JP2014157785A - Drive circuit, and lamp for vehicles - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a noise caused by dimming control.SOLUTION: A drive circuit comprises: a DC/DC converter 2 generating a DC drive current for driving a semiconductor light-emitting element on the basis of a battery voltage; and a control circuit 5 controlling a switching operation of a first switching element SWc included in the DC/DC converter on the basis of a comparison result between a level of the drive current and a reference level so that the level of the drive current is accorded with the reference level. The control circuit comprises a reference level switching part 5A switching the reference level between a first level and a second level on the basis of a PWM signal Spwm. Unlike the conventional art that PWM-drives a switch turning on/off the drive current to perform dimming, no time period in which the drive current becomes at a zero level is generated at dimming, and thereby, a noise can be reduced.

Description

  The present invention relates to a drive circuit for a semiconductor light emitting element driven by a direct current, such as an LED (Light Emitting Diode), and a vehicle lamp provided with the semiconductor light emitting element and the drive circuit.

Japanese Patent No. 5089193

  As a vehicular lamp, a configuration including a semiconductor light emitting element such as a light emitting diode and a DC / DC converter that drives the semiconductor light emitting element at a constant current based on a battery voltage is known. As such a vehicular lamp, a vehicular lamp used in, for example, an electric vehicle or the like, dimming (dimming) a semiconductor light-emitting element according to the traveling environment of the vehicle in order to reduce power consumption and extend a travelable distance. (See, for example, Patent Document 1 above).

  Conventionally, the dimming control as described above is performed by controlling the average level of the drive current of the semiconductor light emitting element using a PWM (Pulse Width Modulation) signal. Specifically, a switch for turning on / off the drive current supplied from the DC / DC converter to the semiconductor light emitting element is provided, and this switch is PWM driven.

  However, in the conventional method in which the switch is PWM-driven as described above, the drive current level of the semiconductor light emitting element fluctuates between 100% and 0% during dimming, which causes a problem that the noise is relatively large.

  In view of the above, an object of the present invention is to overcome the above-described problems and reduce noise associated with dimming control.

  First, a driving circuit according to the present invention includes a DC / DC converter that generates a DC driving current for driving a semiconductor light emitting device based on a battery voltage, and a comparison result between the level of the driving current and a reference level. And a control circuit for controlling a switching operation of the first switching element included in the DC / DC converter so that the level of the drive current matches the reference level, the control circuit based on a PWM signal. A reference level switching unit that switches the reference level between the first level and the second level is provided.

  According to the above configuration, the target value (reference level) of the constant current control system for the drive current is switched by the PWM signal. That is, the average level of the drive current is adjusted by switching the target value of the constant current control system.

Second, in the drive circuit according to the present invention described above, it is preferable that the reference level switching unit switches the reference level between the first level and the second level which are larger than the zero level.
Since the two reference levels switched based on the PWM signal are both non-zero, there is no period during which the drive current is at the zero level.

Thirdly, in the driving circuit according to the present invention, the reference level switching unit includes a first resistance element inserted between a constant voltage source and a ground point, and the first resistance element. A second resistance element connected in series and connected in series to the second resistance element between the second resistance element and the ground point, and in response to the PWM signal. And a second switching element that is turned on / off.
In response to turning on / off of the second switching element by the PWM signal, the level of the output voltage from the connection point between the first resistance element and the second resistance element changes. The reference level is switched using the change in the output voltage.

  The vehicle lamp according to the present invention includes the drive circuit according to the present invention and a semiconductor light emitting element.

According to the present invention, when dimming control of a semiconductor light emitting element is performed by adjusting the average level of the drive current using the PWM signal, a period in which the drive current becomes zero level as in the conventional case using a switch is prevented. Can be.
Therefore, it is possible to reduce noise associated with dimming control.

It is explanatory drawing about the circuit structure of the conventional vehicle lamp. It is a wave form diagram showing the relationship between the PWM signal and the drive current in the conventional vehicle lamp. It is explanatory drawing about the circuit structure of the vehicle lamp of embodiment. It is explanatory drawing about the internal structure of the control circuit with which the vehicle lamp of embodiment is provided. It is a wave form diagram showing the relationship between the PWM signal and drive current in the vehicle lamp of embodiment. It is explanatory drawing about the modification of the reference | standard level switching part with which a control circuit is provided.

Prior to the description of the embodiments of the present invention, a conventional vehicle lamp 1 ′ will be described as a comparative example.
FIG. 1 is an explanatory diagram of a circuit configuration of the vehicular lamp 1 ′.
The vehicular lamp 1 ′ includes a DC / DC converter 2, a detection unit 3, a switch SWa, a light emitting unit 4 having a plurality of light emitting diodes L, a control circuit 5 ′, a positive input terminal Tp, and a ground terminal Tg.
A switch SWb is inserted between the positive terminal and the positive input terminal Tp of the battery BT, and lighting / extinguishing of the vehicular lamp 1 ′ is controlled by turning on / off the switch SWb. As shown in the figure, the ground terminal Tg of the vehicle lamp 1 ′ is connected to the negative terminal of the battery BT through a ground point.

The DC / DC converter 2 includes an inductor Lc, a switch SWc, a diode Dc, and a smoothing capacitor Co, and is configured as a step-up DC / DC converter. As shown, one end of the inductor Lc is connected to the positive input terminal Tp, the other end of the inductor Lc is connected to the anode of the diode Dc, and the series connection circuit of the inductor Lc and the diode Dc is connected in series with the battery BT. Connected to be in a relationship.
The switch SWc is inserted between the connection point between the inductor Lc and the diode Dc and the ground line, and is thus connected in parallel with the battery BT. The switch SWc is composed of a switching element such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
The smoothing capacitor Co has a positive terminal connected to the cathode of the diode Dc, a negative terminal connected to the ground line, and is connected in parallel to the switch SWc.

  The detection unit 3 includes a detection resistor Rd having one end connected to a connection point between the cathode of the diode Dc and the positive terminal of the smoothing capacitor Co. The other end of the detection resistor Rd is connected to the switch SWa. When the switch SWa is turned on, the detection resistor Rd is connected to the light emitting unit 4 through the switch SWa. The detection resistor Rd detects the level of the output voltage of the DC / DC converter 2 applied to the light emitting unit 4. The voltage detection result by the detection resistor Rd is input to the control circuit 5 '.

The light emitting unit 4 is configured by connecting a plurality of light emitting diodes L in series. In this example, six light emitting diodes L1, L2, L3, L4, L5, and L6 are connected in series. The anode of the light emitting diode L1 is connected to the switch SWa, and the cathode of the light emitting diode L6 is connected to the connection point between the negative terminal of the smoothing capacitor Co and the ground line in the DC / DC converter 2. Therefore, when the switch SWa is on, the output voltage of the DC / DC converter 2 as the voltage across the smoothing capacitor Co is applied to the series connection circuit of the light emitting diodes L1 to L6 as the light emitting unit 4.
The drive current ILED flows from the DC / DC converter 2 to the light emitting unit 4 by turning on the light emitting diodes L1 to L6 constituting the light emitting unit 4 in response to the application of the output voltage.

  Based on the result of detecting the level of the drive current ILED based on the result of voltage detection by the detection resistor Rd described above, the control circuit 5 ′ is a DC / DC converter so that the level of the drive current ILED matches the predetermined reference level. The constant current control for the drive current ILED is performed by controlling the on / off duty of the switch SWc in 2.

  Here, in the conventional vehicle lamp 1 ′, dimming control of the light emitting unit 4 is realized by driving the switch SWa with PWM (Pulse Width Modulation) in the above-described configuration. Specifically, the switch SWa is on / off controlled by a control signal Spwm as a PWM signal. With such a configuration, the average level of the drive current ILED of the light emitting diode L constituting the light emitting unit 4 can be adjusted by setting the on / off duty of the control signal Spwm. That is, the light emission amount of the light emitting diode L can be adjusted to a desired light emission amount by adjusting the on / off duty of the control signal Spwm.

FIG. 2 is a waveform diagram showing the relationship between the control signal Spwm and the drive current ILED in the conventional vehicular lamp 1 ′.
FIG. 2 shows an example of a waveform when the on-duty of the control signal Spwm is less than 100% and the light emission amount of the light emitting diode L is dimmed to less than 100% (light emission amount when a rated current is passed). Yes. Specifically, in this example, when the on-duty of the control signal Spwm is 80%, the average level of the drive current ILED is 80%.

  Here, as can be seen with reference to FIG. 2, in the dimming control using the conventional switch SWa, the drive current ILED is 0% (zero) at the time of dimming when the on-duty of the control signal Spwm is less than 100%. Level) and 100% (rated current level). For this reason, the conventional vehicular lamp 1 ′ has a problem that noise associated with the light control becomes relatively large.

Therefore, in the present embodiment, a configuration as shown in FIG. 3 is proposed as the vehicular lamp 1 in order to reduce noise associated with such dimming control. 3, parts that are the same as those already described in FIG. 1 are given the same reference numerals, and descriptions thereof are omitted.
As shown in FIG. 3, the vehicular lamp 1 according to the present embodiment includes a control circuit 5 in place of the switch SWa, the point where the dimming signal input terminal T1 is provided, and the control circuit 5 ′. Is different from the vehicular lamp 1 ′.
In FIG. 3, a portion surrounded by a reference numeral “1A” in the drawing, that is, a portion including the DC / DC converter 2, the detection portion 3, and the control circuit 5 except for the light emitting portion 4 in the vehicular lamp 1, This corresponds to the drive circuit.

A control signal Spwm is supplied to the dimming signal input terminal T1 from an external control unit (not shown) configured by, for example, a microcomputer. The control signal Spwm supplied to the dimming signal input terminal T1 is input to the control circuit 5.
Similar to the control circuit 5 ′, the control circuit 5 performs level detection of the drive current ILED based on the voltage detection result by the detection resistor Rd and constant current control of the drive current ILED based on the level detection result. In particular, the control circuit 5 of the present embodiment switches the reference level used in the constant current control based on the control signal Spwm input through the dimming signal input terminal T1.

FIG. 4 is an explanatory diagram of the internal configuration of the control circuit 5.
In FIG. 4, the DC / DC converter 2 and the detection unit 3 are shown together with the internal configuration of the control circuit 5.
As shown in FIG. 4, the control circuit 5 includes a current detection circuit 6, an error amplifier 7, a comparator 8, a sawtooth wave generation circuit 9, and a reference level switching unit 5A.
The current detection circuit 6, the error amplifier 7, the comparator 8, and the sawtooth wave generation circuit 9 together with the DC / DC converter 2 and the detection unit 3 (detection resistor Rd) constitute a constant current control system for the drive current ILED.
The current detection circuit 6 detects the level of the drive current ILED based on the voltage detection result (the voltage across the detection resistor Rd) by the detection resistor Rd.

The error amplifier 7 is a signal corresponding to the difference between the level of the drive current ILED detected by the current detection circuit 7 and the voltage level (reference level) of a variable voltage source Vv (described later) provided in the reference level converter 5A. Is output to the converter 8.
The converter 8 slices the difference signal from the error amplifier 7 using the sawtooth wave signal input from the sawtooth wave generating circuit 9 as a threshold value. As a result, a signal having an on / off duty corresponding to the difference between the reference level and the level of the drive signal ILED is generated as an output signal of the comparator 8. The output signal from the comparator 8 is supplied as a control signal for the switch SWc of the DC / DC converter 2 as shown in the figure.
By supplying the output signal of the comparator 8 as the control signal of the switch SWc as described above, the on / off duty of the switch SWc is controlled according to the difference between the reference level and the drive current ILED. This realizes constant current control for matching the level of the drive current ILED with the reference level.

