Abstract:
The harvesting Resistor consists of single or dual supply DC to DC converter, which has a current sense resistor in series with its output port. The sensed current magnitude is coupled back to modulate the duty cycle in a way such that a voltage to current together with the power absorbing relationship of a resistor is appearing at the DC to DC converter&#39;s output port. Such an emulated resistor, when connected to an external power source, can efficiently transfer the absorbed energy from an external power source to the single or dual supplies of the DC to DC converter.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to using a DC to DC convertor architecture in a backward fashion with the further addition of providing the means to sense output current in order to adjust the duty cycle. The result is that what is normally used as a DC to DC convertor&#39;s output port, has the voltage to current and power absorbing characteristics of a simple resistor. When an external power source of unknown magnitude, polarity, or shape of waveform is applied to this emulated resistor, the energy that gets absorbed instead gets transferred with high efficiency to the DC to DC convertor&#39;s supplies. 
       BACKGROUND 
       [0002]    The historical way to capture DC power from an AC source is shown in  FIG. 10 . The normal residential alternating voltage AC_ 10  gets transformed down to a lower alternating voltage AC_ 10   b . Diode  10  will peak detect off of AC_ 10   b  to charge up capacitor Cfilter_ 10 . Resistor RLOAD_ 10  represents the circuit being powered up. RLOAD_ 10  causes the voltage VDC_ 10 , which is across Cfilter_ 10 , to drop slightly until the next time Diode_ 10  turns on. 
         [0003]    The key point to  FIG. 10  is that the harvesting of power off of AC_ 10  is both nonlinear and non consistent. But if the power source AC_ 10  is consistent in magnitude, frequency, polarity, and shape, then this method causes few problems. 
         [0004]    This invention harvests power in a much different way in that the input waveform&#39;s characteristics in terms of magnitude, polarity, shape, or up to some limitations frequency, are not critical. And since it works like a resistor, it can be used in dampening and characteristic impedance terminal matching applications. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    This invention provides a way to harvest energy with the voltage to current and power absorbing relationships of a simple resistor. The difference is that power that normally gets dissipated in a resistor will get harvested at high efficiency to the powers supplies instead. This resistance relationship has further applications in dampening mechanical systems, and also providing for good characteristic impedance matching to natural energy waveform sources. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Non-limiting and non-exhaustive embodiments are described with reference to the following drawings: 
           [0007]      FIG. 1  illustrates a prior art DC to DC convertor running off of the single supply VCC 1 . 
           [0008]      FIG. 2  illustrates the circuit of  FIG. 1  having the dual supplies of VCC 2  and VEE 2 . 
           [0009]      FIG. 3  illustrates how the circuit of  FIG. 2  can transfer energy from VCC 3  to VEE 3 . 
           [0010]      FIG. 4  illustrates how the circuit of  FIG. 2  can transfer energy from VEE 4  to VEE 4 . 
           [0011]      FIG. 5  illustrates how the circuit of  FIG. 3  can operating like a 5 Ohm resistor. 
           [0012]      FIG. 6  illustrates how the circuit of  FIG. 4  can operating like a 5 Ohm resistor. 
           [0013]      FIG. 7  illustrates the invention in which the sensed output current is coupled back to adjust the duty cycle to generate a 5 Ohm voltage to current relationship at the output. 
           [0014]      FIG. 8  illustrates the voltage signal waveform of external source VCM 7 , the power loss waveform of VCM 7 , and the power harvested waveform at the supply voltages of VCC 7  and VEE 7 . 
           [0015]      FIG. 9  illustrates the circuit of  FIG. 2  being used to transferring energy from VCC 9  to VEE 9 , by using a low value for RL 9 , and by increasing the duty cycle to 51%. 
           [0016]      FIG. 10  illustrates a standard AC to DC conversion circuit. 
           [0017]      FIG. 11  illustrates how a mechanical system consisting of the mass object MASS_ 11 , the spring object Spring_ 11 , and the shock absorber object Shock_Absorber_ 11  correspond to a critically damped LRC tuned circuit. 
           [0018]      FIG. 12  illustrates how the magnet object Magnet_ 12 , the coil object Coil_ 12 , and the resistor object RL_ 12  can perform the same mechanical function as the shock absorber object Shock_Absorber_ 11  shown in  FIG. 11 . 
           [0019]      FIG. 13  illustrates how the maximum power from a power source object V_equivalent, can be extracted by making the output load resistor RLOAD_ 13  match the equivalent impedance R_equivalent of the power source. 
