JP2767010B2 - Overvoltage protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a switching regulator power supply used for a programmable controller or the like when an output to a load side becomes overvoltage.
The overvoltage is detected and fed back to the switching regulator power supply side, the overvoltage is suppressed by shortening the on-duty of the switching regulator power supply, and the parts on the load side are controlled by stabilizing the output voltage to the load side. The present invention relates to an overvoltage protection circuit configured to protect against overvoltage.

[0002]

2. Description of the Related Art FIG. 3 is a block circuit diagram showing an example of a power supply circuit having an overvoltage protection circuit as described above.

In FIG. 1, reference numeral 2 denotes an input terminal, 4 denotes a fuse, 6 denotes a noise filter, 8 denotes a rectifying and smoothing circuit, and 10 denotes a switching regulator power supply. Switching
The regulator power supply 10 includes a power switching circuit 1
0a, a control circuit section 10b, and a voltage drop circuit section 10c. 12 is a step-down transformer, 14 is a 24V rectifying and smoothing circuit, 16 is a 6V rectifying and smoothing circuit, 18 is a voltage detecting circuit for overvoltage protection, and 20 is an output terminal.

[0004] Energy is stored in the step-down transformer 12 during an on-period of a main power transistor (not shown) in a power switching circuit unit 10a of the switching regulator power supply 10 which is an input side of the step-down transformer 12, and the energy is stored in an off-period. The adjusted energy is output to the secondary side. In the control circuit section 10b, the on-period is lengthened at a fixed rate to gradually increase the output voltage. Therefore, the voltage detection circuit 1
8, a power switching circuit that constantly detects the output voltage and performs feedback control on the voltage drooping circuit section 10c of the switching regulator power supply 10 when an overvoltage occurs, so that the on-duty is shortened. The switching operation of the section 10a is controlled to suppress the overvoltage on the secondary side and stabilize the output voltage at a constant level.

FIG. 2 shows the voltage detection circuit 18 described above.
FIG. 9 is a circuit diagram showing a specific configuration of a conventional example.

In the step-down transformer 12, a primary winding 12a
6V rectifying and smoothing circuit 1
6, a rectifying diode D ₀ and a smoothing capacitor C ₀ composed of an electrolytic capacitor are connected. Smooth from both ends of the capacitor C ₀ and the high-potential power supply line L ₁ and the low potential side power supply line L ₀ is derived, it has each end output terminal 20a, and 20b. High potential side power line L
₁ and a low-potential-side power supply line L ₀ , another smoothing capacitor C _{1 in} parallel with the smoothing capacitor C _0.
And a current fuse F ₀ is inserted in the high-potential-side power supply line L ₁ at a stage subsequent to the output side of the smoothing capacitor C ₁ .

The voltage detecting circuit 18 includes two smoothing capacitors C
₀ , C ₁ and between the high-potential-side power line L ₁ and the low-potential-side power line L ₀ . This voltage detection circuit 18 is composed of the following elements. R ₀ , R ₁
Is a voltage divider connected in series, Comp is a comparator using an operational amplifier, and ZD ₀ is a comparator Comp.
_Is a Zener diode that _produces the reference voltage V _REF . The resistance dividing point of the voltage dividing resistors R ₁ and R ₀ is determined by the comparator Com.
is connected to the non-inverting input terminal of the p (+), the cathode of the Zener diode ZD ₀ comparator Comp an inverting input terminal - is connected to ().

The output terminal of the comparator Comp is connected to the base of an NPN type switching transistor Tr ₀ which is an example of a switching element. Switching regulator power supply 1 which is the input side of step-down transformer 12
At 0, the anode of the light emitting diode LED _PC constituting a photocoupler together with a phototransistor not shown is connected to the high potential side power supply line L ₁ via the current limiting resistor R _2.
And the cathode is the switching transistor Tr
Connected to _zero collector. The emitter of the switching transistor Tr ₀ is connected to the low potential side power supply line L ₀ . Light emitting diode LED for this photo coupler
_PC is an example of a feedback element.

The voltage dividing resistors R ₁ and R ₀ are connected to the high-potential-side power supply line L _{1 at} a stage before the current fuse F _{0 and} on the input side.

Next, the operation of the conventional overvoltage protection circuit configured as described above will be described.

Between a _{0 and} a ₁ , the voltage dividing resistors R ₁ , R
₀ detected voltage divided by V _{DE T} is the reference voltage V _REF
In the following cases, since the output of the comparator Comp is at the “L” level, the switching transistor Tr ₀ keeps the non-conductive state, and the light-emitting diode LED _PC for the photocoupler, which is the feedback element, is in the off state. At this stage, the output voltage V _OUT to the load, which appears at the output terminals 20a and 20b, is at the specified level.

