IES20100461A2 - A control system for PWM-based DC-DC converters - Google Patents
A control system for PWM-based DC-DC convertersInfo
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- IES20100461A2 IES20100461A2 IES20100461A IES20100461A2 IE S20100461 A2 IES20100461 A2 IE S20100461A2 IE S20100461 A IES20100461 A IE S20100461A IE S20100461 A2 IES20100461 A2 IE S20100461A2
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Classifications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
- H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
- H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
- H02M3/157—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators with digital control
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M3/00—Conversion of dc power input into dc power output
- H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
- H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
- H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
Abstract
A control system for PWM-based DC-DC converters. A control circuit for a system comprising an input source based on pulse width modulation, PWM, that includes, input average current control, for conveying the power generated by the input source to the load. The control system comprises a digital feedback circuit which generates a first control signal related to the voltage and the current at the input or the output of the DC-DC converter and an analog feedback circuit which processes the first control signal and the voltage and current values of the input source to generate a second control signal for a PWM generator within the converter. <Figure 5>
Description
Field of the Invention
This invention relates generally to the architecture of a control system for PWM-based DC-DC converters with average current control, used for conveying and maximising the power generated by an input source characterised by a maximum power point, such as a solar panel or fuel cell, to a load of a predominantly capacitive nature, siichns_a_nassive capacitor, or super-capacitor, accumulators and rechargeable batteries. i n
Background to the Invention
One common technique for transferring electrical power from a DC input source to electrical power having different parameters but of the same DC type is through the use of a switch-mode DC-DC converter based on the Pulse Width Modulation (PWM) technique. The converter is placed in a feedback loop built around a high-gain difference amplifier, and receives an input voltage from the source, as Is shown in Figure 1. The amplifier compares the output voltage of the converter with a reference voltage, and derives the voltage applied to the feedback control input of the converter. The duty cycle of the switching elements within the converter is determined by the voltage applied to its feedback input. Thus, the feedback loop controls the power delivered by the DC-DC controller. By virtue of negative feedback, the voltages applied to the difference amplifier are made equal. Hence, the DC-DC converter operates as a controlled voltage source at the output, delivering a voltage equal or proportional to the reference voltage, Vref·
A PWM-based converter placed in the configuration shown in Figure 1 acts as a constant voltage source at its output, while the value of its equivalent input impedance is not directly related to the input source impedance. Therefore, this typical configuration is not suitable for a solar-based (or fuel cell-based) charger of capacitive loads or rechargeable batteries, due to the fact that such loads require a converter that acts as a constant current source at their output, and the fact that the power harvested from a PV cell depends on the matching between the input impedance of the DC-DC converter and the equivalent impedance of the PV cell, being a maximum when the two impedances are equal.
In general, the goal of a solar panel controller should be to maximize its transfer efficiency whilst minimizing the power and consumption and system complexity overheads associated with it. This is especially true for small or micro-solar PV arrays.
if 1 0 0 4 6 1 as their capacity to harvest power is inherently limited by their size. The characteristic power curve of a PV cell or array is shown in Figure 2. It is known that the power versus voltage (P-V) curve in solar cells has a unique maximum at a particular operating voltage. Furthermore, it is known that for a typical solar panel, as its operating conditions change, such as for example as levels of solar insolation change or the ambient temperature of the panel changes, the voltage (Vmpp) or current (Impp) at which the panel should operate to obtain its maximum power output will also change. In addition, for a fixed temperature and solar insolation level, other factors such as panel aging and partial shading can affect the location of the maximum power point.
Considering that PV modules have relatively low conversion efficiency, it is imperative that the maximum possible harvested power is extracted. This may be achieved by the use of maximum power point trackers (MPPT) to track this maximum peak power, in order to ensure that the maximum possible power is always available for delivery to the load.
A number of different schemes may be used to implement maximum power tracking. They can be generally categorized as computational methods, look up table (LUT) methods and perturb and observe (P&O) methods.
In computational methods, the solar cell open circuit voltage (Voc) or short circuit current (Isc) is measured, and used along with an equation based model of the solar cell to predict the maximum power point for a variety of operating conditions. To ensure model accuracy, computational methods require pre-characterization of the PV array. In practice, model inaccuracies and panel deviations mean that a ‘one size fits all’ approach to characterization may not yield optimum results. In addition, a wide variety of conditions affecting real-world panel deployment are not accounted for by computational methods, such as panel aging, scratching and dirt accumulation. Fuzzy logic and neural network based computational methods require a priori knowledge of the deployed PV panel. In addition, Fuzzy logic MPPT solutions depend heavily on the power converter being used, and their performance is very sensitive to the rule-based table employed. Neural network based MPPT solutions require extensive training on the selected type of PV panel, and also need to be re-trained as the panel ages.
Look up table methods based upon the empirical relationship between the solar cell open circuit voltage (Voc) or short circuit current (Isc) and the cell maximum power point have been proposed. However, these solutions come with an inbuilt efficiency penalty. Accurate tracking requires frequent measurement, particularly in rapidly changing operating conditions. While efforts have been made to overcome the if ί 0 0 4 β 1 efficiency shortcomings of LUT based methods, such as by the use of a pilot PV panel, notionally identical to the main PV array, whose open circuit voltage (Voc) or short circuit current (Isc) is constantly measured and used to derive the MPP of the main PV array, this is completely conditional on the matching between the pilot and the main array, not only from the actual matching of the electrical characteristics of the pilot and the PV panel, but also from the matching of their respective operation.
