LED current control circuit and LED lighting device

Abstract

The invention provides an LED current control circuit and an LED lighting device, which can stabilize dimming control near deep dimming. The present invention is provided with a control circuit (20) for controlling an LED current on the basis of a feedback value (FB) obtained by converting a voltage (Vfb) (feedback signal) obtained by adding an offset voltage (Voff) to a detection voltage (Vs) into a digital signal, and for controlling the LED current when a current setting value (SET) is greater than or equal to a depth dimming threshold (TH) for determining whether or not the current setting value is near the depth dimming, the current setting value (SET) is greater than or equal to a depth dimming threshold (TH) for determining whether or not the current setting value (SET) is near the depth dimming. The control circuit (20) controls the LED current on the basis of the deviation between a feedback value (FB) obtained by cancelling an offset value (OFF) corresponding to the offset voltage (Voff) and the current set value (SET), and controls the LED current on the basis of the deviation between the feedback value (FB) obtained by cancelling the current set value (SET) and the offset value (OFF) when the current set value (SET) is less than the deep dimming threshold value (TH), and controls the LED current on the basis of the deviation between the feedback value (FB) obtained by cancelling the current set value (SET) and the offset value (OFF) when the current set value (SET) is less than the deep dimming threshold value (TH).

Description

Technical Field

[0001] Embodiments of this disclosure relate to LED current control circuits and LED lighting devices. Background Technology

[0002] Regarding the dimming of LED lighting devices, after the LED current converted from the voltage of the detection resistor is amplified by the error amplifier, it is read by the A / D converter, and feedback control is performed based on the value (for example, see Patent Document 1).

[0003] Patent Document 1: Japanese Patent Application Publication No. 2016-197570

[0004] Because the LED current is extremely small near the dimming lower limit (deep dimming), the detection voltage generated in the sensing resistor becomes very small (e.g., a few mV to tens of mV). Moreover, when the input voltage is near 0V, the error amplifier exhibits degraded characteristics such as disrupted linearity of the input and output voltages and slower response speed.

[0005] In addition, the LED current is controlled by the ON Duty of the PWM signal. However, since the ON Duty becomes smaller near the dimming lower limit (deep dimming), even a slight change in the ON Duty has a greater impact on the LED current, making dimming unstable. Summary of the Invention

[0006] This disclosure provides an LED current control circuit and an LED lighting device that can stabilize dimming control near the depth dimming range.

[0007] The LED current control circuit disclosed herein generates a PWM signal that controls the LED current supplied to an LED light source. The circuit is characterized by comprising: a current detection circuit that converts the LED current into a voltage signal and detects it; a voltage offset circuit that outputs an offset voltage; and a control circuit that controls the LED current based on a feedback value obtained by adding the offset voltage to the voltage signal and converting it into a digital signal. When the current setpoint is above a depth dimming threshold used to determine whether the current is near a depth dimming threshold, the control circuit controls the LED current based on the deviation between the feedback value (after offsetting the offset value corresponding to the offset voltage) and the current setpoint. When the current setpoint is below the depth dimming threshold, the control circuit controls the LED current based on the deviation between the feedback value (after offsetting the current setpoint) and the offset value.

[0008] The LED current control circuit disclosed herein can stabilize dimming control near the depth dimming range. Attached Figure Description

[0009] Figure 1 This is a circuit diagram illustrating the structure of an embodiment of an LED lighting device.

[0010] Figure 2 This is an explanatory diagram of the offset voltage.

[0011] Figure 3 This is a diagram showing the operation of the setpoint calculation unit.

[0012] Figure 4 It is a graph showing the rate of change for each digit.

[0013] Label Explanation

[0014] 1: LED lighting device; 2: LED power supply device; 3: LED light source unit; 4: Current detection circuit; 10: Power supply circuit; 11: Drive circuit; 20: Control circuit; 21: Set value calculation unit; 22: First multiplier; 23: First adder; 24: Second multiplier; 25: Second adder; 26: PWM generation unit. Detailed Implementation

[0015] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

[0016] Reference Figure 1 The LED lighting device 1 of this embodiment uses DC power supply Vin as the input power supply and has an LED power supply device 2, an LED light source unit 3 and a current detection circuit 4.

