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 6.04kΩ; 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
