JP2004328949A - Switching constant current power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching constant current power supply device in which load current can be stabilized even under the condition that the current flowing to a load (e.g., a display unit, etc. including an LED) repeats an intermittence. <P>SOLUTION: A feedback circuit 7 having a fixed signal generator circuit 8 for generating a second feedback signal having a constant signal level is installed between a detector circuit 5 for generating a first feedback signal in response to load current and a control circuit 4 for driving a power converter 3 for supplying current to an intermittent load 6. A first feedback signal generated in the detector circuit 5 is selectively supplied from the feedback circuit 7 to the control circuit 4 during a current flowing period that current flows to the load 6, and a second feedback signal generated in the fixed signal generator circuit 8 is selectively supplied from the feedback circuit 7 to the control circuit 4 during a current cut-off period that the current does not flow to the load 6. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a switching type constant current power supply device for supplying a stable current to a load that is repeatedly turned on and off.
[0002]
[Prior art]
Generally, a switching type power supply device is often used as a voltage source for supplying a stable voltage to a load. However, it is also possible to use as a current source that supplies a substantially constant current to a load by connecting and configuring the feedback signal according to the output current as shown in FIG.
6, reference numeral 1 denotes an input terminal for receiving power supply from an external battery or the like, and reference numerals 2a and 2b denote output terminals for stably supplying a predetermined current to a load 6 connected therebetween. A power conversion circuit 3 is connected between the input terminal 1 and one output terminal 2a. The power conversion circuit 3 is configured such that the choke coil L1, the switching transistor Q1, the rectifier diode D1, and the smoothing capacitor C1 are connected to form a boost chopper converter. I have.
[0003]
Between the other output terminal 2b and the ground as the reference potential point of the circuit, a current flowing through the load 6 (hereinafter, referred to as a load current) is detected, and a detection for generating a feedback signal corresponding to the load current is performed. Circuit 5 is connected. Then, a control circuit 4 for receiving the feedback signal from the detection circuit 5 and driving the power conversion circuit 3 according to the level of the feedback signal is connected between the power conversion circuit 3 and the detection circuit 5. . (Here, the control circuit 4 is assumed to be a very general PWM control type control IC.)
The power conversion circuit 3, the control circuit 4, and the detection circuit 5 form a switching constant current power supply. The element C0 connected between the input terminal 1 and the ground is an input filter capacitor.
[0004]
The operation of the switching constant-current power supply device of FIG. 6 will be briefly described. The switching transistor Q1 in the power conversion circuit 3 turns on and off according to a signal supplied from the terminal pin OUT of the control circuit 4. As the switching transistor Q1 turns on and off, current flows from the choke coil L1 to the smoothing capacitor C1 via the rectifier diode D1. As a result, the smoothing capacitor C1 is charged to a voltage higher than the input voltage supplied to the input terminal 1, and a current corresponding to the voltage between the terminals of the capacitor C1 flows through the load 6 and the detection circuit 5. Then, a feedback signal corresponding to the load current is generated in the detection circuit 5 and supplied to the feedback signal receiving terminal pin FB of the control circuit 4.
[0005]
The feedback signal provided from the detection circuit 5 to the control circuit 4 is not at a level according to the output voltage as in a normal switching power supply, but at a level according to the output current (= load current). Therefore, the control circuit 4 generates a high-frequency (several hundred kHz) pulse-like signal with an on-duty corresponding to the feedback signal (= load current) in the control logic including the error amplifier EA1, the reference voltage source VR, and the drive circuit DR. And supplies it to the switching transistor Q1 connected to the terminal pin OUT. Then, the switching transistor Q1 performs an on / off operation with an on-duty corresponding to the magnitude of the load current by the pulse signal. For example, when the load current is lower than the stabilization target value, the terminal of the smoothing capacitor C1. The voltage is increased to induce the load current to increase. As a result of such an operation, the load current is stabilized in the device of FIG.