[0017] The LED power supply device 2 has a power supply circuit 10 and a control circuit 20. The LED current controlled by the PWM signal is supplied to the LED light source unit 3. The control circuit 20 generates the PWM signal to drive the power supply circuit 10.

[0018] Figure 1 The power supply circuit 10 shown is a buck converter, connected to the output terminal of the DC power supply Vin. The power supply circuit 10 can also be a boost converter, a buck-boost converter, or any type of non-insulated or insulated converter.

[0019] The power supply circuit 10 includes a switching element Q1, an inductor L1, a diode D1, a capacitor C1, and a drive circuit 11. The switching element Q1 is, for example, an N-channel MOSFET. The capacitor C1 is, for example, an electrolytic capacitor.

[0020] A series circuit consisting of a switching element Q1, an inductor L1, and a capacitor C1 is connected to both ends of a DC power supply Vin. The drain of the switching element Q1 is connected to the positive terminal of the DC power supply Vin, and the source is connected to one end of the inductor L1. The positive terminal of the capacitor C1 is connected to the other end of the inductor L1, and the negative terminal is connected to the negative terminal of the DC power supply Vin.

[0021] Diode D1 is connected in parallel with inductor L1 and capacitor C1. The anode of diode D1 is connected to the cathode of capacitor C1, and the cathode is connected to one end of inductor L1.

[0022] The LED light source unit 3 is composed of multiple LEDs (light-emitting diodes) and is connected between the output terminals of the power supply circuit 10. The anode side of the LED light source unit 3 is connected to the positive terminal of the capacitor C1, and the cathode side is connected to the reference potential via the current detection circuit 4.

[0023] The current detection circuit 4 has a current detection resistor Rs, which converts the LED current flowing through the LED light source section 3 into a detection voltage Vs. The current detection resistor Rs is connected between the cathode side of the LED light source section 3 and the reference potential.

[0024] Furthermore, the current detection circuit 4 includes resistors Ra and Rb, and a capacitor Ca. One end of resistor Rb is supplied with the voltage of power supply Vr, and the other end is connected to the junction of LED light source unit 3 and current detection resistor Rs via resistor Ra. Capacitor Ca is connected between the junction of resistors Ra and Rb and the reference potential, and the voltage Vfb at the junction of resistors Ra and Rb is input to control circuit 20 as a feedback signal.

[0025] The power supply Vr and resistors Ra and Rb function as a voltage offset circuit, such as... Figure 2 As shown, the voltage Vfb is the value obtained by adding the offset voltage Voff to the detection voltage Vs detected by the current sensing resistor Rs. Therefore, the voltage Vfb input to the control circuit 20 will not be near 0V, allowing the use of the region with better linearity of the error amplifier configured in the subsequent stage. Furthermore, by adjusting the power supply Vr and the resistors Ra and Rb, the upper limit voltage of the voltage Vfb can be adjusted.

[0026] The control circuit 20 is an LED current control circuit that generates a PWM signal, which controls the LED current supplied to the LED light source section 3. The control circuit 20 includes: a detection signal input terminal T1, which receives a voltage Vfb from the current detection circuit 4; a dimming signal input terminal T2, which receives a dimming signal; and a PWM signal output terminal T3, which outputs a PWM signal. The control circuit 20 uses the voltage Vfb input from the detection signal input terminal T1 as a feedback signal to generate a PWM signal based on the conduction width of the dimming signal input from the dimming signal input terminal T2. The control circuit 20 outputs the generated PWM signal from the PWM signal output terminal T3 to the drive circuit 11 of the power supply circuit 10.

[0027] The control circuit 20 is an arithmetic processing circuit such as a microcomputer equipped with a CPU (Central Processing Unit), ROM (Read-Only Memory), and RAM (Random Access Memory). The ROM stores a control program for controlling the operation of the control circuit 20. The control circuit 20 reads the control program stored in the ROM, expands the control program into the RAM, and thereby functions as the setpoint calculation unit 21, the first multiplier 22, the first adder 23, the second multiplier 24, the second adder 25, and the PWM generation unit 26.

[0028] The voltage Vfb input from the detection signal input terminal T1 is amplified by the amplifier AMP and then converted into a feedback value FB as a digital signal by the A / D converter (ADC). This feedback value FB is obtained by adding the offset value OFF, which is the offset voltage Voff converted into a digital signal, to the current detection value DET after converting the detection voltage Vs into a digital signal.

[0029] The dimming signal input from the dimming signal input terminal T2 is converted into the current setting value SET by the A / D converter (ADC).

[0030] The setpoint calculation unit 21 calculates the offset value CAN that is canceled (removed) from the feedback value FB and the control value REF that is compared with the feedback value FB that has canceled the offset value CAN, based on the current setpoint SET.

[0031] The setpoint calculation unit 21 outputs the calculated cancellation value CAN to the first multiplier 22. The first multiplier 22 multiplies the cancellation value CAN by -1 to invert the sign and outputs it to the first adder 23. The first adder 23 adds the feedback value FB and the inverted cancellation value CAN, and outputs the feedback value FB (with the cancellation value CAN removed) to the second adder 25.

[0032] The setpoint calculation unit 21 outputs the calculated control value REF to the second multiplier 24. The second multiplier 24 multiplies the control value REF by -1 to invert its sign and outputs it to the second adder 25. The second adder 25 adds the input from the first adder 23 and the inverted control value REF, and outputs the deviation between the feedback value FB and the control value REF to the PWM generation unit 26.

[0033] Reference Figure 3 In case (a), when the current setting value SET is above the depth dimming threshold TH (e.g., TH = 20 digits) used to determine whether it is near the dimming lower limit (depth dimming), that is, when the current setting value SET is not in depth dimming, the setting value calculation unit 21 calculates a preset offset value OFF (e.g., OFF = 20 digits) as the cancellation value CAN. Furthermore, when the current setting value SET is less than the depth dimming threshold TH, that is, when the current setting value SET is in depth dimming, the setting value calculation unit 21 calculates the current setting value SET as the cancellation value CAN.

[0034] Reference Figure 3 (b) When the current setting value SET is above the depth dimming threshold TH, i.e., when the current setting value SET is not in depth dimming mode, the setting value calculation unit 21 calculates the current setting value SET as the control value REF. Furthermore, when the current setting value SET is below the depth dimming threshold TH, i.e., when the current setting value SET is in depth dimming mode, the setting value calculation unit 21 calculates the current setting value SET as the control value REF. Additionally, when the current setting value SET is below the depth dimming threshold TH, i.e., when the current setting value SET is in depth dimming mode, the setting value calculation unit 21 calculates the offset value OFF as the control value REF.

[0035] The PWM generation unit 26 generates a PWM signal through feedback control of an integral control element based on the deviation, thereby eliminating the deviation input from the second adder 25. The PWM generation unit 26 performs feedback control, for example, using PI (Proportional-Integral) control.

[0036] When the current setpoint SET is above the deep dimming threshold TH, the cancellation value CAN becomes the offset value OFF, and the control value REF becomes the current setpoint SET. Therefore, in the first adder 23, the offset value OFF is cancelled from the feedback value FB, and in the second adder 25, the deviation between the current detection value DET(FB-OFF) and the current setpoint SET is calculated. As a result, in the PWM generation unit 26, feedback control based on the integral control element of the deviation between the current detection value DET and the current setpoint SET is executed.

[0037] When the current setpoint SET is less than the deep dimming threshold TH, the cancellation value CAN becomes the current setpoint SET, and the control value REF becomes the offset value OFF. Therefore, the feedback value FB (DET + a portion of OFF) which retains a portion of the offset value OFF in addition to the current detection value DET is input from the first adder 23 to the second adder 25. As a result, in the second adder 25, the deviation between the feedback value FB (which retains a portion of the offset value OFF in addition to the current setpoint SET) and the offset value OFF is calculated, and in the PWM generation unit 26, feedback control based on the integral control element of the deviation between the feedback value FB (which retains a portion of the offset value OFF) and the offset value OFF is executed.

[0038] Figure 4 This shows the rate of change for each digit. (Refer to...) Figure 4 When the current detection value DET is less than 20 digits, the rate of change per digit exceeds 5%, therefore fluctuations in LED current will be identified as flickering. Conversely, when the current detection value DET is 20 digits or more, the rate of change per digit is less than 5%, and fluctuations in LED current will not be identified as flickering.

