Integration Planar Transformer for Reducing Volume, Leakage Inductances and improve EMC disturbances of a radar power supply.

Transcription

1 Integration Planar Transformer for Reducing Volume, Leakage Inductances and improve EMC disturbances of a radar power supply Sega GUEYE*, Brayima DAKYO*, Sylvain ALVES**, Philippe EUDELINE**, Joël CORDIER **, Laboratory GREAH * University of Le Havre, 5 Rue Philippe Lebon Le havre FRANCE segagueye@univ-lehavrefr, dakyo@univ-lehavrefr ** Thales Air Systems, ZI du Mont Jarret 7650 Ymare FRANCE, sylvainalves@thalesgroupcom, philippeeudeline@thalesgroupcom, joelcordier@thalesgroupcom Abstract: This article deals with EMC disturbances caused by leakage inductances and stray capacitances of the internal transformer from a power supply unit for a radar subset A measurement in emissive conductance shows that their frequencies are critical facing MIL-STD-461E EMC standard To cancel their disturbances, reduce the volume and the weight, we use planar technology We also propose new interleaving techniques layers of a planar transformer to further reduce the leakage inductances Key-Words: - PFC, MMF, AC/DC, DC/DC, 3C90 1 Introduction The increasingly compact system developments makes that we must develop a compact, robust power supply and control the interaction between the various parts of the power supply while avoiding the electromagnetic disturbances coming from the external environment and/ or the internal circuits to the module We present in this article a solution allowing to reduce the weight, the volume, the size and disturbances caused by the leakage inductance of the classical power transformer of the radar power supply We will present a technical interleaving of the layers of the planar transformer to further reduce the leakage inductance Synoptic of the module The AC/DC part of the PFC (Power Factor Corrector) Fig1 is composed by a converter which uses a switching transformer which manages the transfer of energy between the network and DC/DC The transformer is susceptible to be source and/or victim of disturbances by wire crosstalk (by its inductances and stray capacitances), by common impedance and coupling field to wire and loop Network 30 V AC Fig1 Synoptic of the radar power supply 3 Problems AC/DC ( PFC ) DC/DC 1 (BUCK) DC/DC (BUCK) DC/DC 8 (BUCK) The work carried out in [] had shown that the main sources of disturbances of the AC/DC part were mainly the leakage inductances, the stray capacitances of the transformer and the inductance of the smoothing circuit We also note that the field lines of the transformer are also found in the wire of the input network of the module Fig V 01 V 0 V 08 POWER AMPLIFIER ISBN:

2 F d = 100kHz and slow down the transfer of energy between primary and secondary and thus increase the leakage flux then the leakage inductances Wires connection to external Fig Spectrum measurement in emissive conduction of the power supply input Measurement in emissive conduction according to standard MIL-STD-461E Fig shows that we find the switching frequency (100 khz) and the resonance frequency between leakage inductances and the stray capacities of the transformer highlighted in [] The energy stored in the leakage inductance creates disturbances, voltage spikes in the terminals of the switching MOSFET power supply Fig3, it also increases the switching losses and therefore reduces the efficiency Fig3 Disturbances in terminals of the Drain-Source V DS of the power MOSFET One of solutions carried out in [] to reduce the leakage inductances and the stray capacitances is to use the planar technology The planar technology presents advantages listed below: Very low profiles, Low leakage inductance, Good MTBF, Increasing of the power density The other motivation to use planar technology is to reduce the volume, the weight, the height of the classical transformer and remove the wires connection of the transformer Fig4 The wires of the transformer can moreover collect disturbances resulting from outside by coupling field, this external field can be found in the magnetic circuit Fig4 Actual transformer use in the power supply 4 Reducing the leakage inductances The comparison of the classical and planar technology Table1 shows that we can reduce the serial resistances of the classical transformer windings, the leakage inductances, the sizes, the volumes but the drawback is the increasing capacitance between windings with planar technology Designation Leakage Inductance Serial resistances of windings Size and Volume Capacitance between layers Classical transformer Planar transformer Table 1 Comparison of classical and planar technology The representation of the primary and secondary windings is shown in Fig5 ISBN:

