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Many solar panel powered models require only power pulses to operate. We need to frequently start data collection or measurement sampling systems, perform measurement or other tasks, send resolved or measured data, and then return to sleep mode. In many cases, sending this data wirelessly consumes most of the output power. Small power sources such as solar panels often struggle to support the power pulses required for the system itself or data transmission. By operating the solar panel at its maximum power point (Mpp) and intelligently obtaining power from the panel, the management of electrical energy is achieved, thereby smoothly driving the pulse load. This article will analyze a simple and cost-effective maximum power point tracking (MppT) processing method for this kind of pulse load system.

Characteristics of solar panels

When operating at maximum power point, the solar panel can provide peak output power. The maximum power point is a voltage and current corresponding to the maximum output power of the battery panel. In the case of changing light levels, the maximum power point tracking method manages the power from the solar panel. A characteristic of solar panels is that the voltage of the panel decreases as the output current of the panel increases. If the output current is too high, the battery board voltage will collapse and the output power will become very low. Figure 1 describes the comparison between the output current and power of a specific solar panel and its output voltage. The maximum power point has been marked. In the figure, a horizontal green line declares the position at which the output power is at least 90% of the maximum power point. Above this line, between "point 1" and "point 2", the maximum output power of the battery board is achieved.

Figure 1: Maximum power point diagram of solar panels

A simple way to make a solar panel powered load operate below 90% of the maximum power point when it only requires power pulses and does not require full time power supply is to turn on the load at "point 1" and close the load at "power point 2". When the load is turned on, it obtains the required power, resulting in a decrease in the voltage of the battery board. In this way, the working point starts moving from "point 1", passes through the maximum power point, and finally exceeds "point 2". At point 2, the load is closed and the battery panel voltage rises again. Even with this simple operation, we still have to deal with three issues.

Firstly, compared to the output of the battery panel, the load may require different voltages. Therefore, we need to use an efficient power supply to convert unstable and relatively high battery panel voltage into a constant voltage available to the load.

Secondly, we need to measure the voltage of the battery board and close or turn on the power supply based on this voltage. Most power supplies utilize a digital input to achieve open or closed functions. This input has an approximate threshold to distinguish between logic low and logic high levels. When using this imprecise threshold, the battery board voltage cannot directly reach the input terminal in the open state. Therefore, we need to use an external circuit with precise thresholds. We can use a power supply voltage monitor, but adding devices will add cost and complexity.

Finally, it is necessary to slow down the rapidly changing battery panel voltage in order to obtain sufficient working time and complete the specified tasks. The voltage of the battery panel changes from "point 1" to "point 2" almost without time - theoretically zero seconds. At this point, when the voltage changes from "point 1" to "point 2", the load power must be turned on, and the load must complete its task. This requires the power supply to have a very fast turn-on ability and be able to maintain the battery board voltage for a long time in order to complete the tasks to be performed.

Maximum power point tracking processing method

We find it difficult to find a single device, low-cost solution. It requires the use of a power limited solar panel input with a wide voltage range, while also being able to efficiently supply a stable output voltage, start quickly, and operate below 90% of the maximum power point. The TITpS62125 is such a device. It can accept an input voltage of up to 17V, has a working efficiency of more than 90%, a starting time of less than 1ms, and has an open input pin that uses an accurate threshold. It can be directly connected to the solar panel voltage to achieve maximum power point tracking. This way, there is no need to add additional devices to achieve this function. Figure 2 shows a complete set of processing methods.

Figure 2: maximum power point tracking Circuit of Pulse Load

A voltage divider composed of R1 and R2 is used to turn on the power at point 1 as shown in Figure 1. Before the power is turned on, the device itself will keep the node between R2 and R3 at ground potential. After the power is turned on, the device releases the node, and R3 becomes a component of the voltage divider. When the voltage of the solar panel drops to "point 2", the device closes the node and once again maintains a low voltage between R2 and R3 nodes. At this point, the battery board voltage starts to rise again until it reaches the opening threshold. This enables a fully programmable on and off voltage that can be used for any solar panel.

The large capacity input capacitor C3 stores energy from the solar panel to supply power to the load at a specified time and to supply power for power startup. The battery board supplies current corresponding to its voltage to the power supply or C3. When the power supply is in a closed state, the solar panel supplies current to the capacitor. When the power is turned on, the capacitor and solar panel supply the required current to the load. Due to the fact that C3 only stores energy and then releases the stored energy over a relatively long period of time, it is considered a low-cost electrolyte capacitor.

