JPH0670538A - Voltage/current conversion circuit - Google Patents

Abstract

PURPOSE:To lessen internal heat generation, to reduce a heat stress thereby, to attain the miniaturization of a circuit, the improvement of reliability and prolongation of the lifetime of a product and to reduce the rush current at the time of a sharp change in a load resistance in a voltage/current conversion circuit. CONSTITUTION:A voltage/time width conversion circuit 2 which converts a DC input voltage VIN into a pulse signal corresponding to the amplitude of the voltage, a switching circuit 3 which makes a chopper operation according to an output pulse of the circuit 2 and outputs a current of an amplitude being proportional to the width of the output pulse, a filter circuit 4 which smoothes the output current of the switching circuit 3, and a feedback resistor 5 and a feedback processing part 1 which detect a load current supplied to an external load R from the filter circuit 4 and subject the input voltage of the voltage/time width conversion circuit 2 to a feedback control, are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage / current conversion circuit (constant current output circuit) used in the field of process control and the like.

[0002]

2. Description of the Related Art As a circuit for converting an input voltage into a current having a constant value corresponding thereto, a circuit generally composed of an operational amplifier, a transistor and other passive elements is constructed. FIG. 16 shows a prior art of this type of voltage / current conversion circuit. In the figure, Q ₁ and Q ₂ are Darlington connected transistors, Q ₃ is an operational amplifier, ZD is a zener diode, and R _{1 to} R ₅ are Resistance, R is an external load. The power supply voltage is V _P and the input voltage is V _IN .

The principle of voltage / current conversion in this circuit will be described below. First, the voltages V ₊ and V _{− at} the non-inverting input terminal and the inverting input terminal of the operational amplifier Q ₃ are represented by Equation 1, and the voltage V _{a at} the connection point of the resistors R ₂ and R ₃ is represented by Equation 2.

[0004]

[Number 1] _{V + = V - = V P} × {R 1 / (R 1 + R 2 + R 4)}

[0005]

[Formula 2] V _a = (V ₋ −V _IN ) × (R ₁ + R ₂ ) / R ₁ + V _IN

Substituting V ₋ in Equation 1 into Equation 2 yields Equation 3
To get

[0007]

## EQU3 ## V _a = V _P × (R ₁ + R ₂ ) / (R ₁ + R ₂ + R ₄ ) −R ₂ × V _IN / R ₁

Assuming that R ₁ and R ₂ >> R ₃ , there is a relationship shown in Formula 4 between V _P and V _a and the current I flowing through the external load R.

[0009]

## EQU4 ## V _P -V _a = I × R ₃

Substituting equation 3 into equation 4 yields equation 5.

[0011]

[Formula 5] V _P × R ₄ / (R ₁ + R ₂ + R ₄ ) + V _IN × R ₂ / R ₁ = I × R ₃

Here, R ₁ = 150 [KΩ], R ₂ = 27
When [KΩ], R ₃ = 45 [Ω], and R ₄ = 50 [Ω], the first term on the left-hand side of Expression 5 can be regarded as almost zero, and thus Expression 6 is obtained.

[0013]

(6) I = (27000 × V _IN ) / (150,000 × 45) = 0.004 V _IN

Therefore, for example, V _IN is 1 [V] to 5 [V]
When the external load R is changed from 4 [mA] to 20 [m
The current [A] flows and voltage / current conversion is performed.

[0015]

In the above-mentioned conventional technique, there is a problem that the amount of power consumption inside the circuit increases due to the size of the external load R and the amount of internal heat generation increases.
Here, the relationship between the magnitude of the external load R and the internal power consumption in the related art will be considered below. That is, assuming that the magnitude of the external load (resistance) is R [Ω] as in the reference numeral, the power consumption P [W] of the current output path is expressed by Equation 7. Further, the internal power consumption P _n [W] is expressed by Equation 8, and all the internal power consumption P _n is internal heat generation.

[0016]

Equation 7] _{^{P = I 2 × R 3 +}} (V P -I × R 3 -I × R) × I + I 2 × R = I × V P

[0017]

Equation 8] ^{_{P n = I 2 × R 3}} + (V P -I × R 3 -I × R) × I = I × V P -I 2 × R

Next, considering the case where the external load is "0", Equation 9 holds.