3 Fig5 Primary and secondary windings The Magneto Motive Force (MMF) variation Fig9 was plotted by assuming that the planar transformer turns ratio is equal to 1 and the total number of turns is 4 ( turns per layer) In reality, the planar transformer turns ratio is equal to 05 and the numbers of primary and secondary turns are equal to 3 and 16 respectively We don t present in this article the design of the planar transformer but the techniques of interleaving layers to improve the leakage inductances of the planar transformer The calculation of the energy stored in the primary and secondary windings is based on the formula below: E Primary Windings 1 1 = B H dv = L f I (1) B: Magnetic flux density, H: Magnetic flux strength, dv: Variation of the volume, Lf: Leakage inductance, I: Rms current Secondary Windings Based on the distribution of the MMF Fig9, the leakage inductances of primary and secondary are obtained by the following formula µ 0 E = h h P S I x I x I 8 dx + 4 dx + hi 0 l hp 0 l hs l I I (hi + h + hi ) + hi + l l L l I 3I (hi + h + hi ) + hi l l 3I 4I + (hi + h + hi ) + hi l l µ L hp + hs L f + 49 h l 3 = I () + h p,h S : Copper thickness of primary and secondary tracks, h I : thickness between layers, L: Total length of primary and secondary tracks l : Width of primary and secondary tracks The lengths of primary and secondary layers are respectively 953 mm and 99 mm The width of the primary and secondary tracks is equal to 8 mm The thicknesses of primary and secondary copper, the insulation between the primary and secondary layers are equal to 00 µm We thus obtain L fp = 01µH and L fs = 10µH L fp and L fs are respectively the leakage of primary and secondary windings For industrial feasibility reasons, the planar transformer PCB is made by two circuits of 5mm each and paste them Fig6 Ferrite: 3C90 Circuit 1 Circuit Fig6 D view of the planar Transformer ISBN:

4 The leakage inductances of the two circuits are in parallel The equivalent primary and secondary inductances are equal to half the leakage inductances calculated above and then become 101µH and 51 µh respectively Connection to external volume, weight and leakage of the transformer is reached S1 I I 3I 4I S Fig7 Example of primary tracks P P S3 P Fig8 Example of secondary tracks The comparison of Fig3 and Fig10 shows that the oscillations on the phase Off of the MOSFET are cancelled with the planar transformer This is mainly due to reduced leakage with planar technology Designation Classical Transformer Planar Transformer Weight Height Volume (g) (mm) (cm 3 ) P S4 P Fig9 Magneto Motive Force (MMF) variation of the circuit 1 of the planar transformer Table Comparison of two transformers Table shows that we almost divide the weight, the height and the volume of classical transformer by two Fig11 and Fig1 The aim to reduce the ISBN:

5 MOSFET On MOSFET Off In our case the number of primary turns is much higher than the secondary ones (3>16) To reduce the leakage inductances we present a technical interleaving PSSPSSPPPPPP (Fig13) that can be applied when the ratio between primary and secondary windings is important That is when the primary turns are above or below the secondary turns and vice versa This technique consists to put the layers that have more turns near the ferrite to reduce leakage The arrangement of the layers of this technique applied to the planar transformer above is shown below P -I -I +I +I Fig10 Drain- source voltage V DS of the switching MOSFET S1 S P H = 54 mm S3 S4 Fig11 Height of the classical transformer P H = 1 mm Fig1 Height of the planar transformer 5 Improving the leakage inductances To reduce the primary and secondary leakage inductances of the planar transformer we expect to use a new technical interleaving The complete interleaving of primary and secondary layers PSPPSPPSPPSP (Fig9) used in the design of the planar transformer has reduced the leakage inductance of the classical transformer P Fig13 New variation of the Magneto Motive Force (MMF) of the circuit 1 of the planar transformer ISBN:

6 The calculation of the primary and secondary leakage inductances associated with this technique gives: µ L hp + hs L f + 18 h l 3 = I (3) Using the same values of width, length and insulation of the planar transformer above, we thus obtain = 8µH and L fs = 4µH The equivalent primary and secondary leakage inductances become 4µH and µh respectively This technique of interleaving greatly reduces the leakage inductance 6 Conclusion We managed to reduce the volume, weight, leakage inductances and the oscillations at the terminals of the power MOSFETs with planar technology The next step of our works is to realize the planar transformer and validate the technique of interleaving that we developed in this paper References: [1] Ziwei Ouyang, Thomsen OC, Andersen MAE, Optimal design and tradeoffs analysis for planar transformer in high power DC-DC converters, International Power Electronics Conference (IPEC), 010, pp [] S Gueye, B Dakyo, J Raharijaona, D Baudry, Z Riah, S Alves, P Eudeline, Analyse, identification, and modeling of electromagnetic disturbance sources Actual signal approach applied to power supply unit, Compatibility and Power Electronics, pp , 0- May 009 [3] Z Ouyang, C Thomsen, A E Andersen, The analysis and comparison of leakage inductance in different winding arrangements for planar transformer, Power Electronics and drive systems, pp ,PEDS 009 [4] Ngo KDT, Alley RP, Yerman AJ, Charles RJ, Kuo MH, Design Issues for the Transformer in a Low-Voltage Power Supply with High Efficiency and High Power Density, IEE Transactions on Power Electronics, Vol 7, 199, pp [5] Hurley WG, Wilcox DJ, Calculation of leakage inductance in transformer windings, IEE Transactions on Power Electronics, Vol 9, 1994, pp [6] Hurley WG, Wilcox DJ, Calculation of leakage inductance in transformer windings, IEE Transactions on Power Electronics, Vol 9, 1994, pp ISBN: