JP5800661B2 - LED power circuit - Google Patents

Description

  The present invention relates to an LED power supply circuit that supplies a direct current for lighting a light emitting diode (LED) such as a full-color LED, and more particularly to an improvement in dimming control thereof.

  In recent years, the performance of LEDs has increased, and lighting fixtures using LEDs are in a state where they can be replaced with conventional light sources due to their long life. If the performance of LEDs is further improved in the future, it will be adopted in the field of general-purpose lighting equipment. Furthermore, full-color LEDs can be freely set in color temperature and freely expressed in color, and are expected to be increasingly used in the field of lighting equipment for production.

  The LED power supply circuit intermittently supplies a direct current to the LED element array, whereby the LED repeats lighting and non-lighting. If this intermittent operation is repeated at a speed equal to or higher than a predetermined speed, it appears to the human eye that the LED is continuously lit. Patent Document 1 discloses a power supply circuit that performs time-averaged dimming control by intermittently driving the LED element array.

JP 2011-70957 A

  However, in the power supply circuit technology described in Patent Document 1, regarding the effect of noise generated by intermittent supply of drive current on the surrounding electromagnetic environment, and the effect of blinking light on the optic nerve cells of the human eye, Detailed examination was not made.

  The present invention has been made in view of the above-described points, and has as its object to provide an LED power supply circuit that has a low noise level to the surrounding environment and that is gentle on the human eye and is less likely to cause fatigue. An object of the present invention is to provide an LED power supply circuit capable of dimming control from zero percent without interrupting the LED drive current.

  In order to solve the above problems, the inventor uses a high-frequency switching circuit such as a flyback converter circuit or a step-down chopper circuit to convert an input current from an external power source into a direct current of a predetermined magnitude. Focusing on the technology to be supplied to the LED element array, we have been diligently working toward the completion of the dimming function of LED lighting fixtures based on this technology.

  A general high-frequency switching circuit includes at least a series circuit of an inductor and a switching element. When the switching element is turned on, a direct current after full-wave rectification flows through the inductor, and magnetic field energy is accumulated in the inductor. When the switching element is turned off, magnetic field energy is released from the inductor. The released magnetic field energy is accumulated or smoothed by the output capacitor and used as a drive current for the LED element array.

  The current (IL) flowing through the inductor repeatedly increases and decreases by the on / off driving of the switching element. If the ON width (ON state period) of the switching element is changed, the magnetic field energy emitted from the inductor can be increased or decreased, and the drive current can be controlled to a predetermined magnitude.

  In the conventional high-frequency switching circuit, the inductor current (IL) is eliminated, and the switching current is controlled so that the current flows again when the inductor current decreases and becomes almost zero. So-called soft switching control has been performed to reduce the switching loss associated with the current flowing through the.

  However, when the drive current is increased or decreased to provide a dimming function, if the drive current is reduced too much in response to a small dimming degree, the switching drive frequency increases too much to maintain soft switching control (drive As the cycle becomes too short, there is a problem that the circuit efficiency is lowered.

  Therefore, when the dimming degree is small, the inventor performs the dimming control without reducing the circuit efficiency by fixing the driving cycle and controlling the magnitude of the driving current only by increasing or decreasing the ON width. As a result, the present invention has been completed.

That is, the LED power supply circuit according to the present invention is an LED power supply circuit that supplies a continuous direct current to the LED load, and performs dimming of the LED load by controlling the magnitude of the direct current,
Conversion means including at least an inductor and a switching element connected in series, and converting and outputting a DC power source into a DC current of a desired magnitude by storing and releasing magnetic field energy of the inductor linked to on / off driving of the switching element; ,
Control means for controlling the driving of the switching element based on the specified dimming degree;
Is provided.
The control means includes
When the dimming level changes in a range equal to or greater than a reference value, the on-width of the switching element is determined according to the dimming level and turned off, and at the timing when the current flowing through the inductor becomes almost zero after the turn-off. Turning on the switching element;
Further, when the dimming degree changes within a range equal to or less than the reference value, the on-width of the switching element is determined according to the dimming degree and turned off, and after the turn-off, the sum of the on-width and the off-width The switching element is turned on with a constant driving period.

