Higher Efficiency Solution without Sacrificing EMI SuperFET II MOSFET for Mobile Charger Applications
Driven by integrated functionality and shrinking size for smart-phone and tablet devices, energy density of lithium-ion batteries have significantly improved during the last few years. Advancements in mobile devices and battery technology have led designers of battery charger to focus on improving power conversion efficiency, power density, operating temperature, charging time and output power with advanced power devices, circuit topologies and control methods. Semiconductor suppliers are playing an important role in development advanced technology to achieve high efficiency and power density.
Driven by integrated functionality and shrinking size for smart-phone and tablet devices, energy density of lithium-ion batteries have significantly improved during the last few years. Advancements in mobile devices and battery technology have led designers of battery charger to focus on improving power conversion efficiency, power density, operating temperature, charging time and output power with advanced power devices, circuit topologies and control methods. Semiconductor suppliers are playing an important role in development advanced technology to achieve high efficiency and power density.
INTRODUCTION
Smart-phones are rapidly developing to support multiple functions and features. It combines the functions of communication, computing, Internet and networking features. As this happens, it requires more chips and more processing cycles, which mean higher power levels. Because of these additional functions, smart-phones require much higher power than before. The conventional linear battery charger no longer adequately meets charge requirements due to its low efficiency and large size. Therefore, the key design challenge for battery charger of smart-phones is high efficiency and power density to meet energy regulation. The Flyback converters are very popular for low power applications such as smart phone, tablet charger or laptop adaptor because of its simplicity and low cost as shown in figure 1.
Figure 1. Typical Flyback Converter Circuit
However, due to the high RMS and peak currents, the MOSFET and output rectifier diode in the flyback have high switching and conduction losses, which results in its relatively low efficiency. Through power loss analysis on 10W Flyback converters, critical power loss factors in primary side MOSFET is switching losses during switch transient especially, when a high drain to source voltage, VDS, apply to the MOSFET.
The most popular approach for increased power density is increasing the switching frequency, which reduces the size of passive components. In order to increase both system efficiency and power density, switching losses on the primary side have to be reduced.
POWER LOSS ANALYSIS IN FLYBACK CONVERTER
The most heat dissipations are created by transformer, primary power MOSFET and secondary diode in low power flyback converters. Especially, power loss is critical for power MOSFETs since the power MOSFETs dissipate much more power than any other devices. It is not only an issue of efficiency, but also of thermal management and reliability. Power dissipation in the MOSFETs is highly dependent on on-resistances, gate charge and current and voltage rise and fall times, as well as the switching frequency and operating temperature. Losses in the power MOSFETs consist of switching loss, conduction loss and gate driving loss. Figure 2 shows the power loss analysis of the MOSFETs in mobile charger application under VIN=230Vac and POUT=10W condition. As shown in figure 2, the switching losses are the most critical losses in 10W flyback converter. As the MOSFET switches on and off, its intrinsic parasitic capacitance stores and then dissipates energy during each switching transition. Those parasitic-related losses are a function of the square of the rectiï¬ed line voltage and the COSS output capacitance of the MOSFET. The losses are proportional to the switching frequency. As the physical size of the MOSFET increases, its capacitance also increases; so, increasing MOSFET size also increases switching loss. In order to increase system efficiency, switching loss has to be reduced rather than conduction loss for primary switch.
Figure 2. Loss analysis in power MOSFET in 10W flyback converter
Since a MOSFET is a unipolar device, parasitic capacitances are the only limiting factors during switching transient. So lower parasitic capacitance is required for smaller switching losses. One way to find out how the output capacitance corresponds to switching losses is by evaluating an effective value of output capacitance. The stored energy in the output capacitance of a MOSFET can be calculated by integrating the product of the output capacitance and drain-source voltage with respect to the drain-source voltage from zero to the drain-source voltage just before the turn-on transient. This stored energy is dissipated through the channel of the MOSFET on every turn-on of the switching cycle. The SuperFET® II MOSFET has approximately 35% in reduced stored energy in output capacitance than the same on-resistance device of the previous generation SuperFET® I MOSFET as shown in figure 3. The SuperFET® II MOSFETs provide not only lower conduction losses but high switching efficiency and low driving losses by smaller gate charge for higher system reliability in 10W mobile charger application.
Figure 3. Stored energy in output capacitance
HIGH EFFICIENCY AND LOW TEMPERATURE SOLUTION
10W flyback converter is designed to evaluate the efficiency of a new super-junction MOSFET. Input voltage of the rectifier is 230VAC, and output voltage and current are set to 5V and 0.5A – 2A, respectively. Power losses, efficiency and radiation EMI of is compared with 600V, 900mΩ SuperFET® II MOSFET and competitor super-junction MOSFET which has same voltage rating and RDS(ON) in IPAK(TO-251)package as shown in table 1. 600V, 900mΩ SuperFET® II MOSFET has robust ESD capability due to integrated zener diode.
Table 1. Critical Specification Comparison of DUTs
| DUTs | BVDSS | RDS(ON) max | EOSS @600V |
Vth | QG | ESR | G-S Zener Diode |
| 600V/900mOhm, SuperFET® II MOSFET FCU900N60Z (IPAK) |
600V | 900mΩ | 3.21µJ | 3V (2.5~3.5V) |
13.63nC | 1Ω | Yes |
| 600V/900mOhm, Competitor SJ MOSFET (IPAK) |
600V | 900mΩ | 3.54µJ | 3V (3.0~4.0V) |
12.03nC | 5.4Ω | No |
As shown in figure 4, Efficiency of SuperFET® II MOSFET increases about 0.69% and 0.41% compared to competitor SJ MOSFET at half load. The major reason for higher efficiency is the reduced switch-off loss and output capacitive loss because of lower Eoss. Figure 5 shows the measured results of horizontal radiated EMI noise between 600V, 900mΩ SuperFET® II MOSFET and 600V, 900mΩ competitor super-junction MOSFET. By using SuperFET® II MOSFET which is optimized for primary switch, enables higher system efficiency without sacrificing in EMI performance thank to its optimized design.
Figure 4. Efficiency Comparison in 10W flyback converter
Figure 5. Radiation EMI Comparison in 10W flyback converter
(under Vin=115Vac, Pout=10W, Horizontal antenna)
SUMMARY
Fast charging, increasing power density and achieving higher efficiency are the most challenging issues in mobile charger applications. To achieve this goal, switching efficiency is the most important. With reduced gate charge and stored energy in output capacitance of SuperFET® II MOSFET, switching efficiency is increased and driving and output capacitive losses are decreased. SuperFET® II MOSFET allows designers to significantly increase system efficiency and power density with good EMI performance for smart-phone or tablet battery charger.
