Abstract:
The active rectifier circuit and related method of operation disclosed herein is self-powered and improves the efficiency and reliability of photovoltaic solar power systems by replacing the conventional bypass and blocking rectifiers used in such systems. The circuit includes a power MOSFET used as a switch between the anode and cathode terminals, and control circuitry that turns on the MOSFET when the anode voltage is greater than the cathode voltage. The method of operation utilizes resonance to produce a large periodic voltage waveform from the small anode-to-cathode dc voltage drop, and then converts the period voltage waveform to a dc voltage to drive the gate of the power MOSFET.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 61/476,391 filed on 18 Apr. 2011. 
    
    
     BACKGROUND 
     The invention relates generally to the field of active rectifier circuits, and more specifically to active rectifier circuits used in photovoltaic (PV) solar power systems as bypass rectifiers and blocking rectifiers. 
       FIG. 1  shows a high level diagram of an example PV solar array consisting of two strings  1  and  2  wired in parallel and feeding their combined currents into a converter  3 . Each string consists of a blocking rectifier  4  and a plurality of solar panels  5 , and each solar panel has an associated bypass rectifier  6 . The purpose of the bypass rectifiers  6  is to protect the solar panels  5  from damage when the PV solar array is partially shaded. For example, in  FIG. 1  one panel in the first string  1  is shaded by some obstruction  7  such as a cloud or tree branch, while the other panels all receive full sunlight. The unshaded panels try to force current to flow through the shaded panel, but the bypass rectifier  6  provides an alternate current path around the shaded panel, thus protecting it from damage. But since the first string  1  contains a shaded panel, its output voltage is lower than that of the second string  2  where all the panels are in full sunlight. The blocking rectifiers  4  prevent the string with the lower voltage  1  from draining current produced by the unshaded string  2 . 
     The bypass and blocking rectifiers degrade the efficiency of the PV solar array because they dissipate significant amounts of power. As current flows through a rectifier in the forward direction (from anode to cathode) a voltage drop develops across the rectifier, resulting in heat being produced inside the rectifier. For example, a typical Schottky used as a bypass rectifier would have a forward voltage drop of about 500 mV at 10 A; thus wasting up to 5 W of power. Additionally, Schottky rectifiers have high reverse leakage currents, typically in the range of 10 mA when the junction temperature is 75° C., so energy is wasted in every bypass rectifier, even when none of the solar panels are shaded. A typical blocking rectifier has an even larger forward voltage drop of about 1.2V at 10 A, thus wasting up to 12 W each. What is more, power is wasted continuously in the blocking rectifiers as long as the sun is out, as opposed to the losses in the bypass rectifiers, which can vary as transient shading conditions come and go throughout the day. 
     The bypass and blocking rectifiers also reduce the systems reliability because of their high junction temperatures. The rectifiers are typically mounted inside enclosures to protect them from moisture and contamination, but these enclosures also have the undesirable effect of acting as thermal insulators that trap the heat. For example, a typical PV solar panel includes three bypass rectifiers inside a plastic enclosure mounted on the back side of the panel. If the entire panel is shaded, then all three bypass rectifiers can dissipate 5 W each, for a total of 15 W dissipation inside the small plastic enclosure. This can easily result in the bypass rectifiers junction temperatures exceeding 200° C., which can severely shorten their lifespans. What is more, Schottky bypass rectifiers are also vulnerable to a destructive phenomenon called thermal runaway because of their high leakage currents, which double for every 10° C. increase in junction temperature. As the leakage current increases, it heats the bypass rectifier further, which produces even more leakage current; this positive feedback loop sometimes leads to a the bypass rectifier failing and becoming a short circuit. 
     When just one bypass or blocking rectifier fails, it prevents the system from operating all the PV cells at their maximum power point. This can decrease the system efficiency by up to 50%. The large number of bypass and blocking rectifiers in a typical PV solar array makes their reliability all the more critical. 
     What is needed to increase the efficiency and reliability of PV solar arrays, are better rectifiers with lower forward voltage drop, lower reverse leakage current, and higher reliability. One solution well known to the art is an active rectifier, which consists of a transistor used as a switch, and circuitry for controlling the transistor such that the switch closes to allow current flow in the forward direction; and the switch opens to prevent current flow in the reverse direction. The forward voltage drop in an active rectifier can easily be ten times lower than in a Schottky rectifier at the same current, and twenty times lower than a silicon rectifier. Additionally, the reverse leakage current in an active rectifier is typically thousands of times lower than with a Schottky, thus eliminating the threat of thermal runaway. This not only boosts system efficiency at the beginning of life, but also helps to maintain peak efficiency over time because their dramatically lower junction temperatures make active rectifiers more reliable. 
     The main challenge faced when designing active rectifier circuits for PV solar arrays, is providing power to run the control circuitry. Most active rectifier circuits are used in the field of power conversion (e.g. switching-mode power supplies) where there is usually a supply voltage available to power the active rectifier control circuitry. In contrast, for bypass and blocking rectifiers in PV solar arrays, there is no readily available power source for the control circuitry; therefore, the active rectifier circuit must power itself. 
     To illustrate the problem of self-powering  FIG. 2  shows an example of prior art disclosed in U.S. patent application Ser. No. 11/094,369 (the &#39;369 application). The &#39;369 application describes an active rectifier circuit that does not self-power, comprising: an anode  8 ; a cathode  9 ; a power MOSFET  10 ; an offset bias voltage source  11 ; and an operational amplifier (opamp)  12 . Before the power MOSFET  10  is turned on, current can flow from anode  8  to cathode  9  via the body diode  14  that forms an integral part of the power MOSFET  10 . The resulting voltage drop across the body diode  14  is larger than the bias voltage  11 , so the differential voltage across the opamp  12  inputs is positive, causing the opamps output to swing high, which turns on the power MOSFET  10 . Once the power MOSFET  10  turns on, the anode-to-cathode voltage drops drastically, but then negative feedback maintains the anode-to-cathode voltage at a constant level, equal to the offset bias voltage  11 . The negative feedback mechanism operates as follows: the differential input voltage to the opamp  12  is the anode-to-cathode voltage minus the bias voltage  11 ; if the anode-to-cathode voltage falls below the offset bias voltage, the opamp  12  decreases the voltage applied to the gate of the MOSFET  10 ; the decreased gate voltage results in increased channel resistance in the MOSFET  10 ; the increased channel resistance results in increased anode-to-cathode voltage, bringing it back up to the level of the bias voltage  11 , and thus closing the negative feedback loop. 
     While the circuit of  FIG. 2  performs the basic functions of an active rectifier, it is not practical for applications such as bypass or blocking rectifiers in PV solar arrays because it is not self-powering and requires an external supply voltage  13  to power the opamp  12 . For example, in  FIG. 1 , the potential between the bypass rectifier  6  at the top of the first string  1  and the bypass rectifier at the bottom of the same string can be hundreds of Volts. So, in order to utilize the circuit of  FIG. 2  to replace the bypass rectifiers in  FIG. 1 , a power supply with many isolated outputs (one output for each active rectifier circuit) would be needed. 
       FIG. 3  shows another example of prior art disclosed in U.S. patent application Ser. No. 12/815,496 (the &#39;496 application). The &#39;496 application describes a self-powered active rectifier circuit that is in many ways analogous to the circuit from the &#39;369 application: both circuits employ a power MOSFET  10  as the switch to conduct current from anode  8  to cathode  9 . A charge pump  19  in  FIG. 3  provides a large voltage gain similar to the opamp  12  in  FIG. 2 . The minimum voltage required to maintain operation of the charge pump  19  in  FIG. 3  is roughly analogous to the offset bias voltage  11  in  FIG. 2 . Thus, both circuits employ similar feedback mechanisms to regulate the voltage from anode to cathode when current is flowing through the circuit in the forward direction. The main difference between the two inventions is that the circuit in  FIG. 3  is self-powering because it is able to produce the power MOSFET gate drive voltage by multiplying the anode-to-cathode voltage with a charge pump  19 . Therefore, an active rectifier like the one in  FIG. 3  can replace the bypass rectifiers  6  in  FIG. 1  without the need for any external power supplies. 