           [0020]      FIG. 14  illustrates how the use of a harvesting resistor for Rmatch_ 14  can match the characteristic impedances of Natural Power Waveforms to fully capture waveform energy. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]      FIG. 1  shows a common DC to DC convertor circuit which consists of a CMOS inverter Invert_ 1 , driving an inductor L 1 , into a capacitor C 1 , to deliver a voltage Out 1 , across a load resistor RL 1 . The inverter needs to have both its NMOS transistor MN 1  and its PMOS transistor PN 1  to be very large such that the CMOS “ON” resistance is very low. The power efficiency of a DC to DC converter approaches 100% when the effective channel resistances of Rp 1  and Rn 1  are small compared to the output load resistor. The supply Vcc 1  is set to 5 Volts. For the sake of simplifying calculations, the resistances will all be modeled at zero. 
         [0022]    When a square wave is applied to the input of the invert_ 1 , the output voltage Out 1  will approach 2.5 Volts. The LRC network at the output of invert_ 1  is just a low-pass filter. Ideally, power is only being dissipated by the load resistor RL 1 . Transistor MP 1  and MN 1  are acting just like switches. Without any “On” resistance, these transistor dissipate no power when either on or off. Their power depends on having both drain current flow at the same time that there is some source drain voltage. These transistors act more like small resisters that dissipate power based upon current and its resulting IR drop. 
         [0023]    The inductor L 1  is seeing an almost constant 2.5V dc value on one end, and a 0 to 5 volt square wave on the other end. The inductor will have an AC current at a +/−60 uA peak value, because any voltage across an inductor will ramp up or down its current. The inductor also has a 500 mA DC current, because it is supplying the current to load resistor RL 1 . The inverter is acting like a 50% multiplexer to the inductor&#39;s dc current. Fifty percent of the time, the inductor&#39;s dc current comes out of VCC. The AC current of the inductor averages out to zero over this time. So a net DC current of 250 mA gets pulled out of VCC. This means VCC is being discharged by 1.25 W. Load resistor RL 1  in the mean time is dissipating the same amount. While there will always be IR drop in real circuits, real world power efficiency for converting a 5 volt supply into a 2.5 volt supply can be in the high ninety percent range. 
         [0024]    The inductor&#39;s 500 mA DC current is also being 50% multiplexed to ground. This will dissipate very little energy. But the inductor is acting like a current pump. Inductors cannot change their current very fast. However the voltage across an inductor can change instantaneously to keep its current constant. With time, the inductor will change is current. But this AC current always averages out to zero over time. This is especially true for the current that gets multiplexed to either VCC 1  or ground. Only the DC current flowing in the inductor needs to be considered. 
         [0025]    So output voltage Out 1 , which is across RL 1  in  FIG. 1 , can be thought of as a DC voltage. Any DC current that flows through RL 1  also flows through L 1  as a DC current. And the inverter invert_ 1  multiplexes that DC current between VCC 1  and ground. 
         [0026]      FIG. 2  is showing a dual supply version of  FIG. 1 . The input to invert_ 2  now needs to swing between both the positive and negative supplies. The output load resistor RL 2  still goes to ground. The 50% duty cycle puts the output at zero volts. Now zero DC current flows through RL 2 . Zero DC current is flowing through L 2 . And neither VCC 2  nor VEE 2  are discharging any power. And the AC currents of L 2  are averaging out. 
         [0027]    A common mode voltage VCM 3  has been added to  FIG. 3 . Now a −2.5V across RL 3  can be applied to such that it draws 500 mA, same as it did in  FIG. 1 . But the other side of RL 3  is still at 0 volts. But now 500 mA of DC current is flowing through L 3 . The 50% duty cycle is multiplexing this current equally between VCC 3  and VEE 3 . For VCC 3 , pulling current out of a +2.5V battery is discharging it. So is it being discharge at 625 mW. But half of L 3 &#39;s 500 mA dc current is also being pull out of the negative end of VEE 3 . Pulling current out of the negative end of a battery is actually charging it. So VCC 3  is being discharge at a 625 mW rate and VEE 3  is being discharged at a −625 mW. So the net power loss for both VCC 3  and VEE 3  is zero. Power is simply being transferred from VCC 3  to VEE 3 . 
         [0028]    In  FIG. 3 , only resistor RL 3  and voltage VCM 3  are drawing any power. One end of RL 3  looks like it is going to ground and the other VCM 3 . So the 1.25 W that is being dissipated by RL 3  is all coming from VCM 3 . The rest of the circuit is not really dissipating any power. It is just rearranging power between VCC 3  and VEE 3 . 
