LED lighting circuit, and LED lighting fixture having the LED lighting circuit

Abstract

A current regulator I1 adapted to supply an adjustment current I, an LED string including at least two LED units D1, D3 connected in series for receiving the adjustment current I, a switch S1 parallel to one of the at least two LED units D1, D3, the switch S1 being adapted to be controlled to be closed to bypass the adjustment current I from the one LED unit D1 or to be opened to allow the adjustment current I to the one LED unit D1, an output capacitor C1 parallel to the LED string, and a control circuit coupled to the switch S1 and adapted to operate the switch S1 to be closed or opened, wherein the control circuit is further adapted to operate the switch S1 in a current adjustment mode to conduct a current I' having a certain deviation from the adjustment current I to adjust the voltage across the output capacitor C1 before operating the switch S1 to be closed or opened. An LED lighting circuit is provided, characterized in that the above is the case.

Description

Technical Field

[0001] The present invention relates to the field of LED lighting circuits.

Background Art

[0002] Figure 1 shows a general topology of a driving device for LEDs. The full current regulator / current source I1 generates a current I to power an LED string having LED units D1 and D3 connected in series. The current regulator / current source can be a single PFC stage or a hysteresis control DC / DC stage. Therefore, the current I1 has a certain ripple rather than a 100% constant current. To smooth the ripple, a relatively large output capacitor is placed in parallel with the LED string. At least one LED unit D1, which may include one or more LED chips, within the LED string is in parallel with a switch M2 called a shunt switch. The shunt switch selectively bypasses or does not bypass the parallel LED unit D1 to regulate whether the current I1 flows into the LED unit D1 or into the shunt switch. Specifically, it closes by entering the saturation mode or opens by entering the cut-off mode. By doing so, the output of the LED unit D1 can be dynamically changed, while the remaining LED unit D3, which may include one or more LED chips, is not affected and is always powered by the current I. In Figure 1, the current regulator I1 floats from the ground, and the shunt switch M2 and the LED D1 are connected to the positive output of the current regulator I1. The shunt switch M2 is a PMOS transistor. A grounded control circuit / gate driver for the PMOS transistor M2 is provided. The control circuit has a resistor R2, a resistor R1, and an NMOS transistor M1 connected between the floating positive output of the current regulator and the ground. The interconnection of the resistors R2 and R1 is connected to the gate of the PMOS transistor M2. In an alternative embodiment where the current regulator I1 is grounded, an NMOS can be used to implement the shunt switch. Note that Figure 1 is merely an example, and instead, the current regulator may or may not be grounded. DE102017200490B3 discloses such a topology. Summary of the Invention Problems to be Solved by the Invention

[0003] Such a topology problem is that when some LEDs are shunted or returned to the unshunted state, the voltage across the output capacitor C1 is different from the effective voltage of the LED string. Specifically, initially, assume that switch M2 is open so that both LED units D1 and D3 can be powered by current I1. Therefore, the voltage across capacitor C1 is equal to the sum of the forward voltages of LED units D1 and D3. When PMOS M2 is closed to shunt / disable LED unit D1, the voltage across capacitor C1 is much higher than the forward voltage of the remaining LED unit D3, so the inrush current passes through PMOS switch M2 and the remaining LED unit D3. The inrush current is much higher than the original current, causing LED D3 to flicker. Figure 2 shows a current curve where the original current of 280 mA rises to about 370 mA. Similarly, when the shunt switch PMOS M2 is open, as shown in Figure 3, a current dip occurs. Initially, assume that switch M2 is closed, bypassing LED unit D1 and allowing only LED unit D3 to be powered by current I1. Therefore, the voltage across capacitor C1 is equal to the forward voltage of LED unit D3. When PMOS M2 is opened to enable LED unit D1 / remove the shunt of LED unit D1, the voltage across capacitor C1 is much lower than the sum of the forward voltages of LED units D1 and D3 (unbalanced), making it difficult to turn on the two LED units D1 and D3, and the LED current drops significantly from 280 mA to only 30 mA. The current returns to I1 when the voltage across both ends of capacitor C1 reaches the sum of the forward voltages of LED units D1 and D3. LED unit D3 has flicker in its light output. If LED units D1 and D3 have different lighting functions or are spatially offset from each other, such flicker is visible to people and is undesirable.

