JP2008295167A - Constant current switching power supply device and light source device - Google Patents

Abstract

Even when a constant current drive is performed in a pulsed manner by a switching power supply, a large current does not flow during the first period when the current starts to flow, and light emission does not become unstable.
An LD switching element connected in series to the LD4, a current detection element 6 for detecting a load current value flowing through the LD4, a target current value flowing through the LD4, and turning on and off the LD switching element5. A control circuit 8 that outputs an on / off control signal, and a switching power source that drives the LD 4 by pulse constant current using a PWM signal created based on the difference between the target current value and the load current value. A predetermined first set value is output as the target current value during the ON period, and for example, zero is output as the target current value during the ON / OFF control signal OFF period.
[Selection] Figure 1

Description

  The present invention relates to a constant current switching power supply device capable of stably supplying a pulse current when a pulsed current is passed through a load by a switching power supply, and a light source device that emits light by the pulse current. .

  Switching power supplies are known as compact, lightweight and highly efficient power supplies, and are used as power supplies for electronic devices in various devices. Generally, it is often used as a constant voltage source for supplying a constant voltage to the load, but of course, it can also be used as a constant current source for supplying a constant current to the load. Recently, there has also been an application in which such a switching power supply is used as a constant current source for supplying a constant current in a pulsed manner. The typical uses will be described below.

  Conventionally, in PTV (projection television), various lamps (white lamps such as discharge xenon lamps, metal halide lamps, halogen lamps, etc.) are used as light sources, and liquid crystal, DMD (digital micromirror device) etc. are used as light valves. A device using a modulation device is commercially available. In recent years, PTV using a light emitting diode (LED) or a semiconductor laser (LD) as a light source is being realized with the aim of extending the life of the light source and expanding the color reproduction range of an image. In such a PTV, when driving each light source, the lamp, LED, and laser are basically driven at a constant current, although there are slight differences in the drive waveforms. By driving at a constant current, the light emission intensity can be made constant with respect to the elapsed time from the start of light emission. The brightness of each pixel is expressed by changing the length of the light emission time.

  In particular, a PTV that uses LEDs and LDs as its light source uses three elements that emit light in single colors of red, green, and blue, respectively, and they are fast enough to be invisible to the human eye (several hundred Hz to several kHz). It is driven so as to emit light separately (see, for example, Patent Document 1). In order to ensure that each color light emitting element emits light stably in a time-sharing manner, a pulse constant is set such that a stable constant current flows through each light emitting element during the light emitting period and no current flows during the non-light emitting period. It is necessary to drive the current at high speed.

  However, when driving at a constant pulse current with a conventional switching power supply at high speed, the output voltage of the power supply rises during a period when no current flows, and a large current flows through the light emitting elements (LED, LD) during the first period when the current begins to flow. There has been a problem that light emission becomes unstable due to deterioration or destruction of the light emitting element.

  As an improvement measure, there is one in which switching of the switching power supply is forcibly turned off during the period in which no current flows, and pulsed constant current driving is performed so as not to increase the output voltage of the power supply (for example, Patent Document 2). reference).

Japanese Patent Laid-Open No. 10-32080 (page 11, FIG. 16) Japanese Patent Laying-Open No. 2005-51883 (page 3, FIG. 1)

  According to the method described in Patent Document 2, the voltage rise during a period in which no current flows can be suppressed by forcibly turning off the switching in a period in which no current flows. However, in Patent Document 2, even in such a state, the drive signal output from the control circuit to the power conversion circuit has an instruction value that causes the maximum current to flow because no current flows through the load. Yes. When switching to a period in which current flows in such a state, there is a problem in that a large current flows in the first period in which current starts to flow, and light emission becomes unstable.

  The present invention has been made in view of the above, and even when a constant current drive is performed in a pulsed manner by a switching power supply, a large current does not flow during the first period when the current starts to flow, and light emission does not become unstable. An object is to obtain a constant current switching power supply device and a light source device.

  In order to solve the above-described problems and achieve the object, the present invention is a constant current switching power supply device that controls a current flowing through a load, the switching element being connected in series to the load, and the current flowing through the load. A current detection element that detects a load current value; a control circuit that outputs a target current value that flows to the load; and that outputs an on / off control signal that turns on and off the switching element; and the target current value and the load current value A switching power supply that drives the load by pulse constant current using a PWM signal created based on the difference, and the control circuit outputs a predetermined first set value as a target current value during a period when the on / off control signal is turned on And a second target current value equal to or lower than the load current value during the OFF period of the switching element is set as a target current value during the OFF period of the ON / OFF control signal. And outputs the value.

