外文翻译(1)

Low Voltage Flyback DC-DC Converter For

Power Supply Applications

Hangzhou Liu1, John Elmes2, Kejiu Zhang1, Thomas X. Wu1, Issa Batarseh1 Department of Electrical Engineering and Computer Science,

University of Central Florida, Orlando, FL 32816, USA

Advanced Power Electronics Corporation, Orlando, FL 32826, USA Abstract ：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 Linear 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.INTRODUCTION

The goal of this project is to develop and build a prototype of a high-efficiency, high-temperature isolated 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 ratings. 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 is﹢16V and the other one is﹣6V. In order to get the desired performance, anLT3574 IC chip from Linear Technology is used. 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 the transformer turns ratio and the flyback loop resistor pairs. Therefore, at the initial design stage, we can choose a convenient turn’s ratio for the transformer, and m odify it later on if necessary to make sure the output performance is desirable and the transformer will not saturate [1].

The relationship between transformers turns ratio and duty cycle can be found as

Where n is the transformer turns ratio, D is the duty cycle, V O` is the sum of the output voltage plus the rectifier drop voltage, V IN is the input voltage of the transformer. The value of feedback resistor can be calculated as

Where R REF is the reference resist or, whose value is typically 6.04k?; α is a constant of 0.986;V BG is the internal band gap reference voltage, 1.23V; and V TC 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 modified 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 conditions.

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 Do well’s curve rule [2].In order to get more accurate values of AC resistance values; we propose to use finite element 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 generated.

III. CONVERTER SIMULATION RESULTS

We choose LT3574 chip in this design. From the simulation results in Figure 1 and Table 1, it clearly shows that the output voltages which are﹢16V and -6V respectively can be regulated pretty well with the input voltage range from 9V to 36V. The voltage tolerance 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 design

Figure 1 . Output voltage and current simulation results

Table 1 . LT Spice simulation resuits

IV.TRANSFORMER SIMULATION RESULTS

With the initial design parameters 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 voltage in the range. Figure 3 shows the current through the primary winding.

Figure 2 . Voltage of the primary winding

Figure 3 . Current of the primary winding

Since 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 structure. 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 marked 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 method

Based on the computer simulation, the 6-layer planar transformer winding 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 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 . Inner 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 transformer

The E-I core transformer PCB in this design will be integrated into the converter’s 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 LAYOUT

In this project, we make the transformer part layout as 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 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 1.4×07, It can always reduce the size of the board by adding 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 converter

VI.CONCLUSION

In this paper, a flyback DC - DC converter for low voltage power supply application has 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 desirable 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.

REFERENCE

[1] Linear Technology Application Notes , Datasheet of Isolated Flyback Converter

Without an Opto-Coupler, https://www.360docs.net/doc/892567901.html,/docs /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 Getting Started: A Transformer Design Example,

https://www.360docs.net/doc/892567901.html,/download/ 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 (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 (NAECON) , vol., no., pp.116-120, 21-23 July 2009.

[7] Ferroxcube Application Notes, Design of Planar Power Transformer,