Backup power switch

A control switch arrangement for supplying power to a load from primary and backup voltage sources of substantially equal potential, each of said voltage sources including a positive terminal and a negative terminal, said control switch arrangement comprising:

• first and second P-channel field-effect transistor means, each including at least a gate electrode, a source electrode and a drain electrode, each of said first and second field-effect transistor means including at least one controllable path for the flow of current between said source and drain electrodes, said controllable path being enabled for the flow of current when the corresponding gate electrode is at a potential which is negative with respect to the potential of said source electrode, each of said first and second field-effect transistor means also including an inherent semiconductor diode, the anode of which is connected to its associated drain electrode, and the cathode of which is connected to its associated source electrode, said drain electrode of said first field-effect transistor means being adapted to be connected to the positive terminal of said primary voltage source, and said drain electrode of said second field-effect transistor means being adapted to be connected to the positive terminal of said backup voltage source;
• interconnection means coupled to said source electrodes of said first and second field-effect transistor means, and to said load, and adapted to be coupled to said negative terminals of said primary and backup voltage sources, for connecting said source electrodes in common to a first terminal of said load, and for connecting a second terminal of said load in common to said negative terminals of said primary and backup voltage sources, whereby said inherent diode of one of said first and second field-effect transistor means is rendered conductive when that one of said primary and backup voltage sources, respectively, to which its anode electrode is connected is energized, to thereby render the potential at said first terminal of said load positive relative to said second terminal, and said load is thereby energized independent of field-effect operation of either said first or second field-effect transistor means, but if both said primary and backup voltage sources is available, power may undesirably be drawn from said backup voltage source, and power is undesirably dissipated due to the flow of load current through said inherent diode of said second field-effect transistor, and the load voltage is also undesirably reduced by the forward junction potential of said inherent diode of said first field-effect transistor;
• control means coupled to said gate electrodes of said first and second field-effect transistor means, and to said interconnection means, for rendering the potential of said gate electrode of said first field-effect transistor means negative relative to source electrode of said first field-effect transistor means, and for rendering the potential of said gate electrode of said second field-effect transistor means positive relative to said source electrode of said second field-effect transistor means for selecting operation from said primary voltage source, whereby said first field-effect transistor means in which said controlled current path is rendered conductive results in a short-circuit of its associated inherent diode to thereby raise said load voltage to substantially equal the potential of said primary voltage source, and to eliminate said undesirable power dissipation in said inherent diode of said first field-effect transistor means, and to reverse-bias said inherent diode of said second field-effect transistor means, and thereby eliminate said undesirable draw of power from said backup voltage source during operation from said primary voltage sources, and for rendering the potential of said gate electrode of said second field-effect transistor means negative relative to said source electrode of said second field-effect transistor means, and for rendering the potential of said gate electrode of said first field-effect transistor means positive relative to said source electrode of said first field-effect transistor means for selecting operation from said backup voltage source;
• wherein each of said first and second field-effect transistor means comprises four paralleled field-effect transistors.