Ideal diode

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.

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. 1illustrates 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 toFIG. 1, an active backflow switch Q1which serves to prevent current from the charged battery from flowing back into the solar array S134when no solar illumination is present thus protecting the battery from discharge. The circuit is composed of a differential amplifier IC144BB OPA349 and a N-channel enhancement mode MOSFET switch Q1Siliconix Si2302DS for example. The battery B138in this example is a 4.2 volt, 2500 mah lithium polymer and the solar array S134is 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 IC1is supplied operational current from the battery B138, positive and negative terminals, and is continually operating even when there is no solar luminance, differential amplifier IC1is selected to have ultralow quiescent current ≈10 microamp such that there would be insignificant current draw from the battery B138. 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 IC1with 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.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 1there is shown a prior art circuit diagram36of an intermittent current limited power source such as a solar array S134connected to a device for storing charge such as a battery B138through an active switch circuit40. The active switch circuit40substantially reduces the power that is lost by conventional reverse current diodes normally associated with solar charging devices. Referring once again toFIG. 1, the active switch circuit40consists of a reverse current detector IC144and a low loss N-channel enhancement mode MOSFET switch Q146having an internal diode D150. In operation, incident solar energy generated by solar array S134causes a counter clockwise current flow to occur as shown by “I-on”48inFIG. 1. The solar array34electromotive force (EMF) forward biases D150and current begins to flow. The reverse current detector IC144, which in this embodiment is shown as a differential amplifier detects the positive difference voltage across D150at the non inverting input which causes Q146to turn on. Q146acts 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 array34causes the current flow described above to stop and current from the battery attempts to flow in the reverse direction as indicated by “I-OFF”52inFIG. 1. The reverse current detector IC144detects a negative differential voltage at the non inverting input and causes Q146to turn off.

More specifically, D150and Q146provide a voltage drop such that IC144is 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 D150additionally would not conduct current in the I-Off direction52.

Referring toFIG. 2there is shown a preferred embodiment whereby differential amplifier IC144now derives operational current from the solar array S134by connecting differential amplifier IC144negative return power input terminal directly to the negative terminal of the solar array S134. In this manner, differential amplifier IC144is only operational when solar array S134is illuminated to the degree that properly biases differential amplifier IC144to turn on N-channel enhancement mode MOSFET switch Q1Siliconix Si2302DS46which then delivers solar array S134current to effectively charge battery B138. When there is no solar illumination of solar array S134, there is no current flow in differential amplifier IC144neither from the solar array S134nor battery B138. The addition of pull down resistor R154ensures that MOSFET switch Q146will be fully turned off when no solar illumination is present.

The circuit and operation ofFIG. 2demonstrates 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. 3conceptually represents such a generic application.

Turning now toFIG. 4conceptually represents the addition of a conventional Current/Voltage Charge Regulator60connected between the current/charge source, the backflow preventer, and the Charge/Energy Storage Device. In the conventional configuration, the Current/Voltage Charge Regulator60may 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 Regulator60. 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 ofFIG. 5gives another preferred embodiment in accordance with the present invention utilizing an active Current/Voltage Charge Regulator62now 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 Regulator62when 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 inFIG. 5would be limited to the leakage current back through the MOSFET switch Q146of 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.