What is the basis for selecting the operating frequency of the switching power supply?
What is the operating frequency of the switching power supply based on? What factors need to be considered when selecting it? Have these issues become your focus when planning the circuit? Here are some elements 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. First, because I don’t know my actual needs well enough, and second, I don’t know enough about some performance parameters, and I don’t have more professional guidance. When a power supply engineer chooses a main control IC, because there are many manufacturers and a variety of parameters, it is difficult to choose a suitable power supply IC for their own use.
When choosing the switching frequency of the AC variable frequency power supply, no engineers say to choose the best switching frequency, but to choose the most suitable one for their own use. Most of the AC-DC switching power supplies on the market now use a switching frequency between 50K and 135K, and this frequency range can meet the needs of most applications. Let's discuss some of the problems encountered when selecting the switching frequency of AC-DC power supplies:
Why generally do not choose a switching frequency below 50K?
We know that the lower the switching frequency is, the smaller the switching loss will be, but on the other hand, the volume of the power equipment will increase with the reduction of the switching frequency, which is not conducive to miniaturization. When the switching power supply is operating, the magnetic core will vibrate due to the switching of the switching equipment. At the same time, the frequency range that can be heard in the ear is about 20~20000Hz. In order to prevent audible noise as much as possible, we generally 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, it is necessary to test the emission of the conduction interference voltage at the power supply end. Although the frequency bands tested by different standards are different, most products are tested in the frequency band of 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 applications 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, the power carrier is mainly used in smart home, telemetry equipment, and remote street lamp control systems. At the same time, my country's power line carrier frequency application scale is 9KHz ~ 500KHz, and the carrier frequency band 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 communication interval of the power carrier, the switching frequency of the power supply for the circuit carrier unit must be far away from our carrier frequency. Now, the more carrier frequencies used in China are 115KHz and 132KHz. In this case, it is necessary to select a power supply with a switching frequency of about 60KHz.
