JP3158239B2 - Overcurrent detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent detection circuit for outputting a detection signal corresponding to the magnitude of an overcurrent.
The switching power supply controls the switching so as to make the output DC voltage constant.Because the switching power supply is a constant voltage power supply, it operates so as to maintain the output DC voltage constant even when the load is short-circuited. A large current will flow. Therefore, a configuration is adopted in which such an overcurrent is detected to drop the output DC voltage. Therefore, it is necessary to detect the overcurrent quickly and accurately.

[0002]

2. Description of the Related Art FIG. 2 is an explanatory diagram of a conventional overcurrent detection circuit based on peak rectification.
32 is a load of various electronic devices, 33 is a current transformer, 34 is a diode, 35 is a capacitor, 36 is a resistor, 37 is a comparator, 38 is a set voltage, 39 is an overcurrent detection circuit, 40 is a switching transistor, and 41 is a switching transistor. A pulse width control circuit, 42 is a transformer, 43 and 44 are diodes, 45 is a choke coil, and 46 is a capacitor.
OUT is an output terminal of a switching control signal, IN is an input terminal for detecting an output DC voltage, and DT is a dead time control terminal.

The switching power supply unit 31 includes a transformer 42
Switching transistor 40 connected to the primary winding
Of the transformer 42 is controlled by a switching control signal from an output terminal of the pulse width control circuit 41.
Is rectified and smoothed by a rectifying and smoothing circuit including diodes 43 and 44, a choke coil 45, and a capacitor 46, and an output DC voltage is applied to the load 32. Is applied to the input terminal IN of the pulse width control circuit 41, is compared with the set reference voltage, and the ON period of the switching transistor 40 is controlled so as to stabilize the output DC voltage.

The load current is detected by a current transformer 33 and applied to a capacitor 35 via a diode 34. The terminal voltage of the capacitor 35 corresponds to the peak value of the load current. In the overcurrent state, the terminal voltage of the capacitor 35 becomes high. Therefore, an overcurrent detection signal is applied from the comparator 37 to the dead time control terminal DT of the pulse width control circuit 41, and the pulse width control circuit 41 Extend the time. That is, control is performed such that the off period of the switching transistor 40 is lengthened. As a result, the output DC voltage is drooped, and an overcurrent can be prevented.

FIG. 3 is an explanatory view of a conventional overcurrent detection circuit. Reference numeral 51 denotes a switching power supply unit, 52 denotes a load of various electronic devices, 53 denotes a current, 54 denotes a diode, 55 denotes a capacitor, and 56 to 63 denote capacitors. Resistor, 64, comparator, 65, capacitor, 66, set voltage, 67, shunt regulator, 68, photocoupler, 69, light emitting diode, 7
0 is a photodiode.

The switching power supply unit 51 has a configuration similar to that of the switching power supply unit 31 shown in FIG. 2, detects a current supplied to a load 52 by a current transformer 53, rectifies the current by a diode 54, and charges a capacitor 55. . Therefore, the terminal voltage of the capacitor 55 corresponds to the peak load current. The terminal voltage of the capacitor 55 is divided by the resistors 57 and 58, and is compared with the set voltage 66 by the comparator 64. If the divided voltage is lower than the set voltage 66, that is, if the overcurrent state is not present, the comparator 64 The output signal becomes low level, and the charge of the capacitor 65 is discharged through the resistor 60 and discharged through the resistors 61 and 62, and becomes almost zero.

When the voltage divided by the resistors 57 and 58 exceeds the set voltage 66, that is, when an overcurrent state occurs,
Since the output signal of the comparator 64 becomes high level, the capacitor 65 is charged by the power supply voltage via the resistors 59 and 60. The terminal voltage of the capacitor 65 is connected to the resistors 61 and 6
When the divided voltage exceeds a predetermined value, the shunt regulator 67 is turned on. As a result, a current flows through the light emitting diode 69 of the photocoupler 68 via the resistor 63 and is incident on the photodiode 70 of the switching power supply unit 51 as an optical signal of overcurrent detection,
The dead time of the switching power supply 51 is extended,
The output DC voltage of the switching power supply 51 drops.

[0008]

The conventional example shown in FIG.
Since the configuration is such that the peak load current is detected, even if the load current temporarily exceeds the set value for some reason, the output DC voltage of the switching power supply unit 31 is dropped as overcurrent detection. There is a problem that the DC voltage to be fluctuated greatly.

On the other hand, in the conventional example shown in FIG. 3, the terminal voltage of the capacitor 55 corresponds to the peak load current. When the current state continues,
An overcurrent detection signal is applied to the switching power supply unit 51 to droop the output DC voltage. Since a temporary peak load current is not detected as an overcurrent, a stabilized DC voltage is supplied to the load 52. be able to. However, when the load current greatly exceeds the set value or slightly exceeds the set value, the overcurrent detection signal is transmitted after a time according to the charging / discharging time constant of the capacitor 65 in the same manner. And there is a problem that a time delay occurs. An object of the present invention is to detect an overcurrent with a time delay corresponding to the magnitude of a load current and to optimize overcurrent protection.

[0010]

An overcurrent detection circuit according to the present invention comprises a current detection section 3 for detecting a current supplied from a switching power supply section 1 to a load 2, and a diode 4 for detecting a detection signal of the current detection section 3. A first capacitor 5 added via
A comparator 14 that sets the output terminal to a high level when the terminal voltage of the first capacitor 5 exceeds a set voltage and indicates an overcurrent state; a second capacitor 15 connected to the output terminal of the comparator 14; A resistor 9 connected to apply the terminal voltage of the first capacitor 5 to the second capacitor 15, and a detection signal for sending an overcurrent detection signal when the terminal voltage of the second capacitor 15 exceeds a predetermined value. An output unit.

When the current supplied from the switching power supply unit 1 to the load 2 is not in an overcurrent state, the output terminal of the comparator 14 is at a low level, the second capacitor 15 is in a discharged state, and the terminal voltage becomes almost zero. It is zero. When the current supplied to the load 2 is in an overcurrent state, the first capacitor 5
Is higher than the set voltage, the output terminal of the comparator 14 becomes high level, and the terminal voltage of the first capacitor 5 is applied to the second capacitor 15 via the resistor 9. At this time, the diode 4 corresponding to the magnitude of the load current
, The rise in the terminal voltage of the second capacitor 15 corresponds to the magnitude of the load current.
When the terminal voltage of the second capacitor 15 exceeds a predetermined value, an overcurrent detection signal is sent to the switching power supply unit 1 from a detection signal output unit including the shunt regulator 17, the photocoupler 18, and the like. Therefore, when the load current greatly exceeds the set value as compared with the case where the load current slightly exceeds the set value, the overcurrent detection signal is output in a short time.

The detection signal output section is provided with a second capacitor 1
The shunt regulator 17 is turned on when this voltage exceeds a predetermined value.
And a light emitting diode 19 of a photocoupler 18 connected in series with the shunt regulator 17 to apply an overcurrent detection signal to the switching power supply 1 as an optical signal.

The shunt regulator 17 is applied with a voltage obtained by dividing the terminal voltage of the second capacitor 15 by the resistors 11 and 12. When the voltage exceeds a predetermined value, the shunt regulator 17 is turned on. That is, when the current supplied to the load 2 is in an overcurrent state, the shunt regulator 17 is turned on,
An electric current flows through the light emitting diode 19 constituting the photocoupler 18 to emit light. This optical signal is incident on the photodiode 20 of the switching power supply unit 1, and the output DC voltage of the switching power supply unit 1 drops.

[0014]

FIG. 1 is an explanatory view of an embodiment of the present invention.
1 is a switching power supply unit, 2 is a load of various electronic devices, etc.
Reference numeral 3 denotes a current transformer constituting a current detector, 4 denotes a diode, 5 denotes a first capacitor, 6 to 13 denote resistors, 14
Is a comparator, 15 is a second capacitor, 16 is a set voltage,
17 is a shunt regulator, 18 is a photocoupler, 1
9 is a light emitting diode and 20 is a photodiode.

The switching power supply unit 1 can employ various known configurations, and applies a stabilized DC voltage to the load 2. The current supplied to the load 2 is detected by the current transformer 3 and rectified by the diode 4 to charge the first capacitor 5. The terminal voltage of the first capacitor 5 is divided by the resistors 7 and 8 and is compared with the set voltage 16 by the comparator 14. When the divided voltage is lower than the set voltage 16, that is, when the load current is equal to or less than the set value, the excess voltage is applied. When not in the current state, the output terminal of the comparator 14 is at a low level.

Therefore, even if the resistors 9 and 10 are connected so that the terminal voltage of the first capacitor 5 is applied to the second capacitor 15, the output terminal of the comparator 14 is at a low level. The second capacitor 15 is discharged via the resistor 10, and its terminal voltage becomes almost zero. The shunt regulator 17 is turned on when a voltage obtained by dividing the terminal voltage of the second capacitor 15 by the resistors 11 and 12 is applied to the gate and this voltage exceeds a predetermined value. When the terminal voltage of the second capacitor 15 is substantially zero, the shunt regulator 17 is off.