Here, in determining the ON width, the control means may use an ON width (a constant value) corresponding to the dimming degree. In addition, the control means uses the reference drive current value corresponding to the dimming degree, compares this reference drive current value with the detected value of the drive current flowing through the LED element array, and sets the ON width so that the difference approaches zero. You may adjust.
According to the present invention, the high frequency switching circuit including the inductor and the switching element connected in series is used to adjust the ON width and the driving cycle of the switching element. Can be controlled. As a result, LED dimming control according to a desired dimming degree can be executed without intermittently driving the LED drive current. Therefore, it is possible to provide an LED power supply circuit that has a low noise level to the surrounding environment and is gentle to human eyes and hardly causes fatigue.
Furthermore, according to the present invention, when the dimming degree becomes small, the drive cycle is fixed, and the magnitude of the drive current is controlled only by increasing / decreasing the ON width. Since the driving cycle is fixed, the switching driving frequency does not increase excessively, and a decrease in circuit efficiency can be avoided. That is, dimming control can be executed without reducing circuit efficiency in a state where the dimming degree is low. Fortunately, when the dimming level is low, the switching loss is small, and the power factor is hardly affected even if the drive cycle is not adjusted (soft switching control). For this reason, even if it is dimming control from zero percent, it can dimm efficiently.

  In the present invention, when the dimming degree shifts from a range greater than or equal to a reference value to a range less than or equal to the reference value, the control means may drive the drive cycle within a range of 2 to 10 times the drive cycle immediately before the transition. To fix.

  According to the present invention, when the driving cycle is fixed, the driving cycle may be fixed using the length of the driving cycle immediately before fixing as it is. However, when the driving cycle at the time of fixing is relatively short, the ON width must be further shortened in order to further reduce the dimming degree. Since the driving cycle is fixed, when the ON width is shortened, the OFF width becomes long, and even when the energy emission from the inductor ends in the OFF period, the turn-on does not occur immediately. For this reason, when the energy release is completed, the electric charge stored in the output capacitance of the switching element starts free vibration between the switching element and the inductor. This free vibration can be captured by detecting the potential difference at the connection point between the switching element and the inductor (see the drain waveform in FIG. 4B and the source waveform in FIG. 6B). When the dimming level is low, turning on while the free oscillation of this charge is occurring may cause problems such as unstable circuit operation, fluctuation, or inability to obtain linearity. There is. Therefore, in the present invention, the drive cycle is fixed at a value twice to 10 times the drive cycle immediately before fixing. In this case, in order to avoid the discontinuity of the magnitude of the direct current flowing through the LED, at the same time as the driving period is fixed, the ON width is enlarged to approximately the same magnification (2 to 10 times). . In this way, even when the dimming degree is further reduced, the actual length of the off width is maintained in a relatively long state, so that the free oscillation of the charge is attenuated to some extent and the turn-on is performed (FIG. 4B ) And the gate signal in FIG. 6B). For this reason, problems such as unstable circuit operation, occurrence of fluctuations, and loss of linearity in a state where the dimming degree is low can be avoided.

Further, in the present invention, when the dimming degree shifts from a range above the reference value to a range below the reference value, the control means uses a reduction reference value as the reference value,
When the dimming degree shifts from a range below the reference value to a range above the reference value, an increase reference value is used as the reference value,
The increase reference value is set to a value larger than the decrease reference value.

  In the present invention, the range in which the dimming degree is equal to or greater than the reference value is set as the soft switching control range, and the range in which the dimming degree is equal to or less than the reference value is set as the frequency fixed control range. According to the present invention, a difference is provided between the decrease reference value when shifting from the soft switching control to the frequency fixed control and the increase reference value when shifting from the frequency fixed control to the soft switching control, and the increase reference value is set. The value was set larger than the reference value for reduction. For this reason, a hysteresis characteristic is provided between the shift of the control method when the dimming degree is increased and the shift of the control method when the dimming degree is decreased. In other words, even if the dimming degree is designated as a value that matches the reference value, the control method does not frequently change, and the number of times of transfer can be reduced and stable control can be executed.

