LED driver circuits, LED power supply circuits and LED lighting devices

Abstract

In order to provide a balance between a fast start-up in a start-up phase and a low light output ripple in a normal operation phase, an LED drive circuit is provided having an input for receiving an LED drive current, a buffer circuit adapted to buffer the LED drive current, and an output connected to an LED and adapted to output the buffered LED drive current to the LED, the LED drive circuit further comprising a buffer adjustment circuit adapted to adjust a buffering capability of the buffer circuit to a first level during a start-up phase of the LED drive circuit when the LED drive circuit begins to receive the LED drive current, and to adjust the buffering capability of the buffer circuit to a second level, greater than the first level, during a normal operation phase after the start-up phase.

Description

[Technical field]

[0001] The present invention relates to an LED driving circuit, and more particularly to an LED driving circuit that has both advantages of less flicker during normal operation and quick start-up. [Background technology]

[0002] With the constant update of technical requirements for LED lighting products, in the new SLR (Single Lighting Regulation) standard released in Europe, there is a clear requirement for SVM (Stroboscopic Visibility Measure) to measure light output ripple. It provisionally requires SVM<0.4, and the final version after consultation and revision may require SVM<0.9. This means that the current to the LED needs to have much less ripple than now.

[0003] Traditionally, LED driver circuits have two stages, i.e. a first stage for power factor correction (PFC) whose output has a large ripple, and a second stage for converting the output of the PFC stage into a smoother current, so that the SVM requirements can be more or less solved. However, there is a new trend to reduce LED driver circuits to one or 1.5 stages, which still need to implement PFC and therefore whose output to the LED still has a significant ripple. A further solution to make the ripple current tolerable is to significantly increase the capacitance of the output capacitor to smooth the ripple. Such output capacitors can be as large as hundreds of μF or even thousands of μF.

[0004] However, a large output capacitor creates another problem. The larger the output capacitor, the longer it takes to charge the output capacitor to a voltage that is sufficient to turn on the LED. According to EU requirements, the start-up time of LED lighting products must be less than 0.5 seconds. When the LED lighting circuit is working in dimming mode and the output current of the power supply is less than the maximum value, it takes a fairly long time to charge the large output capacitor before the LED starts to emit light, which may not meet the above requirement of start-up time.

[0005] US2020205265A1 discloses providing a larger capacitance when the input voltage is between 220V and 240V VAC, and providing a smaller capacitance when the input voltage is between 100V and 120V VAC, in order to meet the requirements for harmonics.

[0006] WO2012059853A1 discloses a valley filling technique in which a capacitor is switched to be charged by the input voltage when the input voltage is sufficient to drive an LED via a power converter, and when the input voltage drops so that energy cannot be drawn from the input voltage, the capacitor is discharged to power the LED. Summary of the Invention [Problem to be solved by the invention]

[0007] Embodiments of the present invention aim to provide both low flicker and fast start-up. [Means for solving the problem]

[0008] The basic idea of ​​the embodiment of the present invention is to dynamically change the buffering capacity of the buffer circuit of the LED driving circuit, providing a relatively small buffering capacity at start-up so that the output voltage can be rapidly increased to turn on the LED, and then increasing the buffering capacity to smooth the ripple during normal operation after start-up, thus shortening the start-up time and reducing the flicker during normal operation.

[0009] According to a first aspect of the present invention, there is provided an LED drive circuit having an input for receiving an LED drive current, a buffer circuit adapted to buffer the LED drive current, and an output connected to an LED and adapted to output the buffered LED drive current to the LED, characterized in that the LED drive circuit further comprises a buffer adjustment circuit adapted to adjust a buffering capability of the buffer circuit to a first level during a start-up phase of the LED drive circuit, when the LED drive circuit starts to receive the LED drive current, and to adjust the buffering capability of the buffer circuit to a second level, greater than the first level, during a normal operation phase after the start-up phase.

[0010] According to this aspect, in the start-up phase, since the buffer capacity is not so high, the buffer circuit quickly reaches a sufficient electrical level to turn on the LED, i.e., start up the LED, and after start-up, the buffer capacity is increased to absorb ripples in the LED driving current and meet the requirement of low flicker, which achieves a balance between fast start-up and low flicker in normal operation for LED lighting.

[0011] In a particular embodiment, the buffer circuit comprises at least one capacitor, and the buffer adjustment circuit is adapted to adjust an effective capacitance of the buffer circuit.

[0012] In this embodiment, by dynamically adjusting the effective capacitance, the time it takes for the buffer circuit to build up voltage to turn on the LED is reduced, and then its buffering capability is increased to meet both fast start-up and low flicker requirements.

[0013] In one embodiment, the start-up phase is a phase in which the voltage at the output and at the at least one capacitor is increased to turn on the LED but is insufficient to turn on the LED, and the normal operation phase is a phase in which the voltage at the output and at the at least one capacitor is increased sufficiently to turn on the LED and provide a nominal light output ripple.

[0014] This embodiment further clarifies that the start-up phase is the phase in which the output voltage is ramped up to reach an LED forward voltage before the LEDs can be turned on, and the normal operation phase is the phase in which the output voltage has reached the LED forward voltage, the LEDs are turned on, and a nominal light output ripple is provided.

[0015] In a detailed embodiment, the buffer circuit comprises a first capacitor and a second capacitor connected in parallel across the output, and the buffer adjustment circuit comprises a switch in series with the second capacitor across the output.

[0016] This embodiment provides a relatively simple circuit that provides an adjustable effective capacitance by providing a first capacitor as the base capacitance and controlling, via a switch, how much of a second capacitor is involved.

[0017] In a further embodiment, the buffer conditioning circuit is adapted to, during the start-up phase, connect the first capacitor across the output and disconnect the second capacitor from the output by opening the switch, and, during the normal operation phase, connect both the first capacitor and the second capacitor across the output by closing the switch.

[0018] This embodiment switches the second capacitor out during the start-up phase and makes the first capacitor form the LED forward voltage, so the start-up time is short, and after start-up, this embodiment switches the second capacitor in to absorb ripple and reduce ripple.

[0019] In a further embodiment, to prevent the second capacitor from being suddenly switched in and causing a large current sinking effect, the buffer adjustment circuit is adapted to operate the switch in a current limiting mode to limit the current through the second capacitor and the switch during a transition of the connection of the second capacitor across the output, and to operate the switch in a full conduction mode in the normal operating mode.

[0020] This embodiment limits the current sinking by the second capacitor, preventing a sudden drop in current to the LED and therefore preventing dips / flickering in the light output.

[0021] In an alternative embodiment, the buffer adjustment circuit further comprises an impedance in series with the second capacitor and in parallel with the switch.

[0022] Instead of switching the second capacitor, this alternative embodiment switches in an additional impedance in series with the second capacitor to change the time response of the second capacitor.

[0023] Preferably, the buffer adjustment circuit is adapted to connect the first capacitor and the series connection of the second capacitor and the impedance across the output during the start-up phase by opening the switch, and to connect the first capacitor and the second capacitor across the output while bypassing the impedance during the normal operation phase by closing the switch.

[0024] In this preferred embodiment, during the start-up phase, the second capacitor is also charged, but slowly, allowing the second capacitor to smoothly transition to the normal operation phase without sinking much current from the first capacitor and the LED, which prevents a large visible light drop from occurring.

[0025] More specifically, the buffer adjustment circuit further includes a bias circuit connected to a control terminal of the switch and adapted to apply a bias voltage to the control terminal to close the switch; and a voltage detection circuit connected to the second capacitor, adapted to detect a signal indicative of a voltage at the second capacitor, to disable the bias voltage when the voltage at the second capacitor is lower than a threshold value, and to enable the bias voltage when the voltage at the second capacitor is higher than the threshold value.

[0026] This embodiment provides detection regarding the charge of the second capacitor, and if it is detected that the second capacitor is already sufficiently charged, the impedance can be bypassed and the second capacitor can be used more efficiently without affecting the light output.

[0027] In a second aspect of the present invention there is provided an LED power supply circuit comprising a power supply adapted to be connected to a power source and to convert energy of the power source into the LED drive current, and an LED drive circuit according to the first aspect.

[0028] This embodiment provides an entire power electronic circuit including power supply and buffer functions, with the advantages of fast start-up and less flicker.The LED power supply circuit can be a stand-alone LED driver.

[0029] In a third aspect of the present invention there is provided an LED lighting device comprising an LED and an LED power supply circuit according to the second aspect.

[0030] This aspect provides an overall lighting device including the LED and the LED power supply circuit. The LED lighting device can be either a light bulb or a luminaire.

[0031] These and other aspects of the invention will be elucidated and elucidated with reference to the following embodiments. [Brief description of the drawings]

[0032] For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which: [Figure 1] 1 illustrates an LED driver circuit according to an embodiment of the present invention. [Diagram 2] 2 shows an improvement of the embodiment shown in FIG. [Diagram 3] 3 shows another LED driving circuit according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The present invention will be described with reference to the drawings.

[0034] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems and methods, are for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects and advantages of the devices, systems and methods of the present invention will become better understood from the following description, appended claims and accompanying drawings. It should be understood that the figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the figures to denote the same or similar parts.

[0035] As shown in Figures 1, 2 and 3, an embodiment of the present invention comprises: - An input Vbus to receive the LED drive current; - buffer circuits C1 and C2 adapted to buffer the LED driving current; - an LED driver circuit having outputs LED+ and LED-, connected to an LED and adapted to output a buffered LED drive current to the LED, The LED driving circuit is a buffer adjustment circuit, comprising, inter alia: In a start-up phase of the LED driving circuit, when the LED driving circuit starts to receive the LED driving current, adjusting a buffering capability of the buffer circuit to a first level; The present invention proposes an LED driving circuit characterized in that, in a normal operation phase after the start-up phase, it further comprises a buffer adjustment circuit including M1 adapted to adjust the buffer capability of the buffer circuit to a second level greater than the first level.

[0036] In a preferred embodiment, the buffer circuit is capacitive and comprises at least one capacitor, and the buffer adjustment circuit is adapted to adjust the effective capacitance of the buffer circuit. Capacitive buffering is a widely used implementation in driving LEDs.

[0037] The start-up phase is a phase in which the voltage at the output and at least one capacitor is raised to turn on the LED but is insufficient to turn on the LED, and the normal operation phase is a phase in which the voltage at the output and at least one capacitor is raised sufficiently to turn on the LED and provide a nominal light output ripple. There may be a transition between the start-up phase and the normal operation phase during which the capacitance is adjusted. The following description will explain this.

[0038] A simple topology for implementing adjustable capacitance is to provide two capacitors, one associated with a switch to control the charging of the capacitor. More specifically, as shown in Figures 1 and 2, the buffer circuit includes a first capacitor C1 and a second capacitor C2 and a buffer adjustment circuit. The buffer adjustment circuit includes a switch M1 in series with the second capacitor C2, and the series connection of the second capacitor C2 and the switch M1 spans the outputs LED+ and LED-. Preferably, the series connection of the second capacitor C2 and the buffer adjustment circuit is placed in parallel with the first capacitor C1.

[0039] In the embodiment shown in FIG. 1, the LED driving current from the power supply flows in at the Vbus terminal. The buffer adjustment circuit is adapted to change the effective capacitance of the two capacitors C1 and C2 by opening the switch M1 during the startup phase, disconnecting the second capacitor C2 from the output or input, while allowing the first capacitor C1 to be connected across the output, and by closing the switch M1 during the normal operation phase, connecting both the first capacitor C1 and the second capacitor C2 across the output. The triggering of the switch M1 may result from the established LED forward voltage. This means that when the LED is turned on and current flows through the LED, the switch M1 closes and connects the second capacitor C2 to the Vbus terminal.

[0040] More specifically, a GS terminal may be tapped from a segment of an LED, the forward voltage of which is preferably 5V or 10V. When the LED is on, this GS voltage is applied to the gate of MOSFET M1, closing it. An optional resistor R0 is connected between the gate and source of MOSFET M1 to protect the gate of MOSFET M1 from overvoltage. The resistance of resistor R0 may be 47kΩ.

