LED driving circuit and dimming control method thereof

Abstract

The application discloses an LED driving circuit and a dimming control method thereof. The driving circuit is connected with a dimmer to obtain a chopped AC input voltage and is connected with an LED to control a load current flowing through the LED. The driving circuit comprises a current control circuit for controlling the load current flowing through the LED and adjusting a duration of the load current in a half power frequency cycle according to a DC bus voltage to realize dimming. The current control circuit obtains a conduction angle range of the dimmer according to a detected value of the load current, selectively operates in one of an open loop mode and a closed loop mode, maintains the load current as a reference current in the open loop mode, and adjusts the load current according to a negative feedback of the DC bus voltage in the closed loop mode. The LED driving circuit controls a conduction state of a transistor in different modes according to different conduction angles of a thyristor in the dimmer, and can balance brightness stability and adjustment response speed of dimming.

Description

Technical Field

[0001] The present application relates to the field of power electronics, and in particular to an LED drive circuit and a dimming control method thereof. Background Art

[0002] Thyristor (SCR) dimming technology is a well-established dimming solution, once widely used for dimming incandescent and CFL lamps. With the increasing popularity of LED lighting products, SCR dimming has also been adopted for LED dimming control. A SCR dimming system consists of a dimmer and an LED driver circuit connected to the dimmer. The dimmer includes a thyristor (SCR), which changes its conduction angle in response to user dimming actions, thereby altering the AC voltage waveform. This dimming action changes the effective value of the LED input voltage, thereby varying the load current and / or effective lighting duration of the LED lamp, thereby adjusting the brightness of the LED lamp.

[0003] However, the dimming performance of thyristor (SCR) dimming systems is affected by the device performance of the thyristor. Due to the inherent characteristics of certain thyristors, the dimmer generates asymmetric AC voltage waveforms between the positive and negative half-cycles of the AC voltage. This asymmetry in the AC voltage causes the LED driver circuit to generate load currents that vary in magnitude during the power frequency cycle. This AC voltage asymmetry is independent of the conduction angle; however, this asymmetry in the AC voltage leads to asymmetric load currents, resulting in flickering.

[0004] It is expected that the brightness stability and adjustment response speed of the entire dimming range can be taken into account in the thyristor dimming system. Summary of the Invention

[0005] In view of the above problems, the object of the present invention is to provide an LED driving circuit and a dimming control method thereof, wherein the load current is adjusted according to the negative feedback of the DC bus voltage, thereby maintaining the brightness stability of the LED and improving the response speed in a continuous half-power frequency cycle.

[0006] According to one aspect of the present invention, there is provided an LED driver circuit, which is connected to a dimmer to obtain a chopped AC input voltage and is connected to an LED to control a load current flowing through the LED. The LED driver circuit comprises:

[0007] a rectifier bridge, configured to convert the chopped AC input voltage into a DC bus voltage having a half power frequency cycle; and

[0008] A current control circuit is connected in series with the LED between the positive output terminal and the negative output terminal of the rectifier bridge, and adjusts the duration of the load current in the half power frequency cycle according to the DC bus voltage to achieve dimming.

[0009] The current control circuit obtains the conduction angle range of the dimmer according to the detected value of the load current and selectively operates in one of an open-loop mode and a closed-loop mode.

[0010] In the open-loop mode, the current control circuit maintains the load current at a reference current.

[0011] In the closed-loop mode, the current control circuit regulates the load current according to the negative feedback of the DC bus voltage.

[0012] Preferably, it also includes:

[0013] a bleeder circuit connected between the positive output terminal and the negative output terminal of the rectifier bridge, for providing a bleeder current, wherein the bleeder current is used to provide a holding current for the dimmer; and

[0014] A diode, wherein the cathode of the diode is connected to the positive output terminal of the rectifier bridge, and the cathode of the diode is connected to the anode of the LED.

[0015] Preferably, the current control circuit includes:

[0016] a transistor, a first sampling resistor, and a second sampling resistor connected in series with the LED;

[0017] a closed-loop control module, configured to generate a first signal according to the DC bus voltage and a first current sampling signal at both ends of the first sampling resistor;

[0018] an open-loop control module, generating a second signal according to a second current sampling signal at both ends of the second sampling resistor; and

[0019] A selection module is connected to the closed-loop control module and the open-loop control module, and selects one of the first signal and the second signal as a control signal for the transistor.

[0020] Preferably, the closed-loop control module includes:

[0021] a comparator, comparing the first current sampling signal with a first reference voltage to generate a detection signal;

[0022] A reference generation module, connected to the comparator, converts the detection signal into a voltage signal;

[0023] an integration module, connected to the reference generation module, for comparing and integrating the first current sampling signal with the voltage signal to generate a compensation signal;

[0024] An addition and subtraction circuit is connected to the integration module and subtracts the compensation signal from the feedback signal of the DC bus voltage to generate the first signal.

[0025] Preferably, the integration module includes:

[0026] a first resistor and a first capacitor; and

[0027] The first operational amplifier,

[0028] The inverting input terminal of the first operational amplifier is connected to the intermediate node of the first sampling resistor and the second sampling resistor via a first resistor to receive the first current sampling signal, the non-inverting input terminal is connected to the reference generation module to receive the voltage signal, the first capacitor is connected between the inverting input terminal and the output terminal of the first operational amplifier, and the output terminal of the first operational amplifier provides the compensation signal.

[0029] Preferably, the closed-loop control module further includes: a second resistor, and the inverting input terminal of the first operational amplifier receives a second reference voltage via the second resistor.

[0030] Preferably, the addition and subtraction circuit further adds the compensation signal to a third reference voltage.

[0031] Preferably, the closed-loop control module further includes: a third resistor and a fourth resistor connected in series between the positive output terminal and the negative output terminal of the rectifier bridge, and the addition and subtraction circuit is connected to the intermediate node of the third resistor and the fourth resistor to obtain the feedback signal of the DC bus voltage.

