WO2013047401A1 - Led driving circuit - Google Patents

Abstract

An LED driving circuit comprises: a coil; a free-wheeling diode; a switching element; and a switching control unit that controls the current flowing through an LED load by on/off controlling the switching element by use of an on-time that is based on an acquired current value of a switching element current at turn-on, a target current value and a current variation rate of the switching element current.

Description

LED drive circuit

The present invention relates to an LED drive circuit for driving an LED.

Conventionally, various LED drive circuits for driving LEDs (Light Emitting Diodes) have been proposed. A conventional example of an LED drive circuit is shown in FIG.

The conventional LED drive circuit shown in FIG. 10 is a circuit for driving an LED array 1 configured by connecting a plurality of LEDs in series, and includes a DC power supply Vdc, a coil 2, a freewheeling diode 3, a capacitor 4, A switching element 5, a current detection resistor 6, a resistor 7, a capacitor 8, a comparator 9, a switching control unit 10, and a reference voltage power source Vref are provided.

The positive terminal of the DC power supply Vdc is commonly connected to the cathode of the freewheeling diode 3 and the anode side of the LED array 1. The LED array 1 and the capacitor 4 are connected in parallel. The cathode side of the LED array 1 is connected to one end of the coil 2, and the anode of the reflux diode 3 and the other end of the coil 2 are connected to the current input terminal of the switching element 5. The capacitor 4 stabilizes the current flowing through the LED array 1.

The current output terminal of the switching element 5 is connected to one end of the current detection resistor 6 and the other end of the current detection resistor 6 is grounded. The current detection resistor 6 converts the current flowing through the switching element 5 into a voltage. The converted voltage is input to a low-pass filter including a resistor 7 and a capacitor 8 connected in series. This low-pass filter is a filter for removing ringing generated when the switching element 5 is turned on / off.

The output of the low pass filter is input to the non-inverting input terminal of the comparator 9. A reference voltage from the reference voltage power supply Vref is input to the inverting input terminal of the comparator 9. The output of the comparator 9 is input to the switching control unit 10, and the switching control unit 10 controls the switching element 5 on and off by sending a control signal to the control terminal of the switching element 5.

The control operation of such a conventional LED drive circuit will be described with reference to the timing chart shown in FIG. 11 shows a current waveform in a steady state, the upper part of FIG. 11 shows the LED current flowing through the LED array 1, and the lower part of FIG. 11 shows the switching element current flowing through the switching element 5.

When the switching element 5 is turned on by the switching control unit 10, the switching element current and the LED current increase. When the output voltage of the low-pass filter exceeds the reference voltage by the reference voltage power supply Vref (that is, when the switching element current exceeds a predetermined peak current value), the output of the comparator 9 becomes High level. As a result, the switching control unit 10 turns off the switching element 5. When the switching element 5 is turned off, the switching element current becomes zero, but the back electromotive force generated in the coil 2 is supplied to the LED array 1 by the turned-on diode 3 turned on, and the LED current decreases. Then, after the switching element 5 is turned on again by the switching control unit 10, the same process is repeated.

Special table 2008-537459 gazette (FIG. 1 etc.)

In the conventional LED driving circuit, when the switching frequency (that is, the switching period Tsw in FIG. 11), the inductance of the coil 2 and the forward voltage of the LED array 1 are as designed, the average value of the LED current is the target. It matches the current value. However, the switching frequency, the inductance of the coil 2 and the forward voltage of the LED array 1 may fluctuate due to manufacturing variations and temperature environments.

Here, the basic characteristic formula regarding the coil 2 is expressed as the following formula.
V = L · di / dt
However, when V: applied voltage of the coil 2, L: inductance of the coil 2, di / dt: forward voltage of the current change rate LED array 1, V in the above basic characteristic equation changes. Further, L in the above basic characteristic equation varies with a temperature change. As described above, when V and / or L in the basic characteristic equation fluctuates, the current change rate di / dt (current increase / decrease slope) fluctuates.

Therefore, when the various constants fluctuate, in the method of limiting the switching element current with the peak current value and defining the on-time of the switching element as in the conventional LED driving circuit, for example, as shown in FIG. The average value Iav is shifted from the target current value Iref, resulting in an error err.

