Low-PF lighting circuit

Abstract

The invention provides a low-PF (power factor) lighting circuit, which comprises a load switch connected in series with an LED (light-emitting diode); the dimming reference generation circuit is used for acquiring an output voltage representing the voltage of a connection node of the load switch and the LED after the startup time of the lighting circuit reaches a first time, generating an error sampling signal according to a reference voltage and the output voltage at the conduction moment of the silicon controlled rectifier dimmer, and outputting the error sampling signal; and adjusting a dimming reference signal generated in the last work period according to the error sampling signal to obtain a dimming reference signal in the current work period, the dimming reference signal being used for controlling the on-off state of the load switch to control the current of the LED. The LED current is stable and free of ripples, the anti-interference capability is high, and the influence of common-mode interference and the like is avoided.

Description

Technical Field

[0001] This invention relates to the field of power electronics, and in particular to a low power factor (PF) lighting circuit. Background Technology

[0002] In a thyristor dimming circuit, the thyristor regulator achieves dimming through phase control, that is, by changing the conduction phase angle to adjust the output current to achieve dimming. In an existing high-PF dimming scheme, such as... Figure 1 As shown, the bus voltage VBUS output by the rectifier circuit powers the LED and charges the capacitor C0. The capacitor C0 is connected in parallel with the LED. When the bus voltage is insufficient to power the LED, the capacitor C0 continues to power the LED. By detecting the conduction angle of the thyristor dimmer, a dimming reference signal is generated based on the size of the conduction angle to control the magnitude of the LED current.

[0003] In low-PF SCR dimming solutions, such as Figure 2 As shown, the AC input VAC is rectified to generate a bus voltage VBUS. This bus voltage charges the input capacitor Cin through diode D0. One end of the input capacitor Cin is connected to the positive terminal of the LED, and the other end is grounded. The voltage on the input capacitor Cin powers the LED. If the LED current control scheme in the high-PF dimming scheme is still used, i.e., the LED current is controlled based on the dimming reference signal generated by the conduction angle, the following problems will occur: When the conduction angle is small, the energy stored in the input capacitor Cin is small. If the output energy, i.e., the LED current, is not controlled and further reduced, the voltage VIN on the input capacitor will experience undervoltage. That is, the voltage VIN is insufficient to fully turn on the load switch M1 in the LED path. The load switch M1 operates in a linear conduction state. At this time, the LED current is no longer controlled by the dimming reference signal but is determined by the voltage VIN. This will cause the LED current to be unstable and the LED to flicker. See [link to relevant documentation]. Figure 3 .

[0004] Reference Figure 3 According to the waveform of the bus voltage VBUS, when the conduction angle is small, as the input capacitor voltage VIN decreases, the LED current ILED is no longer in a constant current state, but decreases as the input capacitor voltage VIN decreases. The LED current ripple is large and unstable. Summary of the Invention

[0005] The purpose of this invention is to provide a low power factor (PF) lighting circuit. The AC power supply voltage is converted to a phase-cut bus voltage via a thyristor dimmer and rectifier circuit, comprising: A diode and an input capacitor are provided. The positive terminal of the diode receives the bus voltage, the negative terminal of the diode is connected to the positive terminal of the LED, the first terminal of the input capacitor is connected to the common connection terminal of the diode and the LED, the second terminal of the input capacitor is grounded, and the input capacitor provides the power supply voltage for the LED. A load switch is connected in series with the LED; The dimming reference generation circuit acquires an output voltage characterizing the voltage at the connection node between the LED and the load switch after the lighting circuit is turned on for a first time. At the turn-on time of the SCR dimmer, it generates an error sampling signal based on a preset reference voltage and the output voltage, and adjusts the dimming reference signal generated in the previous working cycle based on the error sampling signal to obtain the dimming reference signal for the current working cycle. The dimming reference signal is used to control the switching state of the load switch to control the current of the LED. One working cycle is equal to half the size of a power frequency cycle.

