JPS61265609A - Transistor device with current limiter - Google Patents

Abstract

PURPOSE:To maintain a value of an output current flowing through an output transistor, at a prescribed value irrespective of an ambient temperature, by forming a circuit by an output transistor, a current detecting resistor, a comparator, a reference voltage generating circuit, a control transistor, etc. CONSTITUTION:As a current flowing to an output transistor TR0 increases, a voltage drop in a detecting resistor R0 becomes large. Accordingly, a + terminal potential of a comparator 1 becomes higher than a - terminal potential, an output of the comparator 1 becomes a high potential, and a control transistor TR1 is turned on. Therefore, a conducting current is controlled to a prescribed value by decreasing a base current of the transistor TR0. Also, since a temperature characteristic of the resistor R0 and a reference voltage generating circuit 2 is very small, this prescribed current is maintained irrespective of an ambient temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to a circuit configuration of a current-limiting transistor device that limits the output current value of an output transistor to a constant value.

[Prior art and its problems]

As this type of transistor device, one having a circuit configuration as shown in FIG. 3I is known. In this circuit, the current c flowing through the output transistor TRo is detected by the current detection resistor Ro, and when the current exceeds a predetermined IC current value,
The control transistor TR is made conductive through the fixed resistor R1, and the base current of the output transistor TR is limited to a constant value. As a result, the current ■c can be limited to a constant value.

By adjusting the variable resistor R2, c can be set to any current value. However, this circuit has the drawback that the current value, which is intended to be limited to a constant value, changes depending on the temperature, as shown in FIG. This is because at high temperatures, the voltage between the base and emitter of the control transistor TR1 becomes conductive compared to room temperature, that is, with a low voltage drop across the current detection resistor Ro.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a current-limiting transistor device that can always provide a constant output current regardless of the ambient temperature.

[Key points of the invention]

In this invention, for the above purpose, the output transistor, %
A three-terminal circuit is constructed by combining a current detection resistor, a comparator, a reference voltage generation circuit, a control transistor, a fixed resistor, and a variable resistor, and the output is output by the current detection resistor connected in parallel with the output transistor. The current value of the transistor is detected, the detected voltage 2 obtained from this and the reference voltage are connected to the input side of the comparator, and the base potential of the output transistor is controlled to a predetermined value by the output of the ratio converter. .

Further, the power supply for the reference voltage generation circuit and the comparator is taken from the base of the output transistor.

[Embodiments of the invention]

Embodiments of the present invention will be described below based on the drawings. 1st
The figure shows an embodiment of the present invention, in which the terminal 10 of the output transistor TRo is connected to a load (not shown) and the positive side of a power source (not shown), and the base terminals of the output transistor TRo are connected to the base load and the positive side of the power source (not shown). The 0 and 0 terminals connected to each other are connected to the negative terminal of the power supply. The emitter of the output transistor TRo is connected to one end of a current detection resistor Ro, and the other end of the resistor R is connected to terminal 9. Furthermore, a series circuit consisting of a fixed resistor R1 and a variable resistor R2 is connected in parallel to the resistor R.
This variable resistor R2 is adjusted to set a limiting current value.

The connection point between the fixed resistor R1 and the variable resistor R2 is connected to the positive input terminal of the comparator 1, and power to the comparator 1 is supplied from between the base terminals of the output transistor TR.

At the same time, a power source is used between the terminals, and the output of the reference voltage generating circuit 2 is connected to the negative input terminal of the comparator 1, and the output terminal of the comparator 1 is connected to the base of the limiting transistor TR. As the current flowing through the output transistors TRo&C increases, the voltage drop across the detection resistor Ro increases, and the potential of the positive terminal of the comparator 1 becomes higher than the potential of the negative terminal.
The output of comparator 1 becomes high potential and the limiting transistor TR
1 is turned on, the base current of the output transistor TRo is decreased and the conduction current is controlled to a constant value. Since the temperature characteristics of the current detection resistor Ro and the reference voltage generation circuit are extremely small as will be described later, this constant current is maintained regardless of temperature, and the drawbacks of the conventional device can be eliminated.

Here, the circuit of comparator l and reference voltage generation circuit 2 is
This will be explained based on the diagram. First, the comparator l circuit is composed of transistors T R2, T R3, T R4e T 1B, diodes Dl, D=, and fixed resistors a3t R4, and the current is It forms a mirror circuit and provides a bias current for the comparator l. The transistor TR and the transistor TR4 form a non-inverting input and an inverting input of the comparator l, and the transistor TR11 and the diode D form an output circuit that outputs a current according to the input potential difference between the inverting and non-inverting inputs. There is.

Now, when the reference voltage from the reference voltage generation circuit 2 is input to the inverting input of the transistor TR4, a voltage drop occurs across the current detection resistor Ro at the non-inverting input of the transistor TR, and when this voltage drop exceeds the reference voltage, a comparison is made. The base current of the transistor TR connected to the output side of the transistor 1 increases, and conversely, when it falls below, the base current decreases. Therefore, when the circuit is in a steady state, the potentials of the inverting and non-inverting side inputs of comparator 1 are the same and the voltage drop across the current detection resistor R is constant, so the output current IO of the output transistor TR is made constant. Ru.