When the input voltage from the switching regulator power supply 10 increases, the output voltage V _OUT to the load also increases. When the output voltage V _OUT exceeds the specified level and becomes an overvoltage, the detection voltage V _DET detected by the voltage _dividing resistors R ₁ and R ₀ exceeds the reference voltage V _REF and the comparator Com
The output of p is inverted from “L” level to “H” level.
As a result, the base of the switching transistor Tr ₀
A current flows between the emitters, the switching transistor Tr ₀ is turned on, a current flows through the photocoupler light emitting diode LED _PC , and the light emitting diode L
The ED _PC lights up. This overvoltage detection optical signal is fed back to the input side, that is, to the switching regulator power supply 10, and is controlled so as to shorten the on-duty of a main power transistor (not shown), thereby reducing the energy stored in the step-down transformer 12. , The output voltage V _OUT is automatically returned from the overvoltage state to the specified level. Therefore, each component on the load side is protected from overvoltage.

[0013]

By the way, in this conventional overvoltage protection circuit, the voltage dividing resistors R ₁ and R ₀ in the voltage detection circuit 18 are connected to the high potential side power supply line at a stage before the current fuse F ₀ on the input side. to is connected to L _1, as the voltage V _a1 of _a point a, lower than the output voltage V _OUT by the voltage drop due to current fuse F ₀ is adapted to be detected. With each other to input into the comparator Comp as the detection voltage V _DET what this was the a _1-point voltage V _a1 for divided by voltage dividing resistors R _1, R _0. Therefore, there is a disadvantage that the accuracy of the detection voltage V _DET must be low.

In order to avoid this inconvenience and to increase the accuracy of the detection voltage V _DET , as shown by a two-dot chain line in FIG. 2, the connection point of the voltage dividing resistor R ₁ to the high potential side power supply line L _{1 is set} to a ₁ not a point, the output side of the current fuse F ₀ (load side) is considered to downstream of b ₁ point. In this way, it is possible to avoid the influence of the voltage drop due to current fuse F _0.

However, the modification that simply moves the connection point from the point a _{1 to the} point b ₁ has the following problems.

That is, it is assumed that an overcurrent flows due to some abnormality on the load side, and as a result, the current fuse F ₀ is blown. In this case, since the voltage does not appear at the point b ₁ , the detection voltage V _DET divided by the voltage dividing resistors R ₁ and R ₀ between a _{0 and} b ₁ is independent of the fluctuation of the input voltage. It is fixed to 0 [V].

In such a state where the current fuse F ₀ is blown, the output of the comparator Comp does not reverse even if the high-potential-side power supply line L ₁ becomes overvoltage, and the light emitting diode LED _PC for a photocoupler as a feedback element.
Switching regulator power supply 1
Feedback to zero will not work. Then, as a result, further progressed overvoltage condition of the high potential side power supply line L _1, eventually to become an overvoltage exceeding the withstand voltage of the internal components of the voltage detection circuit 18, the internal components is to be destroyed.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and while improving the accuracy of the detection voltage for overvoltage detection, the present invention has been made in the case where an overvoltage occurs even when a fuse is blown. It is an object of the present invention to reliably perform the feedback to the power supply circuit and protect the internal components of the voltage detection circuit from overvoltage.

[0019]

In the overvoltage protection circuit according to the present invention, a high-potential-side power supply line and a low-potential-side power supply line are connected via a voltage-dividing resistor connected in series, and an input is provided. The element for feedback to the side and the switching element are connected via a series circuit, and a fuse is interposed in the high-potential side power supply line at a later stage on the output side than the connection point with the element for feedback. ,
An overvoltage protection circuit configured to conduct the switching element and drive the feedback element when a detection voltage at a resistance division point of the voltage dividing resistor exceeds a reference voltage, wherein the high potential A connection point of the voltage-dividing resistor to the power supply line is provided at a stage subsequent to the fuse on the output side, and an anode is provided between a connection point between the feedback element and the switching element and the low-potential-side power supply line. Are connected to the low potential side power supply line side and the zener diode is connected.

[0020]

[Function] Since the connection point of the voltage-dividing resistor to the high-potential-side power supply line is provided after the fuse on the output side, the detection voltage at the resistance dividing point of the voltage-dividing resistor is affected by the voltage drop due to the fuse. Does not appear, and the accuracy of the detection voltage is high.