The reference level switching unit 5A switches the voltage level of the variable voltage source Vv based on the control signal Spwm.
As shown in the drawing, the reference level switching unit 5A includes a first resistor R1 inserted between the constant voltage source Vin and the ground (grounding point), and a second resistor connected in series with the first resistor R1. And a switching element Q1 connected in series with the second resistor R2 between the second resistor R2 and the ground.

  A connection point between the first resistor R1 and the second resistor R2 is connected to the variable voltage source Vv. The variable voltage source Vv generates a voltage having a level corresponding to the output voltage level from the connection point between the first resistor R1 and the second resistor R2. That is, the output voltage from the connection point functions as an output voltage instruction signal for the variable voltage source Vv.

  The switching element Q1 is an NPN transistor, and has a collector connected to the second resistor R2 and an emitter grounded as shown in the figure. The base of the switching element Q1 is connected to the dimming signal input terminal T1 shown in FIG. 4 via the inverter 10, and an inverted signal of the control signal Spwm is supplied.

Here, the potential of the constant voltage source Vin is “potential V1”, and the potential of the output voltage from the connection point between the first resistor R1 and the second resistor R2 is “potential V2.”
According to the reference level switching unit 5A configured as described above, when the control signal Spwm is on (High), the switching element Q1 is turned off, so that the potential V2 is generated based on the constant voltage source Vin. It has the same potential as the voltage across the first resistor R1. That is, potential V2 = potential V1.
On the other hand, when the control signal Spwm is off (Low), the switching element Q1 is turned on, so that the potential V2 is determined by the voltage division ratio between the first resistor R1 and the second resistor R2 and the potential V1. It becomes a potential. In particular,
V2 = V1 · R2 / (R1 + R2)
It becomes.

  Thus, according to the reference level switching unit 5A, the output voltage level from the variable voltage source Vv to the error amplifier 8 according to the on / off of the control signal Spwm, that is, the reference level (control target value) in the constant current control system. Can be switched. In other words, the reference level in the constant current control system can be switched between the first level and the second level in accordance with on / off of the control signal Spwm.

  In this example, when the reference level is high, the light emitting diode L is lit with a light emission amount of 100%, and when the reference level is low, the light emitting diode L is lit with a light emission amount of 60%. 1A is configured. Specifically, for example, the first resistor R1 = 4 kΩ and the second resistor R2 = 6 kΩ are set, so that switching between 100% and 60% is realized.

  Note that the on-duty of the control signal Spwm is adjusted by the control unit described above. Specifically, for example, it is adjusted according to the traveling environment of the vehicle (for example, brightness of the outside world). According to the vehicular lamp 1 of the present embodiment that performs dimming based on such a control signal Spwm, dimming according to the traveling environment of the vehicle is realized.

FIG. 5 is a waveform diagram showing the relationship between the control signal Spwm and the drive current ILED in the vehicle lamp 1.
FIG. 5 shows an example of a waveform when the on-duty of the control signal Spwm is less than 100% and the light emission amount of the light emitting diode L is reduced to less than 100%.
In the example of this figure, it is shown that when the on-duty of the control signal Spwm is 50%, the average level of the drive current ILED is 80%, which is the same as in the case of FIG.
Further, according to FIG. 5, the level of the drive current ILED is 100% when the control signal Spwm is on, whereas the level of the drive current ILED is 60% when the control signal Spwm is off. Has been.

As described above, in the present embodiment, the reference level (control target value) in the constant current control system of the drive current ILED is switched between the first level and the second level based on the control signal Spwm. I have to. That is, the average level of the drive current is adjusted by switching the target value of the constant current control system.
Thus, unlike the conventional case where the average level of the drive current ILED is adjusted by turning on / off the switch SWa, it is possible to prevent a period during which the drive current ILED is at a zero level during dimming with the control signal Spwm. Therefore, it is possible to reduce noise associated with dimming control.