[0004] WO2021 / 198349A1 proposes a solution to solve such voltage imbalance during shunt switching. WO2021 / 198349A1 has a dedicated circuit including resistors in parallel with all LEDs for discharging the capacitor when some of the switches, capacitors, and LEDs are shunted. US20120299489A1 also discloses a topology including an output capacitor, a string of LEDs, and respective bypass switches in parallel with each LED. US20120299489A1 further has a dedicated capacitor charging constant current section in series with the capacitor for charging and discharging the capacitor. **Means for Solving the Problem**

[0005] The present invention is defined by the claims.

[0006] The applicant has noticed that WO2021 / 198349A1 requires a dedicated circuit to discharge the capacitor, which increases the number of components and cost. Furthermore, the energy released from the output capacitor is dissipated by the resistors in the circuit, which increases the power loss.

[0007] The basic idea of this application is to discharge or charge the output capacitor to adjust the voltage in the output capacitor from the original effective voltage of the LED string to the target effective voltage of the LED string before operating the shunt switch to the closed state or open state to shunt / remove the shunt of the LED unit in the LED string by reusing the shunt switch (and the remaining LED units). The shunt switch is operated in an active (linear) mode, and by providing a control circuit that actively controls the shunt switch to conduct a switch current different from the output current of the current regulator, the differential current between the switch current and the output current is related to the output capacitor and adjusts the voltage in the output capacitor to reach the target voltage.

[0008] A first aspect of the present invention, under the above basic idea, includes a current regulator adapted to supply a regulated current, an LED string including at least two LED units connected in series for receiving the regulated current, a switch parallel to one of the at least two LED units, the switch being adapted to be controlled to be closed to bypass the regulated current from the one LED unit or to be opened to allow the regulated current to the one LED unit, an output capacitor parallel to the LED string, and a control circuit coupled to the switch and adapted to operate the switch to be closed or opened. The control circuit is further adapted to operate the switch in a current regulation mode to conduct a current having a certain deviation from the regulated current to adjust the voltage across the output capacitor before operating the switch to be closed or opened. An LED lighting circuit is provided, characterized in that.

[0009] Reusing the shunt switch to adjust the voltage across the output capacitor reduces the need for a dedicated circuit to discharge the output capacitor in the prior art cited above and reduces the complexity / cost of the LED lighting circuit. Further, the shunt switch conducts current to the remaining LED units, so the remaining LED units help consume the energy of the output capacitor to adjust the voltage, and thus, compared to the prior art cited above, less energy is wasted and the efficiency is increased.

[0010] In a further embodiment, the switch is a semiconductor transistor, and the control circuit is adapted to operate the switch in the active mode as the current regulation mode for conducting the current having the certain deviation from the regulated current, operate the switch in the saturation mode as a closed state, and operate the switch in the cut-off mode as an open state.

[0011] The semiconductor transistor is a common and low-cost component for implementing three modes. Also, implementing a control circuit for driving the semiconductor transistor in the three modes is also simple and low-cost. Therefore, this embodiment has the advantage of low cost.

[0012] In a further embodiment, the voltage of the output capacitor is changed from the original effective forward voltage of the LED string before the switch is operated to the target effective forward voltage of the LED string when the switch is operated to close or open, and then the switch is adapted to be operated to close or open.

[0013] In this embodiment, the voltage of the output capacitor is adjusted to match the new effective forward voltage of the LED string driven by the output capacitor when the switch is to be closed or opened. Since the voltage across the output capacitor is gradually changed to the voltage of the LED string after the switch changes its state, the voltage imbalance is alleviated, which prevents current inrush or dip. Here, the target effective forward voltage when the operation of the switch changes from a close operation to an open operation, or vice versa, results in the new sum of the forward voltages of the LED units driven by the current, excluding the LED unit shunted by the switch.

