JP2014050112A - Dc power supply circuit and led lighting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low cost LED lighting circuit including a DC power supply circuit having a high voltage conversion efficiency and small heat generation while being capable of both step-up and step-down of a DC voltage.SOLUTION: A DC power supply circuit includes a step-up/down chopper DC-DC converter 11 consisting of a series connection of a step-down chopper DC-DC converter 11a for stepping down the DC voltage of an on-vehicle battery 12, and a step-up chopper DC-DC converter 11b for stepping up the DC voltage, between the on-vehicle battery 12 and a light source block 13 and a resistor R, which is a load, and supplying an output voltage Vo, generated by stepping up or down the DC voltage, to the load. During step-up of the DC voltage, only the step-up chopper DC-DC converter 11b is operated and a transistor Q1 is held in the on state, thus stopping operation of the step-down chopper DC-DC converter 11a.

Description

  The present invention relates to a DC power supply circuit and an LED lighting circuit, and more particularly to a DC power supply circuit including a step-up / step-down chopper type DC-DC converter and an LED lighting circuit using the DC power supply circuit.

A headlight of an automobile needs to be able to switch between a high beam, which is a traveling headlamp, and a low beam, which is a passing headlamp, and it is necessary to make the high beam brighter than the low beam.
In addition, when the tail lamp (tail lamp) of an automobile is also used as a brake lamp (stop lamp), it is necessary to make the brake lamp brighter than the tail lamp.
Therefore, some headlamps or taillights using LEDs include an LED lighting circuit that controls the number of LEDs to be lit at the time of a low beam or a tail lamp to be smaller than that at the time of a high beam or a brake lamp.

As disclosed in Patent Documents 1 and 2, a plurality of LEDs are connected in series, all LEDs are turned on during high beam, and a predetermined number of LEDs are short-circuited during low beam, and only the remaining LEDs are turned on. There is known an LED lighting circuit that performs control so as to be controlled.
In Patent Document 1, an isolated flyback DC-DC converter (switching regulator) using a high-frequency transformer is used as a power source for an LED lighting circuit, and a DC voltage of an in-vehicle battery is boosted and then connected in series. A technique for supplying the LED to the LED is disclosed.
Patent Document 2 discloses a technique that uses a DC-DC converter as a power source of an LED lighting circuit, but does not describe a circuit format of the DC-DC converter, and does the DC-DC converter perform boosting? There is also no description on whether to lower the blood pressure.

JP 2008-126958 A JP 2010-143447 A

In recent years, with the improvement of the luminous efficiency of LEDs, an LED lighting circuit in which a plurality of LEDs are connected in series has been able to obtain a sufficient amount of light even if the number of LEDs connected in series is small.
For this reason, the LED lighting circuit is required not only to boost the DC voltage of the vehicle-mounted battery but also to reduce it.

In other words, when the headlamp is a high beam or when the tail lamp is also used as a brake lamp, the number of LEDs connected in series is large, so the total value of the forward voltages of a plurality of LEDs connected in series is Since it becomes higher than the DC voltage of the vehicle-mounted battery, it is necessary to boost the DC voltage of the vehicle-mounted battery and supply it to the LED.
On the other hand, the number of LEDs connected in series decreases when the headlamp is low beam or when the tail lamp is also used as a brake lamp, so the total forward voltage of the LEDs connected in series is reduced. Since it becomes lower than the DC voltage of the in-vehicle battery, it is necessary to step down the DC voltage of the in-vehicle battery and supply it to the LED.

Therefore, it is conceivable to use a step-up / step-down DC-DC converter capable of both boosting and stepping down as a power source for the LED lighting circuit.
The step-up / step-down DC-DC converter includes a method using a high frequency transformer and a method using a choke coil.

  There are various types of DC-DC converters that use high-frequency transformers (insulated flyback type, isolated forward type, half-bridge type, full-bridge type, etc.). (The power loss of the high-frequency transformer is large. The external shape of the high-frequency transformer is large. The high-frequency transformer is expensive.)

A DC-DC converter using a choke coil is called a chopper method (choke converter method).
The step-up / step-down chopper type DC-DC converter is configured by connecting a step-down chopper type DC-DC converter (step-down chopper circuit) and a step-up chopper type DC-DC converter (step-up chopper circuit) in series. Step-up or step-down is performed by synchronizing the circuit and performing pulse width modulation (PWM) control.

Therefore, at the time of boosting, not only the switching elements and diodes constituting the boosting chopper circuit operate, but also the switching elements and diodes constituting the step-down chopper circuit operate.
As a result, in addition to the voltage conversion efficiency of the DC-DC converter being reduced due to the power loss of the switching element and the diode constituting the step-down chopper circuit, in order to prevent a failure due to heat generation of the switching element and the diode, Since a radiator is required, there is a problem that the cost increases.