It is a whole block diagram of the LED lighting fixture which concerns on 1st Embodiment of this invention. It is a graph explaining the relationship between the light control degree in the said LED lighting fixture, and a switching control system. It is a wave form diagram which shows FET drive waveform for every level of the said light control degree. It is a figure which shows the drain waveform at the time of giving the said FET drive waveform in this embodiment to a switching element, a drive voltage, and a drive current, (A) shows the time of soft switching control, (B) shows the time of frequency fixed control. It is a whole block diagram of the LED lighting fixture which concerns on 2nd Embodiment of this invention. It is a figure which shows the drain waveform at the time of giving the FET drive waveform in this embodiment to a switching element, a drive voltage, and a drive current, (A) shows the time of soft switching control, (B) shows the time of frequency fixed control.

DESCRIPTION OF EXEMPLARY EMBODIMENTS First Embodiment Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.
FIG. 1 is an overall configuration diagram of an LED lighting apparatus according to the first embodiment. The LED lighting apparatus includes an LED element array 2 and an LED power supply circuit 4 that supplies a DC power to the LED element array. The LED power supply circuit is supplied with AC power AC.

<Overall configuration of LED power supply circuit>
The LED power supply circuit 4 includes an input unit 6, a conversion unit 8, a voltage detection unit (R2, R3), a current detection unit (R5, OA), and a switching cycle end detection unit 16. The input unit includes a noise removal circuit (C1, T1, C2), a full-wave rectification circuit DB, and a bypass capacitor C3.

  The noise removing circuit has a set of noise preventing capacitors C1 and C2 and a choke coil. The choke coil is composed of a set of coils T1. Two coils T1 are inserted in each line connected to the input terminals 10 and 11 of the AC power supply AC. The first noise prevention capacitor C1 is connected between terminals on the AC power supply side of the coil T1, and the second noise prevention capacitor C2 is connected between terminals on the LED element array side of the coil T1. As a result, the noise in the normal mode included in the AC power is removed by the noise preventing capacitors C1 and C2, and the common mode noise is prevented from entering the conversion unit by the choke coil T1.

  The full-wave rectifier circuit DB is configured by a diode bridge or the like, and is applied with the voltage between both terminals of the second noise removing capacitor C2 as an input voltage. The bypass capacitor C3 connects the output ends of the full-wave rectifier circuit DB. The bypass capacitor C3 is used to partially smooth the rectified current from the full-wave rectifier circuit DB and to interrupt current that is interrupted by on / off driving of the switching element Q1 described later. It is provided as a bypass that prevents the influence from reaching the AC side.

  The converter 8 is a flyback converter connected between the output terminals of the full-wave rectifier circuit DB. The function of the converter 8 is to convert the rectified current from the full-wave rectifier circuit DB into power and charge the output capacitor C5 (electrolytic capacitor) connected between the output terminals 12 and 13 of the converter 8. . In addition, a continuous direct current of a predetermined magnitude is supplied to the LED element array 2 connected to the output terminals 12 and 13 of the conversion unit 8 using the energy stored in the output capacitor C5. Furthermore, the converter 8 also functions as a power factor correction circuit and can shape the alternating current input to the full-wave rectifier circuit DB into a sine wave without distortion.

  The specific configuration of the conversion unit 8 is as follows. The converter 8 includes a flyback transformer T2, a switching element Q1, a secondary side diode D2, an output capacitor C5, and a control circuit 14 that controls the switching element Q1. The transformer T2 applies the secondary voltage to the output capacitor C5 using the rectified voltage after full-wave rectification as the primary voltage.