  According to the present invention, the first set value is set as the target current value during the ON period of the switching element connected in series with the load, and the load current value during the OFF period of the switching element is set as the target current value during the OFF period of the switching element. Since the following second set value is set, even if the load is driven at a constant current in a pulse shape, a large current does not flow during the first period when the current starts to flow, and stable pulse constant current drive Can do. As a result, there is an effect that light emission from the light emitting element can be stabilized.

  DESCRIPTION OF EMBODIMENTS Embodiments of a constant current switching power supply light source device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment 1 FIG.
1 is a block diagram showing a constant current switching power supply apparatus 100 according to Embodiment 1 of the present invention apparatus. A direct current (DC) voltage is input to the switching circuit 1, and an alternating current (AC) voltage is output from the switching circuit 1. The AC voltage output from the switching circuit 1 is input to the rectifier circuit 2 and rectified. The output from the rectifier circuit 2 is input to the smoothing circuit 3 and smoothed. The output of the smoothing circuit 3 is connected to a load. As the load, an LED or an LD is connected. In the present embodiment, the LD 4 is used. The LD switching element 5 is connected in series with the LD 4 as a load, and the drive current of the LD 4 flows through the LD 4, the LD switching element 5, and the current detection element 6. As the LD switching element 5, a semiconductor switch such as a MOSFET or a bipolar transistor is used. The current detection element 6 converts a current value into a voltage value. For example, a shunt resistor, a Hall element, or the like is used. A video synchronization signal is input to the synchronization signal processing circuit 7. A strobe signal is input from the synchronization signal processing circuit 7 to the control circuit 8. The control circuit 8 outputs an ON / OFF control signal for the LD switching element 5 and a current command value to the feedback circuit 9. The feedback circuit 9 outputs a switching signal (PWM signal) that is pulse-width modulated based on the current command value from the control circuit 8 and the current detection value from the current detection element 6 to the switching circuit 1.

  Next, the operation of the first embodiment will be described in detail. In FIG. 1, a DC voltage is input to the switching circuit 1. An example of the switching circuit 1 is shown in FIG. FIG. 2 shows an insulating forward converter switching circuit using a transformer 1a. The switching element 1b is connected to the primary winding, and the primary winding is ON / OFF controlled. By turning ON / OFF the primary side, an alternating current (AC) voltage is induced on the secondary side. The AC voltage induced on the secondary side of the transformer 1a is converted again into a DC voltage by the rectifier circuit 2. An example of the rectifier circuit 2 is shown in FIG. A current flows through the diode 2a when the switching element 1b of the switching circuit 1 is ON, and a current flows through the diode 2b when the switching element 1b is OFF. The output voltage from the rectifier circuit 2 is converted into a DC voltage with ripples taken by the smoothing circuit 3. An example of the smoothing circuit 3 is shown in FIG. This is a circuit in which a coil 3a and a capacitor 3b are connected in an inverted L shape. The current output from the diode 2a during the ON period of the switching element 1b charges the capacitor 3b through the coil 3a, and simultaneously flows through the LD4, the LD switching element 5, and the current detection element 6 as a driving current for the LD4. Further, during the OFF period of the switching element 1b, the driving current of the LD 4 flows through the LD 4, the LD switching element 5, the current detection element 6, and the diode 2b by the energy charged in the coil 3a and the capacitor 3b.

  The synchronization signal processing circuit 7 generates a strobe signal based on the input video synchronization signal so that the output timing of the video data matches the light emission timing of the light source LD4. The strobe signal generated by the synchronization signal processing circuit 7 is input to the control circuit 8. The control circuit 8 outputs an ON / OFF control signal for the LD switching element 5 connected in series to the LD 4 based on the strobe signal and a target current value to be passed to the LD 4 to the feedback circuit 9. The target current value is set in advance in the control circuit 8 or is set in the control circuit 8 from the outside by a microcomputer (not shown).

  The feedback circuit 9 generates a pulse width modulation (PWM) signal that is a switching signal of the switching circuit 1 based on the current command value from the control circuit 8 and the current value from the current detection element 6. An example of the feedback circuit 9 is shown in FIG. The feedback circuit 9 includes an error amplifier 9a and a comparator 9b. The current command value and the current detection value are input to the error amplifier 9a, and the error amplifier 9a amplifies and outputs the difference between these input signals. The output signal of the error amplifier 9a is input to the comparator 9b. The comparator 9b compares the output signal of the error amplifier 9a with, for example, a triangular wave and outputs it as a pulse width modulation (PWM) signal. The error amplifier 9a includes a control circuit such as PID (proportional, integral, derivative) control.

  FIG. 6 shows the relationship between the strobe signal input to the control circuit 8, the light emission waveform of the LD4, and the current passed through the LD4. In a PTV using an LD as a light source, light is emitted in a time division manner for each color, so that the strobe signal for each color is a pulse signal of approximately 1/3 duty. The light emission waveform of the LD 4 may be delayed by a certain time with respect to the strobe signal, but is required to emit light at a constant intensity for the same time as the strobe signal. Since the light emission intensity of the LD is proportional to the current flowing through the LD, a pulse current that requires a constant current for a period equivalent to the light emission period is required as the current flowing through the LD.

  FIG. 7 shows the relationship between the ON / OFF control signal of the LD switching element 5, the timing of the current command value given to the feedback circuit 9, and the current flowing through the LD4. As shown in FIG. 7, the current command value is changed at the same timing as the ON / OFF control signal, and the LD switching element 5 is turned off and simultaneously the current command value is set to zero. . That is, a preset value is output as the target current value during the period when the ON / OFF control signal is on, and zero is output as the target current value during the period when the ON / OFF control signal is off.

  By turning off the LD switching element 5, the current flowing through the LD 4 can be cut off. However, only by that, the current detection value becomes zero and the current command value remains the target current value, so that the feedback circuit 9 outputs a PWM signal that increases the current to the switching circuit 1. Therefore, the switching circuit 1 continues to output current. Then, the capacitor 3b of the smoothing circuit 3 is overcharged while the LD switching element is OFF, and the output voltage of the smoothing circuit 3 increases. When the LD switching element 5 is turned on in such a state, a large current flows through the LD 4 and the light emission of the LD 4 becomes unstable due to degradation or destruction of the LD 4.

  However, as shown in FIG. 7, when the LD switching element 5 is turned OFF and at the same time the current command value is set to zero, the current detection value and the current command value become the same value while the LD switching element 5 is OFF, and the feedback circuit 9 The PWM signal output to the switching circuit 1 is a signal that stops the operation of the switching circuit 1. By doing so, even when the LD switching element 5 is OFF, the output voltage of the smoothing circuit 4 does not increase and remains at a constant value, so that a large current does not flow when the LD switching element 5 is turned ON. . Further, by setting the current command value to the target current value at the same time when the LD switching element 5 is turned ON, a constant current can be stably supplied to the LD 4 while the LD switching element 5 is ON.

  As the current command value during the period when the LD switching element 5 is OFF, since an output that increases the current does not appear from the error amplifier 9a of the feedback circuit 9, the current command value during the period when the LD switching element 5 is OFF. The value may be a value equal to or less than the current detection value during the period when the LD switching element 5 is OFF. In other words, the current detection value during the period when the LD switching element 5 is OFF may not be zero due to various causes. In such a case, the current detection value during the period when the LD switching element 5 is OFF is previously set. The current command value during the period when the LD switching element 5 is OFF may be set to a value equal to or smaller than the current detection value. Further, when the current detection value during the period in which the LD switching element 5 is OFF is zero, the current command value may be set to zero or a negative value.

  Further, it is possible to vary the current flowing through the LD 4 between the target current value and zero by simply changing the current command value without providing the LD switching element 5 connected in series to the LD 4. Since the time constant (several hundred μs or more) of the circuit 9 and the smoothing circuit 3 is long, the rise time and the fall time of the current waveform cannot be made high speed (several μs), and this is applied to the PTV light source drive. I can't.

  As described above, according to the first embodiment, the LD switching element 5 is connected in series to the LD 4 so that the target command value of the current flowing through the LD 4 is changed at high speed (several hundred Hz to several KHz). A predetermined first value is set as a target current value during the ON period of the LD switching element 5 connected to the second switching element, and a second value less than or equal to the load current value during the OFF period of the switching element is set as the target current value during the OFF period of the switching element (For example, zero) is set, so even if the LD 4 is driven at a constant current in a pulse shape, a large current does not flow during the first period when the current starts to flow, and stable pulse constant current driving is performed. Can do. As a result, there is an effect that light emission from the light emitting element can be stabilized.

Embodiment 2. FIG.
Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 8 shows the ON / OFF control signal of the LD switching element 5, the timing for setting the current command value, the output voltage (LD voltage) of the smoothing circuit 3, the current flowing through the LD 4 (LD current), and the light emission waveform of the LD 4. The relationship of (PLD) is shown. In the second embodiment, the target current value is set to the current command value at a timing earlier than turning on the LD switching element 5.

