JP2013142940A - Power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power circuit capable of sufficiently feeding a current to an output section even when the output section is short-circuited to the ground potential.SOLUTION: A power circuit 1 comprises: a threshold voltage generation unit 2 for generating a threshold voltage Vth which increases when a voltage from the rectifier circuit increases, decreases when the voltage from the rectifier circuit decreases, and is fixed to a prescribed voltage when the voltage from the rectifier circuit is a prescribed value or higher; and an output control unit 3 which does not supply a voltage from the rectifier circuit to the output section when an output voltage Vo is higher than the threshold voltage Vth or the voltage from the rectifier circuit is higher than the threshold voltage Vth, supplies the voltage from the rectifier circuit to the output section when the output voltage Vo is not higher than the threshold voltage Vth and the voltage from the rectifier circuit is not higher than the threshold voltage Vth, and supplies the voltage from the rectifier circuit to the output section regardless of a magnitude relation between the voltage from the rectifier circuit and the threshold voltage Vth due to lowering of the output voltage when the output section is short-circuited to the ground potential.

Description

  The present invention relates to a power supply circuit.

  The following power supply circuit is described in Patent Document 1 below. The power supply circuit includes a rectifier circuit that rectifies an input AC voltage, a smoothing unit that smoothes the voltage from the rectifier circuit, and a threshold value that increases when the smoothed voltage increases and decreases when the smoothed voltage decreases. The threshold voltage generation unit that generates the voltage, the voltage from the rectifier circuit and the threshold voltage are compared, and when the voltage from the rectifier circuit is equal to or higher than the threshold voltage, the voltage from the rectifier circuit is not supplied to the output unit, When the voltage from the rectifier circuit is less than the threshold voltage, an output control unit that supplies the voltage from the rectifier circuit to the output unit, and an output unit that generates a DC voltage based on the voltage from the output control unit .

  When the AC voltage increases, the voltage value of the voltage from the rectifier circuit increases. Here, if the threshold voltage is constant, the period during which the voltage is supplied from the rectifier circuit to the output unit via the output control unit varies, the amount of charge supplied to the output unit changes, and the output voltage is a DC voltage. Fluctuates. However, the threshold voltage generated by the threshold voltage generation unit increases as the voltage obtained by smoothing the threshold voltage increases. Therefore, the amount of charge supplied to the output unit does not change, and the DC voltage that is the output voltage does not change. Similarly, when the AC voltage decreases, the threshold voltage generated by the threshold voltage generator decreases as the voltage obtained by smoothing the threshold voltage decreases. Therefore, the amount of charge supplied to the output unit does not change, and the DC voltage that is the output voltage does not change. As described above, according to the power supply circuit, a stable DC voltage can be output.

  However, since the power supply circuit of Patent Document 1 only compares the voltage from the rectifier circuit with the threshold voltage, the output voltage fluctuates when the current flowing from the capacitor of the output unit fluctuates. There is a problem of end. Even when the output capacitor (ie, load) is short-circuited to the ground potential, that is, when the capacitor charging voltage is 0 V (ground potential), the voltage from the rectifier circuit and the threshold voltage Since the current is supplied to the capacitor based only on the comparison result, the current cannot be sufficiently supplied to the output section, the protective fuse cannot be cut, and the protection cannot be activated. is there.

JP 2010-271554 A

  The present invention has been made to solve the above-described conventional problems, and a typical object of the present invention is to provide a power supply circuit capable of flowing a sufficient current to the output section even when the output section is short-circuited to the ground potential. It is to provide.

  A power supply circuit according to a preferred embodiment of the present invention is a power supply circuit that generates a DC voltage based on an input AC voltage, and rectifies the AC voltage, and increases as the voltage from the rectifier circuit increases. A threshold voltage generator that generates a threshold voltage that decreases when the voltage from the rectifier circuit decreases and that is fixed to a predetermined voltage when the voltage from the rectifier circuit is equal to or higher than a predetermined value; and an output of the power supply circuit When comparing the voltage with the threshold voltage and comparing the voltage from the rectifier circuit with the threshold voltage, and the output voltage of the power supply circuit is greater than the threshold voltage, or the voltage from the rectifier circuit Is larger than the threshold voltage, the voltage from the rectifier circuit is not supplied to the output unit, the output voltage from the power supply circuit is less than or equal to the threshold voltage, and from the rectifier circuit When the voltage is equal to or lower than the threshold voltage, the voltage from the rectifier circuit is supplied to the output unit, and when the output unit is short-circuited to the ground potential, the output voltage of the power supply circuit is reduced, thereby The output control unit supplies the voltage from the rectifier circuit to the output unit, and the output of the power supply circuit based on the voltage from the output control unit without being affected by the magnitude relationship between the voltage of the threshold voltage and the threshold voltage And an output unit that generates a DC voltage that is a voltage.

  According to the present embodiment, the output control unit is not affected by the magnitude relationship between the voltage from the rectifier circuit and the threshold voltage due to the decrease in the output voltage from the power supply circuit when the output unit is short-circuited to the ground potential. Supply the voltage from the circuit to the output. Therefore, when the output unit is short-circuited to the ground potential, a sufficient current can be supplied to the output unit, and the fuse can be cut and the protection can be activated. Further, the output control unit compares the output voltage of the power supply circuit with the threshold voltage, and controls whether to supply the voltage from the rectifier circuit to the output unit according to the comparison result. Even when the current flowing through the load fluctuates, the output voltage can be stabilized. Further, the output control unit compares the voltage from the rectifier circuit with the threshold voltage, and controls whether to supply the voltage from the rectifier circuit to the output unit according to the comparison result. Since the voltage from the rectifier circuit is fixed to the predetermined voltage, the voltage from the rectifier circuit is not supplied to the output unit during a period when the amplitude value of the voltage from the rectifier circuit is large. Therefore, power consumption can be reduced.

  In a preferred embodiment, the output control unit compares the output voltage of the power supply circuit with the threshold voltage, compares the voltage from the rectifier circuit with the threshold voltage, and sets the on state or the off state. A first transistor that changes, and a second transistor that changes to an on state or an off state in response to an on state or an off state of the first transistor and switches between supply and non-supply of the voltage from the rectifier circuit to the output unit. An output voltage of the power supply circuit and a voltage from the rectifier circuit are supplied to the base of the first transistor, and the threshold voltage is supplied to the emitter of the first transistor.

  In a preferred embodiment, the current flowing from the second transistor to the output unit is detected to change the second transistor to an off state, and the current flowing from the second transistor to the output unit is limited. And a third transistor.

  The third transistor determines the magnitude of the current flowing from the rectifier circuit to the output unit via the second transistor, and when it detects that the current is excessive, it changes the second transistor to the off state. To function. If a large current flows instantaneously, there are problems such as generation of radiation noise and an increase in power consumption. However, provision of the third transistor can prevent an excessively large current from flowing.

  It is possible to provide a power supply circuit capable of flowing a sufficient current to the output unit even when the output unit is short-circuited to the ground potential.

1 is a circuit diagram showing a power supply circuit according to a preferred embodiment of the present invention. It is a simulation result which shows operation | movement of the power supply circuit of this invention. It is a simulation result which shows operation | movement of the power supply circuit of this invention. It is a simulation result which shows operation | movement of the power supply circuit of this invention. It is a simulation result which shows operation | movement of the power supply circuit of this invention. FIG. 6 is a circuit diagram showing a power supply circuit according to another preferred embodiment of the present invention.

  Hereinafter, although preferable embodiment of this invention is described, this invention is not limited to these embodiment. FIG. 1 is a circuit diagram showing a power supply circuit 1 according to a preferred embodiment of the present invention. The power supply circuit 1 generally includes a full-wave rectifier circuit D1, a threshold voltage generation unit 2, an output control unit 3, and an output unit 4.

  The output control unit 3 compares the output voltage Vo of the power supply circuit 1 with the threshold voltage Vth generated by the threshold voltage generation unit 2, and determines the DC voltage Vd1 from the full-wave rectifier circuit D1 and the threshold voltage Vth. Compare. When the output voltage Vo is greater than the threshold voltage Vth or when the DC voltage Vd1 is greater than the threshold voltage Vth, the output control unit 3 does not supply a current based on the DC voltage Vd1 to the output unit 4 ( DC voltage Vd1 is not supplied to the output unit 4). The output control unit 3 supplies a current based on the DC voltage Vd1 to the output unit 4 when the DC voltage Vd1 is equal to or lower than the threshold voltage Vth and the output voltage Vo is equal to or lower than the threshold voltage Vth (outputs the DC voltage Vd1). Part 4).

  When the output unit 4 is short-circuited with respect to the ground potential, that is, when the output voltage Vo decreases to 0 V (ground potential), the output control unit 3 determines whether the DC voltage Vd1 from the rectifier circuit D1 and the threshold voltage Vth are large or small. A current based on the DC voltage Vd1 is supplied to the output unit 4 over a long period of time without being affected by the relationship (DC voltage Vd1 is supplied to the output unit 4).

  The threshold voltage Vth generated by the threshold voltage generator 2 increases as the DC voltage Vd1 increases, decreases as the DC voltage Vd1 decreases, and the DC voltage Vd1 from the rectifier circuit D1. Is fixed to a predetermined voltage when the value is equal to or higher than a predetermined value (that is, the threshold voltage Vth does not increase beyond the predetermined voltage).

  The full-wave rectifier circuit D1 has two input terminals connected to a secondary winding of a transformer (not shown), and full-wave rectifies the AC voltage input from the transformer to generate a DC voltage (full-wave rectified waveform) Vd1. And output. One output terminal of the full-wave rectifier circuit D1 is connected to the threshold voltage generator 2 and the output controller 3, and the other output terminal is connected to the ground potential. The fuse f is connected to one input terminal of the full-wave rectifier circuit D1.

  The threshold voltage generation unit 2 generates a threshold voltage Vth for determining whether or not the output control unit 3 supplies a current based on the DC voltage Vd1 from the full-wave rectifier circuit D1 to the output unit 4. The generated threshold voltage Vth is supplied to the emitter of the transistor Q1 of the output control unit 3. The threshold voltage Vth varies according to the voltage value of the DC voltage Vd1 from the full-wave rectifier circuit D1. That is, when the DC voltage Vd1 from the full-wave rectifier circuit D1 increases, the threshold voltage Vth generated by the threshold voltage generator 2 also increases, and when the DC voltage Vd1 from the full-wave rectifier circuit D1 decreases. The threshold voltage Vth generated by the threshold voltage generator 2 also decreases. Note that when the DC voltage Vd1 from the full-wave rectifier circuit D1 is equal to or higher than a predetermined voltage, the threshold voltage Vth is fixed without increasing beyond the predetermined voltage.

  The threshold voltage generation unit 2 includes a Zener diode D2 that is a constant voltage generation unit, and a resistor R1. One end of the resistor R1 is connected to the output of the full-wave rectifier circuit D1, and the other end is connected to the emitter of the transistor Q1 and the cathode of the Zener diode D2. The anode of the Zener diode D2 is connected to the ground potential. The threshold voltage Vth is a voltage obtained by subtracting the voltage across the resistor R1 from the DC voltage Vd1 output from the full-wave rectifier circuit D1, and varies depending on the variation of the DC voltage Vd1, but exceeds the Zener voltage of the Zener diode D2. Does not increase. Therefore, it increases or decreases between the Zener voltage of the Zener diode D2 and 0 V as the DC voltage Vd1 from the full-wave rectifier circuit D1 increases or decreases.

  The output control unit 3 compares the output voltage Vo of the power supply circuit 1 with the threshold voltage Vth generated by the threshold voltage generation unit 2, and determines the DC voltage Vd1 from the full-wave rectifier circuit D1 and the threshold voltage Vth. Compare. The output control unit 3 outputs a current based on the DC voltage Vd1 when the output voltage Vo is higher than the threshold voltage Vth or when the DC voltage Vd1 from the full-wave rectifier circuit D1 is higher than the threshold voltage Vth. 4 is not passed through the capacitor C2 (the DC voltage Vd1 is not supplied to the capacitor C2). On the other hand, when the output voltage Vo is equal to or lower than the threshold voltage Vth and the DC voltage Vd1 is equal to or lower than the threshold voltage Vth, the output control unit 3 supplies a current based on the DC voltage Vd1 to the capacitor C2 of the output unit 4 (DC Supply voltage Vd1 to capacitor C2.)

  When one end of the capacitor C2 of the output unit 4 is short-circuited with respect to the ground potential, that is, when the output voltage Vo drops to 0 V (ground potential), the output control unit 3 outputs the DC voltage Vd1 from the full-wave rectifier circuit D1. The current based on the DC voltage Vd1 is supplied to the output unit 4 over a long period without being affected by the magnitude relationship between the threshold voltage Vth and the threshold voltage Vth.

