JP2006141193A - Switching power supply apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply apparatus which can protect a circuit, when a fault occurs. <P>SOLUTION: A voltage VR2 is voltage-divided by resistors R5 and R6, a voltage is generated in the resistor R5, and a charge is started by an electrolytic capacitor C6 connected in parallel with the resistor R5. The voltage of the both ends of the electrolytic capacitor C6 is voltage-divided by resistors R3 and R4, and the voltage VR3 of the both ends of the resistor R3 is supplied as the base voltage of a transistor Q4. A time-constant circuit 8 is formed by the resistor R6 and the electrolytic capacitor C6, and the time duration, from the start of charging in the electrolytic capacitor C6 until the transistor Q4 turns on, is set to be about 1 sec. If the transistor Q4 is turned on, a current flows to resistors R7 and R8, a transistor Q6 will be turned on, a transistor Q1 will be turned off, and the oscillation of an oscillation circuit 3 is forcedly stopped. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a switching power supply device having a protection circuit.

  In recent years, switching power supplies have become increasingly electronic as control systems, have become lighter and smaller, and are widely used as power sources for electronic and electrical equipment. In these switching power supply devices, when an abnormality occurs, a circuit element is destroyed, or a protection circuit that protects the circuit is connected to prevent a fire. For example, when the temperature of the power supply device rises due to an overload or other factors and reaches an abnormal temperature, the operation of the power supply device is stopped (Patent Document 1).

  FIG. 2 is a block diagram showing a switching power supply device 100 which is an example of a conventional switching power supply device. The switching power supply device 100 includes an input power supply circuit 9, an activation circuit 2, an oscillation circuit 3, a drive circuit 4, switching elements Q2 and Q3, a switching transformer T1, a constant voltage circuit 5, and a temperature protection circuit 6. It is comprised by.

  The input power supply circuit 9 includes a plug P1 connected to a commercial AC power supply, a fuse F1, a power switch SW1, a diode D1 that rectifies 100V AC input from the power supply, and an electrolytic capacitor C1 that smoothes the rectified voltage. , C2. The power switch SW1 is a switch for turning on and off the switching power supply apparatus 100, and the fuse F1 is cut when an abnormal current flows through the AC power supply.

  The diode D1 directly rectifies input commercial alternating current (for example, voltage 100V), and the rectified voltage is smoothed by electrolytic capacitors C1 and C2. When the power supply voltage is AC 100V, the voltages of C1 and C2 are about 140V, respectively. Note that the voltage of the electrolytic capacitor C2 is VC2.

  The start-up circuit 2 includes resistors R1 and R2, a Zener diode ZD1, a transistor Q1, and a diode D2. The resistor R1 and the Zener diode ZD1 are connected in series, one end of the resistor R1 is connected to the positive electrode of the electrolytic capacitor C2, and one end of the Zener diode ZD1 is connected to the negative electrode of the electrolytic capacitor C2. The base of the transistor Q1 is connected to the connection point where the resistor R1 and the Zener diode ZD1 are connected. The resistor R2 is connected between the collector of the transistor Q1 and the positive electrode of the electrolytic capacitor C2, and the emitter of the transistor Q1. Is connected to a diode D2.

  When the power switch SW1 is turned on, the voltage VC2 is applied to the resistor R1 and the Zener diode ZD1 connected in series, current flows, and the voltage VZD1 is generated across the Zener diode ZD1. Thereby, the transistor Q1 operates as a series regulator, a current IR2 flows through the resistor R2, and a voltage of (VZD1-VBE (about 0.6 V)) is generated at the emitter of the transistor Q1. VBE is a base-emitter voltage. This current IR2 is supplied to the oscillation circuit 3 through the diode D2. At this time, a voltage VR2 is generated across the resistor R2 connected to the collector of the transistor Q1.

  The oscillation circuit 3 is configured to oscillate at a predetermined frequency, and oscillates with power supplied from the diode D2 of the starter circuit 2 or the diode D3 of the constant voltage circuit 5 as a direct current (DC) power supply. Further, the diode D6 is connected from the temperature protection circuit 6, and the diode D6 is grounded via the transistor Q6, so that the oscillation is stopped.

  The oscillation output of the oscillation circuit 3 is supplied to the drive circuit 4. This drive circuit 4 uses the power supplied through the diode D2 or the diode D3 as a DC power supply in the same manner as the oscillation circuit 3, and drives the switching elements Q2 and Q3 according to the oscillation signal supplied from the oscillation circuit 3, respectively. Generate two drive signals.

  The switching elements Q2 and Q3 are constituted by power MOS-FETs (MOS field effect transistors), the gate electrode of the switching element Q2 is connected to the drive circuit 4, and the source electrode is the minus of the electrolytic capacitor C2 of the input power supply circuit 9. A drain electrode is connected to the source electrode of the switching element Q3 and one end of the primary coil P1 of the switching transformer T1.

  In the switching element Q3, the gate electrode is connected to the other output of the drive circuit 4 similarly to the switching element Q2, the source electrode is connected to the drain electrode of the switching element Q2 and one end of the primary coil P1 of the switching transformer T1, The drain electrode is connected to the positive electrode of the electrolytic capacitor C1 of the input power supply circuit 9.

  In the switching transformer T1, a primary coil P1, a secondary coil S1, and a tertiary coil P2 are formed. One end of the primary coil P1 is connected to the drain electrode of the switching element Q2 and the source of the switching element Q3, and the other end is connected to the negative electrode of the electrolytic capacitor C1 of the input power supply circuit 9. The secondary coil S1 is a power output of the switching power supply 100, and is rectified and transmitted to the device. The tertiary coil P <b> 2 is a coil for forming a power source for the oscillation circuit 3 and the drive circuit 4.

  The constant voltage circuit 5 includes diodes D3 and D4, a series regulator IC1, and electrolytic capacitors C3, C4 and C5. The anode of the diode D4 is connected to one end of the tertiary coil P2, and the cathode is connected to the input terminal of the series regulator IC1. The series regulator IC1 is also called a so-called three-terminal regulator, and has an input terminal connected to the cathode of the diode D4, an output terminal connected to the anode of the diode D3, and a ground terminal connected to the other end of the tertiary coil. The cathode of the diode D3 is connected to the cathode of the diode D2 of the starting circuit 2, and is connected to the power supply of the oscillation circuit 3 and the drive circuit 4.

  The positive electrode of the electrolytic capacitor C3 is connected to the cathode of the diode D3, the positive electrode of the electrolytic capacitor C4 is connected to the output terminal of the series regulator IC1, and the positive electrode of the electrolytic capacitor C5 is connected to the input terminal of the series regulator IC1. The negative electrode is connected to the ground terminal of the series regulator IC1. Therefore, the output of the tertiary coil P2 is rectified and becomes a constant voltage.

  The cathode of the diode D3 is connected to the cathode of the diode D2, which is the output of the starting circuit 2, and forms an OR circuit. Since the voltage made constant by the constant voltage circuit 5 is set higher than the voltage supplied from the starting circuit 2, when the supply of the voltage from the constant voltage circuit 5 is started, the voltage from the starting circuit 2 is set. The supply of is stopped. On the other hand, when the voltage of the power supplied from the constant voltage circuit 5 becomes lower than the voltage supplied from the startup circuit 2 due to startup or some abnormality, the power is supplied from the startup circuit 2.

  As described above, when the oscillation circuit 3 is activated, if there is no abnormality thereafter, the oscillation circuit 3 stably oscillates, and the drive circuit 4 switches the switching elements Q2 and Q3, and the switching transformer T1 Power is supplied by the next coil S2.

  However, the switching power supply apparatus 100 has many heat generating components such as the switching elements Q2 and Q3 therein, and the temperature may rise abnormally due to overload or other factors. In such an abnormal case, the temperature protection circuit 6 is provided to stop the operation of the power supply device.

  The temperature protection circuit 6 includes resistors R20, R21, and R11, a thermistor TH20, a transistor Q6, and diodes D5 and D6. Switching elements Q2, Q3 are attached to a radiator (not shown), and the thermistor TH20 is attached to the radiator. Therefore, when the temperature of switching elements Q2, Q3 increases, the temperature of the thermistor TH20 also increases.

  The thermistor TH20 is connected in series with the resistor R20 between the positive electrode and the negative electrode of the electrolytic capacitor C2, and the resistors R21 and R11 are connected in series at both ends of the thermistor TH20, and the connection point between the resistors R21 and R11 is connected. Is connected to the base of the transistor Q6. The emitter of the transistor Q6 is connected to the negative electrode of the electrolytic capacitor C2, and the collector is connected to the cathodes of the diodes D5 and D6. The anode of the diode D5 is connected to the base of the transistor Q1 of the starting circuit 2, and the anode of the diode D6 is connected to the oscillation circuit 3.

  The thermistor TH20 is a positive temperature coefficient thermistor whose resistance value increases as the temperature rises. If the switching element Q2, Q3 rises for some reason and becomes abnormal, the resistance value of the thermistor TH20 increases. Become. When the resistance value of the thermistor TH20 increases, the potential at the base of the transistor Q6 increases, and the transistor Q6 changes from the off state to the on state.

Therefore, the base potential of the transistor Q1 of the starter circuit 2 decreases via the diode D5, and power supply from the starter circuit 2 to the oscillation circuit 3 and the drive circuit 4 is not performed. At the same time, the oscillation circuit 3 is forcibly stopped via the diode D6, switching is not performed, and power supply is stopped.
JP2003-88100A.

  In the switching power supply device described above, in the case of an abnormality in which the temperature in the device rises, the temperature protection circuit operates and protects the circuit, but in the case of other abnormality, the element that constitutes the start-up circuit is not protected And other circuit elements are destroyed or ignite or smoke. Other abnormalities and failures include, for example, failure of the switching elements Q2 and Q3, failure of the series regulator IC1, disconnection of the coil P1 or P2 of the switching transformer T1, failure of the oscillation circuit 3 or drive circuit 4.

  The present invention has been made to solve the above-described problems, and prevents an element constituting the start-up circuit and other circuit elements from being destroyed, ignited or smoked when an abnormality occurs. It aims at providing the switching power supply device which can be performed.

  In order to achieve this object, the switching power supply device according to claim 1 includes a switching element that switches a rectified input voltage, an oscillation circuit that oscillates at a predetermined frequency, and an output from the oscillation circuit. A drive circuit for driving the power source, and an output switched by the switching element is input to a primary coil to obtain a power output from the secondary coil, and a transformer having a tertiary coil and a voltage output from the tertiary coil of the transformer A constant voltage circuit that rectifies and supplies power as a power source of the oscillation circuit, and supplies power to the oscillation circuit when the power is turned on, and when the power is supplied to the oscillation circuit by the constant voltage circuit, the oscillation circuit And a start circuit for stopping the supply of power, wherein the start circuit supplies power to the oscillation circuit. When the time is longer than the predetermined time, and a protection circuit for controlling to stop the supply of power to the oscillation circuit.

  According to a second aspect of the present invention, there is provided the switching power supply device according to the first aspect, wherein the protection circuit has a time constant circuit in which the voltage gradually changes according to the power supplied from the starter circuit to the oscillation circuit. A switch circuit is provided that stops the supply of power to the oscillation circuit of the starter circuit when the voltage of the time constant circuit reaches a predetermined voltage.

  According to a third aspect of the present invention, there is provided the switching power supply device according to the second aspect, wherein the protection circuit has a time constant circuit in which the voltage gradually changes according to the power supplied from the starter circuit to the oscillation circuit. A switching circuit for stopping supply of power to the oscillation circuit of the start circuit and an oscillation stop circuit for stopping oscillation of the oscillation circuit when the voltage of the time constant circuit reaches a predetermined voltage. Yes.

  The switching power supply according to claim 4 is the switching power supply according to claim 2 or 3, wherein when the voltage of the time constant circuit reaches a predetermined voltage, the protection circuit holds the predetermined voltage. A latch circuit is provided.

  The switching power supply device according to claim 5 is the switching power supply device according to any one of claims 1 to 4, wherein the start-up circuit connects an input voltage to the collector side and supplies power from the emitter side as a power source of the oscillation circuit. And the protection circuit reduces the base voltage of the start switch element when the time for which the start circuit supplies power to the oscillation circuit is longer than a predetermined time. The supply of power to the oscillation circuit is stopped, and the protection circuit has a predetermined time during which the startup circuit supplies power to the oscillation circuit by a current supplied from the emitter of the startup switch element. Whether it is longer or not is detected.

  The switching power supply device according to claim 6 converts the rectified input voltage into a constant voltage lower than the input voltage in the switching power supply device according to any one of claims 1 to 5, and the converted voltage Is provided to the protection circuit.

  According to the switching power supply device of claim 1, a switching element that switches the rectified input voltage, an oscillation circuit that oscillates at a predetermined frequency, a drive circuit that drives the switching element by an output of the oscillation circuit, The output switched by the switching element is input to the primary coil, and the power output is obtained from the secondary coil. At the same time, the transformer having the tertiary coil and the voltage output from the tertiary coil of the transformer are rectified to serve as the power supply for the oscillation circuit. A constant voltage circuit that supplies power to the oscillation circuit when the power is turned on, and a start circuit that stops the supply of power to the oscillation circuit when power is supplied to the oscillation circuit by the constant voltage circuit If the start circuit supplies power to the oscillation circuit for a longer time than the predetermined time, supply power to the oscillation circuit. Since it has a protection circuit that controls it to stop, it is possible to prevent the elements constituting the start-up circuit and other circuit elements from being destroyed, ignited or smoked when an abnormality occurs. is there. Here, the abnormality means that the power supply from the constant voltage circuit is normal due to the failure of the switching element, the failure of the constant voltage circuit, the disconnection of the primary or tertiary coil of the switching transformer, the failure of the oscillation circuit or the drive circuit, etc. This is the case when it is no longer performed.

  According to the switching power supply device according to claim 2, in addition to the effect of the switching power supply device according to claim 1, the protection circuit has a time constant in which the voltage gradually changes according to the power supplied from the start-up circuit to the oscillation circuit. It has a circuit and has a switch circuit that stops the supply of voltage to the oscillation circuit of the start circuit when the voltage of the time constant circuit reaches a predetermined voltage, so it can detect abnormalities with a simple circuit configuration In addition, the operation of the starting circuit can be stopped and the elements constituting the starting circuit can be prevented from being damaged or ignited.

  According to the switching power supply device according to claim 3, in addition to the effect exhibited by the switching power supply device according to claim 2, the protection circuit has a time constant in which the voltage gradually changes according to the power supplied from the starting circuit to the oscillation circuit. And a switch circuit for stopping power supply to the oscillation circuit of the start circuit and an oscillation stop circuit for stopping oscillation of the oscillation circuit when the voltage of the time constant circuit reaches a predetermined voltage. Therefore, when an abnormality is detected, the operation of the startup circuit can be stopped and the oscillation of the oscillation circuit can be stopped. Therefore, breakage or ignition of the switching element or the like can be prevented.

  According to the switching power supply device according to claim 4, in addition to the effect exhibited by the switching power supply device according to claim 2 or 3, when the voltage of the time constant circuit reaches a predetermined voltage, the protection circuit Since the latch circuit that holds the voltage is provided, there is an effect that the operation of the starting circuit and the oscillation operation of the oscillation circuit can be stopped continuously. Therefore, when an abnormality occurs in the switching power supply device, the function can be completely stopped and safety can be ensured.

  According to the switching power supply device according to claim 5, in addition to the effect exhibited by the switching power supply device according to any one of claims 1 to 4, the starting circuit connects the input voltage to the collector side, and the oscillation circuit from the emitter side. And a protection circuit that reduces the base voltage of the startup switch element when the startup circuit supplies power to the oscillation circuit for longer than a predetermined time. The protection circuit detects whether or not the time for which the startup circuit supplies power to the oscillation circuit is longer than a predetermined time based on the current supplied from the emitter of the startup switch element. It is possible to reduce the withstand voltage of an element such as a transistor constituting a circuit that detects whether or not the time for supplying power to the circuit is longer than a predetermined time. Spacing of the wiring of the printed circuit board to be formed can be a low voltage specifications such that the narrowing. Therefore, the switching power supply device can be manufactured in a small size and at low cost as compared with the case of using a high breakdown voltage transistor or increasing the distance between the wirings of the printed circuit board so that there is no problem even at a high voltage. is there.

  According to the switching power supply device of the sixth aspect, in addition to the effect of the switching power supply device according to any one of the first to fifth aspects, the rectified input voltage is converted into a constant voltage lower than the input voltage. Since the protective circuit constant voltage circuit for supplying the converted voltage to the protection circuit is provided, the withstand voltage of the elements such as transistors constituting the protection circuit can be reduced, and the printed circuit board forming the circuit The wiring can be of low voltage. Therefore, the switching power supply device can be manufactured in a small size and at low cost as compared with the case where a high-breakdown-voltage transistor is used or the distance between printed circuit boards is increased so that no problem occurs even at a high voltage. There is.

  Hereinafter, a preferred first embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a block diagram showing an electrical configuration of a switching power supply device 1 according to the first embodiment of the present invention. The same parts as those of the conventional switching power supply device 100 described with reference to FIG. 2 are denoted by the same reference numerals, and the description thereof will be omitted, and only different parts will be described.

  The switching power supply device 1 according to the first embodiment includes a protection circuit 7. The protection circuit 7 includes a transistor Q4, resistors R3, R4, R5, R6, an electrolytic capacitor C6, a switch circuit formed by the transistor Q6, a diode D20, a resistor R20, and diodes D5, D6, a transistor Q5, and a resistor R7, And a latch circuit formed by R8 and R9.

  In the switch circuit, resistors R5 and R6 are connected in parallel with the resistor R2 of the starting circuit 2, and the negative electrode of the electrolytic capacitor is connected to the connection point between the resistors R5 and R6, and the connection point between the resistor R5 and the resistor R6 is opposite to the connection point. The positive electrode of the electrolytic capacitor C6 is connected to Further, a resistor R3 and a resistor R4 are connected in series between the plus electrode and the minus electrode of the electrolytic capacitor C6, and the base of the transistor Q4 is connected to a connection point between the resistor R3 and the resistor R4. The emitter of the transistor Q4 is connected to the positive electrode of the electrolytic capacitor C2 of the input power supply, and is connected to the opposite side of the connection point of the resistor R3 to the resistor R4.

  The collector of the transistor Q4 is connected to the negative electrode of the electrolytic capacitor C2 via the resistors R7 and R8 and to the anode of the diode D20. The cathode of the diode D20 is connected to the transistor Q6 via the resistor R20. The resistor R21 of the temperature protection circuit 6 is connected to the base of the transistor Q6, and an OR circuit is formed with the resistor R20.

  Next, the operation of this switch circuit will be described. As described above, when the power switch SW1 is turned on, Q1 operates as a regulator and supplies power to the oscillation circuit 3 and the drive circuit 4 via the resistor R2. When the oscillation circuit 3 starts oscillating and the drive circuit 4 drives the switching elements Q2 and Q3, a signal switched to the primary coil of the transformer T1 is input, and a voltage is generated in the secondary coil and the tertiary coil. The tertiary coil is connected to the constant voltage circuit 5, and the constant voltage electric power is supplied to the oscillation circuit 3 and the drive circuit 4. Since the constant voltage is set higher than the voltage supplied by the starting circuit 2, the supply of power from the starting circuit 2 is stopped. The time from the supply of power to the oscillation circuit 3 and the drive circuit 4 to the stop by the start circuit 2 is set to be within 100 msec.

  This switch circuit stops the supply of power to the oscillation circuit 3 and the drive circuit 4 by the start circuit 2 when about 1 second or more has elapsed from the supply of power to the oscillation circuit 3 and the drive circuit 4 by the start circuit 2. At the same time, the oscillation operation of the oscillation circuit 3 is set to be stopped.

  When power is supplied to the oscillation circuit 3 and the drive circuit 4 by the starting circuit 2, a voltage VR2 is generated across the resistor R2. This voltage VR2 is divided by the resistors R5 and R6, a voltage is generated in the resistor R5, and charging is started in the electrolytic capacitor C6 connected in parallel to the resistor R5. The voltage across the electrolytic capacitor C6 is divided by the resistors R3 and R4, and the voltage VR3 across the resistor R3 is supplied as the base voltage of the transistor Q4.

  A time constant circuit 8 is formed by the resistor R6 and the electrolytic capacitor C6, and the time from when charging of the electrolytic capacitor C6 is started to when the transistor Q4 is turned on is set to about 1 second.

  When the transistor Q4 is turned on, current flows through the resistors R7 and R8 connected to the collector, the base voltage of the transistor Q6 rises through the diode D20 and the resistor R20 connected to the collector, the transistor Q6 is turned on, and the temperature As in the case of the protection circuit 6, the transistor Q1 is turned off, the starting operation of the starting circuit 2 is stopped, and the oscillating operation of the oscillation circuit 3 is forcibly stopped.

  Next, the latch circuit will be described. A resistor R7 and a resistor R8 are connected in series between the collector of the transistor Q4 of the switch circuit and the negative electrode of the electrolytic capacitor C2 of the input power supply circuit 9, and the base of the transistor Q5 is connected to the connection point of the resistors R7 and R8. It is connected. The emitter of the transistor Q5 is connected to the negative electrode of the electrolytic capacitor C2, the collector is connected to the base of the transistor Q4 and the resistor R3 via the resistor R9, and the other end of the resistor R3 is connected to the positive electrode of the electrolytic capacitor C2. It is connected.

  Next, the operation of this latch circuit will be described. When the switch circuit operates at the time of abnormality, the transistor Q1 is turned off, the current does not flow through the resistor R2, the voltage VR2 decreases, and the voltage VR3 eventually becomes about 0.6 volts or less. As a result, the transistors Q4 and Q6 are turned off, and the activation circuit 2 is activated again. In order to prevent this, the protection operation is latched by the transistor Q5.

  When the transistor Q4 is turned on, a current flows through the resistors R7 and R8, the transistor Q6 is turned on, and a voltage is generated at both ends of the resistor R8. Therefore, the transistor Q5 is turned on with a slight delay before the transistor Q6 is turned on. When the transistor Q5 is turned on, a current flows through the resistors R9 and R3, and the transistor Q4 is kept on by the voltage drop caused by the resistor R3. Therefore, even if the transistor Q1 is turned off by the transistor Q6, the starting circuit 2 is stopped until the power supply of the power supply device 1 is cut off.

  The voltage generated in the thermistor TH20 of the temperature protection circuit 6 is generated by the resistors R7 and R8 connected to the base of the transistor Q6 via the diode D21 and the resistor R21 and connected to the collector of the transistor Q4 of the protection circuit 7. The voltage is connected to the base of the transistor Q6 via the diode D20 and the resistor R20, and is configured such that the transistor Q6 is turned on regardless of whether the protection circuit 7 operates or the temperature protection circuit 6 operates. ing.

  As described above based on the first embodiment, in the switching power supply device 1 according to the present invention, the start-up circuit 2 supplies power to the oscillation circuit 3 when the power is turned on, and the oscillation circuit 3 starts to oscillate. Then, the switching elements Q2 and Q3 perform switching, and the voltage generated in the transformer by the switching is rectified by the constant voltage circuit 5 and supplied to the oscillation circuit 3 as a power source.