[0006]
By the way, display devices and lighting devices of various sizes are mounted on electronic devices in recent years, and light-emitting diodes (hereinafter, referred to as LEDs) are increasingly used as light sources of the display devices and lighting devices. When an LED is used as a light source, it is required to stabilize the current supplied to the LED in order to make the amount of light emitted, the luminance, and the like constant. Therefore, in recent electronic devices, a switching constant-current power supply device as shown in FIG. 6 is provided along with a display device or a lighting device, and a stable current is supplied from the power supply device to the LED. There were things. (See Patent Documents 1 to 3)
[0007]
[Patent Document 1]
JP-A-11-068161 [Patent Document 2]
JP 2001-215913 A [Patent Document 3]
JP-A-2002-203988
[Problems to be solved by the invention]
In recent years, some display devices and lighting devices using LEDs as a light source perform power saving and dimming by repeatedly turning on and off LEDs at a speed (several hundred Hz or more) that cannot be recognized by human eyes. There is something like that. In such a display device or a lighting device, a period during which a current flows through the LED (hereinafter, referred to as a current flowing period) and a period during which no current flows (hereinafter, referred to as a current blocking period) naturally occur. Then, in the switching constant current power supply device in which the power supply for supplying the current to the LED is configured as shown in FIG. 6, during the current interruption period caused by the load interruption, the detection circuit 5 supplies the power to the control circuit 4. The returned feedback signal is almost at zero level.
[0009]
In response to such a feedback signal, the control circuit 4 attempts to set the on-duty of the on / off operation of the switching transistor Q1 to the maximum during the current cutoff period, and to set the on-duty according to the feedback signal during the next current flowing period. Try to set to. Here, when the on-duty becomes maximum during the current cutoff period, the voltage between the terminals of the smoothing capacitor C1 increases rapidly and more than necessary, and during the next load current flow period, the voltage exceeds the stabilization target value for a relatively long time. This causes an undesired instability of the load current that the load current flows.
[0010]
As one of measures against such load current instability, for example, it is conceivable that the feedback signal is smoothed by a capacitor having a relatively large capacity and then supplied to the control circuit 4. However, if a large-capacity capacitor capable of maintaining the feedback signal at a significant level during the current interruption period is provided, the feedback signal processed by the control circuit 4 has an average value over a relatively long period. For this reason, when the load current fluctuates due to aperiodic load interruption or a cause other than the interruption, the load current that deviates from the stabilization target value cannot be quickly returned to the target value. As a result, the load current becomes unstable due to a cause different from the current interruption period.
[0011]
As described above, under the condition that the load is interrupted, the response speed of the control operation of the feedback loop that returns from the control circuit 4 to the control circuit 4 via the switching transistor Q1, the smoothing capacitor C1, the load 6, and the detection circuit 5 and returns to the control circuit 4 is changed. There was a possibility that the load current could not be stabilized because the change could not be followed.
Therefore, an object of the present invention is to provide a switching constant current power supply device capable of stabilizing a load current even under a condition where a load repeats intermittently.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a switching type power conversion circuit for supplying a predetermined current to a load, current detection means for generating a first feedback signal according to the load current, and a reference voltage for obtaining a reference voltage. A switching constant current power supply device comprising a reference voltage source and a control circuit for driving a power conversion circuit so as to stabilize a load current according to the reference voltage and a feedback signal; And a fixed signal generation circuit for generating a second feedback signal having a constant signal level therein, wherein one of the first feedback signal and the second feedback signal is generated in accordance with a load state. A feedback circuit for supplying the control circuit is provided.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
A second feedback signal having a constant signal level is generated between a detection circuit that generates a feedback signal according to the load current and a control circuit that drives a power conversion circuit for supplying a current to the load. The first feedback signal output from the current detection means is supplied to the control circuit when a load current is flowing, and the fixed signal generation circuit outputs when the load current is not flowing. A feedback circuit that supplies a second feedback signal to the control circuit is provided. It is assumed that the signal level (voltage) of the second feedback signal generated in the fixed signal generation circuit is substantially equal to or higher than the reference voltage output from the reference voltage source inside the control circuit.
[0014]
In a switching constant current power supply device incorporating such a feedback circuit, when a load current is flowing, the feedback circuit supplies the first feedback signal to the control circuit, thereby stabilizing the load current as in the conventional circuit. To work.
On the other hand, when the load current is not flowing, the switching constant current power supply device incorporating such a feedback circuit provides the second feedback signal to the control circuit so that the on-off operation of the switching transistor can be performed. Is fixed to a predetermined size. As a result, even if the load is interrupted, the voltage between the terminals of the smoothing capacitor rises more than necessary, or the current control operation of the switching constant current power supply cannot follow the change in load due to the response speed of the feedback loop. This prevents the phenomenon that the current cannot be stabilized.
[0015]
【Example】
FIG. 1 shows an embodiment of a switching constant current power supply according to the present invention.