[0014] Preferably, a voltage detection circuit for detecting the voltage in the output capacitor is provided, and a comparison circuit for comparing the voltage in the output capacitor with a pre-stored value of the target effective forward voltage is provided. When the difference, which means that the voltage in the output capacitor is adjusted to be substantially the same as the future (to-be) forward voltage, is less than the threshold value, the control circuit ends the active mode and transitions to the saturation on mode or the cut-off mode, and operates the switch to proceed to the normal shunt / non-shunt function.

[0015] In certain embodiments, to balance the capacitor voltage in the non-shunted state and the LED string voltage in the shunted state, the control circuit is adapted to operate the switch to conduct a current greater than the regulated current before operating the switch to be closed, whereby the output capacitor is discharged to reduce the voltage across the output capacitor, preferably to the forward voltage of the LED unit connected in series with the switch.

[0016] In this embodiment, since the current through the switch is greater than the current from the current regulator, the output capacitor needs to discharge to supply the differential current, and the voltage across the output capacitor decreases. This embodiment provides an effective way to reduce the voltage and match the voltage to the forward voltage of the LED string in the shunted state.

[0017] In another particular embodiment, to balance the capacitor voltage in the shunted state and the LED string voltage in the non-shunted state, the control circuit is adapted to operate the switch to conduct a current less than the regulated current before operating the switch to be opened, whereby the output capacitor is charged to increase the voltage across the output capacitor, preferably to the forward voltage of the at least two LED units.

[0018] In this embodiment, since the current through the switch is less than the current from the current regulator, a differential current flows into the output capacitor, charging the output capacitor, and the voltage across the output capacitor increases. This embodiment provides an effective way to increase the capacitor voltage and match the voltage to the forward voltage of the two LED units in the non-shunted state.

[0019] In a particular example, the certain deviation is from 5% to 30% of the amplitude of the regulated current.

[0020] In this embodiment, a difference of at least 5% adjusts the voltage in the output capacitor over time, and a difference of up to 30% prevents visible flickering to the human eye in the LED unit that is still operating. This range can balance high-speed response and less flickering.

[0021] In a further embodiment, the LED lighting circuit further has an interface adapted to receive a light setting command, and the control circuit is adapted to operate the switch according to the light setting command.

[0022] In a particular embodiment, the at least two LED units have different lighting functions, and the light setting command is adapted to select one or both of the at least two LED units to be operated.

[0023] In a further particular embodiment, the LED unit in parallel with the switch is for lighting, and the LED unit in series with the switch is for generating ambient light.

[0024] These embodiments apply the present invention to scene / function-switchable lighting, and thus, the occurrence of flickering during lighting scene / function switching can be prevented.

[0025] In a second aspect, there is provided an LED lighting fixture having an LED lighting circuit according to the above aspects and embodiments.

[0026] These and other aspects of the present invention are described and clarified with reference to the following embodiments.

Brief Description of the Drawings

[0027] For a better understanding of the present invention and to more clearly show how the present invention can be implemented, reference is now made, by way of example only, to the accompanying drawings.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5a

Figure 5b

Figure 6

Figure 7a

Figure 7b

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0028] The present invention will be described with reference to the drawings.

[0029] The detailed description and specific examples illustrate exemplary embodiments of the apparatus, system, and method, but are 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 apparatus, system, and method of the present invention will become better understood from the following description, the appended claims, and the accompanying drawings. It is to be understood that the figures are merely schematic and are not drawn to scale. It is also to be understood that the same reference numerals are used throughout the figures to indicate the same or similar parts.

[0030] As shown in FIG. 4, a first aspect of the present invention includes a current regulator I1 adapted to supply an adjustment current I, an LED string including at least two LED units D1, D3 connected in series for receiving the adjustment current I, a switch S1 parallel to one of the at least two LED units D1, the switch S1 being adapted to be controlled to be closed to bypass the adjustment current I from one LED unit D1 or to be opened to allow the adjustment current I to the one LED unit D1, an output capacitor C1 parallel to the LED string, and a control circuit (not shown) coupled to the switch S1 and adapted to operate the switch S1 to be closed or opened, wherein the control circuit is further adapted to conduct a current I' having a certain deviation from the adjustment current I to adjust the voltage across the output capacitor C1 before operating the switch S1 to be closed or opened, and to operate the switch S1 in a current adjustment mode, in other words, as a current regulator / current source I2. An LED lighting circuit is provided, characterized in that.

[0031] More specifically, in the transition state between a state where one LED unit is not shunted and a state where it is shunted, the control circuit draws a current I' to change the voltage of the output capacitor C1 from the original effective forward voltage of the LED string before the switch is operated to the target effective forward voltage of the LED string when the switch is operated. Then, the control circuit operates the switch S1 to shunt or not shunt the LED unit D1. Therefore, when the shunt switch S1 is operated, the capacitor voltage is adjusted near the effective LED string voltage, and the fluctuation of the LED current when the shunt switch connects the output capacitor to the LED unit in the target state is reduced.

[0032] Here, by using FIGS. 7a, 5a, 5b, and 6, how the embodiment operates in the transition from the non-shunted state to the shunted state will be described.

[0033] First, assume that the switch S1 is open to not shunt the LED unit D1. The current regulator I1 outputs a desired current I. The capacitor C1 is charged to the effective forward voltage of the LED string, that is, the sum of the forward voltages of the LED units D1 and D3. All of the current I flows through the LED units D1 and D3. This is illustrated in FIG. 7b.

[0034] To turn off the LED unit D1, a light setting command is transmitted to the LED lighting circuit, preferably via the interface of the LED lighting circuit. The control circuit operates the switch S1 in the active mode / current adjustment mode, and makes the switch S1 into a current regulator I2 for drawing a current I' larger than the current I. This can be seen from the small rising part 62 in the current curve 60 of the LED unit D3 in FIG. 6. The current I' can be from 1.05 times to 1.3 times the current I. This embodiment uses 1.1 as an example. As shown in FIG. 5a, the differential current between the switch current 1.1×I and the output current I from the current regulator I1 is 0.1×I, and the output capacitor needs to supply this differential current. When ΔU = ΔQ / C, where ΔU is the voltage change in the capacitor, C is the capacitance of the capacitor, and ΔQ is the charge change in the capacitor, the voltage in the capacitor C1 is reduced by 0.1×I×t / C, and t is the discharge time when the switch draws a current of 1.1×I. The curve 64 in FIG. 6 shows the voltage in the capacitor C1, and the downward slope 66 shows the voltage reduction due to the discharge by the switch S1.

[0035] In the above embodiment, under the condition that a current of 0.1×I is discharged from the capacitor, this capacitor can be discharged to a sufficiently low voltage in only about 100 ms. Also, the slight increase in the current of the LED unit D3 is only 10%, which will not cause an uncomfortable flicker to the user. It should be noted that this parameter is only an example and not the only applicable parameter, and those skilled in the art can design different parameters to achieve a good balance between less flicker and high-speed voltage regulation.

[0036] When the capacitor voltage is reduced near the forward voltage of the LED unit D3, the control circuit can operate the switch S1 to the saturation mode, and the conduction current I' becomes equal to I, as shown in FIG. 5b.

[0037] Here, by using FIGS. 5b, 7a, 7b, and 8, how the embodiment operates in the transition from the shunted state to the non-shunted state will be described. First, assume that switch S1 is closed to shunt LED unit D1. Current regulator I1 outputs a desired current I. Capacitor C1 is charged only up to the effective forward voltage of the LED string, i.e., the forward voltage of LED unit D3. All of current I flows through LED unit D3. This is very similar to the figure in FIG. 5b.

[0038] To turn on LED unit D1, a light setting command is sent to the LED lighting circuit. The control circuit operates switch S1 in the active mode to make it current regulator I2 for drawing a current I' smaller than current I, as indicated by the small indentation portion 82 in the current curve 80 of LED unit D3 in FIG. 8. Current I' can be from 0.7 times to 0.95 times of current I. This embodiment uses 0.9 as an example. As shown in FIG. 7a, the differential current between the switch current 0.9×I and the output current I from current regulator I1 is 0.1×I, and this differential current needs to flow into output capacitor C1 and charge output capacitor C1. When ΔU = ΔQ / C, where ΔU is the voltage change in the capacitor, C is the capacitance of the capacitor, and ΔQ is the charge change in the capacitor, the voltage in capacitor C1 increases by 0.1×I×t / C, and t is the time when switch S1 conducts a current of 0.9×I. Curve 84 in FIG. 8 shows the voltage in capacitor C1, and the upward slope 86 indicates the increase in voltage due to charging.

