Electromagnetic interference is an unavoidable problem in the design of electronic equipment and systems. The power supply is the part that connects the electronic equipment with the power supply network, so the electromagnetic interference caused by the power supply is an issue that deserves serious attention in the electronic equipment. In fact, electronic equipment is required to reduce the electromagnetic interference (EMI) of the power supply to the specified limits and maintain the electromagnetic compatibility (EMC). Ignoring this requirement will make it impossible for electronic equipment to pass mandatory standard tests and delay the time when electronic equipment is put on the market.

Problems with Switching Frequency Designers should also consider switching power supplies as a source of noise, which can also couple into sensitive equipment. Typically a power supply has a switching circuit which charges the capacitors. Filters can be used to prevent switching signals from flowing into power lines and other equipment. Different industries have different regulations on filtering requirements. Military requirements for switching frequency attenuation are the most stringent. When it comes to launch testing, there's another problem with commercial testing that engineers didn't pay attention to until a few years ago. Gerke notes: “We do conducted emissions tests down from 30MHz and radiated emissions tests up from 30MHz. In most power supplies, switching occurs between 50kHz and 500kHz, so harmonics above 30MHz are not Contains too much energy, and the radiated signal causes little problem. To improve the power efficiency of the power supply, designers increase the switching frequency and make the pulse edges of the switching signal steeper. These changes increase at 30MHz, Gerke explains. radiation in the 100MHz range.

"To meet EMI requirements, we add ferrite toroids to the FET (field-effect transistor) driver circuit to slightly slow down the transistor switching speed," Gerke said. "But power supply designers don't want us to reduce the switching speed because they try to Achieving this speed allows the power supply to operate efficiently.” In practice, reducing the harmonics of the switching signal does not affect efficiency much, often less than 1%. “If we find a 50MHz EMI signal in a power supply operating at 50kHz, we need to remove the harmonics around 50MHz (the 1000th harmonic). In this case, removing these higher harmonics will not lose efficiency. "But if you have a switching power supply operating at 500kHz and you find that there is an EMI problem at 30MHz, filtering the harmonics will lose slightly more efficiency," Gerke said.
(Excerpted from: Big Bit Transformer Forum)