JPH0534028Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to a DC power supply circuit, and particularly to a current limiting circuit that limits the output current for protection during overload.

[Summary of the idea]

This device supplies DC power to the load via a current detection resistor and a current limiting transistor.During overload, the current flowing through the current detecting resistor is detected and the current limiting transistor is controlled to load the load. In a current limiting device, a current detecting resistor may have a small resistance, and the variation in current value during current limiting is small, and the temperature characteristics are good.

[Conventional technology]

For example, if you want to play back images recorded with a camera-integrated VTR on a TV receiver, the camera-integrated VTR
The output must be converted into an RF signal for the TV's open channel, and an RF converter unit is used for this purpose.

When power is supplied to this RF converter unit from the VTR side, there are some that output the power through a mini jack. In such cases, due to incorrect insertion or operation of the plug, or during the plug insertion process, a short circuit may occur at the jack terminals, and at this time, the equipment on the jack side, that is, the VTR.
It is necessary to take protective measures to limit the short-circuit current to a set value or less to prevent the side from being damaged.

FIG. 2 shows an example of a current limiting circuit that achieves this purpose, and 1 is a power supply terminal to which a DC voltage from a DC power supply E is supplied. This power supply terminal 1 is connected to a terminal 4 to which a load Z is connected via a current detection resistor 2 and an emitter-collector of a transistor 3 as a variable impedance element. When this circuit is provided in the above-mentioned camera-integrated VTR, this terminal 4 is a mini jack. The base of transistor 3 is grounded via resistor 5.
The base and emitter of a transistor 6 are connected in parallel to the resistor 2, and the collector of the transistor 6 is connected to the connection point between the base of the transistor 3 and the resistor 5.

In the circuit shown in Fig. 2, when the load Z side is shorted, the load current increases, but when the voltage drop across the resistor 2 becomes 0.6V or more due to this increase in current, the transistor 6 turns on, and as a result, the transistor The base voltage of transistor 3 becomes high, the impedance value of transistor 3 as a variable impedance element becomes high, and the current is limited.
In this case, the current to be limited is determined by the base-emitter voltage V _BE of the transistor 6 and the value R ₂ of the resistor 2.

FIG. 3 shows another example of the current limiting circuit, in which the power supply terminal 11 is connected to the terminal 14 to which the load Z is connected via the resistor 12 and the emitter-collector of the transistor 13. . The base of transistor 13 is grounded via resistor 15.

The collector-emitter of the transistor 16 is connected between the connection point between the power supply terminal 11 and the resistor 12 and the base of the transistor 13, and the power supply terminal 11 is grounded through a series circuit of resistors 17 and 18. The connection point between the resistors 17 and 18 is connected to the base of the transistor 16.

In this example, when the output load current increases due to the shorting of the load Z and the base potential of the transistor 13 falls below the base potential of the transistor 16, the transistor 16 is turned on and the base potential of the transistor 13 increases. The current is limited by

[Problem that the invention attempts to solve]

However, the circuit shown in FIG. 2 and the circuit shown in FIG. 3 have the following drawbacks.

In other words, in the case of the circuit shown in Fig. 2, in order to turn on transistor 6, a voltage drop of approximately 0.6 V or more is required across resistor 2, and if the difference in output current during normal operation and during current limit is small, It is necessary to make the resistance value relatively large. However, if the value of this resistor 2 is increased, the voltage drop across this resistor 2 will increase even during normal operation, and the output power supply voltage supplied to the load Z will decrease.

Further, in the case of the circuit shown in FIG. 2, the effects of variations in the base-emitter voltage V _BE of the transistor 6 and the temperature characteristics are directly added to the limited current value, resulting in variations in the limited current value.

In addition, in the case of the circuit shown in FIG. 3, the value of the current detection resistor 12 can be selected to be smaller than that of the circuit shown in FIG. 2, and the voltage drop across this resistor 12 during normal operation can be made smaller. can.

However, the limited current is affected by variations in the base-emitter voltage V _BE of transistors 13 and 16. Also, during overload, transistor 1
When transistor 3 generates heat, the base-emitter voltage V _BE of transistor 13 also changes, and the current limit becomes unstable.

In order to suppress the relative variations in the voltage V _BE of the transistors 13 and 16, it is possible to consider their characteristics, such as by making them complementary, but care should be taken when managing inventory and selecting substitutes. It has the disadvantage that it requires

This invention attempts to propose something that can eliminate the above-mentioned drawbacks.

[Means for solving problems]

In this invention, a power supply terminal 21 from which a DC voltage can be obtained is connected to a terminal 24 to which a load is connected via a series circuit of a first resistor 22 and a first transistor 23 as a variable impedance element. The connection point between the resistor 22 and the first transistor 23 is the second, third, and fourth series resistor 26, 2.
7, 28, and the power terminal 21 and the first
The collector-emitter of the second transistor 29 is connected to the base of the transistor 23, and the connection point between the second resistor 26 and the third resistor 27 is connected to the base of the second transistor 29,
A diode-connected third transistor 30 is connected in parallel to the second and third series resistors 26 and 27.