In addition, although another conventional example of the LED drive circuit is disclosed in Patent Document 1, this conventional example also has the same problem as described above.

In view of the above problems, an object of the present invention is to provide an LED drive circuit that can accurately control an average value of LED currents with respect to a target current value even when various constants fluctuate.

In order to achieve the above object, the LED drive circuit of the present invention includes:
Coils,
A reflux diode;
A switching element;
A switching control unit for controlling the current flowing through the LED load by controlling the on / off of the switching element according to the on time based on the acquired current value of the switching element current at the time of turn-on, the target current value, and the current change rate of the switching element current; It is set as the structure provided.

According to such a configuration, even when the switching frequency fluctuates or the current change rate fluctuates due to fluctuations in the coil inductance and the forward voltage of the LED load, the average value of the LED current is accurate with respect to the target current value. It becomes possible to control well.

Further, in the above configuration, a current change rate calculation unit that calculates a current change rate of the switching element current;
An on-time calculation unit that calculates an on-time based on an acquired current value of the switching element current at the time of turn-on, a target current value, and a current change rate of the calculated switching element current, and
The switching control unit may be configured to perform on / off control of the switching element according to the calculated on-time.

Further, in the above configuration, the current change rate calculation unit may calculate a current change rate based on a switching element current value acquired during a soft start operation.

Further, in any one of the above configurations, an update unit that recalculates and updates the current change rate of the switching element current is provided,
The on-time calculator may be configured to calculate an on-time based on the updated current change rate.

Further, in any one of the above configurations, a turn-off target current value calculation unit that calculates a turn-off target current value based on the acquired current value and target current value of the switching element current at the time of turn-on,
A comparison determination unit for comparing and determining the calculated turn-off target current value and the acquired current value of the switching element current,
The switching control unit may be configured to turn off the switching element when the comparison determination unit determines that the acquired current value of the switching element current is equal to or greater than the turn-off target current value.

According to the present invention, even when various constants fluctuate, it becomes possible to accurately control the average value of the LED current with respect to the target current value.

It is a figure which shows the structure of the LED drive circuit which concerns on 1st Embodiment of this invention. It is a flowchart regarding control operation of the LED drive circuit according to the first embodiment of the present invention. It is a figure which shows an example of the timing chart of a PWM light control signal and a switching element electric current. It is a figure which shows an example of the timing chart of the switching element electric current in soft start. It is a figure which shows an example of the timing chart of the switching element electric current and LED current in a steady state. It is a figure which shows the structure of the LED drive circuit which concerns on 2nd Embodiment of this invention. It is a flowchart regarding control operation of the LED drive circuit according to the second embodiment of the present invention. It is a figure which shows the structure of the LED drive circuit which concerns on 3rd Embodiment of this invention. It is a flowchart regarding control operation | movement of the LED drive circuit which concerns on 3rd Embodiment of this invention. It is a flowchart regarding control operation | movement of the LED drive circuit which concerns on 3rd Embodiment of this invention. It is a figure which shows the structure of the prior art example of an LED drive circuit. It is a figure which shows an example of the timing chart of LED current by the conventional control system, and switching element current.

(First embodiment)
An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the LED drive circuit according to the first embodiment of the present invention. The LED driving circuit shown in FIG. 1 is a circuit for driving an LED array 1 configured by connecting a plurality of LEDs in series, and includes a DC power supply Vdc, a coil 2, a freewheeling diode 3, a capacitor 4, and a switching element. 5, a current detection resistor 6, a resistor 7, a capacitor 8, and a microcomputer 11.

In the LED drive circuit shown in FIG. 1, the connection relationship of elements other than the microcomputer 11 is the same as that in FIG. Note that the LED driving circuit shown in FIG. 1 is not provided with a comparator as shown in FIG.

In the LED drive circuit according to the first embodiment of the present invention shown in FIG. 1, digital control by the microcomputer 11 is adopted. The microcomputer 11 includes a calculation unit 111, a PWM unit 112, a current value acquisition unit 113, a lighting determination unit 114, and A / D units 115 and 116.