[0039] In the above embodiment, under the condition that a charging current of 0.1×I flows into the capacitor C1, this capacitor C1 can be charged to a sufficient voltage in only about 100 ms. Also, a slight decrease in the current of the LED unit D3 is only 10%, which will not cause an uncomfortable flicker to the user. This parameter is not the only applicable parameter, and it should be noted that those skilled in the art can design different parameters to achieve a good balance between less flicker and high-speed voltage adjustment.

[0040] When the capacitor voltage increases to near the sum of the forward voltages of the LED units D1 and D3, the control circuit can operate the switch S1 to put it into the cut-off mode. The conduction current I' is zero, and the current regulator I1 only drives the LED units D1 and D3 connected in series, and the switch S1 can be practically ignored as shown in Fig. 7b.

[0041] In an embodiment, to monitor the voltage in the output capacitor C1 and control the switch S1 accordingly, a voltage detection circuit for detecting the voltage in the output capacitor is provided, and a comparison circuit for comparing the voltage in the output capacitor with either a target value, the forward voltage of the LED unit D3 when shunting, or the sum of the forward voltages of the LED units D1 and D3 when not shunting is provided. When the difference, which means that the voltage in the output capacitor C1 is adjusted near the forward voltage in the substantially new state, is less than the threshold value, the control circuit can control the switch to change from the active mode to the closed mode for the normal shunting function or the cut-off mode for the non-shunting function.

[0042] Switch S1 is preferably a transistor, and the closed / saturated on mode means that the transistor does not limit the current passing through the transistor and only conducts whatever current is supplied by the upstream power supply. The cutoff mode means that the transistor is open and does not conduct any current. The active mode means that the current passing through the transistor is linearly controlled by the signal applied to the base / gate, and the active mode can also be regarded as the linear mode.

[0043] The transistor is preferably a MOSFET transistor. Controlling the MOSFET in the active mode, as well as in the closed (saturated on) mode and the cutoff mode, can be implemented by controlling the gate voltage of the MOSFET. FIG. 9 shows an example. The current source consists of M1, R1, and C2. The current passing through R1 and M1 is regulated by the voltage across C1. The level shift consists of R2, R3, C3, D2, R4, and M2. The voltage of C2 is controlled by the PWM duty cycle. When the PWM duty cycle is 0 (M2 is always open), the voltage of C2 is zero. Also, when the PWM duty cycle is 1 (M1 is always closed), the voltage of C2 is clamped by D2 (Vd2). Also, when the PWM duty cycle is between 0 and 1, the voltage of C2 is also between 0 and Vd2 accordingly. In the case of the current source, there is a threshold voltage (Vpmos) of the PMOS. When the voltage of C2 is lower than Vpmos, the PMOS operates in the cutoff mode. The current is not bypassed by the current source. When the voltage of C2 is higher than Vpmos and the voltage of D1 is higher than the voltage of C2, the PMOS operates in the variable resistance mode / active mode. The current of D1 is bypassed by the current source, and the current is set by the PWM duty cycle. When the voltage of C2 is higher than Vpmos and the voltage of D1 is lower than the voltage of C2, the PMOS operates in the saturation mode. D1 is short-circuited by the PMOS.

[0044] In another example, switch S1 can also be implemented by a BJT transistor. Controlling the BJT transistor in the active mode, as well as in the closed / saturated on mode and the open / cut-off mode, can be implemented by controlling the base current of the BJT transistor. Designing a circuit for controlling the base current of the BJT transistor to reach the three modes is well known to those skilled in the art, and this specification does not show further details.

[0045] In an embodiment of the scene-switchable LED lighting, the at least two LED units have different lighting functions, the LED lighting circuit further has an interface adapted to receive a light setting command, and the control circuit operates the switch according to the light setting command to select one or both of the at least two LED units to be operated, thereby being adapted to activate different operation scenes of the LED lighting.

