source:Industry News Popular:adapter release time:2021-06-08 14:42:11 Article author:sznbone
There are no special requirements for the input rectifier and energy storage capacitor in the flyback converter. Therefore, as used in other types of converters, it is selected according to the requirements of meeting its rated power and maintaining operation.
The switching transistor in the flyback power adapter is subject to quite high stress. The rated current depends on the maximum load, efficiency, input voltage, operating mode and converter design. First calculate the peak collector current under the minimum input voltage and maximum load. In the example shown in the figure, the peak collector current range is 3 to 6 times the average current, depending on the operating mode.
The maximum collector voltage is also very high. It depends on the maximum input voltage (no load), the flyback coefficient, the overshoot induced by the transformer design, and the buffering method.
For example, when feeding power to an AC power adapter with a rated voltage of 110V, the maximum input voltage value is 137Vrms for this input, and the maximum no-load DC rectified voltage V. (Using voltage doubler input circuit)
Here, Vcc=Vin;
Vin=Maximum AC input voltage, the unit is rms
In this example, Vcc=137×1.42×2=389V
The typical flyback voltage is at least twice the Vcc. In this case, it is 778V. Therefore, a 25% induction overshoot margin is allowed and the peak collector voltage is 972V. A transistor with a V rating of 1000V should be selected.
In addition to meeting these important conditions, the flyback transistor must provide good switching characteristics, low saturation voltage and effective gain margin at peak operating current. Since the selection of transistors must also meet the gain, it determines the requirements for the drive circuit, so the selection of a suitable power transistor may be the most important parameter that determines the efficiency and long-term reliability of the flyback converter.
Note: In order to avoid secondary side open circuit, current distortion and excessive loss in the high voltage bipolar transistor, the correct drive and load circuit shape must be considered.
Appropriate drive circuit, waveform, secondary side open circuit and distortion issues.
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