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1、Low Voltage Flyback DC-DC Converter ForPower Supply ApplicationsHangzhou Liu1, John Elmes2, Kejiu Zhang1, Thomas X. Wu1, Issa Batarseh1Department of Electrical Engineering and Computer Science,University of Central Florida, Orlando, FL 32816, USAAdvanced Power Electronics Corporation, Orlando, FL 32

2、826, USAAbstract In this paper, we design a low voltage DC-DC converter with a flyback transformer. The converter will be used as a biased power supply to drive IGBTs. The flyback transformer using planar EI-core is designed and simulated using ANSYS PExprt software. Besides, anLT3574 IC chip from L

3、inear Technology has been chosen for converter control. Finally, the converter modeling and simulation are presented and PCB layout is designed.Keywords：Flyback, anLT3574IC, PCB I. INTRODUCTIONThe goal of this project is to develop and build a prototype of a high-efficiency, high-temperature isolate

4、d DC-DC converter to be used as a biased power supply for driving a complementary IGBT pair. It is important that the converter can deliver the required power at an ambient temperature of up to 100; therefore it has to be efficient so that its components do not exceed their maximum temperature ratin

5、gs. The final converter will be completely sealed and potted in a metal case. The input voltage range for this converter is from 9V to 36V. The output sides have two terminals, one is16V and the other one is6V. In order to get the desired performance, anLT3574 IC chip from Linear Technology is used.

6、 The key to this design is the flyback transformer. The transformer using planar EI-core is designed and simulated using ANSYS PExprt software. Finally, the PCB layout of the converter will be presented.II. KEY DESIGN OUTLINE For this flyback topology, the output voltage can be determined by both th

7、e transformer turns ratio and the flyback loop resistor pairs. Therefore, at the initial design stage, we can choose a convenient turns ratio for the transformer, and modify it later on if necessary to make sure the output performance is desirable and the transformer will not saturate 1.The relation

8、ship between transformers turns ratio and duty cycle can be found asWhere n is the transformer turns ratio, D is the duty cycle, VO is the sum of the output voltage plus the rectifier drop voltage, VIN is the input voltage of the transformer.The value of feedback resistor can be calculated asWhere R

9、REF is the reference resistor, whose value is typically 6.04k; is a constant of 0.986;VBG is the internal band gap reference voltage, 1.23V; and VTC is normally 0.55V 1.With a specific IC chosen, the converter circuit can be designed based on a demo circuit and some parameters may need to be modifie

10、d if necessary to optimize the performance. Furthermore, in LT Spice, a large number of simulations need to be done with different conditions such as load resistor values and input voltage levels. It is important to make sure that the output voltage can be regulated well with all these different con

11、ditions.The most critical part of the design is the flyback transformer. With high switching frequency, the AC resistance can only be estimated based on some traditional methods such as Dowells curve rule 2.In order to get more accurate values of AC resistance values; we propose to use finite elemen

12、t electromagnetic software ANSYS PExprt to do the design 3. At the initial design stage, key parameters such as the worst-case input voltage, frequency, material, inductance values will be decided. After that, these data will be imported to the software, from which an optimized solution will be gene

13、rated.III. CONVERTER SIMULATION RESULTSWe choose LT3574 chip in this design. From the simulation results in Figure 1 and Table 1, it clearly shows that the output voltages which are16V and -6V respectively can be regulated pretty well with the input voltage range from 9V to 36V. The voltage toleranc

14、e ranges are from 15V to 19V and -12V to - 5V, respectively. In addition, the current is also under control, which is around 100mA in this designFigure 1 . Output voltage and current simulation resultsTable 1 . LT Spice simulation resuitsIV. TRANSFORMER SIMULATION RESULTSWith the initial design para

15、meters of the transformer, we use ANSYS PExprt to simulate and further optimize the transformer 4.Figure 2 shows the primary winding voltage. In order to make the transformer work correctly in all cases, it is important to make sure that it can work at the worst case, which is the minimum input volt

16、age in the range. Figure 3 shows the current through the primary winding.Figure 2 . Voltage of the primary windingFigure 3 . Current of the primary windingSince it is a low power converter in this design, it is critical to minimize the power losses. We choose to use the planar type transformer struc

