JP2797210B2 - Overcurrent protection circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an overcurrent protection circuit for a plurality of switching regulators.

2. Description of the Related Art In an electronic device that requires a plurality of types of stabilized DC voltages and consumes relatively large power, a configuration in which power is supplied using a plurality of switching regulators is employed. In such an electronic device, when the supply current by any voltage increases beyond the rated value, it is considered that an abnormality has occurred and all the supply is stopped. However, in the case of a temporary overcurrent state, the state returns to the normal state in a short time. However, once the overcurrent is detected and the protection action is performed by stopping the power supply, the automatic recovery cannot be performed. It was necessary to restart by such as. Therefore, there is a demand for automatically re-supplying power.

[Conventional technology]

A conventional power supply device using a plurality of switching regulators has, for example, the configuration shown in FIG. 3, in which switching elements such as field-effect transistors and bipolar transistors are connected to primary windings of transformers 31 and 41, ON and OFF are controlled by the width control circuits 35 and 45, respectively. When the switching elements 32 and 42 are turned on, a DC input voltage is applied to the primary windings of the transformers 31 and 41, and a voltage is induced in the secondary windings. Is rectified and smoothed to obtain a DC output voltage. This DC output voltage is set to various voltages in accordance with the demands of the load side.
In addition to the set reference voltages, the ON widths of the switching elements 32 and 42 are controlled in accordance with the error voltage, so that a stabilized DC output voltage is output.

The current supplied to the load is detected by current detectors 34 and 44 comprising resistors, current transformers, and the like, and applied to the overcurrent detection circuit 36. When the current state is determined, an alarm signal is applied to the latch circuit 37 for latching, and the output signal of the latch circuit 37 is applied to the pulse width control circuits 35 and 45 as a stop signal. Thereby, the pulse width control circuits 35, 45
Keeps the switching elements 32 and 42 in the off state.

When restarting such a power supply circuit, the latch circuit
The reset signal is added to 37 to reset the stop signal, and the operations of the pulse width control circuits 35 and 45 are restarted.

The pulse width control circuits 35 and 45 have, for example, the configuration shown in FIG. 4, where 51 is an oscillator, 52 and 53 are comparators, 54 and 55 are error amplifiers, 56 is a flip-flop, 57 is an OR circuit, 58 , 59 are NOR circuits, 60 is an inverter, 61 and 62 are AND circuits 63 and 64, output transistors, RT is a terminal connecting a resistor R, CT is a terminal connecting a capacitor C, DT is a dead time control terminal, + IN, -IN is a rectified smoothed output voltage or an input terminal to which a divided voltage is applied, FB is a feedback terminal, OC
Is an output control terminal, and C1, C2, E1, and E2 are output terminals.

This pulse width control circuit has a configuration applicable to a two-switching type switching regulator. The oscillator 51 charges the capacitor C with a constant current, and when the terminal voltage of the capacitor C reaches a set value, a rapid discharge occurs. In this way, a sawtooth signal is generated, and the resistance R [K
Ω] and the capacitor C [μF], the oscillation frequency f
Is f ≒ 1.2 / (RC) [KHz]. The comparator 53 has a sawtooth signal and error amplifiers 54, 5
The output signals of the output transistors 63 and 64 are controlled via the OR circuit 57 and the NOR circuits 58 and 59 to control the ON period of the output transistors 63 and 64. The ON periods of the switching elements 32 and 42 in the figure are controlled.

Note that even when the dead time control terminal DT is in the no-bias state, the bias voltage is applied to the comparator 52, and the duty
It is configured not to be 100%. When the output control terminal OC is set to “0”, the output signals of the AND circuits 61 and 62 are set to “0”, the output transistors 63 and 64 are simultaneously turned on or off, and when the output control terminal OC is set to “1”, the AND Since the circuits 61 and 62 are opened, the output transistors 63 and 64 perform the opposite-phase operation in response to the inversion operation of the flip-flop 56.

