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ISL6721 Datasheet(PDF) 13 Page - Intersil Corporation |
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ISL6721 Datasheet(HTML) 13 Page - Intersil Corporation |
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13 / 21 page  13 FN9110.4 April 13, 2007 Peak Primary Current: Maximum Primary Inductance: Choose desired primary inductance to be 40 μH. The core structure must be able to deliver a certain amount of energy to the secondary on each switching cycle in order to maintain the specified output power. where Δw is the amount of energy required to be transferred each cycle and Vd is the drop across the output rectifier. The capacity of a gapped ferrite core structure to store energy is dependent on the volume of the airgap and can be expressed as: where Aeff is the effective cross sectional area of the core in m2, lg is the length of the airgap in meters, μ o is the permeability of free space (4 π • 10-7), and ΔB is the change in flux density in Tesla. A core structure having less airgap volume than calculated will be incapable of providing the full output power over some portion of its operating range. On the other hand, if the length of the airgap becomes large, magnetic field fringing around the gap occurs. This has the effect of increasing the airgap volume. Some fringing is usually acceptable, but excessive fringing can cause increased losses in the windings around the gap resulting in excessive heating. Once a suitable core and gap combination are found, the iterative design cycle begins. A design is developed and checked for ease of assembly and thermal performance. If the core does not allow adequate space for the windings, then a core with a larger window area is required. If the transformer runs hot, it may be necessary to lower the flux density (more primary turns, lower operating frequency), select a less lossy core material, change the geometry of the windings (winding order), use heavier gauge wire or multi- filar windings, and/or change the type of wire used (Litz wire, for example). For simplicity, only the final design is further described. An EPCOS EFD 20/10/7 core using N87 material gapped to an AL value of 25nH/N 2 was chosen. It has more than the required air gap volume to store the energy required, but was needed for the window area it provides. Aeff = 31 • 10-6 m2 lg = 1.56 • 10-3 m The flux density ΔB is only 0.069T or 690 gauss, a relatively low value. Since the number of primary turns, Np, may be calculated. The result is Np = 40 turns. The secondary turns may be calculated as follows: where Tr is the time required to reset the core. Since discontinuous MMF mode operation is desired, the core must completely reset during the off time. To maintain discontinuous mode operation, the maximum time allowed to reset the core is tsw - tON(MAX) where Tsw = 1/Fsw. The minimum time is application dependent and at the designers discretion knowing that the secondary winding RMS current and ripple current stress in the output capacitors increases with decreasing reset time. The calculation for maximum Ns for the 3.3 V output using t = tsw - tON (MAX) = 2.75μS is 5.52 turns. The determination of the number of secondary turns is also dependent on the number of outputs and the required turns ratios required to generate them. If schottky output rectifiers are used and we assume a forward voltage drop of 0.45V, the required turns ratio for the two output voltages, 3.3V and 1.8V, is 5:3. With a turns ratio of 5:3 for the secondary windings, we will use Ns1 = 5 turns and Ns2 = 3 turns. Checking the reset time using these values for the number of secondary turns yields a duration of Tr = 2.33 μs or about 47% of the switching period, an acceptable result. The bias winding turns may be calculated similarly, only a diode forward drop of 0.7V is used. The rounded off result is 17 turns for a 12V bias. The next step is to determine the wire gauge. The RMS current in the primary winding may be calculated from: The peak and RMS current values in the remaining windings may be calculated from: I PPK 2I AVG IN () • F sw t ON MAX () • ------------------------------------------- 1.87 == A (EQ. 9) Lp max () V IN MIN () tON MAX () • I PPK --------------------------------------------------------- 43.3 == μH (EQ. 10) ΔwP OUT V OUT Vd + 〈〉 F sw V OUT • ------------------------------------ • = joules (EQ. 11) Vg Aeff lg • 2 μ o Δw • • ΔB 2 ----------------------------- == m 3 (EQ. 12) L p μ o N p 2 Aeff • • lg ---------------------------------------- = μH (EQ. 13) N s Ig Vout Vd + 〈〉 Tr • • N p Ippk μ o Aeff • • • --------------------------------------------------------- ≤ (EQ. 14) I PRMS () I PPK t ON MAX () 3Tsw • --------------------------- • = A (EQ. 15) I SPK 2I OUT t sw • • Tr ------------------------------------- = A (EQ. 16) I RMS 2I OUT • t sw 3Tr • --------------- • = A (EQ. 17) ISL6721 |
Similar Part No. - ISL6721_07 |
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Similar Description - ISL6721_07 |
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