     But there are at least two drawbacks to the circuit in  FIG. 3 . First, it&#39;s poorly suited for use as a blocking rectifier because it&#39;s constructed as an Integrated Circuit (IC). A blocking rectifier typically must withstand at least 300V reverse bias, which is extremely difficult and costly to achieve with an IC. Second, the circuit is very complex; in order to get enough voltage gain, the charge pump  19  must have a large number of stages, each with a capacitor  15 . What is more, the charge pump  19  requires a clock source  16 , and a subcircuit  17  that produces several non-overlapping clock phases. And the charge pump  19  cannot start up on its own; it requires a start-up unit  18  consisting of yet more oscillators and charge pumps. All of this adds significantly to the cost and complexity of the circuit. 
     Accordingly, there is a continuing need in the field of photovoltaic solar power for an active rectifier circuit that is self-powered, has very low forward voltage drop and reverse leakage current, and is extremely reliable. The present invention fulfills these needs and provides other related advantages. 
     SUMMARY 
     The self-powered active rectifier circuit includes an anode terminal; a cathode terminal; a power MOSFET arranged between the anode and cathode; an oscillator subcircuit; a rectifier subcircuit; a bleeder subcircuit; a diode arranged to allow current flow through the oscillator subcircuit when the anode voltage is greater than the cathode voltage; and in some embodiments, a switch arranged in parallel with the diode. The bleeder subcircuit may include a resistor disposed between the gate and source of the power MOSFET. The bleeder subcircuit may also include a bipolar transistor and a base resistor arranged to discharge the power MOSFET gate-to-source capacitance when the cathode voltage is greater than the anode voltage. In some embodiments the rectifier subcircuit simply rectifies the periodic voltage waveform produced by the oscillator subcircuit, while in other embodiments the rectifier subcircuit also multiplies the voltage. In some embodiments the rectifier subcircuit also includes a capacitor to block dc current from the oscillator subcircuit. And, in some embodiments, the oscillator subcircuit is comprised of a transformer with two windings, and a depletion-mode MOSFET. 
     The method of operation of the self-powered active rectifier circuit includes steps of: utilizing the anode-to-cathode voltage to power an oscillator subcircuit when the anode voltage is greater than the cathode voltage; generating a periodic voltage waveform with amplitude that is typically at least fifty times greater than the voltage that powers the oscillator; converting the periodic voltage waveform to a dc voltage; applying the dc voltage between the gate and source of the power MOSFET; and in some embodiments, increasing the dc voltage via a positive feedback mechanism that closes a switch when the dc voltage exceeds a predetermined threshold, where the switch is arranged to increase the supply voltage to the oscillator, which results in increased dc voltage. In some embodiments, the step of converting the periodic voltage waveform to the dc voltage uses rectification and filtering, while in other embodiments the conversion step also includes voltage multiplication. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate the invention. In such drawings: 
         FIG. 1  is an example of a typical photovoltaic solar array to illustrate the need for the self-powered active rectifier circuit disclosed herein; 
         FIG. 2  is a prior art active rectifier circuit that is not self-powering; 
         FIG. 3  is a prior art self-powered active rectifier circuit that includes a complex arrangement of several oscillators and charge pumps; 
         FIG. 4  illustrates a high level block diagram of a self-powered active rectifier circuit disclosed herein; 
         FIG. 5A  illustrates one embodiment of the rectifier subcircuit wherein the periodic input voltage is simply rectified and filtered; 
         FIG. 5B  illustrates a second embodiment of the rectifier subcircuit wherein voltage multiplication is utilized; 
         FIG. 6  illustrates a preferred embodiment of the oscillator subcircuit that is part of the self-powered active rectifier circuit; 
         FIG. 7  illustrates a preferred embodiment of the self-powered active rectifier circuit, including details of the preferred embodiments of the switch and bleeder subcircuit; and 
         FIG. 8  illustrates a high level flow chart diagram that describes a preferred embodiment of the method of operation of the self-powered active rectifier circuit. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in the drawings for purposes of illustration, the present invention for a self-powered active rectifier circuit is shown with respect to  FIGS. 4-7  and the related method of operation is shown generally with respect to  FIG. 8 . The active rectifier circuit is suitable to directly replace the bypass  6  and blocking  4  rectifiers in a PV solar array, such as the one in  FIG. 1  with the affect of improving the efficiency and reliability of the array. 
