Abstract:
A maximum power point tracking method, applied to a tracking device, employs a DC/DC converter connecting with a solar cell array, and including a controller actuating the DC/DC converter to perform an active resistance characteristic; a maximum power point tracking circuit adjusting the active resistance of the DC/DC converter; monitoring a change of an output power of the solar cell array in determining a direction for adjusting the active resistance of the DC/DC converter; and the maximum power point tracking circuit repeatedly adjusting the active resistance of the DC/DC converter. If the change of the output power of the solar cell array is positive, the active resistance of the DC/DC converter is adjusted in the same direction; but, conversely, if the change of the output power of the solar cell array is negative, the active resistance of the DC/DC converter is adjusted in an opposite direction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a maximum power point tracking method and a maximum power point tracking device for a solar power system. Particularly, the present invention relates to the maximum power point tracking method actuating a DC/DC converter to perform an active resistance characteristic so as to supply power to a DC/AC inverter or a DC load. More particularly, the present invention relates to the maximum power point tracking method that adjusts the DC/DC converter for changing its active resistance, and monitors a change of output power of a solar cell array. 
     2. Description of the Related Art 
     In solar power applications, a most significant subject focused today is associated with a technology of maximum power point tracking in addition to other major technologies which have been developed. Presently, a most common technology for maximum power point tracking is a perturbation and observation method. A conventional perturbation and observation method for maximum power point tracking, described in U.S. Pat. No. 5,327,071, controls a DC/DC converter for tracking a maximum power point of a solar cell array. In maximum power point tracking operation, an output voltage of the solar cell array is initially varied. Secondly, an output power of the solar cell array is subsequently detected. Each new value of the detected output power compares with a previous value in determining a perturbation direction of the output voltage of the solar cell array. In this circumstance, the output voltage of the solar cell array is continuously varied in detecting the maximum power point. Once detected a position of the maximum power point, the output voltage of the solar cell array is continuously varied around this position of the maximum power point for repeatedly re-determining it. 
     Another conventional perturbation and observation method for maximum power point tracking, described in U.S. Pat. No. 5,932,994, also controls a DC/DC converter so as to further control output voltage of a solar cell array for tracking a maximum power point. In maximum power point tracking operation, a duty cycle of a power switch in the DC/DC converter is initially varied. An output voltage and an output current of the solar cell array are detected to calculate an output power thereof. Each new value of the calculated output power compares with a previous value in determining a direction of fluctuation of the duty cycle of the power switch. In this circumstance, the duty cycle of the power switch is continuously varied in detecting the maximum power point. Once detected a position of the maximum power point, the duty cycle of the power switch is continuously varied around this position of the maximum power point for repeatedly re-determining it. 
     As has been explained above, these conventional perturbation and observation methods require at least two signals of detected voltages or currents in detecting the maximum power point. However, there exist some problems with practicing these perturbation methods applied to detect the maximum power point. For example, the circuits of maximum power point tracking devices for use in practicing such perturbation and observation methods result in complication of structures, and increase the manufacturing costs. Hence, there is a need for improving these perturbation and observation methods and the maximum power point tracking devices applied thereto. 
     As is described in greater detail below, the present invention intends to provide a maximum power point tracking method and a maximum power point tracking device for a solar power system. A DC/DC converter connecting with a solar cell array is actuated to perform an active resistance characteristic for supplying power to a DC/AC inverter or a DC load. A maximum power point tracking circuit is further used to adjust the DC/DC converter for changing its active resistance, and to monitor a change of the solar cell array in output power. Accordingly, the maximum power point tracking method and the maximum power point tracking device are simplified, and manufacturing cost thereof is reduced. 
     SUMMARY OF THE INVENTION 
     The primary objective of this invention is to provide a maximum power point tracking method and a maximum power point tracking device for a solar power system. A DC/DC converter connecting with a solar cell array is actuated to perform an active resistance characteristic for supplying power to a DC/AC inverter or a DC load. A maximum power point tracking circuit is further used to adjust the DC/DC converter for changing its active resistance, and to monitor a change of the solar cell array in output power. Accordingly, the maximum power point tracking method and the maximum power point tracking device are simplified, and manufacturing cost thereof is reduced. 