  When the supply of power to the oscillation circuit 3 is started by the constant voltage circuit 5, the supply of power from the starting circuit 2 is stopped, but the voltage supplied from the constant voltage circuit 5 drops due to some abnormality. Then, the starting circuit 2 starts supplying power to the oscillation circuit 3.

  The protection circuit 7 stops the supply of power to the oscillation circuit 3 of the activation circuit 2 and the oscillation of the oscillation circuit 3 when detecting that the activation circuit 2 has supplied power to the oscillation circuit 3 for a predetermined time or more. To do. Therefore, it is possible to prevent damage to circuit elements such as the transistor Q1 constituting the starting circuit 2, and to prevent other parts from being affected by being left in an abnormal state.

  Next, a second preferred embodiment of the present invention will be described with reference to FIG. FIG. 3 is a block diagram showing an electrical configuration of the switching power supply device 20 according to the second embodiment of the present invention. The same parts as those of the first switching power supply device 1 described with reference to FIG. 1 are denoted by the same reference numerals, description thereof is omitted, and only different parts will be described.

  In the first embodiment, the commercial AC input is set to 100 V. However, outside of Japan, an AC power supply exceeding 200 V is supplied. When such a high-voltage AC is input, the transistor has a high breakdown voltage. In addition to having to use a device, it is necessary to increase the distance between the lines of the printed circuit board, which increases the size and cost.

  Therefore, in the second embodiment, a portion to which a high voltage is applied is received by a resistor in a protection circuit or the like, and is operated at a low voltage. In the input power supply circuit 9, the plug P1 connected to the commercial AC power supply, the fuse F1, the power switch SW1, the diode D1 that rectifies 100V to 240V AC input from the power supply, and the rectified voltage are smoothed. It is comprised by the electrolytic capacitors C1 and C2. When the power supply voltage is 240V AC, the voltage between the positive electrode of the electrolytic capacitor C1 and the negative electrode of the electrolytic capacitor C2 is about 340V. This voltage is switched by the switching elements Q2, Q3.

  A connection point between the negative electrode of the electrolytic capacitor C1 and the positive electrode of the electrolytic capacitor C2 is connected to one end of the primary coil P1 of the switching transformer T1 via the capacitor 8. The capacitor 8 is called a current resonance capacitor and reduces the generation of EMI noise, and is omitted in the first embodiment.

  The start-up circuit 2 includes resistors R1 and R2, a Zener diode ZD1, and diodes D2 and D7 connected in series with the transistor Q1. The resistor R1 and the Zener diode ZD1 are connected in series, one end of the resistor R1 is connected to the positive electrode of the electrolytic capacitor C1, and one end of the Zener diode ZD1 is connected to the negative electrode of the electrolytic capacitor C2. The base of the transistor Q1 is connected to the connection point where the resistor R1 and the Zener diode ZD1 are connected. The resistor R2 is connected between the collector of the transistor Q1 and the positive electrode of the electrolytic capacitor C1, and the emitter of the transistor Q1. The diodes D2 and D7 are connected in series, and the cathode of the diode D7 is connected to the power supply of the oscillation circuit 3. Since the operation of the starting circuit 2 is the same as that of the first embodiment, the description thereof is omitted.

  Protection circuit 7 includes a switch circuit formed by transistor Q4 and resistors R3, R4, R6 and electrolytic capacitor C6, resistors R17, R18, R20, R11, diode D20, electrolytic capacitor C7, transistor Q6 and diodes D5, D6. A latch circuit formed by a transistor Q5 and resistors R11, R12, R13, and R14, a resistor R15, and a Zener diode ZD2.

  Next, the configuration of the switch circuit will be described. In the switch circuit, a resistor R6 and an electrolytic capacitor C6 are connected in parallel with the diodes D2 and D7 of the starting circuit 2, and one end of the resistor R4 is connected to a connection point between the resistor R6 and the negative electrode of the electrolytic capacitor C6. The other end of the resistor is connected to one end of the resistor R3 with the positive electrode of the electrolytic capacitor C6 and the other end of the resistor R3. One end of the resistor R3 is connected to the base of the transistor Q4, the other end of the resistor R3 is connected to the emitter of the transistor Q4, and one end of the resistor R17 is connected to the collector of the transistor Q4.

  The resistor R17 connected to the collector of the transistor Q4 is connected to one end of the resistor R18, and the other end of the resistor R18 is connected to the negative electrode of the electrolytic capacitor C2. One end of the resistor R18 is connected to the positive electrode of the electrolytic capacitor C7 and the anode of the diode D20. The negative electrode of the electrolytic capacitor C7 is connected to the negative electrode of the electrolytic capacitor C2, the cathode of the diode D20 is connected to one end of the resistor R20, and the other end of the resistor R20 is connected to the base of the transistor Q6 and one end of the resistor R11. ing. The other end of resistor R11 and the emitter of transistor Q6 are connected to the negative electrode of electrolytic capacitor C2, and the collector of transistor Q6 is connected to the cathodes of diodes D5 and D6 and to one end of resistor R12.

  The anode of the diode D5 is connected to the base of the transistor Q1 of the starting circuit 2, and the anode of the diode D6 is connected to a terminal for controlling the oscillation of the oscillation circuit 3.

  Next, the operation of this switch circuit will be described. Similarly to the switch circuit in the first embodiment, this switch circuit is also an oscillation circuit by the startup circuit 2 when about 1 second or more has elapsed from the start of power supply to the oscillation circuit 3 and the drive circuit 4 by the startup circuit 2. 3 and the drive circuit 4 are set to stop supplying power.

  When power is supplied to the oscillation circuit 3 and the drive circuit 4 by the starting circuit 2, a voltage VD (about 1.2 V) is generated across the diodes D2 and D7. This voltage VD is supplied to the time constant circuit 8 formed by the resistor R6 and the electrolytic capacitor C6, and charging of the electrolytic capacitor C6 is started, and the voltage at both ends thereof gradually increases. The voltage across the electrolytic capacitor C6 is divided by the resistors R3 and R4. When the voltage across the resistor R3 is supplied between the base and emitter of the transistor Q4, and the base-emitter voltage exceeds a predetermined voltage, Transistor Q4 is turned on.

  When the transistor Q4 is turned on, a current flows through the resistors R17 and R18, is delayed for a predetermined time by a low-pass filter (time constant circuit) 10 formed by the resistor R17 and the electrolytic capacitor C7, and the resistor R20 and the resistor R11 are connected via the diode D20. Is applied to the base of the transistor Q6, turning on the transistor Q6. When the transistor Q6 is turned on, the start-up operation of the start-up circuit 2 is stopped by setting the base voltage of the transistor Q1 to 0 through the diode D5, the voltage at the oscillation control terminal of the oscillation circuit 3 is set to 0, and the oscillation of the oscillation circuit 3 is Stopped.

  Next, the configuration of the latch circuit will be described. One end of the resistor R12 is connected to the collector of the transistor Q6 and the cathodes of the diodes D5 and D6, and the other end of the resistor R12 is connected to the base of the transistor Q5 and one end of the resistor R13. The other end of the resistor R13 is connected to the emitter of the transistor Q5, one end of the resistor R15, and the cathode of the Zener diode ZD2. The other end of the resistor R15 is connected to the positive electrode of the electrolytic capacitor C1, and the anode of the Zener diode ZD2 is connected to the electrolytic capacitor. Each is connected to the negative electrode of C2. Therefore, a low constant voltage set by the Zener diode ZD2 is supplied to the emitter of the transistor Q5.

  One end of the thermistor TH20 of the temperature protection circuit 6 is connected to the anode of the diode D21, and the cathode of the diode D21 is connected to one end of the resistor R21. The other end of the resistor R21 is connected to the base of the transistor Q6, and the connection point between the base of the transistor Q6 and the resistor R21 is connected to the collector of the transistor Q5 via the resistor R14.

  Next, the operation of the latch circuit will be described. As in the first embodiment, this latch circuit also holds the state of the switch circuit when the switch circuit operates in the event of an abnormality.

  When the transistor Q6 is turned on at the time of abnormality, a voltage set by the Zener diode ZD2 is applied to its collector via the resistors R12 and R13, so that a voltage is generated at both ends of the resistor R13. Q5 turns on. When the transistor Q5 is turned on, the voltage set by the Zener diode ZD2 is supplied to the emitter of the transistor Q5, so that the resistor R14 connected to the collector of the transistor Q5 and the resistor R11 connected to the resistor R14 The divided voltage is supplied to the base of the transistor Q6, and the transistor Q6 is held on. Therefore, once the transistor Q6 is turned on, the transistor Q6 is kept on until the power supply of the switching power supply 10 is cut off.

  In the second embodiment, the two time constant circuits 8 and 10 are provided so that the protection circuit 7 stops the starting operation when the starting circuit 2 performs the starting operation for about 1 second or more. I am going to do it. Here, for example, if the capacitor C6 is a large-capacity capacitor, the time constant circuit can be made one.

