USOO7719866B2
`
`(12) United States Patent
`Boldo
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,719,866 B2
`May 18, 2010
`
`(54) CONTROL CIRCUIT FOR A DC-TO-DC
`SWITCHING CONVERTER, AND THE USE
`THEREOF FOR MAXIMIZING THE POWER
`DELIVERED BY APHOTOVOLTAC
`GENERATOR
`
`(*) Notice:
`
`(75) Inventor: Pablo Rueda Boldo, Katwijk (NL)
`(73) Assignee: Agence Spatiale Europeenne, Paris
`Cedex (FR)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1020 days.
`(21) Appl. No.: 11/415,827
`(22) Filed:
`May 2, 2006
`
`(65)
`
`Prior Publication Data
`US 2007/OO242.57 A1
`Feb. 1, 2007
`
`Foreign Application Priority Data
`(30)
`May 2, 2005
`(FR) ................................... O5 O4441
`
`(51) Int. Cl.
`(2006.01)
`H02M 3/335
`(52) U.S. Cl. ........................... 363/89; 363/97; 323/282;
`323/906
`(58) Field of Classification Search ................. 323/222,
`323/272,273,277, 259,282 290, 268, 269;
`363/41, 65, 49, 16–17,98, 132, 89,97; 307/23,
`307/43, 44, 125
`See application file for complete search history.
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`4, 163,194. A
`7, 1979 Ross .......................... 324,767
`
`4,929,882 A * 5/1990 Szepesi ...................... 323,222
`
`DE
`DE
`
`FOREIGN PATENT DOCUMENTS
`41 O1594 A1
`7, 1992
`19837 862 A1
`3, 2000
`
`OTHER PUBLICATIONS
`Garrigos A. et al., “System model of the sequential Switching shunt
`series regulator for spacecraft regulated high power busses'. Power
`Electronics Specialists Conference, 2004, pp. 3645-3650.
`Perol P. "Another look at the sequential Switching shunt regulator'.
`Proceedings of the European Space Power Conference, vol. 1, Sep.
`21, 1998, pp. 79-84.
`* cited by examiner
`Primary Examiner Rajnikant B Patel
`(74) Attorney, Agent, or Firm—Alston & Bird LLP
`(57)
`ABSTRACT
`
`A control circuit for a switching DC/DC Converter compris
`ing: an input for an indicator signal indicative of an output
`current level from said converter; a peak detector for detect
`ing and storing a maximum value of said indicator signal; a
`comparator element for comparing an instantaneous value of
`said indicator signal with said stored maximum value, and for
`generating a Switching signal when said instantaneous value
`becomes less than a predetermined fraction of said stored
`value; reinitializer means for reinitializing said peak detector
`in response to said Switching signal; and means for generating
`a control signal that Switches between a state in which it
`increases over time and a state in which it decreases overtime
`in response to said Switching signal. A control module for
`photovoltaic generator, the module including such a control
`circuit, and a photovoltaic generator system comprising a
`plurality of Such modules, each controlling a respective pho
`tovoltaic generator.
`
`12 Claims, 4 Drawing Sheets
`
`
`
`CC1
`
`IoRo
`MppeoS
`
`Samsung Electronics Co., Ltd.
`Ex. 1030, p. 1
`
`

`

`U.S. Patent
`
`May 18, 2010
`
`Sheet 1 of 4
`
`US 7,719,866 B2
`
`IsA(A)
`
`DC1
`
`1 OOOW
`
`10 20 30 40 50 60 70 80 90 100 VSA
`
`FG.1
`
`
`
`CHOPPER
`APR
`
`Samsung Electronics Co., Ltd.
`Ex. 1030, p. 2
`
`

`

`U.S. Patent
`
`May 18, 2010
`
`Sheet 2 of 4
`
`US 7,719,866 B2
`
`t1 t2t3 t4
`500ps
`
`FG3
`
`
`
`10 20 30 40 50 60 70 80 90 100ms
`
`. .
`
`.
`
`. .
`
`. .
`
`. . . - - -
`
`- - - - - - - - - - - - - - - - - - - - - - - - - - - e - - - or - - - a
`
`- - - - - - - - - - - - - - - - - - - - - -
`
`-
`
`.
`
`.
`
`.
`
`.
`
`. . . . .
`
`. . .
`
`. . . . .
`
`.
`
`. . .
`
`.
`
`. - - - in a
`
`a a
`
`- M - - - a - - -
`
`- - - - - -
`
`-
`
`- - - - -
`
`-
`
`-
`
`FIG.5 T.
`
`T.
`
`2T3
`
`Samsung Electronics Co., Ltd.
`Ex. 1030, p. 3
`
`

