JPH0246130A - Overload protective circuit for switching regulator - Google Patents

Abstract

PURPOSE:To reduce shortcircuit current by providing a second conduction ratio suppression circuit for suppressing the conduction ratio of intermittent input current when the voltage detected through a voltage detecting means is lower than a predetermined lower limit. CONSTITUTION:When the current (i) flowing through an input side circuit 2 exceeds over a rated current (in) and reaches to the overcurrent set value (io) of a first conduction ratio suppression circuit 20, the first conduction ratio suppression circuit 20 starts to function with protection characteristic (c) where the voltage V drops abruptly but the current (i) continues to increase slightly. When the voltage V drops to the lower limit (vm) according to the characteristic C, a second conduction ratio suppression circuit 40 starts to function. protective characteristic A1 is achieved for maximum suppression of the second conduction ratio suppression circuit 40 and protective characteristic A2 is achieved for minimum suppression of the second conduction ratio suppression circuit 40. By such arrangement, shortcircuit current can be reduced below the overcurrent set value.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a switching regulator used as a DC stable circuit power supply to supply power to various electronic devices, that is, a conduction rate of intermittent current supplied to the input side of a transformer. The present invention relates to a protection circuit for protecting a switching regulator from overload, which adjusts the output side voltage to a constant level by controlling the voltage.

[Conventional technology]

Single or multiple output voltages are supplied from the above-mentioned switching regulator to a load such as an electronic device, but if for some reason a short circuit occurs in the load circuit or the current flowing to the load becomes excessive. It may happen,
In order to protect not only the switching regulator itself but also the load from such short circuits and overcurrents, overload protection circuits have conventionally been built into switching regulators. FIG. 3 shows a conventional example of a switching regulator incorporating an overload protection circuit.

First, to explain the outline of a switching regulator, in the example shown in FIG. This example is a two-output case in which the current flowing through the primary coil is intermittent, and the two output side circuits 3 and 4 handle the intermittent induced voltage received from the secondary coil of the transformer l using diodes 3a and 4a, respectively. After rectifying and smoothing with the inductance of the secondary coil and capacitors 3b and 4b,
Usually, they each output different output voltages Vo.

The output side circuit 3 is the main output circuit, so to speak, and the control circuit 5
0 receives the actual value V of the output voltage of the output side circuit 3 and operates the switching circuit 2C of the input side circuit 2 to obtain the actual value V.
The conduction rate of the intermittent @ current flowing to the input side of the transformer 1 is controlled so that the current becomes a predetermined target value. The output side circuit 4 is a so-called shaft output circuit, and its output voltage is controlled by the control circuit 50 above, but depending on the load conditions, voltage fluctuations different from those of the main output circuit may occur. As an auxiliary voltage control means, a so-called three-terminal regulator 60 is usually incorporated into this shaft output circuit as shown. This three-terminal regulator 60 includes transistors 61. It includes a Zener diode 62 for setting the base potential and a resistor 63 for supplying the Zener diode Ti current and the base current of the transistor.

In order to detect overload conditions such as overcurrent or short circuit, a current detection resistor 3C is inserted in the output side circuit 3, and a current signal representing the electric current it is given to the control circuit 50.

When the value of this current signal exceeds a set value for overcurrent detection, the control circuit 50 reduces or suppresses the conduction rate of the above-mentioned intermittent current via the switching circuit 2C of the input side circuit 2. The current i is limited by A current detection resistor 4c is similarly inserted in the output side circuit 4, but the current signal from now on is output by short-circuiting the Zener diode 62 of the three-terminal regulator 60 by a switch 64, which is indicated by M in the figure. Used to force descent. Switch 64 is a transistor circuit with a predetermined operating threshold.

In the conventional example shown in Figure 3, a current detection resistor is inserted into the output circuit to detect overcurrent, but the resistance value of the current detection resistor must be set quite large to obtain sufficient overcurrent detection sensitivity. There is a problem in that power loss constantly occurs within this resistor, reducing the efficiency of the switching regulator. When there are a plurality of output side circuits as in this example, the decrease in efficiency becomes particularly large.The prior art example shown in FIG.