[0041] When the switch M1 is switched on, the second capacitor may be empty, so the second capacitor tries to sink a large current from the LED driving current or even from the first capacitor C1, which may cause a dip in the LED driving current to the LED. In the improved embodiment shown in FIG. 2, in order to have a smooth connection of the second capacitor C2, the buffer adjustment circuit is adapted to operate the switch M1 in a current limiting mode to limit the current flowing through the second capacitor C2 and the switch M1 during the transition of the connection of the second capacitor C2 across the output. The limited current is preferably of an amplitude that does not cause unacceptable light output degradation. The actual value can be designed according to actual needs by those skilled in the art.

[0042] In the embodiment in FIG. 2, Vcc is used to power the op-amp U1 and to derive a reference signal from Vcc through a voltage divider formed by resistors R5 and R6. The reference signal is connected to the positive input of op-amp U1. A small sense resistor R4 is connected in series with capacitor C2 to sense the charging current of C2, and the current is fed back to the negative input of op-amp U1. To reduce power loss, the resistance of sense resistor R4 can be up to 10 ohms. When the LED is on and Vcc voltage is available, the op-amp is turned on, compares the sensed charging current with a reference, and outputs a signal to drive MOSFET M1 so that the charging current matches the reference. This is a common way of controlling the current of a current source. A detection circuit, not shown, can detect when the second capacitor C2 is charged substantially to the LED forward voltage or to a voltage approximately equal to the voltage on the first capacitor C1, and inject a high-voltage gate drive signal GD to the gate of the MOSFET M1 to override the signal from the op-amp. The high-voltage gate drive signal GD causes the switch M1 to enter a fully conductive mode where it no longer limits the current through the second capacitor C2. In that case, the two capacitors C1 and C2 become substantially a pure parallel connection with a large capacitance that is the sum of both capacitances.

[0043] 1 and 2, the capacitance of C1 is 1 μF and the capacitance of C2 is 150 μF, which is just an example. Those skilled in the art have sufficient knowledge to select the appropriate values ​​according to the actual requirements in start-up time and light output ripple.

[0044] In the above embodiment, the second capacitor is completely disconnected from the output during the start-up phase and is not charged at all. In an alternative embodiment, the LED driving circuit can allow the second capacitor to be charged during the start-up phase as well, but use a large impedance to limit its charging, so that it is still mainly the first capacitor C1 that is charged by the power supply and quickly forms the LED forward voltage. Figure 3 shows this alternative embodiment. The buffer regulation circuit further has an impedance R1 in series with the second capacitor C2 to limit the charging of the second capacitor C2, and the impedance R1 is in parallel with the switch M1.

[0045] The buffer adjustment circuit is adapted to connect the first capacitor C1 and the series connection of the second capacitor C2 and the impedance R1 across the output in a start-up phase by opening the switch M1, and to connect the first capacitor C1 and the second capacitor C2 across the output while bypassing the impedance R1 by closing the switch M1 in a normal operation phase. The buffer adjustment circuit further comprises a bias circuit R7, R2 connected to the control terminal of the switch M1 and adapted to apply a bias voltage to the control terminal to close the switch M1, and a voltage detection circuit including a resistor R3, a resistor R8 and a switch Q1 connected to the second capacitor C2 and adapted to detect a signal indicative of the voltage across the impedance R1. The voltage detection circuit R3, R8, Q1 enables the bias voltage if the voltage across the impedance R1 is lower than a threshold value. This occurs, for example, when the charging current for the second capacitor C2, which also flows through the impedance R1, is too low, indicating that the second capacitor is sufficiently charged. This allows the switch M1 to close and enter into a normal operation mode. The voltage detection circuit R3, R8, Q1 disables the bias voltage when the voltage across the impedance R1 is higher than a threshold, which means that the current through the second capacitor C2 and the impedance R1 is high, indicating that the second capacitor C2 is still charging, and therefore the switch M1 must remain open.