The ‘Perturb and observe’ method employs direct measurement of the operating points of the PV panel as inputs to an algorithm which subsequently drives the panel to its MPP. This method varies the operating current and voltage of the PV array for observation and correction. Although this scheme is blindly adaptive and tracks in real time changes in the operating condition of the panel, it does suffer from inaccuracies in measurement, particularly the measurement of the input current. In addition, the response time of this algorithm can be poor, being limited by the need to balance an agile system dynamic response (especially in conditions of rapidly changing illumination) with a small resolution of ‘perturb’. As the system tracks the MPP, it hunts about within a limit defined by the smallest resolution of voltage step which the system can tolerate without being corrupted by noise. A small step is obviously desirable to maximize system efficiency, however such a small step is contrary to the requirements imposed by tracking under rapidly changing conditions, and can hinder the dynamic response in these cases. In addition, the P&O based method can fail in conditions of rapidly changing illumination.
The use of current mode control in switched mode power supplies is well established and its benefits are clearly understood. A current mode control converter uses a current feedback loop in addition to voltage feedback (the output voltage error signal) as input signals to the PWM modulator. With reference to Figure 3, the inductor current is sensed (through sensing gain Ri), and it is compared to a control voltage Vc (compensator Hc(s) output), which is derived from the output voltage error, with respect to a reference voltage Vref- The circuit implementation of Figure 4 shows that the PWM output comparator will output high (corresponding to the power MOSFET switch being on) until the sensed inductor current equals the control voltage. As soon as this equality is met, the PWM comparator goes low and turns the switch off. In this way, the current of the inductor is precisely controlled by the control voltage through the passing of duty cycle information to the power stage (comprising the output capacitor, inductor and power switches). Intuitively, the current loop causes the converter to act like a current source at the output, supplying current to the output capacitor and load, transforming a second order system into effectively a single pole system.
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The use of current mode control impacts on the performance of the system in three ways. Firstly, the transformation of the converter to effectively a current source at the output provides increased immunity to input voltage disturbance, and thus gives superior line regulation. For application to solar panel systems, this leads to more precise control of the output load current. Secondly, it is much easier to parallel current sources into a load rather than parallel voltage sources. This characteristic makes a modular, extendable solar panel system topology to a common load straightforward, particularly in the case of super-capacitors. Thirdly, inherent current limit control of the power switch is easily implemented, providing protection and immunity from short circuit outputs or overload conditions. This can be achieved by simply clamping the control voltage. In addition, the transformation from a complex second order system to a first order system simplifies the problem of ensuring the system stability.
The two main types of current-mode controls are peak and average, aimed at regulating the peak or the average value respectively of the current through the main inductor within the DC-DC converter. In general, peak current mode control suffers from a number of disadvantages when compared to average current mode control. Firstly, it is extremely susceptible to noise. For application to a solar panel system such as a solar harvester, such degraded immunity to noise will reduce the system efficiency, particularly in low luminosity conditions (corresponding to when Vjn is small). Subharmonic oscillation will further degrade the efficiency through MPP deviation, as the peak current mode control method is inherently unstable at duty cycle ratios exceeding 0.5, resulting in sub-harmonic oscillation. A compensating ramp is usually applied to the comparator input to eliminate this instability. However, for solar harvester applications, this stability restriction may hamper the system tracking dynamics and indeed may cause some MPP algorithms to fail altogether.
In contrast to peak current control, average current control provides for excellent noise immunity as, when the power switch is turned on, the oscillator ramp immediately dives to its lowest level. This characteristic will lead to more reliable operation and improved efficiency for a solar harvester, particularly under conditions of low luminosity. Furthermore, slope compensation is not required, but there is a limit to the loop gain at the switching frequency to guarantee stability. In the context of a solar harvester application, the dynamic performance can be optimized with much looser constraints than is the case with peak current control. In addition, the average current will track a reference set by the control system with a higher degree of accuracy.
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There are a number of disadvantages associated with conventional MPPT control systems. One disadvantage is that they usually require customized converters when used in DC-DC energy conversion for applications such as photovoltaic (PV) - based battery chargers. In order to perform MPPT at the input of such a converter, most systems impose special requirements to the converter that realizes the energy transfer between the input and the output of the system. This leads to low overall efficiency of such systems, significantly inferior to standard DC-DC converters. Another disadvantage of conventional MPPT control systems is that they have an inherent trade-off between precision and response time, leading to solutions that are either fast but inaccurate (usually analog), or accurate but having a slow response to sharp variation of the primary power source output, such as those corresponding to sudden and significant changes in illumination conditions of a PV cell. Furthermore, they are also often based on DC-DC converters employing peak current control. This approach inherently limits both the ability of the system to operate at the Maximum Power Point and the speed of its response to sudden changes in the primary source output. Finally, although the overall efficiency of an MPPT algorithm depends on specific operation conditions, such as the characteristic of the primary source, the DC-DC converter and the load, and the voltage and current levels at the input and output of the system, most MPPT control systems only implement one MPPT algorithm.
US Patent No. 5,604,430 in the name of Decker and International Patent Publication No. WO98/44398 in the name of Lukens are primarily concerned with controlling the current delivered to the load by specific solar-based DC-DC converters (in the case of Decker, in order to precisely supply a load current to an arcjet thruster/system ioad within the confines of a satellite application environment) whilst simultaneously operating the solar panel at, or close to, its maximum power point. It is clear that, as neither the Decker or Lukens topologies are targeted at PWM -based SMPS applications, that the use of average current control is not relevant for MPP tracking. Furthermore, although both the Decker and Lukens power architectures can vary the current delivered to the load, they can only do so by moving the panel away from the MPP. The Decker topology employs P&O MPPT, while the Lukens topology employs RCC (Ripple Correlation Control) MPPT. In addition, both the Decker and Lukens topologies regulate the output power.
