when current flows through the LED device (D10), wherein the current sensor (R3) is serially coupled with the LED device (D10), and wherein the series arrangement of the current sensor (R3) and the LED device (D10) is coupled in parallel with an output capacitor (C2); The output capacitor (C2), wherein the current drive circuit is adapted to drive current through the LED device (D10), the current sensor (R3), and the output capacitor (C2); Control signal (PWM) for setting the current level delivered by the current drive circuit (B1); And Rewriting device (20) for rewriting the current level setting of the control signal to a default current level, Deactivation switch (Q1) for deactivating the rewriting device, wherein the control terminal voltage of the switch is set by the current sensor, wherein the LED circuit (D10) is adapted to deactivate the rewriting device (20) in response to the current sensed by the current sensor (R3) exceeding a threshold current, wherein the default current level is the maximum current level setting, wherein the rewriting device (20) includes a circuit having a pull-up transistor (Q3) that, when conducting, pulls up the control signal (PWM) to a default voltage (V2) and, when off, isolates the default voltage (V2) from the control signal, and wherein the deactivation switch (Q1) turns off the pull-up transistor (Q3) when conducting.

2. The LED circuit (10) according to claim 1, wherein the current sensor (R3) comprises a current sensing resistor (R3) connected in series with the LED device.

3. The LED circuit (10) according to claim 2, comprising an additional capacitor (C3) connected in parallel with the current sensing resistor (R3).

4. The LED circuit (10) according to any one of claims 1 to 3, wherein the deactivation switch (Q1) is a transistor.

5. The LED circuit (10) according to any one of claims 1 to 3, wherein the control signal has a pulse width modulation profile, wherein the duty cycle of the pulse width modulation profile defines the current level, and the rewriting means (20) is adapted to implement an "OR" function between the control signal and a rewrite signal before being applied to the current drive circuit.

6. The LED circuit (10) according to claim 4, wherein the deactivation switch is a bipolar transistor.

7. The LED circuit (10) according to claim 6, wherein the emitter and base of the bipolar transistor deactivation switch are connected to opposite ends of the current sensor.

8. The LED circuit (10) according to any one of claims 1 to 3, wherein the current drive circuit (B1) comprises a linear current source.

9. The LED circuit (10) according to any one of claims 1 to 3, comprising a mains input (V1) and a rectifier (D1 - D4), wherein the output of the rectifier is provided to the current drive circuit (B1), the LED device (D10), and the output capacitor (C2).

10. An LED driving method, comprising: Receiving (30) a control signal setting a current level; During startup, rewriting (32) the current level setting of the control signal to a default current level by the rewriting device; Driving (34) the default current level through a parallel combination of the LED device (D10) and the output capacitor (C2); Sensing (36) using the current sensor (R3) to indicate when current flows through the LED device; Deactivating (38) the rewriting function by the deactivation switch (Q1) in response to the sensing, wherein the control terminal voltage of the switch is set by the current sensor; And After deactivation, driving (40) the received current level setting through the parallel combination of the LED device (D10) and the output capacitor (C2), wherein the current sensor (R3) is serially coupled with the LED device (D10), and wherein the series arrangement of the current sensor (R3) and the LED device (D10) is coupled in parallel with an output capacitor (C2), wherein the default current level is the maximum current level setting, wherein the rewriting device (20) includes a circuit having a pull-up transistor (Q3) that, when conducting, pulls up the control signal (PWM) to a default voltage (V2) and, when off, isolates the default voltage (V2) from the control signal, and wherein the deactivation switch (Q1) turns off the pull-up transistor (Q3) when conducting.

11. The method according to claim 10, wherein sensing the current comprises: Derive the voltage across a current sensing resistor (R3) connected in series with the LED device.

12. The method according to any one of claims 10 to 11, wherein said rewriting comprises: Implement an "OR" function between the current level setting and the rewrite signal.

13. The method according to claim 12, wherein when said rewriting is disabled, the control signal (PWM) has a pulse width modulation profile, wherein the duty cycle of the pulse width modulation profile defines the current level.

Images