     The secondary objective of this invention is to provide the maximum power point tracking method for the solar power system using a maximum power point tracking circuit. The DC/DC converter is actuated to perform an active resistance characteristic, and the maximum power point tracking circuit is used to adjust the active resistance in a direction, and to monitor a change of the solar cell array in output power. If a change of the output power of the solar cell array is positive, the maximum power point tracking circuit continuously adjusts the active resistance so that the active resistance of the DC/DC converter can be successively changed in the same direction. But, conversely, if a change of the output power of the solar cell array is negative, the maximum power point tracking circuit reversely adjusts the active resistance so that the active resistance of the DC/DC converter can be changed in opposite direction. To track the maximum power point of the solar cell array, the maximum power point tracking circuit repeatedly adjusts the DC/DC converter for changing its active resistance. In this manner, the active resistance of the DC/DC converter is continuously varied toward an operation point as well as the maximum power point. 
     The maximum power point tracking device in accordance with an aspect of the present invention includes: a solar cell array for supplying power; a DC/DC converter connecting with the solar cell array, the DC/DC converter including an input capacitor, an inductor, a power electronic switch, a diode, an output capacitor and a controller, the controller generating a driving signal by feeding forward an inductor current which can control turning on or off the power electronic switch of the DC/DC converter such that the controller can actuate the DC/DC converter to perform an active resistance characteristic, and to transfer energies of the DC/DC converter to a DC/AC inverter or a DC load; a maximum power point tracking circuit connecting with the controller of the DC/DC converter, and outputting an active resistance control signal to adjust the DC/DC converter for changing its active resistance; monitoring a change of the output power of the solar cell array. The output power of the solar cell array is obtained from the result of the square of the inductor current of the DC/DC converter multiplied by the active resistance control signal. If a change of the output power of the solar cell array is positive, the active resistance control signal of the maximum power point tracking circuit is continuously adjusted in the same direction. But, conversely, if a change of the output power of the solar cell array is negative, the active resistance control signal of the maximum power-point tracking circuit is adjusted in opposite direction. To track the maximum power point of the solar cell array, the maximum power point tracking circuit repeatedly adjusts the DC/DC converter for changing its active resistance. In this manner, the active resistance control signal of the maximum power point tracking circuit is continuously varied toward an operation point of the maximum power point. 
     The maximum power point tracking method in accordance with a separate aspect of the present invention includes the step of: connecting a DC/DC converter with a solar cell array, and a controller actuating the DC/DC converter to perform an active resistance characteristic; a maximum power point tracking circuit adjusting the active resistance of the DC/DC converter; monitoring a change of an output power of the solar cell array in determining a direction for adjusting the active resistance of the DC/DC converter; and the maximum power point tracking circuit repeatedly adjusting the active resistance of the DC/DC converter. 
     In a further separate aspect, if the change of the output power of the solar cell array is positive, the active resistance of the DC/DC converter is adjusted in the same direction; but, conversely, if the change of the output power of the solar cell array is negative, the active resistance of the DC/DC converter is adjusted in an opposite direction. 
     In a yet further separate aspect, only an inductor current signal is required to be detected and calculated for carrying out a process for tracking the maximum power point of the solar cell array. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic view illustrating a solar power system having a maximum power point tracking device in accordance with a first embodiment of the present invention; 
         FIG. 2  is a schematic circuitry illustrating a DC/DC converter applied in the maximum power point tracing device in accordance with the first embodiment of the present invention, as depicted in  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating a controller of the DC/DC converter applied in the maximum power point tracking device in accordance with the first embodiment of the present invention, as depicted in  FIG. 2 ; 
         FIG. 4  is a flow chart illustrating a maximum power point tracking method for a maximum power point tracking circuit applied in the solar power system in accordance with the first embodiment of the present invention; and 
         FIG. 5  is a schematic view illustrating a solar power system having a maximum power point tracking device in accordance with a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to  FIG. 1 , a schematic view of a solar power system in accordance with a first embodiment of the present invention is illustrated. Referring to  FIG. 1 , a solar power system  1  includes a solar cell array  10  and a DC/DC converter  11 . The solar power system  1  connects with a DC/AC inverter  2  such that a DC power generated from the solar power system  1  can be sent to the DC/AC inverter  2  and converted into AC power to supply to a distribution power system  3 . An output DC voltage of the DC/DC converter  11  controlled by the DC/AC inverter  2  can be constant or varied in response to the changes of the AC voltage of the distribution power system  3 . 
     Turning now to  FIG. 2 , a schematic circuitry of the DC/DC converter applied in the maximum power point tracking device for the solar power system in accordance with a first embodiment of the present invention is illustrated. By referring to  FIG. 2 , the DC/DC converter  11  includes an input capacitor  110 , an inductor  111 , a power electronic switch  112 , a diode  113 , an output capacitor  114  and a controller  115 . 