[0032] Preferably, the reference generation module includes:

[0033] a fifth resistor and a third capacitor connected in series between the comparator and ground to form a low-pass filter, and providing the voltage signal at an intermediate node between the fifth resistor and the third capacitor; and

[0034] A Zener diode is connected between a middle node between the fifth resistor and the third capacitor and the ground, and performs high clamping on the voltage signal.

[0035] Preferably, the open-loop control module includes:

[0036] a second operational amplifier, wherein a non-inverting input terminal of the second operational amplifier receives a fourth reference voltage, and an inverting input terminal is connected to an intermediate node between the transistor and the second sampling resistor to obtain the second current sampling signal;

[0037] The fourth reference voltage is a voltage relative to a potential of an intermediate node between the first sampling resistor and the second sampling resistor.

[0038] Preferably, the reference current is equal to the fourth reference voltage divided by the resistance value of the second sampling resistor.

[0039] Preferably, the selection module includes:

[0040] a first diode and a second diode,

[0041] The anode of the first diode is connected to the output end of the closed-loop control module to obtain the first signal.

[0042] The anode of the second diode is connected to the output terminal of the open-loop control module to obtain the second signal.

[0043] Cathodes of the first diode and the second diode are connected to each other to provide a control signal of the transistor.

[0044] Preferably, the method further comprises: a sixth resistor connected between the control terminal of the transistor and the ground.

[0045] Preferably, the current control circuit can also selectively operate in a mixed mode according to the conduction angle range, in which the load current is adjusted in an open-loop mode and a closed-loop mode in succession during the duration of the load current.

[0046] According to another aspect of the present invention, a dimming control method for an LED driving circuit is provided, comprising:

[0047] generating a chopped AC input voltage based on a dimming action;

[0048] Rectifying the chopped AC input voltage into a DC bus voltage with a half power frequency cycle;

[0049] Using DC bus voltage to supply power, generate load current and bleed current; and

[0050] Adjusting the duration of the load current in the half power frequency cycle according to the DC bus voltage,

[0051] The conduction angle range of the dimmer is obtained according to the detected value of the load current, and the load current is selectively adjusted in one of an open-loop mode and a closed-loop mode.

[0052] In the open-loop mode, the load current is maintained at a reference current,

[0053] In the closed-loop mode, the load current is regulated according to the negative feedback of the DC bus voltage.

[0054] Preferably, the detected value of the load current is an average value of the load circuit in the half power frequency cycle.

[0055] Preferably, in the closed-loop mode, the load current is regulated in a negative feedback manner in response to fluctuations of the DC bus voltage in successive half-power frequency cycles.

[0056] Preferably, according to the conduction angle range, it is also possible to selectively operate in a mixed mode, in which the load current is adjusted in an open-loop mode and a closed-loop mode in succession during the duration of the load current.

[0057] Preferably, the conduction angle range corresponding to the closed-loop mode is greater than the conduction angle range of the hybrid mode, and the conduction angle range of the hybrid mode is greater than the conduction angle range corresponding to the open-loop mode.

[0058] Preferably, in the closed-loop mode, the step of adjusting the load current comprises:

[0059] comparing a first current sampling signal of the load current with a first reference voltage to generate a detection signal;

[0060] Converting the detection signal into a high-clamped voltage signal;

[0061] Comparing and integrating the first current sampling signal with the voltage signal to generate a compensation signal; and

[0062] The compensation signal is subtracted from the feedback signal of the DC bus voltage to generate the first signal.

[0063] Preferably, the step of comparing and integrating the first current sampling signal with the voltage signal includes:

[0064] A second reference voltage is used to obtain a bias voltage.

[0065] Preferably, the method further comprises: adding the compensation signal to a third reference voltage.

[0066] Preferably, in the open-loop mode, the step of adjusting the load current comprises:

[0067] The second current sampling signal of the load current is compared with a fourth reference voltage to generate a second signal.

[0068] Preferably, the load current in the open-loop mode is less than or equal to the load current in the closed-loop mode.

[0069] Preferably, the discharge current and the load current are both greater than the minimum holding current of the thyristor.

[0070] According to the LED drive circuit and dimming control method of the embodiments of the present invention, the current control circuit selects the operating mode of the current control circuit according to the detection value of the load current, maintains the load current as the reference current in the open-loop mode, and adjusts the load current according to the negative feedback of the DC bus voltage in the closed-loop mode.

[0071] In a preferred embodiment, the current control circuit includes a transistor, and a closed-loop control module and an open-loop control module for controlling the conduction state of the transistor. At different conduction angles of the thyristor in the dimmer, one of the closed-loop control module and the open-loop control module of the current control circuit controls the conduction state of the transistor. For example, within a small conduction angle range, the open-loop control module controls the conduction state of the transistor to maintain a constant voltage drop across the sampling resistor, thereby maintaining the load current at a predetermined value. Within a large conduction angle range, the closed-loop control module controls the conduction state of the transistor, providing negative feedback compensation for the load current based on the voltage waveform of the DC bus voltage.

[0072] The brightness of the LED lamp as a load is the superposition effect of the amplitude of the load current and its duration in the half cycle. As mentioned above, in the thyristor dimming control system of the prior art, in the continuous half cycle of the power frequency cycle, the higher the amplitude of the AC input voltage, the longer the load current lasts in the half cycle of the power frequency cycle, and the higher the amplitude of the load current, the brightness of the LED lamp as the load changes with the half cycle, thus causing the "flash light" phenomenon. Unlike the prior art, in the thyristor dimming according to the embodiment of the present invention, it selectively operates in one of the open-loop mode and the closed-loop mode according to the detection value of the load current, and in the closed-loop mode, the detection value of the load current is negatively feedback-regulated in response to the fluctuation of the AC input voltage, thereby maintaining the stable brightness of the LED lamp in the half cycle of the power frequency cycle. This negative feedback regulation also improves the efficiency of the LED drive circuit.

[0073] For large conduction angles, as the AC input voltage increases, the closed-loop control module in the current control circuit provides negative feedback to regulate the load current. This control method not only responds to user dimming actions to adjust the LED's brightness, but also maintains a roughly stable LED brightness over successive half-cycles for the set conduction angle, thereby reducing or even eliminating flickering.