       FIG. 4  shows a high level block diagram of a self-powered active rectifier circuit having an anode terminal  27 ; a cathode terminal  28 ; a power MOSFET  29  with a source  30 , a gate  31 , a drain  32 , and an integral body diode  33  arranged to conduct current from the anode terminal  27  to the cathode terminal  28 ; an oscillator subcircuit  34  which is powered by a voltage potential between its positive power input  35  and its negative power input  36  and produces a large periodic voltage waveform at its output  37 ; a rectifier subcircuit  38  with an input  39  coupled to the output  37  of the oscillator, an output  40  connected to the gate  31  of the power MOSFET  29 , and a common terminal  41  connected to the source  30  of the power MOSFET  29 ; a bleeder subcircuit  42  disposed between the gate  31  and source  30  of the power MOSFET  29 ; a diode  43  arranged to allow current through the oscillator subcircuit power supply inputs  35  and  36  when the anode voltage is greater than the cathode voltage; and in some embodiments, a switch  44  arranged in parallel with the diode  43 . 
     The rectifier subcircuit  38  rectifies the periodic voltage waveform produced by the oscillator  34 , and in some embodiments also multiplies the resulting dc voltage.  FIG. 5A  shows a first exemplary embodiment of the rectifier subcircuit  38  comprised of a rectifier  45  and an output capacitor  46 . In this first example embodiment the dc voltage at the output  40  is simply equal to the amplitude of the ac voltage applied to the input  39 .  FIG. 5B  shows a second exemplary embodiment of the rectifier subcircuit  38  which is a classic voltage doubler comprised of two rectifiers  47  and  48 , an input capacitor  49 , and an output capacitor  46 . In this second example embodiment, the dc voltage at the output  40  is approximately equal to twice the amplitude of the ac voltage applied to the input  39 . Many other embodiments are possible that employ a plurality of capacitors and diodes arranged into well known voltage multiplier circuit topologies. 
     In some embodiments the rectifier subcircuit  38  output capacitor  46  can be omitted because the input capacitance of the power MOSFET  29 , to which the output  40  of rectifier subcircuit is connected, can serve the same function of smoothing the dc voltage output from the rectifier subcircuit. 
       FIG. 6  shows a preferred embodiment of the oscillator subcircuit  34 . In this example, the oscillator subcircuit  34  is comprised of: a depletion-mode MOSFET  51 ; and a transformer  52  with a primary winding  53 , and secondary winding  54 . The primary winding  53  is disposed between the positive power supply input  35  and the drain  55  of the depletion-mode MOSFET  51 . The secondary winding  54  is disposed between the gate  56  and the source  57  of the depletion-mode MOSFET  51  and the source  57  is also connected to the negative power supply input  36 . The two transformer windings  53 ,  54  are arranged to provide voltage feedback from the drain  55  to the gate  56  with phase angle and gain sufficient to assure that the subcircuit oscillates. In some embodiments a capacitor  58  is added in parallel with the secondary winding  54  of the transformer  52  to tune the oscillator subcircuit  34  to a particular frequency. 