  As described above based on the second embodiment, the starting circuit 2 includes the transistor Q1 that connects the input voltage to the collector side and supplies power from the emitter side as the power source of the oscillation circuit 3, and includes a protection circuit. 7 lowers the base voltage of the transistor Q1 when the time for which the starting circuit 2 supplies power to the oscillation circuit 3 is longer than a predetermined time. The protection circuit 7 detects whether or not the time for which the starting circuit 2 supplies power to the oscillation circuit 3 is longer than a predetermined time by the current supplied from the emitter of the transistor Q1. Therefore, it is possible to reduce the withstand voltage of an element such as a transistor that constitutes a circuit that detects whether or not the time for which the starting circuit 2 supplies power to the oscillation circuit 3 is longer than a predetermined time, and the printed circuit board on which the circuit is formed It is possible to achieve low voltage specifications such as narrowing the wiring interval.

  Further, since the rectified input voltage is converted into a constant voltage lower than the input voltage, and the Zener diode ZD2 that supplies the converted voltage to the protection circuit 7 is provided, the transistor Q4 constituting the protection circuit 7 is provided. , Q5 and Q6 can be of a type having a low withstand voltage, and can be of a low voltage with a short distance between wiring lines of a printed circuit board forming a circuit. Therefore, it can be avoided that the switching power supply device becomes large and expensive.

  As described above, the present invention has been described based on the embodiments, but the present invention is not limited to the above-described embodiments, and various improvements and modifications can be easily made without departing from the spirit of the present invention. It can be guessed.

  For example, in the above embodiment, when the time for which the starting circuit 2 supplies power to the oscillation circuit 3 is longer than a predetermined time, the transistor Q1 is turned off and the supply of power to the oscillation circuit 3 is stopped. A relay may be connected between the emitter of Q1 and the diode D2, and the relay may be disconnected when the start-up circuit 2 supplies power to the oscillation circuit 3 for longer than a predetermined time.

It is a block diagram which shows the electric constitution of the switching power supply device of this invention. It is a block diagram which shows the electric constitution of the switching power supply device in a prior art. It is a block diagram which shows the electric constitution of the switching power supply device in 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Switching power supply device 2 Start-up circuit 3 Oscillation circuit 4 Drive circuit 5 Constant voltage circuit 6 Temperature protection circuit 7 Protection circuit 8 Time constant circuit 100 Switching power supply devices D5 and D6 Diode (a part of switch circuit)
Q2, Q3 Field effect transistor (switching element)
Q4 transistor (part of the switch circuit)
Q5 transistor (part of latch circuit)
R3, R4, R5, R6 resistors (part of the switch circuit))
R7, R8, R9 resistors (part of latch circuit)
R12, R13, R14 resistors (part of latch circuit)
T1 switching transformer (transformer)

Claims (6)

A switching element that switches the rectified input voltage, an oscillation circuit that oscillates at a predetermined frequency, a drive circuit that drives the switching element by the output of the oscillation circuit, and an output that is switched by the switching element as a primary coil A transformer having a tertiary coil, a constant voltage circuit for rectifying a voltage output from the tertiary coil of the transformer and supplying power as a power source of the oscillation circuit, and a power source In a switching power supply device comprising a start circuit for supplying power to the oscillation circuit when the power is turned on and stopping power supply to the oscillation circuit when power is supplied to the oscillation circuit by the constant voltage circuit,
A switching power supply device comprising: a protection circuit that controls the oscillation circuit to stop supplying power when the start-up circuit supplies power to the oscillation circuit for longer than a predetermined time.   The protection circuit has a time constant circuit in which the voltage gradually changes according to the power supplied from the start circuit to the oscillation circuit, and when the voltage of the time constant circuit reaches a predetermined voltage, the start circuit The switching power supply device according to claim 1, further comprising a switch circuit that stops supply of power to the oscillation circuit.   The protection circuit has a time constant circuit in which the voltage gradually changes according to the power supplied from the start circuit to the oscillation circuit, and when the voltage of the time constant circuit reaches a predetermined voltage, the start circuit 3. The switching power supply device according to claim 2, further comprising: a switch circuit that stops supply of power to the oscillation circuit; and an oscillation stop circuit that stops oscillation of the oscillation circuit.   4. The switching power supply device according to claim 2, wherein the protection circuit includes a latch circuit that holds a predetermined voltage when the voltage of the time constant circuit reaches a predetermined voltage. The start circuit includes a start switch element that connects an input voltage to the collector side and supplies power from the emitter side as a power source of the oscillation circuit, and the protection circuit supplies the power to the oscillation circuit by the start circuit. When the supply time is longer than a predetermined time, the supply of power to the oscillation circuit is stopped by reducing the base voltage of the start switch element,
The protection circuit is configured to detect whether a time during which the start-up circuit supplies power to the oscillation circuit is longer than a predetermined time based on a current supplied from an emitter of the start-up switch element. The switching power supply device according to claim 1.   A protective circuit constant voltage circuit is provided for converting the rectified input voltage into a constant voltage lower than the input voltage and supplying the converted voltage to the protection circuit. The switching power supply device according to any one of 5.

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