JP4012646B2 - Switching circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a switching circuit, and more particularly, to an isolation type switching circuit in which a switching transistor is driven through a photocoupler.
[0002]
[Prior art]
As shown in FIG. 3, the isolation type switching circuit is configured to drive a switching transistor 30 using, for example, a MOSFET (Metal-Oxide Field Effect Effect Transistor) through photocouplers 40 and 50. .
[0003]
Input signals are given to the light emitting diodes 42 and 52 of the photocouplers 40 and 50 with opposite polarities, whereby one of the phototransistors 44 and 54 is turned on. It operates in a relationship where the other is turned off.
[0004]
When the phototransistor 44 is turned on, the voltage of the DC power supply 80 charged in the capacitor 70 is applied between the gate and the source of the switching transistor 30. As the DC power source 80, a photovoltaic device such as a solar cell is used in terms of easy isolation. The switching transistor 30 is turned on when a voltage is applied between the gate and the source. When the phototransistor 54 is turned on, the gate and source of the switching transistor 30 are short-circuited, and the charge between the gate and source is discharged.
[0005]
The switching transistor 30 is turned on when a voltage is applied between the gate and the source, and turned off when the gate and the source are short-circuited. That is, the switching transistor 30 is turned on / off in response to the input signals of the photocouplers 40 and 50.
[0006]
[Problems to be solved by the invention]
In the switching circuit as described above, there is a moment when the phototransistors 44 and 54 are simultaneously turned on at the time when the on / off is alternated due to the difference in speed between the on operation and the off operation of the phototransistors 44 and 54. This will be described in detail with reference to the time chart shown in FIG. 4. As shown in FIG. 4A, between the two points 1 and 4 upstream of the resistors 62 and 64 as viewed from the input signal source side. When the rectangular wave voltage V14 is applied to the voltage V23, the voltage V23 between the two points 2 and 3 downstream of the resistors 62 and 64 becomes the rectangular wave voltage V23 whose amplitude is reduced by the voltage drop across the resistors 62 and 64. This rectangular wave voltage V23 is applied across the light emitting diodes 42 and 52.
[0007]
As the light emitting diodes 42 and 52 corresponding to the rectangular wave voltage V23 blink, the phototransistors 44 and 54 are turned on and off as shown in FIG. The operation of the phototransistors 44 and 54 is faster when turned on than when turned off, and tON <tOFF. For this reason, a period in which both phototransistors are simultaneously turned on is generated by tOFF-tON. The length of this period is about 10 to 30 μs.
[0008]
When the phototransistors 44 and 54 are turned on at the same time, both ends of the capacitor 70 are short-circuited and electric charges are discharged. For this reason, the voltage across the capacitor 70 rapidly decreases as shown in FIG. The voltage across the capacitor 70 gradually recovers with the charging from the DC power supply 80 after being released from the short circuit. However, since the output current of the DC power supply 80 constituted by a solar cell or the like is as weak as about 1 to 50 μA, the switching transistor It takes much longer than the discharge time to recover to the voltage VGS that turns 30 on. This time is about 0.1 to 10 ms.
[0009]
Meanwhile, since the switching transistor 30 cannot be turned on, a delay of tFETON occurs until the switching transistor 30 is turned on as shown in (d). For this reason, the ON continuation time of the switching transistor 30 becomes shorter than the original ON continuation time indicated by the broken line, and therefore the switching efficiency is lowered. In particular, the shorter the ON / OFF cycle, the more the efficiency is lowered, and in some cases, the operation becomes impossible.
[0010]
The present invention has been made to solve the above-described problems, and an object thereof is to realize an isolation type high-efficiency switching circuit.
[0011]
[Means for Solving the Problems]
(1) A first invention for solving the above-described problem is that a pair of photocouplers that are provided with input signals whose levels are alternated in opposite phases and that are turned on and off in opposite phases, and the pair of photocouplers, A drive signal is applied between the control input terminal and one output terminal through one of the output elements, and the control input terminal and the one output terminal are transmitted through the other output element of the pair of photocouplers. And a switching transistor that is short-circuited between the pair of photocouplers, provided in the input circuit of the pair of photocouplers, and having a time constant for a transient change when the level of the input signal is changed It is a switching circuit characterized by comprising a constant providing means.
[0012]
(2) A second invention for solving the above problem is the switching circuit according to (1), wherein a MOSFET is used as the switching transistor.
[0013]
(3) A switching circuit according to (1) or (2), wherein a third invention for solving the above-described problem uses a voltage of a photovoltaic element charged in a capacitor as the drive signal.
[0014]
(4) A fourth invention for solving the above-mentioned problem is the switching circuit according to any one of (1) to (3), wherein a capacitor is used as the time constant providing means. .
[0015]
(Function)
In the present invention, the time constant is given to the transitional change at the time of the level change of the input signal of the photocoupler by the time constant providing means, and the time when the output element of the photocoupler is turned on is delayed. This eliminates the period during which the two output elements are turned on simultaneously.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment. FIG. 1 shows an electrical connection diagram of the switching circuit. This circuit is an example of an embodiment of the switching circuit of the present invention. As shown in FIG. 1, this circuit has a configuration that is largely in common with the circuit shown in FIG. The substantial difference is that a capacitor 900 is connected between the input signal supply lines of the photocoupler.