The technique described in C. Alippi and C. Galperti “Adaptive System for Optimal Solar Energy Harvesting in Wireless Sensor Network Nodes”, IEEE Trans. Circuits Syst.I, Reg. Papers, vol, 55, no. 6, July 2008 is a boost (step up) converter topology only. In addition, the Alippi scheme uses peak current control to regulate the current.
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Furthermore, the MPPT scheme employed by Alippi is Ripple Correction Control (RCC), which severely constrains the MPP tracking bandwidth, and consequently will reduce efficiency. The Alippi topology charges a NiMH battery, but Is unsuitable for LiIon charging, as there is no mechanism to control and optimise the charging current as the operating conditions of the battery varies. The Alippi scheme can only power track when the output battery voltage is sufficiently high to power the 3V DC-DC converter slaved from the self power point. Alippi uses blind current sweeping to initialise the panel. The Alippi topology also regulates the output power, being the power delivered to the battery.
Object of the Invention
The present invention is concerned with providing an improved MPPT control system for PWM-based DC-DC converters with average current control, which is suitable for use with a primary input source characterized by a maximum power point, such as a solar panel or a fuel cell, which overcomes the shortcomings of the MPPT systems described above.
Summary of the Invention
The present invention provides a control circuit for a system comprising an input source characterized by a maximum power point, a load, and a DC-DC converter based on pulse width modulation, PWM, that includes input average current control, for conveying the power generated by the input source to the load, the control system comprising:
a digital feedback circuit which generates a first control signal related to the voltage and the current at the input or the output of the DC-DC converter; and an analog feedback circuit which processes the first control signal and the voltage and current values of the input source to generate a second control signal for a PWM generator within the converter.
As the analog feedback circuit is always active, it is able to respond very fast to sudden changes in the operation conditions of the input source. Thus, the analog feedback circuit is able to bring the operating point of the DC-DC converter close to the target operating point without having to wait for the digital feedback circuit to respond to new operating conditions. As the digital feedback circuit starts to react to the change in operating conditions, it will then bring the operating point of the DC-DC converter to the target operating point. As a result, the dynamic integral efficiency is maximised.
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The first control signal may be related to the voltage and the current at the input and the output of the DC-DC converter.
The digital feedback circuit may comprise a programmable digital controller and a digital to analog converter, wherein the programmable digital controller processes the voltage and the current levels generated by the input source or the voltage and current levels provided to the load in accordance with a predefined maximum power point tracking algorithm to generate a digital signal, wherein the digital signal is converted by means of the digital to analog converter to provide the first control signal.
The analog feedback circuit may comprise a voltage error amplifier and a current error amplifier, wherein the voltage error amplifier provides as an output a signal corresponding to an amplified difference between the first control signal and a voltage proportional to the voltage provided by the input source, and wherein the current error amplifier generates the second control signal based on a combination of the signal output from the voltage error amplifier and a signal proportional to the average current provided by the input source.
The voltage proportional to the voltage provided by the input source may be generated by a voltage sensor, and the current proportional to the current provided by the input source may be generated by a current sensor.
The voltage error amplifier may comprise a voltage-to-voltage amplifier, such as an operational amplifier, and a negative feedback network having a frequency dependent transfer function.
The current error amplifier may comprise an operational amplifier with a negative feedback network that realizes a proportional-integrative transfer function.
Preferably, the predefined maximum power point tracking algorithm comprises an algorithm that is based on the present and previous values of the voltage and current at the input or output of the DC-DC converter.
Preferably, the programmable digital controller can switch between a plurality of userselectable predefined maximum power point tracking algorithms without interruption of the operation of the converter.
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The maximum power point tracking algorithm may comprise the perturb and observe algorithm.
The load may be a capacitive load.
The capacitive load may comprise one of: a passive capacitor, a supercapacitor, an accumulator or a rechargeable battery.
The input source may comprise at least one photovoltaic cell, or at least one fuel cell.
The control circuit may further comprise a current distributor located between the output of the DC-DC converter and the input to the load, the current distributor controlling the amount of current conveyed from the converter to the load.
The control circuit may further comprise a power supply block for supplying power to the circuit, the power supply block having a selection means for selecting whether power is supplied by the DC-DC converter or by an external power source.
The programmable digital controller may further maintain the output voltage of the system between set limits by means of turning on and off the DC-DC converter or by employing current re-distribution to one or more auxiliary loads via the current distributor.
The control circuit may further perform one or more of: load monitoring, data logging, self-testing, fault detection or data transfer to an external device.
The present invention may also provide a method for controlling a system comprising an input source characterized by a maximum power point, a load, and a DC-DC converter based on pulse width modulation, PWM, that includes input average current control, for conveying the power generated by the input source to the load, the method comprising the steps of:
generating by digital means a first control signal related to the voltage and the current at the input or the output of the DC-DC converter; and processing by analog means the first control signal and the voltage and current values of the input source to generate a second control signal for a PWM generator within the converter.
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The present invention may also provide a computer readable medium comprising instructions that upon execution cause a computer to perform the steps of the method.