The switching constant current power supply device shown in the block diagram in FIG. 1 is the same as the conventional circuit shown in FIG. 6 except that a feedback circuit 7 is provided between the control circuit 4 and the detection circuit 5. The feedback circuit 7 shown in FIG. 1 is roughly divided into a fixed signal generation circuit 8 for generating a second feedback signal, and any one of the first feedback signal output from the detection circuit 5 and the second feedback signal. Selection means 9 for supplying the control signal to the control circuit 4. Here, the selection unit 9 performs a switching operation in conjunction with the operation of the load 6 (in other words, the operation of turning on and off the LED).
[0016]
The feedback circuit 7 is configured as shown in the block diagram of FIG. 2 as an example.
In FIG. 2, the fixed signal generating circuit 8 is composed of a voltage source VC whose output voltage value is almost constant. On the other hand, the selection means 9 includes a first buffer BU1 provided between the detection circuit 5 and the control circuit 4, and a second buffer BU2 provided in series between the fixed signal generation circuit 8 and the control circuit 4. And a switch SW.
[0017]
In the feedback circuit 7 having such a configuration, for example, when a load current flows through the load 6 and the level of the first feedback signal output from the detection circuit 5 is high, the switch SW of the selection unit 9 is turned off. , A first feedback signal is supplied from the buffer BU1 to the control circuit 4. On the other hand, when the load current does not flow through the load 6 and the level of the first feedback signal from the detection circuit 5 is almost zero, the switch SW is turned on and the fixed signal generation circuit 8 generates the signal from the buffer BU2. An operation of supplying the second feedback signal to the control circuit 4 is performed.
[0018]
In the switching constant current power supply device of FIG. 1 provided with such a feedback circuit 7, when a load current is flowing, the first feedback signal from the detection circuit 5 is selectively supplied to the control circuit 4. In this state, the device having the configuration shown in FIG. 1 operates exactly the same as the conventional circuit to stabilize the load current.
On the other hand, when no load current is flowing, the control circuit 4 is selectively supplied with the second feedback signal from the fixed signal generation circuit 8. The control circuit 4 receiving the supply of the second feedback signal fixes the on-duty of the on / off operation of the switching transistor Q1 to a predetermined value.
[0019]
When the on-duty of the switching transistor Q1 is fixed, an increase in the voltage across the terminals of the smoothing capacitor C1 during the current cutoff period is suppressed. For this reason, an undesired phenomenon that a load current that is equal to or more than the stabilization target value flows during the next current distribution period is less likely to occur. Further, when the on-duty of the switching transistor Q1 during the current cutoff period is made substantially the same as when the current of the stabilization target value is actually supplied to the load 6, the state where the load current flows does not flow. At this time, the control operation of the feedback loop works quickly according to the first feedback signal.
As a result of such an operation, the load current can be stabilized even when the load repeats intermittently.
[0020]
FIG. 3 shows a specific circuit example of the switching constant current power supply device according to the present invention. In the circuit of FIG. 3, the feedback circuit 7, which is a main part of the present invention, is configured as follows.
The non-inverting input terminal (+) of the error amplifier EA2 is connected to the detection circuit 5, and the inverting input terminal (-) is connected to the ground via the resistor R1. The resistor R2 is connected between the output terminal of the error amplifier EA2 and the inverting input terminal (-), and the output terminal of the error amplifier EA2 is further connected to the non-inverting input terminal (+) of the error amplifier EA3.
An output terminal of the error amplifier EA3 is connected to a terminal pin FB for inputting a feedback signal of the control circuit 4 via a diode D2 for preventing backflow. An inverting input terminal (-) of the error amplifier EA3 is connected to the diode D2 and the terminal pin FB. To the connection point.
[0021]
A series circuit of the resistors R3 and R4 is connected between the output terminal of the error amplifier EA3 and the ground, and the connection point between the resistors R3 and R4 is connected to the base of the transistor Q2. The emitter of the transistor Q2 is connected to the ground, and the resistor R5 and the voltage source VC are connected between the collector of the transistor Q2 and the ground.
The collector of the transistor Q2 is further connected to the non-inverting input terminal (+) of the error amplifier EA4. The output terminal of the error amplifier EA4 is connected to the terminal pin FB of the control circuit 4 via the diode D3. -) Is connected to the connection point between the diode D3 and the terminal pin FB.
[0022]
In the switching constant current power supply device of FIG. 3, a circuit portion comprising the error amplifier EA3 and the diode D2 constituting the feedback circuit 7 substantially corresponds to the buffer BU1 of FIG. 2, and a circuit portion comprising the error amplifier EA4 and the diode D3. Substantially corresponds to the buffer BU2 in FIG. Further, a circuit portion including the resistors R3, R4, R5 and the transistor Q2 functionally corresponds to the switch SW in FIG.
The circuit portion including the error amplifier EA2, the resistor R1, and the resistor R2 amplifies and adjusts the first feedback signal provided from the detection circuit 5 to a level that can be processed by the control logic inside the control circuit 4. 3 is provided in the feedback circuit 7 in the circuit of FIG. 3, but may be provided in the detection circuit 5 in some cases.
[0023]
In the circuit of FIG. 3 having such a configuration, first, when the load current is flowing, the first feedback signal output from the detection circuit 5 has a level corresponding to the load current. This first feedback signal is amplified by the error amplifier EA2 and supplied to the error amplifier EA3 and the transistor Q2.
At this time, the transistor Q2 is turned on, and the non-inverting input terminal (+) of the error amplifier EA4 is dropped to the ground. As a result, the output of the error amplifier EA4 becomes zero level, and the first feedback signal output from the error amplifier EA3 is selectively supplied to the terminal pin FB of the control circuit 4. As a result, the control circuit 4 drives the switching transistor Q1 with an on-duty corresponding to the load current in order to set the load current to the stabilization target value.
[0024]
Next, when the load current does not flow, the first feedback signal output from the detection circuit 5 becomes almost zero level, and the signal level supplied to the error amplifier EA3 and the transistor Q2 via the error amplifier EA2. Becomes almost zero level.
At this time, the transistor Q2 is turned off, and the second feedback signal from the voltage source VC is supplied to the non-inverting input terminal (+) of the error amplifier EA4. Then, since the output of the error amplifier EA3 is at the zero level, the second feedback signal output from the error amplifier EA4 is selectively supplied to the terminal pin FB of the control circuit 4. As a result, the control circuit 4 drives the switching transistor Q1 with an on-duty fixed to a predetermined size.
[0025]
In the circuits of FIGS. 2 and 3 described above, for example, the output voltage of the voltage source VC (that is, the signal level of the second feedback signal) is substantially equal to the reference voltage output by the reference voltage source VR in the control circuit 4. With the same size, the control circuit 4 drives the switching transistor Q1 with a predetermined on-duty during the current cutoff period. At this time, the on-duty of the switching transistor Q1 is fixed to be substantially the same as the on-duty when the current of the stabilization target value flows through the load 6, and the increase in the voltage between the terminals of the smoothing capacitor C1 during the current cutoff period is suppressed. You.
[0026]
On the other hand, when the output voltage of the voltage source VC is set to a value higher than the reference voltage, for example, about twice the reference voltage, the control circuit 4 turns the switching transistor Q1 into the full off state during the current cutoff period. That is, the on-duty of the on / off operation of the switching transistor Q1 is set to zero. If the switching transistor Q1 maintains the off state, the power conversion circuit 3 substantially stops operating, and the rise of the voltage between the terminals of the smoothing capacitor C1 is completely prevented.
[0027]
For example, when the number of LEDs of the load 6 is large and the current cutoff period is short, the output voltage of the voltage source VC is set to be substantially equal to the reference voltage. When the current cutoff period is short as described above, the amount of increase in the voltage between the terminals of the smoothing capacitor C1 may be small. In addition, when the state is switched so that the load current flows through the load 6, the charge of the LED light emitting layer that has disappeared during the current cutoff period is reduced by the charge accumulated in the smoothing capacitor C1 during the current cutoff period. You can recover quickly. As a result, when the load current changes from a non-flowing state to a flowing state, a large load current does not easily flow, and at the same time, the control operation of the feedback loop can be quickly performed according to the first feedback signal.
[0028]
Conversely, when the number of LEDs is small and the current cutoff period is long, the output voltage of the voltage source VC is set to a value larger than the reference voltage. That is, when the current cutoff period is long, the possibility that the voltage between the terminals of the smoothing capacitor C1 becomes excessive during that period increases. However, if the switching transistor Q1 is maintained in the off state, the voltage across the terminals of the smoothing capacitor C1 does not increase at all during the current cutoff period, so that when the load current changes from a state where no load current flows, a large load current may flow. There is no.
As described above, the output voltage value of the voltage source VC may be selected and set according to conditions such as the number of LEDs of the load 6 and the length of the current interruption period.
[0029]
Some DC-DC converter control ICs on the market have a structure in which a stable voltage generated inside the IC can be led out of the IC. Figure 4 shows the internal structure of the outline of the IC of the exemplary, the regulated voltage _{V 1} of the reference voltage _{V ref} and 2.2V in 1.0V the reference voltage source VR is provided inside the generated . Among these, the reference voltage V _ref is configured to be supplied to the other input terminal of the error amplifier EA1 in which the first or second feedback signal is input to one input terminal, and at the same time, from the REF terminal to the outside of the IC. It is configured so that it can be derived. It is arranged as can be derived from the REG pin outside the IC for stabilized voltage V _1.
[0030]
When a control IC as shown in FIG. 4 is used as the control circuit 4, a voltage source VC can be obtained as shown in FIG. For example, when the signal level of the second feedback signal (= the output voltage of the voltage source VC) is set to a value larger than the reference voltage, the configuration shown in FIG. When the signal level of the second feedback signal is set to be substantially the same as the reference voltage, a configuration shown in FIG. Of course, after configuring the circuit as shown in FIG. 5A, the signal level of the second feedback signal is set to be substantially the same as the reference voltage by adjusting the resistance values of the resistors R6 and R7. It does not matter.
Note that FA7703 (manufactured by Fuji Electric Co., Ltd.) or the like is used as a control IC having a structure capable of leading a stable voltage generated inside the IC to the outside of the IC.
[0031]
In the above description of the embodiments, the power conversion circuit 3 is assumed to be a boost chopper type circuit, and the control circuit 4 is assumed to be a separately-excited PWM type control IC. Is not limited to this. The detection circuit 5 may also use a detection method other than the resistance detection, and it goes without saying that a specific circuit configuration can be modified as long as the gist of the present invention is not changed.
Of course, the load 6 need not be a display device or an illumination device including an LED as long as the load 6 is intermittent during use.
[0032]
【The invention's effect】
As described above, the switching constant current power supply according to the present invention drives the detection circuit that generates the first feedback signal according to the load current and the power conversion circuit that supplies the current to the load that operates intermittently. Circuit provided with a fixed signal generating circuit for generating a second feedback signal having a constant signal level. Then, a first feedback signal is supplied from the feedback circuit to the control circuit during the current flowing period, and a second feedback signal is supplied during the current interruption period.
According to the present invention, it is possible to suppress an increase in the voltage between the terminals of the smoothing capacitor and prevent a situation in which the response speed of the current control operation of the feedback loop cannot follow the load fluctuation, and as a result, the load is interrupted. In such a case, a switching constant current power supply device capable of stabilizing the load current can be provided.
[Brief description of the drawings]
FIG. 1 is a block diagram of a switching constant current power supply device according to the present invention.
FIG. 2 is a block diagram of a feedback circuit forming a main part of the present invention.
FIG. 3 is a specific circuit diagram of a switching constant current power supply device according to the present invention.
FIG. 4 is a block diagram of an internal structure of an example of a DC-DC converter control IC.
FIG. 5 is a configuration example of a voltage source VC for generating a second feedback signal.
FIG. 6 is a block diagram of an example of a conventional switching constant current power supply device.
[Explanation of symbols]
1: input terminal 2a, 2b: output terminal 3: power conversion circuit 4: control circuit 5: detection circuit 6: load (load that repeats intermittent) 7: feedback circuit 8: fixed signal generation circuit 9: selection circuit VR: reference voltage Source VC: voltage source

Claims (5)

A switching type power conversion circuit for supplying a predetermined current to a load, current detection means for generating a first feedback signal corresponding to the load current, a reference voltage source for obtaining a reference voltage, and A switching constant current power supply device including a control circuit that drives the power conversion circuit so as to stabilize the load current in response to the load current.
A fixed signal generating circuit is provided between the current detecting means and the control circuit and generates a second feedback signal having a constant signal level therein. A switching constant current power supply device comprising: a feedback circuit that supplies one of a signal and the second feedback signal to the control circuit. The feedback circuit supplies the first feedback signal to the control circuit when a load current is flowing, and supplies the second feedback signal to the control circuit when a load current is not flowing. The switching constant current power supply device according to claim 1. 3. The switching constant current power supply according to claim 1, wherein a signal level of the second feedback signal generated by the fixed signal generation circuit is substantially equal to the reference voltage. . The signal level of the second feedback signal generated by the fixed signal generation circuit is higher than the reference voltage, and the control circuit receiving the supply of the second feedback signal stops the operation of the power conversion circuit. The switching constant current power supply device according to claim 1 or 2, wherein: The switching constant current power supply device according to any one of claims 1 to 4, wherein the load includes a light emitting diode element that repeats blinking at a high speed.

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