JPH0739344Y2 - Switching power supply circuit with protection circuit - Google Patents

Description

[Detailed Description of the Invention] (a) Industrial field of application This invention is designed to protect the output by keeping the output low while the protection circuit operates when an overcurrent or overvoltage abnormality occurs in the output. The present invention relates to a switching power supply circuit with a protection circuit that is operated.

(B) Prior art FIG. 3 shows a conventional switching power supply circuit with a protection circuit.
There is one shown in FIG. In the switching power supply circuit shown in FIG. 3, the DC input voltage is switched by the switching element Q ₁ , converted by the transformer 1, rectified and smoothed by the rectifier circuit 3, and then output from the output terminals 6a and 6b to the load. It is like this. An overvoltage detection circuit 4 and an output detection circuit 5 are provided between the output terminals 6a and 6b.
The Zener diode ZD ₁ , the resistor R _4, and the light emitting element L _{1 of the} photocoupler PC are connected in series. On the other hand, is connected to the thyristor SCR in parallel to the control circuit 2 are connected in parallel the light receiving element PT and the resistor R ₂ and R ₃ of the photocoupler PC to the thyristor SCR is connected in series, a resistor R ₂ and R ₃ The divided voltage is input as the SCR gate voltage.
Also, the reset switch 7 is connected in parallel with the SCR.

In the switching power supply circuit shown in FIG. 3, when an overvoltage is output, the light emitting element L ₁ of the photocoupler PC emits light in the overvoltage detection circuit 4, and in response to this, the light receiving element PT turns on and the resistance
A current flows through R ₂ and R ₃ , turns on the thyristor SCR, and lowers the power supply voltage of the control circuit 2. As a result, the switching operation of the switching transistor Q ₁ is stopped and the output is held low. When resetting the operation after the protection function has been operated by this switching power supply circuit, by turning on the reset switch 7, the thyristor SCR is short-circuited and the current is turned off, so that the original state is restored.

Further, the switching power supply circuit of FIG. 4 has a main circuit portion similar to that of FIG. 3, and the reset switch 7 has a normally closed switch, and when it is desired to reset, the normally closed switch 7 is turned off. As a result, the current flowing through the thyristor SCR is turned off, and the operation returns to the original state.

(C) Problems to be Solved by the Invention Among the conventional switching power supply circuits described above, the one shown in FIG. 3 has a control circuit 2 when the reset switch 7 is erroneously turned on when the power supply circuit is operating normally. There is a problem that the power supply voltage supplied to the device is lowered, and the switching operation of the switching element Q ₁ is stopped again, resulting in malfunction.

Further, in the switching power supply circuit shown in FIG. 4, there is no problem even if the reset switch 7 is turned off during normal operation,
Thyristor SC even if current flows to the light receiving element PT of photocoupler PC when an error occurs while switch 7 is OFF.
There is a problem that R does not turn on and the protection circuit does not operate.

The present invention has been made by paying attention to the above-mentioned problems. In normal operation, even if the reset switch is operated, it is invalidated, and conversely, in abnormal operation, the protection operation is performed regardless of the state of the reset switch. It is an object of the present invention to provide a switching power supply circuit to be obtained.

(D) Means and Actions for Solving the Problems A switching power supply circuit with a protection circuit of the present invention is a DC voltage source, a switching circuit for switching the output from the DC voltage source, and an input winding for a switched signal. A transformer derived from the output winding, a rectifier circuit that rectifies the output of this transformer, an output abnormality detection circuit that detects an abnormality in the output from this rectification circuit, and an abnormality detection signal of this output abnormality detection circuit. In response to this, the switching operation of the thyristor and the switching circuit that turn on to lower the voltage supply level to the control circuit is controlled, and the switching operation of the switching circuit is stopped when the voltage supply level decreases due to the turning on of the thyristor. Control circuit and reset to turn off the thyristor by operation and restore the switching operation And a level detection circuit for detecting whether the voltage level supplied to the control circuit is higher or lower than a predetermined level, and the operation of the reset switch when the level is high according to the detected voltage level. It is characterized by a reset circuit that disables the switch and enables the operation of the reset switch when it is low.