[0006] Optionally, the dimming reference generation circuit includes a comparison unit and a dimming reference generation unit. The comparison unit is used to compare the error sampling signal with a preset first voltage and a negative first voltage, respectively, wherein the first voltage is greater than zero. The dimming reference generation unit receives the comparison result output by the comparison unit, and adjusts the dimming reference signal generated in the previous working cycle according to the comparison result to obtain the dimming reference signal of the current working cycle.

[0007] Optionally, when the comparison result indicates that the error sampling signal is greater than a negative first voltage and less than a first voltage, the dimming reference generation unit controls the dimming reference signal to remain unchanged; When the comparison result indicates that the error sampling signal is greater than the first voltage, the dimming reference generation unit controls the dimming reference signal to rise. When the comparison result indicates that the error sampling signal is less than the negative first voltage, the dimming reference generation unit controls the dimming reference signal to decrease.

[0008] Optionally, when the comparison result indicates that the error sampling signal is greater than the first voltage, or when the error sampling signal is less than the negative first voltage, the dimming reference generation unit outputs the difference between the dimming reference signal of the previous working cycle and the error sampling signal to obtain the dimming reference signal of the current working cycle.

[0009] Optionally, the dimming reference generation circuit further includes a first error amplifier and a sampling unit, wherein the first error amplifier amplifies the error between the reference voltage and the output voltage to obtain an error amplification signal; When the thyristor dimmer is turned on, the sampling unit is enabled, and the sampling unit samples the error amplification signal to obtain the error sampling signal.

[0010] Optionally, the dimming reference generation circuit further includes a first error amplifier and a sampling unit. When the thyristor dimmer is turned on, the sampling unit is enabled, and the sampling unit samples the output voltage to obtain an output sampling signal. The first error amplifier amplifies the error between the reference voltage and the output voltage to obtain the error sampling signal.

[0011] Optionally, before the power-on time of the lighting circuit reaches the first time, the dimming reference signal generated by the dimming reference generation circuit is a given initial dimming reference signal.

[0012] Optionally, the first time is greater than or equal to one power frequency cycle.

[0013] Optionally, the dimming reference generation unit further includes a first selector, which receives a second voltage and an error sampling signal. Before the power-on time of the lighting circuit reaches a first time, or when the error sampling signal is greater than a negative first voltage and less than the first voltage, the second voltage is selected for output, and the second voltage is a zero voltage signal. After the power-on time of the lighting circuit reaches the first time, when the error sampling signal is greater than the first voltage, or when the error sampling signal is less than the negative first voltage, the error sampling signal is selected to be output; The amount of change of the dimming reference signal in the next working cycle is adjusted according to the output signal of the first selector.

[0014] Optionally, the dimming reference generation unit further includes a second selector and a difference circuit. The second selector receives the dimming reference signal and the initial dimming reference signal from the previous working cycle. Before the power-on time of the lighting circuit reaches the first time, it selects the initial dimming reference signal for output; after the power-on time of the lighting circuit reaches the first time, it selects the dimming reference signal from the previous working cycle for output. The difference circuit outputs the difference between the output signal of the second selector and the output signal of the first selector to obtain the dimming reference signal for the current working cycle.

[0015] Optionally, the dimming reference generation unit further includes a clamping circuit connected to the output terminal of the dimming reference generation circuit, which clamps the dimming reference signal to the regulated value when the dimming reference signal is greater than the regulated value.

[0016] Optionally, the dimming reference generation circuit further includes a conduction timing detection circuit, which receives the bus voltage, determines that the thyristor dimmer is turned on when the bus voltage rises to the bus reference voltage, and outputs an effective conduction detection signal to control the sampling unit to be enabled.

[0017] Optionally, the dimming reference generation circuit obtains the supply voltage and obtains the output voltage based on the supply voltage and the voltage drop of the LED; or, the output voltage is obtained by sampling the voltage at the connection node between the load switch and the LED.

[0018] Optionally, a second error amplifier is also included, whose first input terminal receives the dimming reference signal, whose second input terminal receives the LED current sampling signal, and whose output terminal is connected to the control terminal of the load switch.

[0019] Compared with the prior art, the present invention has the following advantages: The present invention amplifies the error signal by comparing the output voltage and the preset first voltage. When the thyristor dimmer is turned on, the error signal is sampled cycle by cycle to obtain the error sampling signal. The dimming reference voltage is adjusted cycle by cycle according to the error sampling signal to flexibly adjust the LED current, which makes the LED current more resistant to interference, more stable, and ripple-free. Attached Figure Description

[0020] Figure 1 The schematic diagram is of an existing high-PF silicon controlled thyristor dimming circuit. Figure 2 Schematic diagram of a low-PF silicon controlled rectifier dimming circuit; Figure 3 for Figure 2 Waveform diagram of the working state of the thyristor dimming circuit; Figure 4 This is a schematic diagram of the lighting circuit of the present invention; Figure 5 This is a flowchart illustrating the generation of the dimming reference signal in the lighting circuit of the present invention. Figure 6 This is a schematic diagram of the dimming reference generation circuit in the lighting circuit of the present invention; Figure 7 This is a schematic diagram of the dimming reference generation unit of the present invention. Detailed Implementation

[0021] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of the present invention.

[0022] To provide the public with a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the invention, but those skilled in the art can fully understand the invention without these details. The invention is described more specifically in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the drawings are in a simplified form and use non-precise proportions, in order to facilitate and clearly illustrate the purpose of the embodiments of the invention.

[0023] like Figure 4 The diagram illustrates the schematic of the lighting circuit of this invention, including a thyristor dimmer T1, a rectifier circuit, a diode D0, an input capacitor Cin, a load switch M1, a dimming reference generation circuit O1, and an operational amplifier O2. The AC input voltage VAC is converted to a bus voltage VBUS by the thyristor dimmer T1 and the rectifier circuit. The anode of diode D0 receives the bus voltage, and the cathode of diode D0 is connected to the first terminal of the input capacitor Cin. The second terminal of the input capacitor is connected to ground, and the voltage VIN of the input capacitor supplies power to the LED load. The load switch M1 is connected in series with the LED. Preferably, the load switch is an NMOS transistor. The output voltage Vout of the common connection between the LED cathode and the load switch M1 is obtained, which is the voltage of the drain of the load switch. Alternatively, the voltage VIN of the input capacitor can be sampled, and the voltage Vout is obtained based on the difference between VIN and the forward voltage drop of the LED. The dimming reference generation circuit 01 receives the bus voltage VBUS and the output voltage Vout to obtain the dimming reference signal Vref. Operational amplifier 02 receives the dimming reference signal Vref at its non-inverting input and the sampling signal VCS, which characterizes the LED current, at its inverting input. The sampling signal can be obtained by acquiring the voltage across the sampling resistor RCS connected in series with the LED. The output of operational amplifier 02 is connected to the control terminal of load switch M1. (The principle of dimming reference generation circuit 01 generating the dimming reference signal Vref is referenced.) Figure 5 A schematic flowchart.

[0024] See Figure 5 The diagram illustrates the process of generating the dimming reference signal according to the present invention. Further explanation follows: S0-S1: The lighting circuit starts up and provides an initial dimming reference signal Vref0 to adjust the initial current of the LED; S2: Determine whether the power-on time of the lighting circuit is greater than the first time T1. If yes, proceed to step S3. Otherwise, continue to adjust the LED current according to the initial dimming reference signal until the power-on time is greater than the first time. S3-S4: In each working cycle (half a power frequency cycle / half a wave cycle), sample the output voltage Vout of the thyristor dimmer when it is just turned on, and determine the relationship between Vref1-Vout and -V1 and V1 (V1>0); S5 - S6: If Vref1 - Vout < -V1, it indicates that Vout is relatively large, further indicating that the electrolytic capacitor supplies sufficient power to the LED. Based on the dimming reference signal in the previous (last working cycle), the dimming reference signal is further controlled to rise, and then continue to step 3 in the next cycle; S7: If -V1 < Vref1 - Vout < V1, it indicates that Vout is within the appropriate range and the electrolytic capacitor supplies appropriate power to the LED. Keep the dimming reference signal in the previous (last working cycle) unchanged and then continue to step 3 in the next cycle; S8 - S9: If Vref1 - Vout > V1, it indicates that Vout is relatively small, further indicating that the electrolytic capacitor supplies insufficient power to the LED. Based on the dimming reference signal in the previous (last working cycle), the dimming reference signal is further controlled to fall, and then continue to step 3 in the next cycle.