  A series circuit of the switching element Q1 and the primary winding T2a of the transformer T2 is connected between the output terminals of the full-wave rectifier circuit DB. This series circuit corresponds to a series connection of the inductor and the switching element of the present invention. The drain side terminal of the switching element Q1 is connected to the primary winding T2a, and the source side terminal of Q1 is connected to the ground line on the negative terminal side of the full-wave rectifier circuit DB. An N-channel enhancement type MOSFET is used as the switching element Q1. When a drive current is supplied from the drive circuit provided in the control circuit 14 to the gate of the switching element Q1 to generate a gate voltage, a current flows between the drain and the source. This state is called the ON state of the switching element Q1. On the other hand, a state where no drive current is supplied to the gate and no drain current flows is called an off state. A secondary voltage is induced in the secondary winding T2b by the on / off driving of the switching element Q1.

  A series circuit of a secondary diode D2 and an output capacitor C5 is connected between the terminals of the secondary winding T2b. The secondary diode D2 rectifies the generated secondary current. The secondary current after rectification is supplied to the positive electrode of the output capacitor C5 to charge it. A series circuit of the LED element array 2 and the resistor R5 is connected so as to connect both terminals of the output capacitor C5.

  The conversion unit 8 is also provided with a noise removal circuit (D1, C4, R4). The noise removal circuit includes a series circuit of a primary side diode D1 and a noise removal capacitor C4, and a resistor R4. The series circuit of D1 and C4 is connected so as to connect both terminals of the primary winding T2a of the transformer. That is, the anode side terminal of the primary side diode D1 is connected to the connection point between the primary winding T2a and the switching element Q1, and the cathode side terminal is connected to the noise elimination capacitor C4. The noise removal capacitor C4 and the resistor R4 form a parallel circuit.

  The voltage detector is a series circuit of resistors R2 and R3, and is provided between the output terminals of the full-wave rectifier circuit DB. The full-wave rectified waveform generated between the output terminals of the full-wave rectifier circuit DB is divided by resistors R2 and R3 to a voltage value that can be recognized by the control circuit 14, and the divided waveform is sent to the control circuit 14. Based on the resistance values of the resistors R2 and R3, the control circuit 14 can acquire a substantially full-wave rectified waveform from the input divided signal waveform.

  The current detection unit includes a resistor R5 connecting the negative terminal of the output capacitor C5 and the negative terminal of the LED element array 2 and an insulating operational amplifier OA. Then, the terminal voltage on the LED element side of the resistor R5 is detected via the isolated operational amplifier OA, and the detected value is sent to the control circuit 14. In the control circuit 14, the drive current is acquired from the detected value of the voltage of the resistor R5 based on the resistance value of the resistor R5.

  The switching cycle end detection unit 16 detects the potential difference between the drain and source of the switching element Q1 in order to acquire the potential on the drain side of the switching element Q1, and gives the detected value to the control circuit 14. In the control circuit 14, the end timing of the switching drive cycle is acquired based on the detected value of the potential difference.

  Next, a specific configuration example of the control circuit 14 will be described. The control circuit 14 includes a microcomputer (CPU), an AD converter for detecting voltage and the like, a MOSFET drive circuit that supplies a drive current to the switching element Q1, and a ROM and a RAM.

  The CPU calculates the drive current value of the LED element array 2 based on the detection value from the current detection unit (R5, OA). The ON width (ON state period) is determined so that the drive current value becomes a drive current value corresponding to the dimming degree. Further, the CPU acquires the end timing of the switching drive cycle based on the detected value of the potential difference from the switching cycle end detection unit 16. For example, when the switching element Q1 is in the off state, the timing at which the potential difference becomes almost the minimum value may be detected and used as the turn-on timing.

  The CPU sends a command signal having information on the determined switching drive cycle and ON width to the MOSFET drive circuit for the switching element. This drive circuit supplies a drive current based on the command signal to the switching element Q1 to drive the switching element Q1 on and off. By turning on / off the switching element Q1, energy based on the rectified current is stored in the output capacitor C5. Then, a drive current of a predetermined magnitude is supplied to the LED element array 2 by the energy accumulated in the output capacitor C5.

  In the present embodiment, the IC chip built in the conversion unit 8 corresponds to the control circuit 14. The control circuit 14 may be a circuit provided independently of the conversion unit 8 instead of an IC chip.

