LED current control circuit and LED illumination device

Abstract

To provide a technique capable of suppressing "flicker" during no light control even if very small disturbance or the like which interferes with closed-loop control occurs.SOLUTION: A PWM generation part 25 executes closed-loop control for generating a PWM signal with a Ton value (FB) during a change period in which a current setting value is being changed and a count period succeeding the change period, and if a count value (accumulated value) obtained by counting (accumulating) variations in an opposite direction to a light control direction of the current setting value of the Ton value (FB) reaches a predetermined threshold value (th), the PWM generation part 25 executes open-loop control for generating the PWM signal with a Ton value (Hold) until the current setting value is changed.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present disclosure relate to an LED current control circuit and an LED lighting device.

Background Art

[0002] A conventional LED driving device disclosed in Patent Document 1 supplies a DC voltage converted from an AC voltage to a constant current driving unit that generates a driving current by using a driving signal having a predetermined fixed on-pulse period, thereby reducing flicker at low brightness.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Generally, the LED current supplied to the LED light source unit is generated by closed-loop control based on a current set value and a current detection value. In closed-loop control, control is performed so that the current set value and the current detection value become equal. When there is no dimming and the current set value is not changed, the LED current (current set value) becomes constant if there is no disturbance. However, when there are input fluctuations or load fluctuations that interfere with the closed-loop control, or disturbances such as noise superimposed on the analog current set value, even during non-dimming, the LED current (current set value) fluctuates and "flicker" occurs. It is difficult to eliminate "flicker" caused by disturbances by filter control or the like.

[0005] The present disclosure aims to provide a technique capable of suppressing "flicker" during non-dimming even when there are minute disturbances or the like in closed-loop control.

Means for Solving the Problems

[0006] The LED current control circuit of the present disclosure is an LED current control circuit that generates a PWM signal for controlling the LED current supplied to the LED light source unit, and includes an on-width generation unit that generates an on-width specified value for each cycle of a unit time based on the current set value and the current detection value of the LED current, a PWM generation unit that generates the PWM signal with the on-width specified by the on-width specified value for each cycle of the unit time, and an on-width holding unit that holds the on-width specified value used for specifying the on-width by the PWM generation unit as an on-width holding value. During a change period in which the current set value is being changed and a count period following the change period, the PWM generation unit executes closed-loop control to generate the PWM signal with the on-width specified value. When the cumulative value obtained by accumulating the fluctuations in the opposite direction to the dimming direction of the current set value of the on-width specified value reaches a preset threshold value during the count period, until the current set value is changed, the PWM generation unit executes open-loop control to generate the PWM signal with the on-width holding value.

Advantages of the Invention

[0007] According to the present invention, even if there is a minute disturbance or the like in the closed-loop control, the "flickering" during non-dimming can be suppressed.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, components having the same function are denoted by the same reference numerals, and the description thereof will be omitted as appropriate.

[0010] As shown in FIG. 1, the LED lighting device 1 of the present embodiment uses a DC power supply Vin as an input power supply, and includes an LED power supply device 2 and an LED light source unit 3.

[0011] The LED power supply device 2 includes a power supply circuit 10 and a control unit 20 that generates a PWM signal for driving the power supply circuit 10, and supplies an LED current controlled by the PWM signal to the LED light source unit 3.

[0012] The power supply circuit 10 shown in FIG. 1 is a buck converter and is connected to the output terminal of the DC power supply Vin. The power supply circuit 10 may employ a boost converter or a buck-boost converter, and may be either a non-isolated type or an isolated type.

[0013] The power supply circuit 10 includes a switching element Q1, an inductor L1, a diode D1, a capacitor C1, a current detection circuit 11, and a drive circuit 12. The switching element Q1 is composed of, for example, an N-channel MOSFET.

[0014] A series circuit composed of the switching element Q1, the inductor L1, and the capacitor C1 is connected across the DC power supply Vin. The drain of the switching element Q1 is connected to the positive electrode of the DC power supply Vin, and the source is connected to one end of the inductor L1. The positive electrode of the capacitor C1 is connected to the other end of the inductor L1, and the negative electrode is connected to the negative electrode of the DC power supply Vin.

[0015] 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.

[0016] 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 the negative electrode side of the capacitor C1 via the current detection circuit 11.

[0017] The current detection circuit 11 includes a current detection resistor R1 connected between the cathode side of the LED light source unit 3 and the negative electrode side of the capacitor C1, and an amplifier AMP1 that amplifies the potential difference across both ends of the current detection resistor R1. The current detection circuit 11 detects the LED current flowing through the LED light source unit 3 and outputs it as a current detection value.