In FIG. 4, the same parts as in FIG. 3 are given the same reference numerals, and in this example, a single current detection resistor 2d is inserted in the input side circuit 2 as shown. Since the input voltage Vi to the switching regulator is usually much higher than the output voltage Vo, the input current is much smaller than the output current, and the current detection resistor 2d is adjusted so as to generate the desired voltage drop in the event of an overcurrent. Regardless of the resistance value selected, power loss is reduced, and since only a single resistor is required, it is advantageous for improving the efficiency of switching regulators.

[Problem to be solved by the invention]

Conventional example C shown in Fig. 3 above has the problem of large power loss in the current detection resistor and reduced efficiency of the switching regulator as described above, but the conventional example shown in Fig. 4 also has a considerably large short-circuit current. Problems still remain. This will be explained below with reference to FIG.

Figure 2 shows the voltage and voltage during overload protection in the conventional example mentioned above.
Current characteristics are shown in B and C.

Characteristic B is the three-terminal regulator 6 of the output side circuit 4 in Fig. 3.
In this case, when the current i reaches the set value io for overcurrent detection, which is set larger than the rated current In, the voltage V is forcibly lowered, so the current l also decreases at the same time. Therefore, the short circuit current ib becomes a value even lower than the rated current in as shown in the figure.

On the other hand, characteristic C is for the output side circuit 3 in FIGS. 4 and 3, and the control circuit 50 has a current detection resistor 2d.
Or, since the current suppression operation is performed only on the condition that the value of the current l detected at 3c exceeds the overcurrent detection set value io, it is theoretically impossible to suppress the short vII current [1c below the set value io]. Impossible and usually shorted: li
As shown in the figure, the value of current ic increases to more than 1.5 times the rated current in.

During this short circuit, there is no voltage V, so power loss is not very large, but since the short circuit current flows through the secondary coil of transformer L, rectifier diodes, etc., these must have a rated capacity that can withstand large currents continuously. (This would be extremely uneconomical.Although diodes 3a and 4a are simply shown as single diodes in Figures 3 and 4, normally a full-wave rectifier circuit is used, so At least four diodes are required.Also, if the switching regulator is not a flyback type, it is usually necessary to insert a smoothing reactor into the output circuit, so its rated capacity must also be increased. .

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art and provide an overload protection circuit for a switching regulator that can reduce power loss in a current detection resistor and short circuit current.

(Means for Solving the Problems) According to the present invention, this purpose is to provide a switching system in which the output side voltage is adjusted by controlling the conduction rate of the intermittent current supplied to the input side of the transformer, as described at the beginning. A circuit to protect the regulator from overload is configured with a current detection means that detects the input current, and a conduction rate of the input side intermittent current when the current value detected by the current detection means exceeds the overcurrent detection setting value. a first conduction rate suppression circuit that suppresses the conduction rate of the input side intermittent current; a voltage detection means that detects the output side voltage; and a voltage detection means that detects the output side voltage; This is achieved by comprising a second conduction rate suppression circuit that suppresses the current conduction rate.

In principle, the first and second conduction rate suppressing circuits in the above configuration can perform conduction rate limiting operations independently and additively, but the circuit configuration may be changed to an hour wheel. In order to strengthen the operation of the first conduction rate suppression circuit by the second conduction rate suppression circuit, in particular, the overcurrent detection setting value, which is the operating threshold of the first conduction rate suppression circuit, is adjusted. The strength of the suppression action performed by each of the two-way flow rate suppression circuits, which is advantageously increased by decreasing the current rate, can also be selected independently of each other to provide the desired protection characteristics; In this case, it is desirable that the suppression operation of the second conduction rate suppression circuit be stronger than that of the first conduction rate suppression circuit.

[Effect]

In the present invention, first, the overcurrent state for causing the first conduction rate suppressing circuit in the above configuration to start the conductivity rate suppressing operation of the input side intermittent current is detected by the current detecting means in the transformer. By detecting the input current to the current detecting means, power loss generated within the current detecting means can be suppressed to a low level. Therefore, the efficiency of the switching regulator in the case of the present invention is improved compared to the conventional example shown in FIG.

However, it is theoretically impossible to reduce the short-circuit current below the overcurrent detection set value as described above only by limiting the conduction rate by the first conduction rate suppression circuit under the condition of an overcurrent state. Therefore, in the present invention, paying attention to the fact that the output side voltage has a characteristic that decreases as the input side current increases, the voltage detection means and the second conduction rate suppressing circuit as described above are provided, and the voltage detection When the output side voltage detected by the means falls below a predetermined lower limit value, the second conduction rate suppressing circuit further suppresses the conduction rate of the input side intermittent current.

Of course, since it is a switching regulator, when the output side voltage drops, the output side voltage is normally recovered by increasing the conduction rate of the input side intermittent current, but if the output voltage value falls below the lower limit value above. Only occasionally will the output voltage be lowered by suppressing and reducing the current conduction rate.

By this voltage detection means and the second conduction rate suppression circuit, it is possible to newly provide the overload protection circuit with a current suppression effect that is not conditioned on the overcurrent state, thereby reducing the short circuit current (also known as overcurrent detection). The short circuit current can be reduced to a very low value below the rated current value by limiting it to a set value or less, or by providing it with an overload protection characteristic similar to that shown in B in FIG. 2 above.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the drawings. 1st
The figure shows an embodiment of the overload protection circuit according to the present invention incorporated into a switching regulator. In this example, the switching regulator is of the flyback type and separately excited type as in the previous figures 3 and 4. However, the circuit of the present invention is of course not limited to this, and can be similarly applied to forward type and self-help type switching regulators. Note that the same parts in FIG. 1 as in FIGS. 3 and 4 are given the same reference numerals.

In FIG. 1, the input side circuit 2 includes a switching transistor 2a that switches on and off the input side current flowing through the primary coil of the transformer l, and a drive circuit 2b that drives the base of the switching transistor 2a. This corresponds to the switching circuit 2c in FIG. This input side circuit 2 includes a current detection resistor 1 of the current detection means lO in the present invention.
1 is inserted and generates a signal representing the input current l flowing through it, but since the input current is an intermittent current, a stable DC current without pulsation is generated through the smoothing circuit 12 or peak hold circuit. Current detection means I in the form of a signal
Output from O.

In this embodiment as well, the switching regulator has two
Output side circuits 3 and 4 are provided on the output side of the transformer 1, and in this example, both output side circuits are provided with voltage detection means 30 according to the present invention. This voltage detection means 30 has a photodiode 42a of a photocoupler 42 between its pair of output terminals, as shown in detail regarding the output side circuit 3.
It consists of a resistor 31 and a Zener diode 32 connected in series, and the value of the output voltage Vo is the same as that of the second conduction rate suppressing circuit 4.
0 is equal to or higher than the lower limit value vm at which the operation starts, the Zener diode 32 is conductive and the photodiode 42a is lit. However, when the output voltage Vo falls below the lower limit value vw, the Zener diode 32 becomes non-conductive. The photodiode 42a is turned off.

Note that for this output side circuit 3, which is the main output circuit of the switching regulator, it is not necessarily necessary to provide the voltage detection circuit 30 in the above form. Using circuit 3d,
It is also possible to provide the actual value V of the output voltage to the second duty ratio suppression circuit 40.

However, it is desirable to provide the output side circuit 4, which is the shaft output circuit of the switching regulator, with a voltage detection means 30 having the same configuration as above. The signal is sent to the second conduction rate suppression circuit 40 in the form of blinking. In this output side circuit 4, the above-mentioned three-terminal regiator 60 and the capacitor 4d on its input side are provided.

Since FIG. 1 shows the inside of the control circuit 50, which was shown briefly in FIGS. 3 and 4, this will be briefly explained first. The actual voltage value V from the output side circuit 3, which is the main output circuit, is given to the differential amplifier circuit 5, and the difference from the voltage target value νr is given to the voltage adjustment circuit 6, which performs PID operation, for example.

The output of this voltage adjustment circuit 6 is superimposed on a positive setting voltage created from the constant voltage Vc by a setting circuit 7 that sets the reference conduction rate of the input intermittent current, and is connected to one input of the comparator 8 via a diode 8a. given to.