[0074] For small conduction angles, the open-loop control module in the current control circuit maintains the load current at a reference current, making the load current in open-loop mode smaller than that in closed-loop mode. This reduces the impact of AC input voltage fluctuations on the brightness of the LED lamp over successive half-cycles. This control method not only responds to user dimming actions to adjust the LED lamp's brightness, but also maintains a roughly stable LED lamp brightness over successive half-cycles for the conduction angle set for brightness adjustment, thereby reducing or even eliminating flickering.

[0075] In a preferred embodiment, the closed-loop control module in the current control circuit superimposes a compensation signal related to the load current with the third reference voltage to further improve the response speed to fluctuations in the AC input voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings.

[0077] Figure 1 A schematic circuit diagram of a thyristor dimming control system according to the prior art is shown.

[0078] Figure 2 The figure shows the working waveform of the thyristor dimming control system according to the prior art.

[0079] Figure 3 A schematic circuit diagram of a thyristor dimming control system according to an embodiment of the present invention is shown.

[0080] Figure 4 A schematic circuit diagram of a current control circuit in a thyristor dimming control system according to an embodiment of the present invention is shown.

[0081] Figure 5 Show Figure 4 The circuit diagram of the reference generation module in the current control circuit shown.

[0082] Figures 6 to 8 The following respectively illustrate working waveforms of the thyristor dimming control system according to an embodiment of the present invention in different conduction angle ranges.

[0083] Figure 9 The figure shows the working waveform of the thyristor dimming control system transitioning from the open-loop mode to the closed-loop mode according to an embodiment of the present invention. DETAILED DESCRIPTION

[0084] Various embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. In each of the accompanying drawings, identical elements are represented by identical or similar reference numerals. For the sake of clarity, the various parts in the accompanying drawings are not drawn to scale.

[0085] In this application, the term "conduction angle range" refers to the electrical angle range within which the thyristor (SCR) in the dimmer conducts during half the AC voltage cycle when the user dims the light. For example, during the positive half-cycle, the SCR's small conduction angle range is 120 to 180 degrees, while the large conduction angle range is 90 to 180 degrees. The brightness of an LED lamp in the small conduction angle range is lower than that in the large conduction angle range.

[0086] Figure 1 A schematic circuit diagram of a thyristor dimming control system according to the prior art is shown. The thyristor dimming control system 100 includes a dimmer 110 and an LED driving circuit. The LED driving circuit includes a rectifier bridge 120, a bleeder circuit 130, and a main circuit.

[0087] like Figure 1 As shown, the dimmer 110 is connected in series between the AC power supply terminal and the input terminal of the rectifier bridge 120. The dimmer 110 chops the AC input voltage VAC according to the user's dimming action, thereby generating a chopped AC voltage, the conduction angle of which is used to represent the dimming value.

[0088] The rectifier bridge 120 rectifies the AC voltage into a pulsating DC bus voltage VBUS, which has, for example, a half-power frequency cycle. The main circuit and the bleeder circuit 130 are connected in parallel between the positive output terminal and the negative output terminal of the rectifier bridge 120, where the positive output terminal is, for example, the positive terminal and the negative output terminal is, for example, the negative terminal.

[0089] The bleeder circuit 130 includes a transistor Q2 and a resistor RS3 connected sequentially between the positive output terminal and the negative output terminal of the rectifier bridge 120, and a bleeder control module 131 that provides a control signal VG1 to the control terminal of the transistor Q2. The transistor Q2 can be a single bipolar transistor, a metal oxide semiconductor field effect transistor, or a combination of multiple bipolar transistors or metal oxide semiconductor field effect transistors.

[0090] The main circuit includes a diode D01, a load LED, and a current control circuit 140, connected in series between the positive and negative output terminals of the rectifier bridge 120. A capacitor C01 is also connected in parallel with the load LED. The anode of diode D01 is connected to the positive output terminal of the rectifier bridge 120, and the cathode is connected to the anode terminal of the load LED, thereby preventing the load current Io from flowing backward. The current control circuit 140 includes a transistor Q3 and a sampling resistor RS1, connected in series with the load LED. Furthermore, a constant current control module 141 provides a control signal VG2 to the control terminal of transistor Q3. Transistor Q3 can be a single bipolar transistor, a metal-oxide-semiconductor field-effect transistor, or a combination of multiple bipolar transistors or metal-oxide-semiconductor field-effect transistors.

[0091] When thyristor dimming control system 100 is in operation, bleeder circuit 130 and the main circuit together serve as the load for rectifier bridge 120, thereby maintaining normal operation of the thyristor in dimmer 110 during the thyristor off period. Input current Iin provided by rectifier bridge 120 is split into bleeder current Ib and load current Io by bleeder circuit 130 and the main circuit, respectively.

[0092] Transistor Q3 in current control circuit 140 operates in the linear region. Load current Io flows sequentially through transistor Q3 and sampling resistor RS1. The voltage drop across sampling resistor RS1 is a current sampling signal VS, which represents the value of load current Io. In current control circuit 140, constant current control module 141 adjusts the magnitude of control signal VG2 based on current sampling signal VS to maintain a roughly stable load current Io. It also adjusts the duration of load current Io within a half-power cycle based on DC bus voltage VBUS, thereby achieving dimming.

[0093] Figure 2 The following figure shows the working waveform of the thyristor dimming control system according to the prior art. Curves VBUS and Io respectively represent the changes of DC bus voltage and load current over time. In the dimmer, the AC input voltage is chopped according to the user's dimming action. In the LED drive circuit, the rectifier bridge 120 rectifies the chopped AC input voltage into the DC bus voltage VBUS. Figure 2 As shown in the figure, if the DC bus voltage VBUS is greater than the load voltage VLED, a load current Io flows through the LED lamp, illuminating it. If the DC bus voltage VBUS is less than or equal to the load voltage VLED, no load current Io is generated. Therefore, during both the positive and negative half-cycles of the power frequency cycle, the LED lamp maintains an illumination time corresponding to the chopped waveform of the AC input voltage, achieving a brightness corresponding to the dimming operation.