`

`U.S. Patent
`
`May 18, 2010
`
`Sheet 3 of 4
`
`US 7,719,866 B2
`
`
`
`CC1
`
`FG4
`
`Samsung Electronics Co., Ltd.
`Ex. 1030, p. 4
`
`

`

`U.S. Patent
`
`May 18, 2010
`
`Sheet 4 of 4
`
`US 7,719,866 B2
`
`FIG.6
`
`
`
`10 20 30 40 50 60 70 80 90 100 S
`
`-- - - ---N -------
`
`--
`
`FG.7
`
`Io'- Io
`
`Samsung Electronics Co., Ltd.
`Ex. 1030, p. 5
`
`

`

`US 7,719,866 B2
`
`1.
`CONTROL CIRCUIT FOR A DC-TO-DC
`SWITCHING CONVERTER, AND THE USE
`THEREOF FOR MAXIMIZING THE POWER
`DELIVERED BY APHOTOVOLTAC
`GENERATOR
`
`The invention relates to a control circuit for a DC-to-DC
`Switching converter, and to its application to controlling a
`photovoltaic generator. The invention is intended mainly but
`not exclusively for space applications.
`
`10
`
`BACKGROUND OF THE INVENTION
`
`2
`is why analog MPPT controllers have been developed in the
`past, but until now none of them has given complete satisfac
`tion.
`The articles “Electrical power subsystem of Globastar' by
`W. Denzinger, Fourth European Space Power Conference
`1995, and “Power conditioning unit for Rosetta/Mars
`Express' by H. Jensen and J. Laursen, Proceedings of the
`Sixth European Space Power Conference 2002, describe sys
`tems based on the principle that at the maximum power point,
`the absolute value of the dynamic impedance dV/dI of a
`generator is equal to its static impedance VI. This makes it
`possible to avoid multiplying Voltage and current values, and
`consequently to limit the complexity of the circuit. Neverthe
`less, circuit complexity remains excessive for providing inde
`pendent control of a large number of individual generators.
`Thus, a plurality of Solar arrays need to be connected in
`parallel and controlled together; this limits the effectiveness
`of the system and makes it necessary to use protection and
`isolation systems in order to avoid faults propagating.
`U.S. Pat. No. 4,794.272 discloses an MPPT system that
`makes use of a different concept, namely maximizing the
`output current from a DC/DC switching converter (an array
`power regulator (APR)) connected between the photovoltaic
`generators and the power Supply bus bar. This simplification
`is possible because the output voltage from the APR converter
`is equal to the potential of the power supply bus bar, which
`can be considered as being approximately constant. Never
`theless, such a system requires means for modifying the oper
`ating points in order to cause it to oscillate about its maximum
`value, thereby preventing the complexity of the electronic
`circuit from being reduced significantly.
`U.S. Pat. No. 6,316,925 discloses another MPPT system
`that controls a Voltage converter in order to maximize the
`output Voltage by using operations of sampling and compar
`ing output current values. Such a system presents the draw
`back of being synchronous (the sample-and-hold circuit is
`driven by a clock), thereby limiting its performance. Good
`tracking of the optimum operating point can be obtained only
`by using slow oscillations around said optimum point, but that
`implies along acquisition time in order to come close thereto.
`Furthermore, the above two documents disclose in conven
`tional manner, the use of a buck converter (a Voltage-reducing
`switching converter) as the APR converter. That presents
`three major drawbacks:
`firstly, it is necessary to use high Voltage Solar generators
`(operating at several tens of Volts (V) and up to more
`than 100 V), thus running a major risk of failure due to
`electric arcs forming in operation:
`secondly, a buck Switching converter includes a controlled
`switch (typically a metal oxide on silicon field effect
`transistor (MOSFET)) connected in series with each
`generator, as a result, in the event of the Switch failing,
`the generator is permanently isolated from the power
`Supply bus bar,
`and thirdly, when the switch of a buck switching converter