[0019]

## EQU9 ## P = I ² × R ₃ + (V _P −I × R ₃ ) × I = I × V _P = P _n

That is, when the external load resistance R is 0 [Ω], the power consumption of the current output path is all internally generated.
For this reason, it is difficult to form a small circuit in terms of heat capacity, it is difficult to miniaturize the circuit, and reliability is deteriorated due to thermal stress and a product life is shortened. The first to fifth inventions were made in order to solve the above problems, and an object thereof is to suppress the internal heat generation so as to reduce the circuit size and improve the reliability. It is to provide a conversion circuit.

In particular, in the third and fourth inventions, as will be described in detail later, when the external load R changes rapidly in the first and second inventions (for example, the external load R is connected from the unconnected state). In such a case), a current (rush current) more than the set value flows to the external load R, so that this surge current is suppressed to minimize the influence on the external equipment such as the plant controlled by the present invention. An object of the present invention is to provide a highly reliable voltage / current conversion circuit that can be limited. Furthermore,
A fifth aspect of the present invention, as will be described in detail later, when the external load R is connected from the unconnected state in the first and second aspects,
Considering that the time (response time) until the current of the set value is stably output to the external load R becomes long, this can be improved to minimize the influence on external equipment. It is an object of the present invention to provide a voltage / current conversion circuit having a positive polarity.

[0022]

In order to achieve the above object, the first invention is a voltage / current conversion circuit for converting a DC input voltage into a DC current and supplying the DC current to an external load. And a voltage / time width conversion circuit that converts and outputs a pulse signal having a pulse width according to
A switching circuit that operates in a chopper by the output pulse of the time width conversion circuit and outputs a current having a magnitude corresponding to the output pulse width, a filter circuit that smoothes the current output from the switching circuit, and the filter circuit A feedback control circuit for detecting the current supplied to the external load and performing feedback control of the input voltage of the voltage / time width conversion circuit.

The second invention is the same as the first invention,
A Zener diode is connected between a feedback resistor forming a feedback control circuit and an external load.

A third aspect of the present invention is the first or second aspect of the present invention, further including a current limiting circuit that shuts off the output current of the switching circuit when the detected value of the current flowing through the external load exceeds a set value. Is.

In a fourth aspect of the present invention, the output current of the switching circuit is suppressed by detecting that the external load is not connected or the load resistance value is larger than a specified value in the first or second aspect. It is provided with a current limiting circuit.

In a fifth aspect based on the first or second aspect, the voltage / time width conversion circuit includes a comparator for outputting the comparison result of the triangular wave and the output voltage of the feedback control circuit to the switching circuit as a pulse signal. A voltage limiting circuit for limiting the output voltage of the feedback control circuit so as not to deviate from the amplitude range of the triangular wave is connected between the feedback control circuit and the voltage / time width conversion circuit.

[0027]

According to the first aspect of the invention, the output circuit of the voltage / time width conversion circuit causes the switching circuit to perform a chopper operation, and a constant current corresponding to the magnitude of the input voltage flows through the external load. As a result, only the necessary energy is efficiently supplied to the load, and heat generation inside the circuit is reduced. In the second invention, the voltage across the switching circuit decreases due to the Zener voltage, and the pulse duty ratio of the chopper operation for obtaining the same load current increases, so that there is a margin in the minimum switching time of the switching element. it can.

In the third invention, the current flowing through the external load is detected by the current flowing into the switching circuit,
When the value exceeds the set value, the switching element in the switching circuit is turned off to cut off the load current. This prevents the occurrence of inrush current due to a sudden change in external load.

In the fourth invention, it is detected from the voltage at one end of the feedback resistor that the external load is not connected or the load resistance value is larger than the specified value, and at that time, the voltage / time width conversion circuit is detected. The load current is suppressed by controlling the ON period of the switching element in the switching circuit so that the load current takes the minimum value via the. This prevents the occurrence of inrush current due to a sudden change in external load.

In the fifth aspect of the invention, the output voltage of the feedback control circuit is limited so as not to deviate from the output amplitude range of the triangular wave generating circuit in the voltage / time width conversion circuit, so that the switching element of the switching circuit is completely turned on. This prevents inrush current when an external load is connected without setting the state, and also prevents a long response time until the current flowing through the external load becomes constant.