At the time of step-down, not only the switching elements and diodes that constitute the step-down chopper circuit operate, but also the switching elements and diodes that constitute the step-up chopper circuit operate.
As a result, in addition to the voltage conversion efficiency of the DC-DC converter being reduced due to the power loss of the switching element and the diode constituting the step-up chopper circuit, in order to prevent a failure due to heat generation of the switching element and the diode, Since a radiator is required, there is a problem that the cost increases.

  This problem is not limited to the step-up / step-down chopper type DC-DC converter as a DC power supply circuit that supplies a plurality of LEDs connected in series as a load and supplies a DC voltage to each LED. If so, it is a problem even if the DC voltage is supplied to any load.

The present invention has been made to solve the above problems, and has the following objects.
(1) To provide a DC power supply circuit that can increase and decrease DC voltage, has high voltage conversion efficiency, and generates little heat at low cost.
(2) An LED lighting circuit having the advantage of (1) is provided by using the DC power supply circuit of (1).

  As a result of intensive studies in order to solve the above-mentioned problems, the present inventors have arrived at each aspect of the present invention as follows.

<First aspect>
A first aspect is that a step-down chopper type DC-DC converter that steps down a direct current voltage of the direct current voltage source and a step-up chopper type DC-DC converter that steps up the direct current voltage are provided between a direct current voltage source and a load. A DC power supply circuit including a step-up / step-down chopper type DC-DC converter that is connected in series and supplies an output voltage generated by boosting or stepping down the DC voltage to the load. This is a DC power supply circuit that operates only the chopper type DC-DC converter and stops the operation of the step-down chopper type DC-DC converter.

According to the first aspect, when the DC voltage is boosted, the operation of the step-down chopper type DC-DC converter is stopped, so that the power loss of the step-down chopper type DC-DC converter can be reduced, and the voltage conversion is performed. A DC power supply circuit with high efficiency and low heat generation can be provided.
And according to the 1st aspect, since the heat_generation | fever by the power loss of a pressure | voltage fall chopper system DC-DC converter is reduced at the time of a DC voltage boost, the heat radiator for radiating the heat can be reduced in size, Cost reduction can be achieved.

<Second aspect>
The second aspect is the first aspect,
The step-down chopper type DC-DC converter is
A first switching element for switching a current supplied from the DC voltage source;
A choke coil that generates an induced current to prevent a change in current supplied from the DC voltage source;
A first diode that flows a reflux current through the choke coil when the switching element is off,
When the DC voltage is boosted, the operation of the step-down chopper type DC-DC converter is stopped by holding the first switching element in the ON state.

According to the second aspect, by holding the first switching element in the on state (all on state), the operation of the step-down chopper type DC-DC converter is stopped, so that the power loss of the first switching element can be reduced. Thus, the action and effect of the first aspect can be obtained with certainty.
In addition, according to the second aspect, since the first switching element is held in the ON state, the switching noise of the first switching element becomes zero and the noise generation source is reduced. It is possible to simplify the configuration of the noise filter provided to prevent noise, and further cost reduction can be achieved.

<Third aspect>
The third aspect is the first aspect,
The step-down chopper type DC-DC converter is
A first switching element for switching a current supplied from the DC voltage source;
A choke coil that generates an induced current to prevent a change in current supplied from the DC voltage source;
A first diode for flowing a return current through the choke coil when the switching element is off;
A first switch connected in parallel to the first switching element;
When the DC voltage is boosted, the operation of the step-down chopper type DC-DC converter is stopped by closing the first switch and short-circuiting the first switching element.

  According to the third aspect, the operation of the step-down chopper type DC-DC converter is stopped by closing the first switch and short-circuiting the first switching element, so that the power loss of the first switching element is made zero. Since the power loss is further reduced as compared with the second aspect, the action and effect of the first aspect can be obtained more reliably.

<Fourth aspect>
In a second aspect or a third aspect, a fourth aspect includes a second switch connected in series to the first diode, and at the time of boosting the DC voltage, by opening the second switch, Prevent current from flowing through one diode.

  According to the fourth aspect, in order to prevent current from flowing through the first diode, it is possible to make the power loss of the first diode zero, and the operation and effect of the second aspect or the third aspect Can be increased.

<5th aspect>
According to a fifth aspect, in the first to fourth aspects, when the DC voltage is stepped down, only the step-down chopper type DC-DC converter is operated, and the operation of the step-up chopper type DC-DC converter is stopped.

According to the fifth aspect, when the DC voltage is stepped down, the operation of the step-up chopper type DC-DC converter is stopped. Therefore, the power loss of the step-up chopper type DC-DC converter can be reduced, and the voltage conversion is performed. A DC power supply circuit with high efficiency and low heat generation can be provided.
And according to the fifth aspect, when the DC voltage is stepped down, since heat generation due to power loss of the step-up chopper type DC-DC converter is reduced, the radiator for radiating the heat can be miniaturized, Cost reduction can be achieved.