Even when the fuse is blown, when the overvoltage is applied, the driving of the feedback element is not based on the conduction of the switching element by the detection voltage of the voltage dividing resistor, but on the high potential side. Since the feedback element is driven through the overvoltage of the newly provided zener diode to form a series circuit with the feedback element between the power supply line and the low-potential-side power supply line, the input side is configured. Feedback can be applied, and the overvoltage is suppressed by the feedback.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the overvoltage protection circuit according to the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing a specific circuit configuration of the overvoltage protection circuit according to the embodiment.

In the figure, a primary winding 12 a of a step-down transformer 12 is connected to an output side of a switching regulator power supply 10. Secondary winding 12 of step-down transformer 12
b, a rectifier diode D ₀ and a smoothing capacitor C
₀ is connected. Smooth from both ends of the capacitor C ₀ and the high-potential power supply line L ₁ and the low potential side power supply line L ₀ is derived, it has each end output terminal 20a, and 20b. Between the high-potential power supply line L ₁ and the low potential side power supply line L _0, in a parallel state and the smoothing capacitor C _0, another smoothing capacitor C ₁ is connected, the output side of the smoothing capacitor C ₁ current fuse F ₀ to the high potential side power supply line L ₁ is inserted in the subsequent stage.

A voltage detecting circuit 18 which constitutes an overvoltage protection circuit together with a control circuit section and a voltage dropping circuit section (not shown) in the switching regulator power supply 10 includes a high potential side power supply line L between both smoothing capacitors C ₀ and C _{1. 1}
It is formed between the low potential side power supply line L ₀ and. This voltage detection circuit 18 is configured as follows.

The voltage dividing resistors connected in series between the high-potential power supply line L ₁ and the low potential side power supply line L ₀ R _1, R ₀
Connected via However, it is characterized in that the voltage-dividing resistor R ₁ is connected to the high-potential-side power supply line L _{1 at} a point b ₁ on the output side of the current fuse F ₀ .

A current limiting resistor R ₂ , a light emitting diode LED _PC for a photocoupler as a feedback element to the input side, a switching circuit between the high potential power supply line L ₁ and the low potential power supply line L ₀ , A series circuit with an NPN-type switching transistor Tr ₀ as an element is connected. One end of the current limiting resistor R ₂ is connected to the high potential power supply line L _1, the other end a light emitting diode LED _PC
Is connected to the anode, the cathode of the light emitting diode LED _PC is connected to the collector of the switching transistor Tr _0, the emitter of the switching transistor Tr ₀ is connected to the low potential side power supply line L _0. The resistance dividing points of the voltage dividing resistors R ₁ and R ₀ are connected to the non-inverting input terminal (+) of the comparator Comp using an operational amplifier, and the inverting input terminal (−) of the comparator Comp is connected to the reference voltage V of the comparator Comp. The cathode of the first Zener diode ZD ₀ for producing _REF is connected, and the anode thereof is connected to the low potential side power supply line L ₀ . An output terminal of the comparator Comp is connected to the base of the switching transistor Tr _0. The light-emitting diode LED _PC for the photocoupler forms a photocoupler together with a phototransistor (not shown) in the switching regulator power supply 10 which is the input side of the step-down transformer 12. The feedback with the photo coupler is
This is because the secondary side and the primary side are electrically disconnected to prevent the influence of the voltage change on the secondary side from affecting the primary side.

The connection point between the photocoupler light emitting diode LED _PC as the feedback element and the switching transistor Tr ₀ as the switching element is connected to the low potential side power line L ₀ via the second Zener diode ZD _1. Have been. Second anode of the Zener diode ZD ₁ is a low potential side power supply line L ₀ side, the cathode is a light emitting diode LED _PC side. It is also a feature of the present invention in which a Zener diode ZD ₁ of the second.

Next, the operation of the overvoltage protection circuit according to this embodiment configured as described above will be described.

Between a _{0 and} b ₁ , the voltage dividing resistors R ₁ and R
When the detection voltage V _{DE T} which is divided by ₀ is less than or equal to the reference voltage V _REF according to the first Zener diode ZD _0,
Since the output of the comparator Comp becomes “L” level and the switching transistor Tr ₀ is in a non-conducting state, the light-emitting diode LED _PC for a photocoupler is in a non-lighting state. In this case, the output voltage V _OUT to the load, which appears at the output terminals 20a and 20b, is at a specified level.

When the input voltage from the switching regulator power supply 10 rises, the output voltage V _OUT to the load side also rises. When the output voltage V _OUT exceeds a specified level and becomes overvoltage, the voltage dividing resistor R ₁ and R ₀ , the detection voltage V _DET exceeds the reference voltage V _REF and the comparator Comp
Is inverted from "L" level to "H" level.

