Abstract:
An active switch for electrically connecting and disconnecting a power source such as an energy harvester to a charge storage device is disclosed. The active switch allows a minimal amount of reverse back current flow from the charge storage device to the power source having a high off-resistance.

Description:
BACKGROUND 
     Conventional solar battery charging systems employ a “back flow” or reverse current diode to prevent battery current from flowing back through a solar array in the absence of solar energy. Typically, a Schottky diode is used for this reverse current protection due to a low forward voltage drop inherent for Schottky diode operation. The forward voltage drop has a direct impact on charge efficiency such that the less power that is dissipated across the diode, the more charge power is delivered to the battery. The charge efficiency has been acceptable when dealing with macro solar charging systems since the battery charge voltages have been typically “high” in ratio to the Schottky diode forward voltage drop. However, this is not desirable in micro solar charging systems where the charge voltages are not high in ratio to the Schottky diode forward voltage drop. Therefore it would be desirable to provide a circuit that prevents battery back flow current having less forward voltage drop than a Schottky diode. 
       FIG. 1  illustrates a prior art circuit diagram disclosed in U.S. Pat. No. 6,713,989, entitled “Solarswitch” issued to the inventor of this application and incorporated herein in its entirety. Referring now to  FIG. 1 , an active backflow switch Q 1  which serves to prevent current from the charged battery from flowing back into the solar array S 1    34  when no solar illumination is present thus protecting the battery from discharge. The circuit is composed of a differential amplifier IC 1    44  BB OPA349 and a N-channel enhancement mode MOSFET switch Q 1  Siliconix Si2302DS for example. The battery B 1    38  in this example is a 4.2 volt, 2500 mah lithium polymer and the solar array S 1    34  is composed of two triple junction GaAs solar cells providing 250 ma of charge current at approximately 4.5 volts. Also a battery charge voltage regulator is normally present but is not shown for simplicity. 
     Since the differential amplifier IC 1  is supplied operational current from the battery B 1    38 , positive and negative terminals, and is continually operating even when there is no solar luminance, differential amplifier IC 1  is selected to have ultralow quiescent current ≈10 microamp such that there would be insignificant current draw from the battery B 1    38 . This has served to work appropriately for large capacity batteries 500 mah and above. However, battery types commonly used in various ultralow power energy harvesting applications may have capacity ratings of 1 mah or lower and will rapidly discharge while continuously operating a device such as IC 1  with a quiescent current draw near 10 micro amps. Therefore, a backflow switch with zero quiescent current in the non-operational state (off state when no energy is being harvested) is required to prevent discharge of low capacity batteries of 500 mah or less in energy harvesting applications and the like. 
     SUMMARY OF THE INVENTION 
     An ideal diode for use in energy harvesting systems combines a field effect transistor and an operational amplifier together wherein the field effect transistor is operated by the operational amplifier having a pull down resistor for electrically connecting and disconnecting a power source to a device whereby when disconnecting the power source from the device a minimal amount of reverse back current flow from the device to the power source is allowed. The operational amplifier is powered by the power source wherein the pull down resistor ensures the ideal diode shuts off when no power source is available. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art circuit diagram incorporating an active switch circuit in a micro solar charging system; 
         FIG. 2  shows a circuit diagram of one embodiment of the present invention incorporating an active switch circuit in an energy harvesting system; 
         FIG. 3  shows a simplified generic circuit diagram implementing the active switch circuit of  FIG. 2 ; 
         FIG. 4  shows a simplified generic circuit diagram incorporating a conventional current/voltage regulator to the energy harvesting system powered by the battery B 1    38  shown in  FIG. 2 ; 
         FIG. 5  shows a simplified generic circuit diagram incorporating an active current/voltage regulator to the energy harvesting system powered by the solar array S 1    34  shown in  FIG. 2 ; and 
         FIG. 6  shows another simplified circuit diagram incorporating an active current/voltage regulator powered by the solar array S 1    34  to the energy harvesting system shown in  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1  there is shown a prior art circuit diagram  36  of an intermittent current limited power source such as a solar array S 1    34  connected to a device for storing charge such as a battery B 1    38  through an active switch circuit  40 . The active switch circuit  40  substantially reduces the power that is lost by conventional reverse current diodes normally associated with solar charging devices. Referring once again to  FIG. 1 , the active switch circuit  40  consists of a reverse current detector IC 1    44  and a low loss N-channel enhancement mode MOSFET switch Q 1    46  having an internal diode D 1   50 . In operation, incident solar energy generated by solar array S 1    34  causes a counter clockwise current flow to occur as shown by “I-on”  48  in  FIG. 1 . The solar array  34  electromotive force (EMF) forward biases D 1   50  and current begins to flow. The reverse current detector IC 1    44 , which in this embodiment is shown as a differential amplifier detects the positive difference voltage across D 1   50  at the non inverting input which causes Q 1    46  to turn on. Q 1    46  acts a low loss switch with a very low source to drain resistance overcoming the power loss associated with conventional diodes. The absence of solar energy incident upon the solar array  34  causes the current flow described above to stop and current from the battery attempts to flow in the reverse direction as indicated by “I-OFF”  52  in  FIG. 1 . The reverse current detector IC 1    44  detects a negative differential voltage at the non inverting input and causes Q 1    46  to turn off. 