[Effect]

When an overload is connected to the terminal 24, the voltage drop across the first resistor 22 increases and the second transistor 29 turns on. Then, transistor 23
The base potential of will tend to rise, and the current flowing to the load will be limited.

〔Example〕

FIG. 1 shows an embodiment of this invention, in which reference numeral 21 is a power supply terminal to which a constant DC voltage is supplied from a DC power supply E. This power supply terminal 21 is connected to a current detection resistor 22.
It is connected to a terminal 24 to which a load Z is connected via the emitter and collector of a transistor 23 as a variable impedance element. This terminal 24
is sometimes referred to as jack. transistor 2
3 is grounded via a resistor 25.

A connection point between the resistor 22 and the emitter of the transistor 23 is grounded through a series circuit of resistors 26, 27, and 28. Further, the emitter-collector of a control transistor 29 is connected between the power supply terminal 21 and the base of the transistor 23, and the base of the control transistor 29 is connected to the connection point between the resistors 26 and 27.

A diode-connected transistor 30 is connected in parallel to the series circuit of resistors 26 and 27. This transistor 30 has the same characteristics as the transistor 29, and is used as a diode for a constant voltage source.

In this case, the resistors 26 and 27 are for dividing the forward voltage (constant voltage) of the diode 30, and the voltage drop VR ₂₆ across the resistor 26 is approximately constant regardless of the state of the load connected to the terminal 24.

When the load on the terminal 24 is a normal load, the voltage drop VR ₂₆ across this resistor 26 and the resistor 22
Voltage drop due to VR ₂₂ (=I・R ₂₂ ; I is resistance 22
( _R22 is the resistance value of the resistor 22) is set lower than the base-emitter voltage _VBEON at which the transistor 29 is turned on. That is, V _BEON > I·R ₂₂ +VR ₂₆ (1), and the transistor 29 is not turned on. However, at this time, the currents flowing through the resistors 28 and 25 are ignored.

When the load connected to the terminal 24 is overloaded due to short circuit, etc., the voltage drop across the resistor 22
VR ₂₂ increases, which turns transistor 29 on. Then, the base potential of the transistor 23 tends to rise, and the current supplied to the load is limited.

If the current at this time is I _S , then I _S · R ₂₂ + VR ₂₆ = V _BEON ……(2) ∴I _S = V _BEON − VR ₂₆ /R ₂₂ ……(3). Therefore, from equation (3), when the value of the current _IS is fixed, the value R ₂₂ of the resistor 22 can be reduced by increasing the voltage drop VR ₂₆ .
In other words, even if the limiting current value is the same, the value of the resistor 22 can be made smaller by changing the voltage drop caused by the resistor 26.

Therefore, the drop in the output power supply voltage of the terminal 24 during normal use is small.

Also, from equation (3), the accuracy of the limiting current I _S is (V _BEON
-VR26 ) and is affected by temperature characteristics.

However, in this case, since the transistors 29 and 30 have the same characteristics, the temperature characteristics and variations between V _BEON and VR ₂₆ are almost the same. Therefore, variations in (V _BEON −VR ₂₆ ) and temperature characteristics are canceled out and suppressed.

The condition that the transistors 29 and 30 have the same characteristics can be satisfied by mounting transistors of the same lot with well-matched characteristics, for example, manufactured from the same wafer, during mass production. Of course, a dual transistor in which these transistors 29 and 30 are included in a pair in one package may be used.

In addition, temperature compensation is performed by resistor 2 compared to the value of resistor 27.
The larger the value of 6, the better the results.

[Effect of idea]

In this device, the forward drop voltage of the constant voltage diode is divided and this divided voltage is added to the voltage drop of the current detection resistor. This added voltage is detected by a control transistor and the transistor acting as a variable impedance element is controlled to limit the current. This allows the value of the current detection resistor to be reduced by the divided voltage of the diode, thereby reducing the drop in the output voltage value.

By configuring the diode with a transistor having the same characteristics as the control transistor, it is possible to reduce variations in current value during current limitation.
Therefore, there is an effect that no adjustment is necessary. Furthermore, since the temperature characteristics are improved, the usable temperature range becomes wider.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of this invented circuit, and FIGS. 2 and 3 are circuit diagrams of examples of conventional circuits. 21 is a power supply terminal, 22 is a first resistor for current detection, 23 is a transistor as a variable impedance element, 24 is a terminal to which a load is connected, 26,
27 and 28 are second, third, and fourth resistors, 29 is a control transistor, and 30 is a diode-connected transistor.

Claims (1)

[Claims for Utility Model Registration] a. A power supply terminal from which a DC voltage can be obtained is connected to a terminal to which a load is connected via a series circuit of a first resistor and a first transistor as a variable impedance element, and b. A connection point between the first resistor and the first transistor is grounded via second, third, and fourth series resistors, and a second transistor collector
d a connection point between the second resistor and the third resistor is connected to the base of the second transistor; e a diode-connected transistor is connected in parallel to the second and third series resistors; a current limiting circuit to which a third transistor f is connected, and when the second transistor is turned on, the base voltage of the first transistor is controlled to limit the current supplied to the load; .