[0018] The control unit 20 is an LED current control circuit that generates a PWM signal for controlling the LED current supplied to the LED light source unit 3. The control unit 20 includes a detection signal input terminal T1 to which a current detection signal detected by the current detection circuit 11 is input, a dimming signal input terminal T2 to which a dimming signal is input, and a PWM signal output terminal T3 that outputs a PWM signal. The control unit 20 generates a Ton value that specifies the on-width of the PWM signal based on the current detection signal input from the detection signal input terminal T1 and the dimming signal input from the dimming signal input terminal T2. The control unit 20 outputs a PWM signal with an on-width specified by the generated Ton value from the PWM signal output terminal T3 to the drive circuit 12 of the power supply circuit 10.

[0019] The control unit 20 is an arithmetic processing circuit such as a microcomputer including a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), etc. A control program for controlling the operation of the LED lighting device 1 is stored in the ROM. The control unit 20 reads out the control program stored in the ROM and expands the control program in the RAM, thereby functioning as an on-width generation unit 21, a change determination unit 22, an on-width holding unit 23, an on-width selection unit 24, a PWM generation unit 25, a comparison unit 26, and a counter 27.

[0020] The current detection signal input from the detection signal input terminal T1 and the dimming signal input from the dimming signal input terminal T2 are respectively converted into a current detection value and a current setting value, which are digital signals, by an A / D converter (ADC).

[0021] The on-width generation unit 21 functions as an error amplifier and generates a Ton value (FB) such that the current detection value becomes the current setting value. That is, the Ton value (FB) generated by the on-width generation unit 21 is obtained by a feedback operation using the current detection signal input from the detection signal input terminal T1 as a feedback signal. The on-width generation unit 21 outputs the generated Ton value (FB) to the on-width selection unit 24 and the comparison unit 26.

[0022] The change determination unit 22 determines whether or not the current setting value (dimming signal) has been changed. When the current setting value is changed, the change determination unit 22 outputs a close signal instructing closed-loop control to the on-width selection unit 24. When the current setting value has not been changed, the change determination unit 22 outputs a count signal instructing a count for determining a transition to open-loop control to the comparison unit 26 and the counter 27. Further, the change determination unit 22 notifies the comparison unit 26 of the dimming direction (brightening direction, dimming direction) that has reached the current current setting value using the count signal or a separate signal.

[0023] The on-width holding unit 23 holds (holds) the Ton value input from the on-width selection unit 24 as the Ton value (Hold) in closed-loop control. In open-loop control, the on-width holding unit 23 outputs the held Ton value (Hold) to the on-width selection unit 24.

[0024] When the close signal is input, the on-width selection unit 24 outputs the Ton value (FB) generated by the on-width generation unit 21 as the Ton value to the PWM generation unit 25 and the on-width holding unit 23. When the open signal is input, the on-width selection unit 24 outputs the Ton value (Hold) held by the on-width holding unit 23 as the Ton value to the PWM generation unit 25.

[0025] The PWM generation unit 25 generates a PWM signal with a pulse width specified by the Ton value output from the pulse width selection unit 24. The PWM generation unit 25 generates a PWM signal by comparing a triangular wave or a sawtooth wave with the Ton value. The PWM generation unit 25 outputs the generated PWM signal from the PWM signal output terminal T3 to the drive circuit 12 of the power supply circuit 10, thereby performing on / off control of the switching element Q1.

[0026] When the comparison unit 26 receives a count signal from the change determination unit 22, it compares the Ton value (FB) generated by the pulse width generation unit 21 with the Ton value (Hold) held in the pulse width holding unit 23, and determines whether the change in the Ton value (FB) is in the direction opposite to the dimming direction. When the change in the Ton value (FB) is in the direction opposite to the dimming direction, the comparison unit 26 outputs a count-up signal (+1) to the counter 27.

[0027] The counter 27 is an arithmetic unit that accumulates the changes in the Ton value (FB) in the direction opposite to the dimming direction. When the counter 27 receives a count signal from the change determination unit 22, it determines whether the previous output was a close signal. When the previous output was a close signal, when the counter 27 receives a count-up signal (+1) from the comparison unit 26, it increments the count value (the accumulated value of the changes in the Ton value (FB) in the direction opposite to the dimming direction). When the count value is less than the threshold value (th), the counter 27 outputs a close signal to the pulse width selection unit 24. When the count value reaches the threshold value (th), the counter 27 outputs an open signal to the pulse width selection unit 24 and initializes the count value to 0. When the previous output was not a close signal but an open signal, the counter 27 outputs an open signal to the pulse width selection unit 24.

