Design example of transformer in charger

　　The design of the switching transformer in the forward charger converter can be realized in many ways. Manufacturers of chargers should choose a method they are accustomed to.

　　The following example uses a non-rigorous design method, starting from calculating the number of turns on the primary side. The nomogram issued by the manufacturer was used when selecting the core size and the best induction (minimum total loss). In the final prototype evaluation, the transformer is suitable and not far from the best design. It should be remembered that in multi-output applications, it is impossible to always have accurate voltage results for all outputs. This is because the winding can only have 1 turn or in some cases half-turn increments. Furthermore, the core size is often a compromise choice.

　　Except for the calculation of the number of primary turns for the maximum conduction pulse width and the nominal value of the DC voltage, this design method of the forward converter is very similar to the method used in the previous flyback converter. Contrary to the saturation limit design, this is another optimal loss design. The control circuit will prevent saturation. The designed number of primary turns is slightly more than that of the corresponding flyback converter.

　　The reason for the choice of the scanner power adapter is that the output inductor can limit the rate of change of the output current when the load changes suddenly in the forward converter. To compensate for this, the amplifier is controlled to maximize the input pulse width so that the current in the inductor rises as quickly as possible. Under these transient conditions, the plateau side voltage and the maximum pulse width will be simultaneously applied to the primary side of the transformer. Although this situation only occurs for a short time, the core will saturate, unless the transformer is specifically designed for this situation.

　　The design of the control circuit should make the pulse width and rate of change of the control circuit limited under the maximum input voltage. This can prevent the maximum pulse width and the maximum charger voltage from appearing at the same time. This must be checked in the final design.

　　In the forward converter, it is not desirable to store energy in the magnetic core, because the energy must be returned to the charger during the flyback. There is a small air gap in the magnetic core to ensure that the magnetic flux returns to a low remanence level and the maximum magnetic flux during the flyback. The density offset air gap should be as small as possible (50.2µm or 76.2µm is sufficient). The experiment in the figure shows that even if there is a small air gap in the core, the remanent flux can be much lower, and a small air gap can stabilize the magnetic parameters.

　　Step 1. Choose the core size of the transformer

　　The choice of core size is based on the transmitted power, and the manufacturer's recommendation will give the design a good start. The alignment diagram of a typical core selection is shown in the figure. In the following example, the designed transformer is used in a 100W charger with an operating frequency of 30kHz. The output voltage and current are as follows:

　　+5V, 10A=50W

　　+12V, 2A=24W

　　12V, 2A=24W

　　Total power = 98W

　　Input voltage 90~130V or 180~260V, 47~60Hz

　　For each auxiliary charger, the size is allowed to increase by about 3% to provide additional insulation and window space. As can be seen from the figure, the E-type core should be selected for 100W power transmission, and E42-15 is a suitable choice.

　　Magnetic core parameters: effective magnetic core area Ae=181mm

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