JP2012244870A - Protection circuit for switching power source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent elements from generating heat or being destroyed.SOLUTION: A protection circuit for a switching power source comprises a Zener diode 16 which is connected between a transmission line A1 and a ground and yielded when the transmission line A1 is turned to a predetermined voltage higher than rating, a Zener diode D1 which is connected between the transmission line A1 and the ground and yielded when the transmission line A1 is turned to a predetermined voltage higher than the rating and lower than a Zener voltage of the Zener diode 16, a switch section which is inserted to a post-stage rather than a connecting spot of the Zener diodes 16 and D1 in the transmission line A1, outputs a power supply voltage to a load while a predetermined constant voltage is inputted to an output control terminal, and stops outputting the power supply voltage to the load while a ground voltage is inputted to the output control terminal, and a transistor Q1 which is turned on when the Zener diode D1 is yielded, and leads the output control terminal into the ground.

Description

  The present invention relates to a protection circuit for a switching power supply, and in particular, a switching power supply circuit that oscillates continuously when the weight of a connected load is equal to or higher than a predetermined level and intermittently oscillates when the weight of the load is lower than a predetermined level. It relates to a protection circuit.

In general, a switching power supply circuit has a function of preventing secondary overload (overpower).
For example, in the transformer-coupled secondary DC power generation device described in Patent Document 1, when the output becomes an overvoltage, the Zener diode breaks down to turn on the transistor, thereby turning on the switching element and shorting the output line.
Further, the DC-DC converter described in Patent Document 2 pulls the transmission line of the on signal input to the control IC of the switching power supply circuit to the ground by the breakdown of the Zener diode when the secondary voltage exceeds a predetermined value. And an overvoltage prevention circuit for stopping the oscillation control of the control IC of the switching power supply circuit.
Further, in the switching regulator described in Patent Document 3, when the output voltage becomes an overvoltage, the Zener diode breaks down, the transistor is turned on and the error amplifier is bypassed, whereby the current flowing through the light emitting diode constituting the photocoupler in the feedback path The path is changed, thereby making the light emitting diodes in a stronger light emitting state and suppressing the output.
In the switching regulator described in Patent Document 4, when the output exceeds the reference voltage, the Zener diode breaks down and clamps the output.

JP-A-6-14532 JP 2009-171815 A JP 2000-78839 A Japanese Patent Laid-Open No. 6-22544

  The switching power supply circuit may have a plurality of output lines, and the load level connected to each output line is different. In general, on / off control (PWM control or the like) of a voltage applied to a primary winding of a switching transformer of a switching power supply circuit is performed by a power supply control IC, and the power supply control IC is fed back from the secondary side of the switching transformer. The on / off ratio in the on / off control is adjusted based on the feedback signal.

  The power supply control IC has a function to determine the size of the load connected to the secondary winding based on the amount of current flowing in the primary winding of the switching transformer, and detects that the load has become lighter Then, there is a type in which the on / off control of the voltage applied to the primary winding is changed from continuous oscillation to intermittent oscillation in order to automatically reduce the output. A switching power supply circuit including such a power supply control IC detects this when the power supply of the device is turned off and the load connected to the switching power supply circuit becomes light, and shifts to intermittent oscillation. In addition, when the output becomes an overvoltage and the zener diode for overvoltage protection breaks down or is broken by a short circuit, the load is reduced and the operation shifts to intermittent oscillation.

  Here, a first output line that is likely to have a high load level (an increase in processing amount, a fluctuation in output, etc.) connected to many loads, and a second that has a lower load level than the first output line. In a switching power supply circuit including an output line, when the power supply control IC performs on / off control based on a feedback signal based on the voltage of the first output line, feedback is normally performed due to a short-circuit of the feedback line, etc. Think of no case. At this time, the voltage rise of the first output line with a heavy load is slow, and the voltage of the second output line with a light load sharply rises.

  That is, when the output rises to an overvoltage, the protection Zener diode of the second output line is short-circuited, but there is a period during which the protection Zener diode of the first output line is not short-circuited. IC does not shift to intermittent oscillation. Therefore, in the second output line, there is a possibility that other elements generate heat or are destroyed due to short-circuit destruction of the protective Zener diode.

  The present invention has been made in view of the above problems, and is a switching power supply circuit having an auto standby function that performs continuous oscillation at a heavy load and intermittent oscillation at a light load, and the weight of a load to be connected is different. To provide a protection circuit for a switching power supply capable of preventing heat generation and destruction of an element in a switching power supply circuit having a plurality of output lines and a feedback applied to the power supply control IC from an output line with a heavy load And

  One aspect of the present invention is a protection circuit for a switching power supply circuit that continuously oscillates when the weight of a connected load is equal to or higher than a predetermined level and intermittently oscillates when the load is less than a predetermined level. The switching power supply circuit includes at least a first output line and a second output line for outputting a power supply voltage, and a load heavier than the second output line is connected to the first output line, and the protection The circuit has a cathode connected to the first output line, an anode connected to the ground, a first Zener diode that breaks down when the first output line reaches a predetermined voltage higher than a rating, and a cathode connected to the first output line. The first output line is connected to one output line, the anode is connected to the ground, and the first output line is higher than the rating. A second Zener diode that breaks down when it reaches a predetermined voltage lower than the voltage, and is inserted in a stage subsequent to the connection point of the first Zener diode and the second Zener diode in the first output line, and is connected to a predetermined output control terminal. When a constant voltage is input, a power supply voltage is output to the load, and when a ground voltage is input to the output control terminal, a switch unit that does not output a power supply voltage to the load; And an NPN transistor that turns on when the two Zener diodes break down and pulls the output control terminal to the ground.

  In addition, the protection circuit of the switching power supply circuit described above includes various modes such as being implemented in another device or implemented together with another method.

According to the present invention, in a switching power supply circuit having an auto standby function that performs continuous oscillation at a heavy load and performs intermittent oscillation at a light load, the switching power supply circuit includes a plurality of output lines having different load weights. When a feedback is applied to the power supply control IC from a heavy output line, it is possible to provide a protection circuit for a switching power supply that can prevent heat generation and destruction of the element.
According to the invention of claim 2, backflow to the output control terminal can be prevented.

It is the figure which showed the principal part of the switching power supply circuit provided with a protection circuit. 3 is a circuit diagram illustrating a configuration example of an FB circuit 21. FIG. 3 is a circuit diagram showing an example of a circuit diagram of an FB circuit 21. FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(1) Configuration of the present embodiment:
FIG. 1 is a diagram illustrating a main part of a switching power supply circuit including a protection circuit. As shown in the figure, a switching power supply circuit 100 includes a power supply control IC 10, a switching transformer 12, a rectifier circuit 14, a Zener diode 16 for overvoltage protection, a protection circuit 17, a rectifier circuit 18, a Zener diode 19 for overvoltage protection, a constant voltage circuit. A voltage circuit 20, an overvoltage protection Zener diode 22, and a feedback circuit 21 (FB circuit 21) are provided.

  The power supply control IC 10 has an auto standby function, and the switching power supply circuit 100 realizes at least two power supply states of an on state and a standby state by the auto standby function. For example, when the switching power supply circuit 100 is a power supply circuit for a liquid crystal television, a state in which video processing, audio processing, video display on a screen, and the like are performed is on, and only a microcomputer operates and a user operates. The standby state is a state of waiting for an operation input instructing the power-on from.

  The power supply control IC 10 detects the weight of the load connected to the switching transformer 12 (load level) and performs on / off control of direct current application to the primary winding of the switching transformer 12 (hereinafter also referred to as oscillation). An auto standby function can be realized by switching between continuous oscillation and intermittent oscillation. That is, continuous oscillation is performed when the load is equal to or greater than the first threshold, and intermittent oscillation is performed in which continuous oscillation is intermittently performed when the load is less than the first threshold. Here, the first threshold value is set as appropriate by the designer of the switching power supply circuit 100. For example, the state in which only the microcomputer described above is connected to the switching transformer 12 is in the standby state, the backlight or the video. A state where the processing IC, the audio amplifier, and the like are driven is set to an on state, and an intermediate load between the standby state and the on state is set as a first threshold value.