[0046] Preferably, in a large lighting fixture, the LED unit D1 in parallel with the switch S1 is for lighting and is arranged at the center of the lighting fixture, and the LED unit D3 in series with the switch S1 is for generating ambient light and is arranged at the edge of the lighting fixture. Therefore, the lighting function can be selectively activated and deactivated, while the ambient lighting is always on. Also, by using the above embodiments of the present invention, the occurrence of visible flicker in the ambient lighting during scene switching can be prevented.

[0047] In the above embodiment, there is only one shunt switch for the LED unit D1. This does not limit the scope of the present application. There may be two shunt switches respectively for the LED units D1 and D3, and each of the two shunt switches can operate as described above to adjust the capacitor voltage.

[0048] The LED units D1 and D3 only mean to distinguish between the LEDs controlled (shunted) by the switch S1 and the other LEDs. Each of the LED units D1 and D3 may include one or more LED chips.

[0049] Those skilled in the art can understand and achieve modifications to the disclosed embodiments from the study of the drawings, the description, and the appended claims in the implementation of the invention described in the claims. In the claims, the word "comprising" does not exclude other elements or steps, and the singular form does not exclude the plural.

[0050] Merely the fact that certain means are recited in mutually different dependent claims does not indicate that these means cannot be used advantageously in combination.

[0051] It should be noted that when the term "adapted to" is used in the claims or the description, the term "adapted to" is intended to be equivalent to the term "configured to". It should also be noted that when the term "configured" is used in the claims or the description, the term "configured" is intended to be equivalent to the term "system", and vice versa.

[0052] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A current regulator adapted to supply a regulated current, An LED string including at least two LED units connected in series, the LED string being adapted to receive the regulated current, A switch parallel to one of the at least two LED units, the switch being adapted to be controlled to be closed to bypass the regulated current from the one LED unit or to be opened to allow the regulated current to the one LED unit, An output capacitor parallel to the LED string, An LED lighting circuit having a control circuit coupled to the switch and adapted to operate the switch to be closed or opened, The control circuit further being adapted to operate the switch in a current regulation mode to conduct a current having a certain deviation from the regulated current to adjust the voltage across the output capacitor before operating the switch to be closed or opened.

2. The switch is a semiconductor transistor, The control circuit, Operates the switch in the active mode as the current regulation mode for conducting the current having the certain deviation from the regulated current, Operates the switch in the saturation mode as a closed state, The LED lighting circuit according to claim 1, wherein the switch is adapted to operate in an open state in the cut-off mode.

3. The control circuit, Changes the voltage across the output capacitor from the original effective forward voltage of the LED string before the switch is operated to the target effective forward voltage of the LED string when the switch is configured to be closed or opened, and then, Operates the switch to be closed or opened. The LED lighting circuit according to claim 1.

4. The control circuit is adapted to operate the switch to conduct a current greater than the regulated current before operating the switch to be closed, whereby the output capacitor is discharged to reduce the voltage across the output capacitor to the forward voltage of the LED unit connected in series with the switch. The LED lighting circuit according to claim 1.

5. The control circuit is adapted to operate the switch to conduct a current smaller than the regulated current before operating the switch to be opened, whereby the output capacitor is charged to increase the voltage in the output capacitor to the forward voltage of the at least two LED units. The LED lighting circuit according to claim 1.

6. The LED lighting circuit according to claim 4 or 5, wherein the certain deviation is 5% to 30% of the amplitude of the regulated current.

7. The LED lighting circuit according to claim 1, further comprising an interface adapted to receive a light setting command, wherein the control circuit is adapted to operate the switch according to the light setting command.

8. The LED lighting circuit according to claim 7, wherein the at least two LED units have different lighting functions, and the light setting command is adapted to select one or both of the at least two LED units to operate.

9. The LED lighting circuit according to claim 8, wherein the LED unit in parallel with the switch is for lighting, and the LED unit in series with the switch is for generating ambient light.

10. A lighting fixture having the LED lighting circuit according to any one of claims 1 to 9.

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