[0025] The present invention flexibly adjusts the dimming reference signal according to the magnitude of the bus voltage / output voltage, and flexibly adjusts the current of the LED through the dimming reference signal, which can make the anti - interference ability of the LED stronger. At the same time, it can also optimize the large and small wave problems of the bus voltage, making the LED current more stable and ripple - free. According to the above - mentioned adjustment flowchart of the dimming reference signal, an embodiment of a dimming reference generation circuit is referred to Figure 6 as shown.

[0026] See Figure 6, which schematically shows the schematic diagram of an embodiment of a dimming reference generation circuit. The error amplifier 02 amplifies the error between the preset reference voltage Vref1 and the output voltage Vout to obtain an error signal Vdiff0. The conduction time detection circuit 01 receives the bus voltage VBUS and obtains the conduction time of the thyristor dimmer according to the magnitude of the bus voltage, and outputs a conduction detection signal Tr. In one embodiment, the conduction time detection circuit 01 compares the bus voltage with a preset bus reference voltage. When the bus voltage rises to the bus reference voltage, it is determined that the thyristor dimmer conducts, and an effective conduction detection signal Tr is output. The sampling unit 03 receives the error amplification signal Vdiff0 and the conduction detection signal Tr. When the conduction detection signal Tr is effective, it controls the sampling unit 03 to be enabled, and the sampling unit 03 samples the error amplification signal Vdiff0 to obtain an error sampling signal Vdiff1; in another embodiment, when the conduction detection signal is effective, the output voltage Vout is sampled to obtain a voltage sampling signal at this moment, and the voltage sampling signal and the preset reference voltage are amplified by error to obtain an error sampling signal. The comparison unit receives the error sampling signal Vdiff1 and compares the error sampling signal with the preset voltages V1 and -V1 (V1>0) respectively, and generates a logic processing signal Q according to the comparison results; specifically, the comparison unit includes a comparator 04 and a comparator 05. The comparator 04 compares the error sampling signal with the voltage V1 to generate a comparison signal VC1, and the comparator 05 compares the error sampling signal with the voltage -V1 to generate a comparison signal VC2. The logic processing unit receives the comparison signal VC1 and the comparison signal VC2 to generate a logic processing signal Q. When -V1<Vdiff1<V1, the logic processing signal Q is invalid. When Vdiff1<-V1 or Vdiff1>V1, the logic processing signal Q is effective. The power-on timing circuit 07 times the power-on time of the lighting circuit and determines whether the power-on time reaches the first time. If the power-on time reaches the first time, an effective timing discrimination signal TC is output. If the power-on time does not reach the first time, an invalid timing discrimination signal TC is output. The first time is usually greater than one power frequency cycle, preferably 3 to 5 power frequency cycle sizes, so as to adjust the LED current after the lighting circuit starts and stabilizes.The dimming reference generation unit 08 receives a logic processing signal Q, a power-on discrimination signal TC, and an error sampling signal Vdiff1. When the logic processing signal Q or the power-on discrimination signal TC is invalid, the error sampling signal Vdiff1 received by the dimming reference generation unit 08 has no effect (equivalent to not sampling the output voltage or sampling the error signal to obtain the error sampling signal before the power-on time reaches the first time). The generated dimming reference signal remains unchanged. Specifically, if the power-on discrimination signal TC is invalid, the dimming reference signal is the preset initial dimming reference signal (equivalent to the dimming reference signal of each cycle not being adjusted based on the dimming reference signal of the previous cycle). If the logic processing signal Q is invalid, the dimming reference signal remains unchanged based on the previous cycle. When the power-on discrimination signal TC is valid and the logic processing signal Q is valid, the dimming reference generation unit 08 adjusts the magnitude of the dimming reference signal according to the error sampling signal Vdiff1. When the error sampling signal Vdiff1 is large, the dimming reference signal is reduced; when the error sampling signal is small, the dimming reference signal is increased. See the schematic diagram of the dimming reference generation unit 08. Figure 7 .