`is constituted by a MOSFET, the MOSFET must be
`controlled by a floating driver circuit, thereby increasing
`the number of electronic components that are needed.
`The article by A. Boehringer and J. Haussmann entitled
`“Dynamic behavior of power conditioning systems for satel
`lites with a maximum power point tracking system', pub
`lished in the Proceedings of the Spacecraft Power Condition
`ing Electronic Seminar. ESTEC Noordwijk, the Netherlands,
`July 1972, describes an MPPT system based on the above
`mentioned “dV/dI=V/I concept and using a step-up switch
`ing converter operating at the limit between discontinuous
`and continuous conditions.
`
`Satellites and space probes generally include photovoltaic
`generators for powering on-board equipment and for charg
`ing batteries that deliverpower during periods of eclipse. The
`photovoltaic generators, the batteries, and the various items
`of equipment that need to be powered are connected to one
`another by a power Supply bus bar that presents a potential
`that needs to be kept within a predetermined range. Regula
`tors are provided for controlling the magnitude of the currents
`delivered by said photovoltaic generators, in particular as a
`function of the potential of said power supply bus bar.
`The regulator in the most widespread use is the sequential
`switching shunt regulator (S3R or SR) as developed by the
`European Space Agency and as described in the article “The
`sequential switching shunt regular SR'by D. O'Sullivanand
`A. Weinberg, Proceedings of the Third ESTEC Spacecraft
`Power Conditioning Seminar, Noordwijk, the Netherlands,
`Sep. 21-23, 1977. That regulator comprises a plurality of
`individual photovoltaic generators connected in parallel to
`feed a power Supply bus bar. Each individual generator can be
`selectively short-circuited by a controlled switch, and under
`Such circumstances the current it generates is no longer Sup
`plied to the bus bar, but is dissipated; the various short-circuit
`switches are switched ON or OFF as a function of the poten
`tial of the power supply bus bar, and with the help of hyster
`esis comparators having thresholds that are offset from one
`another. Thus, the lower the potential of the bus bar, indicative
`of high consumption by the equipment being powered and/or
`of a low level of charge in the batteries, the greater the number
`of individual generators that are connected to said bus bar.
`Conversely, when the potential of the bus bar is high, genera
`tors are short-circuited, which means that there is excess
`power available.
`The SR regulator constitutes an excellent compromise
`between the requirements for effectiveness and for simplicity,
`however it does not make it possible to optimize the use of the
`power available from the photovoltaic generators. Such gen
`erators present a V-I characteristic curve that presents an
`optimum operating point at which the power extracted is
`maximized; in order to operate at this optimum operating
`point, each generator must be connected to a load that pre
`sents a determined input impedance. However the problem is
`made much more complex by the fact that the characteristic
`curves vary very greatly with aging of the generators, and also
`depend strongly on temperature. That is why, if it is desired to
`make optimum use of the available power, which is very
`important in particular for interplanetary missions directed to
`the outer regions of the Solar systems, it is necessary to
`provide a control system that makes it possible to “track’ the
`maximum power operating point, with this being known as
`maximum power point tracking (MPPT).
`In conventional manner, MPPT systems use microproces
`sor-based controllers, but that is generally not desirable in
`space applications, in particular for reasons of reliability. That
`
`15
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`Samsung Electronics Co., Ltd.
`Ex. 1030, p. 6
`
`