     Still referring to  FIGS. 1 and 2 , the input capacitor  110  is used to stabilize a voltage of the solar cell array  10  while the controller  115  is used to generate a control signal to turn on or off the power electronic switch  112 . If the power electronic switch  112  is turned on, the inductor  111  can be charged by energy generated by the solar cell array  10 . Conversely, if the power electronic switch  112  is turned off, energy stored in the inductor  111  can be transferred to the output capacitor  114  via the diode  113 . Accordingly, the electric power of the solar cell array  10  can be converted into a higher voltage of the DC power. 
     Turning now to  FIG. 3 , a block diagram of the controller  115  of the DC/DC converter applied in the maximum power point tracking device in accordance with the first embodiment of the present invention is illustrated. With reference to  FIG. 3 , the controller  115  includes a current detector  40 , a multiplier  41 , an amplifier  42  and a pulse width modulation circuit  43 . Furthermore, the output of the maximum power point tracking circuit  5  is an input of the multiplier  41 . 
     Referring to  FIGS. 2 and 3 , the current detector  40  can detect a current of the inductor  111  provided in the DC/DC converter  11 . An output of the current detector  40  and an active resistance control signal of the maximum power point tracking circuit  5  are sent to the multiplier  41  and multiplied therein. Subsequently, the result of the multiplier  41  is sent to the amplifier  42  which can amplify it. Subsequently, the result of the amplifier  42  is further sent to the pulse width modulation circuit  43  to perform a modulation signal so as to generate a driving signal. Consequently, the driving signal generated by the pulse width modulation circuit  43  can control the power electronic switch  112  to turn on or off. 
     Still referring to  FIG. 2 , when the power electronic switch  112  is turned on, a voltage across two terminals of the power electronic switch  112  approximates zero. But, conversely, when the power electronic switch  112  is turned off, a voltage across the two terminals of the power electronic switch  112  equals the output voltage of the DC/DC converter  11  since the diode  113  is conducted. A square wave appears across the two terminals of the power electronic switch  112  if turning on or off the power electronic switch  112  is alternatively controlled. In this manner, the voltage across the two terminals of the power electronic switch  112  is alternatively changed between zero and the output voltage of the DC/DC converter  11 . When the current of the inductor  111  is continuously conducted, an average voltage across the two terminals of the power electronic switch  112  is proportional to the time duration for turning off the power electronic switch  112 . 
     Referring again to  FIG. 3 , the modulation signal of the amplifier  42  for the pulse width modulation circuit  43  is proportional to a current signal of the inductor  111 . The modulation signal is sent to the pulse width modulation circuit  43  and compared with a high-frequency triangular wave. When the modulation signal is higher than the high-frequency triangular wave, the controller  115  controls the power electronic switch  112  to turn off. But, conversely, when the modulation signal is lower than the high-frequency triangular wave, the controller  115  controls the power electronic switch  112  to turn on. In this circumstance, the time duration for turning off the power electronic switch  112  is proportional to the modulation signal such that the average voltage across the two terminals of the power electronic switch  112  is proportional to the current passing through the inductor  111 , namely, the DC/DC converter  11  can generate a voltage which is proportional to its input current. Accordingly, the DC/DC converter  11  is controlled to perform an active resistance characteristic such that the DC/DC converter  11  can be regarded as an active resistor. In operation, when the current of the inductor  111  is continuously conducted, the value of active resistor is proportional to the active resistance control signal output from the maximum power point tracking circuit  5 . 
     Turning now to  FIG. 4 , a flow chart of a maximum power point tracking method for the maximum power point tracking circuit applied in the solar power system in accordance with the first embodiment of the present invention is illustrated. Referring to  FIGS. 2 through 4 , firstly, an new interval value (identified as “ΔR(n)”) and an initial value (identified as “R(0)”) of the active resistance control signal are preset. The initial value R(0) of the active resistance control signal is sent to the controller  115  of the DC/DC converter  11  to act as an active resistance control signal. Subsequently, after a time interval, an average current (identified as “I L ”) of the inductor  111  is calculated. The square of the average current I L  of the inductor  111  and the initial value R(0) of the active resistance control signal are multiplied for obtaining an initial value (identified as “P(0)”) of output power of the solar cell array  10 . To measure output power of the solar cell array, there is provided the time interval for stabilizing current and voltage of the DC/DC converter  11  after the active resistance control signal is sent out. 
