Solar Mobile charging Solutions Solar power is one of the widely available energy sources. It has been in focus worldwide and solar installations of capacities in megawatts order are reality today. The efficiency of the solar panels remains low even today and the requirement of ensuring highest efficiency outside the panel remains stringent. Thanks to this, several techniques have been deployed to harness the maximum energy, Maximum Power point tracking being one of them. Solar power is the practical add-on to the existing sources of energy. It effectively supplements the current sources of energy. With intelligent designs, the solar cells can be integrated into the final consumer electronic or portable devices. This reduces the dependence of recharging these with the electricity, thus making this ideal choice for the locations where electricity may not be actually available. The solar panels are costly today but their costs are coming down to affordable levels. Also many a times the government aids are available especially for the remote areas. With the solar panels being available in various form factors, a range of consumer electronic devices making use of the solar panels are making entry into the market place and it is expected that this trend would continue in the future. As of now there are speakers, earphones, calculators and other small power consuming devices already available. The trend is to integrate the solar power in the small form factor that enhances the battery life and acts as an additional source of energy to supplement the battery power from other sources. As the efficiency of the solar panels and conversion would become better, a lesser area would be able to generate more energy and eventually the solar charging may become the sole energy source in many consumer devices. The idea of optimizing available solar power through MPPT is used by ST for varying power needs. Consumer electronic devices typically need few watts and many of these devices/gadgets now use USB port for charging. The standardization to USB voltage presents opportunity to design more and more chargers that conform to this form factor and voltage requirements. Solar charging is slow and this presents another challenge of keeping the devices in the sunlight (daytime). This may not be always practical and hence there is a need to have battery backup solutions. In this article, we present ST s solution to address these requirements. ST has integrated low power MPPT directly inside the device SPV1040. This device is designed to address the consumer electronic devices and with very few additional components, circuits for direct charging of the devices or the battery pack can be realized. Solar USB chargers can be used to charge gadgets like mobiles, PMP; PDA s & even e-book readers or any other devices that use a USB or mini USB interface. It can be especially useful during Conferences/All Day Meetings
Camping trip /Picnic Power outages Natural disasters No access to power outlet At present all the gadgets are charged typically with a 5V to 5.5V source (current could vary depending upon the capacity of the battery, typically limited to 500mA). Hence for charging the voltage provided to the gadgets should not be higher than the above mentioned voltage range. The Solar panel can be housed aesthetically inside a foldable casing with a USB output to charge the gadgets. In fact the electronics designed for this purpose should be small in size with all necessary protection features incorporated for enhanced life. A SPV1040 based boost converter with integrated MPPT function, has been specifically designed with necessary protections for charging the handheld consumer gadgets Explanation of SPV1040: SPV1040 is a high efficiency, low power and low voltage DC-DC converter that provides single output voltage up to 5.2 V. Startup being guaranteed at 0.3 V, the device operates down to 0.45 V while coming out from MPPT mode. It is a 100 khz fixed frequency PWM step-up (or boost) converter able to maximize the energy generated by few solar cells (polycrystalline or amorphous). The duty cycle is controlled by an embedded unit that runs an MPPT algorithm with the goal of maximizing the power generated from the panel by continuously tracking its output voltage and current. MPP (maximum power point) is the working point of the PV cell at which the product of the output voltage and current is at a maximum. In other words, the load resistance is instantaneously equal to the panel source resistance. SPV1040 guarantees safety (both converter and overall application) by stopping the PWM switching in the case of an over current or over temperature condition. Block Diagram of SPV1040 Solar USB Charger The MPPT algorithm follows the Perturb & Observe (P &O) method. The simplicity of P&O method makes it the most popular MPPT algorithm. Figure 1 shows the flow chart of this method. After one perturb operation the power is calculated and compared with the previous value of power (observe)
to determine the change of power (ΔP). If ΔP>0, then the operation continues in the same direction of perturbation. Otherwise the operation reverses the perturbation direction. Measure V(k), I(k) Measure V(k), I(k) Calculate Power P(k)= V(k)xI(k) NO NO P(k)> P(k-1)? YES NO V(k)>V(k-1)? YES NO V(k)>V(k-1)? YES Vref(k-1) +C Vref(k-1) - C Vref(k-1) - C Vref(k-1) +C RETURN Figure1: Flow chart of P&O algorithm The MPP voltage needs to be set for SPV1040 by a simple resistive divider. This can be adjusted for different panels with differing MPP voltages. Once the MPP is set and both the output voltage feedback and current feedback is provided, the MPP tracking is automatically taken care of by the device. Design guidelines: 1. Free vias connected to ground should be suitably placed below the device to provide better heat dissipation. 2. In order to minimize voltage and current ripple, high frequency ringing problems, and electromagnetic interference, it is essential to reduce the length of the paths that carry high frequency switching currents. 3. Large traces for high current paths and an extended ground plane reduces noise and acts as an efficient heat-sink as well. 4. The output and input capacitors should be placed as close as possible to the device. 5. The external resistor dividers should be as close as possible to the V MPP-SET and V CTRL pins of the device and as far as possible from the high current circulating paths to avoid picking up noise. 6. An external Schottky diode between L x and V OUT pins is mandatory in all the applications with V BATT_max > 4.8 V. In fact, voltage on L x pin can go above the maximum absolute voltage