Brief Description of the Drawings
FIGURE 1 is a block diagram of a typical system for DC-DC electrical power conversion based on PWM DC-DC converter;
FIGURE 2 is an illustration of the variation of PV maximum power point with solar irradiation and temperature;
FIGURES 3 and 4 are illustrations of the current-mode control of switched-mode DCDC converters;
FIGURE 5 is a block diagram of the control system of the invention for PWM-based DC-DC converters with average current control;
FIGURE 6 shows a block diagram of the embodiment of the control system applied to a standard boost (step-up) DC-DC converter;
FIGURE 7 is an illustration of the I-V characteristics of the PV panel/cell for two levels of illumination;
FIGURE 8 contains implementation details for the control system applied to a standard buck (step-down) DC-DC converter;
FIGURE 9 is an illustration of a power-versus-voltage curve corresponding to a PV cell for a given set of conditions; points A, B and C indicate three possible operating points of the system when following a Perturb and Observe (P&O) MPPT algorithm; and
FIGURE 10 presents the flow-chart of an improved P&O algorithm that implements a three point weight comparison in order to increase stability and it is able to dynamically adjust the size of the Perturbation step, resulting in faster convergence to the MPP.
Detailed Description of the Drawings
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The control system of the invention for PWM-based DC-DC converters with average current control will now be described with reference to the accompanying figures, It is specifically targeted at SMPS application. It can use any maximum power point tracking algorithm that can be implemented by digital programming, related to either the input power (that is, the power transferred from the primary source to the DC-DC converter) or the output power (that is, the power delivered to the load). It also uses average mode current control to improve the noise performance of the system. This is particularly relevant in micro applications, to ease the stabilization requirements for the system as a whole, and also protect the inductor from saturation. It should be noted that the MPP tracking capability is ‘always on’, whilst still providing for output current control. As a result, it is suitable to charge a wide variety of batteries and loads, including hybrid schemes incorporating super capacitors and parallel charging schemes.
The invention can also close the power loop around either the input or output power, to maximise system efficiency rather than panel efficiency, depending on the efficiency characteristics of the converter chosen,
The invention can also be applied to most types of DC-DC PWM-based converters, including, but not restricted to, the standard boost and buck converter types. The only requirement is that the DC-DC converter employs the average input current control, that is, it comprises a (or can work with an external) feedback loop that regulates the average value of the current taken from the input power source. The inherent efficiency of the DC-DC converter is not affected by the control loop. In fact, it has been shown that it is substantially the same as that achieved by the converter when used in a standard configuration, such as the one presented in Figure 1.
The converter of the present invention is controlled through a feedback amplifier that combines the outputs of two control loops. The two control loops comprise an analog loop, based on the voltage at the input of the converter, and a digital loop, that takes into account the voltage and current levels at both the input and output of the converter. The analog loop provides a fast response to sudden changes of the primary source output. The digital loop enhances accuracy, and allows the implementation of various MPPT algorithms, depending on various factors, such as the parameters of the primary power source, environment conditions such as illumination and temperature, application35 specific requirements, and facilitates the integration of additional control and monitoring features. In one embodiment, the programmable digital controller can switch between a plurality of user-selectable predefined MPPT algorithms. These algorithms can be switched without interruption of the operation of the converter.
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The block diagram of one embodiment of the control system of the invention is shown in Figure 5, In this configuration the DC-DC converters operate as controlled current sources at the output while their input impedance is adjusted so that it matches the output impedance of the input source, for maximum power transfer. It consists of seven main functional sub-systems: a primary input source 1 (such as a PV cell or array of interconnected PV cells); a main DC-DC converter 2, which is a switched-mode DC-DC converter based on the Pulse-Width Modulation (PWM) method, and includes a control loop that regulates the average value of the input current; a load 3, which has a mainly capacitive nature, such as a rechargeable battery and/or supercapacitor; a control system 4, which includes an analog feedback loop that compares the voltage provided by the primary source 1 with a reference provided by a programmable digital controller based on the present and previous values of the voltage and/or current provided by the primary source 1 and/or delivered to the load 3; a current distributor 5, which takes in the current delivered by the main DC-DC converter - which works as a controlled current source in this topology - and delivers it entirely or partially to the load 3, under the command of the control system 4; monitoring circuitry 6, that senses and converts in digital format the voltage and current delivered to the load and feeds this information to the control system 4; and a power supply generator 7, which provides all necessary supply voltages within the system, including the control system 4, the current distributor 5 and the monitoring circuitry 6.
In use, the main DC-DC converter 2 takes in the power provided by the primary input source 1 and conveys it into the load 3 at different voltage and current levels. The converter is a standard switched-mode DC-DC converter based on the PWM method, and employs average current control. It comprises a core DC-DC converter 23, that includes at least one switching element and at least one coil, a PWM generator 22 and a current error amplifier 21. The signal controlling the switching element(s) within the core converter 23 - represented in Figure 5 by Vsw - is generated by the PWM generator 22. It is a square-wave with a duty cycle determined by the control signal represented in Figure 5 by Vpwm· In turn, Vpwm is generated by the Current Error Amplifier (CEA) 21, which is typically implemented by a proportional-integrative (PI) type regulator which controls the average value of the current provided by the primary input source 1. The CEA 21 takes in two signals: a signal proportional with the current provided by the primary source 1 - represented in Figure 5 by Ki*hu - and a signal applied to its FEEDBACK input - represented in Figure 5 by Vfb - that is provided by the control system,
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The control system 4 provides the signal applied at the FEEDBACK input of the main DC-DC converter 2, represented in Figure 5 by the voltage Vfb, which determines, in conjunction with the CEA 21, the duty cycle of the signal controlling the switching elements within the core DC-DC converter 23. The Vfb signal is generated by a Voltage Error Amplifier (VEA) 41, which amplifies the difference between a voltage proportional to the input voltage, Kv*Vin, and a control signal - represented in Figure 5 by Vctrl - provided by a Digital-to-Analog (DAC) converter 42, based on the control word applied to its inputs by a Programmable Digital Controller (PDC) 43. The PDC derives the control word — and thus the voltage Vctrl - following a control algorithm, such as a Maximum Power Point Tracking (MPPT) algorithm, that is pre-programmed into it. The voltage and current levels at the input of the system are sensed by blocks 44 and 45, and are made available to the PDC in digital format by Analog-to-Digital Converters 46 and 47, respectively. The monitoring circuitry 6 makes available to the PDC the voltage and current levels at the output of the system; they are sensed by blocks 61 and 62 and converted in digital format by analog-to-digital converters 63 and 64, respectively. Therefore, the PDC can implement various control algorithms based on the voltage and current levels at the input and or the output of the system, as well as additional monitoring and fail-safe procedures.
