Claims:We Claim:
1. A solar panel charging system (100) comprising of:
a solar panel (105);
a converter module (110) communicatively connected to said solar panel (105);
a feedforward loop (125) including a signal inverting circuit (120) communicatively connected to said solar panel (105) and said converter module (110);
wherein, said signal inverting circuit (120) includes a comparator module (210), an invertor (205) and a pulse width modulation (PWM) controller (215);
a battery (115) receiving power from said solar panel (105) through said convertor module (110);
wherein, an output voltage (Vo) is drawn from said convertor module (110);
wherein, a feedback input for said output voltage (Vo) is received by said convertor module (110);
wherein, on comparison of said output voltage (Vo) with a desired voltage (Vd), if said output voltage (Vo) is less than said desired voltage (Vd); an input voltage (Vin) generated by said solar panel (105) is received by said convertor module (110) and said signal inverting circuit (120);
wherein, said comparator module (210) of signal inverting circuit (120) on receiving said input voltage (Vin) compares said input voltage (Vin) with a reference voltage Vref and generates an error value E;
said error value (E) is sent to said invertor (205) to generate an inverted voltage Vin’;
Wherein, said pulse width modulation unit (215) receives input from said invertor (205) and generates a modulated voltage signal (P); said modulated voltage signal (P) is sent as input to said convertor module (110),
Wherein, modulated voltage signal (P) is then added to said input voltage (Vin) therefore resulting in said output voltage (Vo) equal to said desired voltage (Vd), ensuring extraction of maximum power from said solar panel (105).
2. The invention as claimed in claim 1, wherein, convertor module (110) compares said output voltage (Vo) with a desired voltage (Vd); said desired voltage (Vd) is a standard output voltage (i.e. maximum voltage) of said solar panel at any point of time and said desired voltage (Vd) is in the range of 48V-60V.
3. The invention as claimed in claim 1, wherein, said PWM controller (215) generates said plurality of pulses (P) with a duty ratio (D) corresponding to said error value (E).
4. The invention as claimed in claim 1, wherein, said input voltage (Vin) is sensed using a resistor divider bridge.
5. The invention as claimed in claim 1, wherein, the relationship between said input voltage (Vin) and said duty ratio (D) is directly proportional, when said input voltage (Vin) decreases over a pre-determined reference voltage Vref, said duty ratio (D) decreases to maintain said desired voltage (Vd) corresponding to a maximum power (Pmax).
6. The invention as claimed in claim 1, wherein, said reference voltage Vref is the voltage supplied by said solar panel (105) to produce desired voltage (Vd) corresponding to a maximum power (Pmax).
7. A method of charging a solar panel, said method comprising:
checking if a battery (115) is connected to a solar panel charging system (100);
checking an output voltage (Vo) with respect to a desired voltage (Vd) via a converter module (110);
sensing an input voltage (Vin) from said solar panel (105), if said output voltage (Vo) is greater than said desired voltage (Vd);.
sending said input voltage (Vin) to a signal inverting circuit (120) and said signal inverting circuit (120) generating a modulated voltage signal (P);
comparing said input voltage (Vin) with a reference voltage (Vref) via a comparator module (210) to generate an error value (E);
sending an input signal to an invertor (205) from said comparator module (210) to generate an inverted voltage Vin’;
sending input to a pulse width modulation unit (215) from said invertor (205) and said pulse width modulation unit (215) generating a plurality of modulated signal voltage P with a duty ratio (D) corresponding to said error value (E);
adding said plurality of modulated signal voltage P to said input voltage (Vin) inside said converter module (110) therefore resulting in said output voltage (Vo) equal to said desired voltage (Vd).
, Description:TECHNICAL FIELD
[0001] The present subject matter relates generally to a vehicle. More particularly, the present invention relates to a system and method for a solar panel charging.
BACKGROUND
[0002] A solar vehicle, that is, a vehicle running entirely or partially by on-board solar energy harvesters is an implementable solution to the energy crisis that the world is likely to face in future. Conventional solar panels, such as, the silicon based solar panels may be employed for products or devices with mobility e.g. automobile applications due to high efficiency, less cost, and availability of the silicon based solar panels.
[0003] A solar charge controller, also known as a solar regulator, is essentially a solar battery charger connected between the solar panels and battery. It regulates the battery charging process to ensure that the battery is charged correctly, or more importantly, not over-charged. The direct current (DC) coupled solar charge controllers, have been around for decades and used in almost all small scale off-grid solar power systems. Simple PWM, or pulse width modulation solar charge controllers have a direct connection from the solar array to the battery, and use a basic rapid switch to modulate or control the battery charging. The switch (transistor) is open until the battery reaches the absorption charge voltage. Then the switch starts to open and close rapidly (hundreds of time per second) to reduce the current and maintain a constant battery voltage. The problem with this technology is that the solar panel voltage is pulled down to match the battery voltage. This in turn pulls the panel voltage away from its optimum operating voltage (Vmp) at which it generated maximum power output and reduces the efficiency of the solar panel.

BRIEF DESCRIPTION OF THE DRAWINGS
[0004] The detailed description is described with reference to the accompanying figures. The same numbers are used throughout the drawings to reference like features and components.
[0005] Figure 1 illustrates a block diagram of the solar panel charging system used to enable the present subject matter.
[0006] Figure 2 illustrates a block diagram of the signal inverting circuit.
[0007] Figure 3 illustrates a graphical representation of Voltage-Current and Voltage-Power for the solar panel.
[0008] Figure 4 illustrates a flow chart for main circuit as per an embodiment of the present invention.
[0009] Figure 5 illustrates a flow chart for the signal inverting circuit as per an embodiment of the present invention.
DETAILED DESCRIPTION
[00010] A solar panel is basically a series connection of solar cells (PV - Photovoltaic cells) which are a combination of P-N type of material. Each solar cell has an individual voltage and current rating. In general, all the solar cells should of same ratings to be in series. The solar cells in series aid in achieving voltage of the same current rating. This solar panel is a current source. The Voltage – Current V-I characteristics of a known solar cell is as shown via the graph in Figure 3. Generally solar cell ratings will in terms of Open circuit voltage and Short circuit current. Open circuit voltage means, it is the voltage of solar panel under no load condition. Short circuit current is the current drawn from solar panel when load is short circuited. The Power transferable depends on the operating point i.e. output voltage and output current of a solar panel which are inputs to a DC-DC converter.
[00011] Generally, a (direct current) DC to DC converter takes the voltage from a DC source and converts the voltage of supply into another DC voltage level. They are used to increase or decrease the voltage level. Some devices need a certain amount of voltage to run the device. Additionally, too much of power can destroy the device or less power may not be able to start the device or run the device efficiently. The converter takes the power from the battery and cuts down the voltage level, similarly a converter can step-up the voltage level. The DC-DC converters are meant for step up or step down of the DC voltage without changing the power. In general, the DC to DC converters in electronic circuits use the switching technology. A switched mode DC-DC converter converts the DC voltage level by storing the input energy temporarily and then releasing that energy at different voltage output.
[00012] There are various types of DC-DC converters i.e. Step-Down (Buck) Converter, Step-Up (Boost) Converter, Buck-Boost Converter, etc. A Step-Down Converter is used to generate a voltage lower than the input. The Step-Down Converter is also called a buck. In a Step-Down converter, the polarities are the same as in the input. Also, in a Step-Down Converter the relationship between the voltages and currents is (Output Voltage) Vo = D (Constant)* Vin (Input Voltage); Ii (Input Current) = D (Constant)* Io (Output Current).
[00013] A Step-Up Converter is used to generate a voltage higher than the input voltage. The Step-Up is called as a boost and the polarities are same as in the input. In a Step-Up Converter the relationship between the voltages and currents is Vo (Output Voltage) = [1/(1-D)] * Vin (Input Voltage); Ii (Input Current) = [1/(1-D)] * Io (Output Current). Here, D is constant.
[00014] Further, In a Buck-Boost Converter, the output voltage can be increased or decreased in comparison to the input voltage. The common usage of a Buck-Boost Converter is to reverse the polarity. The relationship between the voltages and currents in a Buck-Boost Converter is Vo (Output Voltage) = [D/ (1-D)] * Vin (Input Voltage); Ii (Input Current) = [D/ (1-D)]* Io (Output Current). Here, D is constant.
[00015] Generally, a maximum power point tracking (MPPT) charge controller ensures that the loads receive maximum current to be used (by quickly charging the battery). Maximum power point is an ideal voltage at which the maximum power is delivered to the loads, with minimum losses. Maximum power point is also commonly referred to as peak power voltage. In general, a maximum power point tracking (MPPT) solar charger works with a micro controller support using different control algorithms to achieve maximum power transfer. This technology uses a switch to control the charging. The switch (transistor) is open until the battery reaches the absorption charge voltage. Once the battery reaches an absorption charge voltage, the switch starts to open and close rapidly to reduce the current and maintain a constant battery voltage towards achieving maximum transfer of power.