`

`US 7,719,866 B2
`
`3
`OBJECTS AND SUMMARY OF THE
`INVENTION
`
`4
`said current-control circuit includes a limiter circuit for
`limiting the value of said second control signal for said
`driver circuit in Such a manner that said target level for
`the output current from the DC/DC converter cannot
`exceed a limit value regardless of the value of the poten
`tial of said power bus bar;
`the module also comprises an activator and deactivator
`circuit for generating a signal for activating said control
`circuit when the value of the potential of said power
`supply bus bar drops below an activation threshold, and
`for generating a signal for deactivating said control cir
`cuit when the value of the potential of said power supply
`bus bar rises above a deactivation threshold, higher than
`said activation threshold.
`The invention also provides a photovoltaic generator sys
`tem comprising a plurality of individual photovoltaic genera
`tors that are isolated from one another and connected to a
`common power Supply bus bar via respective control modules
`of the invention, in which:
`the circuits for activating and deactivating each module
`presents different values for said activation and deacti
`vation thresholds; and
`said activation threshold, for all of the modules, is less than
`the value of the potential of said power supply bus bar at
`which said target level for the output current from said
`DC/DC Converter is equal to said predetermined limit
`value.
`The invention also provides a method of controlling a
`switching DC/DC Converter in such a manner that its output
`current oscillates about its maximum value, the method com
`prising the following operations:
`generating a control signal for controlling said DC/DC
`Converter, which control signal can present a state in
`which it increases over time or a state in which it
`decreases over time;
`continuously acquiring an indicator signal indicative of an
`output current level from said DC/DC Converter;
`continuously detecting a peak value of said indicator signal
`and storing it in a storage element;
`continuously comparing the present value of said indicator
`signal with said stored peak value; and
`when said present value is less than a first predetermined
`fraction of said stored peak value:
`reinitializing said storage element; and
`Switching said control signal between said state of
`increasing relative to time and said state of decreasing
`relative to time.
`
`10
`
`15
`
`25
`
`30
`
`35
`
`40
`
`An object of the present invention is to mitigate at least one
`of the drawbacks of the prior art. In particular, the invention
`makes it possible to obtain an MPPT system which, while
`presenting good performance and good reliability, is suffi
`ciently simple to enable a plurality of individual generators to
`be controlled independently, thereby improving the overall
`effectiveness of the photovoltaic generator system and relax
`ing requirements in terms of protecting and isolating said
`individual generators. In addition, the invention preferably
`uses a step-up Switching converter as the converter power
`stage, thereby Subsequently improving the reliability of the
`photovoltaic generator System.
`The invention thus provides a control circuit for a DC/DC
`Switching converter, the control circuit comprising:
`an input for an indicator signal indicative of an output
`current level from said converter;
`a peak detector for detecting and storing a maximum value
`of said indicator signal;
`a comparator element for comparing an instantaneous
`value of said indicator signal with said stored maximum
`value, and for generating a Switching signal when said
`instantaneous value becomes less than a predetermined
`fraction of said stored value;
`reinitializer means for reinitializing said peak detector in
`response to said Switching signal; and
`means for generating a control signal that Switches
`between a state in which it increases overtime and a state
`in which it decreases over time in response to said
`Switching signal.
`In particular embodiments:
`said control signal varies linearly with time; and
`the circuit also includes an input for an activation/deacti
`Vation signal, and when a deactivation signal is present
`at said input, said control signal is held in a saturated
`State.
`The invention also provides a control module for a photo
`Voltaic generator, the control module comprising:
`a Switching converterconnecting said photovoltaic genera
`tor to a power Supply bus bar;
`a current detector for generating an indicator signal indica
`tive of an output current from said converter;
`a driver circuit connected to said receive a control signal as
`45
`an input and to generate a signal for driving said DC/DC
`converter as a function of said control signal; and
`a control circuitas defined above, connected to receive said
`indicator signal as an input and to generate on its output
`a first control signal for the driver circuit.
`In particular embodiments:
`said DC/DC converter is a step-up switching converter;
`said driver circuit comprises a pulse width modulator for
`generating a driver signal having a duty cycle deter
`mined by said control signal;
`the module also comprises a current-control circuit for
`generating a second control signal for said driver circuit
`such that the output current from said DC-to-DC con
`Verter is maintained at a target level that increases with
`decreasing value of the potential of said power Supply
`bus bar, and in which said control circuit and said cur
`rent-control circuit are connected to said driver circuit in
`Such a manner as to receive as input that one of said first
`and second control signals that determines generating
`the driver signal that corresponds to the higher level for
`the output current from said DC/DC converter;
`said current-control circuit is a conductance control circuit;
`
`50
`
`55
`
`60
`
`65
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Other characteristics, details, and advantages of the inven
`tion appear on reading the following description made with
`reference to the accompanying drawings, given by way of
`example, and showing:
`FIG. 1, two examples of characteristic curves for a solar
`generator at different temperatures;
`FIG. 2, a simplified electrical Schematic diagram of a cir
`cuit for controlling a switching DC/DC Converter of the
`invention;
`FIG. 3, waveforms illustrating the operation of the circuit
`of FIG. 2:
`FIG. 4, a simplified electrical schematic diagram of a con
`trol module for a photovoltaic generator of the invention, in
`particular including the circuit of FIG. 2;
`FIGS. 5 and 6, waveforms illustrating the operation of the
`FIG. 4 module; and
`
`Samsung Electronics Co., Ltd.
`Ex. 1030, p. 7
`
`

`

`US 7,719,866 B2
`
`5
`FIG. 7, a diagram illustrating the operation of a photovol
`taic generator System comprises a plurality of modules of the
`type shown in FIG. 4.
`
`MORE DETAILED DESCRIPTION
`
`10
`
`15
`
`25
`
`30
`
`35
`
`FIG. 1 shows examples of characteristic curves for photo
`voltaic generators. More precisely, the thick-lined curves C1
`and C2 represent the relationship between the Voltage Vs and
`the output current I for a generator at respective temperatures
`of 45° C. and of -90° C. At both temperatures, the character
`istic curves present a first region of approximately constant
`current followed by a second region of approximately con
`stant voltage in which the current drops off rapidly to zero for
`a small increase in Voltage. The power generated is given by
`the relationship Psi-Is (Vs):Vs, and is represented by the
`fine-lined curves P1 and P2. The optimum operating point
`MPP1, MPP2 corresponds to generating maximum power
`and is situated in the region of the inflexion in the character
`istic curves C1, C2: it is of interest to observe that these points
`correspond neither to maximum current nor to maximum
`voltage. The reciprocals of the slopes of the load lines CT1
`and CT2 passing through the origin of the axes and through
`the respective points MPP1 or MPP2, give the values of the
`impedance of the load that should be connected to the output
`of the photovoltaic generator in order to extract maximum
`power therefrom. It will readily be understood that because of
`the temperature variations to which photovoltaic generators
`are subjected, in particular in the context of space applica
`tions, using a load of constant resistance does not enable the
`available power to be used with maximum effectiveness.
`Other major variations in the characteristic curves are due to
`the generators aging.
`That is why it is known to use a Voltage converter to match
`the impedance in Such a manner as to “track the operating
`point that enables maximum power to be extracted from the
`generator, at least whenever that is made necessary by the
`state of charge of the batteries and by the power demand from
`the equipment powered by the power bus bar. The voltage
`converter is generally a Voltage-lowering (buck) or Voltage
`raising (step-up) Switching converter presenting a conversion
`ratio that is determined by a control circuit that seeks to
`maximize the power extracted.
`FIG. 2 is a simplified electrical schematic of a control
`circuit CT of the invention, which receives as an input a
`Voltage signal IRo presenting a value that is proportional to
`the output current from the DC/DC converter (where R is a
`gain that has the dimensions of resistance) and that generates
`at its output a control signal V. A comparator CP2 not
`50
`forming part of the control circuit CT proper, serves to com
`pare V, with a sawtooth signal Vs. that is generated
`locally; the output from the comparator CP2 is a squarewave
`signal that drives a Switching converter APR presenting a duty
`cycle D (fraction of the period during which the signal is at a
`“high value) that is proportional to V. The comparator
`CP2 constitutes a pulse width modulator. The voltage V and
`the current Io at the output from a Switching converter depend
`on the duty cycle; in particular, for a step-up Switching con
`verter that is ideal (lossless), the following apply:
`Io (1-D)Is
`
`40
`
`45
`
`55
`
`60
`
`Vo-Vs/(1-D)
`
`where Vs and Is are respectively the Voltage and the current
`input to the Switching converter, i.e. the Voltage and the cur
`rent at the terminals of the photovoltaic generator.
`
`65
`
`6
`The signal IR input to the control circuit CT is applied to
`the input of a peak detector PK which charges a capacitor
