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SC4809BIMSTRT Datasheet(PDF) 9 Page - Semtech Corporation |
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SC4809BIMSTRT Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 16 page  9 2005 Semtech Corp. www.semtech.com SC4809A/B/C POWER MANAGEMENT Control-to-Output transfer function for the flyback power stage operating in CCM is given by: 2 O 2 O 2 z 1 z P S 2 I O S Q S 1 S 1 S 1 N N ) D 1 ( V dD dV ω + • ω + ω − • ω + • • − = where: C L R x ) D 1 ( Q C L D 1 L D R ) D 1 ( C R 1 1 z SEC SEC O SEC 2 2 z C − ≈ • − ≈ ω • • − ≈ ω • = ω Control-to Output transfer function for the flyback power stage operating in DCM is given by: P SEC S P S I O S 1 1 L 2 T R N N V dD dV ω + • • • • • = where: C R 2 P • = ω Peak current mode control requires simpler compensation, has pulse-by-pulse current limiting, and has better load current regulation. Primary and secondary RMS currents can be up to two times higher for discontinuous mode than for CCM. Discontinuous conduction mode would require using a transistor with a higher current rating. Because the output ripple current is less than it would be continuous mode were used, the output capacitors are smaller. Continuous conduction mode (CCM) was therefore chosen. Application Information (Cont.) The DC transfer function of a CCM flyback converter is: − • = − + max max ) on ( Rds (min) IN D O D 1 D N 1 V V V V where V O = output voltage, V D = forward voltage drop across rectifier D1, N = turns ratio, equal to N P/NS, D = duty cycle. Transformer Design The transformer in a flyback converter is actually a coupled inductor with multiple windings. Transformers provide coupling and isolation whereas inductors provide energy storage. The energy stored in the air gap of the inductor is equal to: () 2 I L E 2 PEAK P • = where E is in Joules, L P is the primary inductance in Henries, and I PEAK is the primary peak current in Amperes. When the switch is on, D1 is reverse biased due to the dot configuration of the transformer. No current flows in the secondary windings and the current in the primary winding ramps up at a rate of: P ) on ( Rds (min) IN L L V V t I − = ∆ ∆ The output capacitor, C OUT, supplies all of the load current at this time. Because the converter is operating in the continuous conduction mode, ∆I L is the change in the inductor current which appears as a positive slope ramp on a step. The step is present because there is still current left in the secondary windings when the primary turns on. When the switch turns off, current flows through the secondary winding and D1 as a negative ramp on a step, replenishing C OUT and supplying current directly to the load. |
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