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Discussion on the design of flyback transformer

Source:http://www.hfztdz.com/en/index.asp?/gnews/1.html  时间:2016-11-1
Flyback switching power supply due to the small size, low cost, simple circuit characteristics, sought after by the designers. Many beginners also chose the flyback transformer design of the starting point of learning and research object, but the network of flyback transformer and more scattered all kinds of learning materials, this thesis will analyze the design of the flyback transformer is from A to Z combing the scattered knowledge integration, and the corresponding analysis, to help you as soon as possible master.
Design flow
Confirm the basic technical parameters
Reminder: we should develop good working habits, no matter how small the power technology, how simple, to produce a detailed technical specifications for each product. First of all, to figure out what they are going to do a product, which will make the design ideas more clearly, and how to start the next step of work.
Technical parameters are divided into two types: basic and detailed.
The basic technical parameters are generally listed as follows (in the case of 60W products):
Minimum input voltage: 85VAC;
Maximum input voltage: 265VAC;
Output voltage current: 12V5A (accuracy 1%);
Minimum efficiency: 85%;
Operating temperature: -25 ~ +60;
Detailed technical parameters of the trouble, according to different circumstances, the need to list the parameters are more and less. Generally include: input and output characteristics, protection characteristics, safety rules, EMC, reliability, application environment, product size, input and output port definition, product labels, shell labels, product packaging, etc..
Input / output characteristics
The range of input voltage, input frequency, power factor, maximum input current, impulse current, output voltage, output current range, voltage regulation, load regulation, voltage accuracy, ripple peak peak value, the efficiency, standby power, turn on delay time, output voltage rise time, capacitive load switch, overshoot and the dynamic response time, dynamic response, amplitude and minimum starting voltage.
Protection characteristic
Input under voltage protection, input under voltage recovery point, input over-voltage protection point, input overvoltage recovery point, output over-voltage protection point, output short circuit protection mode, over temperature protection point, over temperature recovery point.
Tips: for some non-standard products, if you do not know the parameters of the list, it is recommended to refer to competitors product information or the industry's most influential suppliers. If these materials are not, as far as possible to the standard product technical indicators close to.
Design ideas (to make a design proposal and reference calculation)
Identify design difficulties and solutions according to the technical specifications of the products.
Tips: if you want to maximize the degree of avoidance of failure. In the beginning of the project design should have undergone extensive internal discussions, in the end choose what programs (such as special power devices, capacitors, chip), listen to the opinions of those around them, in the course of time will definitely benefit. Because the project is generally informal discussion, if it is new, we must avoid taking up too much time.
Which is the switch chip? How to configure the EMI circuit? How much is the input capacitance? Switching frequency? MOS how to choose? Diode? Magnetic core? Output capacitor? A lot of people in this step do not know how to go down, the next step will focus on analysis.
12V5A, universal input, the standard configuration is 8N60+MBR20100.
It should be noted that this parameter is not "count" out, because the calculated value and the actual situation is often very different, there is a lot of flexibility". For how type selection, the first consideration is what the company's warehouse, can not be used. The design of the product, it should be the design of the transformer parameters (voltage and current stress, etc.) to meet the parameters of these components. Rather than the first design of the transformer, and then to find the semiconductor components, the actual development process and materials are not the same as the. So, first of all to take into account the company currently has no suitable material. Whether it is mould inductor, semiconductor, or electrolytic capacitor, the preferred stock material factors will greatly shorten the development cycle and reduce uncertainty.
Because the switching power supply industry is very competitive, material selection of the second principles: what is the choice of competitors. Or the entire industry is currently the "popular trend", can also be understood that we are so dry. Sometimes industry "default" approach is more important than the first principle. For example, a considerable part of industrial products do not recognize the electrolytic capacitor 400V, are 450V, there are some manufacturers do not recognize domestic. For example, PC power inside the input and output capacitors, such as magnetic core is always so small! But this does not mean that they are cutting corners, the industry is so, otherwise how can so cheap computer. The vast majority of small and medium power products are MOS 600V, 12V output is mostly 100V diodes, etc.......
Material (parameters) selection of the third principles, is to consult a variety of application documents provided by the semiconductor company, evaluation boards, design manuals, etc.. TI, ON, Fairchild, PI, ST, Infineon have a lot of technical articles, and now compared to the previous few years to be "friendly" a lot, are still Chinese, do not see a pity. When selecting material for the network, the efficiency should be the lowest.
For 60W switching power supply in this level, we can use the following three kinds of input circuit, modify some parameters, in front of two kinds of structure of application should be no problem in what 300W (need to consider the lightning protection situation, but also to strengthen the input). Look carefully at these three kinds of structure, will find that they have the same or different, the biggest difference in this one common mode inductance configuration. .

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