JP4558001B2 - Power circuit - Google Patents

Description

  The present invention relates to a power supply circuit that boosts an input voltage from a DC power supply and supplies the boosted voltage to a load, and more particularly, to a power supply circuit in which a boosting operation is repeatedly activated / stopped according to a PWM signal.

  In recent years, as one of the illumination sources (backlights or frontlights) of liquid crystal display devices (LCD) installed in electronic devices such as mobile phones, PDAs (Personal Digital Assistants), digital cameras, durability, luminous efficiency, and occupation White light emitting diodes that are superior in terms of area and the like have been used. This white light emitting diode requires a relatively high forward voltage, and usually, a plurality of white light emitting diodes are used as an illumination source, and the plurality of white light emitting diodes used have uniform brightness of each white light emitting diode. For this reason, the white light emitting diode as the illumination source is driven by a DC voltage higher than the DC voltage from the battery built in the portable device.

  Therefore, as a circuit for driving such a white light emitting diode, a boost type power supply circuit shown in FIG. 9 is conventionally used. FIG. 9 is a circuit block diagram showing an electrical configuration of a conventional power supply circuit. The power supply circuit shown in FIG. 9 includes a DC power supply 1 such as a lithium ion battery, an input capacitor 2, a coil 3, a diode (rectifier element) 4, an output capacitor 5, a resistor (output current detection resistor) R1, and one package. It is composed of a step-up chopper regulator 10 that is made into an IC and performs a step-up operation by switching energy storage / release to the coil 3, and has six white light emission as an illumination source of an LCD mounted on an electronic device such as a cellular phone. The diodes (loads) LEDs 1 to 6 are driven.

  The negative terminal of the DC power source 1 is connected to the ground, and the positive terminal is connected to the ground via the input capacitor 2 and to one end of the coil 3. The other end of the coil 3 is connected to the anode of the diode 4, and the cathode of the diode 4 is connected to the ground via the output capacitor 5. Further, in parallel with the output capacitor 5, a series circuit in which the white light emitting diodes LED1 to 6 and the resistor R1 are connected in series is connected.

  Further, the boost chopper regulator 10 includes a power supply terminal Vi, a power supply terminal GND, an output voltage monitor terminal Vo, a feedback terminal FB, and a control terminal CTRL as terminals for external connection. The power supply terminal Vi is connected to the positive terminal of the DC power supply 1 and the power supply terminal GND is connected to the ground, whereby the boost chopper regulator 10 obtains the DC power supply 1 as its operating power supply. The switch terminal Vsw is connected to the connection point between the coil 3 and the diode 4, the output voltage monitor terminal Vo is connected to the cathode of the diode 4, and the feedback terminal FB is connected to the connection point between the white light emitting diode LED6 and the resistor R1. Has been. In addition, a luminance adjustment signal (external input signal) for adjusting the luminance of white light emitting diodes LED1 to 6 described later is input to the control terminal CTRL.

  Next, the internal configuration of the boost chopper regulator 10 and its connection will be described. The step-up chopper regulator 10 includes N-channel FETs (switching elements) 11 and 12, a drive circuit 13, a current detection comparator 14, an oscillation circuit 15, an amplifier 16, a PWM comparator 17, an error amplifier 18, a reference power supply 19, a resistor R2, R3, R4, soft start circuit 20, ON / OFF circuit (operation / stop circuit) 21, overheat protection circuit 22, and overvoltage protection circuit 23.

  The drains of the FETs 11 and 12 are both connected to the switch terminal Vsw, and the gates of the FETs 11 and 12 are both connected to the drive circuit 13. The source of the FET 12 is connected to the ground, and the source of the FET 11 is connected to the ground via the resistor R2. Both ends of the resistor R2 are connected to two input terminals of the current detection comparator 14, respectively, and the output of the current detection comparator 14 and one output of the two outputs of the oscillation circuit 15 are added by the amplifier 16, and the PWM comparator 17 is added. As one input. Further, the output of the PWM comparator 17 and the other output of the oscillation circuit 15 are supplied to the drive circuit 13, respectively.

  The output of the error amplifier 18 is supplied to the other input terminal of the PWM comparator 17, and one input terminal of the error amplifier 18 is connected to the feedback terminal FB. The other input terminal of the error amplifier 18 is connected to one ends of the resistors R3 and R4, the other end of the resistor R4 is connected to the ground, and the other end of the resistor R3 is connected to the positive terminal of the reference power source 19. The negative terminal of the reference power supply 19 is connected to the ground.

  The outputs of the soft start circuit 20, the ON / OFF circuit 21, the overheat protection circuit 22, and the overvoltage protection circuit 23 are respectively supplied to the drive circuit 13, and the control terminal CTRL is connected to the soft start circuit 20 and the ON / OFF circuit 21. The brightness adjustment signal is supplied through The overvoltage protection circuit 23 is supplied with the output voltage Vout through the output voltage monitor terminal Vo.

  Next, the operation of the power supply circuit having the above configuration shown in FIG. 9 will be described. The power supply circuit shown in FIG. 9 generates an output voltage Vout obtained by boosting the input voltage Vin from the DC power supply 1 at both ends of the output capacitor 5 by turning on / off the FET 12 by the drive circuit 13. That is, when the drive circuit 13 applies a predetermined gate voltage to the gate of the FET 12 and the FET 12 is turned on, the current from the DC power source 1 flows through the coil 3 and energy is stored in the coil 3. When the drive circuit 13 does not apply a predetermined gate voltage to the gate of the FET 12 and the FET 12 is turned off, the stored energy is released to generate a counter electromotive force in the coil 3.

  The back electromotive force generated in the coil 3 is added to the input voltage Vin of the DC power supply 1 and charges the output capacitor 5 via the diode 4. By repeating such a series of operations, a boosting operation is performed, and an output voltage Vout is generated at both ends of the output capacitor 5. The output voltage Vout causes the output current Iout to flow through the white light emitting diodes LED1 to LED6. Light emitting diodes LED1-6 emit light.

  Then, a feedback voltage Vfb obtained by multiplying the current value of the output current Iout by the resistance value of the resistor R1 is supplied to one input terminal of the error amplifier 18 via the feedback terminal FB and supplied to the other input terminal of the error amplifier 18. Is compared with the reference voltage Vref. The reference voltage Vref is a voltage obtained by dividing the voltage of the reference power source 19 by the resistors R3 and R4. Therefore, a voltage corresponding to the difference between the feedback voltage Vfb and the reference voltage Vref appears at the output of the error amplifier 18, and this voltage is supplied to one input terminal of the PWM comparator 17.

  Further, the signal input to the other input terminal of the PWM comparator 17 is obtained by adding the signal proportional to the current flowing through the resistor R2 by turning on the FET 11 and the sawtooth wave signal from the oscillation circuit 15 by the amplifier 16. The amplified signal is compared with the output voltage level of the error amplifier 18 supplied to the one input terminal of the PWM comparator 17. As a result, during a period in which the output voltage level from the error amplifier 18 is higher than the signal level from the amplifier 16, the PWM output of the PWM comparator 17 is at the H (High) level, and the output voltage level from the error amplifier 18 is at the amplifier 16 level. During a period lower than the signal level from the PWM comparator 17, the PWM output of the PWM comparator 17 is at the L (Low) level.

  The drive circuit 13 receives the PWM output of the PWM comparator 17 and turns the FETs 11 and 12 on / off with a duty corresponding to the PWM output. That is, the drive circuit 13 gives a predetermined gate voltage to the FETs 11 and 12 when the PWM output of the PWM comparator 17 is at the H level and at the start of each cycle of the clock signal from the oscillation circuit 15, so that the FET 11 , 12 is turned on. Then, when the PWM output of the PWM comparator 17 becomes L level, supply of the gate voltage to the FETs 11 and 12 is stopped, and the FETs 11 and 12 are turned off.

  When such on / off control of the FETs 11 and 12 is performed, the boosting operation is performed so that the feedback voltage Vfb and the reference voltage Vref are equal. That is, the output current Iout is stabilized to a current value obtained by dividing the reference voltage Vref (= feedback voltage Vfb) by the resistance value of the resistor R1. Further, the signal compared with the PWM comparator 17 is added with a signal corresponding to the current flowing through the resistor R2, that is, a signal corresponding to the current flowing through the coil 3 when the FETs 11 and 12 are turned on. The peak current flowing through the coil 3 is limited.

  Further, since the overvoltage protection circuit 23 detects that the output voltage Vout has exceeded the predetermined overvoltage protection voltage and stops the operation of the drive circuit 13, the white light emission in which the overvoltage exceeding the predetermined overvoltage protection voltage is a load. Application to the diodes LED 1 to 6 and the output capacitor 5 is prevented. In addition, the overheat protection circuit 22 detects the overheat around the FET 12 in particular with the operation of the drive circuit 13 and stops the operation of the drive circuit 13, thereby preventing the boost chopper regulator 10 from being broken or destroyed due to overheating. Yes.

  The ON / OFF circuit 21 activates / stops the driving operation of the FETs 11 and 12 of the drive circuit 13 in accordance with an external input signal input to the control terminal CTRL, and the control terminal CTRL receives this external input signal. The luminance of the white light emitting diodes LED1 to LED6 can be adjusted by inputting a luminance adjustment signal which is a PWM (Pulse Width Modulation) signal. That is, the ON / OFF circuit 21 operates the driving operation of the FETs 11 and 12 of the drive circuit 13 when the luminance adjustment signal input to the control terminal CTRL is at the H level, and outputs the output current Iout to the white light emitting diodes LED1 to LED6. When the brightness adjustment signal is at the L level, the drive operation of the FETs 11 and 12 of the drive circuit 13 is stopped and the output voltage Vout is lowered. Therefore, the average current flowing through the white light emitting diodes LED1 to LED6 depends on the duty of the brightness adjustment signal. Will change. And since the brightness | luminance of white light emitting diode LED1-6 is proportional to this average electric current, the brightness | luminance of white light emitting diode LED1-6 is adjusted in this way.

  The soft start circuit 20 gradually increases the output voltage Vout by gradually changing the output duty of the drive circuit 13 at the start of the operation of the drive circuit 13. If the output voltage Vout is not increased gently, an excessive charging current for charging flows from the DC power source 1 when the output capacitor 5 is not charged, and the DC power source 1 is a battery such as a lithium ion battery. In this case, the battery is burdened, and the battery voltage is lowered by the excessive charging current, which causes a problem that the battery cannot be used up to the original end voltage.