[0031]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the first invention. In the figure, 1 is a feedback processing unit for integrating a difference between an input voltage V _IN and an output side voltage of a feedback resistor 5 which will be described later. The output voltage of the feedback processing unit 1 is input to a voltage / time width conversion circuit 2. ing.
Here, the voltage / time width conversion circuit 2 converts the input voltage into a pulse signal having a pulse width proportional to its magnitude and outputs the pulse signal.

The pulse signal output from the voltage / time width conversion circuit 2 is input as a control signal to the switching circuit 3 to which the power supply voltage V _P is applied. That is, the switching circuit 3 formed of a PNP transistor or the like is chopper-operated by the pulse signal to output a constant current having a magnitude proportional to the pulse width to the external load R side. The constant current output from the switching circuit 3 is smoothed by the filter circuit 4, supplied to the external load R, picked up by the feedback resistor 5 and fed back to the feedback processing unit 1 as a voltage. Here, the feedback resistor 5 and the feedback processing unit 1 constitute a feedback control circuit in the present invention.

Next, referring to FIGS. 2 and 3, the relationship between the input (pulse width) and the output (current) in the switching circuit 3 will be considered. First, assuming that the base current and the collector current of the transistor used in the switching circuit 3 are i _b and i _c , respectively, these values are expressed by Equations 10 and 11.

[0034]

I _b = E / r

[0035]

I _c = E / (R + R _F )

In Equations 10 and 11, E is the emitter potential, r is the base resistance, R is the external load resistance value,
_RF is the resistance value of the feedback resistor 5. Formula 1
0 and equation 11, when the transistors are turned on for τ ₁ of the pulse period τ ₀ as shown in FIG. 2, the average currents I _B and I _C are obtained, and equations 12 and 13 are obtained.

[0037]

[Equation 12]

[0038]

[Equation 13]

Therefore, the collector current is proportional to the pulse width τ ₁ when the pulse period τ ₀ is fixed. Also,
Formula 14 is established from Formula 12 and Formula 13.

[0040]

## EQU14 ## I _B = (R + R _F ) × I _C / r

Next, the power consumptions of this embodiment and the prior art will be compared for the case where the internal heat generation is the largest. From FIG. 3, the power consumption P of the entire circuit in this embodiment is as shown in Expression 15.

[0042]

## EQU15 ## P = I _B ² × r + I _C ² × R + I _C ² × R _F

Here, the loss generated in the transistor means that the transistor is in a saturated state (switching mode).
Ignored it for use in. Since the second term on the right-hand side of Expression 15 is consumed on the external load side, the internal power consumption P _n that causes internal heat generation of the circuit is Expression 16.

[0044]

## EQU16 ## P _n = I _B ² × r + I _C ² × R _F

From this equation 16, it is considered that the internal heat generation of the entire circuit is caused by the resistance r and the feedback resistance 5 (R _F ). Substituting equation 14 into equation 16 yields equation 17.

[0046]

## EQU17 ## P _n = (R + R _F ) ² × I _C ² / r + R _F × I _C ²

From the equation (17), the internal heat generation of the entire circuit does not depend on the power supply voltage but depends on the output current and the external load R. In this embodiment, the internal heat generation becomes maximum when the external load R is maximum. As an example, Equation 17
, R = 750 [Ω], r = 4.7 [KΩ], R
_{When F} = 100 [Ω] and I _C = 20 [mA], the internal power consumption P _n becomes 100 [mW]. On the other hand, in the conventional technique, V _P = 24 [V], I =
If it is 20 [mA], the internal power consumption P _n is 480 [m].
W]. That is, in this embodiment, the internal heat generation amount is about 1/5 of the conventional value. As a result, conventionally, when the external load resistance is 0 [Ω], the power supply voltage × load current appears as it is as internal heat generation. However, in the present embodiment, since all the output current of the transistor flows to the external load, the internal heat generation does not occur. Remarkably smaller than.