<Sixth aspect>
The sixth aspect is the fifth aspect,
The step-up chopper type DC-DC converter is
Sharing the choke coil of the step-down chopper type DC-DC converter,
A second switching element for switching a current supplied from the DC voltage source;
A second diode for preventing the induced current from flowing backward to the DC voltage source;
When the DC voltage is stepped down, the operation of the step-up chopper type DC-DC converter is stopped by holding the second switching element in the OFF state.

According to the sixth aspect, by holding the second switching element in the off state (all off state), the operation of the step-up chopper type DC-DC converter is stopped, so that the power loss of the second switching element is zero. The action and effect of the fifth aspect can be obtained with certainty.
According to the sixth aspect, since the switching noise of the second switching element becomes zero and the noise source is reduced in order to keep the second switching element in the off state, the switching noise is reduced to conduction noise or radiation. It is possible to simplify the configuration of the noise filter provided to prevent noise, and further cost reduction can be achieved.

<Seventh aspect>
In a sixth aspect, the seventh aspect includes a third switch connected in parallel to the second diode, and closes the third switch to short-circuit the second diode when the DC voltage is stepped down. The current is prevented from flowing through the second diode.

  According to the seventh aspect, in order to prevent current from flowing through the second diode, the power loss of the second diode can be reduced to zero, and the action and effect of the sixth aspect can be enhanced. .

<Eighth aspect>
An eighth aspect is an LED lighting circuit including the DC power supply circuit of the first to seventh aspects, wherein the load is a plurality of LEDs connected in series, and when the DC voltage is stepped down, It is an LED lighting circuit including a short circuit that short-circuits a predetermined LED among the LEDs.

According to the 8th aspect, the LED lighting circuit provided with the said advantage of the 1st-7th aspect can be provided by using the DC power supply circuit of the 1st-7th aspect.
In order to apply the LED lighting circuit according to the eighth aspect to a headlamp, the time when the DC voltage is lowered may correspond to the low beam time, and the time when the DC voltage is raised corresponds to the high beam time.
In addition, in order to apply the LED lighting circuit of the eighth aspect to a lighting device in which the tail lamp is also used as a brake lamp, it is possible to make the DC voltage step-down corresponding to the tail lamp and the DC voltage step-up equivalent to the brake lamp. That's fine.

The circuit diagram of the LED lighting circuit 10 provided with the step-up / step-down chopper system DC-DC converter 11 which is the DC power supply circuit of 1st Embodiment which actualized this invention. FIG. 2A is a timing chart for explaining the operation of the LED lighting circuit 10 of the first embodiment. FIG. 2B is a timing chart for explaining the operation of the conventional LED lighting circuit 100. The circuit diagram of the LED lighting circuit 100 provided with the step-up / step-down chopper system DC-DC converter 101 which is the conventional DC power supply circuit. The circuit diagram of the LED lighting circuit 30 provided with the step-up / step-down chopper system DC-DC converter 31 which is the direct-current power supply circuit of 2nd Embodiment which actualized this invention. The timing chart for demonstrating operation | movement of the LED lighting circuit 30 of 2nd Embodiment. The timing chart for demonstrating operation | movement of the LED lighting circuit 30 in the modification of 2nd Embodiment. The circuit diagram of the LED lighting circuit 40 provided with the step-up / step-down chopper system DC-DC converter 41 which is the DC power supply circuit of 3rd Embodiment which actualized this invention. FIG. 8A is a timing chart for explaining the operation of the LED lighting circuit 40 of the third embodiment. FIG. 8B is a timing chart for explaining the operation of the conventional LED lighting circuit 100, and is the same drawing as FIG.

  Hereinafter, embodiments embodying the present invention will be described with reference to the drawings. In each embodiment, the same constituent members and constituent elements are denoted by the same reference numerals, and redundant description of the same contents is omitted.

<First Embodiment>
As shown in FIG. 1, the LED lighting circuit 10 of the first embodiment includes a step-up / step-down chopper type DC-DC converter 11, an in-vehicle battery 12, a light source block 13, a first light source block 14, a second light source block 15, an LED 16, Noise filters 17 and 18, an NMOS transistor Q3, and a resistor R are provided, and are used for automobile headlamps.

The step-up / step-down chopper type DC-DC converter 11 includes a step-down chopper type DC-DC converter (step-down chopper circuit) 11a and a step-up chopper type DC-DC converter (step-up chopper circuit) 11b.
The step-down chopper circuit 11a includes capacitors C1 and C2, an NMOS transistor Q1, a diode D1, a choke coil L, a switch S1, and a control circuit 19.
The switch S1 is a mechanical open / close switch, connected in series to the diode D1, and connected between the anode of the diode D1 and the negative terminal of the in-vehicle battery 12.
The negative terminal of the in-vehicle battery 12 is connected to the ground.
The step-up chopper circuit 11b includes capacitors C1 and C2, an NMOS transistor Q2, a diode D2, a choke coil L, and a control circuit 19.
Both chopper circuits 11 a and 11 b share capacitors C 1 and C 2, choke coil L and control circuit 19.