  10 and 11 are waveform diagrams showing voltage waveforms and current waveforms of respective parts of the power supply circuit shown in FIG. Here, in order to explain the effect of the soft start circuit 20, FIG. 10 shows a waveform when the soft start circuit 20 is not functioning, and FIG. 11 shows a waveform when the soft start circuit 20 is functioning. ing. 10 and 11, (a) shows the voltage waveform of the luminance adjustment signal input to the control terminal CTRL, (b) shows the waveform of the output voltage Vout, and (c) shows the waveform of the input current Iin. 10 and 11, when the brightness adjustment signal first changes to the H level, the power supply circuit shown in FIG. 9 is activated, and at this time, the input voltage Vin is supplied from the DC power supply 1. Until this time, the output voltage Vout is 0 V, and the output capacitor 5 is not charged at all.

  First, the boosting operation when the soft start circuit 20 is not functioning will be described with reference to FIG. In FIG. 10, at the time of start-up, that is, when the luminance adjustment signal first becomes H level (FIG. 10 (a)), the drive circuit 13 starts the boost operation, but the soft start function does not work, so the output voltage Vout Immediately rises to the voltage V1 (FIG. 10B). At this time, the input current Iin becomes a charging current for charging the output capacitor 5 with the voltage V1, and thus becomes an excessive current (FIG. 10C).

  The voltage V1 is an output in which the output current Io flows through the white light emitting diodes LED1 to LED6 and the resistor R1 by the boosted output voltage Vout, whereby the feedback voltage Vfb generated at the feedback terminal FB becomes equal to the reference voltage Vref. This is the voltage Vout. As the charging of the output capacitor 5 proceeds, the input current Iin decreases and thereafter becomes constant (FIG. 10 (c)).

  Next, when the luminance adjustment signal becomes L level (FIG. 10A), the ON / OFF circuit 21 stops the boosting operation of the drive circuit 13, so that the output voltage Vout becomes the input voltage Vin of the DC power source 1 (FIG. 10). 10 (b)), the input current Iin stops flowing (FIG. 10 (c)).

  After that, when the luminance adjustment signal is switched to H level / L level with a predetermined duty for luminance adjustment (FIG. 10A), the output voltage Vout is switched to the voltages V1 and Vin according to the luminance adjustment signal. It changes (FIG. 10B). The input current Iin that flows when the output voltage switches from Vin to V1 is obtained by subtracting the input voltage Vin from the voltage V1 because the output capacitor 5 is already charged to the voltage value of the input voltage Vin. It becomes a charging current for charging, and does not become an excessive current (FIG. 10C).

  The above is the description of the step-up operation in a state where the soft start circuit 20 is not functioning. However, there is a problem that the input current Iin flowing at the time of startup becomes excessive. That is, since an excessive charging current for charging the output capacitor 5 flows out from the DC power source 1 as a battery at the time of starting, the battery is burdened, and the battery voltage is lowered by the excessive charging current, so that the battery is originally terminated. There arises a problem that even the voltage cannot be used. In order to solve this problem, the soft start circuit 20 needs to function.

  Next, the boosting operation when the soft start circuit 20 is functioning will be described with reference to FIG. In FIG. 11, at the time of start-up, that is, when the luminance adjustment signal first becomes H level (FIG. 11A), the drive circuit 13 starts the boosting operation, but the soft start circuit 20 outputs the drive circuit 13. Since the duty is gradually changed, the output voltage Vout gradually reaches the voltage V1 after reaching the input voltage Vin (FIG. 11B). Since the input current Iin at the time of starting becomes a charging current for charging the output capacitor 5 with the input voltage Vin, it is not an excessive current. (FIG. 11 (c)). Then, as the charging of the output capacitor 5 proceeds, the input current Iin decreases and thereafter becomes constant (FIG. 11 (c)).

  Next, when the luminance adjustment signal becomes L level (FIG. 11A), the ON / OFF circuit 21 stops the boosting operation of the drive circuit 13, so that the output voltage Vout becomes the input voltage Vin of the DC power source 1 (FIG. 11). 11 (b)), the input current Iin stops flowing (FIG. 11 (c)).

  After that, when the luminance adjustment signal is switched to H level / L level with a predetermined duty for luminance adjustment (FIG. 11 (a)), the output voltage Vout is obtained when the luminance adjustment signal is switched from L level to H level. The soft start circuit 20 gradually increases toward the voltage V1 by controlling the switching operation of the drive circuit 13, but immediately becomes the voltage Vin when the luminance adjustment signal switches from the H level to the L level (FIG. 11 (b)). The input current Iin that flows when the output voltage Vout rises from the input voltage Vin toward the voltage V1 is already charged to the voltage value of the input voltage Vin. Becomes a charging current for charging, and does not become an excessive current (FIG. 10C). As described above, the function of the soft start circuit 20 can prevent the input current Iin at the start-up from becoming excessive, and prevent the DC power source 1 from being damaged.

A transformer having a primary winding and a secondary winding having a different number of turns from the primary winding; and a switching element connected in series with the primary winding of the transformer between power supply terminals of a DC power supply; Based on the soft start circuit that charges the soft start capacitor when the DC power supply rises and outputs a voltage signal relative to the charge voltage as a soft control signal, and the soft control signal output from the soft start circuit, A switching power supply device comprising a control circuit for controlling the duty ratio of the switching element to gradually increase, wherein a voltage fluctuation that detects a fall of the DC power supply below a predetermined voltage value and outputs a detection signal thereof Based on the detection circuit and the detection signal output from the voltage fluctuation detection circuit, the charging power of the soft start capacitor There is also a switching power supply device provided with a forced discharge circuit for forcibly discharging. (For example, refer to Patent Document 1).
JP 11-69793 A

  However, in the conventional power supply circuit shown in FIG. 9, as shown in FIG. 11, the drive circuit 13 is boosted by the ON / OFF circuit 21 every time the luminance adjustment signal inputted to the control terminal CTRL becomes H level. The soft start circuit 20 operates every time the output is started, and the output voltage Vout gradually rises. Therefore, the output voltage Vout does not immediately rise to the voltage V1, so that a constant output current Io flows through the white light emitting diodes LED1 to LED6. There is a problem that the desired brightness adjustment according to the duty of the brightness adjustment signal cannot be performed.

  Further, in the prior art described in Patent Document 1, even when the switching power supply device is momentarily interrupted or turned on again in a short time, the soft start operation can be surely performed and the switching element can be prevented from being damaged in advance. However, if the output voltage is turned on / off in a short cycle, the output voltage rises gently by the soft start operation every time it is turned on, and thus the output voltage does not rise to a desired voltage. Accordingly, when a white light emitting diode as shown in FIG. 9 is driven and used as a power supply circuit that performs luminance adjustment according to a luminance adjustment signal, there is a problem that desired luminance adjustment cannot be performed.

  In view of the above points, the present invention limits an excessive input current that flows at the time of start-up, and immediately sets the output voltage to a desired voltage even when the boosting operation is repeatedly activated / stopped according to the PWM signal. An object is to provide a power supply circuit that can be started up.

  In order to solve the above-described problem, a power supply circuit according to the present invention is connected to a predetermined power supply, and an output capacitor that generates a voltage by being charged by the power supply, and between the power supply and the output capacitor. A switching element that switches connection / disconnection between a coil provided in the coil, a rectifying element provided between the coil and the output capacitor, an intermediate point between the coil and the rectifying element, and a ground point And a power supply circuit that supplies an output voltage generated at an intermediate point between the rectifying element and the output capacitor to a predetermined load, wherein a control signal in which a first state and a second state appear alternately When the control signal is in the first state, a boosting operation for boosting the output voltage is performed by switching the switching element, while when the control signal is in the second state, the boosting operation is performed. Drive means, and a soft start circuit that performs a soft start process that moderates the rise of the output voltage due to the boosting operation, the soft start circuit having a magnitude of the output voltage, The control signal state is monitored and the soft start process is performed only at a predetermined timing according to the monitoring result (first configuration).

  In the first configuration, the soft start circuit detects, by the monitoring, that the control signal has transitioned to the first state when the output capacitor is not charged, and the boost operation immediately after the transition is performed. Only the above may be configured to perform the soft start process (second configuration).

  In the second configuration, the soft start circuit compares the magnitude of the output voltage with a predetermined threshold value, and determines whether the output capacitor is not charged based on the comparison result. It is good also as a structure (3rd structure) which judges these.

  In the second or third configuration, the switching element is switched according to a predetermined PWM signal generated by the drive means, and the soft start circuit A configuration (fourth configuration) may be employed in which the soft start process is performed by adjusting the duty ratio.

  Further, a power supply circuit according to any one of the first to fourth configurations, wherein an LED is connected as the load and power is supplied to the LED (fifth configuration). Circuits are also useful.

  The power supply circuit according to the present invention is connected to a predetermined power supply, and an output capacitor that generates a voltage by being charged by the power supply and a connection path between the power supply and the output capacitor are used as a predetermined control signal. A power supply circuit configured as a step-up chopper regulator, comprising: a switching element to be grounded in response; and a soft start circuit that performs a limiting process for limiting a magnitude of a current output from the power supply, The start circuit is configured to perform the limiting process (sixth configuration) according to the magnitude of the voltage generated by the output capacitor and the state of the control signal.

  Further, the power supply circuit according to the present invention is connected to a predetermined power supply, and an output capacitor that generates a voltage by being charged by the power supply, a coil provided between the power supply and the output capacitor, Switching that switches connection / disconnection of a rectifying element provided between the coil and the output capacitor, an intermediate point between the coil and the rectifying element, and a ground point in accordance with a predetermined control signal And a soft start circuit that performs a limiting process for limiting the amount of current output from the power source, and outputs an output voltage generated at an intermediate point between the rectifier element and the output capacitor to a predetermined load. The soft start circuit monitors the magnitude of the output voltage and the state of the control signal, and a predetermined timing corresponding to the monitoring result. Timing only be configured to perform the restriction process (seventh configuration).