[00016] In general, a Maximum power point tracking (MPPT) uses a Micro controller Unit (MCU). When the solar input is available to use and battery is available to charge, the MCU has a predefined algorithm to work in the Maximum Power Point region. The inputs received by the MCU are, a solar panel voltage, a solar panel current, an output voltage and a stable power supply value. Further, the MCU should have, at least three ADC channels, at least one PWM channel, at least one 16-bit/ 32-bit processor, a RAM for data storage. The MCU will then generate output as a gate signal to gate driver circuit.
[00017] The MCU will sense the solar voltage and current with the specified sampling rate and multiply the voltage and current for Power, store the power value for reference. During the next sampling, it will again sense the voltage and current and multiply for power, and compare present power with previous power, if power has increased then compare present voltage with previous voltage, if voltage has increased than reduce the duty ratio. Otherwise increase the duty ratio. If present power is less than the previous power, and present voltage is less than previous voltage reduce the duty ratio, otherwise increase the duty ratio. However, the MCU requires a stable and regulated power supply in the order of 3.3V or 5V, which requires to use the one more buck circuit to supply these bias power supply. This in turn increases the component count and complexity. Also, there are other disadvantages, such as more programming effort is required, complexity will increase, an additional circuit is required for the MCU to work, etc. Furthermore, as discussed, the MCU requires stable power supply which needs be derived from the solar panel through an additional buck circuit.
[00018] The problem with this technology is that the solar panel voltage is pulled down to match the battery voltage. This in turn pulls the panel voltage away from its optimum operating voltage (Vo) at which it generates maximum power output and thus, reduces the efficiency of the solar panel. Therefore, there exists a need for an improved solar charging system which overcomes all above problems as well as other problems of known art. In order to overcome the problem of panel voltage not reaching its optimum operating voltage and thereby resulting in reduction in the efficiency of the solar panel, as per an aspect of the present subject matter a solar panel charger is configured with adjustable operating points i.e. input voltage and input current of DC-DC converter. Accordingly, the present invention discloses a solar panel charger capable of regulating the input voltage of DC-DC converter which can be set at one operating point and can maintain that operating point throughout. As per an embodiment of the present invention, the operating point is pre-determined and is selected close to the maximum power point throughout the day.
[00019] In one of the embodiments of the present invention, the solar panel charger includes a combination of a DC-DC converter and an analog PWM controller. As per an embodiment, the DC-DC converter and the analog PWM controller controls the input voltage of the DC-DC converter which is from a current source. The solar panel is a current source whose current value depends up on the irradiance of sun.
[00020] The solar panel V-I (Voltage-Current), V-W (Voltage-Power) characteristics are as shown in figure 3 represented by curves A and W respectively. As shown in figure 3, Vo is the desirable operating voltage because there it will transfer maximum power to the DC-DC converter. As per an embodiment, of the present subject matter, the solar panel charger system has a feed forward loop and a feedback loop to control both input voltage and output voltage respectively. In one of the embodiments of the present subject matter, the input voltage is sensed using a resistor-divider bridge and then said input voltage is compared with a reference voltage. As per an embodiment of the present invention, on comparing the input voltage with the reference voltage an error value is generated. As per an embodiment of the present invention said error value triggers a signal for an invertor to invert the input voltage. In one of the embodiments of the present invention, the invertor sends signal to the PWM controller. In one of the embodiments, on receiving the signal from the inventor as the input, the PWM controller generates pulses with particular duty in correspondence to the signal received. As per an embodiment, the pulses generated by the PWM controller triggers the MOSFET. As per an embodiment of the present subject matter, the relationship between the input voltage and the duty ratio is directly proportional, i.e. if the Vin (Input Voltage) increases beyond a pre-determined value i.e. a desired value at which said solar panel generates required optimum voltage, the duty increases and if the Vin decreases beyond the pre-determined value i.e. the desired value, duty ratio decreases to maintain the desired value of voltage.
[00021] Detailed control flow is as follows, if Vin (Input Voltage) increases, since it is battery load Vo (Output Voltage) is constant so duty ratio D will increase. As the duty ratio D increases the input current Ii will increase, since Ii = D*Io (duty ration multiplied by output current), as the input current Ii increases Vin will reduce. Same will hold good when input voltage reduces. Thereby input voltage Vin will be maintained at the desired value. Output voltage cutoff will be done by disabling the PWM comparator when it crosses a certain value.