  The operation will be described. If the target current value is set as the current command value in a state where the LD switching element 5 is OFF, the LD voltage does not flow and the LD voltage starts to gradually increase. Thereafter, when the LD switching element 5 is turned on at an appropriate timing, the LD current rises faster than usual (indicated by a dotted line) and flows with a slight overshoot because the LD voltage is high. Then, it settles to a certain value.

  Some LD elements (especially blue and green LDs) have a light emission waveform with a slower rise than the rise of the LD current (indicated by a dotted line). This is because the light emission intensity of the LD element is temperature-dependent, and it takes a little time until the temperature inside the element rises to the optimum temperature due to the current of the LD element itself. If the rise of the light emission waveform becomes gradual, the utilization efficiency of the light emission period is deteriorated, so that the light emission efficiency is lowered, and the linearity is deteriorated when gradation control is performed. Therefore, by making the rise of the LD current steep, or by passing the current more like an overshoot, the temperature in the LD element can be raised rapidly, and the rise of the emission waveform can be made steep. The quantitative time difference between the timing to turn on the ON / OFF control signal of the LD switching element 5 and the timing to set the current command value to the target current value is not particularly defined, and the light emission waveform is observed while observing the light emission waveform. What is necessary is just to set to the optimal value which does not overshoot extremely.

  Thus, by setting the target current value as the current command value at an earlier timing than turning on the LD switching element 5, the rise of the LD current becomes faster and the rise of the emission waveform can be made steep. There is an effect.

Embodiment 3 FIG.
Next, a third embodiment of the present invention will be described with reference to FIG. FIG. 9 shows the relationship between the timing for turning off the LD switching element 5, the timing for setting the current command value, the LD voltage, and the LD current. In the third embodiment, the current command value is set to zero at an earlier timing than turning off the LD switching element.

  In general, the capacitor 3b of the smoothing circuit 3 is selected to have a sufficiently large capacity with respect to the load current so that the current can be stably supplied to the load. However, as the capacitance of the capacitor increases, the shape of the capacitor increases and the shape of the power source also increases. In order to reduce the shape of the power supply, it is necessary to reduce the capacitance of the capacitor. The third embodiment relates to a constant current switching power supply device that can supply a stable pulse current even if the capacitance of the capacitor is reduced.

  The operation will be described. The switching element 5 is turned OFF by the ON / OFF control signal, and at the same time, the current command value is set to zero (indicated by a dotted line), so that the switching operation of the switching circuit 1 is stopped, and the voltage is applied to the secondary side of the transformer 1a. Not induced. However, since energy is stored in the coil 3a of the smoothing circuit 3 while the switching element 5 is ON, even if the switching element 5 is turned OFF, the energy charges the capacitor 3b, and the voltage across the capacitor 3b To rise. When the capacitance of the capacitor 3b is large, the voltage rise is slight and does not cause a problem. However, when the capacitance of the capacitor 3b is small, the voltage rise is large (indicated by a dotted line), and is large at the moment when the switching element 5 is turned on. Current will flow.

  On the other hand, by setting the current command value to zero early, the energy accumulated in the coil 3a is consumed through the LD 4, and the voltage of the capacitor 3b increases when the switching element 5 is turned off. Can be suppressed. As a result, a large current does not flow when the switching element 5 is turned on, and light emission does not become unstable due to degradation or destruction of the LD element. The quantitative time difference between the timing at which the LD switching element 5 is turned off and the timing at which the current command value is set to zero is not particularly defined, and a large current is observed at the moment when the switching element 5 is turned on by observing the current waveform. It may be set to an optimal value that does not flow.

  Further, by adjusting the timing for turning off the LD switching element 5 and the timing for setting the current command value to zero, and controlling the voltage increase during the OFF period of the LD switching element 5, the same effect as in the second embodiment can be obtained. it can.

  By setting the current command value to zero earlier than the timing at which the LD switching element 5 is turned off in this way, it is possible to suppress an increase in voltage when the switching element 5 is turned off. As a result, a large current does not flow when the switching element 5 is turned on, and light emission does not become unstable due to degradation or destruction of the LD element.

Embodiment 4 FIG.
Next, Embodiment 4 will be described with reference to FIGS. The fourth embodiment is for improving the problem when the capacitance of the capacitor of the smoothing circuit is reduced as in the third embodiment. FIG. 10 shows a configuration of a load circuit according to the fourth embodiment. A discharge resistor 10 and an R switching element 11 are connected in parallel with the LD 4 and the LD switching element 5. The R switching element 11 uses a semiconductor switch such as a MOSFET or a bipolar transistor similarly to the LD switching element 5.