The transistors Q1 and Q2 switch the current supplied from the in-vehicle battery 12.
The choke coil L generates an induced current so as to prevent a change in the current supplied from the in-vehicle battery 12.
The diode D1 allows a reflux current to flow through the choke coil L when the transistor Q1 is off.
The diode D <b> 2 prevents the induced current generated by the choke coil L from flowing back to the in-vehicle battery 12.
Since the configuration and operation of the DC-DC converter 11 are known, detailed description thereof will be omitted.

The light source block (LED module circuit) 13 includes a first light source block 14 and a second light source block 15 connected in series.
Each light source block 14, 15 is composed of five LEDs 16 connected in series.
That is, the light source block 13 is composed of ten LEDs 16 connected in series.

The transistor Q3 constituting the short circuit of the light source block 15 is connected to the light source block 15 in parallel.
A resistor R for detecting the drive current of each LED 16 flowing in the light source block 13 is connected in series to the light source block 13.
The cathode side of the light source block 13 and the source terminal of the transistor Q3 are connected to the minus side terminal of the in-vehicle battery 12 via a resistor R.
The light source block 13 and the resistor R connected in series are loads of the DC-DC converter 11 that is a DC power supply circuit.

The DC-DC converter 11 is configured such that an input side step-down chopper circuit 11a and an output side step-up chopper circuit 11b are connected in series between an in-vehicle battery 12 and a load (light source block 13 and resistor R). Both the chopper circuits 11a and 11b are boosted or lowered by PWM control.
In other words, the DC-DC converter 11 inputs the DC voltage Vin of the in-vehicle battery 12 as an input voltage Vin through the noise filter 17 and boosts or steps down the input voltage Vin to generate an output voltage Vo that is a DC voltage. The output voltage Vo is supplied to the light source block 13 and the resistor R through the noise filter 18.

The noise filter 17 is inserted between the input side of the DC-DC converter 11 and the in-vehicle battery 12, and switching noise generated by the DC-DC converter 11 is conducted to the in-vehicle battery 12 and becomes conduction noise. To prevent.
The noise filter 18 is inserted between the output side of the DC-DC converter 11 and the light source block 13, and the switching noise generated by the DC-DC converter 11 is conducted to the LED 16 of the light source block 13 to generate radiation noise. To prevent becoming.
The noise filters 17 and 18 may be embodied by any circuit configuration.

The capacitor C <b> 1 is connected in parallel to the in-vehicle battery 12 via the noise filter 17, and has the same operation and effect as the noise filter 17. Note that either one of the noise filter 17 and the capacitor C1 may be omitted.
The capacitor C2 is a smoothing capacitor that smoothes the output voltage Vo of the DC-DC converter 11.

The control circuit 19 detects the voltage Vr between both ends of the resistor R, detects the driving current of each LED 16 flowing through the light source block 13 based on the voltage Vr between both ends, holds the driving current at a constant value, and Control signals (gate signals) G1 to G3 for PWM control of the transistors Q1 to Q3 are generated so that the LED 16 is driven at a constant current.
Control signals G1 to G3 are applied to the gates of the transistors Q1 to Q3, respectively.

[Operation of LED Lighting Circuit 10 of First Embodiment]
The switch S1 is opened and closed in conjunction with a low beam and high beam switch (not shown) operated by the driver.

As shown in FIG. 2A, in the LED lighting circuit 10 of the first embodiment, the switch S1 is closed and the control circuit 19 sets the control signal G3 to the high level when the headlamp is low beam (Lo). The transistor Q3 is turned on, and the control signals G1 and G2, which are square waves, are simultaneously switched on and off, and the transistors Q1 and Q2 are synchronously switched to perform PWM control.
Then, both ends of the second light source block 15 are short-circuited by the transistor Q3 in the ON state, and the output voltage Vo of the DC-DC converter 11 is applied to the first light source block 14 and the resistor R connected in series, and the first light source Only the LED 16 of the block 14 is lit.

  Here, the duty ratio M of each of the control signals G1 and G2 is a value obtained by dividing the pulse width τ of each of the control signals G1 and G2 by the period T as shown in Equation 1.

    M = τ / T (Equation 1)

  When the headlamp is in a low beam, the output voltage Vo of the DC-DC converter 11 is calculated from the input voltage Vin and the duty ratio M as shown in Equation 2, and is lower than the input voltage Vin (Vo). <Vin).

    Vo = Vin × M / (1-M) (Equation 2)

That is, when the headlamp is in a low beam, the total value of the forward voltages of the five LEDs 16 of the first light source block 14 is lower than the DC voltage Vin of the in-vehicle battery 12.
Therefore, the DC-DC converter 11 generates an output voltage Vo obtained by stepping down the DC voltage Vin of the in-vehicle battery 12 and supplies it to the light source block 13 (first light source block 14).