In this switching power supply circuit, a normal voltage is output from the output terminal during normal operation, but if an abnormality such as overcurrent or overvoltage occurs, the output abnormality detection circuit detects the abnormality and responds accordingly. The thyristor turns on, lowering the power supply voltage to the control circuit. Therefore, the voltage drops,
The switching operation is stopped and the output is kept low. To recover from the abnormality, operate the reset switch. When the reset switch is operated, the reset circuit operates to turn off the thyristor, and the control circuit is thereafter supplied with a predetermined power supply voltage. If an error occurs while the reset switch is being operated, the operation of the reset switch is ignored, and the thyristor is turned on by the signal from the error detection output circuit, supplying a low level power supply voltage to the control circuit. After all, the protection circuit function operates. In addition, even if the reset switch is operated during normal operation, if the power supply voltage supply level is above the predetermined level, this reset signal is ignored, and of course the voltage supplied to the control circuit does not drop and remains as it is. Normal operation will continue.

(E) Embodiment Hereinafter, the present invention will be described in more detail with reference to embodiments.

FIG. 1 is a circuit connection diagram of a switching power supply circuit with a protection circuit showing an embodiment of the present invention.

In the figure, the parts denoted by the same reference numerals as those in the circuit diagrams shown in FIGS. 3 and 4 indicate the same parts, and the feature of the switching power supply circuit of this embodiment is that the circuit part shown by the dotted line is added. In point. In this switching power supply circuit, a Zener diode ZD ₂ is connected between the anode of the thyristor SCR and a power supply section for the control circuit 2 and the like, and a transistor Q ₂ is connected in parallel to the thyristor SCR to connect the transistor Q ₂ A time constant circuit consisting of a capacitor C ₃ and a resistor R ₈ is connected between the base and ground, a transistor Q ₃ is connected in parallel to the time constant circuit, and the collector of the transistor Q ₃ and the power supply terminal of the control circuit 2 are connected. The resistor R ₇ and the reset switching 7 are connected in series. Furthermore, connect a series circuit of resistors R ₅ and R ₆ between the power supply terminal and the ground terminal of the control circuit 2,
Its connection point is connected to the base of transistor Q ₃ .

Next, the operation of the switching power supply circuit of this embodiment will be described.

As a basic operation, first turn ON / O switching transistor Q _1.
A voltage is generated in the main winding 1a by the FF, this voltage is led to the output winding 1c of the transformer 1, the derived voltage is rectified and smoothed by the rectifier circuit 3, and supplied to the load from the output terminals 6a and 6b. It A voltage is induced in the control winding 1b of the transformer 1 according to the voltage generated in the main winding 1a, and the control circuit 2 receives the voltage according to the voltage derived from the control winding 1b.
The ON / OFF ratio of the transistor is controlled by the signal from so that the output voltage is stabilized.

Next, when the output is normally output, that is, during normal operation, since the signal from the overvoltage detection circuit 4 is not incident on the light receiving element PT, the phototransistor PT is OFF,
Therefore, the thyristor SCR does not turn on. On the other hand, the transistor Q ₃ is turned on because the power supply voltage v is at a high level, the base voltage of the transistor Q ₂ is low and the transistor Q ₂ is off regardless of whether the switch 7 is turned on or off.
In this state, since the power supply voltage v has a high level, the control circuit 2
Is supplied with a normal power supply voltage, and the transistor Q ₁ also performs switching operation.
Normal voltage is output by 6a and 6b (second
(See the waveform shown in a of the figure).

Next, when an overvoltage is generated between the output terminals 6a and 6b for some reason, the overvoltage detection circuit 4 causes the light emitting element L ₁ to emit light due to this overvoltage, which turns on the phototransistor PT.
Then, turn on the thyristor SCR. The control circuit 2 stops the operation of the zener voltage of the zener diode ZD ₂ and the transistor Q ₂ is set to a driveable value. Therefore, when the thyristor SCR turns on, the voltage v of the control circuit 2 decreases. Stop the operation and keep the state. That is, the holding current of SCR continues to flow through the resistor R ₁ . At this time, the transistor Q ₃ is off because the power supply voltage v is low, and the transistor Q ₂ is off unless the reset switch 7 is turned on (see the first half waveform shown in b of FIG. 2). Next, the return operation will be described.

