As power supply of electronic instruments and equipments DC / DC converters of switching mode are so increasingly adopted that the content of high order harmonic waves in power frequency supply increases and they are restricted by International Electro-technical Commission ( IEC ) .

电子仪器和设备中的电源越来越广泛地采用 开关方式的DC/DC变换器，以致于工频电源中高次谐波的含量增多，国际 IEC对此作了限制。