JP4743710B2 - Power circuit - Google Patents

Description

  The present invention relates to a power supply circuit that can output a voltage without fusing a fuse even when a short circuit occurs between the input and output of a regulator.

  A part of the power supply circuit 20 used in an air conditioner is shown in FIG. A rectifier circuit 14 is connected to the secondary coil NS of the transformer T. A regulator 16 is connected between the rectifier circuit 14 and the output terminal X. The voltage is stepped down to a desired voltage by the regulator 16. For example, the voltage is stepped down from 15V to 5V. An air conditioner microcomputer 18 is connected to the output terminal X to control the operation of the air conditioner. The Zener diode Z1 operates as a stabilization circuit, and changes the duty ratio (on / off ratio), frequency, or both for switching the voltage applied to the primary coil NP of the transformer T by feedback to the control circuit 22. Adjust. That is, the control circuit 22 adjusts the duty ratio, the frequency, or both of the switching of the voltage to the primary coil NP according to the potential of the terminal A. By this adjustment, the output voltage of the secondary coil NS is adjusted.

  When the I terminal and O terminal of the regulator 16 are short-circuited, the voltage at the terminal A is applied to the microcomputer 18 as it is. A voltage more than double the rating is applied to the microcomputer 18. The temperature of the microcomputer 18 rises and the microcomputer 18 is destroyed. In order to protect the microcomputer 18, a fuse F is inserted between the rectifier circuit 14 and the regulator 16, and the fuse F is blown to protect the microcomputer 18.

  As a cause of the short circuit of the regulator 16, dust adheres to the regulator 16. In such a case, if the dust is removed, the regulator 16 need not be replaced. However, since the fuse F is blown, the power supply circuit 20 cannot be restored unless the fuse F is replaced.

  When the fuse F is blown, the voltage at the output terminal X is 0V. The power supply to the microcomputer 18 stops and the air conditioner stops. The user of the air conditioner will determine the failure of the air conditioner himself and contact the maintenance company. When the user cannot determine the failure of the air conditioner, the power button is repeatedly turned on and off. In such a case, if the failure can be notified by an LCD or LED provided in the air conditioner, the convenience for the user is improved. However, since the power supply to the microcomputer 18 is stopped by blowing the fuse F, the display device 24 such as an LCD cannot display. Although the display of the display device 24 can be ensured by providing the battery, the battery needs to be replaced, which may be inconvenient.

  Patent Document 1 below discloses a power supply circuit that does not use the fuse F. Since the load abnormality is detected, when the constant voltage circuit (regulator) is short-circuited, the input voltage of the regulator is applied to the load. That is, Patent Document 1 is not a circuit corresponding to a short circuit of the regulator.

Japanese Patent Laid-Open No. 10-214123

  An object of the present invention is to provide a power supply circuit that can be easily restored after a short circuit between the input and output of a regulator. Another object of the present invention is to provide a power supply circuit configured to output a voltage that does not destroy a load when a short circuit occurs between the input and output of a regulator.

  The power supply circuit of the present invention includes a transformer, a control circuit that adjusts a duty ratio, a frequency, or both for switching the voltage of the primary coil of the transformer, and a rectifier circuit connected to the secondary coil of the transformer. A regulator connected to the rectifier circuit for stepping down a voltage; a first Zener diode that feeds back an output voltage of the rectifier circuit from a contact between the rectifier circuit and the regulator to the control circuit; and And a second Zener diode that feeds back the output voltage of the regulator from the output terminal to the control circuit.

  The present invention can protect a load without a fuse even when the input / output terminals of the regulator are short-circuited. Since no expensive fuse is used, the cost can be reduced. The upper limit value of the voltage applied to the load by the second Zener diode can be limited. Even if the regulator is short-circuited, a predetermined voltage is output without setting the output voltage to 0. Therefore, it is possible to operate the device using the voltage and notify the abnormality.

  A power supply circuit according to the present invention will be described with reference to the drawings. Although the load connected to the power supply circuit will be described as a microcomputer for an air conditioner, it may be used for other electric devices.

  The power supply circuit 10 shown in FIG. 1 includes a transformer T, a control circuit 12 that adjusts a duty ratio for switching a voltage applied to the primary coil NP of the transformer T, a frequency, or both, and a secondary coil of the transformer T. The output voltage of the rectifier circuit 14 is transferred from the rectifier circuit 14 connected to the NS, the regulator 16 connected to the rectifier circuit 14 and a voltage step-down, and the terminal A between the rectifier circuit 14 and the regulator 16 to the control circuit 12. A first Zener diode Z1 that feeds back and a second Zener diode Z2 that feeds back the output voltage of the regulator 16 from the output terminal B of the regulator 16 to the control circuit 12 are included.

  The transformer T is a flyback type transformer. The rectifier circuit 14 is a circuit that uses a half-wave rectifier diode D1 and a smoothing capacitor C1, and converts AC voltage to DC voltage.

