JP3306385B2 - Current detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current detection circuit for detecting an output current or the like of a power supply circuit, and detects a current and at the same time, changes a voltage polarity of a detection signal obtained to a control circuit system. Related to technology.

[0002]

2. Description of the Related Art In recent years, many electronic devices and devices having various functions are mounted on electronic equipment. In order to operate these electronic elements / devices, naturally, specifications (eg, voltage value, polarity, frequency, etc.) suitable for the electronic elements / devices are required.
In such a case, there is a case where a circuit system having a different power supply specification exists within one electronic device.
As an example, consider a fluorescent display tube. In order to drive the fluorescent display tube, a first power source for obtaining a current necessary for heating the filament and a second power source for obtaining a voltage necessary for displaying the fluorescent display tube are provided. And two power sources. Specifically, conventionally, the fluorescent display tube and its peripheral circuits have a configuration as shown in FIG.

[0003] The circuit shown in FIG. 2 is provided with a transformer T, and both ends of a primary winding N 1 are connected to an output terminal of a conversion circuit 2. The input terminal of the conversion circuit 2 is connected to the input terminals 1a and 1b, and the control circuit 3 for controlling the operation is connected to the conversion circuit 2. After forming an inverter type power supply circuit 4 by the transformer T, the conversion circuit 2 and the control circuit 4, the filament of the fluorescent display tube FIT is connected between both ends of the secondary winding N2 of the transformer T. And the secondary winding N2 of the transformer T
Is provided with an intermediate tap P, and the intermediate tap P is connected to the primary winding N3 of the current transformer CT constituting the current detection circuit 7b.
To one end of the voltage source 5. One end of the secondary winding N4 of the current transformer CT is connected to the ground, and the other end is connected to the control circuit 3 of the power supply circuit 4. The other end of the voltage source 5
It is configured to be connected to the grid of the fluorescent display tube FIT via the ground and to the anode of the fluorescent display tube FIT via the ground and the lighting control circuit 6.

In the circuit of FIG. 2 having such a configuration, when a voltage is externally supplied between the input terminals 1a and 1b, the conversion circuit 2 operates and the primary winding N1 of the transformer T is activated.
Is supplied with a pulsed voltage or an AC voltage. Then, an AC voltage having a waveform similar to the waveform of the voltage applied to the primary winding N1 is generated in the secondary winding N2, and an AC current is supplied from the secondary winding N2 to the filament of the fluorescent display tube FIT. . At this time, a voltage source 5 is connected between the filament and the grid of the fluorescent display tube FIT and between the filament and the anode.
Voltage V _B1 generated in is applied. By supplying a current to the filament and applying a voltage between the filament and the grid and between the filament and the anode, the fluorescent display tube F
IT will light up.

Here, when the display state of the fluorescent display tube FIT changes by the operation of the lighting control circuit 6, the flow rate of the current passing between the filament and the grid and between the filament and the anode, that is, the flow rate of the cathode current changes. .
In the circuit shown in FIG. 2, this cathode current is detected by the current transformer CT, and a detection signal is sent to the control circuit 3 constituting the power supply circuit 4. Then, the control circuit 3 controls the control amount of the control signal supplied to the conversion circuit 2 according to the received detection signal,
For example, the conversion circuit 2 changes the voltage value, the pulse width, and the like.
Changes the amount of energy passing through. As a result, the current supplied from the secondary winding N2 to the filament increases or decreases according to the display state of the fluorescent display tube FIT, and acts to stabilize the luminance of the display.

[0006] The fluorescent display tube emits electrons from the filament by heat, accelerates the electrons in an electric field, hits the anode, and lights the anode by emitting light. Therefore, it is basically necessary to keep the filament at a lower potential than the anode and the grid. In the circuit shown in FIG.
At the time of lighting, the potential of the anode and grid is set to the ground potential,
The filament is configured to supply a voltage having a potential lower than the ground potential generated in the voltage source 5, that is, a voltage having a negative voltage polarity. In such a configuration, if a generally-used resistance detection circuit is applied to the current detection circuit 7b to detect the cathode current, the polarity of the voltage generated in the voltage source 5 is negative. The voltage of the detection signal also becomes negative.

However, the conversion circuit 2, the control circuit 3 and the like constituting the power supply circuit 4 in FIG. 2 generally process a signal having a voltage higher than the ground potential, that is, a signal having a positive voltage polarity, that is, a so-called positive logic. Type circuit. Usually, a detection signal having a negative voltage polarity cannot be directly supplied to the control circuit 3 of the positive logic type. Therefore, in the circuit shown in FIG. 2, a current transformer CT is used for the detection circuit 7b for detecting the cathode current, instead of a circuit using the resistance detection method. The configuration is such that the voltage polarity of the detection signal obtained from the secondary winding N4 is positive, and the detection signal is supplied to the control circuit 3.