The output voltage of the low-pass filter composed of the resistor 7 and the capacitor 8 is input to the A / D unit 115. The A / D unit 115 A / D converts the input voltage. The current value acquisition unit 113 acquires voltage data after A / D conversion from the A / D unit 115. That is, the current value acquisition unit 113 acquires a switching element current value flowing through the switching element 5.

The calculation unit 111 performs various calculations based on the current value acquired by the current value acquisition unit 113. The PWM unit 112 performs on / off control of the switching element 5 by PWM (Pulse Width Modulation). PWM is a modulation method in which the duty ratio (on-time ratio) is variable while the switching frequency (cycle) is constant.

The PWM dimming signal is input to the A / D unit 116 from the outside of the LED drive circuit. The PWM dimming signal is, for example, a pulse signal of several hundred Hz (upper stage in FIG. 3 described later). By changing the duty ratio of the PWM dimming signal, the brightness of the LED array 1 can be dimmed (the higher the duty ratio, the brighter it is). The lighting determination unit 114 receives the PWM dimming signal after A / D conversion by the A / D unit 116, and determines that the light is turned on if the PWM dimming signal is at a high level, and is turned off if the level is low. To do.

Next, the control operation of the LED drive circuit (FIG. 1) according to the first embodiment of the present invention will be described with reference to the flowchart of FIG. 2 and the timing charts of FIGS.

When the lighting determination unit 114 detects the rise of the PWM dimming signal to the high level and determines that the lighting is started, the flowchart shown in FIG. 2 starts (timing t0 in FIG. 3). Note that the lower PWM dimming signal in FIG. 3 is an enlarged view of the High level portion of the upper PWM dimming signal. For example, when the PWM dimming signal is several hundred Hz, the switching element current waveform shown in the lower part of FIG. 3 is several hundred kHz.

When the flowchart shown in FIG. 2 is started, first, soft start is started in step S1. Here, the PWM unit 112 performs on / off control of the switching element 5 several times with the on-time fixed. As a result, as shown in the lower part of FIG. 3, several pulses are generated as the waveform of the switching element current. For example, ten pulses are generated with the on-time fixed at 20 msec. The soft start is performed to prevent the inrush current from being generated by charging the capacitor 4.

In such a soft start, the current value acquisition unit 113 acquires the current value when the switching element 5 is turned off every several pulses. In step S2, the calculation unit 111 calculates an average value of the obtained several current values, and divides the calculated current average value by the on-time Tss to thereby determine a current change rate di / dt (ie, (Slope of current increase) is calculated (see FIG. 4). Note that the current value acquisition method is not limited to this. For example, the current value at the time of turn-off may be acquired for any one of several pulses, and the acquired current value may be divided by the on-time. .

In step S3, the lighting determination unit 114 determines whether or not lighting is continued based on the PWM dimming signal. If lighting is continued (Y in step S3), the process proceeds to step S4. In step S4, the PWM unit 112 turns on the switching element 5. At this time, the current value acquisition unit 113 acquires a current value at the time of turn-on.

After step S4, in step S5, the calculation unit 111 determines the ON time of the switching element 5 by calculation. Here, the calculation of the on-time will be described with reference to FIG.

As shown in FIG. 5, when the average value of the switching element current is equal to the target current value Iref, the following equation (1) is established.
Iref = di / dt · Tonα + Istart (1)
Where Iref: target current value, di / dt: current change rate, Tonα: time to target current value, Istart: current value at turn-on

Then, the on time Ton can be expressed by the following equation (2).
Ton = 2Tonα
= 2 × (Iref−Istart) / (di / dt) (2)

Therefore, the on-time Ton is calculated by substituting the target current value Iref, the turn-on current value Istart obtained in step S4, and the current change rate di / dt calculated in step S2 into the above equation (2). .

In step S 6, the calculation unit 111 sets the calculated on-time Ton in the PWM unit 112. The PWM unit 112 turns off the switching element 5 at the timing when the set on-time Ton has elapsed, and waits for the completion of one PWM cycle (step S7).