[0094] Due to the inherent characteristics of thyristors (SCRs), dimmers generate asymmetric AC voltage waveforms during the positive and negative half-cycles of the AC voltage. In existing LED driver circuits, this asymmetric AC voltage waveform results in load currents with varying amplitudes and durations during the positive and negative half-cycles of the power frequency cycle, leading to the "flickering" problem.

[0095] Figure 3 FIG2 shows a schematic circuit diagram of a thyristor dimming control system according to an embodiment of the present invention. The thyristor dimming control system 200 includes a dimmer 110 and an LED driving circuit. The working mode of the dimmer 110 is similar to that of the thyristor dimming control system 200. Figure 1The thyristor dimming control system of the prior art is the same as that shown, and will not be described in detail here. The LED driving circuit includes a rectifier bridge 120, a discharge circuit 130, and a main circuit. The following only describes the working mode of the LED driving circuit in detail.

[0096] The rectifier bridge 120 rectifies the AC voltage into a pulsating DC bus voltage VBUS, which has, for example, a half-power frequency cycle. The main circuit and the discharge circuit 130 are connected in parallel between the positive output terminal and the negative output terminal of the rectifier bridge 120 .

[0097] The bleeder circuit 130 includes a transistor Q2 and a resistor RS3 connected sequentially between the positive output terminal and the negative output terminal of the rectifier bridge 120, and a bleeder control module 131 that provides a control signal VG1 to the control terminal of the transistor Q2. The transistor Q2 can be a single bipolar transistor, a metal oxide semiconductor field effect transistor, or a combination of multiple bipolar transistors or metal oxide semiconductor field effect transistors.

[0098] The main circuit includes a diode D01, a load LED, and a current control circuit 240, connected in series between the positive and negative output terminals of the rectifier bridge 120. Furthermore, a capacitor C01 is connected in parallel with the load LED. The anode of diode D01 is connected to the positive output terminal of the rectifier bridge 120, and the cathode is connected to the load LED, thereby preventing the load current Io from flowing backward.

[0099] The current control circuit 240 includes a closed-loop control module 241, an open-loop control module 242, a selection module 243, and a transistor Q3 and sampling resistors RS2 and RS1 connected in series between the load LED and ground. The closed-loop control module 241 is connected to both ends of the sampling resistor RS1 to obtain a current sampling signal VS1, to the positive output terminal of the rectifier bridge 120 to obtain the DC bus voltage VBUS, or to the intermediate node between the load LED and transistor Q3 to obtain a signal corresponding to the DC bus voltage VBUS. The closed-loop control module 241 generates a first signal VC1. The open-loop control module 242 is connected to both ends of the resistor RS2 to obtain a current sampling signal VS2 and a second signal VC2. The selection module 243 selects one of the first and second signals VC1 and VC2 as the control signal VG2 for transistor Q3, thereby controlling its conduction state. Transistor Q3 can be a single bipolar transistor, a metal-oxide-semiconductor field-effect transistor, or a combination of multiple bipolar transistors or metal-oxide-semiconductor field-effect transistors.

[0100] When the thyristor dimming control system 200 is in operation, the bleeder circuit 130 and the main circuit together serve as the load for the rectifier bridge 120, thereby maintaining normal operation of the thyristor in the dimmer 110 during the thyristor off period. The input current Iin provided by the rectifier bridge 120 is split into a bleeder current Ib and a load current Io by the bleeder circuit 130 and the main circuit, respectively. The bleeder current Ib is maintained at a constant current, for example.

[0101] In the main circuit, the load current Io flows sequentially through the load LED and the current control circuit 240. Furthermore, transistor Q3 in the current control circuit 240 operates in the linear region, and the voltage drops across sampling resistors RS1 and RS2 serve as current sampling signals VS1 and VS2, respectively, representing the value of the load current Io. The closed-loop control module 241 generates a first signal VC1 based on the current sampling signal VS1 and the DC bus voltage VBUS. The open-loop control module 242 generates a second signal VC2 based on the current sampling signal VS2. The selection module 243 selects one of the first and second signals VC1 and VC2 as the control signal for transistor Q3.

[0102] At different conduction angles of the thyristor in the dimmer 110, either the closed-loop control module 241 or the open-loop control module 242 can control the conduction state of transistor Q3. For example, in a small conduction angle range, the open-loop control module 242 controls the conduction state of transistor Q3 to maintain a constant voltage drop across the sampling resistor RS2. Thus, the load current Io is maintained at a predetermined value and does not change with the user's dimming operation. In a large conduction angle range, the closed-loop control module 241 controls the conduction state of transistor Q3 and provides negative feedback compensation for the load current Io based on the voltage waveform of the DC bus voltage VBUS.

[0103] The brightness of the LED lamp, acting as a load, is the combined effect of the load current's amplitude and duration within a half-cycle. As described above, in prior art thyristor dimming control systems, the higher the voltage amplitude during consecutive half-cycles of the power frequency cycle, the longer the load current lasts during that half-cycle. Furthermore, the higher the load current amplitude, the asymmetric AC voltage waveform at the same conduction angle, causing the brightness of the LED lamp, acting as a load, to vary over the half-cycle, resulting in a "flickering" phenomenon. Unlike prior art, in thyristor dimming according to embodiments of the present invention, the higher the voltage amplitude during consecutive half-cycles of the power frequency cycle, the longer the load current lasts during that half-cycle. However, the closed-loop control module 241 in the current control circuit 240 performs negative feedback regulation on the load current Io. This control method not only responds to a user's dimming action to adjust the brightness of the LED lamp to achieve brightness adjustment, but also maintains a substantially stable brightness across consecutive half-cycles for the conduction angle set for the brightness adjustment, thereby reducing or even eliminating the "flickering" phenomenon.

[0104] Figure 4 FIG2 shows a schematic circuit diagram of a current control circuit in a thyristor dimming control system according to an embodiment of the present invention. Figure 3 The current control circuit 240 includes a closed-loop control module 241, an open-loop control module 242, a selection module 243, and a transistor Q3 and sampling resistors RS2 and RS1 connected in series between the load LED and the ground terminal. Figure 4 24 further shows the internal circuit structures of the closed-loop control module 241, the open-loop control module 242 and the selection module 243.