  According to the present invention, at the time of start-up, it is detected that the output voltage is low, the soft start circuit is operated, and the output voltage is gradually increased to prevent the current flowing from the DC power source from becoming excessive. After the output voltage rises, the output voltage can be reduced by not operating the soft start circuit even when the drive operation of the switching element of the drive circuit repeats operation / stop in response to an external input signal. It can be launched immediately.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit block diagram showing an electrical configuration of the power supply circuit according to the first embodiment of the present invention. In FIG. 1, the same parts as those in FIG. 9 are denoted by the same reference numerals, and the description thereof is omitted. The power supply circuit shown in FIG. 1 is different from the power supply circuit shown in FIG. 9 in that an output voltage detection circuit 24 is newly provided in the boost chopper regulator 10.

  The output voltage detection circuit 24 is connected between the output voltage monitor terminal Vo and the soft start circuit 20, and detects that the output voltage Vout given through the output voltage monitor terminal Vo is higher than the set voltage. A detection signal is supplied to the soft start circuit 20. Such an output voltage detection circuit 24 can be configured by a circuit shown in FIG. 2, for example.

  FIG. 2 is a circuit diagram showing an electrical configuration of the output voltage detection circuit 24 shown in FIG. The output voltage detection circuit shown in FIG. 2 includes a comparator 25, a reference power supply 26, and resistors R5 and R6. One input of the comparator 25 is set by dividing the voltage of the reference power supply 26 by resistors R5 and R6. A voltage Vset is applied. The output voltage Vout is applied to the other input of the comparator 25 via the output voltage monitor terminal Vo. The output of the comparator 25 is given to the soft start circuit 24.

  The output voltage detection circuit 24 having such a configuration compares the output voltage Vout with the set voltage Vset and supplies the comparison result signal to the soft start circuit 24. For example, the comparison result signal becomes H level when the output voltage Vout is larger than the set voltage Vset, and becomes L level when the output voltage Vout is smaller than the set voltage Vset. The soft start circuit 20 switches its operation / non-operation in accordance with the comparison result signal from the output voltage detection circuit 24 at the rising edge of the luminance adjustment signal given through the control terminal CTRL. That is, when the comparison result signal is at the H level when the luminance adjustment signal rises, the soft start circuit 20 stops operating, and when the comparison result signal is at the L level, the soft start circuit 20 operates to perform the soft start control of the drive circuit 13. The operation of the power supply circuit shown in FIG. 1 will be described below with reference to FIG.

  FIG. 3 is a waveform diagram showing voltage waveforms and current waveforms of each part of the power supply circuit shown in FIG. 1 similar to FIGS. 3, (a) shows the voltage waveform of the luminance adjustment signal input to the control terminal CTRL, (b) shows the waveform of the output voltage Vout, and (c) shows the waveform of the input current Iin. In FIG. 3, the first time that the luminance adjustment signal changes to the H level is when the power supply circuit shown in FIG. 1 is started. At this time, the input voltage Vin is supplied from the DC power supply 1. Until this time, the output voltage Vout is 0 V, and the output capacitor 5 is not charged at all.

  In FIG. 3, at the time of start-up, that is, when the luminance adjustment signal first becomes H level (FIG. 3A), the output capacitor 5 is not charged, and the output voltage Vout at this time is set. It is smaller than the voltage Vset. Therefore, the output from the output voltage detection circuit 24 at this time is at the L level, and the soft start circuit 20 performs the operation after confirming that the output from the output voltage detection circuit 24 at this time is at the L level. Accordingly, the drive circuit 13 starts the boosting operation, but the soft start circuit 20 controls the drive circuit 13 to gradually change the output duty, so that the output voltage Vout becomes the voltage of the input voltage Vin and then the voltage V1. It gradually rises to (Fig. 3 (b)). Here, since the input current Iin at the time of starting becomes a charging current for charging the output capacitor 5 with the input voltage Vin, it does not become an excessive current (FIG. 3C). Then, as the charging of the output capacitor 5 proceeds, the input current Iin decreases and thereafter becomes constant (FIG. 3C).

  Next, when the luminance adjustment signal becomes L level (FIG. 3A), the ON / OFF circuit 21 stops the boosting operation of the drive circuit 13, so that the output voltage Vout becomes the input voltage Vin of the DC power supply 1 (FIG. 3 (b)), the input current Iin stops flowing (FIG. 3 (c)).

  After that, the luminance adjustment signal is switched to the H level / L level for a luminance adjustment with a predetermined duty (FIG. 3A), but the soft start circuit 20 is in a state when the luminance adjustment signal becomes the H level. After confirming that the output from the output voltage detection circuit 24 is at the H level, the operation is stopped. This is because the output capacitor 5 is charged to the input voltage Vin at this time, so the output voltage Vout is the voltage of the input voltage Vin, and this voltage is higher than the set voltage Vset.

  Therefore, the output voltage Vout is immediately switched between the voltages V1 and Vin in accordance with the luminance adjustment signal (FIG. 3B). Here, the input current Iin that flows when the output voltage switches from Vin to V1 is output at a voltage obtained by subtracting the input voltage Vin from the voltage V1 because the output capacitor 5 has already been charged to the voltage value of the input voltage Vin. It becomes a charging current for charging the capacitor 5 and does not become an excessive current (FIG. 3C).

