AN9008

AN9008 FAIRCHILD | Alldatasheet

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Rev D, July 2000 July, 2000 AN9008 The Use of QFETs in a Flyback Converter By Il Soo Yang Introduction Power supply designers face many challenges in designing more efficient and cost-effective power supplies. Efficiency is a major consideration in designing switching power supplies. Many factors in the design process such as the input filter capacitance, transformer core geometry and construc- tion, output rectifier, and switching device etc., affect the efficiency of switching power supplies. Among the losses all components generate, switching device losses occupy about 30%. Hence, selecting MOSFETs with optimum efficiency and high reliability is very crucial in power supply design. This application note compares the key characteristics, power losses, and efficiency of the new QFET and a conventional MOSFET in a 60 watt flyback converter operated at 180 to 265 VAC. QFET Characteristics Almost all the power supplies used in TVs, VCRs, PCs, fax machines, and other home appliances rely on a switching circuit to convert the AC wall power to DC power or DC to AC. Thus, they are referred to as switched mode power supplies. To obtain high efficiency, it is crucial for designers to select switching MOSFETs to give very low losses in the circuits. MOSFETs must exhibit low con- duction and switching losses with safety qualifications. Fairchild Semiconductor, in extending its commitment to develop high quality MOSFETs, now offers new high efficiency QFETs for switched mode power supply applications. A power QFET, rated at 600V and used in a 60 watt flyback converter, features a gate charge rat- ing which is 45 percent lower than existing devices for improved switching and drive efficiency. Fig- ure 1 compares the new QFET FQP7N60 with its conventional MOSFET counterpart. By using unified singular well stripe technology, the Miller capacitance of the new QFET is reduced by about 40 percent.

Rev D, July 2000 Performance in a Flyback Converter Figure 3 shows the design of a commercially available 60 watt flyback converter with two outputs (+160V, +15V), operating at a switching frequency of 80kHz and an input voltage of 220VAC. This type of switching power supply is used for applications, such as monitors, TVs, and miscellaneous instruments, requiring multiple output voltages . This discontinuous mode flyback converter, using a KA3882 current mode controller, features good voltage tracking with the use of pulse by pulse current sensing on the primary side, and an isolated secondary feedback loop. The PWM IC KA3882 directly drives the power MOSFET. As the power MOSFET sequentially turns on and off, energy is stored in the transformer core dur- ing the on time, and is then transferred to the output capacitor during the off time. When the power MOSFET turns off, the energy stored in the leakage inductance causes a voltage spike across the drain-to-source terminal of the power MOSFET, which amounts to at least twice the input voltage (Vin + nVo + leakage inductance voltage 1). Most applications need clamp circuits to restrict this voltage spike from exceeding the BVdss rating of a MOSFET. A power MOSFET must have high voltage capability with lower on-resistance and smaller gate charge for higher efficiency. Figure 3: Flyback Converter Circuit Diagram 1. 'n' indicates a turns ratio of the transformer windings. The voltage of Vin + nVo + leakage inductance voltage of the transfo rmer appears at the primary side. KA3882 D1 T1 Fuse Vout1 Vout2 QFET(FQP7N60) or Conventional MOSFET Vin=220VAC 1 2 1 5 4 8 R2 12 1 2 1 2 1 2 1 2 R3 12 R5 12 R14 C10 R11 1 2 R11 1 2 R13 1 2C8 R10 12 1 3 12R9 12 R6 12 R7 12 R8 12 R19 12 R201 3 1 2 1 2 C13 12 R14 12 R16 12 R171 2 121 2 C14 C11 R4 12 C12

12 R18

5V 0.8A 160V 0.3A

Rev D, July 2000 Figure 7: Efficiency vs. Frequency (40~140 kHz, @ Vin=220VAC, Pout=60W) The turn-off loss area of QFET(FQP7N60) is half that of of the MOSFET. During turn on and off, there is a short period when there is a significant overlap of voltage and cur- rent across the MOSFET. Figure 5 shows that the QFET(FQP7N60) has a shorter overlap period than the conventional MOSFET, resulting in a lower loss (Figure 6). In Figure 7 the efficiencies of the converter are calculated without D4 (high conduction diode, refer to Figure 3) operating at rated con- ditions of 220 VAC input voltage and 60 watt output as a function of frequency. As shown in Figure 7, the QFET (FQP7N60) design is more efficient than its conventional MOSFET counterpart. The advantage of QFET design is more pronounced as the switching frequency of the power supply increases. These waveforms clearly demonstrate that faster switching translates into lower switch- ing loss and much better efficiency. Summary To ensure high efficiency and reliable performance of the flyback converter, or any other converter, the designer must ensure that the MOSFET operates effectively with lower on-resistance and gate charge in the system. In this application note, that QFET(FQP7N60) flyback design demonstrates higher efficiency than the previous MOSFET design because of the improvement of on-resistance and gate charge. The other series of Fairchild’s QFETs with high voltage ratings (600, 800, and

900 V) allow designers to improve the performance of a switching mode power supply by a signifi-

cant reduction in gate charge and on-resistance. 40 60 80 100 120 140 0.70 0.75 0.80 0.85 0.90 QFET (FQP7N60) Conventional MOSFET Efficiency [η] Frequency [kHz]

Rev D, July 2000 Appendix: A. The printed circuit board layout

Rev D, July 2000 B. Parts List Designator Value Designator Value Designator Value C1, C2, C3, C4 0.0047 µF R1 NTC R19 1.9 k Ω (1/4W) C5 220 µF R2 220 k Ω (1W) R20 500 Ω (variable) C6 0.0022 µF R3 220 k Ω (1W) L1 BSF2125 C7 0.0033 µF R4 220 k Ω (1W) T1 Transformer C8 0.0022 µF R5 68 k Ω (1W) U1 KA3882 C9 100 µF R6 12 k Ω (1/4W) U2 PC817 (Photocoupler) C10 10 nF R7 2.7 k Ω (1/4W) U3 KA431 C11 560 pF R8 100 k Ω (1/4W) Q1 FQP7N60 C12 33µF R9 100 k Ω (1/4W) C13 1000 µF R10 9 k Ω (1/4W) C14 10 nF R11 50 k Ω (1/4W) D1 Bridge Diode R12 1 k Ω (1/4W) D2 1N4937 R13 100 k Ω (1/4W) D3 1N4937 R14 0.5 k Ω (1W) D4 1N4148 R15 5 k Ω (1/4W) D5 1N4744 R16 1 k Ω (1/4W) D6 FR304 R17 33 k Ω (1/4W) D7 UF5404 R18 120 k Ω (1/4W)

The following are registered and unregistered trademarks Fairchild Semiconductor owns or is authorized to use and is not intended to be an exhaustive list of all such trademarks. ACEx™ Bottomless™ CoolFET™ CROSSVOLT™ E 2CMOS™ FACT™ FACT Quiet Series™ FAST FASTr™ GTO™ HiSeC™ ISOPLANAR™ MICROWIRE™ POP™ PowerTrench QFET™ QS™ Quiet Series™ SuperSOT™-3 SuperSOT™-6 SuperSOT™-8 SyncFET™ TinyLogic™ UHC™ VCX™ DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF FAIRCHILD SEMICONDUCTOR INTERNATIONAL. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. PRODUCT STATUS DEFINITIONS Definition of Terms Datasheet Identification Product Status Definition Advance Information Formative or In Design This datasheet contains the design specifications for product development. Specifications may change in any manner without notice. Preliminary First Production This datasheet contains preliminary data, and supplementary data will be published at a later date. Fairchild Semiconductor reserves the right to make changes at any time without notice in order to improve design. No Identification Needed Full Production This datasheet contains final specifications. Fairchild Semicon- ductor reserves the right to make changes at any time without notice in order to improve design. Obsolete Not In Production This datasheet contains specifications on a product that has been discontinued by Fairchild semiconductor. The datasheet is printed for reference information only.