  The power supply control IC 10 can detect the load level connected to the switching power supply circuit 100 based on the voltage of the IS terminal 10a. A sense resistor is connected between the IS terminal 10a and the ground, and the power supply control IC 10 allows the drain current of the built-in FET to flow from the IS terminal to the ground. This FET controls on / off of the current flowing through the primary winding of the switching transformer 12, and the magnitude of the load connected to the switching transformer 12 is reflected in the drain current of the FET. That is, since a voltage corresponding to the magnitude of the load connected to the switching transformer 12 is generated in the sense resistor, the power supply control IC 10 can detect the load level by monitoring the voltage of the IS terminal 10a. . When the load becomes heavier than the first threshold, the power supply control IC 10 sets the power supply state to the on state by performing continuous oscillation, and when the load becomes lighter than the first threshold, the power supply control IC 10 sets the power supply state to the standby state.

  The switching transformer 12 has a direct current input to one end of the primary winding, and the other end of the primary winding is connected to the ground via the source-drain of the FET built in the power supply control IC 10. The power supply control IC 10 can control on / off of the current flowing in the primary winding of the switching transformer 12 by performing on / off control of the FET, and can generate an alternating current in the secondary winding of the switching transformer 12. Note that, for example, a direct current generated by rectifying and smoothing a commercial AC power input from the outside is input to one end of the primary winding of the switching transformer 12.

  The secondary winding of the switching transformer 12 includes a terminal A and a terminal B for taking out a voltage. A rectifier circuit 14 is connected to the terminal A, and the rectifier circuit 14 outputs a direct current generated by rectifying and smoothing an alternating current generated at the terminal A to the transmission line A1. In the present embodiment, the transmission line A1 constitutes the first output line. In FIG. 1, the DC voltage is Va. On the other hand, a rectifier circuit 18 is connected to the terminal B, and the rectifier circuit 18 outputs the alternating current generated at the terminal B to the transmission line B by rectifying and smoothing the direct current. In FIG. 2, the DC voltage is Vb. In the present embodiment, the transmission line B constitutes the second output line.

  Here, the load connected to the secondary winding of the switching transformer 12 will be described. Taking the case where the switching power supply circuit 100 constitutes a power supply circuit for a liquid crystal television as an example, a circuit for generating a power supply voltage for lighting a backlight (hereinafter referred to as a BL lighting circuit) is provided on the transmission line A1. )), A video processing IC that performs digital processing of a video signal, an audio amplifier that amplifies audio, and the like, and a microcomputer is connected to the transmission line B.

  Further, a transmission line A2 branched from the transmission line A1 is also connected to the microcomputer. The microcomputer is supplied with the power supply voltage via the transmission line B in the on state, and is supplied with the power supply voltage via the transmission line A2 branched from the transmission line A1 in the standby state. A constant voltage circuit 20 is provided in the middle of the transmission line A2. FIG. 2 is a circuit diagram illustrating an example of the constant voltage circuit 20. The constant voltage circuit 20 adjusts the voltage of the transmission line A1 to the driving voltage of the microcomputer. Accordingly, when the voltage of the transmission line B becomes lower than the driving voltage of the microcomputer in the standby state, the driving voltage is supplied from the transmission line A1 to the microcomputer via the transmission line A2. Thus, the microcomputer can operate in both the on state and the standby state.

  The FB circuit 21 feeds back a feedback signal generated based on the voltage Va of the transmission line A1 to the power supply control IC 10. The power supply control IC 10 performs the on / off control based on a feedback signal input from the FB circuit 21 to the feedback terminal 10b. FIG. 3 shows an example of a circuit diagram of the FB circuit 21. As shown in the figure, the FB circuit 21 is connected to the transmission line A1, and the Zener diode 21a breaks down when the voltage of the transmission line A1 becomes higher than the rating. Then, the photodiode constituting the photocoupler IC 621 emits light, the phototransistor receives light, and the phototransistor is turned on. Then, a predetermined voltage signal (feedback signal) is input to the feedback terminal 10 b of the power supply control IC 10.

  While the feedback signal is being input, the power supply control IC 10 performs on / off control related to the application of DC to the primary winding so that the voltage generated in the secondary winding of the switching transformer 12 decreases, and the feedback signal When the input is stopped, on / off control for applying a direct current to the primary winding is performed so that the voltage generated in the secondary winding of the switching transformer 12 increases. As a result, the voltage generated on the transmission line A1 converges to a constant value.