         [0029]    Now if one end of RL 3  goes to VEE 3  instead of VCM 3 , then resistor RL 3  would add a +1.25 W of dissipation to VEE 3 &#39;s already −625 mW being dissipated, to yield a total dissipation for VEE 3  of +625 mW, same as for VCC 3 . Then the total power of VCC 3  in series with VEE 3  together would be 1.25 W, just like in VCC 1  in  FIG. 1 . 
         [0030]      FIG. 4  shows the same hold true for current in the opposite direction. In this case power is being transferred from VEE 4  to VCC 4 . 
         [0031]      FIG. 5  is like  FIG. 4  with the duty cycle changed from 50% to 75%. Now one end of RL 5  sees +2.5V, and the other end see 1.25V. So now L 5  sees a dc current of 250 mA. But the duty cycle is now 75%. So most of that DC current is going in to charge up VCC 5  by 469 mW, and VEE 5  is being discharged by 156 mW. 
         [0032]    Resistor RL 5  sees 1.25V across it, and is dissipating 312 mW. VCM 5  is drawing 250 mA and being drained by 625 mW. The inductor, inverter and two supplies are receiving 250 mA of current from RL 5 . They are producing +1.25V in return. The V to I relationship is that of a 5 Ohm resistor. And the net discharge rate of both VCC 5  and VEE 5  together is −312 mW. Of the 625 mW of power coming out of VCM 5 , half of it is being dissipated as heat by RL 5 , and the other half is being supplied to the VCC 5  and VEE 5 . The DC to DC convertor&#39;s output port is acting like a 5 Ohm resistor. Except that it is harvesting to its supplies the power that a normal 5 Ohm resistor would otherwise dissipate as heat. 
         [0033]      FIG. 6  show that changing the direction of the L 6 &#39;s DC current and that changing the duly cycle to 25% still produces a V to I relationship of 5 Ohms, with the same harvesting of power. Now VEE 6  is harvesting most of the power. 
         [0034]    The invention simply consists of monitoring output current, and adjusting the duty cycle of a DC to DC convertor to product an output voltage to current relationship of a simple resistor. What is normally used as a power output port, is in this case being used as a power input port instead. The power flow is apparently efficient in both directions. The PulseWidthModulator_ 7  circuit of  FIG. 7  is designed to start off at 50% duty cycle, and then read the voltage across current sense resistor RS 7  to detect incoming current. 
         [0035]    The schematic of PulseWidthModulator_ 7  is a simple behavioral model. A zero to one volt 100 KHz triangle wave is fed to one input of comparator_ 7 . The other input sees the voltage Vduty 7 , which can be raised or lowered. Gain of OTA 7 , together with the value of Rb 7 , and the IR drop across RS 7 , are adjusted to produce an offset of 500 mV when 500 mA flow thru L 7 . So when 500 mA flows into L 7 , the duty cycle will be 100%. The output voltage is then 2.5, and the V to I relationship is that of 5 Ohms. 
         [0036]    It is possible to filter the currents across Rp 7  and Rn 7  and RL 7  to monitor the power waveforms of VCC 7 , VEE 7 , and VCM 7 .  FIG. 8  shows a 50 Hz waveform for VCM 7 , and the power waveforms of VCM 7  and VCC 7 +VEE 7  along side. Using VCM 7  as a power source, a +/−2.5 volt swing across VCM 7  draws from its swing the power from 0 to 1.25 W peak. The power being discharged from VCM 7  is that of a 5 Ohms resistor. This same power waveform appears in the opposite polarity by the discharge powers of VCC 7  and VEE 7 . So while it looks like VCM 7  is seeing a 5 Ohm resistor, and is dissipating the expected amount of power, that power is really being ending up in both the VCC 7  and VEE 7  supplies. 
         [0037]      FIG. 8  shows that power is being transferred on a continuous basis. It does not depend on magnitude or polarity. But there is one potential problem.  FIG. 5  shows that if the VCM 5  power source is a DC 5 volts, then while VCC is getting charged, VEE 5  is getting discharged. That can&#39;t go on forever.  FIG. 9  shows how the principle shown in  FIGS. 3 and 4  can be recruited to do some high efficiency power rearrangement. The RL 9  resistor in  FIG. 9  has been made very small at 0.1 Ohms. Now increasing the duty cycle by 1% will transfer about one watt from VCC 9  to VEE 9 . This technique might also find some use in battery applications where power needs to be moved efficiently between several cells in series. 
         [0038]    The circuit of  FIG. 9  is not required if the external power source is of a single polarity and the DC to DC converter is running off a single supply as in  FIG. 11   f  the polarity of the current in  FIG. 1  were to be reverse by taking the end of RL 1  to an external 5V rather than to ground. Then VCC 1  would be harvesting 1.25 Watts. A Single Supply Harvesting Resistor can be made using the same feedback of output current to duty cycle. But a need for the AC version of an Energy Harvesting Resistor may be more common. 