As a result, the switching transistor Tr
₀ becomes conductive, and the light emitting diode L for the photocoupler
Electric current flows through the ED _PC and it lights up. This overvoltage detection optical signal is fed back to the switching regulator power supply 10 on the input side, and control is performed so as to shorten the on-duty of a main power transistor (not shown) in the switching regulator power supply 10, and is stored in the step-down transformer 12. The output voltage V _OUT is automatically returned from an overvoltage state to a specified level by reducing the energy to be supplied. Thus, each component on the load side is protected from overvoltage.

In the detection of the overvoltage, the connection point of the voltage-dividing resistors R ₁ and R _{0 to} the high-potential-side power supply line L ₁ is provided after the point b ₁ and the output side of the current fuse F ₀ . The detection voltage V _DET at the resistance division point of the voltage dividing resistors R ₁ and R ₀ does not have the influence of the voltage drop due to the current fuse F ₀ , and the detection voltage V _DET has high accuracy. I have.

[0034] Now, then, consider the case where current fuse F ₀ overcurrent flows due to some abnormality of the load side is blown. b stop seeing the voltage in _one point, a ₀ detection voltage V _DET in the resistor division point -b dividing resistors R ₁ in between _1, R ₀ is fixedly 0 regardless of the fluctuation of the input voltage [V], and the output of the comparator Comp is fixed at the “L” level. Therefore, the switching transistor Tr ₀ is not turned on by the comparator Comp even when the overvoltage state occurs.

However, the current limiting resistor R ₂ inserted between the high-potential-side power supply line L ₁ and the low-potential-side power supply line L ₀
And the series circuit composed of the photocoupler light-emitting diode LED _PC and second Zener diodes ZD ₁ Tokyo, always the input voltage to the second cathode of the Zener diode ZD ₁ is applied. Therefore, when the input voltage becomes an overvoltage, the second Zener diode ZD ₁ conducts surrendered, current flows through the light emitting diode LED _PC photocoupler, feedback from the light emitting diode LED _PC to the switching regulator power supply 10 side That would be. This feedback allows switching
Control is performed so as to shorten the on-duty of a main power transistor (not shown) in the regulator power supply 10, the energy stored in the step-down transformer 12 is reduced, and the input voltage is automatically returned from the overvoltage state to a specified level. Thus, the internal components of the voltage detection circuit 18 can be protected from overvoltage.

[0036]

As described above, according to the present invention, the voltage-dividing resistor is connected to the high-potential-side power supply line after the fuse and on the output side, so that the voltage drop due to the fuse is not affected. Therefore, the detection voltage at the resistance dividing point of the voltage-dividing resistor can be made highly accurate. And yet,
A zener diode is connected in series with the feedback element between the high-potential-side power supply line and the low-potential-side power supply line.If an overvoltage occurs even when the fuse is blown, the feedback element is connected through conduction of the zener diode. Since it can be driven and fed back to the input side, overvoltage can be suppressed and the internal components of the voltage detection circuit can be protected.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a specific configuration of an overvoltage protection circuit according to one embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific configuration of a conventional overvoltage protection circuit.

FIG. 3 is a block circuit diagram showing a power supply circuit including a conventional overvoltage protection circuit.

[Explanation of symbols]

REFERENCE SIGNS LIST 10 switching regulator power supply 12 step-down transformer 18 voltage detection circuit D ₀ rectifier diode C ₀ , C ₁ smoothing capacitor L ₀ low-potential-side power supply line L ₁ high-potential-side power supply line R ₀ , R ₁ voltage-dividing resistor R ₂ current limit Resistor Comp Comparator V _DET detection voltage V _REF reference voltage ZD ₀ First Zener diode to generate reference voltage LED Light emitting diode for _PC photocoupler (Feedback element) Tr ₀ Switching transistor (Switching element) F ₀ Current fuse b ₁ High Connection point of voltage-dividing resistor to potential side power supply line ZD ₁ Newly provided second Zener diode for driving light emitting diode at overvoltage

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H02H 9/04 G01R 19/165 H02M 3/28

Claims (1)

(57) [Claims] 1. A series circuit including a high-potential-side power supply line and a low-potential-side power supply line connected via a voltage-dividing resistor connected in series, and a feedback element to the input side and a switching element. Connected via
In the high-potential-side power supply line, a fuse is inserted after the connection point with the feedback element on the output side, and the detection voltage at the resistance division point of the voltage-dividing resistor exceeds a reference voltage. An overvoltage protection circuit configured to drive a feedback element by conducting a switching element, wherein a connection point of the voltage-dividing resistor to the high-potential-side power supply line is provided at a stage after an output side of the fuse. And a Zener diode is connected between the connection point of the feedback element and the switching element and the low-potential-side power supply line with the anode on the low-potential-side power supply line side. Overvoltage protection circuit.