  When the control circuit 14 is expressed by functional blocks as shown in FIG. 1, the control circuit 14 includes a dimming degree determination unit 20, a drive cycle determination unit 22, and an ON width determination unit 24. In the following description, these functional blocks will be used. The dimming degree determination unit 20 uses two preset reference values (decrease reference value and increase reference value), and the specified dimming degree is larger or smaller than one of the reference values. Determine whether.

  FIG. 2 shows a case where the dimming degree is changed in a mountain shape with the passage of time, with the vertical axis representing the dimming level and the horizontal axis representing the time axis. In addition, two reference values for the dimming degree are provided on the vertical axis. The lower set value Lo is a decrease reference value, and the higher set value Hi is an increase reference value.

<About soft switching control when the dimming degree is larger than the reference value>
First, the case where the dimming degree changes in a range larger than the set value Hi (the process of gradually decreasing the dimming degree from the maximum set value in FIG. 2) will be described.
The on-width determining unit 24 reads the driving current value corresponding to the designated dimming degree from the data table, and adjusts the on-width of switching of the switching element Q1 so that the detected value of the driving current approaches the driving current value of the data table. To do.
Further, the drive cycle determination unit 22 acquires the end timing of the switching drive cycle based on the detected value of the potential difference from the switching cycle end detection unit 16, and turns on at this end timing. As a result, the switching drive frequency changes according to the detected value, and soft switching control is executed.

  FIG. 3 shows an FET drive waveform sent to the gate of the switching element Q1. The driving waveform when the dimming degree is “large” is shown in FIG. 5A, and the driving waveform when the dimming degree is “medium” is shown in FIG. Both are driving waveforms in the soft switching control region, and the driving cycle becomes shorter as the dimming degree decreases. As for the ratio of the ON width to the drive cycle (ON duty), the ON duty decreases as the dimming degree decreases.

  In the on state of the switching element Q1, the full-wave rectified current is accumulated as magnetic field energy in the magnetic body of the transformer T2, and in the off state, a current flows based on the secondary voltage induced on the secondary side, and the output capacitor C5 Charged. By adjusting the ON width by the control circuit 14, the drive current of the LED element array 2 can be controlled to a predetermined magnitude.

<Switching frequency fixed control when the dimming degree is smaller than the reference value>
A case where the dimming degree becomes smaller than the set value Lo will be described with reference to FIG.
The on-width determining unit 24 reads the driving current value corresponding to the designated dimming degree from the data table, and controls the on-width of switching of the switching element Q1 so that the detected value of the driving current approaches the driving current value of the data table. To do.

  On the other hand, the drive cycle determination unit 22 fixes the drive cycle. In the present embodiment, when the drive cycle is fixed, the drive cycle immediately before fixing is not used as it is, but the drive cycle is fixed at a value twice to 10 times the drive cycle immediately before fixing. Further, in order to avoid the discontinuity of the magnitude of the direct current flowing through the LED, the ON period determining unit 24 fixes the ON period at the same time as the driving period determining unit 22 fixes the driving period. (2 times to 10 times). Further, even when the dimming degree is reduced, the actual length of the off width is maintained in a relatively long state, so that problems such as unstable circuit operation, occurrence of fluctuation, and loss of linearity can be avoided.

  3B and 3C show changes in the FET drive waveform when the control method of the switching element Q1 is shifted. Assume that the light control level “medium” in FIG. 5B is the light control level immediately before reaching the set value Lo. The driving waveform at the dimming level “small” after the dimming level decreases to reach the set value Lo and the control method shifts from the soft switch control to the frequency fixed control is shown in FIG.

  Due to the shift of the control method, the drive cycle immediately after the shift is expanded to approximately seven times the drive cycle immediately before the shift. After that, even if the dimming degree decreases, the driving cycle does not change and becomes constant. The on-width immediately after the transition also increases approximately seven times the on-width immediately before the transition (indicated by the broken line in FIG. 3C). Thereafter, the ON width becomes narrower as the amount after dimming decreases (indicated by the solid line in FIG. 3C).

  In addition, when the drive cycle immediately after the transition is left as it is immediately before, when the dimming control is performed only by adjusting the ON width, the peak current flowing through the primary winding T2a is intermittently interrupted. The heat generation of the switching element Q1 is increased. When the drive cycle is set to 1 to 2 times immediately before the transition, the primary winding T2a is turned on in a state where the amplitude of the free vibration state is still large, and thus the vibration component is included in the current flowing through the LED. On the other hand, when the drive cycle exceeds 10 times just before the transition, the switching drive frequency becomes too low and enters the audible frequency band, and noise is generated. In addition, the peak current flowing through the primary winding T2a becomes too large, which may cause magnetic saturation. Therefore, in the present invention, when the driving cycle is fixed, the driving cycle is fixed at a value 2 to 10 times the driving cycle immediately before the transition. The fixed range of the drive cycle is also within a range that can be easily controlled when drive control is performed using a microcomputer (MCU).

  In the on state of the switching element Q1, the full-wave rectified current is accumulated as magnetic field energy in the magnetic body of the transformer T2, and in the off state, a current flows based on the secondary voltage induced on the secondary side, and the output capacitor C5 Charged. By adjusting the ON width by the control circuit 14, the drive current of the LED element array 2 can be controlled to a predetermined magnitude.

<About hysteresis characteristics>
Next, the hysteresis characteristic in the switching drive control of the present invention will be described.
As shown in FIG. 2, when the dimming degree is decreased, the set value Hi is used as a decrease reference value. When the dimming degree increases, the set value Lo set lower than the set value Hi is used as the increase reference value. In other words, there is a difference between the decrease reference value when shifting from soft switching control to frequency fixed control and the increase reference value when shifting from frequency fixed control to soft switching control. The value was larger than the reference value.

  In this embodiment, a hysteresis characteristic is provided between the shift of the control method when the dimming degree is increased and the shift of the control method when the dimming degree is decreased. That is, when the reference value for increase and the reference value for decrease are the same value, if the dimming degree is specified to a value that matches the reference value, the control method performs between soft switching control and frequency fixed control. Each time, the MCU performs a calculation for shifting the control method, which may cause flickering. If the hysteresis characteristics are given, even if the dimming degree matches the reference value, the control method does not frequently change, and the number of times of the transfer is reduced and stable control is executed. be able to.

As described above, in the present embodiment, the high frequency switching circuit including the series connection of the inductor and the switching element is used to adjust the ON width and the driving cycle of the switching element. The magnitude of the current can be controlled. As a result, LED dimming control according to a desired dimming degree can be executed without intermittently driving the LED drive current. Therefore, it is possible to provide an LED power supply circuit that has a low noise level to the surrounding environment and is gentle to human eyes and hardly causes fatigue.
Furthermore, according to the present embodiment, when the dimming degree is reduced, the drive cycle is fixed, and the magnitude of the drive current is controlled only by increasing / decreasing the ON width, thereby reducing the circuit efficiency. Dimming control can be executed without any problem. For this reason, dimming control from zero percent can be executed efficiently.

  Furthermore, according to the LED power supply circuit 4 of the present embodiment in FIG. 1, it is possible to supply a direct current with a low power factor or to supply a direct current with a high power factor. The low power factor indicates a supply method when power factor improvement control is not executed, and the high power factor indicates a supply method when power factor improvement control is executed.

  In the low power factor supply method, by adopting a capacitor having a large capacity as the bypass capacitor C3, the ripple voltage is smoothed and reduced by the capacitor C3. If constant current control (dimming) is performed in this state, a direct current with a relatively small ripple component can be supplied to the LED element array 2.