     Still referring to  FIGS. 2 through 4 , the initial value R(0) of the active resistance control signal is regarded as an old (previous) value R(n-1) while the initial value P(0) of output power of the solar cell array  10  is regarded as an old (previous) value P(n-1). In addition to this, a new value R(n) of the active resistance control signal is obtained by adding the old value R(n-1) of the active resistance control signal and a new interval value ΔR(n), and is sent to controller  115  of the DC/DC converter  11  to act as an active resistance control signal. In this circumstance, the new interval value ΔR(n) has replaced the old interval value ΔR(n-1). Subsequently, after a time interval, an average current I L  of the inductor  111  is calculated, and the square of the average current I L  of the inductor  111  and the new value R(n) of the active resistance control signal are multiplied for obtaining a new value (identified as “P(n)”) of output power of the solar cell array  10 . 
     To track the maximum power point, the new value P(n) of output power of the solar cell array  10  is compared with the old value P(n-1), with continued reference to  FIGS. 2 through 4 . If the new value P(n) of output power of the solar cell array  10  is greater than the old value P(n-1), the new interval value ΔR(n) of the active resistance control signal is not changed, and is identical with the old (previous) interval value ΔR(n-1) (namely, ΔR(n)=ΔR(n-1)). But, conversely, if the new value P(n) of output power of the solar cell array  10  is less than the old value P(n-1), the new interval value ΔR(n) of the active resistance control signal is changed to reverse direction, and is opposite to the old (previous) interval value ΔR(n-1) (namely, ΔR(n)=−ΔR(n-1)). Finally, the new value P(n) of output power of the solar cell array  10  has replaced the old value P(n-1), and the new value R(n) of the active resistance control signal has also replaced the old value R(n-1) at the same time. 
     Subsequently, a new series of steps is repeated and circulated continuously by the previous steps until a maximum power point of output power is tracked. Once detected an operation point for the maximum power point, the maximum power point tracking circuit  5  controls the output power of the solar cell array  10  continuously perturbing around the operation point for the maximum power point. In a preferred embodiment, the interval value “ΔR” of the active resistance control signal is constant or variable. 
     In an alternative embodiment, when the interval values “ΔR” of the active resistance control signal are variable values, each of which is proportional to a difference between the new value P(n) and the old value P(n-1) of output power of the solar cell array  10 . If a difference between the new value P(n) and the old value P(n-1) of output power of the solar cell array  10  becomes greater, it represents a position having a perturbation point far away from the exact position of the maximum power point of output power that enlarges the interval values “ΔR” of the active resistance control signal. Accordingly, it would be advantageous that the processing time for tracking the maximum power point is speeded up. But, conversely, if a difference between the new value P(n) and the old value P(n-1) of output power of the solar cell array  10  becomes smaller, it represents a position having a perturbation point approaching the exact position of the maximum power point of output power in such a way as to reduce interval values “ΔR” of the active resistance control signal. Accordingly, it would be advantageous that the perturbation of output power of the solar cell array  10  around the exact maximum power point is small, and the power loss is reduced. 
     Referring back to  FIGS. 3 and 4 , the maximum power point tracking method and the maximum power point tracking circuit in accordance with the present invention only requires detecting the current of the inductor  111  of the DC/DC converter  11  for tracking the maximum power point. Consequently, it would be advantageous that this method and this circuit can simplify the entire structure, and reduce manufacturing cost. Conversely, the conventional perturbation methods require at least two signals of detected voltages or currents in detecting the maximum power point. Inevitably, such a conventional method results in a complicated structure and an increase of manufacturing cost. 
     Turning now to  FIG. 5 , a schematic view of a solar power system having a maximum power point tracking device in accordance with a second embodiment of the present invention is illustrated. Referring to  FIG. 5 , the solar power system  1  includes a solar cell array  10  and a DC/DC converter  11 . The DC/DC converter  11  further includes a maximum power point tracking circuit  5 . In the second embodiment, the solar power system  1  supplies DC power to a battery  6  and/or a DC load  7 . The operational steps for maximum power point tracking method in accordance with the second embodiment of the present invention has similar to those of the first embodiment and detailed descriptions may be therefore omitted. 
     Although the invention has been described in detail with reference to its presently preferred embodiment, it will be understood by one of ordinary skill in the art that various modifications can be made without departing from the spirit and the scope of the invention, as set forth in the appended claims.