[0008]

As described above, a current transformer is useful for detecting a current and transmitting and receiving a detection signal obtained by the detection between circuit systems having different voltage polarities. However, the current transformer is a winding component, and each winding has a stray capacitance and a leakage inductance. Therefore, when the display state of the fluorescent display tube FIT changes and the cathode current changes stepwise, the signal waveform sent from the current transformer CT to the control circuit 3 includes noise generated due to stray capacitance and leakage inductance. There was a problem of being superimposed.

If noise is actually superimposed on the signal waveform, the control system cannot accurately detect the amount of current, and the output control of the power supply circuit to be controlled cannot be performed correctly. As a result, if the load is a fluorescent display tube, there is a possibility that an undesired phenomenon such as an unstable display brightness or a shortened life of the tube may occur. Therefore, the present invention
It is an object of the present invention to provide a current detection circuit capable of detecting a current, setting the voltage polarity of the detection signal to a polarity suitable for a circuit system in a subsequent stage, and preventing noise from being superimposed on the detection signal.

[0010]

A current detection circuit according to the present invention includes a detection resistor inserted in series in a power transmission line, and a difference between two terminal voltages appearing at both ends of the detection resistor. In a current detection circuit for detecting a flowing current, a reference voltage source having one electrode connected to a reference potential point is provided, and a voltage signal corresponding to two terminal voltages is biased with a reference voltage of the reference voltage source, respectively. , A signal corresponding to the flow rate of the current flowing through the transmission line is obtained from the two voltage signals. Alternatively, there is provided a reference voltage source having one electrode connected to a reference potential point, and first and second voltage dividing circuits respectively connected to both ends of a detection resistor.
Biasing the first divided voltage appearing at the dividing point of the voltage dividing circuit and the second divided voltage appearing at the dividing point of the second voltage dividing circuit with the reference voltage of the reference voltage source, A signal corresponding to the flow rate of the current flowing through the transmission line is obtained from the biased first and second divided voltages.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A reference voltage source having a detection resistor connected in series to a power line and a negative terminal connected to the ground is provided. A first resistor having a first resistor and a second resistor connected in series;
Is connected to one end of the detecting resistor, and a voltage dividing circuit having a third resistor and a fourth resistor connected in series is connected to the other end of the detecting resistor. A positive terminal of a reference voltage source is connected to each of the first and second voltage dividing circuits, and a voltage signal that appears at a connection point of the first and second resistors and a voltage signal that appears at a connection point of the third and fourth resistors. The applied voltage signal is biased with a reference voltage obtained from a reference voltage source. An error amplifier is provided, and two voltage signals biased by a reference voltage are input to each input terminal. The output signal obtained by the error amplifier is used as a detection signal corresponding to the current flowing through the power line, and is input to a subsequent circuit.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A fluorescent display tube and its peripheral circuits, including an embodiment of a current detection circuit according to the present invention capable of detecting a current and setting the voltage polarity of a detection signal to a polarity suitable for a circuit system in a subsequent stage, are described. As shown in FIG. In the circuit shown in FIG. 1, the current detection circuit 7a according to the present invention is configured as follows. 1 and 2 have the same configuration except for the current detection circuit 7a.

A reference voltage source 8 is provided in which a detection resistor R1 is connected in series to a power line connecting an intermediate tap P of the transformer T and one end of a voltage source 5, and a negative terminal is connected to ground. A voltage dividing circuit composed of a series circuit of resistors R2 and R3 is connected between one end of the detection resistor R1 and the positive terminal of the reference voltage source 8, and the other end of the detection resistor R1 and the reference voltage source 8 are connected. A voltage dividing circuit composed of a series circuit of a resistor R4 and a resistor R5 is connected between the positive terminal. An error amplifier EA is provided, one input terminal is connected to a connection point (voltage division point) between the resistors R2 and R3, and the other input terminal is connected to a connection point (voltage division point) between the resistors R4 and R5. The output terminal of the error amplifier EA is connected to the control circuit 3.

The current detection circuit 7 having such a circuit configuration
In FIG. 1A, when the entire circuit of FIG. 1 operates and the fluorescent display tube FIT is turned on, one end (A) of the detection resistor R1 on the voltage source 5 side is connected to the negative voltage V _B1 of the voltage source 5. Appears. On the other hand, the other end (B) of the detecting resistor R1 on the transformer T side
, A negative voltage having a magnitude obtained by subtracting a voltage drop generated in the detection resistor R1 from the voltage _VB1 appears. The voltage appearing at both ends (A) and (B) of the detection resistor R1 is
The voltage is divided by a voltage dividing circuit of the resistors R2 and R3 or the resistors R4 and R5 connected to both ends of the detecting resistor R1, respectively. The current detection circuit 7a of the present invention has a configuration in which one ends of the resistors R3 and R5 are connected to the reference voltage source 8. For this reason, the divided voltage appearing at the connection point between the resistors R2 and R3 and the divided voltage appearing at the connection point between the resistors R4 and R5 are equal to the positive voltage V generated at the reference voltage source 8.
A bias having a magnitude corresponding to _B2 is received.