[0027] See Figure 7This diagram illustrates the principle of the dimming reference generation unit 08 of the present invention, including a first selector 701, a second selector 703, a difference circuit 702, and a clamping circuit 704. The first selector 701 selects one of the error sampling signal Vdiff1 and a preset second voltage V2 based on the logic processing signal Q and the power-on discrimination signal TC. Specifically, when the logic processing signal Q or the power-on discrimination signal TC is invalid, the first selector 701 selects the preset second voltage V2 for output. When both the logic processing signal Q and the power-on discrimination signal TC are valid, the first selector 701 selects the error sampling signal Vdiff1 for output. When the power-on discrimination signal TC is valid, the second selector 703 selects the dimming reference signal Vref from the previous cycle for output. When the power-on discrimination signal TC is invalid, the second selector 703 selects the initial dimming reference signal Vref0 for output. The difference circuit 702 calculates the difference between the output signal of the second selector 703 and the output signal of the first selector 701 to obtain the dimming reference signal Vref for the current cycle. In this circuit, the second voltage V2 is zero. This means that when the logic processing signal Q or the power-on detection signal TC is invalid, the output of the first selector is zero, and the output of the difference circuit 702 is the same as the output of the second selector 703. In other words, the dimming reference signal remains unchanged based on the dimming reference signal of the previous cycle (or the initial dimming reference signal). Optionally, the output of the difference circuit 702 is also connected to a voltage regulator circuit 704. When the output signal of the difference circuit 702 is greater than a preset reference threshold, the dimming reference signal is clamped at the reference threshold to ensure that the obtained dimming reference signal is always less than or equal to the reference threshold, thus avoiding excessively large dimming reference signals and unstable LED current.

[0028] The adjustment range of the dimming reference signal is positively correlated with the conduction angle of the SCR dimmer, enabling rapid LED dimming. After the lighting circuit has been powered on for a period of time, by acquiring the differential signal obtained through the feedback output voltage at the conduction moment of the SCR dimmer, the dimming reference signal can be stabilized under the large and small wave sizes of the positive and negative half-waves within the same power frequency cycle, avoiding the influence of large and small wave size issues on LED current ripple.

[0029] Although the embodiments are described and illustrated separately above, some common technologies are involved. Those skilled in the art can replace and integrate them between the embodiments. If there is any content not explicitly described in one embodiment, then another embodiment that is described can be referred to.

[0030] The embodiments described above do not constitute a limitation on the scope of protection of this technical solution. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the above embodiments should be included within the scope of protection of this technical solution.

Claims

1. A low-PF lighting circuit, wherein the AC power supply voltage is converted into a phase-cut bus voltage via a thyristor dimmer and a rectifier circuit, characterized in that, include: A diode and an input capacitor are provided. The positive terminal of the diode receives the bus voltage, the negative terminal of the diode is connected to the positive terminal of the LED, the first terminal of the input capacitor is connected to the common connection terminal of the diode and the LED, the second terminal of the input capacitor is grounded, and the input capacitor provides the power supply voltage for the LED. A load switch is connected in series with the LED; The dimming reference generation circuit acquires an output voltage characterizing the voltage at the connection node between the LED and the load switch after the lighting circuit is turned on for a first time. At the turn-on time of the SCR dimmer, it generates an error sampling signal based on a preset reference voltage and the output voltage, and adjusts the dimming reference signal generated in the previous working cycle based on the error sampling signal to obtain the dimming reference signal for the current working cycle. The dimming reference signal is used to control the switching state of the load switch to control the current of the LED. One working cycle is equal to half the size of a power frequency cycle.

2. The lighting circuit according to claim 1, characterized in that: The dimming reference generation circuit includes a comparison unit and a dimming reference generation unit. The comparison unit is used to compare the error sampling signal with a preset first voltage and a negative first voltage, respectively, wherein the first voltage is greater than zero. The dimming reference generation unit receives the comparison result output by the comparison unit, and adjusts the dimming reference signal generated in the previous working cycle according to the comparison result to obtain the dimming reference signal of the current working cycle.

