LED current control circuit and LED illumination apparatus

Abstract

To provide an LED current control circuit capable of stabilizing dimming control in the vicinity of deep dimming.SOLUTION: An LED current control circuit comprises a control circuit 20 which controls an LED current based on a feedback value FB resulting from converting a voltage Vfb (feedback signal) which is obtained by adding an offset voltage Voff to a detection voltage Vs into a digital signal. In a case where a current setting value SET is equal to or more than a deep dimming threshold TH for determining whether or not dimming control is in the vicinity of deep dimming, the control circuit 20 controls the LED current based on a deviation between the feedback value FB which cancels an offset value OFF corresponding to the offset voltage Voff and the current setting value SET and in a case where the current setting value SET is less than the deep dimming threshold TH, the control circuit controls the LED current based on a deviation between the feedback value FB which cancels the current setting value SET and the offset value OFF.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The embodiments of the present disclosure relate to an LED current control circuit and an LED lighting device. [Background technology]

[0002] To dim an LED lighting device, the LED current converted to a voltage by a detection resistor is amplified by an error amplifier, and then read by an A / D converter, and feedback control is performed based on this value (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-197570 Summary of the Invention [Problem to be solved by the invention]

[0004] Near the lowest dimming limit (deep dimming), the LED current is minute, so the detection voltage generated in the detection resistor is very small (for example, a few mV to a dozen mV). When the input voltage approaches 0V, the error amplifier begins to show degradation in its characteristics, such as a loss of linearity in the input / output voltage and slower response speed.

[0005] Furthermore, the LED current is controlled by the on-width (ON Duty) of the PWM signal, but since the on-width becomes smaller near the lower limit of dimming (deep dimming), even a small change in the on-width has a large effect on the LED current, making dimming unstable.

[0006] The present disclosure aims to provide an LED current control circuit and an LED lighting device that can stabilize dimming control near deep dimming. [Means for solving the problem]

[0007] The LED current control circuit of the present disclosure generates a PWM signal that controls the LED current supplied to an LED light source unit, and includes: 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 converting a feedback voltage obtained by adding the offset voltage to the voltage signal and converting it into a digital signal, wherein the control circuit controls the LED current based on the deviation between the feedback value obtained by canceling an offset value corresponding to the offset voltage and the current setting value when the current setting value is equal to or greater than a deep dimming threshold for determining whether or not the current setting value is close to deep dimming; and controls the LED current based on the deviation between the feedback value obtained by canceling the current setting value and the offset value when the current setting value is less than the deep dimming threshold. [Effects of the Invention]

[0008] The LED current control circuit of the present disclosure can stabilize dimming control near deep dimming. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a circuit diagram showing a configuration of an embodiment of an LED lighting device. [Figure 2] FIG. 10 is an explanatory diagram of an offset voltage. [Figure 3] FIG. 10 is a diagram illustrating the operation of a setting value calculation unit. [Figure 4] FIG. 10 is a diagram showing the rate of change per digit. DETAILED DESCRIPTION OF THE INVENTION

[0010] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.

[0011] Referring to FIG. 1, an LED lighting device 1 of this embodiment uses a DC power supply Vin as an input power supply, and includes an LED power supply device 2, an LED light source unit 3, and a current detection circuit 4.

[0012] The LED power supply device 2 includes a power supply circuit 10 and a control circuit 20 that generates a PWM signal for driving the power supply circuit 10, and supplies an LED current to the LED light source unit 3 that is controlled by the PWM signal.

[0013] 1 is a step-down converter connected to the output terminal of a DC power supply Vin. The power supply circuit 10 may be a step-up converter or a step-up / step-down converter, and may be either a non-insulated or an insulated type.

[0014] 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 configured, for example, by an N-channel MOSFET. The capacitor C1 is configured, for example, by an electrolytic capacitor.

[0015] A series circuit consisting of a switching element Q1, an inductor L1, and a capacitor C1 is connected across 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.

[0016] The diode D1 is connected in parallel to the inductor L1 and the capacitor C1. The anode of the diode D1 is connected to the negative electrode of the capacitor C1, and the cathode is connected to one end of the inductor L1.

[0017] The LED light source unit 3 is composed of a plurality of 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 electrode of the capacitor C1, and the cathode side is connected to a reference potential via the current detection circuit 4.

[0018] The current detection circuit 4 includes a current detection resistor Rs that converts the LED current flowing through the LED light source unit 3 into a detection voltage Vs. The current detection resistor Rs is connected between the cathode end of the LED light source unit 3 and a reference potential.

[0019] The current detection circuit 4 also includes resistors Ra and Rb, and a capacitor Ca. One end of the resistor Rb is applied with the voltage of the power supply Vr, and the other end is connected via the resistor Ra to the connection point between the LED light source unit 3 and the current detection resistor Rs. The capacitor Ca is connected between the connection point between the resistors Ra and Rb and a reference potential, and the voltage Vfb at the connection point between the resistors Ra and Rb is input to the control circuit 20 as a feedback signal.

[0020] The power supply Vr and resistors Ra and Rb function as a voltage offset circuit, and the voltage Vfb is the sum of the detection voltage Vs detected by the current detection resistor Rs and the offset voltage Voff, as shown in Figure 2. Therefore, the voltage Vfb input to the control circuit 20 does not become close to 0 V, allowing the error amplifier located downstream to use a region with good linearity characteristics. In addition, the upper limit voltage of the voltage Vfb can be adjusted by adjusting the power supply Vr and resistors Ra and Rb.

[0021] The control circuit 20 is an LED current control circuit that generates a PWM signal that controls the LED current supplied to the LED light source unit 3. The control circuit 20 includes a detection signal input terminal T1 to which the voltage Vfb from the current detection circuit 4 is input, a dimming signal input terminal T2 to which a dimming signal is input, and a PWM signal output terminal T3 from which a PWM signal is output. The control circuit 20 uses the voltage Vfb input from the detection signal input terminal T1 as a feedback signal and generates a PWM signal with an ON width based on 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.

[0022] The control circuit 20 is an arithmetic processing circuit such as a microcomputer equipped with a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), etc. A control program for controlling the operation of the control circuit 20 is stored in the ROM. The control circuit 20 reads the control program stored in the ROM and expands the control program in the RAM, thereby functioning as a set value calculation unit 21, a first multiplier 22, a first adder 23, a second multiplier 24, a second adder 25, and a PWM generation unit 26.

[0023] The voltage Vfb input from the detection signal input terminal T1 is amplified by the amplifier AMP and then converted into a digital feedback value FB by an A / D converter (ADC). This feedback value FB is the sum of the current detection value DET, which is obtained by converting the detection voltage Vs into a digital signal, and the offset value OFF, which is obtained by converting the offset voltage Voff into a digital signal.

[0024] The dimming signal input from the dimming signal input terminal T2 is converted into a current setting value SET by an A / D converter (ADC).

[0025] The set value calculation unit 21 calculates a cancellation value CAN to be cancelled (removed) from the feedback value FB and a control value REF to be compared with the cancelled feedback value FB by using the current set value SET.

[0026] The set value 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 the result to the first adder 23. The first adder 23 adds the feedback value FB and the cancellation value CAN with the inverted sign, and outputs the feedback value FB from which the cancellation value CAN has been removed to the second adder 25.

[0027] The set value 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 the sign and outputs the result to the second adder 25. The second adder 25 adds the input from the first adder 23 and the control value REF whose sign has been inverted, and outputs the deviation between the feedback value FB and the feedback value FB to the PWM generation unit 26.

[0028] 3(a), when the current setting value SET is equal to or greater than a deep dimming threshold TH (e.g., TH=20 digits) for determining whether the current setting value SET is near the dimming lower limit (deep dimming), i.e., when the current setting value SET is not deep dimming, the set value calculation unit 21 calculates a preset offset value OFF (e.g., OFF=20 digits) as the cancellation value CAN. Also, when the current setting value SET is less than the deep dimming threshold TH, i.e., when the current setting value SET is deep dimming, the set value calculation unit 21 calculates the current setting value SET as the cancellation value CAN.

[0029] 3(b), when the current setting value SET is equal to or greater than the deep dimming threshold TH, i.e., when the current setting value SET is not deep dimming, the set value calculation unit 21 calculates the current setting value SET as the control value REF. Furthermore, when the current setting value SET is less than the deep dimming threshold TH, i.e., when the current setting value SET is deep dimming, the set value calculation unit 21 calculates the current setting value SET as the control value REF. Furthermore, when the current setting value SET is less than the deep dimming threshold TH, i.e., when the current setting value SET is deep dimming, the set value calculation unit 21 calculates the offset value OFF as the control value REF.

[0030] The PWM generating unit 26 generates a PWM signal by feedback control based on an integral control element of the deviation so as to eliminate the deviation input from the second adder 25. The PWM generating unit 26 performs the feedback control by, for example, PI (Proportional-Integral) control.

[0031] When the current set value SET is equal to or greater than the deep dimming threshold TH, the cancellation value CAN becomes the offset value OFF, and the control value REF becomes the current set value SET. Therefore, the first adder 23 cancels the offset value OFF from the feedback value FB, and the second adder 25 calculates the deviation between the current detection value DET (FB-OFF) and the current set value SET. As a result, the PWM generation unit 26 performs feedback control based on the integral control element of the deviation between the current detection value DET and the current set value SET.

[0032] When the current setting value SET is less than the deep dimming threshold TH, the cancellation value CAN becomes the current setting value SET, and the control value REF becomes the offset value OFF. Therefore, the feedback value FB (DET+part of OFF), which is the current detection value DET plus a portion of the offset value OFF remaining, is input from the first adder 23 to the second adder 25. As a result, the second adder 25 calculates the deviation between the feedback value FB, which is the current setting value SET plus a portion of the offset value OFF remaining, and the offset value OFF, and the PWM generation unit 26 performs feedback control based on the integral control element of the deviation between the feedback value FB, which is the current setting value SET plus a portion of the offset value OFF remaining, and the offset value OFF.

[0033] Figure 4 shows the rate of change per digit. Referring to Figure 4, when the detected current value DET is less than 20 digits, the rate of change per digit exceeds 5%, so fluctuations in the LED current are perceived as flickering. In contrast, when the detected current value DET is 20 digits or more, the rate of change per digit is 5% or less, so fluctuations in the LED current are not perceived as flickering.

[0034] Therefore, when the current setting value SET is less than the deep dimming threshold value TH, the control circuit 20 executes feedback control using the feedback value FB, which is the current detection value DET plus a portion of the offset value OFF remaining, thereby making it possible to set the feedback value FB to a large value (for example, 20 digits or more) and suppressing the rate of change per digit.

[0035] In addition, since small values ​​(calculations using only the lower bits) are no longer used during intermediate calculations, errors due to rounding are reduced, improving calculation accuracy.

[0036] The features of the above embodiment are summarized below. This embodiment is an LED current control circuit that generates a PWM signal to control the LED current supplied to an LED light source unit 3, and includes a current detection circuit 4 that converts the LED current into a detection voltage Vs, which is 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, which is a digital signal obtained by converting a voltage Vfb (feedback signal) obtained by adding the offset voltage Voff to the detection voltage Vs. When the current setting value SET is equal to or greater than a deep dimming threshold TH for determining whether or not the current is close to deep dimming, the control circuit 20 controls the LED current based on the deviation between the feedback value FB, which cancels the offset value OFF corresponding to the offset voltage Voff, and the current setting value SET. When the current setting value SET is less than the deep dimming threshold TH, the control circuit 20 controls the LED current based on the deviation between the feedback value FB, which cancels the current setting value SET, and the offset value OFF. With this configuration, by setting the feedback value FB to a large value (for example, 20 digits or more), the rate of change per digit can be suppressed, and dimming control near deep dimming can be stabilized.

[0037] Furthermore, according to this embodiment, the deep dimming threshold value TH is set to the same value as the offset value OFF. With this configuration, the lower limit of the feedback value FB can be set by the deep dimming threshold value TH and the offset value OFF.

[0038] It is clear that the present invention is not limited to the above-described embodiments, and that each embodiment can be appropriately modified within the scope of the technical concept of the present invention. Furthermore, the number, position, shape, etc. of the above-described components are not limited to the above-described embodiments, and the number, position, shape, etc. can be set to be suitable for implementing the present invention. Note that the same components are denoted by the same reference numerals in each drawing. [Explanation of symbols]

[0039] 1 LED lighting device 2 LED power supply 3 LED light source section 4 Current detection circuit 10 Power circuit 11 Drive circuit 20 Control circuit 21 Setting value calculation section 22 First multiplier 23 First adder 24 Second Multiplier 25 Second adder 26 PWM generation section

Claims

1. An LED current control circuit that generates a PWM signal to control an LED current supplied to an LED light source unit, 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; a control circuit that controls the LED current based on a feedback value obtained by converting a feedback signal obtained by adding the offset voltage to the voltage signal into a digital signal, The control circuit When the current setting value is equal to or greater than a deep dimming threshold value for determining whether the current setting value is close to deep dimming, the LED current is controlled based on a deviation between the feedback value obtained by canceling an offset value corresponding to the offset voltage and the current setting value; When the current setting value is less than the deep dimming threshold, the LED current is controlled based on the deviation between the feedback value and the offset value, which cancels the current setting value.

1. An LED current control circuit comprising:

2. The deep dimming threshold is set to the same value as the offset value.

2. The LED current control circuit according to claim 1.

3. an LED light source unit; a power supply circuit that supplies an LED current generated by on / off control of a switching element to the LED light source unit; an LED current control circuit that generates a PWM signal that controls the on / off of the switching element, The LED current control circuit includes: 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; a control circuit that controls the LED current based on a feedback value obtained by converting a feedback signal obtained by adding the offset voltage to the voltage signal into a digital signal, The control circuit When the current setting value is equal to or greater than a deep dimming threshold value for determining whether the current setting value is close to deep dimming, the LED current is controlled based on a deviation between the feedback value obtained by canceling an offset value corresponding to the offset voltage and the current setting value; When the current setting value is less than the deep dimming threshold, the LED current is controlled based on the deviation between the feedback value and the offset value, which cancels the current setting value. An LED lighting device characterized by:

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