FIG. 4 is a circuit diagram showing a specific structure of the embodiment shown in FIG. In the figure, in the feedback processing unit 1, the input voltage V _IN is input to the inverting input terminal of the operational amplifier Q ₁ via the resistor R ₁ , and the non-inverting input terminal is connected to the 0V line. Further, the voltage from the feedback resistor 5 is input to the inverting input terminal via the resistor R ₂ . The operational amplifier Q ₁ is for integrating the difference between the two input voltages with the capacitor C ₁ and outputting it.

In the voltage / time width conversion circuit 2, the output voltage of the feedback processing unit 1 is input to the positive input terminal of the comparator Q ₂ , and its negative input terminal has a triangular wave generation circuit 21.
The triangular wave from is input. This comparator Q ₂
Operates so as to compare two input voltages and output a pulse that becomes a “Low” level when the triangular wave exceeds the output voltage of the feedback processing unit 1. The pulse width of the output pulse of the comparator Q ₂ is proportional to the output voltage of the feedback processing unit 1.

The switching circuit 3 comprises a PNP transistor Q ₃ and a resistor R ₃ , and the transistor Q ₃ is turned on by the output pulse of the comparator Q ₂ to turn on the base current i _1.
Flows in the direction of the solid arrow in the figure through the resistor R ₃ . The collector current i ₂ flows in the direction of the broken line arrow in the figure. The filter circuit 4 is composed of an LC filter and smoothes the output current of the transistor Q ₃ with the inductor L and the capacitor C. The smoothed current is supplied to the external load R.

Further, the feedback resistor 5 is composed of only the resistor R ₄ , and picks up the output current and returns it as a voltage to the feedback processing unit 1 to correct the input voltage V _IN and stabilize the system. Note that FIG. 5 is a time chart showing the voltage and current of each part (-) in FIG.

Next, FIG. 6 shows an embodiment of the second invention. In this embodiment, a Zener diode 6 is connected in series to the feedback resistor 5 of the embodiment shown in FIG. 1 so as to compensate the minimum voltage time for which the transistor of the switching circuit 3 can be turned on. According to this embodiment, the switching circuit 3 can stably output the minimum current when the external load R is light.
That is, in this embodiment, the collector current of the transistor in the switching circuit 3 can be expressed by Expression 18, and the average collector current I _C can be expressed by Expression 19. In these equations, V _Z is the Zener voltage.

[0053]

I _c = (E−V _Z ) / (R + R _F )

[0054]

[Formula 19]

In Equation 19, the collector of the transistor
Since the voltage between the emitters is reduced by the Zener voltage as compared with the above Expression 13, the duty ratio τ ₁ / τ ₀ of the transistor can be increased correspondingly to obtain the same output current. Therefore, a margin can be given to the minimum switching time of the transistor, and stable operation becomes possible.

In each embodiment, the specific configuration of the feedback processing unit 1, the voltage / time width conversion circuit 2, the switching circuit 3, the filter circuit 4, etc. is not limited to the illustrated example.

By the way, in the embodiment shown in FIG. 4, since the current does not flow through the feedback resistor 5 unless the external load R is connected, the feedback processing unit 1 and the voltage / time width conversion circuit 2 are provided with the transistor Q of the switching circuit 3. _{Turn 3} on completely. If the external load R is connected in this state, the external load R
An inrush current will flow into.

FIG. 7 shows the configuration of a conventionally known current limiting circuit. When the current value detecting unit 7 detects a supply current exceeding a set value, the current interrupting unit 8 interrupts the supply current. , Limit the current. FIG. 8 is a specific circuit example of FIG. 7. When a current flows through the resistor R ₁₀ , a voltage drop occurs across the resistor R _{10. When the} transistor TR is turned on by this voltage, the field effect transistor FET is turned off.
The current is cut off. Therefore, by appropriately selecting the value of the resistor R ₁₀ , it becomes possible to interrupt the current exceeding the allowable value. The voltage dividing resistors R ₁₁ and R ₁₂ are the transistors T
It is for selecting the gate voltage for turning on the field effect transistor FET when R is off.

However, when the current limiting circuit having such a configuration is added to the embodiment of FIG. 4 to limit the current, the circuit configuration becomes complicated, and the cost increases due to the increase in the number of parts. Therefore, the third invention not only prevents the inrush current due to the rapid change of the external load R, but also limits the current by utilizing a part of the circuit configuration of the first invention.
This is to reduce the number of parts and cost.