[0105] The closed-loop control module 241 includes a resistor R3, a resistor R4, a resistor R5, a resistor R6, a capacitor C2, a first reference voltage source providing a reference voltage REF1, a second reference voltage source providing a reference voltage REF2, a third reference voltage source providing a reference voltage REF3, a comparator B1, an operational amplifier A1, an addition and subtraction circuit U1, and a reference generation module U2.

[0106] Comparator B1's non-inverting input is connected to the midpoint between sampling resistors RS1 and RS2 to receive the current sampling signal VS1, while its inverting input receives the reference voltage REF1. Specifically, the positive and negative terminals of a first reference voltage source are connected to the inverting input of comparator B1 and ground, respectively. Comparator B1 compares the current sampling signal VS1 with the reference voltage REF1 to generate a detection signal. The input of reference generation module U2 is connected to the output of comparator B1 to obtain the detection signal and generate a voltage signal corresponding to the detection signal.

[0107] Operational amplifier A1's non-inverting input is connected to the output of reference generator module U2 to receive a voltage signal. Its inverting input receives reference voltage REF2 via resistor R5 and is connected to the node between sampling resistors RS1 and RS2 via resistor R6. Capacitor C2 is connected between the output and inverting input of operational amplifier A1. Operational amplifier A1, resistor R6, and capacitor C2 form an integration module. Operational amplifier A1 compares the voltage signal provided by reference generator module U2 with reference voltage REF2 and performs an integration operation to obtain a compensation signal COMP.

[0108] Adder / subtractor circuit U1 has a first positive input connected to the output of operational amplifier A1 to receive compensation signal COMP. A second positive input receives reference voltage REF3. A negative input is connected to the node between resistors R3 and R4 to obtain a feedback signal. An output provides a second signal VC2. Resistors R3 and R4 are connected in series between the positive and negative output terminals of rectifier bridge 120 to obtain DC bus voltage VBUS.

[0109] The open-loop control module 242 includes a fourth reference voltage source providing a reference voltage REF4 and an operational amplifier A2 .

[0110] One end of resistor RS2 is connected to the inverting input of operational amplifier A2, and the other end is connected to resistor RS1. The non-inverting input of operational amplifier A2 receives reference voltage REF4. The negative terminal of the fourth reference voltage source is connected to the midpoint between sampling resistors RS1 and RS2. That is, reference voltage REF4 is a predetermined voltage relative to the potential of the midpoint between sampling resistors RS1 and RS2. The inverting input of operational amplifier A2 is connected to the midpoint between transistor Q3 and sampling resistor RS2. Operational amplifier A2 compares current sampling signal VS2 with reference voltage REF4 to generate a second signal VC2.

[0111] Selection module 243 includes diodes D2 and D3. The anode of diode D2 is connected to the output of closed-loop control module 241 to receive the first signal VC1. The anode of diode D3 is connected to the output of open-loop control module 242 to receive the second signal VC2. The cathodes of diodes D2 and D3 are connected to each other and to the control terminal of transistor Q3. Selection module 243 compares the first signal VC1 and the second signal VC2 and selects the one with the higher signal level as the control signal VG2 of transistor Q3.

[0112] In a preferred embodiment, current control circuit 240 further includes a resistor R7 connected between transistor Q3 and ground. Resistor R7 serves as a discharge resistor from the control terminal to ground. After the control terminal of transistor Q3 attains a high voltage, if it regains a low voltage, the control terminal discharges through resistor R7, thereby returning to a low-voltage control state.

[0113] exist Figure 4 In the illustrated current control circuit 240, the selection module 243 selects one of the first signal VC1 and the second signal VC2. Therefore, the closed-loop control module 241 and the open-loop control module 242 are selectively used to control the load current. Accordingly, the current control circuit 240 operates in one of the closed-loop mode, the open-loop mode, and the mixed mode. In the mixed mode, the current control circuit 240 operates in the open-loop mode and the closed-loop mode sequentially during the duration of the load current.

[0114] In the case where the signal level of the first signal VC1 is less than the signal level of the second signal VC2 (for example, a small conduction angle range of the thyristor), the current control circuit 240 operates in an open-loop mode. When the DC bus voltage VBUS is less than or equal to the load voltage VLED, the transistor Q3 cannot operate. When the DC bus voltage VBUS is greater than the load voltage VLED, the transistor Q3 operates. If Io > (REF4 / RS2), the output of the operational amplifier A2 is at a low level, the open-loop control module 242 is ineffective, and the closed-loop control module 241 is effective; if Io <= (REF4 / RS2), the operational amplifier A2 quickly adjusts the output voltage so that Io = (REF4 / RS2). This moment is very short and can almost be considered to be completed immediately. The open-loop control module 242 is effective, and the closed-loop control module 241 is ineffective.

[0115] When the open-loop control module 242 is effective, the load current Io is maintained at a predetermined value. At this time, Io = REF4 / RS2. Therefore, REF4 / RS2 represents the reference current, which should be greater than the minimum holding current of the thyristor.

[0116] In the case where the signal level of the first signal VC1 is greater than or equal to the signal level of the second signal VC2 (for example, a large conduction angle range of the thyristor), the current control circuit 240 operates in a closed-loop mode. REF1 < REF4*RS1 / RS2. When the load current Io is greater than zero, the detection signal output by the comparator B1 is at a high level, which represents the duration of the load current Io. The larger the conduction angle, the longer the duration of the load current Io. The reference generation module U2 converts the square wave of the detection signal into a voltage signal. The longer the duration of the load current Io, the higher the level of the voltage signal. When the duration of the load current Io increases to a certain value, even if the duration further increases, the voltage signal no longer increases, that is, the load current is high-clamped.

[0117] Operational amplifier A1, resistor R6, and capacitor C2 form an integration module that generates compensation signal COMP. Resistors R5 and R6 form a superposition circuit. Because resistors R5 and R6 are much larger than sampling resistor RS1, a bias voltage is present at the inverting input of operational amplifier A1 even when there is no voltage across sampling resistor RS1. This bias voltage is Vp = REF2 * R6 / (R5 + R6). Due to the bias voltage at the inverting input of operational amplifier A1, when the voltage signal generated by reference generation circuit U2 is less than or equal to (Vp + D * REF4 * RS1 * R5 / RS2 / (R5 + R6)), where D is the duty cycle of the main circuit operating time during half the power frequency cycle, operational amplifier A1 is saturated, and compensation signal COMP is low. When the voltage signal generated by the reference generation circuit U2 is greater than (Vp + D*REF4*RS1*R5 / RS2 / (R5+R6)), the compensation signal COMP generated by the operational amplifier A1 flips to a high level, causing the closed-loop control module 241 of the current control circuit 240 to control the conduction state of the transistor Q3, thereby adjusting the detected value of the load current Io. The addition and subtraction circuit U1 generates a first signal VC1 based on the DC bus voltage VBUS, the compensation signal COMP, and the reference voltage REF3. Resistors R3 and R4 divide the DC bus voltage VBUS into the feedback signal Vin. The first signal VC1 generated by the addition and subtraction circuit U1 is calculated as COMP + REF3 - Vin. Therefore, the higher the amplitude of the AC input voltage, the smaller the control signal VG2 of the transistor Q3, and accordingly, the smaller the instantaneous load current Io.

[0118] When the closed-loop control module 241 is in effect, in the case of asymmetry in the AC voltage, negative feedback regulation is performed on the load current Io in response to fluctuations in the positive and negative half-cycles of the AC voltage in the power frequency cycle, so that the brightness of the LED lamp can be maintained approximately stable in consecutive half-cycles.

[0119] It will be appreciated that the aforementioned small conduction angle range and large conduction angle range are relative terms, intended merely to facilitate understanding of the embodiments of the present invention, and are not intended to limit the embodiments of the present invention. For example, in the current control circuit 240, the value of the reference voltage REF1 is set to achieve the small conduction angle range and the large conduction angle range required by a predetermined circuit design.

[0120] Figure 5 Show Figure 4 The circuit diagram of the reference generation module in the current control circuit shown in FIG. The reference generation module U2 converts the square wave of the detection signal into a voltage signal.

[0121] Reference generation module U2 includes a Zener diode Z1, a capacitor C11, and a resistor R11. Resistor R11 and capacitor C11 are connected in series between the input of reference generation module U2 and ground, forming a low-pass filter that filters the square wave of the detection signal into a voltage signal corresponding to its detection value. The node between resistor R11 and capacitor C11 is connected to the output of reference generation module U2 to provide a voltage signal. Zener diode Z1, connected between the output of reference generation module U2 and ground, provides a high-level clamp for this voltage signal.

[0122] Therefore, the magnitude of the voltage signal generated by the reference generation module U2 is related to the duration of the load current Io. The longer the duration of the load current Io, the higher the level of the voltage signal. When the duration of the load current Io increases to a certain value, even if the duration increases further, the voltage signal will no longer increase due to the high clamp.

[0123] Figures 6 to 8 The following diagrams illustrate the operating waveforms of a thyristor dimming control system according to an embodiment of the present invention at different conduction angle ranges. In the diagram, curves VG1, VBUS, Io, and REF4 / RS2 respectively represent the control signal of transistor Q2 in the bleeder circuit 130, and the DC bus voltage, load current, and reference current in the current control circuit over time.

[0124] like Figure 6As shown, during the power frequency half-cycle from t0 to t3, the conduction angle range of the thyristor in the dimmer 110 is, for example, 30 degrees to 180 degrees, and the current control circuit 240 in the LED driver circuit accordingly operates in closed-loop mode. During the time periods from t0 to t1 and t2 to t3, the DC bus voltage VBUS does not exceed the load voltage VLED, and transistor Q3 of the current control circuit 240 is off. During the time period from t1 to t2, the DC bus voltage VBUS exceeds the load voltage VLED, and transistor Q3 of the current control circuit 240 is turned on. The closed-loop control module 241 of the current control circuit 240 controls the conduction state of transistor Q3, thereby controlling the load current Io. Because the load current Io lasts for a long time in this conduction angle range, the voltage signal output by the reference generation module U2 is highly clamped. Operational amplifier A1 generates a compensation signal COMP based on the voltage signal. Adder-subtractor circuit U1 generates a first signal VC1 based on the DC bus voltage VBUS and the compensation signal COMP, which serves as a control signal VG2 for transistor Q3 and is provided to the control terminal of transistor Q3. In this mode, the higher the DC bus voltage VBUS, the lower the control signal VG2 for transistor Q3. Therefore, the load current Io exhibits a U-shaped waveform as shown in the figure. Because the operating current Io is always greater than the set reference current REF4 / RS2, only the closed-loop control module 241 in the current control circuit 240 controls the conduction state of transistor Q3, while the open-loop control module 242 is inactive.

[0125] The transistor Q2 of the discharge circuit 130 is turned on during the time periods t0 - t1 and t2 - t3 , and is turned off during the time period t1 - t2 .

[0126] like Figure 7As shown, during the power frequency half-cycle from t0 to t4, the conduction angle range of the thyristor in the dimmer 110 is, for example, 100 to 180 degrees. The current control circuit 240 in the LED driver circuit operates in a hybrid mode, that is, operating in open-loop mode and closed-loop mode. During the time periods from t0 to t1 and t3 to t4, the DC bus voltage VBUS does not exceed the load voltage VLED, and the transistor Q3 of the current control circuit 240 is off. During the time period from t1 to t3, the DC bus voltage VBUS exceeds the load voltage VLED, and the transistor Q3 of the current control circuit 240 is turned on. In the closed-loop control module 241 of the current control circuit 240, the comparator B1 and the reference generation module U2 convert the duration of the load circuit Io into a corresponding voltage signal. During the time period t1-t2, because the addition and subtraction circuit U1 of the closed-loop control module 241 negatively compensates the DC bus voltage, the load current Io is less than the reference current REF4 / RS2. Therefore, in the current control circuit 240, only the open-loop control module 242 controls the conduction state of the transistor Q3, while the closed-loop control module 241 is inoperative. The load current Io remains at the reference current REF4 / RS2 until it can no longer be maintained constant and increases. During the time period t2-t3, the load current Io exceeds the reference current REF4 / RS2. Therefore, in the current control circuit 240, only the closed-loop control module 241 controls the conduction state of the transistor Q3, while the open-loop control module 242 is inoperative.

[0127] When the open-loop control module 242 of the current control circuit 240 is in effect, the current control circuit 240 maintains a constant load current Io. When the closed-loop control module 241 of the current control circuit 240 is in effect, the current control circuit 240 performs negative feedback regulation on the load current Io in response to fluctuations in the AC voltage during the positive and negative half-cycles of the power frequency cycle, thereby maintaining a substantially stable brightness of the LED lamp over consecutive half-cycles.

[0128] The transistor Q2 of the discharge circuit 130 is turned on during the period t0 - t1 ′ and turned off during the period t1 ′ - t4 .

[0129] like Figure 8As shown, during the power frequency half-cycle from t0 to t3, the conduction angle range of the thyristor in the dimmer 110 is, for example, 135 degrees to 180 degrees, and the current control circuit 240 in the LED driver circuit accordingly operates in open-loop mode. During the time periods t0 to t1 and t2 to t3, the DC bus voltage VBUS does not exceed the load voltage VLED, and the transistor Q3 of the current control circuit 240 is off. During the time period t1 to t2, the DC bus voltage VBUS exceeds the load voltage VLED, and the transistor Q3 of the current control circuit 240 is turned on. During the time period t1 to t2, in the closed-loop control module 241, the voltage signal generated by the reference generation module U2 is less than or equal to (Vp + D*REF4*RS1*R5 / RS2 / (R5+R6)), and the compensation signal COMP generated by the operational amplifier A1 is saturated and at a low level. At this time, the load current Io is fully controlled by the open-loop control module 242. Therefore, in the current control circuit 240, only the open-loop control module 242 controls the conduction state of the transistor Q3, while the closed-loop control module 241 does not function. The load current Io is maintained at the reference current REF4 / RS2.

[0130] When the open-loop control module 242 of the current control circuit 240 is active, the current control circuit 240 maintains a constant load current Io. From a waveform perspective, when the dimmer generates an asymmetrical AC voltage waveform during the positive and negative half-cycles of the AC voltage, the peak value of the load current Io generated by the current control circuit 240 in consecutive half-cycles is the same, but the duration is asymmetrical. When the current control circuit 240 operates in open-loop mode, the open-loop control module 242 maintains a low load current, thereby reducing LED brightness fluctuations and significantly alleviating the "flickering" phenomenon.

[0131] The transistor Q2 of the discharge circuit 130 is turned on during the time period t0 - t1 ′ and turned off during the time period t1 ′ - t3 .

[0132] Figure 9 The following is a working waveform diagram of the thyristor dimming control system according to an embodiment of the present invention transitioning from an open-loop mode to a closed-loop mode. In the diagram, the curve COMP represents the variation of the compensation signal in the current control circuit over time.

[0133] In the closed-loop control module 241 of the current control circuit 240 , the addition and subtraction circuit U1 superimposes the DC bus voltage VBUS, the compensation signal COMP, and the reference voltage REF3 to generate a first signal VC1 .

[0134] If reference voltage REF3 is not superimposed, the transition from open-loop mode to closed-loop mode occurs between t5 and t7. If reference voltage RFF3 is superimposed, the transition from open-loop mode to closed-loop mode occurs between t5 and t6. Therefore, superimposing reference voltage REF3 on compensation signal COMP improves the response speed of the transition from open-loop mode to closed-loop mode, significantly alleviating the problem of sudden current changes caused by response time.

[0135] The embodiments of the present invention are described above. These embodiments do not describe all details in detail, nor do they limit the invention to the specific embodiments described. Obviously, based on the above description, many modifications and variations can be made. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can make good use of the present invention and modify and use it based on the present invention. The scope of protection of the present invention shall be based on the scope defined by the claims of the present invention.

Claims

1. An LED driver circuit connected to a dimmer to obtain a chopped AC input voltage and connected to an LED to control a load current flowing through the LED, the LED driver circuit comprising: A rectifier bridge, used for converting the chopped AC input voltage into a DC bus voltage with a half power frequency cycle; as well as A current control circuit is connected in series with the LED between the positive output terminal and the negative output terminal of the rectifier bridge, and adjusts the duration of the load current in the half power frequency cycle according to the DC bus voltage to achieve dimming. The current control circuit obtains the conduction angle range of the dimmer according to the detected value of the load current, selectively operates in one of an open-loop mode and a closed-loop mode, and performs negative feedback regulation on the detected value of the load current in response to fluctuations of the AC input voltage in the closed-loop mode. In the open-loop mode, the current control circuit maintains the load current at a reference current. In the closed-loop mode, the current control circuit adjusts the load current according to the negative feedback of the DC bus voltage. The dimmer includes a thyristor, and the reference current is greater than a minimum holding current of the thyristor.

2. The LED driving circuit according to claim 1, further comprising: A discharge circuit, connected between the positive output terminal and the negative output terminal of the rectifier bridge, for providing a discharge current, wherein the discharge current is used to provide a holding current for the dimmer; as well as A diode, wherein the cathode of the diode is connected to the positive output terminal of the rectifier bridge, and the cathode of the diode is connected to the anode of the LED.

3. The LED driving circuit according to claim 1, wherein: The current control circuit comprises: a transistor, a first sampling resistor, and a second sampling resistor connected in series with the LED; a closed-loop control module, configured to generate a first signal according to the DC bus voltage and a first current sampling signal at both ends of the first sampling resistor; an open-loop control module, generating a second signal according to a second current sampling signal across the second sampling resistor; and A selection module is connected to the closed-loop control module and the open-loop control module, and selects one of the first signal and the second signal as a control signal for the transistor.