Also, the load current rises and the first capacitor 5
Terminal voltage rises corresponding to the load current, and the resistance 7,
When the divided voltage by the voltage 8 exceeds the set voltage 16, the comparator 1
The output terminal 4 is at a high level. Thereby, the terminal voltage of the first capacitor 5 is changed to the second voltage via the resistors 9 and 10.
, And the terminal voltage of the second capacitor 15 increases according to the charging time constant.

When the voltage obtained by dividing the terminal voltage of the second capacitor 15 by the resistors 11 and 12 exceeds a predetermined value,
The shunt regulator 17 is turned on, a power supply voltage is applied to the light emitting diode 19 of the photocoupler 18 via the resistor 13, and the light emitting diode 19 emits light. That is, an optical signal for overcurrent detection is transmitted. In the switching power supply unit 1, the photodiode 20 of the photocoupler 18 receives an optical signal of overcurrent detection, for example, extends the dead time, causes the output DC voltage to drop, and reduces the load current. That is, overcurrent protection can be performed.

In this case, the terminal voltage of the second capacitor 15 rises faster as the load current increases, so that the overcurrent detection signal can be sent in a short time and the load current slightly increases. When the current exceeds the set value, the overcurrent detection signal can be transmitted after a time sufficient to determine whether the current is a temporary overcurrent or a continuous overcurrent. That is, the overcurrent detection signal can be transmitted in a relationship inversely proportional to the magnitude of the overcurrent.

The present invention is not limited to the above-described embodiment, but can be variously added and changed.
For example, a resistor may be used instead of the current transformer 3 constituting the current detection unit. Also, a case is shown in which the terminal voltages of the first and second capacitors 5 and 15 are divided by a resistor, but the voltage can be compared without dividing the voltage. The detection signal output unit has a configuration including the shunt regulator 17, the light emitting diode 19 of the photocoupler 18, and the resistor 13. The shunt regulator 17 is used as a comparator, and the set voltage of the terminal voltage of the second capacitor 15 is set. It is also possible to use a configuration for comparing with.

Although the case where an overcurrent detection signal is transmitted to the switching power supply unit 1 by the photocoupler 18 is shown, the signal may be transmitted directly to the switching power supply unit 1 as an electric signal or transmitted via a transformer. To prevent the influence of noise, a resistor is connected in parallel with the first capacitor 5, a diode is connected between the first capacitor 5 and the resistor 7, and a capacitor is connected in parallel with the resistors 7 and 8. Can also be connected.

[0022]

As described above, according to the present invention, the terminal voltage of the first capacitor 5 for detecting and charging the load current is connected to the second capacitor 15 in the overcurrent state by the resistor 9,
10 through the second capacitor 1
Since the terminal voltage of the terminal 5 rises in accordance with the magnitude of the current in the overcurrent state, in an overcurrent state such as a load short circuit, an overcurrent detection signal is quickly transmitted to protect the switching power supply unit 1. can do. Therefore, the switching power supply unit 1
Overcurrent protection of the switching power supply unit 1 can be optimized and heat generation at the time of overload of the switching power supply unit 1 can be reduced, so that there is an advantage that downsizing can be achieved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional overcurrent detection circuit based on peak rectification.

FIG. 3 is an explanatory diagram of a conventional overcurrent detection circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Switching power supply part 2 Load 3 Current transformer 4 Diode 5 First capacitor 6-13 Resistance 14 Comparator 15 Second capacitor 17 Shunt regulator 18 Photocoupler

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01R 19/00-19/32 G05F 1/00-1/70 H02H 9/00-9/08 H02M 1 / 00-1/30

Claims (2)

(57) [Claims] 1. A current detection unit for detecting a current supplied from a switching power supply unit to a load, a first capacitor for applying a detection signal of the current detection unit via a diode, and a terminal voltage of the first capacitor. A comparator that sets an output terminal to a high level when an overcurrent state is indicated by exceeding a set voltage; a second capacitor connected to an output terminal of the comparator; and a terminal of the first capacitor connected to the second capacitor. An overcurrent detection circuit, comprising: a resistor connected to apply a voltage; and a detection signal output unit for transmitting an overcurrent detection signal when a terminal voltage of the second capacitor exceeds a predetermined value. . 2. The shunt regulator according to claim 1, wherein the detection signal output unit applies a voltage obtained by dividing a terminal voltage of the second capacitor, and turns on when the voltage exceeds a predetermined value. 2. The overcurrent detection circuit according to claim 1, further comprising: a light-emitting diode of a photocoupler connected to the switching power supply unit to connect the overcurrent detection signal to the switching power supply unit as an optical signal.