Double flux effect

　　The difference in the working methods of single-ended transformers and push-pull balanced transformers cannot always be fully identified. The single-ended forward or flyback SAA power adapter converter only uses a quarter-quadrant B/H magnetic ring, the residual flux is B, and the holding range of the inductance is usually quite small.

　　In push-pull transformers, it is usually assumed that the entire B/H magnetic ring can be used. In each cycle, B increases from a Bmax to +Bmax, so the total change of △B during a conduction period is (2×B), and this value is usually used Calculating the number of turns on the primary side, the result is that the number of turns required for a push-pull transformer is half of that required for a single-ended transformer. therefore

　　However, the charger designer should be careful when using this method, because △B is not always 2B. For example, consider the situation when the converter is just turned on. The magnetic core flux density is a value between -B, and +B, (residual value). Therefore, in the first half of the cycle, the 2B magnetic flux change may make the magnetic core into saturation (so called the "double flux effect"). Therefore, in the first few cycles of work, the full voltage and maximum pulse width cannot be applied, and a soft start is required (that is, the pulse width must be increased slowly in the first few cycles of work). Furthermore, if the step-shaped saturation effect causes the magnetic core to saturate under steady-state conditions, the entire 2B range is invalid in both directions (this is important in transient conditions that require rapid pulse width changes). Therefore, symmetry correction is indeed required in the push-pull converter.

　　For this reason, the range of △B is often reduced in practice, and the number of turns used is more than the number of turns calculated by the above formula in order to provide a good working margin for any startup and symmetry problems. The required margin depends on the quality of the above control effect.

　　EMC test content and troubleshooting methods

　　EMC is the ability of a power adapter module device or system to work normally in its electromagnetic environment, and does not constitute an unbearable electromagnetic disturbance to anything in the environment.

　　EMC testing includes two major aspects: testing the electromagnetic disturbance intensity sent to the outside world to confirm whether it meets the limit value requirements specified by the relevant standards; conducting sensitivity testing under the MTD2002 electromagnetic environment conditions that specify the electromagnetic disturbance intensity, In order to confirm whether it meets the immunity requirements of the relevant standards. For engineering and technical personnel engaged in the design of single-chip application systems, it is very necessary to master certain EMC testing technology.

　　Electromagnetic compatibility includes MTD2002 electromagnetic interference (EMI) and electromagnetic susceptibility (EMS) two parts. Since electrical products have electromagnetic interference to other electrical appliances during use, or are subject to electromagnetic interference from other electrical appliances, it is not only related to the reliability and safety of the product, but also may affect the normal operation of other power adapter modules and electrical appliances, and even cause safety hazards. .

　　EMC test of single chip microcomputer system

　　(1) Test environment

　　In order to ensure the accuracy and reliability of the test results, electromagnetic compatibility measurement has higher requirements for the test environment. The measurement site includes an outdoor open field, a shielded room or an anechoic chamber, etc.

　　(2) Test equipment

　　Electromagnetic compatibility measurement equipment is divided into two categories: one is electromagnetic interference measurement equipment, which can be measured by connecting appropriate sensors to the equipment; the other is electromagnetic susceptibility measurement in the power adapter module, MTD2002 equipment simulates different interference The source, through appropriate coupling/decoupling networks, sensors or antennas, is applied to various types of equipment under test for sensitivity or interference measurement.

　　(3) Measuring method

　　Electromagnetic compatibility testing is based on different standards. There are many measurement methods, but they can be summarized into 4 categories; conducted emission test, radiated emission test, conducted sensitivity (immunity) test, and radiation sensitivity (immunity) test.

　　(4) Test and diagnosis steps

　　Figure 1 shows the electromagnetic interference emission and failure analysis steps of a device or system.

　　Following this step can improve the efficiency of test diagnosis.

　　(5) Test preparation

　　① Test site conditions: EMC test laboratory is a semi-dark room and a shielded room. The former is used for radiation emission and radiation sensitivity testing, and the latter is used for conducted emission and conducted sensitivity testing.

　　②Environmental level requirements: The electromagnetic environment level of conduction and radiation should be far lower than the limit value stipulated by the standard. Generally, the environment level of the power adapter module should be at least 6dB lower than the limit value.

　　③Test table.

　　④Isolation of the measuring equipment and the equipment under test.

　　⑤ Sensitivity judgment criteria: generally provided by the tested party, and monitor and judge the truth, and determine the degree of performance degradation by means of measurement and observation.

　　⑥The placement of the tested equipment: In order to ensure the repeatability of the experiment, there are usually specific regulations on the placement of the tested equipment.

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