  The output control unit 3 compares the output voltage Vo with the threshold voltage Vth, compares the DC voltage Vd1 with the threshold voltage Vth, and outputs a comparison result, and the transistor Q1 And a transistor Q3 that switches supply / non-supply of the DC voltage Vd1 to the capacitor C2 in accordance with the comparison result. Output control unit 3 further includes resistors R2 to R5, a capacitor C1, and a transistor Q2.

  The base of the transistor Q1 is connected to one end of the capacitor C2 of the output unit 4 through the resistor R4 to be supplied with the output voltage Vo, and is connected to the output of the full-wave rectifier circuit D1 through the resistor R5. Vd1 is supplied. The transistor Q1 has an emitter connected to a connection point between the resistor R1 and the cathode of the Zener diode D2, is supplied with a threshold voltage Vth, and a collector is connected to the ground potential via the resistor R2. The transistor Q1 is turned off when the output voltage Vo is larger than the threshold voltage Vth or when the DC voltage Vd1 is larger than the threshold voltage Vth, and the output voltage Vo is equal to or lower than the threshold voltage Vth. In addition, the DC voltage Vd1 is turned on when it is equal to or lower than the threshold voltage Vth. Actually, since the on-state or off-state of the transistor Q1 is determined in consideration of the conduction start voltage of the transistor Q1 of 0.6 V and the voltage across the resistor R4 or R5, the threshold voltage generator 2 The threshold voltage Vth generated by adding the conduction start voltage 0.6V of the transistor Q1 and the voltage across the resistor R4 or R5 can be said to be a broad threshold voltage.

  The base of the transistor Q2 is connected to the collector of the transistor Q1 via the resistor R3, is connected to the ground potential via the resistor R2, and is connected to the ground potential via the capacitor C1, and the collector is connected via the resistor R6. The emitter of the transistor Q3 is connected to the ground potential.

  The base of the transistor Q3 is connected to the collector of the transistor Q2 via the resistor R6, the emitter is connected to one output terminal of the full-wave rectifier circuit D1, and the collector is connected to the anode of the diode D3 of the output unit 4. Yes. The transistor Q3 is turned on when the transistor Q1 is turned on, passes a current based on the DC voltage Vd1 from the full-wave rectifier circuit D1 to the capacitor C2 of the output unit 4, and is turned off when the transistor Q1 is turned off. Thus, the current based on the DC voltage Vd1 from the full-wave rectifier circuit D1 is not passed through the capacitor C2 of the output unit 4.

  The output unit 4 generates a smoothed DC voltage that is the output voltage Vo of the power supply circuit 1 by charging the capacitor C2 with a current supplied from the full-wave rectifier circuit D1 via the output control unit 3. The output unit 4 includes a diode D3 and a capacitor C2. The diode D3 has an anode connected to the collector of the transistor Q3 and a cathode connected to one end of the capacitor C2 and one end of the load RL. Capacitor C2 has one end connected to the cathode of diode D3, resistor R4, and one end of load RL, and the other end connected to the ground potential. The other end of the load RL is connected to the ground potential.

  The operation of the power supply circuit 1 having the above configuration will be described. FIG. 2 shows simulation results obtained by measuring the input AC voltage to the full-wave rectifier circuit D1, the output voltage Vo, and the current flowing from the transistor Q3 to the capacitor C2. Hereinafter, description will be made with reference to FIGS. 1 and 2.

  The full-wave rectifier circuit D1 performs full-wave rectification on an AC voltage input from a transformer (not shown), generates a DC voltage Vd1, and supplies the DC voltage Vd1 to the threshold voltage generator 2 and the output controller 3. The threshold voltage generator 2 generates a threshold voltage Vth based on the DC voltage Vd1 from the full-wave rectifier circuit D1, and supplies it to the emitter of the transistor Q1. The output voltage Vo, which is the charging voltage of the capacitor C2, is supplied to the base of the transistor Q1 via the resistor R4, and the DC voltage Vd1 from the full-wave rectifier circuit D1 is supplied via the resistor R5. The transistor Q1 compares the magnitude relationship between the output voltage Vo and the threshold voltage Vth, and compares the magnitude relationship between the DC voltage Vd1 and the threshold voltage Vth.

  In the period T1 in FIG. 2, since the amplitude value of the input AC voltage is small, the threshold voltage Vth generated from the input AC voltage is also small. Therefore, the output voltage Vo becomes larger than the threshold voltage Vth (actually, the conduction start voltage of the transistor Q1 and the voltage across the resistor R4 are considered, and so on). Therefore, the transistor Q1 is turned off because the base-emitter voltage has not reached the conduction start voltage. When the transistor Q1 is turned off, the transistor Q2 is turned off because the base is connected to the ground potential. Accordingly, when the transistor Q2 is turned off, the transistor Q3 is released from the ground potential, and thus is turned off.

  Therefore, when the transistor Q3 is turned off, a current based on the DC voltage Vd1 from the full-wave rectifier circuit D1 does not flow to the capacitor C2. As a result, during the period T1 when the output voltage Vo is greater than the threshold voltage Vth, the capacitor C2 is not charged, and the output voltage Vo is consumed by the load and gradually decreases.

  In the period T2, the threshold voltage Vth increases as the amplitude value of the input AC voltage increases. On the other hand, the output voltage Vo decreases due to the operation in the period T1. Therefore, the output voltage Vo is equal to or lower than the threshold voltage Vth, and the DC voltage Vd1 is equal to or lower than the threshold voltage Vth. Actually, it is necessary to consider the conduction start voltage of the transistor Q1 and the voltage across the resistor R5. The resistance value of the resistor R5 is determined based on the DC voltage Vd1 and the base of the transistor Q1 in the periods T2 and T4. Is set to a value such that the voltage supplied to the threshold voltage is equal to or lower than the threshold voltage Vth. Accordingly, the transistor Q1 is turned on when the base-emitter voltage reaches the conduction start voltage. When the transistor Q1 is turned on, the transistor Q2 is turned on because the base is connected to the emitter of the transistor Q1 and the threshold voltage Vth is supplied. Accordingly, when the transistor Q2 is turned on, the transistor Q3 is turned on because the base is connected to the ground potential.

  Accordingly, when the transistor Q3 is turned on, a current based on the DC voltage Vd1 from the full-wave rectifier circuit D1 flows to the capacitor C2 via the transistor Q3 and the diode D3, and the capacitor C2 is charged, and the output voltage Vo A certain DC voltage increases little by little.

  In the period T3, the threshold voltage Vth is fixed to the Zener voltage of the Zener diode D2 and does not increase by further increasing the amplitude value of the input AC voltage. However, the base of the transistor Q1 is supplied from the full-wave rectifier circuit D1 through the resistor R5. The DC voltage Vd1 supplied to the voltage continues to increase. Therefore, the DC voltage Vd1 is larger than the threshold voltage Vth. The transistor Q1 is turned off because the base-emitter voltage does not become the conduction start voltage. When the transistor Q1 is turned off, the transistor Q2 is turned off because the base is connected to the ground potential. Accordingly, when the transistor Q2 is turned off, the transistor Q3 is released from the ground potential, and thus is turned off.

  Therefore, when the transistor Q3 is turned off, a current based on the DC voltage Vd1 from the full-wave rectifier circuit D1 does not flow to the capacitor C2. As a result, during the period T3 when the DC voltage Vd1 is greater than the threshold voltage Vth, the capacitor C2 is not charged, and the output voltage Vo is consumed by the load RL and gradually decreases.

  In the period T4, the amplitude value of the input AC voltage decreases compared to the period T3, so that the DC voltage Vd1 supplied from the full-wave rectifier circuit D1 to the base of the transistor Q1 via the resistor R5 is smaller than that in the period T3. Become. The output voltage Vo is lowered by the operation in the period T3. Therefore, similarly to the period T2, the output voltage Vo is equal to or lower than the threshold voltage Vth, and the DC voltage Vd1 is equal to or lower than the threshold voltage Vth. Accordingly, the transistor Q1 is turned on when the base-emitter voltage becomes the conduction start voltage. When the transistor Q1 is turned on, the transistor Q2 is turned on because the base is connected to the emitter of the transistor Q1 and the threshold voltage Vth is supplied. Accordingly, when the transistor Q2 is turned on, the transistor Q3 is turned on because the base is connected to the ground potential.

  Accordingly, when the transistor Q3 is turned on, a current based on the DC voltage Vd1 from the full-wave rectifier circuit D1 flows to the capacitor C2 via the transistor Q3 and the diode D3, and the capacitor C2 is charged, and the output voltage Vo A certain DC voltage increases little by little.

  In the period T5, the amplitude value of the input AC voltage further decreases, and the value of the threshold voltage Vth decreases. Therefore, the output voltage Vo becomes larger than the threshold voltage Vth. Accordingly, the transistor Q1 is turned off because the base-emitter voltage does not become the conduction start voltage. When the transistor Q1 is turned off, the transistor Q2 is turned off because the base is connected to the ground potential. Accordingly, when the transistor Q2 is turned off, the transistor Q3 is released from the ground potential, and thus is turned off.

  Therefore, when the transistor Q3 is turned off, a current based on the DC voltage Vd1 from the full-wave rectifier circuit D1 does not flow to the capacitor C2. As a result, during the period T5 when the output voltage Vo is greater than the threshold voltage Vth, the capacitor C2 is not charged, and the output voltage Vo is consumed by the load RL and gradually decreases.

  As described above, according to the power supply circuit 1, not only the output voltage Vo but also the DC voltage Vd1 from the full-wave rectifier circuit D1 controls the on / off state of the transistor Q3 compared to the threshold voltage Vth. Further, the threshold voltage Vth is fixed to the Zener voltage of the Zener diode D2 when the input AC voltage increases to a predetermined value or more. Therefore, in the period T3 in which the amplitude value of the input AC voltage is very large, the transistor Q3 is turned off without being affected by the value of the output voltage Vo, and no current flows from the transistor Q3 to the capacitor C2. That is, according to the power supply circuit 1, the transistor Q3 is turned on only during a period in which the amplitude value of the input AC voltage is low, and current flows from the transistor Q3 to the capacitor C2, so that power consumption can be reduced.

  FIG. 3 shows simulation results obtained by measuring the input AC voltage to the full-wave rectifier circuit D1, the output voltage Vo, and the current flowing from the transistor Q3 to the capacitor C2 when the input AC voltage fluctuates. Va corresponds to Ia, Vb corresponds to Ib, and Vc corresponds to Ic. Even when the input AC voltage fluctuates, in the period T11 in which the amplitude value of the input AC voltage increases, it can be seen that the transistor Q3 is turned off and no current flows from the transistor Q3 to the capacitor C2. Therefore, as described above, power consumption can be reduced. Further, the output voltage Vo is compared with the threshold voltage Vth and the transistor Q3 is controlled to be turned on / off, so that it can be seen that the output voltage Vo can be kept constant even if the input AC voltage fluctuates.

  FIG. 4 shows a simulation result of measuring the input AC voltage to the full-wave rectifier circuit D1, the output voltage Vo, and the current flowing from the transistor Q3 to the capacitor C2 when the current flowing from the capacitor C2 to the load RL fluctuates. It is. When the current flowing from the capacitor C2 to the load RL fluctuates, the output voltage Vo tries to fluctuate. However, since the transistor Q3 is controlled on and off by comparing the output voltage Vo and the threshold voltage Vth, the output voltage Vo Variations can be suppressed. That is, when the current flowing from the capacitor C2 to the load RL increases, the voltage Vo of the capacitor C2 tends to decrease, but the period during which the transistor Q3 is turned on increases, and a large amount of current flows from the transistor Q3 to the capacitor C2. , A decrease in the voltage Vo of the capacitor C2 is suppressed. As a result, the output voltage Vo can be kept constant.

  FIG. 5 shows a full-wave rectifier circuit when one end of the capacitor C2 (one end of the load RL) is short-circuited and connected to the ground potential, that is, when the charging voltage of the capacitor C2, which is the output voltage Vo, becomes 0V. It is the simulation result which measured the input alternating voltage to D1, and the electric current which flows into the capacitor | condenser C2 from the transistor Q3. In this case, since the output voltage Vo is 0 V, the output voltage Vo is always equal to the threshold voltage regardless of the magnitude of the threshold voltage Vth, except for a few periods T31 and T33 in which the threshold voltage Vth is around 0 V. Vth or less, and the transistor Q1 is turned on. This is because the DC voltage Vd1 is about to be supplied to the base of the transistor Q1 via the resistor R5, but the capacitor C2 is short-circuited to the ground potential, so that the base of the transistor Q1 is brought to the ground potential via the resistor R4. This is because the base voltage of the transistor Q1 cannot be increased even by the DC voltage Vd1. Accordingly, the transistor Q3 is turned on, and a current flows from the transistor Q3 to the capacitor C2 over a long period of the period T32. Therefore, when the load RL is short-circuited, the fuse f can be blown and the protect operation can be activated.

  FIG. 6 is a circuit diagram showing a power supply circuit 11 according to another embodiment of the present invention. The same parts as those in FIG. Power supply circuit 11 further includes transistors Q4 and Q5 and resistors R7 and R8. In particular, by including the transistor Q5 and the resistor R7, it is possible to prevent the current flowing through the capacitor C2 via the transistor Q3 from becoming too large. The transistor Q4 is a transistor that transmits the on / off states of the transistors Q2 and Q5 to the transistor Q3.

  The transistor Q5 has a base connected to one end of the resistor R7 and the emitter of the transistor Q3, a collector connected to a connection point between the base of the transistor Q4 and the resistor R6, an emitter connected to the other end of the resistor R7, and a full-wave rectifier circuit. Connected to the output of D1.

  The transistor Q5 is supplied with a voltage obtained by subtracting a voltage generated at both ends of the resistor R7 from the DC voltage Vd1 output from the full-wave rectifier circuit D1 due to the current flowing to the capacitor C2 via the transistor Q3. Is supplied with the DC voltage Vd1 from the full-wave rectifier circuit D1. Therefore, when the current flowing through the capacitor C2 through the transistor Q3 becomes excessive, the voltage at the base of the transistor Q5 becomes smaller than the emitter voltage, and the transistor Q5 is turned on.

  When the transistor Q5 is turned on, the transistor Q4 is turned off, and as a result, the transistor Q3 is turned off. Therefore, the current flowing through the capacitor C2 through the transistor Q3 is cut off, and it is possible to prevent the current flowing through the capacitor C2 through the transistor Q3 from becoming too large. Actually, before the transistor Q5 is completely turned on, the current flowing to the capacitor C2 through the transistor Q3 is limited so that the condition for turning on the transistor Q5 is not satisfied, and the transistor Q5 is turned off. Return immediately.

  As described above, by limiting the current flowing through the capacitor C2 via the transistor Q3 so as not to increase to a predetermined value or more, radiation noise can be prevented and power consumption can be prevented from increasing. .

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment. The polarity of each transistor is not limited to the above embodiment.

  The present invention can be suitably employed in a power supply circuit of an audio device such as an amplifier.

DESCRIPTION OF SYMBOLS 1 Power supply circuit 2 Threshold voltage generation part 3 Output control part 4 Output part

Claims (3)

A power supply circuit that generates a DC voltage based on an input AC voltage,
A rectifier circuit for rectifying the AC voltage;
A threshold value that increases when the voltage from the rectifier circuit increases, decreases when the voltage from the rectifier circuit decreases, and generates a threshold voltage that is fixed to a predetermined voltage when the voltage from the rectifier circuit is equal to or higher than a predetermined value. A voltage generator;
When the output voltage of the power supply circuit is compared with the threshold voltage, and the voltage from the rectifier circuit is compared with the threshold voltage, and the output voltage of the power supply circuit is greater than the threshold voltage, or When the voltage from the rectifier circuit is larger than the threshold voltage, the voltage from the rectifier circuit is not supplied to the output unit, the output voltage from the power supply circuit is equal to or lower than the threshold voltage, and from the rectifier circuit When the voltage of the power supply circuit is equal to or lower than the threshold voltage, the voltage from the rectifier circuit is supplied to the output unit. An output control unit that supplies the voltage from the rectifier circuit to the output unit without being affected by the magnitude relationship between the voltage from the threshold voltage and the threshold voltage;
A power supply circuit comprising: the output unit that generates a DC voltage that is an output voltage of the power supply circuit based on a voltage from the output control unit. The output control unit compares the output voltage of the power supply circuit with the threshold voltage, compares the voltage from the rectifier circuit with the threshold voltage, and changes to an on state or an off state; A second transistor that changes to an on state or an off state in response to an on state or an off state of the first transistor and switches between supply and non-supply of the voltage from the rectifier circuit to the output unit,
The power supply according to claim 1, wherein an output voltage of the power supply circuit and a voltage from the rectifier circuit are supplied to a base of the first transistor, and the threshold voltage is supplied to an emitter of the first transistor. circuit.   A third transistor for detecting an excessive current flowing from the second transistor to the output unit, changing the second transistor to an off state, and limiting a current flowing from the second transistor to the output unit; The power supply circuit according to claim 2 provided.