`C; in the simplified schematic, PK is represented as a
`simple diode, but it is preferably implemented in the form of
`an active circuit, in known manner. At each instant, the poten
`tial difference across the terminals of said capacitor C is
`thus equal to the maximum value that has been taken by the
`signal IoRo since the most recent reinitialization of the peak
`detector; this potential difference is multiplied by a factor
`K<1 (typically in the range 0.95 to 0.99, e.g. equal to 0.97)
`and is applied to the inverting input of a comparator CP1; the
`non-inverting input of the comparator has a signal applied
`thereto that is equal to IoRo when the output from the com
`parator CP1 is high (diode D2 non-conductive) and equal to a
`value that is less than IoRo when the output from the com
`parator CP1 is low (diode D2 conductive and voltage divided
`by resistors R1 and R2). An operational amplifier F1 con
`nected as a Voltage follower, although not essential is never
`theless useful in ensuring that the value of the input signal
`IoRo is not disturbed.
`The operation of the control circuit CT is described in
`detail below. Understanding is made easier on examining
`FIG. 3 which shows, amongst other things, the way in which
`signals S1, S2, and S3 vary over time, which signals corre
`spond respectively to the non-inverting input, to the inverting
`input, and to the output of the comparator CP1.
`Consideration is given initially to circumstances in which
`the input signal IR increases over time and the comparator
`CP1 presents an output S3 that is high. Under such circum
`stances, the signal S2 at the inverting input of CP1 is equal to
`KIR (K=0.97<1), since the instantaneous value of IoRo
`coincides with its peak value, and the signal S1 at the non
`inverting input is equal to IR since the diode D2 is non
`conductive and thus no current is flowing through the Voltage
`divider R1/R2; this situation is compatible with the initial
`assumption that the output from the comparator is high. It can
`be seen that the diode D1 connecting the capacitor C to the
`output of the comparator CP1 is also non-conductive: as a
`result the capacitor cannot discharge.
`At an instant t1, IR begins to decrease: the signal S1 at the
`non-inverting input CP1 also decreases, while the signal S2 at
`its inverting input remains constant: S2 Kmax(IR). At an
`instant t2, the instantaneous value of IoRo drops below
`S2=K max(IR) and the output S3 of the comparator CP1
`switches to its low level. At this instant, the diodes D1 and D2
`become conductive: the signal S1 of the non-inverting input
`of CP1 goes from IR to IRR2/(R1+R2), and the capacitor
`C discharges quickly through D1, thereby reinitializing the
`peak detector. When at an instant t3 the voltage S2 at the
`inverting input of CP1 drops below S1=IRR2/(R1+R2), the
`output S3 of the comparator returns to a high level and causes
`the diodes D1 and D2 to become non-conductive. The capaci
`tor C can then start to charge again until at instant ta. it
`"catches up with the input signal IR.
`In conclusion, while the instantaneous value of the signal
`IR indicative of an output current level from the converter
`APR, becomes less than a predetermined threshold K of its
`maximum value, as Stored by the capacitor C of the peak
`detector PK:
`said peak detector PK is reinitialized by the capacitor C.
`discharging through the diode D1; and
`a pulse S3 is generated at the outlet from the comparator
`CP1.
`The output S3 of the comparator CP1 is connected to the
`clock input CK of a D type bistable B1, having its D input
`connected to its Q output. When the bistable B1 receives the
`pulse S3 coming from the comparator CP1 on its clock input
`
`Samsung Electronics Co., Ltd.
`Ex. 1030, p. 8
`
`

