JPH0230206B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bidirectional solid state switch using field effect transistors.

Conventionally, so-called mechanical contact switches, mercury relays, and the like have been used as switches for relatively high alternating current voltages such as commercial voltages, and direct current voltages. In particular, when the potential levels of the switch control circuit system and the switched circuit system are different, or when you want to keep both circuits electrically independent, it is necessary to electrically isolate both circuits, and the above switch is advantageous. It was hot. However, as electronic circuits become more solid-state and more integrated, and electronic devices as a whole become smaller, there is a desire to create a compact, high-performance bidirectional solid-state switch that can replace the conventional mechanical contact switch, and various developments are underway. ing. FIG. 1 is an example of a conventional solid-state AC switch in which a field effect transistor is used as a switching transistor, and an input signal circuit and a switched circuit are insulated and separated by a transformer. This solid-state AC switch has various advantages over switches using bipolar elements, such as high reliability as a high-output and high-voltage switch, but since it uses a transformer, which has a limit to miniaturization, the overall switch size The drawback was that it could not be miniaturized or integrated.

FIG. 2 cites an invention filed by the present inventors on the same day as the present invention.

FIG. 2 shows a solid-state AC switch that uses a capacitor at the input end to solve this problem, and the input signal circuit and the circuit to be switched are separated in terms of direct current by the capacitor. Since the capacitance of the capacitor used may be relatively small, ranging from several tens _{of pF} to several hundred _pF , the switch has the advantage of being easy to downsize and integrate as a whole.

By the way, a switch using a transistor is
Generally, due to the on-resistance of the transistor itself, there is a drawback that the series resistance is higher than that of conventional mechanical contact switches. Since the series resistance needs to be low enough to not interfere with the switch function, in the case of the solid-state switches shown in Figures 1 and 2, it is necessary to reduce the on-resistance by increasing the size of the field effect transistor. . In fact, in field effect transistors, an on-resistance of about 1Ω can be easily obtained by increasing the gate width, but at the same time, the drain capacitance increases significantly as the drain area increases. have For example, in the case of a high-voltage field effect transistor with a drain breakdown voltage of 150V or more and an on-resistance of about 1Ω, the drain capacitance is
It can be as high as 1000 _pF . If such a field effect transistor is used in a solid-state AC switch as shown in FIGS. 1 and 2, the input capacitance at the switched end of the solid-state switch will be approximately 1000 _pF , and the frequency characteristics of the switch will be significantly degraded. For example, for a 1KHz AC switched input signal, the switch will output approximately 160K
For an AC switched input signal of Ω10KHz,
It only acts as an impedance of about 16KΩ. Therefore, if the load resistance is 10KΩ, 1K
A partial voltage of about 6% for the Hz AC voltage and 38% for 10KHz appears at the load end even when the switch is turned off, and the switch does not operate satisfactorily.

The switch shown in FIG. 2 has improved frequency characteristics compared to the switch shown in FIG. 1 because it uses a capacitor instead of a transformer, which has a slower response.

In the switch shown in FIG. 2, an alternating current switched voltage is applied to the two transistors from terminals 4 and 5, so that a large positive voltage to a large negative voltage is applied. While a large voltage is applied, the PN junction at the drain of either transistor is reverse-biased, reducing the capacitance and reducing the overall capacitance of the switch. However, while a small voltage near OV is applied, the drain capacitance becomes extremely large. Therefore, this switch has a low impedance and has a limited frequency response, necessitating improvement.

An object of the present invention is to provide a novel solid-state AC switch that has both low on-resistance and good frequency characteristics, which have conventionally been contradictory characteristics, and is capable of being miniaturized and integrated.

According to the present invention, two sets of diodes are connected in series between the drain and source terminals of a field effect transistor having a gate shunt resistance, with the anode facing the drain side for a p-type transistor and the cathode facing the drain side for an n-type transistor. The middle point of each diode row is the switched end, and the output end of the driving pulse power source, the gate electrode side end of the field effect transistor, and the ground electrode end of the driving pulse power source and the source electrode side of the field effect transistor. Among the ends, at least the output end of the driving pulse power source and the gate electrode side end of the field effect transistor are connected by a capacitor, and the driving pulse input voltage train connects the gate electrode side end of the capacitor and the source. The pulse voltage train generated between the electrode side ends was rectified and smoothed by a rectification and smoothing circuit provided between the gate electrode side end of the capacitor and the gate electrode of the field effect transistor, and was used as a DC gate input of the field effect transistor. A solid-state AC switch is obtained.

Hereinafter, the present invention will be explained in detail using drawings showing one embodiment.

FIG. 3 is a circuit diagram of a solid state switch showing one embodiment of the present invention. The solid state AC switch according to the present invention transfers the input pulsed drive voltage to the diode 10 and the resistor 1 through the capacitors 6 and 7.
2 performs rectification and smoothing, and operates the field effect transistor 1 by making it conductive. As shown in the figure, bidirectional signals are turned on and off by inserting the field effect transistor 1 into a bridge configuration of diodes 13 to 16. be able to.