FIG. 9 is a block diagram showing an embodiment of the third invention. As is clear from comparison with FIG. 1, in this embodiment, the current detection circuit 9 is added between the power supply voltage and the switching circuit 3. That is, the current detection circuit 9 monitors the value of the current flowing into the switching circuit 3, and when the value exceeds a certain set value, the switching element of the switching circuit 3 is turned off and the external load R
Cut off the current to the.

FIG. 10 shows a concrete circuit configuration of the embodiment shown in FIG. 10, a current detection circuit 9 has a resistor R ₆ for detecting a current i ₃ flowing into the switching circuit 3, an emitter and a base connected to both ends of the resistor R _6, and a collector connected to a transistor Q in the switching circuit 3.
_And a transistor Q ₄ connected to the base of ₃ .

In the above structure, the current detection circuit 9, the transistor Q ₃ of the switching circuit 3 and the like constitute the current limiting circuit of the present invention.

Next, the operation of this embodiment will be described. Figure 1
At 0, the current amplification factor h _fe of the transistor Q ₃ is _divided by 1
Flows to the comparator (op amp) Q ₂ ,
The current i ₂ flowing through the external load R is substantially equal to the current i ₃ flowing into the switching circuit 3. Therefore, the current i ₃ is applied to the resistor R
₆ , and when the voltage drop (i ₃ × R ₆ ) becomes the ON voltage of the transistor Q ₄ (about 0.6 [V]), the transistor Q ₄ is turned on. Thus, the emitter of the transistor Q ₃ - voltage between the base emitter of the transistor Q ₄ - becomes below the voltage between the collector, the transistor Q ₃ is turned off. Thus, by appropriately setting the value of the resistor R ₆ , the current i ₂ flowing in the external load R can be limited. For example, when the current i ₂ is limited to 100 [mA], R ₆ = 6 [Ω] may be set.

According to this embodiment, the switching circuit 3
The transistor Q ₃ can also be used for current limiting, and the inrush current due to the abrupt change of the external load R can be suppressed only by adding the current detection circuit 9 having a simple structure. In the embodiment, the current i ₂ flowing through the external load R is
Is estimated by the current i ₃ which is almost equal to this, but the detection position of the current i ₂ is not limited to this embodiment.

Next, an embodiment of the fourth invention will be described. Like the third invention, this invention also prevents the inrush current due to the abrupt change of the external load R, and limits the current by utilizing a part of the circuit configuration of the first invention, thereby reducing the number of parts, This is a cost reduction. FIG. 11 is a block diagram showing the configuration of this embodiment. As compared with the embodiment of FIG. 1, in this embodiment, between the one end of the feedback resistor 5 on the external load side and the voltage / time width conversion circuit 2, The voltage detection circuit 10 is added. This voltage detection circuit 10
Is to detect the fact that the external load R is not connected or the load resistance value is larger than the specified value by detecting that the voltage at one end of the feedback resistor 5 is smaller than a certain value, and to set the current to flow to the external load R. The load current is finally limited by forcibly controlling the input voltage of the voltage / time width conversion circuit 2 so that the value becomes the minimum value.

FIG. 12 shows a concrete circuit configuration of this embodiment. That is, the voltage detection circuit 10 includes the resistor R ₉ and the Zener diode ZD _{2 to which the} power supply voltage V _P is applied.
A series circuit of a connection point of these series circuits is connected to the negative input terminal, a comparator Q ₅ to the positive input terminal is connected to one end of the feedback resistor 5, the resistor R ₈ to its output terminal connected to one end And a resistor R ₇ connected between the other end and the output terminal of the comparator Q ₁ , and the connection point of the resistors R ₇ and R ₈ is the voltage / time width conversion circuit 2
Is connected to the positive input terminal of the internal comparator Q ₂ .

In the above structure, the voltage detection circuit 10, the voltage / time width conversion circuit 2, the switching circuit 3 and the like constitute the current limiting circuit of the present invention.

