JP2011130588A - Power supply circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power supply circuit which has the protective circuit capable of preventing cost increase while improving detection accuracy, in the power supply circuit having the protective circuit for protecting a product even if the product is connected to a power supply of rated voltage or more by mistake. <P>SOLUTION: The power supply circuit for receiving a supplied power from an external power supply and switching an FET to generate power has a transformer for outputting a predetermined voltage upon receiving power, a feedback winding for outputting a feedback voltage on the basis of the output value of the transformer, a Zener diode connected via an anode to the output side of the feedback winding, and outputting a breakdown current when the output voltage reaches a predetermined voltage, and a protective circuit for always maintaining a gate current of the FET at high level when the breakdown current flows. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a power supply circuit, and more particularly to a power supply circuit that protects a product when connected to a power supply having a rated voltage or higher.

  2. Description of the Related Art Conventionally, a power supply circuit that generates and supplies DC power from AC power to drive a product is known. For example, the power supply circuit generates a DC power from a commercial power supply of AC 100 volts and supplies it to each part constituting the product (see, for example, Patent Documents 1-3).

Japanese Patent Laid-Open No. 4-200276 JP-A-5-76185 JP 2008-199728 A

  In power supply circuits, when a user accidentally connects to a power supply that exceeds the rated voltage, a power supply circuit is known that prevents damage to the product by short-circuiting (or destroying) a part of the circuit. ing. In a power supply circuit having such a protection function, overvoltage detection is performed by using a diode or the like, but overvoltage detection using only a diode has a detection error and it is difficult to detect overvoltage with higher accuracy. Further, when an element such as a thyristor is used, the cost of the product is increased.

  The present invention has been made in view of the above problems, and in a power supply circuit having a protection circuit for protecting a product even when erroneously connected to a power supply of a rated voltage or higher, suppressing an increase in cost while increasing a detection system. An object of the present invention is to provide a power supply circuit using a protection circuit capable of performing the above.

  In order to solve the above problems, in the present invention, in a power supply circuit that receives power supply from an external power supply and generates power by switching an FET, a transformer that receives power supply and outputs a predetermined voltage, and an output of the transformer A feedback winding that outputs a feedback voltage based on the value, a Zener diode that is connected to the output side of the feedback winding with an anode and outputs a breakdown current when the output value reaches a predetermined voltage, and the breakdown And a protection circuit that always keeps the gate current of the FET at a high level when a current flows.

  In the invention configured as described above, a Zener diode is connected to the output side of the feedback winding that outputs the feedback voltage based on the output value of the transformer at the anode, and the output value of the feedback winding reaches a predetermined voltage. Output a breakdown current. The protection circuit always keeps the gate current of the FET at a high level when the breakdown current flows. As a result, the FET is short-circuited and driving of the power supply circuit is stopped.

  Therefore, since the overvoltage detection is performed using the output voltage of the feedback winding that outputs a voltage having a certain relationship with the output value of the transformer, the detection system of the image voltage can be enhanced. Moreover, since an effective element such as a thyristor is not used for overvoltage detection, an increase in cost can be suppressed.

In addition, a control circuit for switching the FET by controlling a gate voltage supplied to the gate of the FET, the protection circuit stops the operation of the control circuit when the breakdown current is input, The gate voltage may be constantly supplied.
In the invention configured as described above, since the gate voltage is continuously supplied to the FET by controlling the operation of the control circuit, the present invention can be realized with a simple configuration using an existing circuit.

The control circuit may be configured to control the supply of the gate voltage by outputting a PWM signal.
In the invention configured as described above, the present invention can be applied to a separately excited oscillation type power supply circuit.

Furthermore, it is good also as a structure which has a fuse which acts as a safety circuit.
In the invention configured as described above, the driving of the power supply circuit can be stopped more safely by blowing the fuse.

  According to another aspect of the present invention, the control circuit includes a control circuit that controls a gate voltage supplied to the gate of the FET to switch the FET, and a fuse that functions as a safety circuit. The gate voltage supply may be controlled by a signal output, and the protection circuit may stop the operation of the control circuit and always supply the gate voltage when the breakdown current is input.

As described above, according to the present invention, it is possible to suppress an increase in cost while enhancing a detection system in a power supply circuit including a protection circuit for protecting a product even when the power supply is accidentally connected to a power supply having a rated voltage or higher.
In the invention according to claim 2, since the gate voltage is continuously supplied to the FET by controlling the operation of the control circuit, the present invention can be realized with a simple configuration using an existing circuit.
In the invention according to claim 3, the present invention can be applied to a separately excited oscillation type power supply circuit.
Furthermore, in the invention according to claim 4, the driving of the power supply circuit can be stopped more safely by blowing the fuse.

It is a figure explaining the structure of the power supply circuit 100 concerning this embodiment.

Embodiments of the present invention will be described below in the following order with reference to the drawings.
1. First embodiment:
2. Other embodiments:

1. First embodiment:
FIG. 1 is a diagram for explaining a configuration of a power supply circuit 100 according to the present embodiment. The power supply circuit 100 is connected to each circuit constituting the television receiver on the output side of the transformer 95, generates a stabilized power supply from the power supplied via the outlet 90, and outputs it to each circuit. In addition, the power supply circuit 100 according to the present embodiment controls the operation of the FET 94 by the separately excited oscillation method.

  The power supply circuit 100 includes the following main parts. Reference numeral 91 denotes a fuse, which acts as a safety circuit when a current exceeding a specified value flows in the power supply circuit 100. Reference numeral 92 denotes a rectifier circuit that rectifies the AC power supplied through the outlet 90. Reference numeral 93 denotes an electric field capacitor (smoothing circuit) that smoothes and outputs the rectified power supply. Reference numeral 94 denotes an FET, which generates AC power from a power source smoothed by a switching operation and supplies it to the primary winding of the transformer 95. Reference numeral 95 denotes a transformer, which increases the voltage value of the AC power source supplied to the primary side winding in accordance with the winding ratio, and supplies it to the circuit connected to the secondary side winding. Reference numeral 96 denotes a feedback winding, which generates and outputs a feedback signal corresponding to the output voltage of the transformer 95. Reference numeral 80 denotes an oscillation circuit that controls the switching operation of the FET 94 by controlling the supply of the gate voltage of the FET 94 in accordance with the PWM signal.

  Furthermore, the power supply circuit 100 includes the following parts. Reference numeral 70 denotes a Zener diode, the anode of which is connected to the output side of the feedback winding 96. Reference numeral 60 denotes a protection circuit, which is connected to the cathode of the Zener diode 70 and to the oscillation circuit 80 and a gate voltage supply line 98 that supplies a gate voltage to the FET 94.

  The oscillation circuit 80 includes a control circuit 81 and a bridge circuit 82. The control circuit 81 is connected to the base of the transistor Q2 constituting the bridge circuit 82 via the PWM signal output terminal. The bridge circuit 82 is a collector of the transistor Q2 and is connected to the input line 97. Therefore, when a PWM signal is output from the control circuit 81, the transistor Q2 switches on / off in accordance with the duty of the PWM signal, and the input voltage after rectification / smoothing applied to the input line 97 is used as the gate voltage of the FET 94. Supply to the gate. Further, the control circuit 81 is connected to the feedback winding 96 and controls the duty ratio of the PWM signal by a feedback signal output from the feedback winding 96.

  The feedback winding 96 is configured as a part of the transformer 95, and is set so that the winding ratio of the secondary winding is N: M (N <M). Therefore, when an output voltage is generated in the secondary winding of the transformer 95, a feedback signal N / M times the output voltage is output to the feedback winding 96.

  The Zener diode 70 is connected to the output side of the feedback winding 96 at the anode via the diode D1 and the resistor R1, and is connected to the protection circuit 60 at the cathode. Here, the condition for the breakdown of the Zener diode 70 is set such that the output voltage of the transformer 95 is equal to or higher than “output voltage Vo1 · N / M”. Here, the output voltage value Vo1 is a value set based on, for example, an output voltage output when the power supply circuit 100 is connected to an external power supply of 200 volts. N / M is a winding ratio of the secondary winding and the feedback winding 96 in the transformer 95.

  The protection circuit 60 includes a transistor Q1 and resistors R2 and R3. The transistor Q1 is connected to the connection point between the resistor R2 and the resistor R3 at the base, the collector is connected in parallel to the connection line between the PWM signal output terminal of the control circuit 81 and the transistor Q2, and the emitter is grounded. The resistor R2 is connected to the resistor R3 connected to the ground, and when the Zener diode 70 supplies a breakdown current, the current value is divided and a base current is output to the base of the transistor Q1. Therefore, when the transistor Q1 is turned on, the transistor Q2 is turned off, so that the control circuit 81 does not output a PWM signal.

=== Operation of the Power Supply Circuit 100 ===
With the above configuration, when an input voltage of 100 VAC is supplied to the power supply circuit 100 through the outlet 90, the rectifier circuit 92 and the smoothing circuit 93 rectify and smooth the input voltage and output it. The oscillation circuit 80 supplies the gate voltage to the FET 94 in accordance with the duty of the PWM signal, and switches the FET 94. Therefore, the rectified and smoothed input voltage is converted into an AC voltage by the FET 94 and supplied to the primary side winding of the transformer 95. The transformer 95 transforms the input voltage and outputs it from the secondary winding. Further, the feedback winding 96 outputs a feedback signal corresponding to the winding ratio to the oscillation circuit 80. Therefore, the oscillation circuit 80 adjusts the switching of the FET 94 by changing the duty of the PWM signal in accordance with the feedback signal.

  Here, when the user connects the power supply circuit 100 to a power supply having a voltage lower than the rated voltage (for example, 100 volt commercial power supply) via the outlet 90, the output value of the transformer 95 does not exceed the output voltage Vo1 and the feedback winding. The Zener diode 70 is not broken down by the feedback signal output from 96. Therefore, the protection circuit 60 does not work and the FET 94 operates normally.

  On the other hand, when the user connects the power supply circuit 100 to a power supply having a rated voltage or higher via the outlet 90, the output value of the transformer 95 becomes a larger value than usual. For example, when the user mistakenly connects the outlet 90 to a 200 volt AC power supply, the output voltage output by the transformer 95 is equal to or higher than the output voltage Vo1, which is higher than when the power supply is connected to a 100 volt commercial power supply.

  Therefore, a feedback signal having a voltage value equal to or higher than Vo1 · (N / M) corresponding to the output voltage of the transformer 95 is output from the feedback winding 96, and the Zener diode 70 is broken down. Then, the transistor Q1 of the protection circuit 60 is kept on by the breakdown current flowing from the Zener diode 70, and the transistor Q2 is turned off. Therefore, the gate voltage continues to be supplied to the gate of the FET 94 through the input line 97. Then, after a certain period of time has elapsed, the FET 94 is short-circuited, and further the fuse 91 is blown. As a result, the driving of the power supply circuit 100 is safely stopped by the melting of the fuse 91.

  As described above, even when the user mistakenly connects to a power supply that exceeds the rated voltage, the power supply circuit 100 causes the protection circuit 60 to short-circuit the FET 94 by the feedback signal whose value changes due to the incorrect connection. The power supply circuit 100 is stopped. Therefore, no overvoltage is supplied to the circuit connected to the power supply circuit 100. In addition, since the power supply circuit 100 shorts the FET 94 using a feedback signal that maintains a certain relationship (Vo1 · (N / M)) with the output value of the transformer 95, the detection circuit is configured only by a Zener diode or the like. Compared to the above, the detection accuracy can be further improved. Furthermore, since it is not the structure which improves a detection precision using a thyristor etc., a cost increase can be suppressed.

2. Other embodiments:
There are various modifications of the present invention.
The power supply circuit 100 is driven by a separately excited oscillation method, and may be driven by a self-excited oscillation method.
Further, the circuit configuration for operating the protection circuit by the breakdown current is an example, and the present invention is not limited to this.

Needless to say, the present invention is not limited to the above embodiments. It goes without saying for those skilled in the art,
・ Applying mutually interchangeable members and configurations disclosed in the above embodiments by appropriately changing the combination thereof.− Although not disclosed in the above embodiments, it is a publicly known technique and the above embodiments. The members and configurations that can be mutually replaced with the members and configurations disclosed in the above are appropriately replaced, and the combination is changed and applied. It is an embodiment of the present invention that a person skilled in the art can appropriately replace the members and configurations that can be assumed as substitutes for the members and configurations disclosed in the above-described embodiments, and change the combinations and apply them. It is disclosed as.

  DESCRIPTION OF SYMBOLS 60 ... Protection circuit, 70 ... Zener diode, 80 ... Oscillation circuit, 81 ... Control circuit, 82 ... Bridge circuit, 90 ... Outlet, 91 ... Fuse, 92 ... Rectification circuit, 93 ... Smoothing circuit (electric field capacitor), 94 ... FET, 95 ... transformer, 96 ... feedback winding, 97 ... input line, 98 ... gate voltage supply line, 100 ... power supply circuit

Claims (5)

In a power supply circuit that accepts power supply from an external power supply and generates power by switching FETs,
A transformer for receiving a power supply and outputting a predetermined voltage;
A feedback winding that outputs a feedback voltage based on the output value of the transformer;
A Zener diode connected to the output side of the feedback winding at the anode and outputting a breakdown current when the output value reaches a predetermined voltage;
And a protection circuit that keeps the gate voltage of the FET always at a high level when the breakdown current flows. A control circuit for controlling the gate voltage supplied to the gate of the FET to switch the FET;
2. The power supply circuit according to claim 1, wherein when the breakdown current is input, the protection circuit stops the operation of the control circuit and constantly supplies the gate voltage.   The power supply circuit according to claim 2, wherein the control circuit controls supply of the gate voltage by output of a PWM signal.   The power supply circuit according to any one of claims 1 to 3, further comprising a fuse that functions as a safety circuit. A control circuit for controlling the gate voltage supplied to the gate of the FET to switch the FET;
A fuse that acts as a safety circuit,
The control circuit controls the supply of the gate voltage by the output of a PWM signal,
2. The power supply circuit according to claim 1, wherein when the breakdown current is input, the protection circuit stops the operation of the control circuit and constantly supplies the gate voltage.

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