The current distributor 5 allows the system to limit the current delivered to the load 3 to a pre-programmed value while maintaining the operating point with respect to the primary source 1. In normal operation, the entire available current is delivered to the load 3. However, when Iout reaches a threshold set by the control system 4, a current switch 51 starts diverting the excess current away from load 3, and into the dummy load 52 or into a second, third, etc, load, illustrated in Figure 5 by outputs OUT2, ..., OUTn.
The power supply generator 7 provides the supply voitage(s) required by entire system, represented in Figure 5 by Vdd· It comprises two power conditioning blocks, 71 and 72, that take in power from the primary source 1 and from the load 3, respectively. A Vsupply Manager 73 chooses between these two possible supply providers and delivers the supply voltage Vdd. After start-up, the Vsupply Manager re-assesses periodically its decision regarding the most suitable power source for Vdd based on information provided by the PDC 43. This arrangement allows the system to operate without an external supply voltage irrespective of the actual voltage levels present at the input or output of the system in a particular application. For example, the system can be used to charge supercapacitors starting from a very low level (virtually from zero), as well as accumulators and rechargeable batteries over a wide range of voltage levels. The
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Vsupply Manager 73 ensures that the most energy-efficient power source is used for powering the system in all cases.
The control system can turn off and on the main DC-DC converter - a feature necessaiy for the start-up procedure. It may also provide self testing and fault testing functionality, In some embodiments of the invention, the control system may also include further functionality, such as load monitoring, data logging and data transfer to an external device. The implementation details of such features would be well known to a person skilled in the art, and are therefore not elaborated in further detail here.
A typical application for the control system of the present invention is that of highefficiency chargers for rechargeable batteries or supercapacitors based on PV cells or arrays or PV cells and implementing a MPPT algorithm. Figure 6 shows an embodiment of the control system of the invention driving a standard boost converter. In this case, the core DC-DC converter consists of a coil Ly, switching transistor My and diode Dy and realizes a step-up voltage conversion, that is, the targeted voltage level at the output of the converter, VOut, has a higher value than the voltage available at the input of the converter, Vin.
The PWM generator 22 is implemented by a voltage comparator and a signal generator that provides a saw-tooth voltage to one input of the comparator - marked in Figure 6 as OA22 and SG22, respectively. The Current Error Amplifier 21 is implemented by an operational amplifier with large open-loop voltage gain and a negative feedback network - marked in Figure 6 as OA21 and C21A-C21B-R21, respectively. Overall, this circuit realizes a proportional-integrative transfer function, the voltage it provides to the second input of the comparator within the PWM generator being a combination of the average value of the input current, Iin, and the voltage VFb applied to the noninverting input of the Operational Amplifier,
The Voltage Error Amplifier 41 is implemented by another operational amplifier with large open-loop voltage gain, and a negative feedback network which determines the frequency characteristics of the VEA and the stability of the entire system, marked in Figure 6 as OA41 and C41. The VEA 41 provides the voltage Vfb to the CEA 21, as a frequency-dependent combination of the input voltage, Vjn, and the voltage Vctrl supplied by the Digital-to-Analog converter 42 under the control of the Programmable Digital Controller 43.
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The software pre-programmed into the Programmable Digital Controller 43 implements a Maximum Power Point Tracking (MPPT) algorithm that aims to maximize the power harvested from the PV input source 1U, which is characterized by a Maximum Power Point (MPP).
Figure 7 shows the typical current-voltage (I-V) characteristics of the PV cell/panel that provides electrical power at the input of the system, for two levels of illuminations; the bottom curve corresponds to the lower level of illumination of the PV source. The aim of the control system is to ensure that the PV panel operates at its Maximum Power
Point (MPP) for every illumination level; represented in Figure 7 by points MPP1 and MPP2. This implies three steps:
1) bringing the operating point of the system (represented here by the levels of the voltage and current at its input) close to the MPP of the PV input source corresponding to the current level of illumination;
2) maintaining the operation of the system at, or around, this MPP position, as long as the output of the PV source remains constant; and
3) changing the operating point of the system when the PV source conditions change, for example due to illumination or temperature variations, so that the new operating point corresponds to the new MPP of the PV source.
The operation of the control system for each of these situations are described in detail below.
1). bringing the PV to operate at the MPP corresponding to the current illumination level
Let us assume that initially the operating conditions of the PV source correspond to the bottom I-V characteristic shown in Figure 7. At power-up the PDC 43 disables the main DC-DC converter 2U, by applying the appropriate level on the “Enable” control input of the converter. Therefore, the PV source will be unloaded or loaded only by the power conditioning block 71. The corresponding operating point of the system is represented in Figure 7 by point A. The resulting value of the input voltage is read by the PDC via the Input Voltage sense 44 and the corresponding ADC, 46. Based on this value, the PDC derives the first value of the control voltage Vctrl to be applied to the Voltage
Error Amplifier 41 via the DAC 42. Once the voltage Vctrl is generated, the PDC enables the main DC-DC converter 2U. As a result, the operating point of the system moves to the point marked in Figure 7 by point B.
IE 1 0 0 4 6 1
From this point on, the control system follows the MPPT algorithm pre-programmed into the PDC. In general, the MPPT algorithm implements an iterative search for the MPP: the next value of the control voltage Vctrl is decided based on the input power levels (the product between the voltage and current levels at the input) corresponding to the present and previous operating points of the system. Alternatively, the output power can be monitored and maximised.
The Voltage Error Amplifier 41 has a very large voltage gain, so that the overall gain of the control loop is also very large. By virtue of negative feedback, the control loop will force the equality between the voltages applied to the inputs of the difference amplifier, so that the voltage delivered by the PV source will be forced to become equal to the reference voltage set by the PDC. Thus, the voltage generated by the PV source is forced towards the level Vrefi, which corresponds to the MPP1 of the PV source. This is accomplished by using the feedback voltage Vfb generated by the VEA as the reference for the Current Error Amplifier 21 within the main DC-DC converter 2U. Thus, the feedback voltage provided by the VEA determines the duty cycle of the switching transistor Mu within the core converter, so that it forces the average value of the input current to follow Vfb- This is equivalent to changing the equivalent input impedance of the converter. In fact, the effect of the control loop is to adapt the equivalent input impedance of the main DC-DC converter to the equivalent impedance of the PV source, ensuring maximum power transfer between the PV source and the DC-DC converter.
This way, the operating point of the system moves towards, and eventually reaches, the Maximum Power Point corresponding to the initial condition of the solar panel, marked
MPP1 in Figure 7. This process is relatively slow, as the operating point the PV cell starts from what may be relatively far from the wanted MPP, so that the control system will need several iterations to derive the suitable feedback voltage, Vfb- It will be appreciated that the actual time required to reach the MPP depends on the MPPT algorithm run on the PDC, the characteristics of the main DC-DC controller and the frequency characteristics of the analog feedback loop formed by the Input Voltage
Sense 44, the Voltage Error Amplifier 41, the Current Error Amplifier 21 and the input stage of the core DC-DC converter 23U.
2) maintaining the operation of the system at, or around, the MPP of the PV source as long as the conditions (illumination and temperature) remain constant
IE 1 Ο Ο 4 6 1
The system behavior depends on the MPPT algorithm programmed into the controller but, in general, the reference voltage will dither about the Vrefi level; as a result the system will operate around the MPP1 point.
3) reaching a new MPP when the conditions change
Let us now assume that a sudden change in the operating conditions occurs, so that the corresponding PV panel/cell characteristic is now represented by the top curve in Figure
7. The control voltage Vctrl applied to the Voltage Error Amplifier 41 by the DAC 42 will not change immediately, as the PDC 43 needs time to determine the appropriate response. However, as the analog control loop is always active, it forces the voltage at the input of the main DC-DC converter to remain at the previous value, that is Vren. Therefore, the operating point of the system is shifted immediately to the coordinates (I2, Vrefi), denoted point C in Figure 7. As the PDC 43 completes its calculations, it starts adjusting the control voltage applied to the difference amplifier through the DAC, leading the PV operating point towards the MPP corresponding to the new conditions, marked MPP2 in Figure 7.
As the analog feedback loop operates continuously, the difference between the operating point C (I2, Vren) and the MPP2 is relatively small, depending mainly on the speed the operating conditions of the PV cell change and the delay through the analog loop. Therefore, the system will reach the MPP corresponding to the new conditions, point MPP2, in a relatively short time. It should be appreciated that this time will be far faster than the time required for a digital-only system, for which the control voltage of the DC-DC converter is derived solely by a digital controller.
Another embodiment of the high-efficiency charger for rechargeable batteries or supercapacitors based on PV cells or arrays and using the control system of the present invention is presented in Figure 8, in the form of a standard buck (step down) converter,
In this case, the core DC-DC converter consists of a coil Lp, switching transistor M[> and diode Dd, arranged in a standard buck topology that realizes a step-down voltage conversion, that is, the targeted voltage level at the output of the converter, Vout, has a lower value than the voltage available at the input of the converter, Vnq.
The PWM generator 22 is implemented by a voltage comparator and a signal generator that provides a saw-tooth voltage to one input of the comparator, marked in Figure 8 as OA22 and SG22, respectively.
IE 1 Ο Ο 4 6 1
The Current Error Amplifier 21 is implemented by an operational amplifier with large open-loop voltage gain and a negative feedback network, marked in Figure 8 as OA21 and C21A-C21B-R21, respectively. Overall, this circuit realizes a proportionalintegrative transfer function, as the voltage it provides to the second input of the comparator within the PWM generator, OA22, being a combination of the average value of the input current, , and the voltage Vfb applied to the noninverting input of the operational amplifier OA21.
The Voltage Error Amplifier 41 is implemented by another operational amplifier with large open-loop voltage gain and a negative feedback network, marked in Figure 8 as OA41 and C41, which determines the frequency characteristics of the VEA and the stability of the entire system. The VEA provides the voltage Vfb to the CEA 21, as a linear (but frequency-dependent) combination of the input voltage, Vjn, and the voltage Vctrl supplied by the Digital-to-Analog converter 42 under the control of the
Programmable Digital Controller 43.
The software pre-programmed into the Programmable Digital Controller 43 implements a Maximum Power Point Tracking (MPPT) algorithm that aims to maximize the power delivered to the load, taking into consideration the efficiency of the core DC-DC converter and the fact that the PV input source 1 is characterized by one - and only one - Maximum Power Point. By comparing Figures 6 and 8 it becomes obvious that the circuit implementation of the control system is essentially the same in both cases.
In another embodiment of the invention, an enhanced version of the Perturb and
Observe (P&O) MPPT algorithm is used instead of the standard point-by-point P&O MPPT algorithm. This algorithm can have either fixed or variable step.
This is described with reference to Figures 9 and 10. Figure 9 presents the powerversus-voltage curve corresponding to a PV cell for a given set of conditions; points A,
B and C indicate three possible operating points of the system when following a P&O MPPT algorithm. The curve has a maxima (the maximum power point, MPP) which corresponds to the point marked A in Figure 9. The goal is to reach and maintain the operating point of the system at, or very close around, the MPP, over a wide range of luminosity and temperature conditions. Obviously, the time necessary to reach MPP has to be as short as possible.
Let us assume that the PV array is operating at point A. In the standard P&O algorithm, the operating voltage of the PV array is perturbed by a small increment, the resulting
IE 1 0 0 4 6 1 change in power, ΔΡ, is measured and it leads to a decision regarding the next operating point of the array as follows:
- If ΔΡ > 0, it means that the perturbation of the operating voltage has moved the operating point of the PV array closer to the MPP. Thus, the operating point of the array following the perturbation is maintained and further voltage perturbations are made in the same direction (that is, with the same algebraic sign), assuming this should move the operating point towards the MPP.
- If ΔΡ < 0, it means that the system operating point has moved away from the MPP.
Therefore the system is brought back to the initial operating point, that is the one before applying the perturbation, and the algebraic sign of the next perturbation is reversed.
The flow chart of the enhanced P&O algorithm is presented in Figure 10. It implements a three point weight comparison in order to increase stability. At set time intervals the ] 5 control system perturbs the converter with both upwards and downwards excitations, and compares the resulting PV output power on three points of the V-P curve. The three points are: the current operation point (represented in Figure 9 by point A), and the result of the downward and upward perturbations (represented in Figure 9 by points B and C, respectively).
If the power corresponding to point A is greater than or equal to that of point B, the status is assigned a positive weighting, Otherwise, the status is assigned a negative weighting. Similarly, if the power corresponding to point C is smaller than that of point A, the status is assigned a positive weighting, otherwise, the status is assigned a negative weighting. Of the three measured points, if two are positively weighted, the duty cycle of the converter should be increased. On the contrary, when two are negatively weighted, the duty cycle of the converter should be decreased. In the case of one positive and one negative weighting, the system assumes that either the MPP has been reached or the solar radiation has changed rapidly, and it will not change the control signal.
The size of the perturbation step is dynamically changed as follows: if there are two positive/negative consecutive perturbations of the control signal, then the size of the perturbation step is increased. This only happens if the current operating point is far from the wanted MPP. If there is only one positive/negative perturbation, and after that a negative/positive perturbation takes place, or the operating point does not change, the size of the perturbation step is decreased.
IE 1 Ο Ο 4 61
The words “comprises/comprising” and the words “having/including” when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.
Claims (20)
1. A control circuit for a system comprising an input source characterized by a maximum power point, a load, and a DC-DC converter based on pulse width modulation, PWM, that includes input average current control, for conveying the power generated by the input source to the load, the control system comprising: a digital feedback circuit which generates a first control signal related to the voltage and the current at the input or the output of the DC-DC converter; and an analog feedback circuit which processes the first control signal and the voltage and current values of the input source to generate a second control signal for a PWM generator within the converter.
2. The control circuit of Claim 1, wherein the first control signal is related to the voltage and the current at the input and the output of the DC-DC converter.
3. The control circuit of Claim 1, wherein the digital feedback circuit comprises a programmable digital controller and a digital to analog converter, wherein the programmable digital controller processes the voltage and the current levels generated by the input source or the voltage and current levels provided to the load in accordance with a predefined maximum power point tracking algorithm to generate a digital signal, wherein the digital signal is converted by means of the digital to analog converter to provide the first control signal.
4. The control circuit of any of Claims 1 to 3, wherein the analog feedback circuit comprises a voltage error amplifier and a current error amplifier, wherein the voltage error amplifier provides as an output a signal corresponding to an amplified difference between the first control signal and a voltage proportional to the voltage provided by the input source, and wherein the current error amplifier generates the second control signal based on a combination of the signal output from the voltage error amplifier and a signal proportional to the average current provided by the input source.
5. The control circuit of Claim 4, wherein the voltage proportional to the voltage provided by the input source is generated by a voltage sensor, and the current proportional to the current provided by the input source is generated by a current sensor.
6. The control circuit of Claim 4, wherein the voltage error amplifier comprises a voltage amplifier and a negative feedback network having a frequency dependent transfer function. IE 1 0 0 4 6 1
7. The control circuit of Claim 4, wherein the current error amplifier comprises an operational amplifier with a negative feedback network that realizes a proportionalintegrative transfer function.
8. The control circuit of any of Claims 3 to 7, wherein the predefined maximum power point tracking algorithm comprises an algorithm that is based on current and previous values of the voltage and current at the input or output of the DC-DC converter.
9. 10 9. The control circuit of any of Claims 3 to 8, wherein the programmable digital controller can switch between a plurality of user-selectable predefined maximum power point tracking algorithms without interruption of the operation of the converter. 10. The control circuit of Claim 8, wherein the maximum power point tracking 15 algorithm is the perturb and observe algorithm.
