Component Selection Principle in Switching Power Supply Design

In the switching power supply, the voltage and current waveforms are suddenly pulsed. The voltage or current that the component is subjected to is divided by the voltage applied to the components, the induced voltage caused by the inductance components in the circuit, and the charging current of the capacitor. This complicates the choice of components.

In fact, the switching power supply is an AC/DC or DC/DC converter with a voltage regulation function. Even if it is called DC/DC conversion, the intermediate part still has to pass the pulse state as a conversion medium. The actual process is: DC first reversed into pulsed AC, and then pulsed rectification and filtering became DC voltage. In this process, the requirements of rectifier and filter components are also significantly different from those of industrial-frequency rectifier circuits. The maximum value, average value and effective value of the power frequency sine wave AC power supply have a fixed proportional relationship according to the sine function, and can accurately calculate the rated parameters of the components.

However, the relationship between pulse wave, voltage, and current values ​​is not constant, but varies greatly depending on the pulse waveform and the nature of the load.

Even if the integration method is used to calculate the average value of the pulse waveform, the pulse waveform is required to have a certain rule, and the instability of the relationship between the waveform amplitude and time makes this calculation often difficult to be accurate. In particular, the quantitative measurement of pulse waveforms is not exactly what a simple instrument can measure accurately. In addition to pulse oscilloscopes, there is no simpler method, such as the reverse voltage value of a switching power supply switch. It is more difficult to measure the effective value of the pulse wave in some cases. For example, using a line retrace pulse to supply power to a CRT filament requires an RMS value of 6.3V. It does not seem to be any other way than the magnetoelectric meters or high-frequency electrodynamic meters that consist of thermocouple sensors.

In other words, it is impossible for a component working in a pulse circuit to select its performance through measured voltage and current parameters. As for the theoretical calculation, it can only reach the degree of approximate estimation. The specific parameter selection is based on the calculation results and narrowly used. The most obvious example is a single-ended switching circuit. Theoretically, the switch back-pressure should be twice the maximum input voltage. In practical applications, the pulse waveform added to the switch collector is affected by the lumped parameters of the energy storage inductor, the distribution parameters, and the nature of the power load. The back pressure value of the switch tube will exceed the theoretical calculation range.

Because the inductive potential of the inductor is not only a function of the current change, but also inversely proportional to the time when the current changes. In addition, the process of the inductor coil is almost difficult to artificially control the distribution parameters, but also make the induced potential greatly exceed the calculated value. Therefore, in the pulse state, regardless of passive components or active components, the performance of the choice is different from ordinary analog circuits.