  In this way, when starting up, the soft start circuit 20 gently raises the output voltage Vout to limit the input current Iin from becoming excessive, and the boosting operation is repeatedly activated / stopped according to the luminance adjustment signal. The output voltage Vout can be immediately raised to a desired voltage. As a result, it is possible to realize a power supply circuit capable of performing a desired brightness adjustment according to a brightness adjustment signal given from the outside without damaging the DC power supply 1.

  FIG. 4 is a circuit diagram showing another electrical configuration of the output voltage detection circuit 24 shown in FIG. 4, the same parts as those in FIG. 2 are denoted by the same reference numerals, and the description thereof is omitted. The output voltage detection circuit 24 shown in FIG. 4 is different from the output voltage detection circuit 24 shown in FIG. 2 in that a comparator 27 having hysteresis is provided instead of the comparator 25. One input of the comparator 27 is supplied with a set voltage Vset obtained by dividing the voltage of the reference power supply 26 by resistors R5 and R6. This set voltage Vset has a hysteresis corresponding to the output of the comparator 27. . For example, the set voltage Vset when the output of the comparator 27 is L level is 4.6V, and the set voltage Vset when it is H level is 3.4V.

  The operation of the power supply circuit shown in FIG. 1 when the output voltage detection circuit 24 shown in FIG. 4 is used will be described with reference to FIG. In FIG. 3, at the time of start-up, that is, when the luminance adjustment signal first becomes H level (FIG. 3A), the output capacitor 5 is not charged, and the output voltage Vout at this time is 4 Since it is smaller than .6 V, the output from the output voltage detection circuit 24 is at L level.

  The soft start circuit 20 performs a soft start operation when the output from the output voltage detection circuit 24 is at the L level, and stops operating when the output from the output voltage detection circuit 24 is at the H level. . Accordingly, the drive circuit 13 starts the boosting operation, but the soft start circuit 20 gradually changes the output duty of the drive circuit 13, so that the output voltage Vout becomes 4.6 V after reaching the input voltage Vin. Until the comparator 27 in the output detection circuit 24 is inverted (FIG. 3B).

  When the output voltage Vout exceeds 4.6V and the comparator 27 in the output detection circuit 24 is inverted and the output becomes H level, the soft start circuit 20 stops its operation, so the output voltage Vout immediately rises to the voltage V1. To do. However, the input current Iin flowing at this time is not an excessive current because the output capacitor 5 has already been charged to 4.6 V and becomes a charging current at a voltage increase exceeding that. At this time, since the output of the comparator 27 becomes H level, the set voltage Vset changes to 3.4V.

  Next, when the luminance adjustment signal becomes L level (FIG. 3A), the ON / OFF circuit 21 stops the boosting operation of the drive circuit 13, so that the output voltage Vout becomes the input voltage Vin of the DC power supply 1 (FIG. 3 (b)), the input current Iin stops flowing (FIG. 3 (c)).

  After that, the luminance adjustment signal is switched to H level / L level with a predetermined duty for luminance adjustment (FIG. 3A), but the soft start circuit 20 outputs the output voltage from the output voltage detection circuit 24 at the H level. The operation has stopped because there is. This is because the output capacitor 5 is charged to the input voltage Vin at this time, so the output voltage Vout is the voltage of the input voltage Vin, and this voltage is higher than 3.4V.

  Therefore, the output voltage Vout is immediately switched between the voltages V1 and Vin in accordance with the luminance adjustment signal (FIG. 3B). Here, the input current Iin that flows when the output voltage switches from Vin to V1 is output at a voltage obtained by subtracting the input voltage Vin from the voltage V1 because the output capacitor 5 has already been charged to the voltage value of the input voltage Vin. It becomes a charging current for charging the capacitor 5 and does not become an excessive current (FIG. 3C).

In this way, when starting up, the soft start circuit 20 gently raises the output voltage Vout to limit the input current Iin from becoming excessive, and the boosting operation is repeatedly activated / stopped according to the luminance adjustment signal. The output voltage Vout can be immediately raised to a desired voltage. This effect is the same even when the output detection circuit 24 shown in FIG. 2 is used. By using the output detection circuit 24 shown in FIG. 4, the soft start circuit 20 operates in accordance with the output from the output detection circuit 24. It is only necessary to switch between non-operation and the soft start circuit 20 can be configured in a simple manner.

  Further, as described above, by setting the setting voltage Vset to 4.6 V, when a lithium ion battery is used for the DC power supply 1, the soft start function is maintained until the maximum charging voltage of the lithium ion battery is maintained. be able to. In addition, by providing a hysteresis of 1.2V to the set voltage Vset and setting the voltage at which the soft start function works again to 3.4V after the output voltage Vout rises, the softening is achieved up to the lowest end voltage of the lithium ion battery. The start function can be disabled and the battery can be used effectively.

  FIG. 5 is a circuit block diagram showing an electrical configuration of the power supply circuit according to the second embodiment of the present invention. In FIG. 5, the same parts as those in FIG. The power supply circuit shown in FIG. 5 is different from the power supply circuit shown in FIG. 1 in that a feedback detection circuit 28 is provided instead of the output voltage detection circuit 24.

