Designing Simple but High Performance Chargers

Travel adapter and mobile phone charger designs are facing more and more challenges such as technical, cost, size, and reliability requirements. A good design should have very low no-load power dissipation, less BOM cost, and tight constant voltage/current tolerance. The primary-side regulation technique is promising way to achieve low cost and less BOM count. There was, however, wide belief than primary-only regulation is a trade-off between performance and cost. Here, performance is output voltage regulation performance that is primary function of charger. The wider variation of the charger output voltage is relevant to constant current regulation as well as wider variation of the constant-voltage (CV) to constant-current (CC) transition point. It would be good to understand battery characteristics little bit at this point. Li-ion technology is currently the battery technology of choice for small portable devices because of its high energy density, low self-discharge characteristics, and high flexibility in size and shape. Li-ion battery cells are generally suited to a CC/CV charging strategy. The charger operates in a constant current charging mode when the battery voltage is low. Once the battery has charged to the float voltage (battery voltage at zero current, usually around 4.2 V) the system will begin to reduce the charging current to maintain the desired voltage and thus switch to constant-voltage mode. Although this charging process looks simple, it actually requires precise control of the float voltage region to obtain to maximize battery capacity and improving service life. Imprecise battery voltage regulation could undercharge the battery, with a large decrease in battery capacity. Or, if the voltage is too high, the battery cycle life is significantly degraded. Excessive overcharging of a Li-ion cell could also result in catastrophic failure of the unit.

Compared to early primary-side regulation designs, new controllers provide much improved performance. For an example, FAN104W performance is CV +/- 5% and CC +/- 7% with cable compensation, and ripple is less than 80mV.

Fig. 1 Regulation principle for CV and CC

Fig. 1 shows how FAN104W regulates output. The CV and CC are controlled by two feedback loops, and lower feedback level is dominating by peak current mode. Although it is pure primary-side only regulation solution, no more concern on output regulation performance. Fig. 2 shows another example of load regulation performance. The load regulation has been crucial problem of pure primary-side regulation solution. In the test, output current has changed from 0A to 500mA with USB cable attached and Cout=660uF, and FAN104 shows 0.88V drop at output voltage. This is huge improvement when compared to previous generation of pure primary-side regulation solution which shows 3.5V drop at output voltage under same conditions. Thanks to this good load regulation performance, FAN104 can eliminate additional component helping load regulation in secondary-side that has been required by conventional primary-side regulation solutions. All of these features have also verified by major OEMs of mobile devices and charger manufacturers.

Fig. 2 Dynamic response at Po=5W, Vin=264VAC, Io transient from 0A to 500mA

Another critical measure of chargers is no-load power dissipation. There are over four billion mobile phone users worldwide and most of them leave their chargers plugged-in even when the phone is disconnected. There are a lot of other handheld devices requiring charger too such as tablets, digital still cameras, or music players. According to one of major cell-phone manufacturers, up to two-thirds of the electricity consumed by a mobile device is lost when in no-load mode. It is quite easy to understand why governments or manufacturers are so keen on reducing standby power of chargers. The industry average is in the range of 150 to 300mW in no-load power consumption. A new regulation has been taken for both efficiency and no-load power dissipation. This stringent requirement is a voluntary charger star rating system agreement put forth by the world’s five largest mobile phone makers. Chargers will be labeled starting at zero star for >0.5W standby power consumption, and going up to the five star for consumption of <0.03W. Under rating system, people always look for the best rating products and so OEMs of mobile devices demand <0.03W performance from chargers. The FAN104W can provide 0.025W no load power consumption when designed for 5W or 7.5W charger. Table 1 shows performance test result from evaluation board, FEBFAN104WMX.

Table 1 No load power consumption

 About the Author

Won-Seok Kang has worked for Fairchild Semiconductor since 2006 and is currently a Principal Technical Marketer for Fairchild’s Switch Mode Power Supply in the Power Conversion Division located in Korea. His research interests are in high performance travel adapter, mobile power system, resonant and soft switching dc/dc converters. He has a B.S. and M.S. degree in electronics engineering

source: http://www.powerpulse.net/powerViews.php?pv_id=83&page=1