As shown in FIG. 2A, at the time of high beam (Hi) of the headlamp, the switch S1 is opened, and the control circuit 19 sets the control signal G3 to the low level to turn off the transistor Q3, and the control signal G1 is fixed at a high level to keep the transistor Q1 in an ON state, and only the transistor Q2 is switched by a control signal G2 that is a square wave to perform PWM control.
Then, the output voltage Vo of the DC-DC converter 11 is applied to the light source block 13 and the resistor R connected in series, and all the LEDs 16 of the light source block 13 are lit.

  When the headlamp is in a high beam, the output voltage Vo of the DC-DC converter 11 is calculated from the input voltage Vin and the duty ratio M as shown in Equation 3, and is higher than the input voltage Vin (Vo). > Vin).

    Vo = Vin / (1-M) (Equation 3)

That is, during the high beam of the headlamp, the total value of the forward voltages of the ten LEDs 16 of the light source block 13 is higher than the DC voltage Vin of the in-vehicle battery 12.
Therefore, the DC-DC converter 11 generates an output voltage Vo obtained by boosting the DC voltage Vin of the in-vehicle battery 12 and supplies it to the light source block 13 (first light source block 14 and second light source block 15).

[Operation of Conventional LED Lighting Circuit 100]
As shown in FIG. 3, the conventional LED lighting circuit 100 is different from the LED lighting circuit 10 of the first embodiment in that the step-up / step-down chopper type DC-DC converter 11 is replaced with a step-up / step-down chopper type DC-DC converter 101. It is only a point that is.
The step-up / step-down chopper type DC-DC converter 101 includes a step-down chopper circuit 101a and a step-up chopper circuit 11b.
The step-down chopper circuit 101a differs from the configuration of the step-down chopper circuit 11a of the first embodiment only in that the switch S1 is omitted.

As shown in FIG. 2B, in the conventional LED lighting circuit 100, the operation of the headlamp during the low beam (Lo) is the same as that of the LED lighting circuit 10 of the first embodiment.
However, in the conventional LED lighting circuit 100, the control circuit 19 during the high beam (Hi) of the headlamp sets the control signal G3 to the low level to turn off the transistor Q3, and each control signal G1 that is a square wave. , G2 are simultaneously switched on and off, and the transistors Q1 and Q2 are synchronously switched to perform PWM control.
Then, the output voltage Vo of the DC-DC converter 11 is applied to the light source block 13 and the resistor R connected in series, and all the LEDs 16 of the light source block 13 are lit.

[Operations and effects of the first embodiment]
According to the first embodiment, the following actions and effects can be obtained.

[1] As shown in FIG. 2 (B), in the conventional LED lighting circuit 100, the step-down chopper circuit 101a and the step-up chopper circuit 11b are operated synchronously when the headlamp is in the high beam, as in the low beam.
Therefore, when the headlamp is in a high beam (step-up), not only the transistor Q2 and the diode D2 of the step-up chopper circuit 11b operate, but also the transistor Q1 and the diode D1 of the step-down chopper circuit 101a operate.

  As a result, in addition to the voltage conversion efficiency of the DC-DC converter 101 being reduced due to the power loss of the transistor Q1 and the diode D1 of the step-down chopper circuit 101a, in order to prevent a failure due to heat generation of the transistor Q1 and the diode D1, However, there is a problem that the cost increases.

  [2] As shown in FIG. 2A, in the LED lighting circuit 10 of the first embodiment, when the headlamp is in a high beam, the switch S1 is opened so that no current flows through the diode D1, and the transistor Q1 is turned on. By maintaining the state (all on state), the operation of the step-down chopper circuit 11a is stopped and only the step-up chopper circuit 11b is operated.

Therefore, in the first embodiment, it is possible to reduce the power loss of the transistor Q1 in the step-down chopper circuit 11a and make the power loss of the diode D1 zero when the headlamp is in a high beam (step-up). The voltage conversion efficiency of the DC-DC converter 11 can be improved.
In addition, since the heat generation of the transistor Q1 is reduced and the heat generation of the diode D1 becomes zero, the radiator (not shown) provided for preventing failure due to the heat generation of the transistor Q1 and the diode D1 can be reduced in size.
As a result, according to the LED lighting circuit 10 of the first embodiment, the cost can be reduced as compared with the conventional LED lighting circuit 100.

  [3] In the first embodiment, since the transistor Q1 is held in the on state when the headlamp is in a high beam (when boosting), the switching noise of the transistor Q1 becomes zero and the noise source is reduced. The configuration of the filters 17 and 18 can be simplified, and the cost can be reduced.

Second Embodiment
As shown in FIG. 4, the LED lighting circuit 30 of the second embodiment is different from the LED lighting circuit 10 of the first embodiment in that the step-up / step-down chopper type DC-DC converter 11 is a step-up / step-down chopper type DC-DC converter. It is only the point which is replaced by 31.

The step-up / step-down chopper type DC-DC converter 31 includes a step-down chopper circuit 31a and a step-up chopper circuit 11b.
The step-down chopper circuit 31a differs from the configuration of the step-down chopper circuit 11a of the first embodiment only in that the switch S2 is connected in parallel to the transistor Q1.
The switch S2 is a mechanical open / close switch, and the open / close is switched in conjunction with the switch S1.

[Operation of LED lighting circuit 30 of the second embodiment]
As shown in FIG. 5, since the switch S2 is opened during the low beam (Lo) of the headlamp, the LED lighting circuit 30 of the second embodiment has the same configuration as the LED lighting circuit 10 of the first embodiment. The operation of the LED lighting circuit 30 is the same as that of the LED lighting circuit 10.

At the time of high beam (Hi) of the headlamp, the switch S1 is opened and the switch S2 is closed, and the control circuit 19 sets the control signal G3 to the low level to turn off the transistor Q3 and each of the square waves. The control signals G1 and G2 are simultaneously switched on and off, and the transistors Q1 and Q2 are synchronously switched to perform PWM control.
Then, the output voltage Vo of the DC-DC converter 11 is applied to the light source block 13 and the resistor R connected in series, and all the LEDs 16 of the light source block 13 are lit.

[Operation and Effect of Second Embodiment]
According to the second embodiment, the following actions and effects can be obtained.

  [4] As shown in FIG. 5, in the LED lighting circuit 30 of the second embodiment, during the high beam of the headlamp, the switch S1 is opened so that no current flows through the diode D1, and the switch S2 is closed to turn on the transistor Q1. By short-circuiting between the source and the drain, the operation of the step-down chopper circuit 11a is stopped, and only the step-up chopper circuit 11b is operated at the time of high beam of the headlamp.

  Therefore, in the second embodiment, it is possible to reduce the power loss of the transistor Q1 in the step-down chopper circuit 11a to zero and the power loss of the diode D1 during the high beam of the headlamp (during boosting). The voltage conversion efficiency of the DC-DC converter 11 can be further improved as compared with the first embodiment.

  For example, when the power loss of the headlamp is 1.5 W, the power loss of the transistor Q1 is 1.5 W, the power loss of the diodes D1 and D2 is 0.8 W, and the power loss of the transistor Q2 is 2.1 W, respectively, as shown in FIG. The power loss of the DC-DC converter 101 of the LED lighting circuit 100 is 5.2 W, whereas the power loss of the DC-DC converter 31 of the LED lighting circuit 30 of the second embodiment is 2.9 W. The power loss of the second embodiment is 3/5 of the prior art.

In addition, in the second embodiment, since the heat generation of the transistor Q1 becomes zero and the heat generation of the diode D1 becomes zero, a radiator (not shown) provided to prevent failure due to heat generation of the transistor Q1 and the diode D1. Can be further reduced in size compared to the first embodiment.
However, in the second embodiment, there is a disadvantage that the entire apparatus is increased in size by the installation space of the switch S2, and the LED lighting circuit 30 is increased in cost by the component cost of the switch S2.

[5] In the second embodiment, the transistor Q1 is turned on and off according to the control signal G1 that is a square wave when the headlamp is in a high beam (at the time of boosting), but the switch S2 is closed to connect the source and drain of the transistor Q1. Since the transistor Q1 is short-circuited, the transistor Q1 is in the same state as being held in the on state, and no actual switching operation is performed.
Therefore, the switching noise of the transistor Q1 becomes zero and the number of noise generation sources is reduced. Therefore, the configuration of the noise filters 17 and 18 can be simplified, and the cost can be reduced.

[6] In the second embodiment, the transistor Q1 is turned on and off according to the control signal G1 that is a square wave when the headlamp is in a high beam (step-up).
However, during the high beam of the headlamp, the switch S2 is closed and the source and drain of the transistor Q1 are short-circuited.
Therefore, as shown in FIG. 6, the control circuit 19 may fix the control signal G1 to the low level and hold the transistor Q1 in the off state. In this case, the control circuit 19 is similar to [4] and [5]. The following effects can be obtained.

<Third Embodiment>
As shown in FIG. 7, the LED lighting circuit 40 of the third embodiment is different from the LED lighting circuit 10 of the first embodiment in that the step-up / step-down chopper type DC-DC converter 11 is a step-up / step-down chopper type DC-DC converter. Only 41 is replaced.