  The feedback detection circuit 28 is connected between the feedback terminal FB and the soft start circuit 20, detects that the feedback voltage Vfb given via the feedback terminal FB is higher than the set voltage, and soft-starts the detection signal. This is applied to the circuit 20. The power supply circuit shown in FIG. 5 is obtained by changing the output voltage Vout to the feedback voltage Vfb to detect the operation / non-operation of the soft start circuit 20, and the feedback voltage Vfb is a voltage proportional to the output voltage Vout. Therefore, the feedback voltage detection circuit 30 can be realized by changing the voltage level of the set voltage Vset, for example, to have the same circuit configuration as the output voltage detection circuit 24 shown in FIGS. Accordingly, the power supply circuit shown in FIG. 5 performs the same operation as that of the power supply circuit shown in FIG.

  FIG. 6 is a circuit block diagram showing an electrical configuration of the power supply circuit according to the third embodiment of the present invention. In FIG. 6, the same parts as those in FIG. The power supply circuit shown in FIG. 6 is different from the power supply circuit shown in FIG. 1 in that an overvoltage protection circuit 29 that also functions as the output voltage detection circuit 24 is provided instead of the overvoltage protection circuit 23.

  The overvoltage protection circuit 29 is a combination of the output voltage detection circuit 24 and the overvoltage protection circuit 23 shown in FIG. The output voltage detection circuit 24 and the overvoltage protection circuit 23 shown in FIG. 1 are both circuits that compare the output voltage Vout with a predetermined voltage set in each circuit and output a signal of the comparison result. The overheat protection circuit 29 combining these two circuits can be easily realized. For example, as shown in FIG. 7, it can be realized by extracting a voltage for detecting an overvoltage and a voltage for detecting an output voltage from the output voltage Vout by resistance division. In this way, the circuit configuration of the power supply circuit can be simplified.

  FIG. 8 is a circuit block diagram showing an electrical configuration of the power supply circuit according to the fourth embodiment of the present invention. In FIG. 8, the same parts as those in FIG. The power supply circuit shown in FIG. 8 is different from the power supply circuit shown in FIG. 1 in that an input voltage detection circuit 30 is provided instead of the output voltage detection circuit 24.

  The input voltage detection circuit 30 is connected between the power supply terminal Vi and the soft start circuit 20, and detects that the input voltage Vin given through the power supply terminal Vi is higher than the set voltage, and the detection signal is softened. This is given to the start circuit 20. The power supply circuit shown in FIG. 7 is obtained by changing the output voltage Vout to the input voltage Vin to detect the operation / non-operation of the soft start circuit 20, and the input voltage detection circuit 30 includes, for example, FIG. This can be realized by changing the voltage level of the set voltage Vset to the same circuit configuration as the output voltage detection circuit 24 shown in FIG.

  When the input voltage Vin from the DC power source 1 is applied at the start-up, the input capacitor 2 is charged, and the terminal voltage of the input capacitor 2 increases accordingly. Therefore, since the input voltage Vin detected by the input voltage detection circuit 30 until the input voltage Vin completely rises is lower than the set voltage Vset, the input voltage detection circuit 30 gives a detection signal to the soft start circuit, The soft start circuit 20 soft-starts the drive circuit 13 to gently increase the output voltage Vout, thereby limiting the input current Iin. In addition, after the activation, the input voltage Vin detected by the input voltage detection circuit 30 becomes higher than the set voltage Vset, and the soft start circuit 20 stops its operation because no detection signal is given to the soft start circuit 20. Therefore, the output voltage Vout rises immediately.

  As a result, in the power supply circuit shown in FIG. 8, the soft start circuit 20 gradually increases the output voltage Vout at the start-up so that the input current Iin does not become excessive, and the boost operation is activated according to the luminance adjustment signal. When repeating / stop, the output voltage Vout can be immediately raised to a desired voltage.

  Further, in the power supply circuit shown in FIG. 8, the input voltage detection circuit 30 is configured by a circuit having hysteresis in the set voltage Vset as shown in FIG. 4, and the set voltage Vset is set to 4.2 V. When a lithium ion battery is used, it is possible to keep the soft start function working up to the maximum charging voltage of the lithium ion battery. In addition, by providing a hysteresis of 1.2V to the set voltage Vset and setting the voltage at which the soft start function works again after the output voltage Vout rises to 3.0V, the softening is achieved up to the lowest end voltage of the lithium ion battery. The start function can be disabled and the battery can be used effectively.

  Further, when the power supply circuit of the embodiment described above is mounted on an electronic device such as a mobile phone provided with the white light emitting diodes LED1 to 6, a current flowing in a battery such as a lithium ion battery built in the electronic device at the time of start-up is obtained. Limiting and using this battery up to the end voltage, an electronic device capable of adjusting the luminance of the white light emitting diodes LED1 to LED6 can be realized.

  In addition, this invention is not limited to the above-mentioned embodiment, In the range which does not deviate from the meaning of this invention, it is also possible to change suitably the structure of each part, etc. and to implement.

  As described above, according to the present invention, a coil connected to one terminal of a DC power supply, a switching element having one end connected to the coil and the other end connected to the other terminal of the DC power supply, A series circuit of a rectifying element and an output capacitor connected in parallel to the switching element, a series circuit of a load and an output current detection resistor connected in parallel to the output capacitor, and an output current flowing through the load are constant In a power supply circuit including a drive circuit that drives the switching element based on a feedback voltage from the output current detection resistor, the output voltage generated at the connection point between the rectifying element and the output capacitor is adjusted from the outside An operation / stop circuit for operating / stopping the driving operation of the switching element of the drive circuit according to an external input signal; An output voltage detection circuit that detects that the output voltage is higher than a predetermined voltage and outputs a detection signal, and a detection signal from the output voltage detection circuit when the external input signal is input Is not operated, and when not given, a soft start circuit is provided to control the drive circuit so that the output voltage rises slowly. By operating the soft start circuit and gradually increasing the output voltage, the current flowing from the DC power supply is prevented from becoming excessive. Once the output voltage rises, the drive circuit Even when the driving operation of the switching element is repeatedly activated / deactivated, the output voltage can be quickly adjusted by not operating the soft start circuit. It can be started up to.

  Further, by making an electronic device using such a power supply circuit, it is possible to limit a current from a DC power source built in the electronic device at the time of startup, and from the power supply circuit to the electronic device after startup. An electronic device capable of adjusting a voltage supplied to an internal load by a signal from the electronic device can be realized. Further, the load is a light emitting element of a backlight or a front light of a liquid crystal display device, and the operation / stop circuit is In the backlight or front light brightness adjustment circuit of the liquid crystal display device, the current flowing to a battery such as a lithium ion battery built in the electronic device is limited at the time of start-up, and the battery is used up to the end voltage. An electronic device capable of adjusting the brightness of the backlight or the front light of the liquid crystal display device by the power supply circuit There can be realized.

1 is a circuit block diagram showing an electrical configuration of a power supply circuit according to a first embodiment of the present invention. FIG. 2 is a circuit diagram showing an electrical configuration of an output voltage detection circuit shown in FIG. 1. It is a wave form diagram which shows the voltage waveform and current waveform of each part of the power supply circuit shown in FIG. FIG. 3 is a circuit diagram showing another electrical configuration of the output voltage detection circuit shown in FIG. 1. It is a circuit block diagram which shows the electric constitution of the power supply circuit of 2nd Embodiment of this invention. It is a circuit block diagram which shows the electrical constitution of the power supply circuit of 3rd Embodiment of this invention. It is a figure for demonstrating the overheat protection circuit shown in FIG. It is a circuit block diagram which shows the electric constitution of the power supply circuit of 4th Embodiment of this invention. It is a circuit block diagram which shows the electrical structure of the conventional power supply circuit. It is a wave form diagram which shows the voltage waveform and current waveform of each part of the power supply circuit shown in FIG. FIG. 10 is a waveform diagram showing a voltage waveform and a current waveform in another state of each part of the power supply circuit shown in FIG. 9.

Explanation of symbols

1 DC power supply 2 Input capacitor 3 Coil 4 Diode (rectifier element)
5 Output capacitor 10 Boost chopper regulator 11, 12 FET (switching element)
13 Drive circuit 14 Current detection comparator 15 Oscillation circuit 16 Amplifier 17 PWM comparator 18 Error comparators 19 and 26 Reference power supply 20 Soft start circuit 21 ON / OFF circuit (actuation / stop circuit)
22 Overheat protection circuit 23 Overvoltage protection circuit 24 Output voltage detection circuit 25, 27 Comparator 28 Feedback voltage detection circuit 29 Overvoltage protection circuit (output voltage detection circuit)
30 Input voltage detection circuit R1 Resistance (output current detection resistance)
R2 to R6 Resistors LED1 to LED6 White light emitting diode (load)

Claims (5)

An output capacitor connected to a predetermined power source and generating a voltage by being charged by the power source;
A coil provided between the power source and the output capacitor;
A rectifying element provided between the coil and the output capacitor;
Switch and an intermediate point between said coil and said rectifying element, and a ground point, a connection / disconnection, a switching element,
A power supply circuit that supplies an output voltage generated at an intermediate point between the rectifier element and the output capacitor to a predetermined load,
When a control signal in which the first state and the second state appear alternately is received and the control signal is in the first state, a boosting operation for boosting the output voltage is performed by switching the switching element, while the control signal is Drive means for preventing the step-up operation during the two states;
A soft start circuit that performs a soft start process that moderates the rise of the output voltage due to the boost operation; and
The soft start circuit
A power supply circuit that performs the soft start processing only at a predetermined timing according to the magnitude of the output voltage and the state of the control signal. The soft start circuit
Based on the magnitude of the output voltage and the state of the control signal, detecting that the control signal has transitioned to the first state when the output capacitor is not charged,
2. The power supply circuit according to claim 1, wherein the soft start process is performed only at a timing at which the boosting operation is performed immediately after the transition. The soft start circuit
The magnitude of the output voltage is compared with a predetermined threshold value, and based on the comparison result, it is determined whether or not the output capacitor is not charged. Power supply circuit. The switching element is to be switched according to a predetermined PWM signal generated by the drive means,
4. The power supply circuit according to claim 2, wherein the soft start circuit performs the soft start process through adjustment of a duty in the PWM signal. 5. A power supply circuit according to any one of claims 1 to 4,
An LED driving power supply circuit, wherein an LED is connected as the load, and power is supplied to the LED.

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