[00022] The present invention along with all the accompanying embodiments and their other advantages would be described in greater detail in conjunction with the figures in the following paragraphs. The present subject matter is further described with reference to accompanying figures. It should be noted that the description and figures merely illustrate principles of the present subject matter. Various arrangements may be devised that, although not explicitly described or shown herein, encompass the principles of the present subject matter. Moreover, all statements herein reciting principles, aspects, and examples of the present subject matter, as well as specific examples thereof, are intended to encompass equivalents thereof.
[00023] Figure 1 illustrates a block diagram of the solar panel charging system as per the present subject matter, depicting the interaction of each component for input and output voltage control. As per an embodiment of the present subject matter, a solar panel charging system (100) includes a solar panel (105), a converter module (110), a feed forward loop (125), and a battery (115). In one of the embodiments of the present invention, said feed forward loop (125) includes a signal inverting circuit (120). In one of the embodiment of the present invention, said converter module (110) is a DC-DC converter. In one of the embodiments of the present subject matter said solar panel (105) acts as the current source for the circuit. According to an embodiment of the present subject matter, said solar panel (105) sends input voltage Vin to said converter module (110) as well as said signal inverting circuit (120). As per an embodiment of the present invention, said signal inverting circuit (120) includes a comparator module (210) (shown in Figure 2), an invertor (205) (shown in Figure 2) and a PWM controller (215) (shown in Figure 2).
[00024] In one of the embodiment of the present subject matter said converter module (110) comprises of a buck converter (not shown) and an analog controller (not shown). According to an embodiment of the present subject matter, said signal inverting circuit (120) modulates the input voltage Vin to a modulated signal voltage P and sends it to said converter module (110). In one of the embodiments of the present invention, said plurality of modulated signal voltage P is then added to said input voltage (Vin) therefore resulting in said output voltage (Vo) equal to said desired voltage (Vd) sent to said battery (115), ensuring extraction of maximum power from said solar panel (105) with a simple and non-expensive circuit. Further, as per an embodiment of the present invention a part of said output voltage Vo is sent as feedback to said converter module (110) and compared with a desired voltage Vd (shown in Figure 3).
[00025] Figure 2 illustrates a block diagram of the present subject matter depicting the interaction of each component of said signal inverting circuit (120). Accordingly, as per an embodiment of the present invention, said solar panel charging system (100) comprises of both a feed forward loop (125) and feedback loop (not shown) to control both of said input voltage Vin and said output voltages Vo respectively. In one of the embodiments of the present invention, said input voltage Vin is sensed using a resistor divider bridge (not shown). As per an embodiment of the present invention, said signal inverting circuit (120) includes a comparator module (210), an invertor (205) and a PWM controller (215). As per an embodiment of the present invention, said comparator module (210) compares said input voltage Vin with a pre-determined reference voltage Vref. As per an embodiment of the present invention, on comparison of said input voltage Vin and said reference voltage Vref, said comparator module (210) generates an error value E. In one of the embodiments of the present invention, said error value E generates a signal input for said invertor (205). In one of the embodiments of the present invention, said invertor (205) inverts said input voltage Vin to an inverted voltage Vin’. As per an embodiment of the present invention, said invertor (205) sends signal to said PWM controller (215).
[00026] In one of the embodiments, on receiving input from said inventor (205), said PWM controller (215) generates a plurality of modulated signal voltage P with a particular duty D corresponding to said error value E. As per an embodiment, plurality of modulated signal voltage P generated by said PWM controller (215) triggers a MOSFET (metal–oxide–semiconductor field-effect transistor) (not shown). As per an embodiment of the present subject matter, the relationship between said input voltage Vin and said duty ratio D is directly proportional, i.e. if said input voltage Vin increases beyond a pre-determined value i.e. a reference voltage Vref, said duty ratio D increases and if said input voltage Vin decreases beyond the pre-determined value i.e. said reference voltage Vref, said duty ratio D decreases to maintain said desired value of voltage Vd corresponding to a maximum power Pmax (shown in Figure 3) generation. As per an embodiment of the present invention, said reference voltage Vref is the voltage at which said solar panel (105) generates maximum power. In one of the embodiment of the present invention said desired voltage (Vd) value is in the range of 48V to 60V.
[00027] According to an embodiment of the present invention, when said input voltage Vin increases, since said battery load/ output voltage Vo is constant, said duty ratio D increases. In one of the embodiments of the present invention, when said duty ratio D increases, an input current Ii increases [since (input current) Ii = (duty ratio) D * Io (an Output current)] i.e. input current Ii is equal to duty ratio D multiplied by an output current Io. Further, as per an embodiment of the present invention, when said input current Ii increases, said input voltage Vin reduces. Similarly, as per an embodiment of the present invention, when said input voltage Vin reduces, the relationship as explained above will increase said input voltage Vin to a desired value so as to generate said maximum power Pmax. Therefore, as per an embodiment of the present invention, said input voltage Vin is always maintained at said reference voltage Vref so as to generate said maximum power Pmax. Furthermore, as per an embodiment of the present subject matter, said output voltage cut-off as discussed above is done by disabling said PWM comparator (215) when it crosses a pre-determined value i.e. said reference voltage Vref. Figure 3 illustrates a graphical representation of Voltage-Current and Voltage-Power for said solar panel (105), where said output voltage Vo is equal to said desired voltage Vd for achieving said maximum power Pmax and power line is represented by W and current line via A.
[00028] Figure 4 illustrates a flow chart for main circuit as per an embodiment of the present invention. According to an embodiment of the present invention, when said solar panel charging system (100) is ON or started, the first step (405) is to check whether said battery (115) is connected. If the battery is not connected said solar panel charging system (100) turns OFF or is stopped. The next step (410) involves checking said output voltage Vo with respect to said desired voltage Vd. If said output voltage Vo is greater than said desired voltage Vd, said input voltage Vin from said solar panel (105) is sensed at step (415). However, at step (410) when Vd is not greater than Vo, said solar panel charging system (100) turns OFF or is stopped. Further, the next step is to compare said input voltage Vin with said reference voltage Vref (420). If said input voltage Vin is greater than said reference voltage Vref then said signal inverting circuit (120) modulates said input voltage Vin to generate a plurality of modulated signal voltage P (425). However, at step (420) if said input voltage Vin is not greater than said reference voltage Vref, then said solar panel charging system (100) turns OFF or is stopped. In one of the embodiment of the present invention, said Vin is compared with said reference voltage Vref to generate and error value E. Said error value E is then sent as input signal to said invertor (205) to invert said input voltage Vin to said inverted voltage Vin’. Next step is to send input from said invertor (205) to said PWM comparator (215) wherein said PWM comparator (215) generates said plurality of modulated signal voltage P. Accordingly, based on the steps inside said signal inverting circuit (120) as will be explained in detail in Figure 5, said duty ratio D increases (430). As a result of increase in said duty ratio D, based on the relationship as explained via figure 2 said input voltage Vin decreases (435). Next step (440) is to check whether output voltage Vo is equal to said desired voltage Vd or not. Furthermore, when said output voltage Vo is equal to said desired voltage Vd the process stops (440). However, if at step (440) Vo is not equal to said desired voltage Vd, said solar panel charging system (100) turns OFF or is stopped.
[00029] Figure 5 illustrates a flow chart for said signal inverting circuit (120) as per an embodiment of the present invention. According to an embodiment of the present invention, first step (505) for said signal inverting circuit (120) is to check whether said input voltage Vin is available from step (420) (figure 4). However, if at step (505) said input voltage Vin is not available, said system stops. Further, if said input voltage Vin is available, the next step involves scaling down said input voltage to a signal level (510). Next step is to send the scaled down said input voltage Vin to an inverting amplifier (not shown) to amplify said input voltage Vin (515). Next, step (520) involves sending amplified version of said input voltage Vin signal to said PWM Controller (215) after which the flow chart for said signal inverting circuit (120) stops.
[00030] As per an embodiment of the present invention, said pulse width modulation unit (215) generates said plurality of modulated signal voltage P which is sent to said convertor module (110). In one of the embodiments of the present invention, said plurality of modulated signal voltage P is then added to said input voltage (Vin) therefore resulting in said output voltage (Vo) equal to said desired voltage (Vd), ensuring extraction of maximum power from said solar panel (105) with a simple and non-expensive circuit.
[00031] Many modifications and variations of the present subject matter are possible in the light of above disclosure. Therefore, within the scope of claims of the present subject matter, the present disclosure may be practiced other than as specifically described.

List of Reference Numerals:
100 Solar Panel Charging System
105 Solar Panel
110 Converter Module
115 Battery
120 Signal inverting circuit
125 Feed forward loop
205 Invertor
210 Comparator module
215 pulse width modulation (PWM) controller
Vo Output Voltage
Vd Desired Voltage
Vin Input voltage
Vin’ Inverted input voltage
Vref Reference Voltage
P Plurality of modulated signal voltage
E Error value
D Duty Ratio
Pmax Maximum generated power