JPH0535447B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a current limiting device that prevents excessive current from flowing through a load in, for example, a power supply device.

[Conventional technology]

FIG. 3 is a configuration diagram of a conventional current limiting device shown in, for example, Japanese Patent Publication No. 56-40364. In the figure, 1 is a power source, and 2 is a transistor whose collector terminal is connected to the positive terminal of this power source 1. , 3 is a load whose one end is connected to the emitter of transistor 2, 4
5 is an operational amplifier, 6 is a reference voltage source, and 7 is a control circuit for the transistor 2.

A conventional current limiting device is configured as described above,
The current flowing through the load 3 is converted into a current detection voltage V _R by the resistor 4, and when the operational amplifier 5 determines that it matches the voltage V _S of the reference voltage source 6, the transistor 2 is connected via the control circuit 7. control and limit or cut off overcurrent.

[Problem that the invention seeks to solve]

In the conventional current limiting device as described above, the limiting current is determined only by the voltage V _S of the reference voltage source 6 and the value of the resistor 4, and does not depend on the voltage of the power source 1. Therefore, when the current of the load 3 changes depending on the voltage, especially in the case of a constant power load such as a DC/DC converter device, overcurrent protection of the load 3, power source 1
There was a problem in that the power supply was not effectively limited. The reason for this will be explained below.

FIG. 4 is a voltage-current characteristic diagram of a constant power load, where A shows the current limiting characteristic and B shows the constant current load characteristic. In general, constant power loads have a maximum voltage V _H and a minimum voltage V _L that allow normal operation. If the current consumption at each voltage is I _L and I _H , then V _L ×
There is a relationship where I _L almost matches V _H × I _H , and I _L and I _H are
The larger V _H −V _L is, the greater the difference. When the voltage of the power source 1 changes between V _L and V _H , the limiting current I _S of this current limiting device must be selected by adding an appropriate margin to the maximum current consumption I _L of the load 3. The failure of load 3 happens to be caused by the voltage of power source 1.
If the current occurs near V _L , the current will be limited to an appropriate value compared to the normal value I _L , and the power flowing into load 3 and the power supplied from power source 1 will be approximately V _S ×V _L
limited by.

However, if load 3 fails when the voltage of power source 1 is close to V _H , the current will not be limited until it reaches I _S , which is much larger than the normal value I _H , and the power flowing into load 3 will be I _S × V _H. There was a problem in that the value became large, damaging the load, and the power supplied from the power source 1 also became I _S ×V _H , causing the power source 1 to be overloaded.

The present invention was made to solve such problems, and an object of the present invention is to obtain a current limiting device in which the current is always limited to a normal value with an appropriate margin even if the voltage of the power source 1 fluctuates. shall be.

[Means for solving problems]

The current limiting device according to the present invention includes a reference voltage source
V _S is generated from a power source using a resistor divider circuit.

[Effect]

In this invention, the voltage of the reference voltage source changes depending on the voltage of the power source, so even if the voltage of the power source fluctuates, the current limit value always changes with an appropriate margin around the normal value. , properly protect loads and power sources.

〔Example〕

FIG. 1 is a block diagram of a current limiting device showing an embodiment of the present invention, and 1 to 4 and 7 are completely the same as the conventional device described above, and in the figure, 1 is a power source;
2 is a transistor whose collector terminal is connected to the positive terminal of this power source 1; 3 is a load whose one end is connected to the emitter of transistor 2; 4 is a first resistor;
7 is an operational amplifier, and 7 is a control circuit for the transistor 2. The first resistor 4 is for current detection as in the conventional device described above, and converts the current flowing through the load 3 into a current detection voltage V _R. This resistor has a small value to reduce power loss. . The second resistor 8 and the third resistor 9 are a resistive divider connected in series between the positive terminal of the power source 1 and one end of the first resistor 4 . The circuit consisting of the operational amplifier 5, the fourth resistor 10, and the fifth resistor 11 connects the midpoint of the series resistance consisting of the second resistor 8 and the third resistor 9 to the first resistor 4.
This is an operational amplifier circuit that amplifies the voltage difference at the other end, and the fourth resistor 10 and the fifth resistor 11 are used to set the output voltage of the operational amplifier circuit.
In addition, the control circuit 7 includes an operational amplifier 5 and a transistor 2.
This control circuit is connected between the bases of the operational amplifier 5 and controls the transistor 2 by the output of the operational amplifier 5.