When one PWM cycle is completed, the process returns to step S3 to determine whether or not the lighting is continued. If the lighting is continued, the process proceeds again to step S4, and thereafter the same operation as described above is performed. On the other hand, if it is determined in step S3 that the low level of the PWM dimming signal is detected and the light is turned off (timing t1 in FIG. 3), the process ends with the switching element 5 turned off (end), and the LED array 1 Turns off.

Further, after the soft start, the value of the target current value Iref at the time of calculating the on-time Ton (step S5) is gradually increased at each calculation step and then made constant at a certain value. As a result, after the soft start, after a transient state in which the average value of the switching element current gradually increases, a transition is made to a steady state in which the average value of the switching element current coincides with the target current value Iref (FIG. 3). (However, the waveform in the transient state is not shown in FIG. 3). Accordingly, the average value Iav of the LED current flowing through the LED array 1 also gradually increases in the transient state, and then becomes constant in accordance with the target current value Iref in the steady state (see FIG. 5).

According to the first embodiment of the present invention, even when the switching frequency (period) fluctuates or the current change rate di / dt fluctuates due to the inductance of the coil 2 and the forward voltage of the LED array 1. Since the ON time is determined such that the average value of the switching element current matches the target current value, and the turn-off is performed at the timing when the determined ON time elapses, the average value of the LED current is set to match the target current value. It becomes possible to control to.

(Second Embodiment)
Next, FIG. 6 shows the configuration of the LED drive circuit according to the second embodiment of the present invention. The LED drive circuit shown in FIG. 6 is similar in configuration to the first embodiment (FIG. 1) described above except that the microcomputer 12 is provided.

The microcomputer 12 includes a calculation unit 121, a PWM unit 122, a current value acquisition unit 123, a current determination unit 124, a lighting determination unit 125, and A / D units 126 and 127.

As a difference from the microcomputer 11 in the first embodiment (FIG. 1), the microcomputer 12 includes a current determination unit 124. As will be described in detail later, the current determination unit 124 compares and determines the turn-off target current value calculated by the calculation unit 121 and the current value acquired by the current value acquisition unit 123.

Next, the control operation of the LED drive circuit according to the second embodiment of the present invention will be described with reference to the flowchart of FIG.

When the lighting determination unit 125 detects the rise of the PWM dimming signal to the high level and determines that the lighting is started, the flowchart shown in FIG. 7 starts.

When the flowchart shown in FIG. 7 starts, first, soft start is started in step S11. Here, as in step S1 of FIG. 2 described above, a waveform of several pulses is generated in the switching element current. This prevents inrush current from occurring.

After the soft start, in step S12, the lighting determination unit 125 determines whether or not lighting is continued based on the PWM dimming signal. If lighting is continued (Y in step S12), the process proceeds to step S13.

In step S13, the PWM unit 122 turns on the switching element 5. At this time, the current value acquisition unit 123 acquires a current value at turn-on.

In step S14, the calculation unit 121 calculates a turn-off target current value. Here, calculation of the turn-off target current value will be described with reference to FIG.

As shown in FIG. 5, when the average value of the switching element current is equal to the target current value Iref, the following equation (3) is established.
Ioffref = Iref + (Iref−Istart) (3)
Where Ioffref: turn-off target current value, Iref: target current value, Istart: current value at turn-on

Therefore, the turn-off target current value Ioffref is calculated by substituting the target current value Iref and the turn-on current value Istart acquired in step S13 into the above equation (3).

Next, proceeding to step S15, the current determination unit 124 determines whether or not the turn-off target current value calculated by the calculation unit 121 exceeds the current value acquired by the current value acquisition unit 123. Continues the determination in step S15 (that is, turn-on continues) (Y in step S15).

When the acquired current value becomes equal to or greater than the turn-off target current value due to the increase of the switching element current (N in Step S15), the process proceeds to Step S16, and the PWM unit 122 turns off the switching element 5. Thereafter, the completion of one PWM cycle is awaited (step S17). When one cycle of PWM is completed, the process returns to step S12, and it is determined whether or not lighting is continued.

If the lighting is continued (Y in step S12), the process proceeds to step S13 again, and thereafter the same operation as described above is performed. On the other hand, when the Low level of the PWM dimming signal is detected and it is determined that the light is extinguished, the process ends with the switching element 5 turned off (end), and the LED array 1 is extinguished.

