The emergence of the energy crisis and the worsening environmental pollution, solar energy as a clean renewable energy countries in the world attach great importance. With the development of solar technology in recent years, ultra-thin, ultra-light photovoltaic cells in portable electronic devices has also been a great development. The solar cell phone lithium battery charger to get rid of the shackles of the traditional charging power supply, energy saving and environmental protection, and has good development prospects. [1]
Output power of photovoltaic cells by light, temperature and other environmental factors, a solar charger to the AC mains charger for lithium-ion battery for safe and fast charging, but also address the following issues [1]: ① The how changes in environmental temperature, light intensity and other external conditions into the charge control method; ② charge controller topology; ③ how to maximize the photovoltaic cell output power. In response to these issues, this paper analyzed three commonly used cell phone lithium battery charger topology and control method, pointed out that there are some limitations in the use of solar energy battery-powered. And designed a BUCK converter constitutes pulse charging control, solar cell phone lithium battery charger based on microprocessor controlled through computer simulation to verify the validity of this charger.
Analysis of a cell phone lithium battery charger Business phone lithium-ion battery charger has three types: linear chargers, pulse chargers and switch mode charger [2]. Table 1, these three charger circuit topology and charge control methods for comparison.
Advantages and disadvantages of the three types of chargers, linear chargers, linear regulator for the charging current and voltage regulation, adjust the tube's power-hungry, need to take effective measures to solve the heat problem can be seen from Table 1. Switch-mode charger by changing the switch turn-off duty cycle to achieve a constant current and constant voltage, the switch in the switch state, the loss is small, but the circuit is more complex. The pulse charger both the advantages of both, but the charger itself does not regulate the current, so a limit of the flow pattern between the power adapter used in conjunction with
In Table 1 The features and structures of three The types chargers
Charger type
Main circuit structure
Charging process
Charge termination method to judge
Charging current control mode
Advantage
Shortcoming
Linear type
Linear regulator, the linear charge control circuit
The prefilled → constant current → constant pressure
The minimum charge current, time
Linear adjustment tube to adjust
Circuit is simple, small size, low cost
Loss, low efficiency, severe fever
Pulse
Switch tubes, pulsed charge control circuit
Prefilled → fast → pulse
Ton / the Toff ratio
Rely on external current limiting power adapter
Circuit is simple, small size, low power consumption, high efficiency
Be current-limited power adapter
Switch-
DC / DC converter, PWM charge control circuit
The prefilled → constant current → constant pressure
The minimum charge current, time
Adjust the duty cycle of the DC / DC converter
Low power consumption, high efficiency, the charge current, wide input voltage range
Large volume, high cost of
The following analyzes the applicability of the above three chargers when the charger to provide power from photovoltaic cells.
(1) solar charger should be a DC / DC converter topology. The output characteristics of photovoltaic cells has a strong nonlinearity, the output of the influence of ambient temperature, light intensity and load. Can be seen from Figure 1, the output power of photovoltaic cells increased with increasing light intensity, and decrease with increasing ambient temperature. When light and temperature changes, real-time adjustment of the operating voltage of the photovoltaic cell allows it to operate near the maximum power point is the maximum power point tracking (maximum power point of tracking, MPPT) [3]. Therefore, in order to improve the output power of photovoltaic cells, the charger can adjust the input voltage to adjust the operating point of the photovoltaic cells. Linear and pulse charger input voltage is determined by the external power supply and therefore can not achieve the maximum power point tracking function. Switch-mode charger input voltage can be achieved by adjusting the duty cycle D BUCK converter input and output voltage relationship: Vo = D * Vin (Vo lithium Acer Aspire 5720 Battery terminal voltage Vin is the solar cell output voltage). Lithium battery voltage change is slow, so change the switch duty cycle D can be changed Vin in order to achieve the MPPT function.
Figure 1 photovoltaic cells under different illumination (a) and temperature (b) the P-V curve
Fig.1 The PV curves of the PV module with varying irradiation (a) and temperature (b)
(2) The solar charger should be used to pulse charge termination judgment method. Linear and switching charge mode commonly used in the two control algorithms to determine if the Panasonic Nv-ds27 Battery is full, the first detection of the minimum charge current law, the second timing method. As mentioned earlier, the output of the photovoltaic cell by the external environment. In the process of charging, due to changes in the weather or changes in the environment around the charge current drops, and drop to set the termination charge current value below the detection error minimum current law terminates the charge. Long is difficult to predict, therefore, from time to time terminate the charging method can not guarantee full of battery due to charging of the battery charge of the current instability. Pulse charging curve shown in Figure 2, the pulse charging when the battery voltage is below 4.2V turn-on switch tube to charge the battery, turn off the switch to stop charging when the battery voltage exceeds 4.2V, when Ton / Toff value below the set value that the battery is full and terminates the charge. If the charge current drops due to changes in the external environment, the battery voltage corresponding decline in Ton / the Toff value increases only when the Apple Macbook 13-Inch mb061b/A Battery is full of Ton, / the Toff will be below the set value, it will not cause the miscarriage of justice.
Fig.2 The curves of The pulse mode charge
By the above analysis, the lithium battery charger is basically a prerequisite for stable power supply design, by solar power can not be well adapted to light, temperature and other environmental changes.
2 solar cell phone lithium-ion battery charger
2.1 System Architecture
This paper, the design of the solar cell phone lithium-ion Acer Aspire 6920 Battery charger system block diagram shown in Figure 3. The charger consists of three parts: photovoltaic cells, the MCU controller with a step-down DC-DC converter.
Fig.3 of The system architecture
BUCK converter's main function is to: output a small current in the pre-charge stage Imin; solar cell maximum power point tracking in the fast-charge phase, when the current reaches Imax limit the current to the constant current charging; pulse charging the battery voltage value output current or stop output. MCU functions are: the acquisition of lithium-ion Acer Aspire 5720 Battery voltage Vbatt current Ibatt, the temperature Tbatt and solar cell voltage Vpv current Ipv; charger controlled by a software program to convert between different charge states; in a different state of charge calculate the duty cycle D, output PWM drive pulse according to the control objectives.
2.2 charging control method
In this paper, the design of the solar charge controller is a multi-objective control system [4], control the duty cycle of the target is also different in different stages of charging, the charging state transitions shown in Figure 4.
Fig. 4 of The change of charge the state
Conversion conditions: 1: Vbatt <Vmin and; 2, (3): Vbatt ≥ Vmin and; ④: Ibatt ≥ Imax; ⑤ The: Ibatt <Imax; ⑥, ⑦: Vbatt ≥ Vmax of; ⑧: Ton / the Toff ≤ settings.
(1) pre-charge: When the lithium battery into the charger, the Canon mv600 battery voltage is below the set threshold Vmin and the microcontroller on the duty cycle of the control objectives is to output a very narrow PWM pulse, a small current of the converter output Imin, over-discharge of lithium-ion battery repair and charging; if the battery voltage ≥ Vmin and can skip the pre-charge stage and go straight to the fast charge stage.
(2) fast charge: When the battery current Ibatt ≥ Imax, microcontroller-controlled constant current PWM pulse width converter output; when the output power of solar cells fall in Ibatt <Imax, then transferred to the maximum power point tracking state to adjust the duty photovoltaic cells than to change the BUCK input voltage in the vicinity of the maximum power point, back and forth to the size of the charger according Ibatt conversion MPPT with constant current between the two states until Vbatt ≥ Vmax of (4.2V), transferred to pulse charging phase.
(3) Pulse Charging: at this stage converter intermittent, constant current output, from time to time ΔT to judge the battery voltage value, if Vbatt <Vmax of output constant current to charge the battery; Vbatt ≥ Vmax of, then stop charge the HP Compaq Business Notebook NX6110 Battery. The switch drive pulse diagram shown in Figure 5. Ton is the time of the output PWM drive pulse, the Toff to stop charging time. Ton during the switch duty cycle D conduction and turn-off to the constant current control, D = ton / ts. Cycle ts determined by the switching frequency and magnitude in μs. Ton and Toff is an integer multiple of ΔT, ΔT ms order of magnitude. The beginning of the pulse charging, Ton / the Toff ratio larger, longer and longer as the battery is gradually filled, Toff Ton / the Toff is less than the set value, the battery is full, the charge termination.
