AN3180 Application note A 200 W ripple-free input current PFC pre-regulator with the L6563S Introduction A major limitation of transition-mode-operated PFC pre-regulators is their considerable input ripple current, which requires a large differential mode (DM) line filter to meet EMI requirements.
Contents AN3180 Contents 1 Basic topologies with zero-ripple current . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Zero-ripple current phenomenon: theory . . . . . . . . . . . . . . . . . . . . . . . . 6 3 Sensitivity of zero-ripple current condition . . . . . . . . . . . . . . . . . . . . . 10 4 Zero-ripple current phenomenon: practice . . . . . . . . . . . . . . . . . . . . . . 13 5 Capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AN3180 List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Figure 30. EVL6563S-200ZRC 200W PFC demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Basic topologies with zero-ripple current 1 AN3180 Basic topologies with zero-ripple current Coupled magnetic devices have been around since the early days of electronics, and their application to power switching circuits dates back to the late 70's with the experiments on the Cuk converter, from which “magnetic integration” originated.
AN3180 Basic topologies with zero-ripple current All converter topologies are capable of producing zero-ripple current phenomenon, provided there are two or more inductors which have equal (or, more generally, proportional) voltages of equal frequency and phase. Some topologies, such as Cuk and SEPIC, have two inductors, which can be coupled on a common magnetic core: so they immediately lend themselves to ripple-steering.
Zero-ripple current phenomenon: theory 2 AN3180 Zero-ripple current phenomenon: theory Zero-ripple current in one of a two-winding coupled inductor, having self-inductances L1 and L2, can be achieved if the coupling coefficient k, given by: Equation 1 M k= L1 L 2 (M is their mutual inductance), and the effective turns ratio ne defined as: Equation 2 ne = L2 L1 is such that either k ne = 1 or k = ne, provided the windings are fed by the same voltage.
AN3180 Zero-ripple current phenomenon: theory Figure 5.
Zero-ripple current phenomenon: theory AN3180 Equation 6 k ne = k L2 M = =1 L1 L1 Equation 5 and 6 are noteworthy because of their concision in expressing the conditions for zero-ripple current phenomenon to occur, but unfortunately its physical nature is not shown. To provide some physical insight, let us consider the a = n coupled inductor model (n is the physical turn ratio N2/N1) excited by equal terminal voltages v(t), shown in Figure 6. Figure 6.
AN3180 Zero-ripple current phenomenon: theory To summarize the main results of this brief analysis: 1. Ripple-current can be reduced to zero in either winding of a two-winding coupled inductor, but not simultaneously 2. The only conditions imposed on the voltages that excite the windings, in order to get ripple current steering, is that they are proportional, with the same frequency and phase 3.
Sensitivity of zero-ripple current condition 3 AN3180 Sensitivity of zero-ripple current condition In real-world coupled inductors it is unthinkable to reduce the ripple current in a winding to exactly zero and produce a perfect ripple steering. There are two basic reasons for this: ● Zero-ripple condition mismatch. In practice, the inductance of a winding is determined by the number of turns and the average permeability of the associated magnetic circuit.
AN3180 Sensitivity of zero-ripple current condition If the attenuation A is defined as the ratio of the residual ripple di2(t)/dt, given by Equation 7 or 8, to the ripple that would be there without the coupled inductor (di1(t)/dt =v1(t)/L1, equal to the actual ripple on the cancellation winding, as L1 is unchanged), it is possible to write for the worst case scenario: Equation 10 di 2 ( t ) ⎛ Δv(t) ⎞ L di 2 ( t ) =ρ⎜ + δ⎟ A = dt = 1 ⎜ v ( t) ⎟ di1 (t) v1 (t) dt ⎝ 1 ⎠ dt where Δv(t) = v2(t)-v1(t) is the
Sensitivity of zero-ripple current condition Figure 7.
AN3180 4 Zero-ripple current phenomenon: practice Zero-ripple current phenomenon: practice Before giving details of the practical realization of a coupled inductor able to provide ripple steering, it is useful to draw some conclusions of considerable practical interest from the theoretical analysis carried out in the previous sections: 1.
Zero-ripple current phenomenon: practice Figure 9. AN3180 Two-section slotted bobbin suggested for the realization of a coupled inductor - top view Figure 10. Two-section slotted bobbin suggested for the realization of a coupled inductor - side view 6HFRQGDU\ SULPDU\ ZLQGLQJ VORW 3ULPDU\ VHFRQGDU\ ZLQGLQJ VORW 6HFRQGDU\ SULPDU\ ZLQGLQJ VORW 3ULPDU\ VHFRQGDU\ ZLQGLQJ VORW !- V 3.
AN3180 Zero-ripple current phenomenon: practice At this point, all the elements needed to outline a step-by-step practical design procedure are in place. For the details of steps 2 to 4, please refer to the algorithm described in References 3. 1. The case of a conventional inductor is considered.
Zero-ripple current phenomenon: practice AN3180 Equation 17 N2 = N1 L L − L LK where, in this case, L is the measured inductance of the AC winding. Add 5 % to the result, to account for the slight coupling improvement that there is as the DC winding is entirely in place, and round up to the next integer. Let this number be N2 7. After removing the layer used for the preliminary measurement and the core, wind the N2 turns of the DC winding.
AN3180 Zero-ripple current phenomenon: practice bobbin, unless mounting clips or gluing are used (quite impractical to use during the cutand-try phase). The position of the core inside the bobbin, especially along the direction of the legs, is critical because it changes the position of the air gap with respect to the windings.
Capacitor selection 5 AN3180 Capacitor selection It is obvious that the capacitance of the smoothing transformer capacitor should be as large as possible and its ESR as low as possible to minimize the impressed voltage mismatch. Although this assertion is always true, the use in a PFC pre-regulator to minimize the input current ripple poses an important limitation to the capacitance value that can be used.