[Problems to be solved by the invention]

As described above, when a plurality of switching regulators are used to supply a stabilized DC voltage of various voltage values to an electronic device, when an overcurrent is detected, the operation of all switching regulators is stopped. Further, when the overcurrent state is released, the latch circuit 37 must be manually reset, and there is a problem in operability.

An object of the present invention is to automatically restore a normal state when the overcurrent state is released by limiting the supply current without completely stopping when the overcurrent state occurs. It is.

[Means for solving the problem]

The overcurrent protection circuit of the present invention performs overcurrent protection using the dead time control terminal of the pulse width control circuit, and will be described with reference to FIG.

A switching element 2 such as a transistor connected to a primary winding of a transformer 1; a rectifying / smoothing circuit 3 connected to an arbitrary number of secondary windings of the transformer 1; and a current detecting unit for detecting an output current of the rectifying / smoothing circuit 3 4 and a pulse width control circuit 5 for controlling the ON width of the switching element 2, the dead time control of the pulse width control circuit 5 corresponding to the plurality of switching regulators in the overcurrent protection circuit of the plurality of switching regulators A comparison circuit 7 is commonly connected to the terminal 6, a set reference voltage is applied to one terminal of the comparison circuit 7, and a detection signal of the current detection unit 4 corresponding to each rectifying / smoothing circuit 3 is wired-OR connected to the other terminal. When the detection signal is larger than the set reference voltage, it is determined that an overcurrent state has occurred, and a voltage that increases the dead time is supplied to the dead time control terminal 6. It was configured to be pressurized.

[Action]

Since the detection signal from the current detection unit 4 is wired-OR connected and input to the other terminal of the comparison circuit 7, the detection signal having the largest value is input to the comparison circuit 7. That is, the maximum detection signal among the detection signals normalized corresponding to various rated output currents is input to the comparison circuit 7 and compared with the set reference voltage. When the detection signal is larger than the set reference voltage, it is determined that an overcurrent has been supplied to the load, and a control voltage is applied to the dead time control terminal 6 of the pulse width control circuit 5 so that the pulse width control circuits 5 of all the switching regulators are controlled. Prolong the dead time at As a result, the output is limited, and overcurrent can be suppressed.

When the overcurrent state is released, the voltage applied from the comparison circuit 7 to the dead time control terminal becomes zero.
It will automatically return to the original control state.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a main part circuit diagram of an embodiment of the present invention.
Is a switching regulator, 11, 21 is a transformer, 12, 22
Are switching elements such as field effect transistors and bipolar transistors, 13a, 13b, 23a and 23b are rectifying and smoothing circuits, 1
4a, 14b, 24a, 24b are current detectors using current transformers and resistors, etc., 15 and 25 are pulse width control circuits, DT is a dead time control terminal, VI is a voltage detection terminal, 16 and 26 are drive transformers, 1
7 is an operational amplifier, D1 to D5 are diodes, and R1 to R7 are resistors.

The switching regulators 10 and 20 show a case of a multi-output configuration.In general, the DC output voltages of the switching regulators 10 and 20 have different values according to the demands of the load side, but the pulse width control circuits 15 and 25 have the same configuration. Has a dead time control terminal DT,
It has a voltage detection terminal VI and the like, and controls on / off of the switching elements 12 and 22 via the drive transformers 16 and 26. A set voltage obtained by dividing the + V voltage by the resistors R1 and R2 is applied to the dead time control terminal DT, and the minimum dead time is set.

The rectifying / smoothing circuits 13a, 13b, 23a, and 23b each include a rectifying diode (not shown), a smoothing capacitor, an inductance, and the like. Is detected by the diode D
Wired or connected via 1 to D4. Therefore, the detection signal of the maximum value is input to the + terminal of the operational amplifier 17 forming the comparison circuit via the diode.

In this case, the detection signals of the current detectors 14a, 14b, 24a, 24b are directly output from the operational amplifiers 17 via the diodes D1 to D4 when the rated output currents from the rectifying and smoothing circuits 13a, 13b, 23a, 23b are the same. However, if the rated output currents are different from each other,
For example, the rectifying / smoothing circuits 13a, 23a, and 24b have the same rated output current, and the rectifying / smoothing circuit 13b has a rated output current that is twice the rated output current. In this case, the current detector 14b corresponding to the rectifying and smoothing circuit 13b
Is set to a half value by a resistor or the like.

