What criteria is used to choose the switching power supply's working frequency?
What is the basis for the selection of the operating frequency of the switching power supply? What factors need to be considered when selecting it? Have these issues become your focus when designing the circuit? Let me share some factors for selecting the operating frequency.
As an electronic engineer, have you encountered "difficulty in selection" when selecting equipment? Let me diagnose the cause for you. One is that I am not clear enough about my actual needs, and the other is that some performance parameters are not well understood and there is no more professional guidance. When power supply engineers select the main control IC, because there are many manufacturers and various parameter selections, it is difficult to select a power supply IC suitable for their own use.
When choosing the switching frequency of the AC variable frequency power supply, no engineer said to choose the best switching frequency, but to choose the most suitable one for your own use. Most AC-DC switching power supplies on the market now use a switching frequency between 50K and 135K, which can meet the needs of most applications. Let's discuss some problems encountered when selecting the switching frequency of AC-DC power supply:
Why don't you generally choose a switching frequency lower than 50K?
We know that the lower the switching frequency is, the smaller the switching loss will be. On the other hand, the size of the power device will increase with the decrease of the switching frequency, which is not conducive to miniaturization. When the switching power supply is working, the magnetic core will vibrate due to the switching of the switching device. At the same time, the frequency range that can be heard in the ear is about 20~20000Hz. In order to avoid the noise that can be heard as much as possible, we usually choose the frequency away from 20KHz. .
What are the disadvantages of choosing an AC-DC control IC with a switching frequency higher than 135K?
When testing the electromagnetic interference emission of electronic and electrical equipment, the emission of the conducted interference voltage at the power supply terminal must be tested. Although the frequency bands tested by different standards are different, most of the product test frequency bands are 150K~30M. It can be seen from the lower limit of the test frequency that the closer the operating frequency of the switching power supply is to 150K, the more difficult it is to deal with the conducted disturbance.
What special application occasions have requirements for the switching frequency of the power supply?
In some special occasions, the circuit operation is more sensitive to the switching noise of the power supply. Such as power carrier, wireless communication, wireless identification and other occasions. Let's discuss the requirements for power transfer applications.
At present, power carrier is mainly used in smart homes, telemetry equipment, and remote street lamp control systems. At the same time, the application range of power line carrier frequency in my country is 9KHz to 500KHz, and the carrier frequency bandwidth is 4KHz. Because of the impedance change of the power line and the noise interference of the power line, the development of the power carrier is greatly restricted. In order to increase the power carrier communication distance, the switching frequency of the power supply for the circuit carrier unit must be far away from our carrier frequency. At present, the carrier frequencies used in China are 115KHz and 132KHz. In this case, it is necessary to use a power supply with a switching frequency of about 60KHz.
