Abstract:
A bilateral switch derives internal power for the operation of the switch control circuitry from a converter powered by the input signal, whether the input is AC or DC, and also operates the internal voltage isolated circuits powered by this from the input signal. The internal circuitry operates a pair of similar FET switches switching power on and off between their drain connections, relying on a reverse protection diode across the FETs to allow passage of either AC current or DC current in either direction.

Description:
TECHNICAL FIELD 
       [0001]    The invention generally relates to electronic bilateral switches. 
         [0002]    More particularly the invention relates to bilateral switches operated by a control input. 
       BACKGROUND ART 
       [0003]    Electronic or solid state switches are well known and are normally produced as suitable for AC or for DC typically with a DC control voltage applied. Operation of such switches at greater than a rate in the low kilohertz range requires measures which produce costly switches. Switches which can handle either AC input or DC input are even more costly to produce. 
         [0004]    Therefore a need exists for a solution to the problem of providing a solid state switch which can switch a large current at a high rate and which meets cost requirements. 
         [0005]    The present invention provides a solution to this and other problems which offers advantages over the prior art or which will at least provide the public with a useful choice. 
         [0006]    All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein; this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. 
       SUMMARY OF THE INVENTION 
       [0007]    In one exemplification the invention consists in a bilateral switch having a converter which internally converts an input signal to provide an electrically isolated power supply, the electrically isolated power supply connected to and providing current to a low current switch operated by the input signal via an electrically isolated connection, the low current switch connected to and operating at least a totem pole pair of semiconductor devices also powered from the electrically isolated power supply, the at least one totem pole pair of semiconductor devices connected to and operating at least one similar pair of FET (Field Effect Transistor) transistor devices connected to bilaterally switch current to a circuit to be switched on or off in response to the input signal. 
         [0008]    Preferably the input signal is applied to an IRLED (Infra-Red Light Emitting Diode) of an optocoupler in which the optocoupler output device is the low current switch. 
         [0009]    Preferably the input signal is applied as the power source of an oscillator whose isolated output provides the electrically isolated power supply. 
         [0010]    Preferably the totem pole pair of semiconductor devices is connected to the conjoined gates of two similar MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistors forming the pair of FET transistor devices, that the sources of the MOSFETs are connected together and that the circuit to be switched is connected to the drain of each MOSFET and a reverse protection diode is connected across the drain and source of each MOSFET. 
         [0011]    Preferably each MOSFET has an internal diode connected across the drain and source. 
         [0012]    Preferably the current path for the circuit to be switched on and off extends through a biased on MOSFET and the reverse protection diode across the drain and source of the other MOSFET. 
         [0013]    Preferably the input signal is either AC or DC. 
         [0014]    Preferably the current in the switched circuit is either AC or DC. 
         [0015]    In an alternative embodiment the invention consists in a method of providing a bilateral switch by deriving an electrically isolated power supply from a switch input signal, operating an electrically isolated low power switch powered from the power supply, operating from the electrically isolated low power switch at least a pair of totem pole semiconductor devices powered from the electrically isolated power supply, operating from the at least one pair of totem pole semiconductor devices at least one pair of FET devices, the FET devices switching current through a connected circuit on or off in response to the input signal. 
         [0016]    Preferably the totem pole pair of semiconductor devices is connected to the conjoined gates of two similar MOSFET transistors forming the pair of FET transistor devices, that the sources of the MOSFETs are connected together and that the circuit to be switched is connected to the drain of each MOSFET and a reverse protection diode is connected across the drain and source of each MOSFET. 
         [0017]    These and other features of as well as advantages which characterise the present invention will be apparent upon reading of the following detailed description and review of the associated drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a circuit diagram of the bilateral switch of the invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0019]    Referring now to  FIG. 1  a bilateral switch circuit  100  is shown. The circuit includes input terminals  101 ,  102  with a noise frequency filter capacitor  118  across them and connecting to a converter made up of transformer  104 , transistor  103  and a tuned feedback loop of resistor  106  and capacitor  105  and the inductance of transformer  104 . The voltage on the output side of the transformer is rectified in voltage doubling diodes  107 ,  108 . The converter operates at between 100 KHz and 300 KHz with the output of from 20 to 40 volts being smoothed by capacitors  109 ,  110  before being limited to 9 volts after passing through resistor  111  by zener diode  112  and held in storage capacitor  114 . 
         [0020]    The input terminals  101 ,  102  also supply an IRLED  122  forming part of a optoelectronic coupling pair of IRLED diode  122  and phototransistor  113 , which may be implemented as an integrated optocoupler. An LED  120 , with current limited by resistor  119 , provides an indication when an input signal is present. A preset resistor  121  is provided to set the standing current in the optocoupler IRLED for best frequency response. In the other polarity of input diode  123  limits the voltage across the optocoupler IRLED. 
         [0021]    When optocoupler IRLED  122  is off transistor  113  is non-conductive, allowing resistor  115  to pull down the bases of totem pole connected transistors  116 ,  117 . This in turn pulls the emitters of transistors  116 ,  117  low and pulls the gates of N-channel MOSFET transistors  125  and  126  low, rendering them non-conductive. As a result no current flows between output terminals  129 ,  130 . 
         [0022]    When optocoupler IRLED  122  is provided with voltage from the input terminals  101 ,  102  it illuminates transistor  113  and brings it into conduction. This raises the voltage at the bases of totem pole transistors  116 ,  117 , in turn raising the emitter voltages. This in turn biases on the gates of similar MOSFETs  125 ,  126 , bringing them into conduction and providing a path between output terminals  129 ,  130 . 
         [0023]    Because transistors  125  and  126  are connected to the output terminals in opposition one of the MOSFETs will always have a drain negative with respect to the other and to its own source terminal, and thus although it may be biased on it cannot conduct in the normal manner. Instead the protective diode  127  or  128  of the respective transistor will conduct. This allows a circuit to be established between the output terminals regardless of the polarity at the output terminals. The conduction voltage across the protective diode is greater than the forward voltage across a fully conductive MOSFET because it is normally a parasitic transistor in the MOSFET which will give a higher power dissipation in the reverse polarity MOSFET than in the forward polarity MOSFET. 
         [0024]    The input voltage signal may be AC or DC but must be sufficient to both power the optocoupler and the totem pole transistor pair as well as driving the gate capacitance of the MOSFET transistors. The current through the optocoupler IRLED is adjusted with preset resistor  121  for best frequency response at the MOSFET, that is, for the highest frequency at which the MOSFETs will still switch on and off without exceeding the power dissipation rating of any heatsink. 
         [0025]    It should be noted that with an AC input at terminals  101 ,  102  the converter will be running only for the positive half cycles, however this should still provide sufficient power to operate the MOSFET switching circuitry which will have a reasonably constant rectified supply and will be switched by the optocoupler. 
         [0026]    Totem pole connected transistors  116 ,  117  provide a sufficiently low resistance current path when on to quickly discharge the gate capacitance, thereby providing a quick turnoff of the active MOSFET. 
         [0027]    The bilateral switch shown will operate with either pulsed AC or DC at the input, and with either AC or DC at the output. It provides a maximum switching frequency which is normally between 10 KHz and 20 KHZ with commercially available components and can provide a controlled current of up to 80 amps with such components. 
         [0028]    The components used in the embodiment are described below in Table 1: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Reference 
                 Component 
                 Value or type 
               