[0017]
This circuit is configured to drive a switching transistor 300 using, for example, a MOSFET or the like via photocouplers 400 and 500. Switching transistor 300 is an example of an embodiment of a switching transistor in the present invention. Photocouplers 400 and 500 are examples of embodiments of photocouplers in the present invention. Input signals are given to the light-emitting diodes 420 and 520 of the photocouplers 400 and 500 with opposite polarities, whereby the phototransistors 440 and 540 operate so that when one is turned on, the other is turned off. Phototransistors 440 and 540 are an example of an embodiment of an output element in the present invention.
[0018]
When the phototransistor 440 is turned on, the voltage of the DC power supply 800 charged in the capacitor 700 is applied between the gate and the source of the switching transistor 300. As the DC power source 800, for example, a photovoltaic device such as a solar cell is used. When the phototransistor 540 is turned on, the gate and the source of the switching transistor 300 are short-circuited, and the charge between the gate and the source is discharged.
[0019]
The switching transistor 300 is turned on when a voltage is applied between the gate and the source, and turned off when the gate and the source are short-circuited. That is, the switching transistor 300 is turned on / off in response to the input signal of the photocouplers 400 and 500.
[0020]
FIG. 2 shows a time chart of the operation of this circuit. As shown in FIG. 6A, a rectangular wave voltage V14 is applied between two points 1 and 4 upstream of the resistors 620 and 640 as viewed from the input signal source side. At this time, the voltage V23 between the two points 2 and 3 downstream of the resistors 620 and 640 is a trapezoid having an amplitude reduced by a voltage drop in the resistors 620 and 640 and a rising time constant determined by the resistors 620 and 640 and the capacitor 900. It becomes a wave-like voltage V23. This trapezoidal voltage V23 is applied across the light emitting diodes 420 and 520. A circuit including resistors 620 and 640 and a capacitor 900 is an example of an embodiment of a time constant providing unit in the present invention.
[0021]
As the light emitting diodes 420 and 520 corresponding to the trapezoidal voltage V23 blink, the phototransistors 440 and 540 are turned on and off as shown in FIG. Here, since the light emitting diodes 420 and 520 emit light when the trapezoidal wave voltage V23 becomes equal to or higher than the light emission voltages V420 and V520, the light emission delay time tON (based on the time constant of the rise and fall of the trapezoidal wave voltage V23) LED) and quenching delay time tOFF (LED).
[0022]
The time constant of rising and falling of the trapezoidal voltage V23 is set so that the light emission delay time tON (LED) is larger than the sum of the extinction delay time tOFF (LED) and the off delay time tOFF of the phototransistors 440 and 540. Therefore, the phototransistors 440 and 540 are not turned on at the same time.
[0023]
Therefore, a period in which the capacitor 700 is short-circuited does not occur, and the charge is not discharged. For this reason, the switching transistor 300 is turned on at the same time as the phototransistor 440 is turned on and turned off at the same time as the phototransistor 540 is turned on, as shown in FIG. In this way, the on-period of the switching transistor 300 can be prevented from being shortened, and efficient switching can be performed.
[0024]
As described above, the example in which the MOSFET is used as the switching transistor has been described. However, the present invention is not limited to the MOSFET, and various types of transistors such as a junction FET and a bipolar transistor may be used. Needless to say, the energy source of the drive signal for the switching transistor is not limited to a photovoltaic device such as a solar cell, and a normal battery may be used.
[0025]
【The invention's effect】
As described above in detail, according to the present invention, an isolation type highly efficient switching circuit can be realized.
[Brief description of the drawings]
FIG. 1 is an electrical connection diagram of an exemplary switching circuit according to an embodiment of the present invention.
FIG. 2 is a time chart of the operation of the switching circuit shown in FIG.
FIG. 3 is an electrical connection diagram of a conventional switching circuit.
4 is a time chart of the operation of the switching circuit shown in FIG. 3;
[Explanation of symbols]
300 switching transistor 400, 500 photocoupler 420, 520 light emitting diode 440, 540 phototransistor 620, 640 resistor 700 capacitor 800 battery 900 capacitor

Claims (4)

A pair of photocouplers which are provided with input signals whose levels are changed in opposite phases and which are turned on and off in opposite phases;
A drive signal is applied between a control input terminal and one output terminal through one output element of the pair of photocouplers, and the control input terminal is transmitted through the other output element of the pair of photocouplers. And a switching transistor in which the one output terminal is short-circuited;
A switching circuit comprising
A time constant providing means which is provided in an input circuit of the pair of photocouplers and which gives a time constant to a transitional change at the time of level change of the input signal;
A switching circuit comprising: MOSFET is used as the switching transistor,
The switching circuit according to claim 1. Using the voltage of the photovoltaic element charged in the capacitor as the drive signal,
The switching circuit according to claim 1 or 2, wherein A capacitor is used as the time constant providing means.
The switching circuit according to any one of claims 1 to 3, wherein

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