  In the high power factor supply system, a bypass capacitor C3 having a small capacity is employed. And a ripple voltage is suppressed as follows. First, in order to adjust the ON width of switching, the average value of the drive currents of the LED element array 2 in a period that is an integral multiple of one cycle of the AC power supply AC (for example, 10 msec in the case of 50 Hz) is detected. The ON width is adjusted so that the average value becomes a drive current value corresponding to the dimming degree. The control circuit 14 turns off the switching element Q1 using this ON width. Thereby, constant current control (dimming control) is executed. However, since the ripple component of the direct current flowing through the LED element array 2 becomes large as it is, soft switching control is further executed. In the soft switching control, the control circuit 14 acquires the end timing of the switching drive cycle based on the detected value of the potential difference from the switching cycle end detection unit 16, and turns on the switching element Q1 at this timing. With the above soft switching control, the input current from the AC power supply AC has a waveform close to a sine wave, and a high power factor of 90% or more can be realized.

  FIG. 4 shows the drain waveform, drive voltage, and drive current when the FET drive waveform (gate signal) in this embodiment is applied to the switching element. FIG. 4A shows the soft switching control, and FIG. 4B shows the frequency fixing control. The drain waveform is a waveform of a potential difference between both terminals of the switching element Q1 detected by the switching cycle end detection unit 16. In FIG. 5B, since the frequency is fixed and the dimming is executed only with the ON width, the drain waveform is disturbed in the second half of the OFF period. This is because the charge stored in the output capacitance of the switching element Q1 starts free vibration between the switching element Q1 and the primary winding T2a after the energy release of the primary winding T2a is completed. In the present embodiment, due to the shift of the control method, the drive cycle immediately after the shift is expanded to approximately seven times the drive cycle immediately before the shift. Similarly, the off width is almost 7 times larger. For this reason, the above-described free vibration is attenuated to some extent and then turned on, and problems such as circuit operation instability, fluctuations, and loss of linearity are avoided. Therefore, even if the switching drive cycle is fixed, the influence of the LED drive current is small, and almost constant current control can be executed.

Second Embodiment FIG. 5 is an overall configuration diagram of an LED lighting apparatus according to a second embodiment. The LED lighting apparatus includes an LED element array 102 and an LED power supply circuit 104 that supplies a DC power to the LED element array. The LED power supply circuit is supplied with AC power AC.

  The LED power supply circuit 104 includes an input unit 106, a conversion unit 108, a current detection unit R5, and a switching cycle end detection unit 116. The input unit 106 includes a full-wave rectifier circuit DB that rectifies the AC output of the AC power supply AC, and a smoothing capacitor C0 that is connected between the output terminals of the DB and smoothes the output of DB. Rectify and smooth to DC voltage.

  The converter 108 is a step-down chopper circuit, which converts the rectified current from the full-wave rectifier circuit DB into power and charges the output capacitor C6 connected between the output terminals of the converter 108. In addition, a continuous direct current of a predetermined magnitude is supplied to the LED element array 102 connected to the output terminal of the conversion unit 108 using the energy stored in the output capacitor C6.

  Specifically, the converter 108 includes a switching element Q1, an inductor L1, a diode D1, an output capacitor C6, and a control circuit 114 that controls on / off of the switching element Q1, and an LED is connected to both ends of the output capacitor C6. The element row 102 is connected. Here, the switching element Q1, the diode D1, and the inductor L1 constitute a step-down chopper circuit. One end of the switching element Q1 is connected to one output end of the full-wave rectifier circuit DB, and an inductor L1 and an output capacitor C6 are connected in series between the other output end of DB and the other end of the switching element Q1. When the switching element Q1 is turned on and magnetic energy is accumulated in the inductor L1 by the rectified current from DB, and the switching element Q1 is switched from on to off, the current from the inductor L1 due to the release of magnetic energy is transmitted via the LED element array. In order to return to the inductor L1 again, both ends of the series circuit of the inductor L1 and the capacitor C6 are connected by a diode D1 that allows current to flow only in the direction returning to the inductor L1.

  The control circuit 114 detects the drive current of the LED element array 102 via the resistor R5, and adjusts the ON width of the switching element Q1 according to the detected value of the drive current. Thus, the direct current supplied to the LED element array 102 is controlled to have a predetermined magnitude.