`

`7
`CK, the signal presentatits Q output goes from a high level to
`a low level, or vice versa. It is assumed that prior to the rising
`front of the pulse S3 (tist3) Q is low, and that for to t3. Q
`passes to a high level (see FIG. 3).
`The Q output of the bistable B1 is delivered at an input to
`an integrator INT, essentially constituted by an operational
`amplifier, a capacitor C, and a resistor R3. A reference
`voltage Ref int of value intermediate between the high and
`low values of Q is applied to the inverting input of the inte
`grator. Mere inspection of the circuit shows that when Q is
`high, the signal V
`at the output from the integrator
`decreases linearly, in other words it presents a waveform of
`downward slope, and while Q is low, the signal V,
`increases linearly, in other words it presents a waveform of
`rising slope.
`The signal Vee constitutes a control signal for the Volt
`age converter APR since its value acting via the comparator
`CP2 determines the duty cycle D of the squarewave signal
`that drives said converter, and thus determines the operating
`point of the photovoltaic generator connected to its input.
`The overall operation of the circuit is considered below.
`Initially, for t<t1, Q is low and the control signal V,
`increases linearly over time, progressively reducing the duty
`cycle D of the signal driving the converter APR. It is assumed
`initially that D>D. D., being the optimum duty cycle
`enabling the highest output current Io to be obtained (which
`corresponds approximately to maximum power, since the
`voltage of the power supply bus bar to which the output of the
`converter is connected varies very little); it follows that the
`signal IRo increases likewise over time. At instant t1, D
`30
`becomes equal to D and then it drops below this optimum
`value, thereby causing the output current to decrease, and thus
`causing IR to decrease. At instant t2, IR has dropped far
`enough below its maximum value to trigger reinitialization of
`the peak detector PK and to generate a pulse at the output of
`35
`CP1. At instant t3, this pulse cause the bistable B1 to change
`state: the output Q goes to the high level. V
`begins to
`decrease linearly, Dagain returns towards D and the out
`put current Io from the converter begins to increase again. In
`this way, the output current Io oscillates about its maximum
`40
`value, which corresponds to the fact that the operating point
`of the photovoltaic generator oscillates about its maximum
`power point (MPP1, MPP2 in FIG. 1). The frequency of the
`oscillation depends in particular on the capacitance of the
`capacitor C, in the example of FIG. 3, this frequency is
`equal to about 150 hertz (Hz).
`From a more general point of view, it should be considered
`that the assembly comprising the bistable B1 and the integra
`tor INT constitutes means for generating a control signal
`V
`that is time-dependent, and that Switches between a
`state in which it increases over time and a state in which it
`decreases over time in response to the Switching signal S3
`generated by the comparator CP1. Terms such as “increasing
`and “decreasing should be understood broadly: in other
`embodiments of the invention, VA
`could present fre
`quency modulation, for example: under Such circumstances,
`it would be its frequency that increases and decreases, and not
`its amplitude. The essential point is that after receiving the
`Switching signal, the operating point of the generator changes
`the direction in which it travels along the characteristic curve.
`On a close examination of FIG. 3, it can be seen that the
`output current Io and the signal IoRo do not begin to increase
`immediately after the bistable B1 has switched. This effect is
`due mainly to the fact that the transfer function I/D of the
`voltage converter APR presents a right-half-plane Zero effect
`at high frequencies. Because of this delay in the response of
`Io it necessary to ensure that the capacitor C does not
`
`45
`
`50
`
`55
`
`60
`
`65
`
`US 7,719,866 B2
`
`5
`
`10
`
`15
`
`25
`
`8
`recharge too quickly, so that the Voltage at this terminal can
`not "catch up with the signal IR while it is still decreasing,
`since that would lead to untimely switching of the bistable.
`By way of example, a satisfactory charging time has been
`obtained using a capacitor C having a capacitance of 220
`nanofarads (nF) and charging it through a resistance of 1
`kilohm (kS2) connected in series with the peak detector PK.
`The person skilled in the art can easily modify these values in
`order to adapt them to various embodiments of the invention.
`A signal MPPT-INAC can be applied to the “SET'input of
`the bistable B1, thereby forcing its Q output to the high value.
`Consequently, V
`decreases until the integrator INT
`becomes negatively saturated: MPPT-INAC is thus a signal
`for inactivating the control circuit CT. Naturally, in another
`embodiment of the invention, the circuit CT could be inacti
`vated by bringing Veto its positive saturation level.
`In practice, the control circuit CT forms part of a control
`module for a photovoltaic generator. It is not always neces
`sary to extract maximum power from each generator, so it is
`necessary to provide another control circuit suitable for
`adjusting the current that is generated. In addition, auxiliary
`functions of reinitializing and activating/deactivating the cir
`cuit CT are needed in order to ensure that the module operates
`properly under all circumstances.
`The control module shown in FIG. 4 includes in particular
`a current-control circuit C COND that enables the DC/DC
`converter APR to be controlled in linear manner as a function
`of the potential of the power supply bus bar (referenced BDP).
`In other words, the current-control circuit C COND gener
`ates a second control signal V, for the driver circuit
`constituted by the comparator CP2 and the driver stage CRV.
`such that the output current I from the converter APR is
`maintained at a target level, which level rises with decreasing
`potential of the power Supply bus bar (and thus of V). In
`particular, it is advantageous to use a current-control circuit
`based on the conductivity control principle as described for
`example in the article by D. O. Sullivan, H. Spruijt, A.
`Crausaz, published in ESA Journal 1989, Vol. 13, pp. 33-46,
`and in particular because of the width of its passband; never
`theless, other control techniques could validly be used. Such
`as peak current control mode. The current-control circuit
`receives as an input a Voltage signal V that is proportional
`to the difference between the potential of the power supply
`bus bar BDP and a reference signal. An error amplifier Ac
`compares V
`with a signal Is Rs that is proportional to the
`current passing through the controlled Switch of the Voltage
`converter APR. The signal V, taken from the output of
`the amplifier Ac is delivered to the input of the comparator
`CP2 that determines duty cycle D of the signal driving the
`converter APR. In practice, the duty cycle D decreases with
`decreasing V, thus having the effect of increasing