Here, in explaining the operation of this embodiment, referring to FIG. 15, the output voltage of the feedback processing section 1, the output waveform of the triangular wave generating circuit 21, the comparator Q
The relationship between the _two output waveforms will be described. When the output waveform of the triangular wave generation circuit 21 is larger than the output voltage of the feedback processing unit 1, the output waveform of the comparator Q ₂ becomes the “Low” level, and conversely, when the output voltage is larger than the output waveform. , Output waveform is "High"
It becomes a level. The longer the output waveform is "Low", the more the current (load current) flows through the external load R.
Grows. Now, the output voltage of the feedback processing unit 1 when the load current is set to the minimum is defined as V _min, and the waveform of each part at this time is shown in FIG. Further, the output voltage of the feedback processing unit 1 when the load current is set to the _maximum is defined as V _max, and the waveform of each unit at this time is shown in FIG.

Here, considering the case where the external load R is not connected or the load resistance value is larger than the specified value, the output voltage of the feedback processing unit 1 becomes smaller than V _max , and the triangular wave generation circuit 21. No longer crosses the output waveform of. Therefore, the output waveform of the comparator Q ₂ remains at “Low” level, and the transistor Q ₃ of the switching circuit ₃ is completely turned on. The waveform of each part at this time is shown in FIG. Therefore, if the external load R is connected in this state, the inrush current will flow due to the delay time of the feedback system.

Based on the above points, the operation of this embodiment will be described. That is, in this embodiment, when the external load R is not connected or the load resistance value is larger than the specified value so that the state of FIG.
The output voltage of the feedback processing unit 1 is forcibly fixed to V _min as shown in (a). That is, in FIG. 12, the voltage of the feedback resistor 5 is compared with the voltage of the Zener diode ZD ₂ by the comparator Q ₅ . The voltage of the feedback resistor 5 is the Zener diode ZD ₂
Is higher than the voltage of the comparator Q ₅ , the output of the comparator Q ₅ is at “High” level (the output voltage of the comparator Q ₅ is V _H
In the opposite case, the output of the comparator Q ₅ is at “Low” level (the output voltage of the comparator Q ₅ is V _L
And). When the external load R is connected, the output voltage of the comparator Q ₅ is V _L , so the output of the comparator Q ₁ of the feedback processing unit 1 is V _IN ,
When the voltage at the positive input terminal of the comparator Q ₂ of the voltage / time width conversion circuit 2 is V _OUT , Formula 20 is obtained.

[0071]

[Formula 20] V _OUT = R ₈ (V _IN −V _L ) / (R ₇ + R ₈ ) + V _L

When the external load R is not connected or when the load resistance value is larger than the specified value, the output voltage of the comparator Q ₅ becomes V _H , which is the voltage V shown in the equation 21.
_{In order} to obtain _OUT , the resistors R ₇ and R in the voltage detection circuit 10
Adjust the value of ₈ .

[0073]

[Expression 21] V _OUT = R ₇ (V _H −V _IN ) / (R ₇ + R ₈ ) + V _IN = V _min

[0074] Incidentally, the Zener diode ZD ₂ resistor R ₉ connected in series is for voltage generation of the Zener diode ZD _2, due to the series circuit of a resistor R ₉ and a resistor in place of the Zener diode ZD ₂ The reference voltage of the comparator Q ₅ can also be obtained by the voltage division.

As described above, according to this embodiment, when the external load R is not connected or the load resistance value is larger than the specified value, this is detected by the voltage detection circuit 10 and the voltage is detected. By forcibly fixing the voltage of the positive input terminal of the comparator Q ₂ of the time width conversion circuit 2 to V _min , the state of FIG. 15A is realized, and the ON period of the transistor Q ₃ of the switching circuit 3 is realized. The current is limited so that the load current is minimized by limiting. Therefore, even if the load resistance value changes abruptly by connecting the external load R in this state, it is possible to prevent the inrush current from flowing.

Next, an embodiment of the fifth invention will be described.
The present invention is intended to prevent an increase in the response time until the external load R stabilizes at the set value due to disturbance of the load current when the external load R is connected from the unconnected state. 4 and 5 described as the embodiment of the first invention, since the current does not flow through the feedback resistor 5 when the external load R is not connected, the output of the comparator Q ₁ in the feedback processing unit 1 is negative. Maximum value. Therefore, the triangular wave generation circuit 21 in the voltage / time width conversion circuit 2 is
If the amplitude width of the output of the comparator is smaller than the power supply range of the comparator (op amp) Q ₁ (it is usually difficult to set the amplitude level of the triangular wave to the full power supply range), the output of the comparator Q ₁ is Since it has a more negative value than the output, the output of the comparator Q ₂ does not have a pulse waveform and has a negative maximum value.