The step-up / step-down chopper type DC-DC converter 41 includes a step-down chopper circuit 101a and a step-up chopper circuit 41b.
The step-down chopper circuit 101a is the same as the step-down chopper circuit 101a of the conventional LED lighting circuit 100 shown in FIG.
The step-up chopper circuit 41b is different from the step-up chopper circuit 11b in the first embodiment only in that the switch S3 is connected in parallel to the diode D2.
The switch S3 is a mechanical open / close switch, and is opened and closed in conjunction with a low beam / high beam switch (not shown) operated by the driver.

[Operation of LED lighting circuit 40 of the third embodiment]
As shown in FIG. 8A, in the LED lighting circuit 40 of the third embodiment, the switch S3 is closed during the low beam (Lo) of the headlamp, and the control circuit 19 sets the control signal G3 to the high level. The transistor Q3 is turned on, the control signal G2 is fixed at a low level, the transistor Q2 is held in an off state, and only the transistor Q1 is switched by the control signal G1, which is a square wave, to perform PWM control.
Then, both ends of the second light source block 15 are short-circuited by the transistor Q3 in the ON state, and the output voltage Vo of the DC-DC converter 11 is applied to the first light source block 14 and the resistor R connected in series, and the first light source Only the LED 16 of the block 14 is lit.

  As shown in FIG. 8A, in the LED lighting circuit 40 of the third embodiment, the switch S3 is opened during the high beam (Hi) of the headlamp, so that the LED lighting circuit 40 of the third embodiment is conventional. The LED lighting circuit 100 has the same configuration, and the operation of the LED lighting circuit 40 is the same as that of the LED lighting circuit 100.

[Operation and Effect of Third Embodiment]
According to the third embodiment, the following actions and effects can be obtained.

[7] As shown in FIG. 8B, in the conventional LED lighting circuit 100, the step-down chopper circuit 101a and the step-up chopper circuit 11b are operated synchronously even when the headlamp is in the low beam, as in the case of the high beam.
Therefore, when the headlamp is in the low beam (step-down), not only the transistor Q1 and the diode D1 of the step-down chopper circuit 101a operate, but also the transistor Q2 and the diode D2 of the step-up chopper circuit 11b operate.

  As a result, in addition to the voltage conversion efficiency of the DC-DC converter 101 being reduced due to the power loss of the transistor Q2 and the diode D2 of the boost chopper circuit 11b, in order to prevent a failure due to heat generation of the transistor Q2 and the diode D2, However, there is a problem that the cost increases.

  [8] As shown in FIG. 8A, in the LED lighting circuit 40 of the third embodiment, when the headlamp is in a low beam, the switch S3 is closed to short-circuit between the anode and the cathode of the diode D2, and the diode D2 is connected. By stopping the current from flowing and keeping the transistor Q2 in the OFF state (all OFF state), the operation of the step-up chopper circuit 41b is stopped and only the step-down chopper circuit 101a is operated.

Therefore, in the third embodiment, it is possible to reduce the power loss of the transistor Q2 in the step-up chopper circuit 41b to zero and the power loss of the diode D2 at the time of low beam (during step-down) of the headlamp. The voltage conversion efficiency of the DC-DC converter 41 can be improved.
In addition, since the heat generation of the transistor Q2 becomes zero and the heat generation of the diode D2 becomes zero, a radiator (not shown) provided for preventing failure due to heat generation of the transistor Q2 and the diode D2 can be reduced in size.
As a result, according to the LED lighting circuit 40 of the third embodiment, the cost can be reduced as compared with the conventional LED lighting circuit 100.

  [9] In the third embodiment, since the transistor Q2 is held in the off state when the headlamp is in a low beam (step-down), the switching noise of the transistor Q2 becomes zero and the noise source is reduced. The configuration of the filters 17 and 18 can be simplified, and the cost can be reduced.

<Another embodiment>
The present invention is not limited to the above-described embodiments, and may be embodied as follows. Even in this case, operations and effects equivalent to or higher than those of the above-described embodiments can be obtained.

  [A] In the LED lighting circuit 10 of the first embodiment and the LED lighting circuit 30 of the second embodiment, the switch S1 may be connected to the cathode side of the diode D1.

  [B] In the LED lighting circuit 10 of the first embodiment and the LED lighting circuit 30 of the second embodiment, the switch S1 may be omitted, and even in that case, the power loss of the transistor Q1 during the high beam of the headlamp can be reduced. .

  [C] In the LED lighting circuit 40 of the third embodiment, the switch S3 may be omitted, and even in that case, the power loss of the transistor Q2 during the low beam of the headlamp can be reduced.

  [D] The NMOS transistors Q1 and Q2 may be replaced with PMOS transistors or other switching elements (bipolar transistors, static induction transistors, thyristors, etc.).

[E] The number of LEDs 16 constituting the first light source block 14 and the second light source block 15 is not limited to five, and may be an appropriate number.
Further, the number of LEDs 16 constituting the first light source block 14 may be different from the number of LEDs 16 constituting the second light source block 15.

  [F] The switches S1 to S3 may be embodied by semiconductor switches instead of mechanical open / close switches.