  The regulator 16 which is a constant voltage circuit steps down the output voltage of the rectifier circuit 14 to the rated voltage of the microcomputer 18. For example, the voltage is stepped down from 15V to 5V. When normal, this voltage of 5 V is applied from the output terminal X to the microcomputer 18. The regulator 16 also functions to stabilize the unstable rectified voltage of the rectifier circuit 14. Capacitors C2 and C3 and a diode D2 are connected to the regulator 16 to stabilize the regulator 16 and prevent oscillation.

  The anodes of the first and second Zener diodes Z1 and Z2 are connected to the control circuit 12. Feedback is performed to the control circuit 12 from the first Zener diode Z1 and the second Zener diode Z2. The control circuit 12 is protected by the resistor R. Although the anodes of the first and second Zener diodes Z1 and Z2 are connected to each other, they may be separated and connected to the control circuit 12. Further, separate control circuits 12 may be connected to the respective Zener diodes Z1 and Z2 and fed back to each.

  In the present invention, instead of inserting the fuse F between the rectifier circuit 14 and the regulator 16, a second Zener diode Z2 for feeding back the voltage at the terminal B on the output side of the regulator 16 is provided. Therefore, a voltage higher than the breakdown voltage of the second Zener diode Z2 does not appear at the output terminal X, and functions as a protection circuit for the microcomputer 18. The voltage at terminals A and B to which the cathodes of the first and second Zener diodes Z1 and Z2 are connected will be described as a breakdown voltage.

  The breakdown voltage of the second Zener diode Z2 is set lower than the breakdown voltage of the first Zener diode Z1. Furthermore, the breakdown voltage of the second Zener diode Z2 is set higher than the output voltage during normal operation of the regulator 16. For example, the breakdown voltage of the first Zener diode Z1 is 15V, the breakdown voltage of the second Zener diode Z2 is 7V, and the output voltage of the regulator 16 is 5V. The output of the rectifier circuit 14 is 15V. Thus, during normal operation, the first Zener diode Z1 operates to provide feedback. When the terminal I and the terminal O of the regulator 16 are short-circuited, the second Zener diode Z2 operates to provide feedback. This operation will be described in detail later.

  The control circuit 12 varies the duty ratio, the frequency, or both of the switching with respect to the voltage applied to the primary coil NP of the transformer T depending on the voltage fed back from the first Zener diode Z1 or the second Zener diode Z2. The control circuit 12 adjusts the output voltage of the secondary side NS of the transformer T to be lowered at the time of feedback from the second Zener diode Z2. At the time of feedback from the second Zener diode Z2, the output voltage of the secondary side NS of the transformer T is not set to zero. Therefore, the breakdown voltage of the second Zener diode Z2 is applied to the microcomputer 18, and the microcomputer 18 can be operated.

  The present invention detects the output voltage of the display device 24 or the indicator lamp and the regulator 16, and when the output voltage is the breakdown voltage of the second Zener diode Z2, the circuit 26 or the display for displaying an abnormality on the display device And a circuit for lighting the lamp. The display device 24 is an LCD or the like, and the indicator lamp is an LED or the like. These LCDs and LEDs may be those provided in an air conditioner. The LCD and the LED may be used at the same time. The circuit 26 may be built in the microcomputer 18. Even if the regulator 16 is short-circuited, the output voltage is not set to 0 V, so that the microcomputer 18 can operate. When the regulator 16 is short-circuited, the display device 24 or the indicator lamp notifies the user of an abnormality, and the user can recognize that an abnormality has occurred in the power supply circuit.

  Next, an example of the operation of the circuit 10 of the present invention will be described. Description will be made assuming that the breakdown voltage of the first Zener diode Z1 is 15V, the breakdown voltage of the second Zener diode Z2 is 7V, the output voltage of the rectifier circuit 14 is 15V, and the output voltage of the regulator 16 is 5V.

  During normal operation, terminal A is 15V and terminal B is 5V. A voltage of 5V is applied to the microcomputer 18. The first Zener diode Z1 is a breakdown region. Further, the second Zener diode Z2 is not higher than the breakdown voltage. Therefore, the first Zener diode Z1 is turned on, and the potential of 15V is fed back to the control circuit 12. Thus, if the regulator 16 is operating normally, the control circuit 12 controls the potential of the secondary coil NS to be the breakdown voltage (15 V) of the first Zener diode Z1. In addition, although there exists resistance R etc., resistance R etc. are disregarded and demonstrated.

  When the regulator 16 is short-circuited, the potential of the terminal A and the potential of the terminal B are the same, so the potential of the terminal B tends to be 15V. However, the second Zener diode Z2 is connected to the terminal B. Therefore, the potential of the terminal B does not become higher than 7V. The potential of the terminal B becomes 7V, and the potential of the terminal A becomes slightly over 7V. The first Zener diode Z1 is below the breakdown voltage, and the second Zener diode Z2 is a breakdown region. The second Zener diode Z2 is turned on, and a potential of 7V is fed back to the control circuit 12. Table 1 shows a comparison of the potential of the terminal A and the terminal B with the conventional one.

  When there is feedback from the second Zener diode Z2, the control circuit 12 adjusts the switching duty ratio with respect to the voltage applied to the primary coil NP of the transformer T, and lowers the potential of the secondary coil NS. When the potential of the secondary coil NS is set to be equal to or higher than the breakdown voltage of the second Zener diode Z2 and equal to or lower than the breakdown voltage of the first Zener diode Z1, feedback is performed from the second Zener diode Z2. In order to protect the circuit 10 and the microcomputer 18, the breakdown voltage of the second Zener diode Z2 is set to a voltage that can protect the microcomputer 18. Therefore, in the above example, the potential of the secondary coil NS is slightly over 7V.

  A voltage of 7 V is applied to the microcomputer 18. Although a voltage higher than the rated voltage is applied, it is less than half of the conventional 15V voltage. Although the microcomputer 18 generates heat somewhat, it does not break down.

  When the display device 24 or the circuit 26 for lighting the display lamp detects a voltage of 7 V, the display device 24 displays an abnormality or turns on the display lamp to notify the abnormality. The user can know the abnormality of the product, stop the operation of the product, and contact the service center.

  As described above, the present invention can protect the microcomputer 18 without cutting the fuse F even if the terminal I and the terminal O of the regulator 16 are short-circuited. If the regulator 16 is dusty and short-circuited, the circuit 10 can be easily restored. Further, since a voltage higher than the breakdown voltage of the second Zener diode Z2 is not applied to the microcomputer 18, the microcomputer 18 can be always protected. Furthermore, since the breakdown voltage of the second Zener diode Z2 can be output even if the regulator 16 is short-circuited (see Table 1), it is possible to notify the abnormality with a display device etc. unlike the conventional case, which is more convenient than the conventional case. Is good.

  As mentioned above, although embodiment was described about the power supply circuit of this invention, this invention is not limited to said embodiment. For double protection of the microcomputer 18, a fuse F may be inserted in the same place as before. If the microcomputer 18 abnormally generates heat contrary to the circuit design, the output of the power supply circuit 10 can be set to 0 V by fusing the fuse F, and the microcomputer 18 can be protected.

  Further, in addition to providing the display device 24 and the indicator lamp, a communication device that automatically notifies the service center via a network such as the Internet may be provided.

  The transformer T may be either a forward coupling method or a flyback method. The rectifier circuit 14 and the like are changed accordingly.

  The circuits and elements described above can be replaced with other circuits and elements having the same functions and effects. Other circuits and elements may be added to the power supply circuit 10 of the present invention as needed as long as the same operation and effect as the power supply circuit 10 of the present invention are exhibited.

  In addition, the present invention can be carried out in a mode in which various improvements, modifications, and changes are added based on the knowledge of those skilled in the art without departing from the spirit of the present invention.

It is a circuit diagram of the power supply circuit of this invention. It is a circuit diagram of the conventional power supply circuit.

Explanation of symbols

10, 20: Power supply circuit 12, 22: Control circuit 14: Rectifier circuit 16: Regulator 18: Microcomputer (load)
24: Display device 26: Display circuit

Claims (2)

A power supply circuit for supplying power to a load,
A transformer including a primary coil and a secondary coil;
A control circuit connected to a primary coil of the transformer and adjusting a switching duty ratio, a frequency, or both to a voltage applied to the primary coil;
A rectifier circuit connected to the secondary coil of the transformer;
A regulator connected to the rectifier circuit and stepping down the output voltage of the rectifier circuit to a voltage applied to the load;
A first Zener diode that feeds back an output voltage of the rectifier circuit from a contact point between the rectifier circuit and the regulator to the control circuit;
A second Zener diode that feeds back the output voltage of the regulator from the output terminal of the regulator to the control circuit;
Only including,
The breakdown voltage of the first Zener diode is higher than the breakdown voltage of the second Zener diode, and the breakdown voltage of the second Zener diode is higher than the output voltage during normal operation of the regulator;
When the regulator is short-circuited, the second Zener diode is turned on and the output voltage of the regulator is fed back to the control circuit. During normal operation, the first Zener diode is turned on and the output voltage of the rectifier circuit is supplied to the control circuit. Give feedback,
The control circuit varies a duty ratio, a frequency, or both of switching with respect to a voltage applied to the primary coil of the transformer according to a difference in voltage fed back from the first Zener diode or the second Zener diode,
When the breakdown voltage of the first Zener diode is fed back to the control circuit, the control circuit causes the output voltage of the secondary coil of the transformer to be the breakdown voltage of the first Zener diode, and the regulator reduces the output voltage. Applied to the load,
When the breakdown voltage of the second Zener diode is fed back to the control circuit, the control circuit lowers the output voltage of the secondary coil of the transformer and applies the breakdown voltage of the second Zener diode to the load. circuit. The load includes a microcomputer;
A display device or indicator lamp provided in the microcomputer ;
A circuit built in the microcomputer and detecting a breakdown voltage of the second Zener diode, a circuit for displaying an abnormality on the display device or a circuit for lighting the indicator lamp;
The power supply circuit according to claim 1 .

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