Therefore, the transistor Q ₃ of the switching circuit 3 remains in the completely ON state. As described above, if the external load R is connected in this state, inrush current will flow, and the output of the comparator Q ₁ will change from the negative maximum value to the triangular wave generation circuit 2 due to the influence of the capacitor C _{1 and the} like.
A delay time occurs until it returns to the range of the amplitude width of 1, and the response time until the current flowing through the external load R stabilizes at the set value becomes long. Therefore, the present invention is designed to eliminate such an inconvenience.

FIG. 13 is a block diagram showing the structure of this embodiment. Comparing the present embodiment with the embodiment of FIG. 1, a voltage limiting circuit 11 is added between the feedback processing unit 1 and the voltage / time width conversion circuit 2. The voltage limiting circuit 11 limits the output voltage of the feedback processing unit 1 so as not to become smaller than V _max even when the external load R is not connected or the load resistance value is larger than the specified value. It is a thing.

FIG. 14 is a circuit diagram showing a specific structure of FIG. This voltage limiting circuit 11 includes a resistor R ₁₃ connected between the output terminal of the comparator Q ₁ of the feedback processing unit 1 and the positive input terminal of the comparator Q ₂ of the voltage / time width conversion circuit 2, and a comparator R ₂ . Positive input terminal and 0
It is composed of a diode D ₁ connected to the V line.

The operation of this embodiment will be described. The circuit constants (resistance values of R ₁ , R ₂ , R _4, etc., the triangular wave of the triangular wave generating circuit 21, etc.) are adjusted, and V shown in FIG. _max is −
It should be 0.6 [V]. The diode D ₁ is connected between the positive input terminal of the comparator Q ₂ and the 0V line, and the output voltage of the feedback processing unit 1 is V _max ˜.
If it is within V _min , it has no effect. However, when the external load R is not connected, or when the load resistance value is larger than the specified value, when the output voltage of the feedback processing unit 1 tries to become smaller than V _max (= −0.6 [V]). , The current is 0 through the diode D _1.
The voltage is bypassed to the V line, and a voltage of -0.6 [V] or less is not applied to the positive input terminal of the comparator Q ₂ . The resistor R ₁₃ is for limiting the bypass current.

With the configuration as described above, the voltage can be limited so that the output voltage of the feedback processing unit 1 does not deviate from the output amplitude range of the triangular wave generating circuit 21.
As a result, the transistor Q ₃ can be prevented from being completely turned on, so that no rush current flows when the external load R is connected, and the output of the comparator Q ₁ is caused by the capacitor C ₁ or the like. Since there is no delay time until the voltage returns to within the output amplitude range of the triangular wave generating circuit 21, the response time until the load current stabilizes can be shortened even if the external load R changes suddenly. Although the output voltage of the feedback processing unit 1 is limited by the diode D ₁ in this embodiment, the voltage limiting circuit 11 can be easily constructed by using a transistor or a Zener diode, and the limiting voltage value can be changed. It is also easy to do.

The above-described third to fifth inventions are all based on the embodiment of FIGS. 1 and 4 according to the first invention, but the third to fifth inventions are the second embodiment. The present invention (the embodiment of FIG. 6) can also be applied.

[0083]

As described above, according to the first to fifth inventions, the output circuit of the voltage / time width conversion circuit choppers the switching circuit to supply a constant current to the external load. Since only the required energy is supplied to the load to reduce losses such as internal heat generation, it is possible to downsize the circuit.Also, by relaxing the thermal stress on the components, reliability and product life can be improved. It can be extended. Further, since the internal heat generation of the circuit is small, it is suitable for making the entire circuit into an LSI, and there is an advantage that the application range of the voltage / current conversion circuit or the constant current output circuit is expanded.

According to the second invention, it is not necessary to use a high speed switching element as a switching circuit,
This has the effect of reducing switching loss.