[0046] More specifically, referring to FIG. 3, the LED driving circuit includes a first capacitor C1 and a second capacitor C2, as well as M1, R2, D3, Q1, R7, R3, R8, and R1. Before the start-up phase, there is no charge in this circuit. Both capacitors C1 and C2 are completely empty, their voltages are 0V, and since there is no voltage on both capacitors, MOSFET M1 is open and transistor Q1 is also open. When the power supply starts to output the LED driving current at the Vbus terminal, the start-up phase begins and the LED driving current charges the output capacitors C1 and C2. C1 is fine to be charged at a normal rate, but capacitor C2 is charged at a smaller current / slower rate due to the impedance R1 implemented by a resistor in series with capacitor C2. R1 could be a fixed resistor (providing a fixed slow charging rate) or even a PTC resistor (providing an increasingly slow charging rate as the PTC resistor heats up due to the current flowing through it). When the capacitor C2 is still being charged, there is a charging current flowing through the resistor R1 as well, so there is a voltage VR1>0.7*(R3+R6) / R6 across the resistor R1, and 0.7V is the close threshold of the transistor Q1, so the transistor Q1 is closed through the voltage divider R3 and R6, the transistor Q1 shunts the gate of the MOSFET M1 to ground, and the MOSFET M1 remains open. The power supply is mainly loaded by the capacitor C1 and very slightly by the series combination of the capacitor C2 and the resistor R1. The LED drive current flows mostly through the capacitor C1 to charge the capacitor C1. When the voltage on the capacitor C1 reaches the LED forward voltage, the LED starts to provide light, although it does not provide the nominal low ripple temporarily. The start-up phase ends. During the transition phase from the start-up phase to the normal operation phase, capacitor C2 charges to the voltage of capacitor C1 and the LED forward voltage, and then stops substantially charging (except for absorbing a relatively small ripple in current). The charging current of capacitor C2 decreases, and therefore the voltage across resistor R1 decreases.When the voltage VR1 on the resistor R1 is <0.7*(R3+R6) / R6, the transistor Q1 is opened. When the transistor Q1 is opened, the MOSFET M1 is closed by the bias voltage from Vbus, which is provided by the bias circuit R4, which is connected to Vbus, and optionally through R2. Therefore, the capacitor C2 is directly connected to Vbus without any additional impedance R1. To prevent the capacitor C2 from sinking a large current, thereby preventing a dip in the LED drive current to the LED and preventing a flicker in the light output, it is better to select the appropriate values ​​of those components so that the capacitor C2 is at approximately the same level as the capacitor C1 when the MOSFET M1 is closed. Then, the two capacitors C1 and C2 are in parallel, providing a large capacitance to absorb the ripple in the LED drive current, providing a nominal light output ripple, and the normal operation phase begins. After the MOSFET M1 is closed, VR1=0, and the transistor Q1 remains open thereafter. In this way, MOSFET M1 is always closed and latched as long as the power supply is still powered up and providing LED drive current. The effective capacitance / buffering capability of the circuit in normal operation phase is higher than that in start-up phase. The Zener diode D3 at the gate of MOSFET M1 is to protect the gate of MOSFET M1 from too high voltage. Zener diode D3 can be replaced by a resistor.

[0047] An advantage of the second embodiment is that the voltage detection circuit connects to the second capacitor rather than directly to the LED, thus saving wiring to the LED board in cases where the LED drive circuit is located on a different board than the LED board, such as when the LED drive circuit is located on a stand-alone driver separate from the LED board.

[0048] The capacitance of capacitor C1 may be 330 μF, and the capacitance of capacitor C2 may be 660 μF. The resistances of R1, R2, R3, R4, and R6 may be 150 Ω, 22 Ω, 100 kΩ, 1000 kΩ, and 100 kΩ, respectively. All of these values ​​are for example purposes only.

[0049] The LED driver circuit described above may be part of a stand-alone LED power supply circuit, such as a power electronics driver for LEDs, comprising a power supply adapted to connect to a power source, such as an AC mains power source, and to output an LED drive current, and an LED driver circuit as described above.

[0050] The above LED power supply circuit and the LEDs driven by the LED power supply circuit can be combined together to form a stand-alone LED lighting device, which can be as small as a light bulb or as large as a luminaire.

[0051] Although the above embodiment uses a capacitor to describe the buffering circuit, the applicant suggests that this is not the only option. In another example, the buffer circuit can also be implemented in an inductive manner, since inductance can also buffer the ripple in the LED driving current. More specifically, in the start-up phase, the buffer circuit is adjusted by a buffer adjustment circuit with a smaller inductance to quickly provide the driving current to the LED, and in the normal operation phase, the buffer circuit is adjusted by a buffer adjustment circuit with a larger inductance to buffer the ripple. Therefore, the manner of dynamically changing the inductance is similar to the manner of dynamically changing the capacitance described above, in that it provides a fast start-up in the start-up phase and a low ripple in the normal operation phase. Although the illustrated description mainly describes the present invention by taking capacitive buffering as an example, the present application does not provide a detailed description, since a person skilled in the art would implement the present invention by inductive buffering.

[0052] Those skilled in the art can understand and effect variations to the disclosed embodiments in the practice of the claimed invention, from a study of the drawings, the specification and the appended claims. In the claims, the word "comprises" does not exclude other elements or steps, and the singular form "a" does not exclude a plurality.

[0053] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0054] Please note that when the term "adapted to" is used in the claims or specification, the term "adapted to" is intended to be equivalent to the term "configured to."

[0055] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. an input for receiving an LED drive current; a buffer circuit adapted to buffer the LED drive current; an output coupled to an LED and adapted to output the LED drive current to the LED; The LED driving circuit is 1. A buffer adjustment circuit, comprising: adjusting a buffering capability of the buffer circuit to a first level during a start-up phase of the LED driver circuit, the start-up phase being a phase in which the LED driver circuit begins to receive the LED drive current; The LED driver circuit further comprises a buffer adjustment circuit adapted to adjust the buffering capability of the buffer circuit to a second level, greater than the first level, in a normal operation phase after the start-up phase.

2. 2. The LED driver circuit of claim 1, wherein the buffer circuit comprises at least one capacitor, and the buffer adjustment circuit is adapted to adjust an effective capacitance of the buffer circuit.

3. the start-up phase is a phase in which the voltage at the output and at the at least one capacitor is increased to turn on the LED, but is insufficient to turn on the LED; 3. The LED driver circuit of claim 2, wherein the normal operation phase is a phase in which the voltage at the output and at the at least one capacitor is sufficiently increased to turn on the LED and provide a nominal light output ripple.

4. The buffer circuit is a first capacitor and a second capacitor connected in parallel across the output; 4. The LED driver circuit of claim 3, wherein the buffer conditioning circuit comprises a switch in series with the second capacitor across the output.

5. The buffer adjustment circuit, during the start-up phase, connecting the first capacitor across the output and disconnecting the second capacitor from the output by opening the switch; 5. The LED driver circuit of claim 4, adapted to connect both the first capacitor and the second capacitor across the output by closing the switch during the normal operation phase.

6. The buffer adjustment circuit, operating the switch in a current limit mode to limit current through the second capacitor and the switch during a transition of a connection of the second capacitor across the output; 6. The LED driver circuit of claim 5, adapted to operate the switch in a fully conductive mode in the normal operating mode.

7. 5. The LED driver circuit of claim 4, wherein the buffer conditioning circuit further comprises an impedance in series with the second capacitor and in parallel with the switch.

8. The buffer adjustment circuit, during the start-up phase, connecting the first capacitor and the series combination of the second capacitor and the impedance across the output by opening the switch; 8. The LED driver circuit of claim 7, adapted to connect the first capacitor and the second capacitor across the output while bypassing the impedance by closing the switch during the normal operation phase.

9. The buffer adjustment circuit, a bias circuit connected to a control terminal of the switch and adapted to apply a bias voltage to the control terminal to close the switch; a voltage detection circuit coupled to the second capacitor for detecting a signal indicative of a voltage across the second capacitor; disabling the bias voltage when the voltage on the second capacitor is below a threshold; 8. The LED driver circuit of claim 7, further comprising: a voltage detection circuit adapted to enable the bias voltage if the voltage on the second capacitor is higher than the threshold value.

10. a power supply adapted to connect to a power source and convert energy of the power source into the LED drive current; An LED power supply circuit comprising: an LED drive circuit according to any one of claims 1 to 9.

11. LEDs, An LED lighting device comprising the LED power supply circuit according to claim 10.

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