               
                   
                   
               
             
             
               
                   
                 103 
                 Transistor 
                 MJE13003 
               
               
                   
                 105 
                 Capacitor 
                 100 pF 
               
               
                   
                 106 
                 Resistor 
                 22 KΩ 
               
               
                   
                 107, 108 
                 Diode 
                 1N4148 
               
               
                   
                 109, 110 
                 Capacitor 
                 .01 μF 
               
               
                   
                 111 
                 Resistor 
                 1 KΩ 
               
               
                   
                 112 
                 Zener diode 
                 9 V 
               
               
                   
                 113, 122 
                 Optocoupler 
                 4N25 
               
               
                   
                 114 
                 Capacitor 
                 1 μF 
               
               
                   
                 115 
                 Resistor 
                 10 KΩ 
               
               
                   
                 116 
                 Transistor 
                 BC548 
               
               
                   
                 117 
                 Transistor 
                 BC558 
               
               
                   
                 118 
                 Capacitor 
                 .01 μF 
               
               
                   
                 119 
                 Resistor 
                 4 KΩ 
               
               
                   
                 120 
                 LED 
               
               
                   
                 121 
                 Variable resistor 
                 10 KΩ 
               
               
                   
                 123 
                 Diode 
                 1N4148 
               
               
                   
                 124 
                 Resistor 
                 10 KΩ 
               
               
                   
                 125, 126 
                 MOSFET 
                 1XFX80N50P 
               
               
                   
                 127, 128 
                 Protective diode 
                 Integral with MOSFET 
               
               
                   
                   
               
             
          
         
       
     
         [0029]    The components used are all easily available and cheap and the overall current rating is a property of the MOSFET type used and easily variable. 
         [0030]    While the implementation described uses an optocoupler to drive the low power switches implemented as a totem pole pair other methods of deriving the same drive signals are optional, for instance a suitably programmed microprocessor may be used. Similarly the derivation of the isolated power supply from the input signal may be differently provided in order to be totally polarity independent. 
         [0031]    Similarly while the circuit describes the use of two similar MOSFET transistors the switch may equally use any even number of such transistors with half having the drain connected to each of the output terminals. The totem pole pair may require multiplication to provide adequate MOSFET drain capacitance drive. 
         [0032]    It is to be understood that even though numerous characteristics and advantages of the various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functioning of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail so long as the functioning of the invention is not adversely affected. For example the particular elements of the bilateral switch may vary dependent on the particular application for which it is used without variation in the spirit and scope of the present invention. 
         [0033]    In addition, although the preferred embodiments described herein are directed to a bilateral switch for use in controlling a current within the capabilities of the FET transistors of the system, it will be appreciated by those skilled in the art that variations and modifications are possible within the scope of the appended claims. 
       INDUSTRIAL APPLICABILITY 
       [0034]    The bilateral switch of the invention is used in the switching of AC or DC power in numerous industrial applications, for instance in phase correction circuitry and are employed in the electrical industry. The present invention is therefore industrially applicable.