     The oscillator subcircuit  34  of  FIG. 6  has two characteristics that are critical to the operation of the self-powered active rectifier circuit: the ability to operate at extremely low supply voltage; and the ability to produce an output waveform with amplitude many times greater than the supply voltage. The low voltage operation is achieved by the use of the depletion-mode MOSFET  51  which has low resistance between its drain  55  and source  57  while the voltage from its gate  56  to its source  57  is zero. Thus, current flows through the depletion-mode MOSFET  51  and the primary winding  53  of the transformer  52 , even at start-up when the gate-to-source voltage is initially zero. The high output amplitude is achieved because the insulated gate  56  of the depletion-mode MOSFET  51  does not clip or clamp the output voltage, and because the transformer  52  turns ratio (the number of turns in the secondary winding  54 , divided by the number of turns in the primary winding  53 ) is very large, for example 100:1 or greater. Thus, a small voltage swing on the drain  55  produces a large voltage swing at the gate  56 . 
       FIG. 7  discloses more details of a preferred embodiment of the self-powered active rectifier circuit where the switch  44  is a MOSFET and the diode  43  is implemented by the body diode  43  which is an integral part of the MOSFET  44 . 
       FIG. 7  also discloses the bleeder subcircuit  42  including a passive bleeder in the form of a resistor  61 ; and optionally, an active bleeder comprised of a bipolar transistor  62  and a base resistor  63 . When the forward current through the self-powered active rectifier circuit drops toward zero, the anode-to-cathode voltage also approaches zero; at some threshold, typically in the range of 30 mV to 60 mV, the anode-to-cathode voltage becomes too small to sustain the operation of the oscillator subcircuit  34 . Consequently, the oscillator subcircuit shuts down, and the rectifier subcircuit  38  can no longer output current to sustain the voltage across the passive bleeder  61 . Thus, the gate-to-source capacitance discharges, the gate-to-source voltage drops, and the power MOSFET  29  soon begins to turn off. However, as the power MOSFET  29  starts to turn off, the drain-to-source voltage can increase rapidly (because a positive voltage may be applied from cathode to anode by an external source) resulting in charge flowing into the gate  31  via the drain-to-gate capacitance of the power MOSFET  29 ; this charge flow partially cancels the current in the passive bleeder  61  and thus slows the turn-off process. The optional active bleeder ( 62  and  63 ) speeds up the turn-off process. As the drain  32  voltage increases with respect to the source  30 , current flows through the base resistor  63  into the base of the bipolar transistor  62 . Consequently, the bipolar transistor  62  turns on, and rapidly pulls down the gate  31  of the power MOSFET  29 . 
       FIG. 8  illustrates a method of operation of the self-powered active rectifier circuit. The method of operation is comprised of several steps. In the first step  64 , the anode-to-cathode voltage is utilized to power the oscillator subcircuit  34 . When the voltage at the anode  27  is greater than the voltage at the cathode  28 , the diode  43  allows current to flow through the oscillator subcircuit  34 . In the second step  65 , the oscillator generates a periodic voltage waveform at its output  37  and the amplitude of the periodic voltage waveform builds up to a level typically fifty to one hundred times greater than the anode-to-cathode voltage that powers the oscillator. In the third step  66 , the periodic voltage waveform is converted to a dc voltage by the rectifier subcircuit  38 ; optionally, this third step may include voltage multiplication as well as rectification. In the fourth step  67 , the dc voltage produced by the rectifier subcircuit is applied between the gate  31  and the source  30  of the power MOSFET  29 . Some embodiments, the method of operation also includes the optional fifth step  68 , where positive feedback mechanism is utilized to further increase the power MOSFET gate-to-source voltage. 