[0028] The control unit 20 acquires a current detection signal for each predetermined clock (for each cycle of a unit time) and generates a PWM signal. Hereinafter, the PWM signal generation operation by the control unit 20 will be described in detail with reference to FIGS. 2 to 4. FIG. 2 shows the PWM signal generation operation executed for each predetermined clock (unit time). FIGS. 3 and 4 show examples of changes in the Ton value accompanying changes in the current set value (dimming signal).

[0029] The on-width generation unit 21 generates a Ton value (FB) that specifies the on-width of the PWM signal so that the detected current value becomes the current set value (step S101).

[0030] The change determination unit 22 determines whether the current set value (dimming signal) has been changed (step S102). If the current set value is changed in step S102, the change determination unit 22 outputs a close signal, and the close signal is input to the on-width selection unit 24 (step S103). When the close signal is input in step S103, the on-width selection unit 24 outputs the Ton value (FB) generated by the on-width generation unit 21 as the Ton value to the PWM generation unit 25 and the on-width holding unit 23 as closed-loop control (step S104). Thereby, the PWM generation unit 25 generates a PWM signal having an on-width specified by the Ton value (FB) which is a variable value. Also, the on-width holding unit 23 holds (holds) the Ton value (FB) input from the on-width selection unit 24 as the Ton value (Hold) in the closed-loop control (step S105), and returns to step S101.

[0031] That is, the change period during which the current set value changes from time t1 to t2 shown in FIGS. 3 and 4 is the closed-loop control period in which the closed-loop control is executed, and the on-width of the PWM signal is specified by the Ton value (FB) which is a variable value.

[0032] If the current set value has not been changed in step S102, the change determination unit 22 outputs a count signal to the comparison unit 26 and the counter 27. When the counter 27 receives the count signal from the change determination unit 22, it determines whether the previous output was a close signal (step S106).

[0033] Also, when the comparison unit 26 receives the count signal from the change determination unit 22, it compares the Ton value (FB) generated by the on-width generation unit 21 with the Ton value (Hold) held in the on-width holding unit 23, and determines whether the change in the Ton value (FB) is in the opposite direction to the dimming direction (step S107).

[0034] When the previous output is a close signal in step S106 and the variation of the Ton value (FB) is in the direction opposite to the dimming direction in step S107, the comparison unit 26 outputs a count-up signal (+1) to the counter 27, and the counter 27 counts up the count value (step S108).

[0035] That is, when the change period of the current set value ends at time t2 shown in FIGS. 3 and 4, it becomes a count period for counting the count value.

[0036] When the counter value is counted up in step S108, the counter 27 determines whether the counted-up counter value has reached the threshold value (th) (step S109). When the counter value reaches the threshold value (th) in step S109, the counter 27 outputs an open signal and resets the counter value to 0 (initial value) (step S110).

[0037] The open signal from the counter 27 is input to the on-width selection unit 24 (step S111). When the open signal is input in step S111, the on-width selection unit 24 outputs the Ton value (Hold) held in the on-width holding unit 23 as the Ton value to the PWM generation unit 25 as open-loop control (step S112), and returns to step S101. Thereby, the PWM generation unit 25 generates a PWM signal with an on-width specified by the Ton value (Hold) which is a fixed value.

[0038] That is, when the threshold value (th) = 2, when the count value reaches the threshold value (th) = 2 at time t3 shown in FIGS. 3 and 4, it becomes the open-loop control period. During the open-loop control period, open-loop control is executed, and the on-width of the PWM signal is specified by the Ton value (Hold) which is a fixed value.

[0039] As shown in Fig. 3, when the dimming direction is the direction of increasing brightness, the variation in the Ton value (FB) decreases in the direction opposite to the dimming direction, and the counter 27 increments the count value. As shown in Fig. 4, when the dimming direction is the direction of decreasing brightness, the variation in the Ton value (FB) increases in the direction opposite to the dimming direction, and the counter 27 increments the count value.

[0040] If in step S106 the previous output was not a close signal but an open signal, the counter 27 outputs an open signal and proceeds to step S111. In this route, open-loop control is continued.

[0041] That is, the open-loop control period from time t3 shown in Figs. 3 and 4 continues until the current set value is changed, and the on-width of the PWM signal is specified by a fixed value of the Ton value (Hold).

[0042] If in step S106 the previous output was a close signal and in step S107 the variation in the Ton value (FB) is not in the direction opposite to the dimming direction, the counter 27 outputs a close signal and proceeds to step S103. In this route, closed-loop control is continued.

[0043] If in step S109 the count value is less than the threshold value (th), the counter 27 outputs a close signal and proceeds to step S103. In this route, closed-loop control is continued.

[0044] That is, when the change period of the current set value ends at time t2 shown in Figs. 3 and 4, it becomes a count period for counting the count value until the count value reaches the threshold value (th) at time t3. The count period is a closed-loop control period during which closed-loop control is executed, and the on-width of the PWM signal is specified by the variable Ton value (FB).