AN3180 6 A 200 W ripple-free input current PFC pre-regulator A 200 W ripple-free input current PFC pre-regulator Figure 13 shows the electrical schematic of the EVL6563S-ZRC200 demonstration board, a 200 W zero-ripple-current PFC pre-regulator (see BOM in Table 1) based on the boost topology modified by adding the cancellation winding to the boost inductor and a capacitor, to make a smoothing transformer to minimize the input ripple.
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AN3180 Table 1. A 200 W ripple-free input current PFC pre-regulator Bill of material Ref. Value Description Supplier BD1 D15XB60 15 A/600 V single phase bridge rectifier SHINDENGEN C1 220 nF-630 V 630 V polypropylene film capacitor AVX C2 1.
A 200 W ripple-free input current PFC pre-regulator Table 1. AN3180 Bill of material (continued) Ref.
AN3180 A 200 W ripple-free input current PFC pre-regulator Table 3. 200 W PFC pre-regulator with ripple-free input current: coupled inductor specification Item Description Core 2xEFD35, PC40 material grade Bobbin 2-slot, 14 pins, horizontal mounting Inductance AC winding inductance (pin 3-1): 260 µH DC winding inductance (pin 12-8): 490 µH leakage inductance (pin 12-8 with pin 1-3 shorted): 255 µH Winding Pin S/E Wire Turns Notes AC 3/1 100 x 0.1 mm 46 Wound in slot 1 DC 12/8 0.
A 200 W ripple-free input current PFC pre-regulator AN3180 Figure 16.
AN3180 A 200 W ripple-free input current PFC pre-regulator Figure 17. Line current and voltage @ full load Figure 18. Line current and voltage @ full load (200 W) - Line current and voltage (200 W) - Line current and voltage @ 115 Vac -200 W @ 230 Vac -200 W Figure 20. Figure 19.
A 200 W ripple-free input current PFC pre-regulator AN3180 The zoomed image of Figure 20 shows a residual ripple current in the DC winding of 114 mA pk-pk, against a 2.962 A pk-pk of the AC winding current, and so resulting in 0.114/2.962 = 38.5*10-3 (-28.3 dB) attenuation.
AN3180 7 Conclusions Conclusions The ripple-steering technique, with its ability to reduce an inductor ripple current theoretically to zero, has been discussed and its theoretical base has been outlined. The construction of a coupled inductor is addressed and a practical design guide given to enable its successful implementation.
References 8 28/39 AN3180 References 1. “A 'Zero' Ripple Technique Applicable to Any DC Converter”, Power Electronics Specialists Conference, 1999. PESC 99. 30th Annual IEEE (1999 Volume 2) 2. “The Coupled Inductor Filter: Analysis and Design for AC Systems”, IEEE Transactions on Power Electronics, Vol. 45, No. 4, August 1998, pp. 574-578 3. “Inductor and Flyback Transformer Design”, Unitrode Magnetics Design Handbook (MAG100A), Section 5 4.
AN3180 Electrical equivalent circuit models of coupled inductors and transformers Appendix A Electrical equivalent circuit models of coupled inductors and transformers A system of coupled inductors is a set of coils that share one or more common magnetic paths because of their proximity. Because of this, magnetic flux changes in any one coil do not only induce a voltage across that coil by self-induction, but also across the others by mutual induction.
Electrical equivalent circuit models of coupled inductors and transformers AN3180 Figure 23. Coupled inductors 0 0 L W Y W L W L W / / Y W Y W 0! L W / / Y W 0 !- V The mutual inductance M cannot be assigned arbitrarily but must fulfill the inequality: Equation 22 M ≤ L1 L 2 The case M = L1 L 2 is that of perfectly coupled inductors, that is when the magnetic flux generated by either winding is totally linked to the other.
AN3180 Electrical equivalent circuit models of coupled inductors and transformers Figure 24.
Electrical equivalent circuit models of coupled inductors and transformers AN3180 inductance Lb is associated to the secondary leakage flux, the flux generated by the secondary winding and not completely linked to itself nor to the primary winding: it is called secondary leakage inductance and is designated by Ll2.
AN3180 Electrical equivalent circuit models of coupled inductors and transformers Figure 26. Model of coupled inductors with a = ne (a = ne model) L W N / Y W QH N / L W QH / Y W LGHDO !- V Another interesting and quite frequently used choice is a = 1.
Electrical equivalent circuit models of coupled inductors and transformers AN3180 Figure 28.
AN3180 Electrical equivalent circuit models of coupled inductors and transformers is associated to the magnetic flux that flows mostly through the magnetic core and so, is affected by the non-linearity typical of magnetic materials.
Measuring transformer and coupled inductor parameters Appendix B AN3180 Measuring transformer and coupled inductor parameters From a practical point of view there is the need to measure the parameters of coupled inductors (transformers) L1, L2 and M or, equivalently, k, and derive those of the equivalent circuits. Needless to say that it is of particular interest to obtain the parameters of the “physical model” with a = n, LM, Ll1 and Ll2. L1 and L2 are directly measurable.
AN3180 Measuring transformer and coupled inductor parameters Figure 30. Winding connections: aiding flux (left), opposing flux (right) / / 0 0 / / / $ / / 0 / 2 / / 0 !- V It is always LA > LO, therefore, as already mentioned: Equation 32 M= L A − LO 4 ⇒ k= L A − LO 4 L1 L 2 The advantage of this method is its low sensitivity to winding resistance and to the impedance of the wire used for connecting the windings.
Revision history AN3180 Revision history Table 4. 38/39 Document revision history Date Revision 14-Sep-2010 1 Changes Initial release.
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