JP2011151325A - Led driving circuit, lamplight device, and signal lamplight device - Google Patents

Abstract

To realize an LED driving circuit that keeps an LED current constant and requires less power loss.
In an LED drive circuit, a predetermined set voltage Vs is applied to an LED array by a feedback control unit based on a voltage between both ends of a detection resistor connected in series to the LED array. As described above, the control voltage Vx to be applied to the LED array 100 is automatically controlled.
[Selection] Figure 1

Description

  The present invention relates to an LED drive circuit and the like.

In recent years, traffic signal lamps using LEDs (light emitting diodes) have rapidly become widespread. The rated current I ₀ (for example, 20 mA) is determined for the LED, and it is necessary to control the applied voltage so that the current flowing through the LED (LED current) does not exceed the rated current I _0. There is a characteristic that it fluctuates greatly by a slight fluctuation of the applied voltage. In addition, an overcurrent flows through the LED, such as the temperature rises due to heat generated by lighting, the forward voltage Vf decreases, the LED current increases, and the current further increases due to the temperature increase. There is a problem of thermal runaway. For this reason, when driving the LED, a limiting resistor for current suppression is connected in series to the LED (see, for example, Patent Document 1).

JP 2009-295571 A

  However, when a limiting resistor is connected to the LED, there is a problem of power loss at this limiting resistor. FIG. 8 is a diagram for explaining the power loss due to the limiting resistor. As shown in FIG. 8A, a limiting resistor R is connected in series to the LED, and a voltage V is applied. FIG. 8B is a graph showing the relationship between the applied voltage V and the LED current I in this circuit when the limiting resistor R is not connected (R = 0) and when the limiting resistor R1 is connected (R = R1). In the case where the limiting resistor R2 is connected (R = R2> R1).

As shown in FIG. 8B, the LED current I is directly proportional to the applied voltage V, and the slope thereof decreases as the limiting resistance R increases. Further, the applied voltage V necessary for setting the LED current I to the rated current I ₀ varies depending on the size of the limiting resistor R. Specifically, the applied voltage V required increases as the limiting resistor R increases.

The power consumption P in this circuit is P = V × I ₀ , and the power consumption P in the entire circuit increases as the applied voltage V increases. Further, the power consumption P of the entire circuit is LED and the power consumption _{P L} of the sum of the power _{P R} of the limiting resistor _{R (P = P L + P} R). When the forward voltage Vf of the LED to be constant, although the LED power _{P L} of a constant, the power consumption _{P R} of the limiting resistor R _{is, P R = (V-Vf} ) × I 0, next, the applied voltage V is power consumption P _R larger increases. That is, as the limiting resistor R is large, it is necessary to increase the applied voltage V, a result, increases the power consumption P _R or power loss in the limiting resistor R.

  Further, in the LED, the LED current increases as the temperature rises, but the LED current becomes more stable as the limiting resistance R connected in series with the LED is larger. FIG. 9 is a diagram illustrating an increase in LED current due to a temperature rise. As shown in FIG. 9A, a limiting resistor R is connected in series to the LED, and a constant voltage V is applied. FIG. 9B is a graph showing the relationship between the applied voltage V and the LED current I in this circuit. When the limiting resistor R is present (R = R1) and when the limiting resistor R is not present (R = 0).

As shown in FIG. 9 (b), by the presence or absence of the limiting resistor R, different applied voltage V required to the LED current I and the rated current I _0, specifically, more of there limiting resistor (V = V1), which is smaller than that without the limiting resistor (V = Vf <V1). When the LED forward voltage Vf decreases due to a temperature rise, the graph showing the relationship between the applied voltage V and the LED current I changes from a solid line to a broken line. Then, since the applied voltage V is constant, the LED current I increases, and the degree of the increase in the LED current I depends on the presence or absence of the limiting resistor R. Specifically, the greater the limiting resistance R, the smaller the degree of increase in the LED current I. That is, by increasing the limiting resistance R, an increase in the LED current I due to a temperature rise can be suppressed and stabilized.

  Thus, thermal runaway can be suppressed by increasing the limiting resistance R connected in series with the LED, but there is a problem that the power loss due to Joule heat generated by the limiting resistance R increases as the limiting resistance increases. The present invention has been made in view of the above circumstances, and an object of the present invention is to realize an LED drive circuit that keeps the LED current constant and that requires less power loss.

The first form for solving the above problem is
A power supply unit (for example, PWM control unit 30 in FIG. 1) that supplies driving power to the LED unit;
A resistance portion (for example, the detection resistor 10 in FIG. 1) connected in series to the LED portion;
A resistance voltage detection unit (for example, the resistance voltage detection unit 20 in FIG. 1) for detecting a voltage applied to the resistance unit;
The supply voltage of the power supply unit is variably controlled based on the detected voltage so that the current flowing through the LED unit obtained from the voltage detected by the resistance voltage detection unit satisfies the rated condition of the LED unit. A control unit (for example, the feedback control unit 50 in FIG. 1);
It is the LED drive circuit provided with.

  According to the first embodiment, the voltage applied to the resistance unit connected in series with the LED unit is detected, and the current flowing through the LED unit obtained from the detected voltage is detected so as to satisfy the rated condition of the LED unit. The driving voltage supplied to the LED unit is automatically controlled based on the determined voltage. Thereby, the thermal runaway of the LED portion does not occur. Moreover, since the resistance part connected in series with the LED part is provided in order to detect an electric current in the LED part, the resistance value may be small. For this reason, less power is consumed in the resistance section.

As a second form, in the LED drive circuit of the first form,
The power supply unit converts an externally supplied power source into a PWM method applied voltage corresponding to an instruction voltage input from the control unit, and outputs the converted voltage.
An LED drive circuit may be configured.

  According to the second embodiment, the supply voltage to the LED unit is generated by converting the power supplied from the outside into a PWM applied voltage corresponding to the instruction voltage. Since the supply voltage to the LED unit is variably controlled based on the voltage applied to the resistor unit, the command voltage varies, but by converting the external power supply to the PWM method applied voltage, The driving power to the LED unit can be supplied with high power efficiency according to the fluctuation.

As a third form, in the LED drive circuit of the first or second form,
When the LED unit is turned on, the control unit controls the supply voltage of the power supply unit so as to reach a predetermined lighting voltage over a predetermined settling time (for example, feedback in FIG. 1). Having a control unit 50),
An LED drive circuit may be configured.

  According to the third embodiment, when the LED unit is lit, the supply voltage is controlled so as to reach a predetermined lighting voltage over a predetermined settling time. Thereby, generation | occurrence | production of the high frequency component which arises when changing the applied voltage to a LED part rapidly can be suppressed.

As a fourth form,
An LED unit;
An LED drive circuit of any one of the first to third forms;
You may comprise the lamp provided with.

As a fifth form,
An LED unit comprising a lamp unit of a signal lamp;
An LED drive circuit of any one of the first to third forms;
A signal lamp with
The control unit controls power supply by the power supply unit according to whether the LED unit is turned on, turned off, or blinking.
You may comprise a signal lamp.

The block diagram of a LED drive circuit. The circuit block diagram of a lag reed filter. Operation | movement explanatory drawing of a level converter. The block diagram of a LED drive circuit. The block diagram of the LED drive circuit at the time of considering noise. An example of the initial response Vx (t) with respect to the reference voltage Vs. The relationship diagram of the lighting / extinction command with respect to LED array, and a control voltage. Explanatory drawing of the power loss by a limiting resistance. Explanatory drawing of the LED current increase by a temperature rise.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, below, although the case where this invention is applied to a traffic signal lamp is demonstrated, embodiment which can apply this invention is not limited to this.

[Constitution]
FIG. 1 is a configuration diagram of an LED drive circuit 1 in the present embodiment. The LED drive circuit 1 is a circuit that drives the LED array 100 of each color of the traffic signal lamp, and includes a detection resistor 10, a resistance voltage detection unit 20, a PWM control unit 30, and a feedback control unit 50. It is prepared for.

  The LED array 100 is formed by serially connecting M columns of N LEDs of a predetermined light color (red, blue, or yellow) connected in series, that is, a total of N × M LEDs are in a matrix. It is arranged in a shape. Then, the LED array 100 is supplied with the voltage Vx by the PWM control unit 30 so that each LED emits light and the corresponding signal lamp color is turned on.

  The detection resistor 10 is for detecting the total current I flowing through the LED array 100 (the sum of the currents flowing through the M series LED series lines), and is connected to the LED array 100 in series.

  The resistance voltage detection unit 20 includes, for example, a voltage sensor, and detects the voltage Vr across the detection resistor 10. When the resistance value of the detection resistor 10 is “R”, the detected resistance voltage Vr is Vr = I × R.

The PWM control unit 30 performs PWM control to convert the DC voltage E supplied from the DC power supply 40 into a pulse voltage having a duty ratio corresponding to the control voltage Vx calculated by the feedback control unit 50, and thereby a PWM control voltage. Vx is applied to the LED array 100 and the detection resistor 10. The generated pulse voltage is generated so that the pulse width Tw satisfies Vx = (Tw / T ₀ ) · E, where the period of the pulse voltage is “T ₀ ”.

The feedback control unit 50 uses the resistance voltage Vr detected by the resistance voltage detection unit 20 and the reference voltage Vs given from the instruction circuit 200 in order to keep the current flowing through each LED of the LED array 100 at the rated current I _0. Then, a control voltage Vx to be applied to the LED array 100 and the detection resistor 10 is calculated.

  The instruction circuit 200 is a circuit that controls the display of the traffic signal lamp, generates a lighting / lighting-off command that instructs lighting and extinguishing of each of the red, blue, and yellow lighting colors, and generates the lighting / lighting-off generated. A reference voltage Vs corresponding to the command is output to each LED drive circuit 1 corresponding to each lamp color. Specifically, Vs = Vs is output when the lighting command is issued, and Vs = 0 is output when the light extinction command is issued. That is, the reference voltage Vs is switched in two stages.

The reference voltage Vs is obtained from the reference current Is to be passed through the LED array 100 and the resistance value R of the detection resistor 10, and Vs = R × Is. The reference current Is is obtained from the rated current I _{0 of} each LED constituting the LED array 100 and the number M of LED series lines arranged in the LED array 100, and is Is = I ₀ × M. Incidentally, the rated current I ₀ is because it varies depending on the lighting color of the corresponding LED, the rated current I ₀ corresponding to the corresponding light color used.

  The feedback control unit 50 includes a comparator 51, a lag lead filter 52, and a level converter 53, and controls the control voltage Vx so that the resistance voltage Vr follows the reference voltage Vs.

  The comparator 51 calculates a difference voltage ΔV (= Vs−Vr) between the reference voltage Vs and the resistance voltage Vr.

  The lag lead filter 52 is a kind of LPF and is configured as shown in FIG. That is, the lag lead filter 52 converts the difference voltage ΔV calculated by the comparator 51 into a voltage Vdc obtained by attenuating a high frequency component and outputs the voltage.

  The level converter 53 converts the level of the voltage Vdc output from the lag lead filter 52. FIG. 3 is a diagram for explaining level conversion by the level converter 53. FIG. 3 shows the relationship between the input voltage Vdc and the control voltage Vx, with the horizontal axis representing the input voltage Vdc from the lag lead filter 52 and the vertical axis representing the control voltage Vx that is the output voltage of the level converter 53. As shown in the figure, the level converter 53 converts the input voltage Vdc from the lag lead filter 52 so as to be a positive control voltage Vx. That is, when the input voltage Vdc is in a predetermined voltage range “−Vdcm ≦ Vdc ≦ + Vdcm”, the input voltage Vdc is directly converted to the control voltage Vx so that the control voltage Vx is in the voltage range “0 ≦ Vx ≦ Vxm”. When the input voltage Vdc is in the voltage range “Vdc <−Vdcm”, the control voltage Vx is “Vx = 0”, and when the input voltage Vdc is the voltage “Vdc> + Vdcm”, the control voltage Vx is “ Vx = Vxm ”. Note that Vxm ≦ E.

  Thus, the LED drive circuit 1 is a feedback control system that feeds back the resistance voltage Vr corresponding to the LED current I and controls the control voltage Vx supplied to the LED array 100.

  FIG. 4 is a block diagram of the LED drive circuit 1 which is a feedback control system. As shown in FIG. 4, the LED drive circuit 1 can be regarded as a feedback control system that receives a reference voltage Vs (s) and controls the control voltage Vx (s).

但し、Ｔ１＝Ｒ１・Ｃ、Ｔ２＝Ｒ２・Ｃ、である。 The transfer function KF (s) of the lag reed filter 52 is given by the following equation (1).

However, T1 = R1 · C and T2 = R2 · C.

The transfer function Kv (s) of the level converter 53 is given by the following equation (2).

ここで、

とおくと、式（３）は、次式（６）となる。

The transfer function H (s) of the LED drive circuit 1 is given by the following equation (3).

here,

Then, equation (3) becomes the following equation (6).

The initial responses (transient responses) Vx (s) and Vx (t) of the transfer function H (s) are expressed by the following equations (7) and (8), respectively.

  Next, consider a case where noise is taken into consideration in the LED driving circuit 1. FIG. 5 is a block diagram of the LED drive circuit 1 when noise is taken into consideration. The noise considered here is (1) fluctuation dV of the forward voltage Vf applied to each LED in the LED array 100, and (2) PWM ripple (harmonic noise) rip in the PWM control unit 30.

  The transfer function H (s) of the LED drive circuit 1 when noise is taken into consideration is given by the above equation (3) as in the case where noise is not taken into consideration.

Then, the initial response Vo (s) of the transfer function H (s) when noise is taken into consideration is expressed by the following equation (9).

The steady deviation of the initial response Vo (s) is expressed by the following equation (10).

Accordingly, the initial response Vo (t) of the transfer function H (s) when noise is taken into consideration is expressed by the following equation (11).

  FIG. 6 is a diagram showing an initial response of the LED drive circuit 1. As shown in the figure, the control voltage Vx (t) changes following the reference voltage Vs (t). The ease of following at this time is determined by the damping factor ζ given by the equation (4), It depends on the lock time tn. Here, as specific set values, for example, damping factor ζ = 0.707, lock time (settling time) tn = 50 [ms], and ωn · tn = 4.5 [rad] are set.

  FIG. 7 is a diagram showing the relationship between the lighting / extinction command for the LED array 100 and the control voltage Vx (t). The reference voltage Vs is switched in two stages according to the lighting / extinction command of the lamp color corresponding to the LED array 100. As shown in the figure, at the time of switching the lighting / extinction command (that is, at the time of switching the reference voltage Vs), the control voltage Vx (t) takes a predetermined lock time (settling time) tn. It will be controlled to switch.

  By the way, if the control voltage Vx is changed simultaneously with the change of the reference voltage Vs at the time of switching the reference voltage Vs, the LED current I changes abruptly and high frequency noise is generated. However, as in this embodiment, when the reference voltage Vs is switched, the switching is performed slowly over a predetermined time tn, so that generation of high-frequency noise at the time of switching can be suppressed.

[Action / Effect]
Thus, in the LED drive circuit 1 of the present embodiment, a predetermined reference current Is is applied to the LED array 100 based on the voltage Vr across the detection resistors 10 connected in series to the LED array 100 by the feedback control system. The control voltage Vx to be applied to the LED array 100 is automatically controlled to flow. Thereby, for example, even if the LED current increases due to the heat generation of the LED, the control voltage Vx is controlled so as to suppress this increase. Therefore, a constant reference current Is always flows in the LED array 100, and the LED There is no such thing as thermal runaway.

  Further, since the voltage applied to the LED array 100 is a PWM wave generated by PWM control of the DC power supply E, power efficiency is good. In addition, since an arbitrary reference current Is can be set, dimming control of the signal lamp is easy. Further, even if the lamp color of the LED array to be driven is changed, it is only necessary to set the reference current Is according to the changed lamp color. In addition, since a time delay element is included in the response characteristic of the control voltage Vx with respect to the fluctuation of the reference voltage Vs, high-frequency noise generated in the LED current at the time of switching between lighting / extinction is suppressed.

[Modification]
It should be noted that embodiments to which the present invention can be applied are not limited to the above-described embodiments, and can be appropriately changed without departing from the spirit of the present invention.

  For example, in the above-described embodiment, the case where the present invention is applied to an LED traffic signal lamp has been described. However, the present invention can be applied to any LED lamp such as a railway signal lamp.

  In this embodiment, the PWM control unit 30 generates and applies the control voltage Vx from the power supply voltage E by PWM control. This is inferior in terms of power efficiency and response speed. The control voltage Vx may be generated from the power supply voltage E by a control method other than the above.

DESCRIPTION OF SYMBOLS 1 LED drive circuit 10 Resistance for detection, 20 Resistance voltage detection part 30 PWM control part, 40 DC power supply 50 Feedback control part 51 Comparator, 52 Lag lead filter, 53 Level converter 100 LED array

Claims (5)

A power supply unit for supplying driving power to the LED unit;
A resistance portion connected in series to the LED portion;
A resistance voltage detection unit for detecting a voltage applied to the resistance unit;
The supply voltage of the power supply unit is variably controlled based on the detected voltage so that the current flowing through the LED unit obtained from the voltage detected by the resistance voltage detection unit satisfies the rated condition of the LED unit. A control unit;
LED drive circuit with The power supply unit converts an externally supplied power source into a PWM method applied voltage corresponding to an instruction voltage input from the control unit, and outputs the converted voltage.
The LED drive circuit according to claim 1. The control unit includes a lighting-time control unit that controls a supply voltage of the power supply unit so as to reach a predetermined lighting voltage over a predetermined settling time when the LED unit is lit.
The LED drive circuit according to claim 1 or 2. An LED unit;
The LED drive circuit according to any one of claims 1 to 3,
With a lamp. An LED unit comprising a lamp unit of a signal lamp;
The LED drive circuit according to any one of claims 1 to 3,
A signal lamp with
The control unit controls power supply by the power supply unit according to whether the LED unit is turned on, turned off, or blinking.
Signal lamp.

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