         [0039]      FIG. 10  shows the prior art as using Diode_ 10 , which tend to only draw power from source AC_ 10 B at a very small duty cycle. Diode_ 1  only turns on when its anode is 0.6V above the voltage across Cfilter_ 10 . It is certainly not harvesting the power of AC_ 10   b  at all times. This invention can harvest power in a much different fashion. It doesn&#39;t depend on the waveform&#39;s shape, magnitude, or polarity. But most important, it is operating linearly. The ability to do linear energy harvesting has some important implications when applied to the mechanical world. 
         [0040]    The mapping between the mechanical world and the electronic world is shown in  FIG. 11 . Inductors are like Mass in that they have a momentum to keep them from changing their current instantaneously. Capacitors are like springs which resist charge by developing voltage. Resistors are like shock absorbers in that they dissipate energy proportional to velocity of charge. In  FIG. 11 , Shock_Absorber_ 11  is shown as a metal disk suspended in a viscous fluid. The resistance will be proportional to the metal disk velocity. 
         [0041]    The shock absorbers used on an automobile are often chosen to give a critical damping response when combined with the mass of the automobile and the strength of the springs. It is possible to use a magnet, a coil, and a resistor to perform the same function as a shock absorber. If Magnet_ 12  in  FIG. 12  were to move in and out of Coil_ 12 , then the change in magnetic field will produce a voltage across Coil_ 12 . If resistor RL 12  across Coil_ 12  and is small enough in resistance, this will produce a lot of current in Coil_ 12 , which will produce its own opposing magnetic field to resist the movement of Magnet_ 12 . 
         [0042]    The higher the speed of movement for Magnet_ 12 , the higher is the resistance to movement. The energy that is put into pushing and pulling Magnet_ 12  into Coil_ 12  all gets dissipated as heat in RL_ 12 . In  FIG. 11 , Shock_Absorber_ 11  is dissipating the energy of movement in a viscous liquid. In  FIG. 12 , energy of movement of Magnet_ 12  is instead being dissipated in resistor RL_ 12 . 
         [0043]    This invention efficiently harvests energy as a linear resistor. Shock_Absorber_ 11  of  FIG. 11  could be replaced to the electrical version of  FIG. 12 , and then resister RL_ 12  could be replaced by an Energy Harvesting Resistor. The equivalent resistance would need to be adjusted to critically dampen Mass_ 11  and Spring_ 11 . But the result would be that any energy that gets dissipated as heat in Shock_Absorber_ 11  could instead be harvested. So shock absorbs could be converted into generators. 
         [0044]    It is common practice to model power sources in electronics as equivalent voltage sources having an equivalent output impedance. The maximum energy that can be derived from a power source is when the load impedances matches the output impedance. When a harvesting resistor such as RLOAD_ 13  as shown in  FIG. 13  is applied to power source V_equivalent_ 13 , it is very easy to adjust the impedance of RLOAD_ 13  to match R_equivalent_ 13 . A 1% change in duty cycle changes the output voltage by 1% of the supply voltage. Setting that 1% change to corresponds to a change in current at the output give the desired impedance. But there are more important applications to matching resistance. 
         [0045]    Every media that can carry a wave does so with a characteristic impedance. For sound, the mass of air plays the roll of the inductor and compression of the air the capacitor. For a ribbon of steel, there is mass and springiness. For ocean waves, there is mass and gravity, etc. When a wave in a swimming pool hits a wall, it gets reflected. If ocean waves could be terminated with something that matches their characteristic impedance, the termination can absorbs all the energy of the waves without reflection. 
         [0046]    Consider the pipe carrying the exhaust of an automobile engine. The mass and springiness of the pipe can be modeled as the inductors and capacitors forming a transmission line in  FIG. 14 . The sound wave traveling within the metal of the pipe is seeing a mechanical transmission line. A proper impedance size of Rmatch_ 14  can capture all Waveform_ 14 &#39;s energy without a reflection. Now if Rmatch_ 14  happens be a the circuit of  FIG. 12  which is using an Energy Harvesting Resistor for RL_ 12 , then perhaps a muffler could be turned into an electrical power generator. 
         [0047]    While the invention has been shown in this particular embodiment, it will be understood by those skilled in the art, that different methods for DC to DC conversion, and different methods of sensing current, and different methods to adjust duty cycle, can be substituted, as long as the end result generates a voltage to current relationship of a simple resistor, and as long as the intention of is to harvest energy from an external power source as an equivalent resistor. All of these substitutions can all be made with out departing from the spirit and scope of the invention.