Protecting circuit against short-circuit of output terminal of AC adapter

The present invention relates to a protecting circuit which can control an output current, against short-circuit of an output terminal of an AC adapter.

In conventional AC adapters for charge, an output voltage thereof is temporarily zero, when a large current flows or an output terminal of an AC adapter is short-circuited. Thereafter, the conventional AC adapters for charge can be charged again by flowing a load current of approximately 500 mA. To prevent heating due to the charge, means for preventing the heating is provided for a casing of the AC adapters.

However, as mentioned above, the output voltage is temporarily zero and the load current of approximately 500 mA thereafter flows to charge the AC adapter again, when the output terminal of the AC adapter is short-circuited and, then, there is a danger in that the output terminal of the AC adapter is repeatedly short-circuited. In this case, since the means for preventing the heating has limitation, there is a danger in that the load is damaged.

Accordingly, it is an object of the present invention to provide a preventing circuit against short-circuit of an output terminal of an AC adapter, which needs no means for preventing the heating by controlling an output current.

The present invention is devised by paying attention to a fact that a value of a controlled constant-voltage is reduced to make the above means for preventing the heating unnecessary, only when it is within an available range.

A protecting circuit in the present invention comprises a current output switching transistor, a first switching transistor, a second switching transistor, a switch control amplifier, and a constant-current control amplifier.

A power source is connected to an emitter of the current output switching transistor, and an output terminal is connected to a collector of the current output switching transistor. The first switching transistor is turned on when a value of a controlled constant-voltage is approximately a value when a voltage difference between a voltage of the power source and an output voltage is over an available range. The second switching transistor is connected to be turned on when the first switching transistor is turned on. The switch control amplifier which controls a switching operation so that the current output switching transistor is turned off when a voltage of the power source is a predetermined voltage or less. The constant-current control amplifier controls a constant current when the second switching transistor is turned on and enters a protecting operation state.

FIG. 1 is a diagram showing the structure of a protecting circuit when an output terminal of an AC adapter is short-circuited in the present invention; and

FIG. 2 is a graph showing a relationship between a load current I and an output voltage V OUT (ΔV F1 ) in the case of a preventing operation.

Hereinbelow, a description is given of a protecting circuit according to an embodiment of the present invention when an output terminal of an AC adapter is short-circuited. FIG. 1 is a circuit diagram showing the structure of the protecting circuit when the output terminal of the AC adapter is short-circuited. Referring to FIG. 1, the protecting circuit when the output terminal of the AC adapter is short-circuited in the present invention, comprises a current output switching transistor Tr 3 , a switch control amplifier, a switching transistor Tr 1 a switching transistor Tr 2 , and a constant-current control amplifier.

The switch control amplifier controls an on/off operation of the current output switching transistor Tr 3 . A load detecting resistor R Ls is connected between an emitter and a collector of the current output switching transistor Tr 3 . A load detecting resistor R LB is connected to a base of the current output switching transistor Tr 3 . A power source V cc is connected to the emitter of the current output switching transistor Tr 3 , and an output terminal OUT is connected to the collector of the current output switching transistor Tr 3 .

The power source V cc is connected to an emitter of the switching transistor Tr 1 . The output terminal OUT is connected to a base of the switching transistor Tr 1 . A collector of the switching transistor Tr 1 is connected to a base of the switching transistor Tr 2 .

Resistors R 1 , R 2 , and R 3 are connected to a negative input terminal (−) of the constant-current control amplifier. A reference voltage source Vz is connected to one end of the resistor R 2 . A current detecting resistor R cs is connected to a positive input terminal of the constant-current control amplifier.

Next, a description is given of operations of the protecting circuit when the output terminal of the AC adapter is short-circuited. FIG. 2 is a graph showing a relationship between an output voltage V OUT (ΔV F1 ) and a load current I (charge current) which flows to the load shown in FIG. 1 . In the protecting circuit having the above-mentioned structure, a constant current is controlled during charging by a current I 1 which is shown by the following expression. I 1 = ( Vz × R 1 R 1 + R 2 ) / Rcs ( 1 )

On the other hand, the switch control amplifier is set so that the switching transistor Tr 3 is turned off so as to prevent the decrease in voltage of the power source V cc when the power source V cc is 2.4V (4 VF) or less. When the load is increased in this state, a voltage difference between the power source V cc and the output terminal OUT is 0.6V (1 VF) or more. Then, the switching transistor Tr 1 is turned on. Accordingly, the switching transistor Tr 2 is turned on and the voltage (V 2 ) of the negative terminal of the constant-current control amplifier is given in the following expression (2). V 2 = ( Vz ÷ ( R 1 × R 3 R 1 + R 3 + R 2 ) ) × ( R 1 × R 3 R 1 + R 3 ) ( 2 )

Therefore, the protecting operation of the protecting circuit in FIG. 1 is controlled by a current I 2 which is shown by the following expression (3).

I 2 =V 2 /Rcs   (3)

FIG. 2 shows the relationship between the load current I and the output voltage V OUT in the protecting operation. A start voltage in the protecting operation is determined by the output voltage V OUT within an available range shown in FIG. 2 .

Referring to FIG. 2, in the case of a curve formed by the output voltage V OUT and the load current I in the above operation when the available range is within 2V to 5V and the output voltage is below 2V, the output current is reduced for the protecting operation. Conventionally, even if the available range is below 2V, the load current I is I 1 (approximately 500 mA). When the output terminal of the AC adapter is short-circuited, the output voltage is temporarily 0V and the load current I 2 (approximately 160 mA) thereafter flows to the load. When the output voltage is over the available range, the load current I 1 flows again. When the output voltage is close to an upper limit value within the available range, the load current is gradually reduced. That is, the current can be controlled to have ideal characteristics.

In the present invention, when the output terminal is short-circuited and the output terminal is thereafter is below the available range, the output current can be protected and the current output switch can also be protected. The means for preventing the heating can be made unnecessary by controlling the output current.