Characteristics and selection of power adapter components

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Characteristics and selection of power adapter components

In the power adapter, the voltage and current waveforms are all in a sudden pulse state. In addition to the power supply voltage applied to the components, the voltage or current borne by the components also includes the induced voltage caused by the inductance component in the circuit and the charging current of the capacitor, which complicates the selection of components.

In fact, the power adapter belongs to the AC / DC or DC / DC converter with voltage stabilizing function. Even the so-called DC / DC converter, the intermediate link still needs to use the pulse state as the conversion medium. The actual process is that the DC first reverses to the pulse state AC, and then the pulse rectifier filters to the DC voltage. In this process, the requirements of rectifying and filtering components are different from those of power frequency rectifying circuits. The maximum value, average value and effective value of power frequency sine wave AC power supply have a fixed proportion relationship according to the sine function, which can calculate the rated parameters of components very accurately. However, the relationship among the values of pulse wave, voltage and current is not constant, but varies greatly with the characteristics of pulse wave and load. Even if the integral method is used to calculate the average value of the pulse waveform, it requires a certain regularity of the pulse waveform, and the instability of the relationship between the waveform amplitude and time makes this calculation difficult to be accurate. In particular, the quantitative measurement of pulse waveform is not the same as that of general simple instruments. Besides pulse oscilloscope, there is no simpler way, such as the reverse voltage value of power adapter power adapter. In some cases, it is more difficult to measure the effective value of pulse wave. For example, to supply power to CRT filament with line reverse pulse requires effective value of 6.3V. There seems to be no other way to accurately measure CRT filament except magnetoelectric instrument or high frequency electric instrument composed of thermocouple sensor.

That is to say, it is impossible for the components working in the pulse circuit to choose their performance through the measured voltage and current parameters. As for the theoretical calculation, only approximate estimation can be achieved. The specific parameter selection is based on the calculation results. The most obvious example is the single ended switch circuit. Theoretically, the back voltage of the power adapter should be twice the maximum input voltage. In practical application, the pulse waveform of the collector of the power adapter is affected by the lumped parameter, the distributed parameter and the load property of the power supply, and the back pressure value of the power adapter will exceed the theoretical calculation range. Because the induced potential of the inductance coil is not only a function proportional to the current change, but also inversely proportional to the time of current change. In addition, the distribution parameter of inductance coil, which is almost difficult to control artificially, also makes the induced potential greatly exceed the calculated value. Therefore, the performance selection of passive components and devices in pulse state is different from that of ordinary analog circuits.