17、ture. After doing the winding interleaving, the power loss can be reduced by approximately 25% and the temperature rise can be reduced by approximately 15% 5.The structure can be found in Figure 4. The primary winding is marked in yellow, which has 6 turns in series. The first secondary winding is m

18、arked in red, which has 3 turns in parallel. The second secondary winding is marked in blue, which has 1 turn. It will be totally 6 layers in the multi-layer transformer structure 6.Figure 4 . Winding geometry by interleaving methodBased on the computer simulation, the 6-layer planar transformer win

19、ding structure can be drawn in Figures 5 -10. The primary side winding has 6 turns in series. In Figures 6 and 9, it clearly shows that the turns in different layers are connecting through via hole. In one of the secondary winding which is the +16V one, it has 3 turns in parallel as shown in Figures

20、 5, 8 and 10. The one turn secondary winding (6V) is shown in Figure 7.Figure 5 . Top layer winding structure (secondary 1)Figure 6 . Inner Layer 1 winding structure (primary)Figure 7 . Inner Layer 2 winding structure (secondary 2)Figure 8 . Inner Layer 3 winding structure (secondary 1)Figure 9 . In

21、ner Layer 4 winding structure (primary)Figure 10 . Bottom layer winding structure (secondary 1)The core loss of the transformer is approximately 47mW, comparing to the winding loss of 154mW, it i s about 30%, as shown in Figure 11 7.Figure 11. Power loss of transformerThe E-I core transformer PCB in

22、 this design will be integrated into the converters PCB, rather than a separate board being added to the whole circuit 8, which will reduce the cost of the PCB fabrication since multi-layer PCB layout is expensive.V. CONVERTER CIRCUIT PCB LAYOUTIn this project, we make the transformer part layout as

23、 one component; it will be integrated into the whole circuit PCB layout. It has 6 layers totally. The isolation requirement is 1500V, so the layout takes a little more space than the one without any isolation rules. In Figure 12, we make the primary side components all in the right hand side of the

24、board, the secondary sides all in the left hand side of the board, and the transformer in between them.The wire traces have been marked with different colors in order to show the specific layer that the traces are on The board area is about 1.407, It can always reduce the size of the board by adding

25、 more layers. However, the cost will be more expensive. It is important to balance these factors. The size of the PCB board meets the specs of the project.Figure 12. PCB layout of the flyback converterVI. CONCLUSIONIn this paper, a flyback DC - DC converter for low voltage power supply application h

26、as been designed. The modeling and simulation results are presented. Based on the design specifications, a suitable IC from Linear Technology is chosen. A large amount of circuit simulations with different conditions such as load resistor values and input voltage levels are presented to get the desi

27、rable output voltage and current performance. The transformer has been designed including electrical, mechanical and thermal properties. With all the specific components decided, the PCB layout of the converter has been designed as well.REFERENCE1 Linear Technology Application Notes , Datasheet of I

28、solated Flyback Converter Without an Opto-Coupler, /Datasheet/3574f.pdf.2 P.L.Dowell, “Effect of eddy currents in transformer windings” Proceedings of the IEE, NO.8 PP.1387-1394, Aug 1966. 3 S.Xiao, “Planar Magnetics Design for Low- Voltage DC-DC Converters” MS, 2004.4 ANSYS Application Notes, PEmag

29、 Getting Started: A Transformer Design Example, EDA/Maxwell9/planarGS0601.pdf.5 K. Zhang; T. X.Wu; H.Hu; Z. Qian; F.Chen.; K.Rustom; N.Kutkut; J.Shen; I.Batarseh; Analysis and design of distributed transformers for solar power conversion 2011 IEEE Applied Power Electronics Conference and Exposition

30、(APEC), v l., no., pp.1692-1697, 6-11 March 2011. 6 Zhang.; T.X.Wu.; N.Kutkut; J.Shen; D.Woodburn; L.Chow; W.Wu; H.Mustain; I. Batarseh; ,Modeling and design optimization of planar power transformer for aerospace applic ation, Proceedings of the IEEE 2009 National, Aerospace & Electronics Conference