  The switching cycle end detection unit 116 detects the potential difference between the terminals of the diode D1 and provides the detected value to the control circuit 114 in order to acquire the source-side potential of the switching element Q1. In the control circuit 114, the end timing of the switching drive cycle is acquired based on the detected value of the detection unit 116 that has been sent. For example, in the OFF state of the switching element Q1, the timing at which the potential difference between the terminals of the diode D1 increases may be detected and used as the turn-on timing.

  Also in the LED lighting apparatus of the present embodiment, soft switching control is executed when the dimming degree is larger than the reference value, and switching frequency fixing control is executed when the dimming degree is smaller than the reference value, as in the first embodiment. The effect is obtained.

  FIG. 6 shows the source waveform, drive voltage, and drive current when the FET drive waveform in this embodiment is applied to the switching element Q1. FIG. 4A shows the soft switching control, and FIG. 4B shows the frequency fixing control. The source waveform is a waveform of a potential difference between both terminals of the diode D1 detected by the switching cycle end detection unit 116. In FIG. 5B, the dimming is performed in the second half of the off period because the dimming is executed only with the on width while the frequency is fixed. However, as in the first embodiment, the drain waveform is disturbed. After the free vibration, which is the cause of noise, is attenuated to some extent, it is turned on, and problems such as unstable circuit operation, occurrence of fluctuations, and loss of linearity are avoided. Therefore, even if the switching drive cycle is fixed, the influence of the LED drive current is small, and almost constant current control can be executed.

Modified Example The LED power supply circuit of the present invention can also be applied to an LED lighting apparatus that uses a DC power supply DC instead of an AC power supply AC. In this case, the diode bridge DB may be omitted and only the bypass capacitor C1 may be used.

  Further, the arrangement of the switching period end detection unit 116 used for the soft switching control in the second embodiment may be arranged so as to detect, for example, a potential difference between both terminals of the switching element Q1 in FIG. However, since the ground level of the detection unit 116 and the ground level of the microcomputer of the control circuit 114 are different, it is necessary to use an element such as a photocoupler when transmitting the detection signal. If the detection unit 116 is arranged so as to detect the potential on the source side of the switching element Q1 as in the present embodiment, an element such as a photocoupler becomes unnecessary.

2,102 LED element array (LED load)
4,104 LED power circuit 6,106 Input unit 8,108 Conversion unit (conversion means)
14,114 Control circuit (control means)
16, 116 Switching cycle end detection unit 20 Dimming degree determination unit 22 Drive cycle determination unit 24 On width determination unit L1 Inductor Q1 Switching element R2, R3 Voltage detection unit resistance R5 Current detection unit resistance T2a Primary winding (inductor)

Claims (2)

An LED power supply circuit for dimming the LED load by supplying a continuous direct current to the LED load and controlling the magnitude of the direct current,
Conversion means including at least an inductor and a switching element connected in series, and converting and outputting a DC power source into a DC current of a desired magnitude by storing and releasing magnetic field energy of the inductor linked to on / off driving of the switching element; ,
Control means for controlling the driving of the switching element based on the specified dimming degree;
With
The control means includes
When the dimming level changes in a range equal to or greater than a reference value, the on-width of the switching element is determined according to the dimming level and turned off, and at the timing when the current flowing through the inductor becomes almost zero after the turn-off. Turning on the switching element;
Further, when the dimming degree shifts from a range above the reference value to a range below the reference value , the driving cycle is fixed in a range of 2 to 10 times the driving cycle immediately before the transition,
The on-width of the switching element is determined according to the dimming degree and turned off, and after the turn-off, the switching element is turned on with a driving cycle that is the sum of the on-width and off-width being constant. LED power supply circuit characterized by this. The LED power circuit according to claim 1,
The control means uses a decrease reference value as the reference value when the dimming degree shifts from a range above the reference value to a range below the reference value,
When the dimming degree shifts from a range below the reference value to a range above the reference value, an increase reference value is used as the reference value,
The LED drive circuit, wherein the increase reference value is set to a value larger than the decrease reference value.

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