JPH0832166B2 - Overcurrent detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention detects when the output voltage of a main circuit such as a switching regulator droops due to a short circuit or the like and shuts off the output of the switching regulator or the like. The present invention relates to an overcurrent detection circuit.

[Conventional technology]

Conventionally, for example, in a switching regulator or the like, a desired output is obtained by pulse-controlling an input DC voltage. However, when this output drops or becomes zero due to, for example, a short circuit. ,
By detecting this by the overcurrent detection circuit and appropriately controlling the control circuit for performing the pulse control, the output of the switching regulator or the like is cut off to protect the switching regulator or the like.

As such a switching regulator, for example, one having a configuration shown in FIG. 2 is known. That is, in the switching regulator 1 of FIG. 2, the portion shown by the chain line constitutes the overcurrent detection circuit 1a, and the full-wave rectification is performed by the diode from the main circuit 2 of the AC portion of the switching regulator through the current transformer 3. The voltage of the detected output smoothed by the smoothing capacitor 4 and the smoothing resistor 5 is input to one input terminal 7a of the differential amplifier 7 incorporated in the control IC 6, and the other input of the differential amplifier 7 is also input. A reference voltage divided by a resistor is input from a constant voltage power supply to the terminal 7b, and the running amplifier 7 compares the voltages input to both input terminals 7a and 7b and outputs the difference. The signal is output to the base of the switching transistor Tr of the switching regulator 1 via the terminal 8.

According to the switching regulator 1 configured as described above, when the output of the switching regulator 1 is equal to or higher than the predetermined voltage, a detection output of a normal value is generated in the secondary coil of the current transformer 3, and this output is Full-wave rectification by diode, smoothing capacitor 4 and smoothing resistor 5,
Since it is smoothed and input to one input terminal 7a of the differential amplifier 7, the differential amplifier 7 compares the voltage of this detection output with the reference voltage input to the other input terminal 7b. Since the voltage of the detection output is higher, a signal is output from the output terminal thereof, so that the switching transistor Tr is turned on and the operation of the switching regulator is continued.

[Problems to be solved by the invention]

Here, when the output of the switching regulator 1 drops or becomes zero due to a short circuit, damage, etc.,
Since the voltage V _{0 of the} detection output generated in the secondary coil of the current transformer 3 is lowered and the input to the input terminal 7a of the differential amplifier 7 is also lowered, no signal is output from the output terminal 8. , Which makes the switching transistor Tr
Is turned off, and damage to the transistor Tr is prevented, but at this time, the differential amplifier 7 increases the difference in input voltage, so that the output of the switching regulator 1 is increased as the output voltage drops. A signal corresponding to the above voltage difference is output to a pulse width control circuit (not shown).

Accordingly, the output generated in the secondary coil of the current transformer 3, and in the main circuit 2 of the switching regulator 1, the voltage V ₀ drops with a gradient from point B of droop begins as shown by the dotted line A in FIG. 3 , Then the voltage V ₀
In the vicinity of zero, a considerably large current I ₁ will flow compared with the current I ₀ at the start of drooping.

Therefore, the current transformer 3, the control IC 6 including the diode and the differential amplifier 7, the switching transistor Tr, and the circuit elements such as the diode D and the choke coil L that form the main body of the switching regulator 1 have the above-mentioned current I ₁ However, it is necessary to use a current container having a sufficient resistance to the above, and there is a problem that the size of the entire apparatus increases and the cost increases.

In view of the above points, an object of the present invention is to provide an overcurrent detection circuit in which a large current does not flow when the output of a switching regulator or the like droops.

[Means and Actions for Solving Problems]

According to the present invention, the above object is to provide an input side connected to a primary side of a current transformer through a switching element connected to a primary side of a main transformer from one input terminal for detecting an overcurrent, and It has a main circuit consisting of a circuit connected to the output terminal from the secondary side of the main transformer, and inputs the output voltage generated in the secondary side coil of the current transformer to one input terminal via a smoothing circuit, and , The other input terminal has a differential amplifier for inputting a constant voltage as a reference voltage divided by a resistor, and the external output terminal connected to the output terminal of the differential amplifier is connected to the detector transformer. In an overcurrent detection circuit, which is connected to the primary side and the secondary side of the detector transformer is connected to the drive circuit of the main circuit, the secondary side of the current transformer is connected. The auxiliary resistor connected in parallel to the smoothing resistor of the smoothing circuit connected in parallel to the secondary coil on the output side of the rectifier, and the main resistor connected in series to the auxiliary resistor. This is achieved by the fact that the switching element, which is driven off when the output voltage of the circuit becomes equal to or lower than the predetermined voltage, is connected to the output side of the rectifier and is connected in parallel to the output side of the main circuit as a whole. .