3. The lighting circuit according to claim 2, characterized in that: When the comparison result indicates that the error sampling signal is greater than the negative first voltage and less than the first voltage, the dimming reference generation unit controls the dimming reference signal to remain unchanged. When the comparison result indicates that the error sampling signal is greater than the first voltage, the dimming reference generation unit controls the dimming reference signal to rise. When the comparison result indicates that the error sampling signal is less than the negative first voltage, the dimming reference generation unit controls the dimming reference signal to decrease.

4. The lighting circuit according to any one of claims 2 or 3, characterized in that: When the comparison result indicates that the error sampling signal is greater than the first voltage, or when the error sampling signal is less than the negative first voltage, the dimming reference generation unit outputs the difference between the dimming reference signal of the previous working cycle and the error sampling signal to obtain the dimming reference signal of the current working cycle.

5. The lighting circuit according to claim 1, characterized in that: The dimming reference generation circuit further includes a first error amplifier and a sampling unit. The first error amplifier amplifies the error between the reference voltage and the output voltage to obtain an error amplification signal. When the thyristor dimmer is turned on, the sampling unit is enabled, and the sampling unit samples the error amplification signal to obtain the error sampling signal.

6. The lighting circuit according to claim 1, characterized in that: The dimming reference generation circuit further includes a first error amplifier and a sampling unit. When the thyristor dimmer is turned on, the sampling unit is enabled, and the sampling unit samples the output voltage to obtain an output sampling signal. The first error amplifier amplifies the error between the reference voltage and the output voltage to obtain the error sampling signal.

7. The lighting circuit according to claim 1, characterized in that: Before the power-on time of the lighting circuit reaches the first time, the dimming reference signal generated by the dimming reference generation circuit is a given initial dimming reference signal.

8. The lighting circuit according to claim 1, characterized in that: The first time is greater than or equal to one power frequency cycle.

9. The lighting circuit according to claim 2, characterized in that: The dimming reference generation unit further includes a first selector, which receives a second voltage and an error sampling signal. Before the power-on time of the lighting circuit reaches a first time, or when the error sampling signal is greater than a negative first voltage and less than the first voltage, the second voltage is selected for output, and the second voltage is a zero voltage signal. After the power-on time of the lighting circuit reaches the first time, when the error sampling signal is greater than the first voltage, or when the error sampling signal is less than the negative first voltage, the error sampling signal is selected to be output; The amount of change of the dimming reference signal in the next working cycle is adjusted according to the output signal of the first selector.

10. The lighting circuit according to claim 9, characterized in that: The dimming reference generation unit also includes a second selector and a difference circuit. The second selector receives the dimming reference signal and the initial dimming reference signal from the previous working cycle, and selects the initial dimming reference signal to output before the power-on time of the lighting circuit reaches the first time. After the power-on time of the lighting circuit reaches the first time, the dimming reference signal of the previous working cycle is selected for output; The difference circuit outputs the difference between the output signal of the second selector and the output signal of the first selector to obtain the dimming reference signal for the current working cycle.

11. The lighting circuit according to claim 1, characterized in that: The dimming reference generation unit also includes a clamping circuit connected to the output terminal of the dimming reference generation circuit. When the dimming reference signal is greater than the regulated value, the dimming reference signal is clamped to the regulated value.

12. The lighting circuit according to any one of claims 5 or 6, characterized in that: The dimming reference generation circuit also includes a conduction timing detection circuit, which receives the bus voltage and determines that the thyristor dimmer is turned on when the bus voltage rises to the bus reference voltage, and outputs an effective conduction detection signal to control the sampling unit to enable.

13. The lighting circuit according to claim 1, characterized in that: The dimming reference generation circuit obtains the supply voltage and obtains the output voltage based on the supply voltage and the voltage drop of the LED; or, it obtains the output voltage by sampling the voltage at the connection node between the load switch and the LED.

14. The lighting circuit according to claim 1, characterized in that: It also includes a second error amplifier, whose first input terminal receives the dimming reference signal, whose second input terminal receives the LED current sampling signal, and whose output terminal is connected to the control terminal of the load switch.

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