This comparator 8 gives a switching command specifying the conduction rate to the above-mentioned drive circuit 2b, and the other human power is supplied from a triangular wave generating circuit 9 which operates in synchronization with the pulse from the pulse generating circuit 9a within that frame. In this example, the switching transistor 2a is specified to be turned on only for the time when the triangular wave signal value exceeds the voltage value received at one input. Outputs Pln1 type switching command.

Therefore, when the actual voltage value V is higher than the target value vr and one input side of the comparator 8 rises, the conduction rate specified by the switching command given to the drive circuit 2b is lowered, and a reduction in the output voltage is commanded. Conversely, when the actual voltage value is lower than the target value, an increase in the output voltage is commanded.

The first duty ratio suppression circuit 20 connects the positive output voltage of the differential amplifier circuit 21 to the comparator 8 via the diode 8b.
This reduces the throughput rate by giving one hand to the human power. The differential amplifier circuit 21 receives the current signal value from the above-mentioned current detection circuit 10 at its non-inverting input, receives the overcurrent detection setting value at its inverting input, and always outputs the difference between the two values, but the current signal value is in an overcurrent state higher than the overcurrent detection setting value, and therefore, only when its output voltage is positive, it is applied to the comparator 8 via the diode 8b, and the conduction rate is suppressed.

The overcurrent detection set value that specifies the starting point of this conduction rate suppression operation is created by a voltage divider circuit consisting of a pair of series-connected resistors 22 and 23 receiving a constant voltage Vc. However, in this embodiment, a resistor 41 connected in parallel to the phototransistor 42b of the photocoupler 42 of the second conduction rate suppressing circuit 40 is inserted between these resistors 22 and 23, but the output side circuit 3 and 4, while the output voltage value is equal to or higher than the lower limit value at which the second duty ratio suppression circuit 40 starts operating, the phototransistor 42b is on and the resistor 41 is short-circuited, so the overcurrent The detection set value is determined by the voltage division ratio of resistors 22 and 23.

The second conduction rate suppression circuit 40 in this embodiment is configured to suppress the conduction rate by lowering the above-mentioned overcurrent detection setting value of the first conduction rate suppression circuit 20. . A photocoupler 4 in which a photodiode 42a is connected in series to a voltage detection circuit 30 provided in output side circuits 3 and 4.
The second phototransistor 42b is connected to the resistor 41, for example, as shown in the figure, and when the output voltage of any of the output side circuits falls below the above-mentioned lower limit value, the resistor 41 is released from the short-circuited state and detects an overcurrent. The setting value decreases. As a result, the output voltage of the differential amplifier circuit 21 of the first conduction rate suppression circuit 20 increases, and the conduction rate is suppressed more strongly. The degree to which the conduction rate is suppressed by the second conduction rate suppression circuit 40 can of course be appropriately set by the ratio of the resistance values of the resistors 22, 23 and 41.

In FIG. 2, the protection characteristic obtained by the above-described overload protection circuit according to the present invention is indicated by A.

When the current l flowing through the input circuit 2 exceeds its rated current in and reaches the overcurrent detection setting value to of the first conduction rate suppression circuit 20, the first conduction rate suppression circuit 20 starts operating. However, the protection characteristic at that time becomes the above-mentioned characteristic C, in which the voltage V falls quite rapidly, but the current i still continues to increase slightly.

When the voltage V drops to the lower limit value V- along the characteristic line C,
This time, the second conduction rate suppression circuit 40 starts operating. In Figure 0, the protection characteristic when the second conduction rate suppression circuit 40 is maximally suppressed is AI, and the protection characteristic is AI when the effect is the lowest. In the present invention, the characteristic A in the range indicated by hatching in the figure between the two characteristic lines is selected by setting the resistance value ratio of the three resistors described above, for example. Can be done.

As can be seen from the figure, it is desirable to set the minimum effect characteristic A2 of the second duty ratio suppressing circuit so that the short circuit current becomes the overcurrent detection setting value io, and the short circuit current in the case of the maximum effect characteristic AI is It is possible to reduce the conduction rate to a low value corresponding to the characteristic line B described above, and in this case, it is appropriate to reduce the conduction rate to about 10%.