In FIG. 2, a second resistor 8 and a third resistor 9
is a resistor divider that generates a reference voltage V _S that depends on the voltage of the power source 1, and in the figure, the voltage V ₊ at the + terminal of the operational amplifier 5 is V ₊ = V _S = (V _IN − V _R ) · R ₉ /R ₈ +R ₉ V _IN・R ₉ /R ₈ +R ₉ ...(1). Here, V _IN is the voltage of voltage source 1, and R ₈ and R ₉ are the resistance values of resistors 8 and 9, respectively.As mentioned above, the resistance value of the first resistor 4 is selected to be sufficiently small, so the voltage drop V _R is Since it is sufficiently small compared to V _IN (1)
The approximate expression of Eq. On the other hand, operational amplifier 5
The voltage V _- at one terminal of is V _- = V _OUT・R ₁₀ −V _R・R ₁₁ /R ₁₀ +R ₁₁ ……(2). Here, V _OUT is the output voltage of operational amplifier 5,
V _R is the current detection voltage of resistor 4, and R ₁₀ and R ₁₁ are the resistance values of resistors 10 and 11, respectively. Formula (1), Formula (2)
From the equation, assuming that the gain of the operational amplifier is sufficiently large and setting V ₊ = V _- , we obtain equation (3).

V _OUT =V _IN・R ₉ /R ₈ +R ₉・R ₁₀ +R ₁₁ /R ₁₀ +V _R R ₁₁ /R ₁₀ ...(3) Figure 2 is a diagram showing the setting of the current limit value in this invention. A shows the current limit characteristics, and B shows the constant power load characteristics. In Figure 2, I ^L _S is the current limit value when the voltage of power source 1 is the minimum value V _L , and I ^H _S
is the current limit value when the voltage of the power source 1 is the maximum value _VH . The present invention provides a current limiting device having a current limiting characteristic (current limiting characteristic A in the figure) that is linearly approximated between these two limiting values in ^FIG . R ₈ suitable for _S , I ^L _S ,
It is shown below that R ₉ , R ₁₀ , and R ₁₁ can be designed. It is assumed that the control circuit 7 controls the transistor 2 at a certain constant voltage _VX . _{Here ,} R ₈ , _{R 9} , ^R ₁₀ , _and R ₁₁ are ^expressed as _{R 11} / _{R 10} = _V
(4) R ₉ / R ₈ + R ₉ = V _X・(V ^L / _R − V ^H / _R ) / V ^L / _R
・V _H −V ^H / _R・V _L / {1+V _X・(V _H −V _L )/V ^L / _R・V _H −
V ^H / _R・V _L }……(5) Here, V ^H _R ; I ^H _S・R ₄ V ^L _R ; I ^L _S・R ₄ R ₄ ; Resistance value of the first resistor 4 for current detection If selected so that (in equations (4) and (5), V _H > V _L ,
Since V ^L _R > V ^H _R , the right sides of equations (4) and (5) are positive, and appropriate resistance values can be determined for R ₈ to _{R 11} ), V _X = V _H・R ₉ /R ₈ +R ₉・R ₁₀ +R ₁₁ /R ₁₀ +V ^H _R・R ₁₁ /R ₁₀ ……(6) V _X =V _L・R ₉ /R ₈ +R ₉・R ₁₀ +R ₁₁ /R ₁₀ +V _LR・R ₁₁ /R ₁₀ ...(7). That is, when the voltage of the power source 1 is V _L , the voltage across the first resistor 4 for current detection is V ^L _R , the output of the operational amplifier 5 is V _X , and the power source 1
When the voltage at both ends is _VH , the voltage at both _ends is ^VHR , and the output of the operational amplifier 5 becomes _VX . This can be realized by designing the control circuit 7 to operate at voltage _VX .
This means that the limiting currents when the voltages of the power source 1 are V _L and V _H are I ^L _S and I ^H _S , respectively, and the current limiting characteristics shown in FIG. 2 can be achieved.

〔Effect of the invention〕

As explained above, this invention uses the simple method of generating a reference voltage source dependent on the voltage of the power source to provide normal operation over a wide voltage range for loads whose current changes depending on the voltage. This has the effect that the current can be limited with an appropriate margin at all times.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a current limiting device showing an embodiment of the present invention, FIG. 2 is a diagram showing voltage-current characteristics of the current limiting device of the present invention, and FIG. 3 is a block diagram of a conventional current limiting device. , FIG. 4 is a diagram showing voltage-current characteristics of a conventional current limiting device. In the figure, 1 is a power source, 2 is a transistor, 3 is a load, 4 is a first resistor, 5 is an operational amplifier, 6 is a reference voltage source, 7 is a control circuit, 8, 9, 10, 11 are second and third , fourth and fifth resistors. It should be noted that the same or corresponding parts in the figures are indicated by the same reference numerals.

Claims (1)

[Claims] 1 a power source, a transistor having its collector connected to the positive terminal of the power source, a load having one end connected to the emitter of the transistor, and a first transistor connected between the other end of the load and the negative terminal of the power source. resistance and
second and third resistors connected in series to the positive terminal of the power source and one end of the first resistor; and a midpoint between the second and third resistors connected in series and the other end of the first resistor. 1. A current limiting device comprising: an operational amplifier circuit for amplifying a voltage difference; and a control circuit connected between an output terminal of the operational amplifier circuit and a base of the transistor to control conduction of the transistor.