     The positive feedback mechanism operation is described by steps  69 ,  70 ,  71 , and  72 . In the first step  69  of the positive feedback mechanism, when the output voltage from the rectifier subcircuit  38  exceeds a threshold that is less than or equal to the gate-turn-on threshold of the power MOSFET  29 , the transistor switch  44  begins to turn on. As a result of the transistor switch  44  starting to conduct, the voltage drop across the diode  43  is reduced. Therefore, as shown in the second step  70  of the positive feedback mechanism, the supply voltage applied to the oscillator subcircuit  34  is increased. Consequently, the amplitude of the periodic voltage waveform produced by the oscillator subcircuit increases as stated in the third step  71  of the positive feedback mechanism. Thus, the voltage output by the rectifier subcircuit  38  also increases as stated in the fourth step  72  of the positive feedback mechanism. The increased output voltage from the rectifier subcircuit then turns on the transistor switch  44  more fully, thus closing the positive feedback loop. 
     The self-powered active rectifier circuit disclosed herein can improve the efficiency and reliability of PV solar arrays like the one shown in  FIG. 1  by directly replacing each of the bypass  6  and blocking  4  rectifiers. For example, when the current in the first string  1  is 10 A, the power dissipation in the associated blocking rectifier  4  can be up to 12 W; however, when the conventional blocking rectifier  4  is replaced by the self-powered active rectifier circuit disclosed herein, the power dissipation drops to about 1 W, for an energy savings of 92%. Similar energy savings are attained when replacing each of the bypass rectifiers  6  with the self-powered active rectifier circuit; while a solar panel  5  is shaded, the power dissipation in the associated bypass rectifier  6  would be up to 5 W if a Schottky is used, but only about 0.5 W with the self-powered active rectifier circuit, for an energy savings of 90%. The greatly reduced power dissipation also provides the benefit of greatly reduced junction temperatures, which translates to improved reliability. 
     Another advantage of the self-powered active rectifier circuit disclosed herein, is that it is simple enough to be economically and reliably manufactured from discrete components. In contrast, prior art such as disclosed in the &#39;496 application is so complex that it can only be produced in the form of an integrated circuit or multi-chip module. Using discrete components is an advantage because it allows the circuit to be easily adapted to the specific application. For example, when the self-powered active rectifier circuit is used as a bypass rectifier, the power MOSFET  29  requires a drain-to-source breakdown voltage rating of only about 30V. At such low voltage, there are many low-cost power MOSFETs available with very low on-resistance. However, when the self-powered active rectifier circuit is used as a blocking diode, a power MOSFET  29  with a much higher breakdown voltage, 300V for example, would be chosen. 
     Yet another advantageous aspect of the preferred embodiment of the self-powered active rectifier circuit shown in  FIG. 7  is that it prevents the power MOSFET  29  from turning on inadvertently due to leakage currents when the cathode voltage is greater than the anode voltage. All semiconductors have some finite leakage current which increases with temperature. The diode  43  and the switch  44  will leak some small current into the oscillator subcircuit negative supply input  36 . In some embodiments of the oscillator (such as the one shown in  FIG. 6 ) the leakage current will be conducted through the oscillator to its output  37 , and thence to the input  39  of the rectifier subcircuit  38 . The capacitor  49  in the rectifier subcircuit  38  blocks this leakage current from reaching the gate  31  of the power MOSFET  29 . Additionally, in some extreme cases (such as a nearby lightning strike) it may be possible for the power MOSFET  29  to be slightly damaged, resulting in a small leakage current from its drain  32  to its gate  31 . The active bleeder ( 62  and  63 ) provides a discharge path for this leakage current, thus preventing the power MOSFET  29  from turning on. 
     Although several embodiments have been described in detail for purposes of illustration, various modifications may be made to each without departing from the scope and spirit of the invention. Accordingly, the invention is not to be limited, except as by the appended claims.