Datenblatt-Suchmaschine für elektronische Bauteile |
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FAN6921 Datenblatt(PDF) 9 Page - Fairchild Semiconductor |
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FAN6921 Datenblatt(HTML) 9 Page - Fairchild Semiconductor |
9 / 16 page AN-6921 APPLICATION NOTE © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 8/24/10 9 Once Lm is determined, the maximum peak current and RMS current of the MOSFET in normal operation are obtained as: .. max min . PK OPFC L DS mS QR VD I Lf ⋅ = (24) max 3 RMS PK DS DS D II = (25) The MOSFET non-conduction time at heavy load and low line is obtained as: max . . (1 ) OFF L min SQR D t f − = (26) The MOSFET non-conduction time at heavy load and high line is obtained as: .. .. .. .. . . O PFC L O PFC H RO OFF H OFF L O PFC H O PFC L RO VV V tt VV V + =⋅ ⋅ + (27) To guarantee the first valley switching at high line and heavy-load condition, tOFF.H should be larger than 8µs. Figure 12. Switching Timing of QR Flyback Converter When designing the transformer, the maximum flux density swing in normal operation (B) as well as the maximum flux density in transient (Bmax) should be considered. The maximum flux density swing in normal operation is related to the hysteresis loss in the core, while the maximum flux density in transient is related to the core saturation. The minimum number of turns for the transformer primary side to avoid the over temperature in the core is given by: min PK mDS P e LI N AB = Δ (28) where B is the maximum flux density swing in Tesla. If there is no reference data, use B =0.25~0.30T. Once the minimum number of turns for the primary side is determined, calculate the proper integer for N S so that the resulting N P is larger than Np min as: min PS P Nn NN =⋅ > (29) The number of turns of the auxiliary winding for VDD is given as: () nom DD FA AUX S OF VV NN VV + = ⋅ + (30) where VDD nom is the nominal VDD voltage, which is typically 18V and VFA is forward voltage drop of VDD diode. Once the number of turns of the primary winding is determined, the maximum flux density when the drain current reaches its pulse-by-pulse current limit level should be checked to guarantee the transformer is not saturated during transient or fault condition. The maximum flux density (Bmax) when drain current reaches ILIM is given as: max mLIM sat eP LI B B AN =< (31) Bmax should be smaller than the saturation flux density. If there is no reference data, use B sat =0.35~0.40T. (Design Example) Setting the minimum frequency is 52kHz and the falling time is 0.8µs: min max . .. 36 (1 ) 130 (1 52 10 0.8 10 ) 0.319 130 260 RO SQR F RO O PFC L V Df t VV − =⋅ − ⋅ + =⋅ − × ⋅ × = + 2 .. max min . 2 3 () 2 0.95 (260 0.319) 700 252 10 90 QR O PFC L m SQR O VD L fP H η μ ⋅⋅ = ⋅ ⋅⋅ == ⋅ ×⋅ 63 260 0.319 2.28 70010 5210 PK DS I A − ⋅ == ×⋅ × max . 3 . (1 ) 10.319 13 52 10 OFF L min SDD D ts f μ − − == = × .. .. .. .. . . 260 400 130 13 11.48 8 400 260 130 O PFC L O PFC H RO OFFH OFFL O PFC H O PFC L RO VV V tt VVV s ss μ μμ + =⋅ ⋅ + + =⋅ ⋅ = > + Assuming POT3319 (Ae=159mm2) core is used and the flux swing is 0.26T |
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Ähnliche Beschreibung - FAN6921 |
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