31、 (NAECON) , vol., no., pp.116-120, 21-23 July 2009. 7 Ferroxcube Application Notes, Design of Planar Power Transformer, 低电压反激式DC-DC转换器的在电源中的应用Hangzhou Liu1, John Elmes2, Kejiu Zhang1, Thomas X. Wu1, Issa Batarseh1Department of Electrical Engineering and Computer Science, University of Central Florid

32、a, Orlando, FL 32816, USAAdvanced Power Electronics Corporation, Orlando, FL 32826, USA摘要：在本文中，我们设计了一个低电压反激式DC-DC转换器。该转换器将被用来作为一个偏置电源来驱动器的IGBT。该反激式变压器采用平面EI磁芯设计并且使用ANSYS PExprt软件仿真。此外，该变换器控制芯片选用凌力尔特公司的anLT3574IC芯片。最后，介绍该转换器的建模与仿真和PCB布局设计。关键词：反激式,，anLT3574IC，PCB1引 言该项目的目标是开发和建立一个高效率的原型，高温隔离式DC-DC转换器，

33、作为偏置电源用来驱动一对互补的IGBT。更重要的是，该转换器可提供所需的功率，在环境温度高达 100，因此它必须是高效的，以便它的组件不超过其最大额定温度。最终转换器将用金属外壳完全密封。该转换器的输入电压范围为9V至36V。输出端有两个接地端子，一个是+16 V，而另一个是6V。为了获得所需的性能，采用了凌力尔特anLT3574 IC芯片。这种设计的关键是在反激式变压器。变压器采用平面EI磁芯设计且使用ANSYS PExprt软件仿真。最后，介绍了该转换器的印刷电路板的布局。2关键的设计大纲对于这种反激式拓扑结构，输出电压可以通过变压器匝数比和反激回路电阻确定。因此，在最初的设计阶段，我们可

34、以选择一个简便的变压器匝数比，以后如有必要通过修改变压器匝数比，确保变压器的输出性能是可靠的，不会饱和1。变压器匝数比和占空比之间的关系可以得到其中，n是变压器的匝数比，D为占空比，VO是输出电压加上整流电压降的总和，Vin是变压器的输入电压。反馈电阻的值可以计算为其中，RREF是参考电阻，其值通常是6.04k。是一个常数0.986 VBG是内部带隙参考电压，1.23V和V TC 通常是0.55V 1。与一个特定的集成电路的选择相比，转换器电路设计的基于一个参数可修改的演示电路，如有必要，修改某些可能需要修改的参数来优化电路性能。此外，在LT Spice，需要大量的做不同的条件下的仿真，如负载

35、的电阻值和输入电压值。更重要的是要确保，在所有这些不同的条件下，输出电压是可调节的。最关键的部分的设计是反激式变压器。在高开关频率下，交流电阻参数只能基于在一些传统的方法上，如Dowell的曲线规则2，进行估计。为了得到更准确的值的交流电阻值，我们建议使用电磁学有限元发分析软件ANSYS PExprt做设计3。在最初的设计阶段，关键的参数，例如在最坏情况下的输入电压，频率，素材，电感值将被决定。之后，这些数据将被导入到软件中，从中将产生一个优化的解决方案。3. 转换器的仿真结果在这个设计中我们选择的LT3574芯片。从图1和表1的仿真结果，它清楚地表明，输出电压分别是+16 V，-6V在从9V

36、至36V的输入电压范围可调节效果相当不错。输出电压所能承受的电压范围分别是从+15 V至+19 V和-12V - 5V。另外，电流也在控制范围中，其中在本设计中是大约100毫安。图1. 输出电压和电流的模拟结果表1 . LT Spice仿真结果4. 变压器仿真结果与最初的设计参数的变压器中，我们使用ANSYS PExprt仿真来进一步优化变压器设计4。图2 示出的初级绕组电压。为了使变压器在所有的情况下的正常工作，更重要的是要确保，它可以在最坏的情况下工作，且在输入电压最低限度范围内。图3显示了通过初级绕组的电流 。图2在初级绕组的电压 图 3.初级绕组的电流在本设计中因为它是一个低功耗的转换

