Small power switch power adapter on conducted interference test, because itself is less power, and the radiation received signal strength is much weaker than that of the high power adapter, therefore in the process of test engineer need comprehensive reference conducted interference amplitude size, when the design will need to fully consider the impact of electromagnetic interference.
Below we will through the analysis of the small power power adapter general mechanism, to further explain the reason of electromagnetic interference and noise.
At present, the production of small power switching power supply, the basic principle of its operation according to the circuit structure is different, can be subdivided into series-parallel type and dc transform type.
After the mains through a rectifying filter will be dc voltage, and through the circuit to switch inverter circuit system.
Inverter circuit dc into several hundred MHZ to several thousand Hertz high frequency rectangular wave, high frequency rectangular wave is obtained by high frequency transformer coupling to the secondary coil to small voltage alternating current (ac), the alternating current (ac) after secondary rectifier filter output to drive the load.
At the same time, the output voltage output of the error amplifier circuit, sampling and feedback to give pulse control circuit.
Pulse control circuit will feedback after the voltage and the reference voltage value than a pulse duty ratio of the corresponding change, will produce strong noise in the process of switch action, by the power cord in common mode and differential mode way transmission to the outside.
This is currently the biggest cause of most of the small power adapter noise problems.
Summary in addition to the above mentioned causes, the noise of the switch power adapter is a very sensitive device, by grid invasion of external noise is passed to the internal electronic circuit also can produce interference in the process.
So in the design of a switching power supply should be given at the beginning of electromagnetic interference.