10. 11. The control circuit of any of Claims 1 to 10, wherein the load is a capacitive load.
11. 12. The control circuit of Claim 11, wherein the capacitive load comprises one of: a 20 passive capacitor, a supercapacitor, an accumulator or a rechargeable battery.
12. 13. The control circuit of any of Claims 1 to 12, wherein the input source comprises at least one photovoltaic cell, or at least one fuel cell. 25
13. 14. The control circuit of any of Claims 1 to 13, further comprising a current distributor located between the output of the converter and the input to the load, the current distributor controlling the amount of current conveyed from the converter to the load,
14. 15. The control circuit of any of Claims 1 to 14, further comprising a power supply 30 block for supplying power to the circuit, the power supply block having a selection means for selecting whether power is supplied by the DC-DC converter or by an external power source.
15. 16. The control circuit of Claim 14, wherein the programmable digital controller further 35 maintains the output voltage of the system between set limits by means of turning on and off the DC-DC converter or by employing current re-distribution to one or more auxiliary loads via the current distributor. IE 1 0 0 4 6 1
16. 17. The control circuit of any of Claims 1 to 16, wherein the control circuit further performs one or more of: load monitoring, data logging, self-testing, fault detection or data transfer to an external device.
17. 18. A method for controlling a system comprising an input source characterized by a maximum power point, a load, and a DC-DC converter based on pulse width modulation, PWM, that includes input average current control, for conveying the power generated by the input source to the load, the method comprising the steps of: generating by digital means a first control signal related to the voltage and the current at the input or the output of the DC-DC converter; and processing by analog means the first control signal and the voltage and current values of the input source to generate a second control signal for a PWM generator within the converter.
18. 19. A computer readable medium comprising instructions that upon execution cause a computer to perform the steps of the method of Claim 18.
19.
20. A control circuit substantially as described herein with reference to and/or as illustrated in the accompanying Figures 5 to 10.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IES20100461 IES20100461A2 (en) | 2010-07-22 | 2010-07-22 | A control system for PWM-based DC-DC converters |
| PCT/EP2011/062389 WO2012010613A1 (en) | 2010-07-22 | 2011-07-19 | Improved mppt- control for pwm - based dc -dc converters with average current control |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IES20100461 IES20100461A2 (en) | 2010-07-22 | 2010-07-22 | A control system for PWM-based DC-DC converters |
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| IES20100461 IES20100461A2 (en) | 2010-07-22 | 2010-07-22 | A control system for PWM-based DC-DC converters |
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| WO (1) | WO2012010613A1 (en) |
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| CN116107383A (en) * | 2023-04-12 | 2023-05-12 | 华南理工大学 | Maximum power point tracking circuit applied to piezoelectric energy collection interface |
Families Citing this family (13)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN102609030B (en) * | 2012-03-15 | 2015-04-15 | 大连海事大学 | Maximum power point tracking device of marine photovoltaic power generation system |
| AT513941B1 (en) * | 2013-01-15 | 2018-03-15 | Siemens Ag Oesterreich | On-board unit with microcontroller and accumulator |
| KR20160047344A (en) | 2014-10-22 | 2016-05-02 | 주식회사 엘지화학 | Apparatus and method for controlling current |
| US10486836B2 (en) | 2016-11-10 | 2019-11-26 | Hamilton Sundstrand Corporaration | Solar powered spacecraft power system |
| US10110000B2 (en) | 2017-02-27 | 2018-10-23 | Hamilton Sundstrand Corporation | Power management and distribution architecture for a space vehicle |
| US10488879B2 (en) | 2017-03-09 | 2019-11-26 | Ecole Polytechnique Federale De Lausanne (Epfl) | Device and method for performing maximum power point tracking for photovoltaic devices in presence of hysteresis |
| CN107168450A (en) * | 2017-06-14 | 2017-09-15 | 刘博文 | A kind of solar energy power generating MPPT maximum power point tracking control system and method |
| CN107734613B (en) * | 2017-10-17 | 2021-03-16 | 北京小米移动软件有限公司 | Power management method and device |
| US20220255097A1 (en) * | 2019-07-17 | 2022-08-11 | Universitat De Barcelona | Self-powered system and method for power extraction and measurement of energy-generator units |
| CN111666658B (en) * | 2020-05-15 | 2023-06-23 | 中国能源建设集团甘肃省电力设计院有限公司 | Simplified model modeling method for multiple photovoltaic power generation devices in large photovoltaic power generation system |
| US11567551B2 (en) | 2020-07-28 | 2023-01-31 | Rohde & Schwarz Gmbh & Co. Kg | Adaptive power supply |
| CN113690962B (en) * | 2021-07-29 | 2024-04-05 | 广东博力威科技股份有限公司 | MPPT control method, device, equipment and storage medium for different input sources |
| DE102022209547A1 (en) | 2022-09-13 | 2024-03-14 | Zf Friedrichshafen Ag | Method for determining a control signal for controlling a switch of a boost converter, power supply system for a vehicle and vehicle |
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| US5604430A (en) | 1994-10-11 | 1997-02-18 | Trw Inc. | Solar array maximum power tracker with arcjet load |
| US5923100A (en) | 1997-03-31 | 1999-07-13 | Lockheed Martin Corporation | Apparatus for controlling a solar array power system |
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| CN116107383A (en) * | 2023-04-12 | 2023-05-12 | 华南理工大学 | Maximum power point tracking circuit applied to piezoelectric energy collection interface |
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