37、器，关键是尽量减少功率损耗。我们选择使用平面型变压器的构造。交错绕组后，功率损耗可以降低约25，并且可减少约15的温升。这种交错绕组的结构，可以参见在图4。初级绕组被标记为黄色，有6匝串联。第一次级绕组被标记为图 4.绕组几何交织方法红色，其中有3圈并联。第二次级绕组被标记为蓝色，其中有1圈。这将是总共6层的多层变压器结构6。根据在计算机上仿真，6层的平面变压器绕组结构可以如图5 -10中那样绘制。初级侧绕组串联有6匝。在图6和9中，它清楚地表明，在不同的层中的匝数，通过导通孔连接。+16 V一个绕组与一个次级绕组连接，它具有图5，图8和图10中3匝并联连接3匝。图7中所示为次级绕组的一匝（6

38、V）。图5. 顶层绕组结构（中一） 图6. 内1层绕组结构（主） 图7.内部第2层绕组结构（中2） 图8.内部第3层绕组结构（中一） 图 9. 内4层绕组结构（主） 图10.底层绕组结构（中1）该变压器的磁心损耗是约47MW，同154mW的绕组损耗相比，它的损耗约30，如在图11中所示的7。（左图） 图11.变压器的功率损耗该EI芯变压器印刷电路板的计将被集成到转换器的印刷电路板上，而不是一个单独的电路板被添加的整个电路中8，这将降低由多层PCB布局而产生昂贵的PCB制造工艺的成本。5. 转换电路 PCB布局在这个项目中，我们使变压器部分布局的一个组成部分，它将被集成到整个电路的PCB布局。它

39、 总共 有 6层。隔离的要求是1500V，所以布局需要更多一点的空间比一个没有任何隔离规则。如图12中，我们将初级侧元件放置在印刷电路板的右边，二级侧元件放置在印刷电路板的左边，转换器放置在他们中间。电线的痕迹已用不同颜色标记，以显示指定层的痕迹是在电路板面积大约是1.40.7英寸。它总是可以减少电路板的大小，通过添加更多的层。然而，成本将更加昂贵。重要的是要平衡这些因素。在PCB板的尺寸符合该项目的规格。图12. 反激式转换器的PCB布局6.结论在本文中，反激式直流- 直流转换器的低电压电源应用已经设计完毕。它的建模与仿真结果都已介绍。根据设计规格，从凌力尔特公司产品中选择一个合适的IC。在

40、不同的条件，做了大量的仿真，如负载的电阻值和输入电压电平的电路仿真，有助于以获得所需要的输出电压和电流的性能。包括电气，机械和热性能的变换器就已经设计完成了。随着所有特定元器件的确定，该转换器的印刷电路板布局已经被设计完成了。参考文献1 Linear Technology Application Notes , Datasheet of Isolated Flyback Converter Without an Opto-Coupler, /Datasheet/3574f.pdf.2 P.L.Dowell, “Effect of eddy currents in transformer win

41、dings” Procedings of the IEE, vol.113, No.8, pp.1387-1394, Aug 1966.3 S.Xiao, “Planar Magnetics Design for Low- Voltage DC-DC Converters” MS thesis, University of Central Florida, 2004.4 ANSYS Application Notes, PExprt Getting Started: A Transformer Design Example,5 K. Zhang; T. X.Wu; H.Hu; Z. Qian;

42、 F.Chen.; K.Rustom; N.Kutkut; J.Shen; I.Batarseh; , Analysis and design of distributed transformers for solar power conversion 2011 IEEE Applied Power Electronics Conference and Exposition (APEC), v l., no., pp.1692-1697, 6-11 March 2011.6 K. Zhang.; T.X.Wu.; N.Kutkut; J.Shen; D.Woodburn; L.Chow; W.

43、Wu; H.Mustain; I. Batarseh; , Modeling and design optimization of planar power transformer for aerospace applic ation, Proceedings of the IEEE 2009 National, Aerospace & Electronics Conference (NAECON) , vol., no., pp.116-120, 21-23 July 2009.7 erroxcube Application Notes, Design of Planar Power Transformer, 16