Calculation method for required large capacity input capacitance

The first step of maximum power point tracking circuit design is to determine the power demand of the load, and then calculate the size of the required large capacity input capacitance according to the power requirements and the selected solar panel. For example, suppose a remote sensing circuit requires a voltage of 3.3V (825mW) at a current of 250mA and a power supply time of 15ms. This is a basic requirement for systems containing measurement devices, microsolutions, and RF transmitters.

After determining the power demand of the load, we need to calculate the required value of C3. Firstly, the input current required to power the load can be calculated using Equation 1:

The VIN is the uniform solar panel voltage between "Point 1" and "Point 2" as shown in Figure 1, while η The power efficiency for a given output power. Please note that when the VIN is approximately 7.8V and the output power is 825mW, the power efficiency is generally around 87%. Using these values, it can be calculated that IIN=122mA, which is much greater than the output capacity of the solar panel shown in Figure 1. Therefore, C3 must store enough energy to supply the missing current, and the duration is 15ms. Equation 2 calculates the required C3 value based on load requirements and solar panel characteristics:

Vp1 and Vp2 are the voltages between "point 1" and "point 2". When using this solar panel, they are approximately 9V and 6.5V, respectively, which correspond to the voltage changes of C3 during discharge. The required load working time for tON is 15ms. Finally, Ipanel (Avg) is the uniform current supplied by the solar panel when it operates below 90% of its maximum power point. As shown in Figure 1, the current is approximately 19mA.

From equation 2, we can understand that C3 should be greater than 618 μ F。 Using 680- μ F capacitors can provide a certain margin during device operation.

Enable pin voltage divider calculation

R1, R2, and R3 together form a fully configurable voltage divider with open (EN) pin hysteresis. Equations 3 and 4 are used to set the resistor value:

We first choose R1, and 1M Ω is a more suitable starting value. In this way, through calculation, it can be obtained that R2 is 153.8k Ω. We chose the closest standard value of 154k Ω. R3 should be 60.9k Ω, while 60.4k Ω is the closest standard value.

Other maximum power point tracking circuit configuration

Another configurable feature that can benefit typical usage is the use of power good (pG) output to control the load on (EN) input. When the power supply is closed, the pG pin remains low. Only when the power supply is on and the output voltage is in the regulated state, pull up resistor R6 to pull it up. Directly connecting the pG output to the load EN input can keep the load closed until the input voltage rises above Vp1 and the output voltage is high enough to drive the load normally. Due to the power supply being closed when the input voltage drops to Vp2, the pG pin is actively pulled down, causing the load to be closed. This configuration ensures that the load is only turned on when the power supply voltage is in a regulated state, thereby guarding against low power supply voltages that may damage load performance or data.

Detection results

Figure 3 shows the maximum power point tracking circuit in operation. The battery panel voltage VIN remains between 9V and 6.5V (Vp1 and Vp2, respectively). Once VOUT enters the regulating state, the load is turned on and a 250mA current is obtained. When the voltage of the battery board drops to 6.5V, the VOUT is closed and thus the load current is closed. The solar panel always supplies a uniform current of 19mA. In Figure 3, the load has a running time of approximately 18ms, which can meet the requirement of 15ms. The running time is not completely consistent with the above calculation results, and the C3 value has been added, exceeding the calculation results.

Figure 3: maximum power point tracking circuit operating below 90% of the maximum power point

Figure 4 replaces the output voltage trajectory line shown in Figure 3 with the trajectory line of the current ICap from C3. When the VIN drops, the current from the capacitor is positive - the capacitor supplies its stored energy to the power supply, which is then obtained by the load. Once the load is closed, due to the battery panel voltage dropping to 6.5V and the power supply being closed, the current supplied by C3 becomes negative - the capacitor is recharged through the battery panel to store energy for the next cycle. When the battery panel voltage is high enough, when the power is turned on, the current from C3 briefly peaks before the load is turned on. During startup, C3 needs to supply more input current.

Figure 4: High capacity input capacitor (C3) for circuits operating below 90% maximum power point

conclusion

This paper describes a simple and low-cost maximum power point tracking circuit for solar panels, which is suitable for pulse load systems, such as telemetry systems that transmit data through RF transmitters. In addition, we can also configure this topology structure accordingly for any solar panel and pulse load system.

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