The -V terminal of the operational amplifier 17 is connected to the resistors R3, R
Divided by 4, added as a set reference voltage, compared with the maximum value detection signal, the comparison output signal is diode D5
To the dead time control terminal DT of the pulse width control circuits 15 and 25.

When any one of the rectifying / smoothing circuits 13a, 13b, 23a, and 23b enters an overcurrent state in which a current exceeding the rated output current is supplied, a detection signal applied to the + terminal of the operational amplifier 17 via the diodes D1 to D4 is generated. , And the positive reference signal of the operational amplifier 17 is applied to the dead time control terminal DT of the pulse width control circuits 15 and 25 via the diode D5. Is controlled to be longer. That is, in the pulse width control circuit shown in FIG. 4, when the voltage applied to the dead time control terminal DT is increased, the period in which the level of the sawtooth wave signal by the oscillator 51 is high with respect to this voltage is shortened, and the error amplifier The dead time becomes longer regardless of the output of 54 and 55. Therefore, each DC output current of the switching regulators 10, 20 is limited, and the load and the switching regulators 10, 20 can be protected from overcurrent.

When such an overcurrent state is released, the operational amplifier 17
Since the detection signal applied to the + terminal is smaller than the set reference voltage applied to one terminal, the polarity of the output signal of the operational amplifier 17 is inverted, and the output signal is connected to the dead time control terminal DT by the diode D5. Then, only the voltage preset by the resistors R1 and R2 is applied to the dead time control terminal DT, and a normal control state is set.

The embodiment described above has two switching regulators 1
Although the case of a configuration using 0, 20 is shown, the same configuration can be applied when a larger number of switching regulators are used.

〔The invention's effect〕

As described above, according to the present invention, the comparison circuit 7 is commonly connected to the dead time control terminals 6 of the pulse width control circuits 5 corresponding to a plurality of switching regulators, and the set reference voltage is applied to one terminal of the comparison circuit 7. The detection signal of the current detection unit 4 is input to the other terminal by wired-OR connection, and when the detection signal is larger than the set reference voltage, it is determined that an overcurrent state has occurred, and the voltage is applied to the dead time control terminal 6. However, since the dead time is lengthened and the output current is suppressed, the load and the switching regulator can be protected from overcurrent. When the overcurrent state is released, no voltage is applied to the dead time control terminal 6 from the comparison circuit 7, so that the control state returns to the normal state. That is, there is an advantage that the normal power supply state can be automatically returned to at the same time when the abnormal load state is restored.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention, FIG. 2 is a circuit diagram of a main part of an embodiment of the present invention, FIG.
The figure is a block diagram of the pulse width control circuit. 1 is a transformer, 2 is a switching element, 3 is a rectifying and smoothing circuit, 4 is a current detector, 5 is a pulse width control circuit, 6 is a dead time control terminal, and 7 is a comparison circuit.

Claims (1)

(57) [Claims] 1. A switching element (2) connected to a primary winding of a transformer (1); a rectifying and smoothing circuit (3) connected to an arbitrary number of secondary windings of the transformer (1); Overcurrent of a plurality of switching regulators each including a current detection unit (4) for detecting an output current of a circuit (3) and a pulse width control circuit (5) for controlling an ON width of the switching element (2). In the protection circuit, a comparison circuit (7) is commonly connected to a dead time control terminal (6) of the pulse width control circuit (5) corresponding to the plurality of switching regulators, and one of the comparison circuits (7) is connected. A set reference voltage is applied to a terminal, and a detection signal of the current detection unit (4) corresponding to each of the rectification / smoothing circuits (3) is input to the other terminal by wire-OR connection, and the detection signal is obtained from the set reference voltage. Is big An overcurrent protection circuit, characterized in that it is determined that an overcurrent state has occurred, and a voltage that increases the dead time is applied to the dead time control terminal (6).