Fig. 5 The drive pulse of the MOSFET
When the voltage reaches Vmax pulse charger is still applied to the lithium high-current charging pulses slightly exceeds the Vmax of the battery voltage during the pulse, but this does not prevent the battery from premature aging. On the contrary, the pulse charging can reduce the impact of polarization in the process of charging, effectively increase the charge capacity, improve the charging efficiency [5].
3 simulation results
In order to verify the feasibility of the solar cell phone lithium Acer Aspire 3682wxmi Battery charging method using matlab simulink toolbox to build a system simulation model [6-8], Simulink simulation model shown in Figure 6. Shown in the PV array output power is 5W, the lithium-ion battery rated capacity of 850mAh. Precharge, the MPPT, CC, Pulse module to achieve pre-charge, the maximum power point tracking constant current pulse charging function, multi-channel switch controlled by the Control module to convert. Maximum power point tracking PV array output power results directly change the duty cycle of DC / DC converter mountaineering method [9]. By changes in light intensity of the signal module signal1 simulation. Pre-charge voltage threshold is set to 2.9V, precharge flow Imin is set to 0.1C (C is the rated capacity of the lithium battery). Constant current charging current is set to 1C, taking into account the influence of the inductor current ripple, set MPPT with constant-current conversion threshold 0.95Imax. ΔT is set to 64ms, Ton / the Toff is less than 1/256 that the battery is fully charged.
Fig. 6 the System simulation model in. Simulink
Figure 7 shows the light intensity, the simulation results shown in Figure 8 (a) as shown. Conversion in Figure 8 (c) can be seen from the charging status when the illumination changes of the 1000s and 2000s, the charger can automatically switch between the constant current with MPPT charge optimization charging current Ibatt of. Pulse charging stage, when the light shines in the 4000s at the drop, the charger continues charging until Ton / the Toff is less than the set value, the battery is full. By amplification of the current waveform in Figure 8 (b) can be seen as the charging time of growth Toff progressively larger process. Voltage Vbatt curve in line with the charging characteristics of lithium batteries, the Apple a1280 Battery is full after the voltage is maintained at 4.2V.
Figure 9 Simulation results when the charger does not have the MPPT function in the same light. Be seen from the graph the charging current when the fall of the 1000s-2000s light was significantly lower than the current 8 (a), the lithium battery voltage increases slowly, resulting in the total charging time of about 400s.
Figure 10 Figure 6 Pulse module with a constant voltage module instead of the simulation results, the minimum detectable current method (less than 0.1C) for charge termination. Into the constant voltage charging phase, the decline of the 4000s at the light charger immediately stop charging, however, the current decline is due to external factors, not the battery is fully charged, the lithium battery voltage quickly decreased.
4 Conclusion
In this paper, the design of solar cell phone lithium battery charger with DC / DC converter topology to achieve the maximum power point tracking, to maximize the efficiency of photovoltaic cells, charge termination judgment, taking into account the light intensity, ambient temperature and other factors on The lithium Canon Bp-2l12 battery charging curve, accurate charge termination judgment, so that the lithium battery rated capacity.
Fig.7 Change of the irradiation
Figure 8 (a) The pulse rechargeable lithium battery voltage, current waveform diagram (b) enlarged map (c) charge state transition diagram
Fig.8 (a) Voltage and current wave of the Li-ion battery in the pulse charge mode (b) local zoom (c) The change of charge state
Fig.9 Voltage and current wave of the Li-ion Acer Aspire 5740 Battery without the MPPT
Fig.10 Voltage and current wave of the Li-ion battery in the CC-CV charge mode
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