Specifically, the ratio of the resistances R2 and R3 and the resistance R
4 and R5 are the same, and the voltage V _B2Voltage value for each resistor
Resistance values of anti-R4 and R5 and voltage V_B1Equation from the voltage value of
｛(R4 / R5) · V_B1Greater than the value required by｝
In this case, the polarity of the voltage at each voltage dividing point is positive. This
Then, each divided voltage that has become positive polarity is supplied to the error amplifier EA.
input. Then, the output signal of the error amplifier EA has the voltage polarity
Becomes positive, and the cathode current flowing through the detection resistor R1 changes.
In this case, the amount of change in the signal
It depends on the amount of conversion. Therefore, the current detection circuit of the present invention
7a adjusts the voltage polarity of the detection signal to the control circuit 3 at the subsequent stage.
It is possible to do. Further, the current detection circuit 7 of the present invention
a is different from the case where a current transformer is used.
No elements such as stray capacitance or leakage inductance inside
No. As a result, there is no noise superposition, and
Can be sent to the subsequent control circuit 3
You.

In the circuit shown in FIG. 1, a positive logic circuit is assumed for the error amplifier EA. Therefore, the circuit system is substantially the same as the control circuit 3,
It is also possible to configure the error amplifier EA integrally with the control circuit 3. Also it is possible to receive a voltage supplied from the power source for driving the control circuit 3 for power V _C required to drive the error amplifier EA. In the above-described embodiment of the current detection circuit according to the present invention, the configuration for detecting the current flowing through the circuit system having the negative voltage polarity and outputting the detection signal to the positive logic circuit system has been described. The present invention is not limited to such a configuration, and can be applied to a configuration in which the voltage polarity and the logic type are reversed. Naturally, the circuit system in which the current detection circuit of the present invention is incorporated is not limited to the inverter circuit for lighting the fluorescent display tube as shown in the embodiment.

[0017]

As described above, in the current detection circuit according to the present invention, a detection resistor is connected in series to a power transmission line, and a reference voltage is applied to each voltage signal corresponding to a voltage appearing at both ends of the resistor. A bias is applied with a reference voltage from a source, and a detection signal corresponding to a current flowing through the power transmission line is obtained from two biased voltage signals. In the current detection circuit of the present invention, the voltage polarity of the signal can be manipulated by applying a bias with the reference voltage, and the voltage polarity of the detection signal can be adapted to the circuit system at the subsequent stage. Also, since there are no factors such as stray capacitance and leakage inductance in the circuit, noise is not superimposed on the signal.
Therefore, a signal that correctly reflects the current flow rate can be directly input to the control circuit 3 at the subsequent stage, and a highly reliable current detection circuit can be provided.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of a current detection circuit according to the present invention, and a fluorescent display tube and peripheral circuits thereof.

FIG. 2 is a circuit diagram of a conventional fluorescent display tube and its peripheral circuits.

[Explanation of symbols]

1a, 1b: input terminal 2: conversion circuit 3: control circuit 4: power supply circuit 5: voltage source 6:
Lighting control circuit 7: Current detection circuit 8: Reference voltage source CT: Current transformer EA: Error amplifier FIT: Fluorescent display tube R1: Detection resistor R2: Resistance (first resistor) R3: Resistance (second resistor) R4 : Resistance (third resistance) R
5: (Fourth resistor)

Claims (3)

(57) [Claims] 1. A current detection circuit for connecting a detection resistor in series to a power transmission line and detecting a current flowing through the transmission line from a difference between two terminal voltages appearing at both ends of the detection resistor. A reference voltage source having one electrode connected to a reference potential point; and first and second voltage dividing circuits respectively connected to both ends of the detection resistor, and a voltage dividing circuit of the first voltage dividing circuit. The first that appeared on the point
And the second divided voltage appearing at the dividing point of the second voltage dividing circuit are respectively biased by the reference voltage of the reference voltage source, and the biased first and second divided voltages are applied. A current detection circuit for obtaining a signal corresponding to the flow rate of the current flowing through the transmission line from the divided voltage of (2). 2. The reference voltage of the reference voltage source is large enough to convert both the first and second divided voltages biased by the reference voltage from a negative voltage value to a positive voltage value. The current detection circuit according to claim 1. 3. A detection resistor connected in series to a power transmission line, a reference voltage source having one electrode connected to a reference potential point, and first and second resistors connected in series. A first voltage dividing circuit connected between one end of the detection resistor and the other electrode of the reference voltage; a third and a fourth resistor connected in series; A second voltage dividing circuit connected between the other end of the resistor and the other electrode of the reference voltage; a first input terminal connected to a connection point between the first and second resistors; A current detection circuit comprising: an error amplifier having an input terminal connected to a connection point between the third and fourth resistors.