  FIG. 11 shows the relationship between the timing at which the LD switching element 5 is turned off, the timing at which the current command value is set to zero, and the timing at which the R switching element 11 is turned on. At the same time when the LD switching element 5 is turned OFF, the current setting value is set to zero, and the R switching element 11 is turned ON simultaneously.

  The operation will be described. When the LD switching element 5 is turned off and the R switching element 11 is turned on simultaneously, the energy accumulated in the coil 3a is discharged through the discharge resistor 10. As a result, the capacitor 3b is not charged with energy and the voltage does not increase. In the third embodiment, the light emission of the LD 4 is attenuated before the LD switching element 5 is turned off. However, the light emission of the LD 4 is reduced by adding the discharge resistor 10 and the R switching element 11 as in the fourth embodiment. It is no longer attenuated before the switching element 5 is turned off, and the light emission during the ON period of the LD switching element 5 can be stabilized. The time interval for turning on the R switching element 11 is not particularly determined, and may be set to an optimum value so that the voltage does not increase excessively by observing the LD voltage.

  Further, by adjusting the time interval during which the R switching element 11 is turned on and controlling the voltage increase during the OFF period of the LD switching element 5, the same effect as in the second embodiment can be obtained.

  In this way, the discharge resistor 10 and the R switching element 11 are connected in parallel with the LD 4 and the LD switching element 5 so that the R switching element 11 is turned on at the same time that the LD switching element 5 is turned off. An increase in voltage during the OFF period can be suppressed, and a large current does not flow when the switching element 5 is turned on. As a result, light emission does not become unstable due to degradation or destruction of the LD element. At the same time, the light emission of the LD 4 is not attenuated before the LD switching element 5 is turned off, and the light emission during the ON period of the LD switching element 5 can be stabilized.

  As described above, the constant current switching power supply device according to the present invention is useful for PTV using LEDs and LDs as light sources.

BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram which shows the structure of Embodiment 1 of the constant current switching power supply device concerning this invention. It is a figure which shows an example of a switching circuit. It is a figure which shows an example of a rectifier circuit. It is a figure which shows an example of a smoothing circuit. It is a figure which shows an example of a feedback circuit. 3 is a time chart showing the relationship between a strobe signal, a light emission waveform, and an LD current in the first embodiment. 4 is a time chart showing the relationship between an ON / OFF signal of a switching element, a current command value, and an LD current in the first embodiment. 10 is a time chart showing a relationship among an ON / OFF signal, a current command value, a voltage, an LD current, and a light emission waveform of the switching element in the second embodiment. 10 is a time chart showing a relationship among an ON / OFF signal, a current command value, a voltage, and an LD current of a switching element in Embodiment 3. FIG. 10 is a circuit diagram illustrating a partial configuration used in the fourth embodiment. 10 is a time chart showing the relationship between the ON / OFF signal of the switching element, the current command value, and the ON / OFF signal of the R switching element in the fourth embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Switching circuit 1a Transformer 1b Primary side switching element 2 Rectifier circuit 2a Diode 2b Diode 3 Smoothing circuit 3a Coil 3b Capacitor 4 Semiconductor laser 5 LD switching element 6 Current detection element 7 Synchronous signal processing circuit 8 Control circuit 9 Feedback circuit 9a Error amplifier 9b Comparator 10 Discharge resistor 11 R switching element

Claims (6)

A constant current switching power supply device for controlling a current flowing in a load,
A switching element connected in series to the load;
A current detection element for detecting a load current value flowing through the load;
A control circuit that outputs a target current value to flow to the load and outputs an on / off control signal for turning on and off the switching element;
A switching power supply for driving the load with a pulse constant current by a PWM signal created based on a difference between the target current value and the load current value;
With
The control circuit outputs a predetermined first set value as a target current value during a period when the on / off control signal is on, and an off period of the switching element as a target current value during a period when the on / off control signal is off. A constant current switching power supply apparatus that outputs a second set value equal to or less than the load current value.   2. The constant current switching power supply device according to claim 1, wherein the control circuit outputs the first set value as a target current value at a timing earlier than turning on the on / off control signal.   2. The constant current switching according to claim 1, wherein the control circuit switches the target current value from a first set value to a second set value at an earlier timing than turning off the on / off control signal. Power supply. Connecting a discharge resistor and a second switching element in parallel with the load and the switching element;
2. The constant current switching power supply device according to claim 1, wherein the control circuit turns on the second switching element for a predetermined period when the on / off control signal is turned off.   The constant current switching power supply device according to claim 1, wherein the second set value is a value equal to or less than zero. The load is a light emitting element,
A light source device comprising the constant current switching power supply device according to claim 1.

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