The solution of the influence of Class D audio
For Class D amplifiers, there are two main ways to reduce EMI: 1. Spread spectrum technology is adopted for the sampling frequency of the power amplifier, so that the EMI interference spectrum caused by the sampling frequency is relatively average, and the purpose of reducing EMI is achieved. 2. Control the turn-on and turn-off time of the output tube, and then control the edge EMI interference. It is generally believed that the second point is the main factor affecting the EMI of Class D amplifiers. The frequency spectrum component of the PWM signal is simplified to its frequency and rise time. The clock or system frequency establishes the time reference of the circuit, but its edge rate forms interference harmonics. The energy of EMI mainly depends on the change time and the magnitude of change. There are two ways of electromagnetic interference (EMI) propagation: conduction and radiation. That is, electromagnetic interference is divided into conductive electromagnetic interference and radiated electromagnetic interference. When the frequency of the electromagnetic interference wave is less than 30MHz, the electromagnetic interference mainly produces conductive noise in the electronic equipment by conduction; when the frequency of the electromagnetic interference wave is higher than 30MHz, the electromagnetic interference mainly produces radiated noise in the electronic equipment by radiation . The current common EMI standards are FCC and CISPR, the main concern is interference in the 30MHz band. The relationship between the maximum frequency of EMI and the edge change time corresponds to the following formula. For example, when the edge change time is 10ns, the calculated fmax = 31.8MHz, that is, the system will be affected by the frequency within 31.8MHz. Class D amplifiers also have shortcomings. When the output signal of class D amplifiers is a high-current and high-speed pulse width modulation switching signal, when the switching signal is transmitted to the speaker through the speaker wire, it will indirectly cause electromagnetic radiation and produce electromagnetic interference ). This EMI interference contains a wide frequency spectrum, different frequency bands interfere with different receivers, and even interfere with non-receiver electronic products.