[0039] Therefore, when the current setpoint SET is less than the depth dimming threshold TH, the control circuit 20 uses a feedback value FB, which contains a portion of the offset value OFF in addition to the current detection value DET, to perform feedback control. This allows the feedback value FB to be a large value (e.g., 20 digits or more), suppressing the rate of change per digit.

[0040] Furthermore, even during intermediate calculations, smaller values ​​are not used (only the lower bits are calculated), thus suppressing rounding errors and improving calculation accuracy.

[0041] The following is a summary of the features described in the above embodiments.

[0042] This embodiment is an LED current control circuit that generates a PWM signal to control the LED current supplied to the LED light source unit 3. The LED current control circuit includes: a current detection circuit 4 that converts the LED current into a detection voltage Vs as a voltage signal and detects it; a voltage offset circuit (power supply Vr and resistors Ra, Rb) that outputs an offset voltage Voff; and a control circuit 20 that controls the LED current based on a feedback value FB obtained by converting a voltage Vfb (feedback signal) obtained by adding the offset voltage Voff to the detection voltage Vs into a digital signal. When the current setpoint SET is above the depth dimming threshold TH used to determine whether it is near depth dimming, the control circuit 20 controls the LED current based on the deviation between the feedback value FB (after offsetting the offset value OFF corresponding to the offset voltage Voff) and the current setpoint SET. When the current setpoint SET is less than the depth dimming threshold TH, the control circuit controls the LED current based on the deviation between the feedback value FB (after offsetting the current setpoint SET) and the offset value OFF.

[0043] Based on this structure, by making the feedback value FB a large value (e.g., more than 20 digits), the rate of change per digit can be suppressed, thereby stabilizing the dimming control near the depth dimming.

[0044] Furthermore, according to this embodiment, the depth dimming threshold TH is set to the same value as the offset value OFF.

[0045] Based on this structure, the lower limit of the feedback value FB can be set by using the depth dimming threshold TH and the offset value OFF.

[0046] Furthermore, the present invention is not limited to the embodiments described above, and it is understood that appropriate modifications can be made to each embodiment within the scope of the technical concept of the present invention. In addition, the number, position, shape, etc., of the above-described structural components are not limited to the embodiments described above, and can be any number, position, shape, etc., suitable for implementing the present invention. Furthermore, in the accompanying drawings, the same structural elements are labeled with the same reference numerals.

Claims

1. An LED current control circuit that generates a PWM signal, the PWM signal controlling the LED current supplied to an LED light source, characterized in that, This LED current control circuit has the following features: A current detection circuit that converts the LED current into a voltage signal and detects it; A voltage offset circuit, which outputs an offset voltage; as well as The control circuit controls the LED current based on the feedback value obtained by converting the feedback signal obtained by applying the offset voltage to the voltage signal into a digital signal. When the current setpoint is above the depth dimming threshold used to determine whether the area is near deep dimming, the control circuit controls the LED current based on the deviation between the feedback value (after offsetting the offset value corresponding to the offset voltage) and the current setpoint. When the current setting value is less than the depth dimming threshold, the control circuit controls the LED current based on the deviation between the feedback value after offsetting the current setting value and the offset value.

2. The LED current control circuit according to claim 1, characterized in that, The depth dimming threshold is set to the same value as the offset value.

3. An LED lighting device, comprising: an LED light source unit; A power supply circuit that supplies LED current generated by controlling the on / off state of a switching element to the LED light source; and an LED current control circuit that generates a PWM signal to control the on / off state of the switching element, characterized in that... The LED current control circuit has the following features: A current detection circuit that converts the LED current into a voltage signal and detects it; A voltage offset circuit, which outputs an offset voltage; as well as The control circuit controls the LED current based on the feedback value obtained by converting the feedback signal obtained by applying the offset voltage to the voltage signal into a digital signal. When the current setting value is above the depth dimming threshold used to determine whether it is near the depth dimming threshold, the control circuit controls the LED current based on the deviation between the feedback value after offsetting the offset value corresponding to the offset voltage and the current setting value. When the current setting value is less than the depth dimming threshold, the control circuit controls the LED current based on the deviation between the feedback value after offsetting the current setting value and the offset value.

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