The solid-state AC switch of this construction is capable of sufficiently switching high-frequency AC signals even though a field effect transistor with an on resistance of 1 Ω or less and a drain capacitance of 1000 _pF or more is used as a switching transistor. This means that when field effect transistor 1 is off, field effect transistor 1
Since the intersections 17 and 18 of the diodes 17 and 18 are DC biased to constant positive and negative voltages, respectively, with respect to the switched AC input signal of constant amplitude, the input signal is always present at the drain-source terminal of the transistor 1. This is due to the application of a DC voltage equal to the maximum amplitude of . This is a major difference from the conventional example. The reason why the terminal 18 is biased to a negative potential and the terminal 17 is biased to a positive potential is that the electric charge that has once flowed in cannot flow out due to the rectifying action of the diode of the diode bridge. In this way, a DC voltage equal to the maximum amplitude of the input signal is applied between the drain and source of the transistor 1, so that the drain region of the transistor is reverse biased and depleted, reducing the drain capacitance to several tenths.
In this way, the capacitance of the transistor 1 is kept small, so that only the capacitance of the diode appears as the series equivalent capacitance of the switch with respect to the input signal. The capacitance of a diode is originally much smaller than the drain capacitance of a transistor. In addition, diodes 16, 15, and 13 forming a diode bridge
and 14 are always reverse biased, resulting in low capacitance. As a result, the overall capacity is low. It should be noted that the capacitance of a transistor is reduced by reverse bias, as described by J.
Sze) “Physics of Semiconductor Devices” (Corona Publishing) No. 1
It is described in Volume 86 and Page 87 that when changing from forward bias to reverse bias, the capacitance of the PN junction decreases by 1/100.
It can be seen from FIG. 9 that the degree of change varies. Incidentally, the capacitance of a diode is at most several tens _{of pF} even if the diode has a withstand voltage of several hundred volts and an average rectified current of about 1 A. For example, if the series input capacitance of the solid state AC switch of the present invention is 10 _pF , the impedance for a 1KHz AC signal in the previous example is 16MΩ, and for a 10KHz AC signal it is 1.6MΩ, which is sufficient. It can be seen that a switch effect can be obtained.

In other words, in the solid state AC switch of this configuration, although a field effect transistor with a large drain capacitance and a capacitor are used on the gate input side for electrical isolation from the switch control circuit, the Since the series equivalent input capacitance can be made extremely small, a solid-state AC switch can be realized that is compact, can be integrated, and can sufficiently switch high-frequency signals.

The solid state AC switch circuit shown in Figure 3 is as follows:
This is merely for explaining one embodiment of the present invention, and it goes without saying that various different circuit configurations can be selected in accordance with the spirit of the present invention.
For example, in the solid state AC switch of the present invention, even if the capacitor 7 connecting the ground side end of the driving pulse power source and the source electrode side of the field effect transistor is omitted,
Due to the presence of parasitic capacitance in the switched circuit system, sufficient operation is possible when there is an equivalent closed circuit connecting the switch control circuit and the switched circuit with respect to the driving pulse. In addition, the rectifying and smoothing circuit for the drive pulse input can be selected depending on the purpose of the switch, ranging from a simple configuration as shown in the figure to a more complex configuration that reduces ripples at the DC gate input. can do. Various field effect transistors can be used as the transistors, such as so-called p-channel and n-channel MOS transistors, external junction field effect transistors, and electrostatic induction field effect transistors.

[Brief explanation of drawings]

FIG. 1 shows the circuit configuration of a conventional solid-state AC switch using a transformer, FIG. 2 shows the circuit configuration of a conventional improved fixed AC switch that can be downsized, and FIG. 3 shows the circuit configuration of the present invention, which can be downsized and An example of the circuit configuration of a solid-state AC switch with excellent high frequency characteristics is shown. In each figure, 1 and 2 are field effect transistors, 3 is a transformer, 4 and 5 are switched ends, 6 and 7 are input signal end capacitors, 8, 9 is the input signal end of the switch,
10 is a rectifying diode, 11 is a gate shunt resistor, 12 is a smoothing resistor, 13 to 16 are diodes for forming a bidirectional switch, 1
7 and 18 indicate the source and drain electrode ends of the field effect transistor, respectively.

Claims (1)

[Claims] 1. Two sets of diodes connected in series are provided between the drain and source terminals of a field effect transistor having a gate shunt resistance, with the anode facing the drain side for a p-type transistor and the cathode facing the drain side for an n-type transistor. The middle point of the diode string is the switched end, and the output end of the driving pulse power source and the gate electrode side end of the field effect transistor, and the ground electrode end of the driving pulse power source and the source electrode side end of the field effect transistor are connected to each other. At least the output end of the driving pulse power source and the gate electrode side end of the field effect transistor are connected by a capacitor, and the voltage between the gate electrode side end and the source electrode side end of the capacitor is connected by a driving pulse input voltage train. The pulse voltage train generated in the capacitor is rectified and smoothed by a rectification and smoothing circuit provided between the gate electrode side end of the capacitor and the gate electrode of the field effect transistor, and is used as a DC gate input of the field effect transistor. Solid AC switch.