Note that the lower limit value vs of the output voltage is preferably set to a value that is approximately 10% lower than the rated value vn.

〔Effect of the invention〕

In the present invention, an overload protection circuit for a switching regulator in which the output side voltage is adjusted by controlling the conduction rate of intermittent current supplied to the input side of the transformer is provided with a current detecting means for detecting the input side current and a current detecting means for detecting the input side current. a first conduction rate suppression circuit that suppresses the conduction rate of the input side intermittent current when the current value detected by the detection means exceeds an overcurrent detection set value; a voltage detection means that detects the output side voltage; The second conduction rate suppression circuit suppresses the conduction rate of the input side intermittent current when the voltage value detected by the voltage detection means is below a predetermined lower limit value, so power loss can be reduced by reducing the conduction rate of the input side intermittent current. In addition to suppressing the current generated in the current detection means that receives the input side current to a small value, when an overload condition occurs and the output side voltage decreases to below the lower limit value, the input current is suppressed by the second duty ratio suppression circuit. It is possible to effectively suppress the conduction rate of the side current and reduce the short-circuit current ffi value to below the overcurrent detection setting value.

According to the present invention, it is possible to simultaneously satisfy the demands for improving the efficiency of a switching regulator and reducing the required rating values of its output side circuit components. By providing the protection circuit with various protection characteristics, the switching regulator and its load can be most rationally and safely protected from troubles such as damage and burnout.

[Brief explanation of the drawing]

1 and 2 relate to the present invention, FIG. 1 is a circuit diagram illustrating a switching regulator incorporating an overload protection circuit according to the present invention, and FIG. FIG. Figures 3 and 4 are circuit diagrams of switching regulators incorporating different conventional overload protection circuits. ng transistor drive circuit,
2C switching circuit, 2d: current detection resistor, 3: output side circuit or main output circuit, 3a: rectifier diode,
3b: capacitor, 3C: current detection resistor, 3d: voltage division circuit, 4: output side circuit or shaft output circuit, 4a: rectifier diode, 4b = capacitor, 4C: current detection resistor,
4d: Capacitor, 5: Differential amplifier circuit, 6: Voltage adjustment circuit, 7: Setting circuit, 8: Comparator, 8a, 8b:
Diode, 9: Triangular wave generation circuit, 9a: Pulse generation circuit, lO: Current detection means, 11: Current detection resistor, 12
: Smoothing circuit, 20: First conduction rate suppression circuit, 21: Differential amplifier circuit, 22° 23: Overcurrent detection value setting resistor, 30:
voltage detection means, 31: resistor, 32: Zener diode,
40: second conduction rate suppression circuit, 41: suppression effect setting resistor, 42: photocoupler, 42a: photodiode, 4
2b: Phototransistor, 50: Control circuit, 60:3
Terminal regulator, 61 Nigiries dropper transistor, 62: Zener diode, 63: Resistor, 64: Transistor switch, A: Protection characteristics according to the present invention, Al-
Protection characteristics when conduction rate suppression effect is maximum, A2: Protection characteristics when conduction rate suppression effect is minimum, B, C: Protection characteristics according to conventional technology, i: current, ib, ic: short circuit current, in: rated current, io=overcurrent detection setting value, vC: constant voltage, vi: input voltage, vO: output voltage, ■=voltage, v■: lower limit value at which the second conduction rate suppression circuit starts operating, vn: Fig. 1

Claims (1)

[Claims] A circuit for protecting a switching regulator whose output side voltage is adjusted from overload by controlling the conduction rate of an intermittent current supplied to the input side of a transformer, comprising: current detection means for detecting input side current; a first conduction rate suppression circuit that suppresses the conduction rate of the input side intermittent current when the current value detected by the current detection means exceeds an overcurrent detection set value; and a voltage detection means that detects the output side voltage. an overload protection circuit for a switching regulator, comprising: a second conduction rate suppression circuit that suppresses the conduction rate of an intermittent current on the input side when the voltage value detected by the voltage detection means falls below a predetermined lower limit value; .