According to the present invention, when the output of the main circuit such as the switching regulator is normal, the output voltage is equal to or higher than the predetermined voltage, so that the switching element is on. The output voltage is input to the output side via the combined resistance of the smoothing resistance and the auxiliary resistance, and the detection output generated in the secondary coil of the current transformer is compared with the reference voltage by the differential amplifier, and the difference When the signal is output from the external output terminal, the input side of the main circuit is controlled via the transformer for the detector and the switching element to continue the operation of the main circuit. In the case of drooping due to short circuit, damage, etc., when the output voltage falls below a specified voltage, the switching element turns off, which causes the output voltage to drop. Auxiliary resistance against is blocked,
Since the combined resistance of the smoothing resistance and the auxiliary resistance becomes substantially higher, the load impedance to the current transformer rises, so the output voltage input to one input terminal of the differential amplifier becomes higher, and therefore the detection output When the output current of the main circuit increases due to the action of the differential amplifier due to the drooping, the reference voltage of the differential amplifier is substantially switched to a relatively low reference voltage. The voltage difference input to the differential amplifier when the voltage droops to near zero does not become so large, and the signal input from the differential amplifier to the detector transformer becomes a relatively small signal. Output current does not increase much.

〔Example〕

Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 shows an embodiment of a switching regulator as a main circuit incorporating an overcurrent detection circuit according to the present invention.

In the switching regulator 10, the portion shown by the chain line constitutes an overcurrent detection circuit 11, and a current transformer in which a primary side coil 13a is connected in series to an AC portion circuit 12 of the present switching regulator 10 in which an overcurrent should be detected. 13
And two diodes 14, 14 ′ whose negative side is connected to both ends of the secondary side coil 13 b of the current transformer 13 and whose positive sides are connected to each other, and the + side of the diodes 14, 14 ′ and the current transformer 13 A capacitor 15 and a smoothing resistor 16 respectively connected between the secondary coil 13b and the intermediate point thereof.
And the + side of the smoothing resistor 16 is connected to one input terminal 17a, and a constant voltage is applied to the other input terminal 17b by the resistor 1
The differential amplifier 17 built in the control IC 18 that is divided by 9,20 and input as the reference voltage, and the base of the switching transistor Tr of the switching regulator 10 that is connected to the output terminal of the differential amplifier 17 An external connection terminal 21 for outputting a signal is included.

The above configuration is the same as that of the conventional switching regulator 1 shown in FIG. 2, and in the switching regulator 10 according to the present invention, the overcurrent detection circuit 11 is
Furthermore, the + side of the smoothing resistor 16, that is, the input terminal 17 of the differential amplifier 17
The collector is connected to the a side via the auxiliary resistor 22 and the emitter is connected to the negative side of the smoothing resistor 16, that is, the ground, the base is connected to the ground via the resistor 23, and the zener diode 24 and the resistor 25 are connected. And a transistor 26 as a switching element connected to the output of the switching regulator 10.

The resistance value of the auxiliary resistance 22 is selected so that the resistance value of the combined resistance with the smoothing resistance 16 becomes equal to the resistance value of the smoothing resistance 5 in the conventional example.

The switching regulator 10 according to the present invention is configured as described above, and when the output of the switching regulator 10 is normal, the switching regulator 10
Since the output voltage of is higher than the predetermined voltage, an H level signal is input to the base of the transformer 26 via the resistor 25 and the Zener diode 24. Therefore, the transistor 26 is turned on, which causes the smoothing resistor 16
Since the auxiliary resistor 22 is connected in parallel with respect to the input terminal 17a of the differential amplifier 17, the detection output of the normal value generated in the secondary coil 12b of the current transformer 12, the diode 14, It is full-wave rectified by 14 ', smoothed by the capacitor 15, smoothing resistor 16 and auxiliary resistor 22 and input. As a result, the differential amplifier 17 outputs the signal of the difference from the reference voltage input to the other input terminal 17b to the output terminal 21b.
Output to the primary side of the transformer T _{1 for} a detector via, and control is performed so that the operation of the switching regulator 10 is continued.

Here, when the output voltage of the switching regulator 10 droops due to a short circuit, breakage, or the like, when the output becomes a predetermined voltage V ₁ or less, the Zener diode 24 is turned off, whereby an L level signal is supplied to the base of the transistor 26. Is entered. As a result, the transistor 26 is turned off and the auxiliary resistor 22 is separated from the smoothing resistor 16, and the resistance value of the smoothing resistor is substantially equal to the resistance value of the combined resistance of the smoothing resistor 16 and the auxiliary resistor 22. Increases to 16 independent resistances. Therefore, since the load impedance for the current transformer 13 increases, the detection output generated in the secondary coil of the current transformer 13 is input to the input terminal 17a of the differential amplifier 17 at a higher output voltage level. Therefore, the differential amplifier 17 is substantially switched to a relatively low reference voltage when the output of the switching regulator 10 becomes a certain voltage or less, so that the secondary coil 13b of the current transformer 13 is switched. in the main circuit of the detection output, and the switching regulator 10 generates, as indicated by a solid line C in FIG. 3, first, the voltage V ₀ drops with a gradient from point B starting droop but constant voltages V ₁ or less When this happens, the auxiliary resistor 22 is disconnected, and the output voltage level of the detection output input to the input terminal 17a of the differential amplifier 17 rises, so the signal output from the output terminal 21 of the control IC 18 is detected. Controlled to a lower output for the dexterous transformer T ₁ ,
As a result, the drooping characteristic is shifted in the direction in which the current becomes smaller.Thus, even when the output voltage V ₀ is near zero, the current is too large compared to the current I ₀ at the start of drooping.
I ₁ does not flow, and a current I ₂ lower than this I ₁ flows.

Therefore, the control IC 18 including the current transformer 13, the diodes 14 and 14 'and the differential amplifier 17, the switching transistor Tr, and the circuit elements such as the diode D and the choke coil L that form the main body of the switching regulator 1 are described above. Since it suffices to use a device having a current capacity that can sufficiently withstand a relatively low current I ₂ , it is possible to use a small-capacity and small-sized circuit element for each of the above-mentioned circuit elements, and therefore, the overall size of the device can be reduced. As a result, the cost can be reduced.

〔The invention's effect〕

As described above, according to the present invention, when the output of the main circuit such as the switching regulator is normal, the output voltage is equal to or higher than the predetermined voltage. The output voltage is input to one input terminal of the amplifier through the combined resistance of the smoothing resistance and the auxiliary resistance, and the detection output generated in the secondary coil of the current transformer is compared with the reference voltage by the differential amplifier. By comparing and outputting the signal of the difference from the external output terminal, the output of the main circuit is controlled, for example, the transformer for the detector that constitutes the pulse width control circuit, the switching element is controlled, and the operation of the switching regulator, etc. To continue.

On the other hand, when the output voltage of the main circuit droops due to a short circuit, damage, etc., the switching element is turned off when the output voltage becomes equal to or lower than a predetermined voltage, whereby the auxiliary resistance is cut off against the output voltage. , The combined resistance of the smoothing resistance and the auxiliary resistance is substantially increased, and the load impedance to the current transformer is increased. Therefore, the output voltage input to one input terminal of the differential amplifier is increased, and therefore the detection output is increased. When the output current of the main circuit increases due to the action of the differential amplifier due to the drooping of the differential amplifier, the reference voltage of the differential amplifier is substantially switched to a relatively low reference voltage. The voltage difference that is input to the differential amplifier when the voltage droops to near zero does not become too large, and the detector transistor that constitutes the pulse width control circuit Signal input to the scan also becomes relatively small signal, so that the output current of the switching regulator will not be so much increased.

Thus, according to the present invention, when the output of the main circuit droops, the current value of the output does not increase so much, so that the circuit elements such as the diodes and transistors forming the main circuit have a relatively low current capacity. Therefore, it is possible to provide a very excellent overcurrent detection circuit which can reduce the cost as well as the size of the entire device.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a switching regulator incorporating an overcurrent detection circuit according to the present invention. FIG. 2 is a circuit diagram showing an example of a switching regulator incorporating a conventional overcurrent detection circuit, and FIG. 3 is a graph comparing the output drooping states of the present invention and the conventional overcurrent detection circuit. 10 ... Switching regulator; 11 ... Overcurrent detection circuit; 12
... Detection circuit; 13 ... Current transformer; 14,14 '... Diode; 15 ... Capacitor; 16,19,20,22,23,25 ... Resistance; 17 ... Differential amplifier; 18 ... Control IC; 21 ... Output terminal; 24 ... Zener diode; 26 ... Transistor.

Claims (1)

[Claims] 1. An input side connected to a primary side of a current transformer through a switching element connected to a primary side of a main transformer from one input terminal for detecting an overcurrent, and a secondary side of the main transformer. Has a main circuit consisting of a circuit connected to the output terminal from the side, and outputs the output voltage generated in the secondary coil of the current transformer to one input terminal via a smoothing circuit and the other input. The terminal has a differential amplifier that inputs a constant voltage as a reference voltage divided by a resistor, and the external output terminal connected to the output terminal of the differential amplifier is connected to the primary side of the detector transformer. And a rectifier connected to the secondary side of the current transformer in an overcurrent detection circuit in which the secondary side of the detector transformer is connected for the drive circuit of the main circuit. The output of the main circuit, which is connected in parallel to the smoothing resistance of the smoothing circuit connected in parallel with the secondary coil on the output side of, and is connected in series to the auxiliary resistance, and An overcurrent detection characterized in that a switching element that is driven off when the voltage becomes a predetermined voltage or less is connected to the output side of the rectifier and is connected in parallel as a whole to the output side of the main circuit. circuit.