Next, the circuit of the reference voltage generation circuit 2 includes transistors T a,
t TR7e T R8e T R9* T RIG
e T all * T R12#TR engineering,, TR□
4 e T axs and fixed resistors R, , R, , R, and makes the voltage Vo generated at the emitter of the transistor TR a constant voltage. Is this a fixed resistor R? e
R1# R9 and transistor TFt, , TR,,
The voltage Vo ft is kept constant over a wide range of the power applied between the terminals by controlling the base current of the transistor TR with the sequential drop between the base and emitter of TR and the transistor TR. It is from.

The transistors TRl0, TRII, and TR□2 form a bias source that supplies base current to the transistor TR. In particular, the transistor TR uses a voltage-operated field effect transistor, so it can be biased independently of the power supply voltage. The current can be made constant, the voltage V can be stabilized with high precision, and the current consumption of the circuit itself can be reduced. The transistors TR13, TR14, and TR15 form a current amplifier, and by amplifying the collector current of the transistor TR, the bias current of the bias source is reduced in current consumption, and the power supply voltage applied to the terminals is The minimum operating voltage of the transistor TR15 is determined by the j@ drop between the base and emitter of the transistor TR15 and the transistor TR1,
The value is set such that the sum of the saturation voltages between the collector, emitter, and rib exceeds the voltage Vo. The fixed resistors R5s and R6 divide the voltage ■o to generate a reference voltage VREF.

Voltage v obtained by the circuit configuration described above.

Let the currents flowing through the fixed resistors R7 and R8 be Il and I2, respectively, and the transistor TR.

Since the order drop and drop between the base, emitter, and rear of TR8 are approximately equal, if this is taken as VBE, the following equation holds true.

Vo = IIRy +VBE ・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
・・・・・・(1) IIRy=I2R8・・・・・・
・・・t・・・・・・・・・・・・・・・・・・
・・・・・・・・・(2) Here: Boltzmann constant, q
: [Charge amount of child.

T: Absolute temperature By substituting equations (2) and (3) into equation (1), we can find the temperature coefficient of voltage vo at this time: In other words, a resistor R7e that satisfies this condition is
By selecting the resistance value of R9, a constant constant voltage output independent of temperature can be obtained. At this time, when calculating the constant voltage value ■O, dVBg when the transistor is made of silicon.
2 mV/'C"C", T = 273°K at room temperature
.

case? - Since VBg = 0.6 to 0.7v, these are (4
), V V6 = 2 mV/'c x +0,65 at 273°
:1.2 Since (7), the output current 1° of the output transistor TR is calculated as follows. In other words, the voltage drop across the current detection resistor Ro is VIs, which is the voltage tn that is input to the non-inverting input of the comparator 1 by dividing vl by the fixed resistor R and the variable resistor R2.
Then, in the steady state, VBEF '' Vlfl, so the following equation is established!

Here, in order to find the temperature coefficient of constant current output of 0 in equation (11), in actual use, the output current of several A is converted to a constant current with a voltage drop of several hundred mV, so the current detection The resistance value of the resistor Ro is in the range of tens to hundreds, and since metal is used as the material of the resistor, its temperature coefficient is almost zero. Therefore, by using a current limiting transistor device that satisfies the condition of equation (6), a constant 11E output without temperature dependence can be obtained.

〔Effect of the invention〕

According to this invention, an output transistor, a current detection resistor,
Since the circuit is composed of a comparator, a reference voltage generation circuit, a control transistor, a fixed resistor, and a variable resistor, temperature changes in the reference voltage generation circuit, comparator, and current detection resistor are extremely small, and the output flowing to the output transistor is minimized. It is possible to keep the current value constant regardless of the ambient temperature.

[Brief explanation of drawings]

FIG. 11 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram of the reference voltage generation circuit and comparator in FIG. 1, FIG. 3 is a circuit diagram of a conventional device, and FIG. 4 is a diagram showing the relationship between output current and temperature in the device shown in FIG. 3. FIG. 1: Comparator, 2: Basic voltage generation circuit, 10.20, 30: Terminal, Ro: Current detection resistor, K1
= fixed resistor, R2 variable resistor, TR (,: output transistor, TR,: control transistor. Fig. 2

Claims (1)

[Claims] 1) A three-terminal circuit is constructed by combining an output transistor, a current detection resistor, a comparator, a reference voltage generation circuit, a control transistor, a fixed resistor, and a variable resistor, and a current detection resistor is connected in series with the output transistor. The current value of the output transistor is detected by the body, the detected voltage and reference voltage obtained from this are connected to the input side of the comparator, and the base potential of the output transistor is controlled to a predetermined value by the output of the comparator. A current-limiting transistor device characterized by: