20W Flyback Converter using FSQ0365RN
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The figure above shows an example application designed in our Global Power Resource Center in Germany. R103-5, D104 and C103 form the extra components needed for the detection of the minimum voltage levels on the drain. The FOD2741 is an error amplifier combining the functions of a standard optocoupler and an industry standard '431 reference in one package. The remaining components are standard for a flyback converter.
The no-load standby power was measured to be less than 130mW in the 175VAC to 265VAC range used for the design. At lower input voltages the standby power is even lower for similar designs. The full load efficiency was greater than 86% for the whole voltage range, which is very high for a multiple output flyback power supply at this power level. Line regulation was excellent " the measured voltages did not really change when the input voltage was modified. Load regulation for the regulated output was well within 5%.
The availability of low power quasi-resonant devices has opened up interesting new possibilities. The Global Power Resource Center has developed a quasi-resonant buck circuit using the FSQ311 device for a 20V/100mA output. Here the standby power with a 10mA load was measured. The total power consumption was less than 400mW (including the 200mW base load) over the full 85VAC to 265VAC range, and less than 350mW for the range up to 180VAC. The better performance at low input voltage is due to the troughs of the drain voltage being far closer to zero than at higher input voltage. Moreover, it shows that it is possible to use this design for a 0.2W power supply having 50% efficiency.
For 85VAC to 160VAC the full load efficiency was greater than 80%, dropping to 73% for the range up to 265VAC. This is excellent performance for such a small power supply, entirely attributable to the use of quasi-resonant techniques. Both line and load regulation were well within 1% over the full operating range. The temperature rise of the device without heatsink was measured to be 15 degrees at room temperature. A 1.2mH inductor and a 220nF capacitor provided sufficient filtering to meet standard EMI specifications.
REFERENCES
[1] R. W. Erickson and D. Maksimovic, "Fundamentals of Power Electronics", Second Edition, Springer, 2001, Chapter 19, ISBN 0-7923-7270-0