As described above during the protection circuit operation, the transistor Q ₃ is OFF, the shorting switch 7, the transistor Q ₂
Of the zener diode ZD ₂ is higher than the voltage of the zener diode ZD ₂ though the base voltage of the transistor Q ₂ is low and the power supply voltage v is low, the transistor Q ₂ can be driven and the transistor Q ₂ is turned on. When the transistor Q ₂ turns on, the anode and cathode of the thyristor SCR are short-circuited, and the holding current stops flowing in the SCR, so the SCR turns off. However, the power supply voltage v is still low because the transistor Q ₂ is ON, and the control circuit 2 is not operating (see the latter half waveform shown in b of FIG. 2).

Next, when the switch 7 turns from short-circuited to open, the transistor Q ₂ is turned off and the thyristor SCR is also turned off first, so the power supply voltage v rises due to the current flowing from R _1, and the operation of the control circuit 2 When the voltage is exceeded, the control circuit 2 starts operating and returns. That is, if the reset switch 7 is short-circuited during the operation of the protection circuit, normal operation is restored when the switch 7 is opened (see the first half waveform shown in c of FIG. 2).

Assuming that the reset switch 7 is short-circuited during normal operation, that is, in a state where outputs are normally output from the output terminals 6a and 6b, in this case, the power supply voltage v
Is above the normal value, so transistor Q ₃ is ON.
is doing. Therefore, even if the reset switch 7 is turned on, the transistor Q ₂ remains off regardless of this, and the SCR is not turned on during normal operation, so that the power supply voltage v of the control circuit 2 is not lowered. Therefore, the power output is not reduced. If an overvoltage abnormality occurs while the reset switch remains ON, the overvoltage detection circuit 4 detects this and the phototransistor PT of the photocoupler PC turns on.
To reason, the thyristor SCR is turned ON, a current flows to the Zener diode ZD _2, the power supply voltage v becomes a predetermined level or less, the operation is stopped control circuit 2, output to c of suppressing be (Figure 2 lower (See the second half waveform shown and the first half waveform shown in d).

That is, even if an abnormality occurs while the reset switch 7 is turned on, the protection circuit operates reliably.

(F) Effect of the device According to this device, the voltage level detection circuit for detecting whether the voltage level supplied to the control circuit is higher or lower than a predetermined level, and the operation of the reset switch when the voltage level is higher than the predetermined level. Since it has a reset circuit that disables the power supply and enables the operation of the reset switch when the voltage level is lower than the predetermined level, it is possible to operate the reset switch by mistake while the power supply circuit itself is operating normally. Since the voltage supplied to the control circuit does not decrease, the operation of the reset switch is ignored and the output does not decrease. Further, if an abnormality occurs even during the period when the reset switch is being operated, it is possible to reliably maintain the output reduction.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switching power supply circuit with a protection circuit showing an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation of the switching power supply circuit, and FIGS. 3 and 4 are It is a circuit diagram which shows the conventional switching power supply circuit with a protection circuit. 1: trans, 2: control circuit, 3: rectifier circuit, 4: overvoltage detection circuit, 6a · 6b: Output terminal, 7: reset switch, SCR: a thyristor, Q ₂ · Q _3: the transistor.

Claims (1)

[Scope of utility model registration request] 1. A DC voltage source, a switching circuit for switching an output from the DC voltage source, a transformer for receiving a switched signal in an input winding and deriving it from an output winding, and a rectifying output of the transformer. Rectifier circuit,
An output abnormality detection circuit that detects an abnormality in the output from the rectifier circuit, a thyristor that turns ON in response to an abnormality detection signal of the output abnormality detection circuit, and reduces the voltage supply level to the control circuit, and the switching circuit. Of the thyristor by controlling the switching operation of the thyristor and stopping the switching operation of the switching circuit when the voltage supply level is lowered by turning on the thyristor.
A reset switch for resetting the switching operation to reset the switching operation, and a voltage level detection circuit for detecting whether the voltage level supplied to the control circuit is higher or lower than a predetermined level, and the voltage level is higher than the predetermined level. A switching power supply circuit with a protection circuit, comprising: a reset circuit that invalidates the operation of the reset switch in this case, and validates the operation of the reset switch when the voltage level is lower than a predetermined level.