[0045] The characteristic points in the above embodiment are summarized below. This embodiment is a control unit 20 (LED current control circuit) that generates a PWM signal for controlling the LED current supplied to the LED light source unit 3, and includes an on-width generation unit 21 that generates a Ton value (FB) (on-width designation value) for each cycle of a unit time based on the current set value and the current detection value of the LED current, a PWM generation unit 25 that generates a PWM signal with an on-width designated by the Ton value (FB) for each cycle of a unit time, and an on-width holding unit 23 that holds the Ton value (FB) used for designating the on-width by the PWM generation unit 25 as a Ton value (Hold) (on-width hold value). During a change period in which the current set value is being changed and a count period following the change period, the PWM generation unit 25 executes closed-loop control for generating a PWM signal with the Ton value (FB). When a count value (accumulated value) obtained by counting (accumulating) fluctuations in the Ton value (FB) in a direction opposite to the dimming direction of the current set value reaches a preset threshold value (th) during the count period, the PWM generation unit 25 executes open-loop control for generating a PWM signal with the Ton value (Hold) until the current set value is changed. Due to this feature, since the Ton value (FB) during non-dimming switches to open-loop control in a stable state, even if there are minute disturbances or the like that interfere with the closed-loop control, it is possible to suppress the "flickering" during non-dimming that does not change the dimming signal. When the dimming signal is changed, rapid dimming is executed by the closed-loop control.

[0046] In this embodiment, the on-width generation unit 21 functions as an error amplifier (error compensator) based on the current set value and the current detection value to control the Ton value (FB).

[0047] In this embodiment, the PWM generation unit 25 generates a PWM signal by comparing a triangular wave or a sawtooth wave with the Ton value (FB).

[0048] The present invention has been described based on the embodiment. It is understood by those skilled in the art that this embodiment is an exemplification, and various modifications are possible for combinations of these respective components and the like, and such modifications are also within the scope of the present invention.

Description of Reference Numerals

[0049] 1 LED Lighting Device 2 LED Power Supply Device 3 LED Light Source Unit 10 Power Circuit 11 Current Detection Circuit 12 Drive Circuit 20 Control Unit 21 On - Width Generation Unit 22 Change Judgment Unit 23 On - Width Holding Unit 24 On - Width Selection Unit 25 PWM Generation Unit 26 Comparison Unit 27 Counter AMP1 Amplifier R1 Current Detection Resistor Vin: DC Power Supply

Claims

1. An LED current control circuit that generates a PWM signal for controlling an LED current supplied to an LED light source unit, an on-width generation unit that generates an on-width specified value for each cycle of a unit time based on a current set value and a current detection value of the LED current; a PWM generation unit that generates the PWM signal having an on-width specified by the on-width specified value for each cycle of the unit time; an on-width holding unit that holds the on-width specified value used for specifying the on-width by the PWM generation unit as an on-width holding value, and comprising: during a change period in which the current set value is being changed and a count period following the change period, the PWM generation unit executes closed-loop control to generate the PWM signal with the on-width specified value; in the count period, when a cumulative value obtained by accumulating fluctuations in the on-width specified value in a direction opposite to the dimming direction of the current set value reaches a preset threshold value, until the current set value is changed, the PWM generation unit executes open-loop control to generate the PWM signal with the on-width holding value. An LED current control circuit characterized by this.

2. 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 lighting device comprising an LED current control circuit that generates a PWM signal for on / off control of the switching element, wherein the LED current control circuit an on-width generation unit that generates an on-width specified value for each cycle of a unit time based on a current set value and a current detection value of the LED current; a PWM generation unit that generates the PWM signal having an on-width specified by the on-width specified value for each cycle of the unit time; an on-width holding unit that holds the on-width specified value used for specifying the on-width by the PWM generation unit as an on-width holding value, and comprising: during a change period in which the current set value is being changed and a count period following the change period, the PWM generation unit executes closed-loop control to generate the PWM signal with the on-width specified value; in the count period, when a cumulative value obtained by accumulating fluctuations in the on-width specified value in a direction opposite to the dimming direction of the current set value reaches a preset threshold value, until the current set value is changed, the PWM generation unit executes open-loop control to generate the PWM signal with the on-width holding value. An LED lighting device characterized by this.

3. The LED lighting device according to claim 1 or 2, wherein the on-width generation unit controls the on-width specified value with an error compensator based on the current set value and the current detection value.

4. The LED lighting device according to claim 1 or 2, wherein the PWM generation unit generates the PWM signal by comparing a triangular wave or a sawtooth wave with the on-width specified value.

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