  [G] Although the first to third embodiments are LED lighting circuits used for automobile headlamps, they may be applied to an LED lighting circuit used when a tail lamp is also used as a brake lamp. The headlight high beam time corresponds to the brake lamp time, and the headlight low beam time corresponds to the taillight.

  [H] The present invention is not limited to an LED lighting circuit used in an automobile, but can be applied to any LED lighting circuit that controls to change brightness by changing the number of LEDs to be lit. Good.

  [I] The present invention is not limited to a step-up / step-down chopper type DC-DC converter as a DC power supply circuit that uses a plurality of LEDs connected in series as a load and supplies a DC voltage to each LED. If it is a DC converter, it can be applied to supplying a DC voltage to any load.

  [J] The embodiments described above may be implemented in combination as appropriate. In that case, the functions and effects of the combined embodiments can be combined or a synergistic effect can be obtained.

  The present invention is not limited to the description of each aspect and each embodiment. Various modifications are also included in the present invention as long as those skilled in the art can easily conceive without departing from the scope of the claims. The contents of papers, published patent gazettes, patent gazettes, etc. specified in this specification are incorporated by reference in their entirety.

DESCRIPTION OF SYMBOLS 10, 30, 40, 100 ... LED lighting circuit 11 ... Buck-boost chopper system DC-DC converter (DC power supply circuit)
11a, 31a, 101a ... step-down chopper type DC-DC converter (step-down chopper circuit)
11b, 41b ... step-up chopper type DC-DC converter (step-up chopper circuit)
12 ... In-vehicle battery (DC voltage source)
13 ... Light source block (load)
14 ... 1st light source block 15 ... 2nd light source block 16 ... LED
17, 18 ... Noise filter 19 ... Control circuit Vin ... DC voltage (input voltage)
Vo ... Output voltage R ... Resistance (load)
Q1 ... NMOS transistor (first switching element)
Q2 ... NMOS transistor (second switching element)
Q3 ... NMOS transistor (short circuit)
C1, C2 ... Capacitor D1 ... First diode D2 ... Second diode L ... Choke coil G1-G3 ... Control signal S1 ... Second switch S2 ... First switch S3 ... Third switch

Claims (8)

A step-down chopper type DC-DC converter that steps down the DC voltage of the DC voltage source and a step-up chopper type DC-DC converter that steps up the DC voltage are connected in series between the DC voltage source and the load, and the DC A DC power supply circuit including a step-up / step-down chopper type DC-DC converter that supplies an output voltage generated by stepping up or down a voltage to the load,
A DC power supply circuit that operates only the step-up chopper type DC-DC converter and stops the operation of the step-down chopper type DC-DC converter when the DC voltage is boosted. The step-down chopper type DC-DC converter is
A first switching element for switching a current supplied from the DC voltage source;
A choke coil that generates an induced current to prevent a change in current supplied from the DC voltage source;
A first diode that flows a reflux current through the choke coil when the switching element is off,
2. The DC power supply circuit according to claim 1, wherein during the step-up of the DC voltage, the operation of the step-down chopper type DC-DC converter is stopped by holding the first switching element in an ON state. The step-down chopper type DC-DC converter is
A first switching element for switching a current supplied from the DC voltage source;
A choke coil that generates an induced current to prevent a change in current supplied from the DC voltage source;
A first diode for flowing a return current through the choke coil when the switching element is off;
A first switch connected in parallel to the first switching element;
2. The DC power supply circuit according to claim 1, wherein when the DC voltage is stepped up, the operation of the step-down chopper type DC-DC converter is stopped by closing the first switch and short-circuiting the first switching element. A second switch connected in series to the first diode;
4. The DC power supply circuit according to claim 2, wherein when the DC voltage is boosted, the second switch is opened to prevent a current from flowing through the first diode. 5.   5. The DC power supply circuit according to claim 1, wherein when the DC voltage is stepped down, only the step-down chopper type DC-DC converter is operated and the operation of the step-up chopper type DC-DC converter is stopped. . The step-up chopper type DC-DC converter is
Sharing the choke coil of the step-down chopper type DC-DC converter,
A second switching element for switching a current supplied from the DC voltage source;
A second diode for preventing the induced current from flowing backward to the DC voltage source;
6. The DC power supply according to claim 1, wherein when the DC voltage is stepped down, the operation of the step-up chopper type DC-DC converter is stopped by holding the second switching element in an OFF state. circuit. A third switch connected in parallel to the second diode;
The DC power supply circuit according to claim 6, wherein when the DC voltage is stepped down, the third switch is closed and the second diode is short-circuited so that no current flows through the second diode. An LED lighting circuit comprising the DC power supply circuit according to any one of claims 1 to 7,
The load is a plurality of LEDs connected in series,
An LED lighting circuit comprising a short circuit that short-circuits a predetermined LED among the plurality of LEDs when the DC voltage is stepped down.

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