     More specifically, D 1   50  and Q 1   46  provide a voltage drop such that IC 1    44  is able to detect current flow without the addition of a series resistor that is normally present in current sensing applications. Thus the directional current sense detector is able to determine current flow without the usual power losses associated with series resistors. In this manner, it is a virtually a “loss less” current detection device since it adds no significant power losses to the system. It should be understood that D 1   50  additionally would not conduct current in the I-Off direction  52 . 
     Referring to  FIG. 2  there is shown a preferred embodiment whereby differential amplifier IC 1    44  now derives operational current from the solar array S 1    34  by connecting differential amplifier IC 1    44  negative return power input terminal directly to the negative terminal of the solar array S 1    34 . In this manner, differential amplifier IC 1    44  is only operational when solar array S 1    34  is illuminated to the degree that properly biases differential amplifier IC 1    44  to turn on N-channel enhancement mode MOSFET switch Q 1  Siliconix Si2302DS  46  which then delivers solar array S 1    34  current to effectively charge battery B 1    38 . When there is no solar illumination of solar array S 1    34 , there is no current flow in differential amplifier IC 1    44  neither from the solar array S 1    34  nor battery B 1    38 . The addition of pull down resistor R 1    54  ensures that MOSFET switch Q 1    46  will be fully turned off when no solar illumination is present. 
     The circuit and operation of  FIG. 2  demonstrates a solar charging battery as an example use of the preferred embodiment. However, those skilled in the art may apply the preferred embodiment to any number of like energy harvesting applications where ever a low loss, zero quiescent current backflow switch is required.  FIG. 3  conceptually represents such a generic application. 
     Turning now to  FIG. 4  conceptually represents the addition of a conventional Current/Voltage Charge Regulator  60  connected between the current/charge source, the backflow preventer, and the Charge/Energy Storage Device. In the conventional configuration, the Current/Voltage Charge Regulator  60  may consist of components that require a small amount of current drain such as a voltage divider used to compare or reference the output voltage to a known voltage reference for proper voltage regulation of the Current/Voltage Charge Regulator  60 . While this current draw may be acceptable for charge storage devices of 500 mah or more, there may be excessive current draw for energy harvesting charge storage devices less than 500 mah capacity. 
     The circuit configuration of  FIG. 5  gives another preferred embodiment in accordance with the present invention utilizing an active Current/Voltage Charge Regulator  62  now being connected between the Charge Current Source Device and the Idea Diode Zero Quiescent Back Flow Switch. In this configuration there will be no current drain through the active Current/Voltage Charge Regulator  62  when there is no solar illumination present since the Idea Diode Zero Quiescent Back Flow Switch now effectively provides an open circuit to the Charge/Energy Storage Device. The voltage drop across a conventional backflow diode would prohibit the use of a voltage regulator in this configuration due to imprecise voltage feedback to Current/Voltage Charge Regulator. 
     However, since the Diode Zero Quiescent Back Flow Switch exhibits insignificant voltage drop in typical energy harvesting low current applications, the Current/Voltage Charge Regulator can be placed in this configuration with insignificant regulation error. 
     Therefore, the only current drain of Charge/Energy Storage Device in  FIG. 5  would be limited to the leakage current back through the MOSFET switch Q 1    46  of the Diode Zero Quiescent Back Flow Switch which is compatible with low power energy harvesting applications. 
     It should further be noted that numerous changes in details of construction, combination, and arrangement of elements may be resorted to without departing from the true spirit and scope of the invention as hereinafter claimed.