  In the BL lighting circuit, the output of the power supply voltage to the backlight is controlled by the control IC B / L that drives the circuit, and the video processing IC, the audio amplifier, and the like are transmitted via regulators IC1 to IC3 that adjust the voltage. Connected to line A1. The control IC B / L and the regulators IC1, IC2 and IC3 have output control terminals T1 to T4 for controlling outputs, and a predetermined voltage signal is applied to the output control terminals T1 to T4 under the control of a microcomputer which will be described later. When input, the drive voltage generated using the voltage of the transmission line A1 is supplied to each load, and when input of a predetermined voltage signal to the output control terminals T1 to T4 is stopped, the drive voltage for each load is reduced. Stop supplying. Although not shown in the figure, the microcomputer is also connected to the transmission line B and the transmission line A1 through a regulator that adjusts the voltage as necessary.

  A Zener diode 16 is connected between the transmission line A1 and the ground with the cathode facing the transmission line A1 and the anode facing the ground. The Zener diode 16 breaks down when the alternating current generated at the terminal A becomes an overvoltage and the voltage Va of the transmission line A1 exceeds a predetermined value. A Zener diode 19 is connected between the transmission line B and the ground with the cathode facing the transmission line B and the anode facing the ground. In the present embodiment, the Zener diode 16 constitutes a first Zener diode.

  The Zener diode 19 breaks down when the alternating current generated at the terminal B becomes an overvoltage and the voltage Vb of the transmission line B exceeds a predetermined value. Between the transmission line B and the ground, there is a Zener diode 22 at a stage after the output voltage of the constant voltage circuit 20 is connected to the transmission line B, with the cathode facing the transmission line B and the anode facing the ground. It is connected. The Zener diode 22 breaks down when the alternating current generated at the terminal A in the standby state becomes an overvoltage and the voltage Vc of the transmission line A2 exceeds a predetermined value.

  Here, if the load of the transmission line A1 is at an average level, for example, when the feedback line of the feedback circuit 21 is short-circuited in the ON state and feedback is not normally applied and each transmission line is in an overvoltage state, The zener diode is designed to break down almost simultaneously, and the power supply control IC 10 shifts to intermittent oscillation. However, if the load on the transmission line A1 is heavier than usual due to an increase in luminance, an increase in volume, an increase in digital processing, etc., the voltage increase on the transmission line A1 is slower than the transmission line B. The Zener diodes 19 and 22 that detect the overvoltage of the transmission line B are broken down or short-circuited earlier than the Zener diode 16 that protects the transmission line A1 from the overvoltage. Since a large current flows through the transmission line B during this time lag, elements such as the rectifier circuit 18 of the transmission line B and the backflow prevention diode 23 may generate heat. Therefore, in this embodiment, a protection circuit 17 is provided.

  The protection circuit 17 instructs the control IC B / L and the output control terminals T1 to T4 of the regulators IC1, IC2, and IC3 to output by turning on the NPN transistor Q1 when the Zener diode D1 breaks down. By pulling a signal transmission line for inputting a predetermined voltage signal to the ground, the outputs of the control IC B / L and the regulators IC1, IC2, and IC3 are stopped. The B / L output control terminal T4 is, for example, an output control terminal of a control IC that drives an inverter circuit when the backlight is formed of a cold cathode fluorescent tube, and the backlight is formed of an LED. Is an output control terminal of the control IC of the LED drive circuit.

  More specifically, the protection circuit 17 includes a Zener diode D1, a resistor R1, an NPN transistor Q1, and diodes D2 to D5. The Zener diode D1 and the resistor R1 connect between the transmission line A1 and the transistor Q1, and the Zener diode D1 has the transmission line A1 and the transistor Q1 with the anode facing the transmission line A1 and the cathode facing the base of the transistor Q1. Are connected. The Zener voltage of the Zener diode D1 is set lower than the Zener diode 16 and higher than the output rating of the transmission line A1. For example, when the output rating of the transmission line A1 is 24V, the Zener voltage of the Zener diode 16 is The voltage obtained by combining the Zener voltage of the Zener diode D1 and the base-emitter voltage of the transistor Q1 is set to, for example, 30V between 24 to 33V. That is, in the present embodiment, the Zener diode D1 constitutes the second Zener diode.

  Therefore, when the voltage of the transmission line A1 exceeds the sum of the Zener voltage of the Zener diode D1 and the base-emitter voltage of the transistor Q1, the Zener diode D1 breaks down and the transistor Q1 is turned on, and the control IC B / L or The output control terminals T1 to T4 of the regulators IC1, IC2 and IC3 are drawn to the ground. Then, since the ON signal is not input to the output control terminals T1 to T4, the BL lighting circuit stops supplying the power supply voltage to the backlight, and the regulators IC1 to IC3 also stop outputting the power supply voltage. That is, no load is connected to the transmission line A1. As a result, the power supply control IC 10 detects a state in which no load is connected to the switching transformer 12 and shifts to intermittent oscillation. Therefore, the overcurrent flowing through the transmission line B is suppressed, and the heat generation of the element is suppressed.

  The diodes D2 to D5 connect the output control terminals T1 to T4 and the collector of the transistor Q1, respectively. The diode D2 connects the output control terminal T4 and the transistor Q1 with the anode facing the output control terminal T4 and the cathode toward the collector of the transistor Q1, and the diode D3 connects the transistor with the anode facing the output control terminal T1. The cathode is directed to the collector of Q1, and the output control terminal T1 and the transistor Q1 are connected. The diode D4 has the anode directed to the output control terminal T2 and the cathode directed to the collector of the transistor Q1, and the output control terminal T2 and the transistor. The diode D5 is connected between the output control terminal T3 and the transistor Q1 with the anode facing the output control terminal T3 and the cathode facing the collector of the transistor Q1. The diodes D2 to D5 prevent backflow from the transistor Q1 to the output control terminals T1 to T4 of the control IC B / L and the regulators IC1, IC2, and IC3, and prevent current from flowing between the output control terminals T1 to T4. . Therefore, malfunction or failure of the device can be prevented.

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 10 ... Power supply control IC, 10a ... IS terminal, 10b ... Feedback terminal, 12 ... Switching transformer, 14 ... Rectifier circuit, 16 ... Zener diode, 17 ... Protection circuit, 18 ... Rectifier circuit, 19 ... Zener diode, 20 ... Constant voltage Circuit, 21 ... Feedback circuit, 21a ... Zener diode, 22 ... Zener diode, 23 ... Diode, 100 ... Switching power supply circuit, A ... Terminal, A1 ... Transmission line, A2 ... Transmission line, B ... Terminal, B ... Transmission line, B / L ... control IC, D1 ... Zener diode, D2-D5 ... diode, IC621 ... photocoupler, IC1-IC3 ... regulator, Q1 ... transistor, R1 ... resistor, T1-T4 ... output control terminal, Va ... voltage, Vb ... Voltage, Vc ... Voltage

Claims (2)

In the protection circuit of the switching power supply circuit that continuously oscillates when the weight of the connected load is greater than or equal to a predetermined level and intermittently oscillates when the load weight is less than the predetermined level,
The switching power supply circuit includes at least a first output line and a second output line for outputting a power supply voltage, and a load heavier than the second output line is connected to the first output line,
The protection circuit is
A first Zener diode having a cathode connected to the first output line, an anode connected to the ground, and a breakdown when the first output line reaches a predetermined voltage higher than a rating;
A cathode is connected to the first output line, an anode is connected to the ground, and a breakdown occurs when the first output line becomes a predetermined voltage higher than the rating and lower than the Zener voltage of the first Zener diode. 2 Zener diodes,
In the first output line, when a predetermined constant voltage is input to the output control terminal after the first Zener diode and the second Zener diode are connected to each other, a power supply voltage is applied to the load. When the ground voltage is input to the output control terminal, the switch unit that does not output the power supply voltage to the load,
A protection circuit comprising: an NPN transistor that turns on when the second Zener diode breaks down and pulls the output control terminal to ground.   2. The switching according to claim 1, further comprising: a diode inserted on a line connecting the output control terminal and the transistor with the anode facing the output control terminal and the cathode facing the transistor. Power supply protection circuit.

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