threshold (5.5 V) due to the voltage drop on the high side integrated switch when this is off (discontinuous mode) and current needs to flow from input to output. 7. Inductor size also affects the maximum current delivered to the load. The saturation current of the choke should be higher than the peak current limit (1.8A) of the input source. Thus the suggested saturation current is > 1.8 A. Smaller inductance values guarantee both faster response to load transients. Inductors with low series resistance are suggested in order to minimize Ohmic losses. The SPV1040 based charger has low component count with all features as mentioned earlier. A typical PCB layout size will be of 22x16mm which can be provided inside a small USB dongle form factor (typical size 35 x20mm). Typical Solar Based USB charger Specifications: PARAMETERS RATING/FEATURES Output 3W (5V,600mA) Efficiency (Overall Electronics) >90% Solar Panel Rating 3W Solar Panel Open Circuit Voltage 4.2V (Voc) Solar Panel MPP Voltage (Vmp) 3.1V Protections Over current, Over temperature Power pack for use with the SOLAR USB CHARGER. This is an extension to the solar USB charger just explained above. In many cases, when the mobile user is travelling, on a camping vacation or in rural/hilly areas where access to electricity is minimal or non-existent, this solution comes in very handy. The system basically consists of a LiIon battery of suitable capacity, a DC DC converter to boost the battery voltage from a nominal 3.7V to 5V output and of course, a LiIon battery charger. Figure 2. The Power Pack
Block diagram of Power Pack As can be seen, the system consists of four major blocks: 1) The input stage 2) The single cell Li Ion battery charger 3) A reservoir Li Ion battery, typically 3.7V, 1800mAH 4) A DC DC boost converter for converting the battery voltage back to 5V for the load. The input stage: This is typically a USB connector, which means it is rated to accept typically a 5V, 500mA power source. In this case, this voltage comes typically from the Solar USB charger or it may as well be a USB adaptor or any adaptor with a USB jack. This means, that there should be some amount of protection built into the system such that it can withstand excessive over voltage or reverse polarity of some kind, at the input. Without this, the field reliability of the system becomes questionable. It has been implemented by using a couple of passive devices. A shunt connected clamp diode and a series connected resettable fuse. The resettable fuse is rated at 650 ma in this case and the shunt clamp is rated at 5.6V. It might seem enough to use any Zener diode for the clamp, but this may not work. It might not have the surge capacity to withstand the energy till the resettable fuse (Fr) trips. The TRANSIL clamp diodes have the capability to withstand these shocks and are designed for that purpose. As long as the input voltage is less than the clamp diode voltage, the clamp diode behaves as an open circuit. Negligible leakage current flows through the device in normal reverse bias (typically 200nA). But, the moment the input voltage exceeds the clamp rating, the device breaks down. So a huge current tries to pass through the circuit, which is when the resettable fuse opens, and disconnects the circuit. Thus the circuit is protected from excessive and persistent over voltages at the input. In fact, this scheme may be added as input protection in several other applications including the solar USB charger as described earlier.
The single cell Li Ion battery charger: The Li Ion cell is something that must be charged with care to enhance useful life and safety of the user. To optimally charge Li Ion batteries, precise monitoring and charge controlling is a must, while tracking for excessive temperature rise during the process. Most Li Ion batteries now come with inbuilt protection features, to make the battery immune from excessive discharge, extended overload, short circuits, over heat etc. There are several charger ICs dedicated solely to cater to the single cell Li Ion battery charging. The STBC08 is a simplistic approach; it can work in a standalone fashion, quite suited for this application. Typically, this device has been programmed to a charge current of C/4, or 400mA for a 1600mAH battery. The scheme is a constant current/constant voltage charger for single cell Li-Ion batteries. No external sense resistor or blocking diode is required. It is designed to work within USB power specifications. An internal block regulates the current when the junction temperature increases, in order to protect the device when it operates in high power or high ambient temperature conditions. The charge voltage is fixed at 4.2V, and the charge current can be programmed using a single resistor connected between PROG pin and GND. The charging is automatically terminated when the current flowing to the battery is 1/10 of the programmed value. If the external adaptor is removed, the STBC08 turns off and a 2µA current can flow from the battery to the device. The device can be put into Shutdown Mode, reducing the supply current to 25µA. The device also has a charge current monitor, under voltage lockout, automatic recharge. The charge termination and input voltage presence are indicated by two separate status pins. The DC DC boost converter: To provide power to loads like mobiles or MP3 players etc, it is required to maintain the voltage at 5V, as a standard. The Li Ion battery typically would provide 3.7V nominally. So this calls for a boost converter ahead of the battery and something with a high efficiency to maximize the reservoir battery life. This particular solution uses a L6920 device which is a boost converter with integral power switch and synchronous rectifier. The L6920 is a high efficiency step-up controller requiring only three external components to realize the conversion from the battery voltage to the selected output voltage. Internal synchronous rectifier is implemented with a 120m Ω P-channel MOSFET and, in order to improve the efficiency, a variable frequency control is implemented. To program the device to 5V it is necessary to just tie to FB pin to ground. An overload protection scheme would also be required at the output for long term short circuit or overload protection. A resettable fuse will successfully do the job. Although the device has inherent current limit, adding this passive part will drastically enhance reliability in the field. The entire system can be easily designed on a small PCB. Thanks to the high efficiency and small form factor of the key devices, optimal pad and track sizes take care of heat-sinking very well. Judicious use of copper plane, appropriate ground and supply track widths and thermal pads with vias on the bottom of the devices will enhance continuous operation at rated load with minimal heat generation. Whenever there are tracks overlapping on the top and bottom layer, use of multiple vias, will reduce thermal resistance to a great degree, allowing for a compact design. It is possible to pack the entire solution in a sealed box, to make it waterproof, with minimal temperature rise. A generic scheme has been discussed above, and it is possible to use variants of the parts indicated for other voltage and current ratings, according to the user end requirement. So far we have explained the simplest possible implementation of a Solar Charger/Power pack system. It is possible to have the entire implementation done with discrete devices and a low power MCU. References 1. SPV1040 datasheet
2. AN3319 3. L6920 datasheet 4. STBC08 datasheet This article was originally published in ArrowTimes.