In the present embodiment, the reference level switching unit 5A switches the reference level between the first level and the second level that are greater than the zero level.
According to this, since the two reference levels switched based on the control signal Spwm are both non-zero, there is no period during which the drive current ILED is at the zero level. Therefore, noise can be reduced.

Further, in the present embodiment, the reference level switching unit 5A is connected in series with the first resistor R1 inserted between the constant voltage source Vin and the ground point and the first resistor R1. A second resistor R2 and a switching element Q1 connected in series with the second resistor R2 between the second resistor R2 and the ground point, and turned on / off in response to the control signal Spwm. It is prepared for.
Thereby, the configuration for switching the reference level according to the control signal Spwm can be realized by a simple configuration of at least two resistance elements and one switching element.

The present invention should not be limited to the specific examples described above.
FIG. 6 is an explanatory diagram of a modification of the reference level switching unit.
As shown in the drawing, in the control circuit 5 in this case, a reference level switching unit 5B as a modification is provided instead of the reference level switching unit 5A. The difference from the reference level switching unit 5A is that a third resistor R3 is connected in parallel to the switching element Q1.

  In the reference level switching unit 5B, during the period in which the switching element Q1 is turned off in response to the control signal Spwm being on, the connection point between the first resistor R1 and the second resistor R2 is connected to the variable voltage source Vv. The potential V2 of the output voltage is a potential determined by the voltage dividing ratio between the resistor R1, the resistor R2, and the resistor R3 and the potential V1. That is, V2 = V1 · (R2 + R3) / (R1 + R2 + R3). On the other hand, when the control signal Spwm is turned off and the switching element Q1 is turned on, the potential V2 becomes a potential determined by the voltage dividing ratio between the resistor R1 and the resistor R2 and the potential V1. That is, V2 = V1 · R2 / (R1 + R2).

  The reference level switching unit 5B as such a modification can also switch the reference level according to the on / off state of the control signal Spwm. Also according to this modified example, switching of the reference level according to the control signal Spwm can be realized with a simple configuration using at least a resistance element and a switching element.

In the above description, the case where the light emitting diode L is used as the semiconductor light emitting element has been exemplified. However, the semiconductor light emitting element in the present invention is limited to the light emitting diode L as long as it is a light emitting element driven by a direct current. is not. Moreover, although the structure which connected six semiconductor light emitting elements in series was illustrated, the number of the semiconductor light emitting elements which comprise a light emission part is not limited to six.
Further, a step-down DC / DC converter or a DC / DC converter capable of switching between step-up / step-down can be used instead of the step-up DC / DC converter.

  DESCRIPTION OF SYMBOLS 1 ... Vehicle lamp, 1A ... Drive circuit, 2 ... DC / DC converter, 4 ... Light emission part, 5 ... Control circuit, 5A, 5B ... Reference level switching part, L1-L6 ... Light emitting diode

Claims (4)

A DC / DC converter that generates a direct current drive current for driving the semiconductor light emitting element based on the battery voltage;
A control circuit for controlling a switching operation of the first switching element included in the DC / DC converter so that the drive current level matches the reference level based on a comparison result between the drive current level and the reference level; Prepared,
The control circuit comprises:
A drive circuit comprising a reference level switching unit that switches the reference level between a first level and a second level based on a PWM signal. The reference level switching unit is
The drive circuit according to claim 1, wherein the reference level is switched between the first level and the second level that are greater than zero level. The reference level switching unit is
A first resistance element inserted between the constant voltage source and the ground point;
A second resistive element connected in series with the first resistive element;
And a second switching element connected in series with the second resistance element between the second resistance element and the ground point, and turned on / off in response to the PWM signal. The drive circuit according to claim 1 or 2. A vehicle lamp comprising the drive circuit according to any one of claims 1 to 3 and the semiconductor light emitting element.

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