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AN2644 Datasheet(PDF) 9 Page - STMicroelectronics |
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AN2644 Datasheet(HTML) 9 Page - STMicroelectronics |
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9 / 64 page  AN2644 The LLC resonant half-bridge converter 9/64 and, in certain conditions, the initial imbalance of the V·s applied to the transformer at start up (see "Converter's start-up" section). Additionally, it makes the input current to the converter look like that of a full-bridge converter, as shown in Appendix E, with a resulting reduction in both the input differential mode noise and the stress of the input capacitor. Obviously, the currents through Q1 and Q2 will be unchanged. It is easy to recognize that the two Cr/2 capacitors are dynamically in parallel, so that the total resonant tank's capacitance is again Cr. The system appears quite bulky, with its three magnetic components. However, the LLC resonant topology lends itself well to magnetic integration. With this technique inductors and transformers are combined into a single physical device to reduce component count, usually with little or no penalty to the converter's characteristics, sometimes even enhancing its operation. To understand how magnetic integration can be done, it is worth looking at the well-known equivalent schematics of a real transformer in Figure 5 and comparing them to the inductive component set of Figure 3. Lp occupies the same place as the magnetizing inductance LM, Ls the same place as the primary leakage inductance LL1. Then, assuming that we are going to use a ferrite core plus bobbin assembly, Lp can be used as the magnetizing inductance of the transformer with the addition of an air gap into the magnetic circuit and leakage inductance can be used to make Ls. Figure 5. Equivalent schematic of a real transformer (left, tapped secondary; right, single secondary) To do so, however, a leaky magnetic structure is needed, which is contrary to the traditional transformer design practice that aims at minimizing leakage inductance. The usual concentric winding arrangement is not recommended here, although higher leakage inductance values can be achieved by increasing the space between the windings. Figure 6. Example of high-leakage magnetic structures (cross-section) i1(t) n : 1 v1(t) LM LL1 i M(t) ideal LL2 i 2(t) v2(t) i1(t) n : 1 v1(t) v1(t) LM LL1 i M(t) ideal LL2 i 2(t) v2(t) v2(t) i1(t) n : 1 : 1 v1(t) LM LL1 i M(t) ideal LL2 i 2(t) LL2 v2(t) v2(t) i1(t) n : 1 : 1 v1(t) v1(t) LM LL1 i M(t) ideal LL2 i 2(t) LL2 v2(t) v2(t) v2(t) v2(t) Windings on separate legs of an EE core Primary winding Secondary winding Primary winding Secondary winding Side-by-side windings (EE or pot core) Windings on separate legs of an EE core Primary winding Secondary winding Primary winding Secondary winding Side-by-side windings (EE or pot core) |
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