4. The LED driving circuit according to claim 3, wherein: The closed-loop control module includes: a comparator, comparing the first current sampling signal with a first reference voltage to generate a detection signal; A reference generation module, connected to the comparator, converts the detection signal into a voltage signal; an integration module, connected to the reference generation module, for comparing and integrating the first current sampling signal with the voltage signal to generate a compensation signal; An addition and subtraction circuit is connected to the integration module and subtracts the compensation signal from the feedback signal of the DC bus voltage to generate the first signal.

5. The LED driving circuit according to claim 4, wherein: The integration module includes: a first resistor and a first capacitor; and The first operational amplifier, The inverting input terminal of the first operational amplifier is connected to the intermediate node of the first sampling resistor and the second sampling resistor via a first resistor to receive the first current sampling signal, the non-inverting input terminal is connected to the reference generation module to receive the voltage signal, the first capacitor is connected between the inverting input terminal and the output terminal of the first operational amplifier, and the output terminal of the first operational amplifier provides the compensation signal.

6. The LED driving circuit according to claim 5, wherein: The closed-loop control module further includes a second resistor, and the inverting input terminal of the first operational amplifier receives a second reference voltage via the second resistor.

7. The LED driving circuit according to claim 4, wherein: The addition and subtraction circuit further adds the compensation signal to a third reference voltage.

8. The LED driving circuit according to claim 4, wherein: The closed-loop control module also includes: a third resistor and a fourth resistor connected in series between the positive output terminal and the negative output terminal of the rectifier bridge, and the addition and subtraction circuit is connected to the intermediate node of the third resistor and the fourth resistor to obtain the feedback signal of the DC bus voltage.

9. The LED driving circuit according to claim 4, wherein: The benchmark generation module includes: a fifth resistor and a third capacitor connected in series between the comparator and ground to form a low-pass filter, and providing the voltage signal at an intermediate node between the fifth resistor and the third capacitor; and A Zener diode is connected between a middle node between the fifth resistor and the third capacitor and the ground, and performs high clamping on the voltage signal.

10. The LED driving circuit according to claim 3, wherein: The open-loop control module includes: a second operational amplifier, wherein a non-inverting input terminal of the second operational amplifier receives a fourth reference voltage, and an inverting input terminal is connected to an intermediate node between the transistor and the second sampling resistor to obtain the second current sampling signal; The fourth reference voltage is a voltage relative to a potential of an intermediate node between the first sampling resistor and the second sampling resistor.

11. The LED driving circuit according to claim 10, wherein: The reference current is equal to the fourth reference voltage divided by the resistance value of the second sampling resistor.

12. The LED driving circuit according to claim 3, wherein: The selection module includes: a first diode and a second diode, The anode of the first diode is connected to the output end of the closed-loop control module to obtain the first signal. The anode of the second diode is connected to the output terminal of the open-loop control module to obtain the second signal. Cathodes of the first diode and the second diode are connected to each other to provide a control signal of the transistor.

13. The LED driving circuit according to claim 3, further comprising: a sixth resistor connected between the control terminal of the transistor and ground.

14. The LED driving circuit according to claim 1, wherein: The current control circuit may also selectively operate in a mixed mode according to the conduction angle range. In the mixed mode, the load current is adjusted in an open-loop mode and a closed-loop mode in sequence during the duration of the load current.

15. A dimming control method for an LED drive circuit, comprising: generating a chopped AC input voltage based on a dimming action; Rectifying the chopped AC input voltage into a DC bus voltage with a half power frequency cycle; It uses DC bus voltage to supply power and generates load current and discharge current; as well as Adjusting the duration of the load current in the half power frequency cycle according to the DC bus voltage, The conduction angle range of the dimmer is obtained according to the detected value of the load current, the load current is selectively adjusted in one of an open-loop mode and a closed-loop mode, and in the closed-loop mode, negative feedback adjustment is performed on the detected value of the load current in response to fluctuations of the AC input voltage. In the open-loop mode, the load current is maintained at a reference current, In the closed-loop mode, the load current is regulated according to the negative feedback of the DC bus voltage. The dimmer includes a thyristor, and the reference current is greater than a minimum holding current of the thyristor.

16. The control method according to claim 15, wherein: The detected value of the load current is an average value of the load circuit in the half power frequency cycle.

17. The control method according to claim 15, wherein: In the closed-loop mode, the load current is regulated in a negative feedback manner in response to fluctuations of the DC bus voltage in successive half-power frequency cycles.

18. The control method according to claim 15, wherein: Depending on the conduction angle range, it is also possible to selectively operate in a mixed mode, in which the load current is regulated successively in an open-loop mode and a closed-loop mode during the duration of the load current.

19. The control method according to claim 18, wherein: The conduction angle range corresponding to the closed-loop mode is greater than the conduction angle range of the hybrid mode, and the conduction angle range of the hybrid mode is greater than the conduction angle range corresponding to the open-loop mode.

20. The control method according to claim 15, wherein: In the closed-loop mode, the step of regulating the load current includes: comparing a first current sampling signal of the load current with a first reference voltage to generate a detection signal; Converting the detection signal into a high-clamped voltage signal; Comparing and integrating the first current sampling signal with the voltage signal to generate a compensation signal; and The compensation signal is subtracted from the feedback signal of the DC bus voltage to generate a first signal.

21. The control method according to claim 20, wherein: The steps of comparing and integrating the first current sampling signal with the voltage signal include: A second reference voltage is used to obtain a bias voltage.

22. The control method according to claim 20, further comprising: The compensation signal is added to a third reference voltage.

23. The control method according to claim 15, wherein: In the open-loop mode, the step of regulating the load current includes: The second current sampling signal of the load current is compared with a fourth reference voltage to generate a second signal.

24. The control method according to claim 15, wherein: The load current in the open-loop mode is less than or equal to the load current in the closed-loop mode.

25. The control method according to claim 15, wherein: The discharge current and the load current are both greater than the minimum holding current of the thyristor.

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