Further, after the soft start, the value of the target current value Iref at the time of calculating the turn-off target current value Ioffref (step S14) is gradually increased at each calculation step and then becomes constant at a certain value. Thereby, after the soft start, after passing through a transient state in which the average value of the switching element current gradually increases, a transition is made to a steady state in which the average value of the switching element current coincides with the target current value Iref. Accordingly, the average value of the LED current flowing through the LED array 1 also gradually increases in the transient state, and then becomes constant in accordance with the target current value Iref in the steady state.

According to the second embodiment of the present invention, even when the switching frequency (period) fluctuates or the current change rate di / dt fluctuates due to the inductance of the coil 2 and the forward voltage of the LED array 1. Since the turn-off target current value is determined so that the average value of the switching element current matches the target current value, the current value is monitored, and the turn-off is performed when the current value exceeds the turn-off target current value. It is possible to control the average value so as to match the target current value.

(Third embodiment)
Next, FIG. 8 shows a configuration of an LED drive circuit according to the third embodiment of the present invention. The LED drive circuit shown in FIG. 8 is the same in configuration as the above-described second embodiment (FIG. 6) except that the microcomputer 13 is provided.

The microcomputer 13 includes a counter determination unit 131, a counter 132, a calculation unit 133, a PWM unit 134, a current value acquisition unit 135, a current determination unit 136, a lighting determination unit 137, and A / D units 138 and 139. have.

As a difference from the microcomputer 12 in the second embodiment (FIG. 6), the microcomputer 13 has a counter determination unit 131 and a counter 132. The counter determination unit 131 determines the count value counted by the counter 132.

Next, the control operation of the LED drive circuit according to the third embodiment of the present invention will be described using the flowcharts shown in FIGS. 9A and 9B.

When the lighting determination unit 137 detects the rise of the PWM dimming signal to the high level and determines that the lighting is started, the flowchart illustrated in FIG. 9A starts.

When the flowchart shown in FIG. 9A starts, first, soft start is started in step S21. Here, as in step S1 of FIG. 2 described above, a waveform of several pulses is generated in the switching element current. This prevents inrush current from occurring. At this time, as in step S1, the current value acquisition unit 135 acquires the current value when the switching element 5 is turned off.

After the soft start, in step S22, the calculation unit 133 calculates the current change rate di / dt in the same manner as in step S2 in FIG.

In step S23, the lighting determination unit 137 determines whether or not lighting is continued based on the PWM dimming signal. If lighting is continued (Y in step S23), the process proceeds to step S24.

In step S24, the PWM unit 134 turns on the switching element 5. At this time, the current value acquisition unit 135 acquires a current value at turn-on.

Next, in step S25, the counter determination unit 131 determines whether or not the count value counted by the counter 132 has reached a predetermined value equivalent to several seconds (for example, equivalent to 10 seconds). Initially, the counter 132 is reset here.

If the count value has not reached the predetermined value (N in step S25), the process proceeds to step S26. In step S26, as in step S5 of FIG. 2 described above, the calculation unit 133 calculates the on-time Ton based on the current value Istart at turn-on, the target current value Iref, and the current change rate di / dt.

In step S27, the arithmetic unit 133 sets the calculated on-time Ton in the PWM unit 134. The PWM unit 134 turns off the switching element 5 at the timing when the set on-time Ton has elapsed, and waits for completion of one PWM cycle (step S28).

When one cycle of PWM is completed, the process returns to step S23 to determine whether or not lighting is continued. If lighting is continued, the process proceeds again to step S24.

In step S25, if the count value reaches the predetermined value (Y in step S25), the counter 132 is reset, and the process proceeds to step S29 in FIG. 9B.

In step S29, similarly to step S14 of FIG. 7 described above, the calculation unit 133 calculates the turn-off target current value Ioffref based on the current value Istart at turn-on and the target current value Iref. In step S30, the current determination unit 136 determines whether the current value acquired by the current value acquisition unit 135 matches the target current value Iref. If they do not match (N in Step S30), the process proceeds to Step S31, and the current determination unit 136 determines whether or not the turn-off target current value Ioffref exceeds the current value acquired by the current value acquisition unit 135. If so (Y in step S31), the process returns to step S30.

If the acquired current value matches the target current value Iref in step S30 due to the increase in switching element current (Y in step S30), the process proceeds to step S32. In step S32, the calculation unit 133 calculates and updates the current change rate di / dt based on the following equation (4).
di / dt = (Iref−Istart) / Tonα (4)
Where Iref: target current value, Istart: current value at turn-on, Tonα: time from turn-on until the acquired current value reaches the target current value Iref

After step S32, the process proceeds to step S31. After the process proceeds to step S32, when the acquired current value becomes equal to or greater than the turn-off target current value Ioffref due to the current increase (N in step S31), the process proceeds to step S33. In step S33, the PWM unit 134 turns off the switching element 5.

After step S33, the completion of one PWM cycle is awaited (step S28 in FIG. 9A), and the process returns to step S23. In the subsequent calculation of the on-time Ton in step S26, the value of the current change rate di / dt updated in step S32 is used.

If the low level of the PWM dimming signal is detected in step S23 and it is determined that the light is extinguished (N in step S23), the process ends with the switching element 5 turned off (end), and the LED array 1 is Turns off.

Further, after the soft start, the value of the target current value Iref at the time of calculating the on time Ton (step S26) is gradually increased at each calculation step and then made constant at a certain value. Thereby, after the soft start, after passing through a transient state in which the average value of the switching element current gradually increases, a transition is made to a steady state in which the average value of the switching element current coincides with the target current value Iref. Accordingly, the average value of the LED current flowing through the LED array 1 also gradually increases in the transient state, and then becomes constant in accordance with the target current value Iref in the steady state.

When the duty ratio of the PWM dimming signal is 100%, the inductance of the coil 2 may change (decrease) due to the influence of temperature change due to heat generation. In that case, the current change rate di / dt changes, and if the use of di / dt calculated in the past is continued, the control accuracy of the average value of the LED current deteriorates. Therefore, in the third embodiment of the present invention, the current change rate is recalculated and updated once every several seconds, for example. Thereby, the control precision of the average value of LED current can be maintained satisfactorily.

The embodiment of the present invention has been described above, but the embodiment can be variously modified within the scope of the gist of the present invention.

The LED drive circuit according to the present invention can be applied to various devices using LEDs as a light source, and in particular, applied to backlight devices (edge light type, direct type, etc.) using LEDs as a light source. Is preferred.

DESCRIPTION OF SYMBOLS 1 LED array 2 Coil 3 Reflux diode 4 Capacitor 5 Switching element 6 Current detection resistance 7 Resistance 8 Capacitor 9 Comparator 10 Switching control unit 11, 12, 13 Microcomputer Vdc DC power supply Vref Reference voltage power supply

Claims (5)

Coils,
A reflux diode;
A switching element;
A switching control unit for controlling the current flowing in the LED load by controlling on / off of the switching element according to the on time based on the acquired current value of the switching element current at the time of turn-on, the target current value, and the current change rate of the switching element current;
An LED driving circuit comprising: A current change rate calculating unit for calculating a current change rate of the switching element current;
An on-time calculation unit that calculates an on-time based on an acquired current value of the switching element current at the time of turn-on, a target current value, and a current change rate of the calculated switching element current, and
The LED driving circuit according to claim 1, wherein the switching control unit performs on / off control of the switching element according to the calculated on-time. 3. The LED driving circuit according to claim 2, wherein the current change rate calculation unit calculates a current change rate based on a switching element current value acquired during a soft start operation. An update unit that recalculates and updates the current change rate of the switching element current,
The LED driving circuit according to claim 2, wherein the on-time calculating unit calculates an on-time based on the updated current change rate. A turn-off target current value calculation unit for calculating a turn-off target current value based on the acquired current value and the target current value of the switching element current at the time of turn-on;
A comparison determination unit for comparing and determining the calculated turn-off target current value and the acquired current value of the switching element current,
The switching control unit turns off the switching element when the comparison determination unit determines that the acquired current value of the switching element current is equal to or greater than the turn-off target current value. Item 5. The LED drive circuit according to Item 4.

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