According to the third invention, the current cutoff portion of the current limiting circuit is shared with the switching circuit, and the output current of the switching circuit is cut off when the current flowing through the external load exceeds the set value. By adding a few circuits to the first invention and the like, it is possible to suppress the inrush current due to the rapid fluctuation of the external load and minimize the influence on the external equipment such as the plant controlled by the present invention. it can.

According to the fourth invention, it is detected that the external load is not connected or the load resistance value is larger than the specified value, and the switching circuit is controlled so that the load current takes the minimum value. Because it suppresses the load current,
Similar to the third aspect, by adding a few circuits to the first aspect and the like, it is possible to suppress the inrush current due to the rapid change of the external load and minimize the influence on the external device.

According to the fifth aspect of the invention, the output voltage of the feedback processing unit is restricted so as not to deviate from the output amplitude range of the triangular wave generation circuit in the voltage / time width conversion circuit, thereby preventing the generation of inrush current. It is possible to shorten the response time until the load current is disturbed and becomes constant in response to a sudden change in the external load, and the influence on the external device can be minimized.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a first invention.

FIG. 2 is an explanatory diagram of a current flowing through a transistor.

FIG. 3 is a circuit diagram showing a main part of the embodiment shown in FIG.

FIG. 4 is a circuit diagram showing a specific configuration of the embodiment shown in FIG.

5 is a time chart showing the operation of the circuit of FIG.

FIG. 6 is a block diagram showing an embodiment of the second invention.

FIG. 7 is a block diagram showing a conventional technique of a current limiting circuit.

FIG. 8 is a circuit diagram showing a specific configuration of FIG.

FIG. 9 is a block diagram showing an embodiment of the third invention.

FIG. 10 is a circuit diagram showing a specific configuration of the embodiment shown in FIG.

FIG. 11 is a block diagram showing an embodiment of the fourth invention.

FIG. 12 is a circuit diagram showing a specific configuration of the embodiment of FIG.

FIG. 13 is a block diagram showing an embodiment of the fifth invention.

FIG. 14 is a circuit diagram showing a specific configuration of the embodiment shown in FIG.

FIG. 15 is an operation waveform chart of each part for explaining the operation of the fourth and fifth embodiments of the invention.

FIG. 16 is a circuit diagram showing a conventional technique.

[Explanation of symbols]

 1 Feedback processing unit 2 Voltage / time width conversion circuit 3 Switching circuit 4 Filter circuit 5 Feedback resistor 9 Current detection circuit 10 Voltage detection circuit 11 Voltage limiting circuit R External load

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoji Daito 1 Fujimachi, Hino City, Tokyo Fujifacom Control Co., Ltd.

Claims (5)

[Claims] 1. A voltage / current conversion circuit for converting a DC input voltage into a DC current and supplying the same to an external load, the voltage / current being converted into a pulse signal having a pulse width corresponding to the magnitude of the input voltage / output voltage. A time width conversion circuit, a switching circuit that performs a chopper operation by an output pulse of the voltage / time width conversion circuit, and outputs a current having a magnitude corresponding to the output pulse width, and a current output from the switching circuit is smoothed. A voltage / current, comprising: a filter circuit; and a feedback control circuit that detects a current supplied from the filter circuit to the external load and feedback-controls an input voltage of the voltage / time width conversion circuit. Conversion circuit. 2. The voltage / current conversion circuit according to claim 1, further comprising a Zener diode connected between a feedback resistor forming a feedback control circuit and an external load. 3. The voltage / current conversion circuit according to claim 1, further comprising a current limiting circuit that shuts off the output current of the switching circuit when the current flowing through the external load exceeds a set value. Voltage / current conversion circuit. 4. The voltage / current conversion circuit according to claim 1, wherein the output current of the switching circuit is suppressed by detecting that an external load is not connected or the load resistance value is larger than a specified value. A voltage / current conversion circuit comprising a current limiting circuit. 5. The voltage / current conversion circuit according to claim 1, wherein the voltage / time width conversion circuit includes a comparator that outputs a comparison result of the triangular wave and the output voltage of the feedback control circuit to the switching circuit as a pulse signal. A voltage / current converter characterized in that a voltage limiting circuit for limiting the output voltage of the feedback control circuit so as not to deviate from the amplitude range of the triangular wave is connected between the feedback control circuit and the voltage / time width conversion circuit. circuit.