Design of EMI input filter for switching power supply system
The efficient design of EMI conduction in switching power supply system is to optimize the design of EMI filter;
Switching power supply electromagnetic compatibility advanced optimization design; for products and control systems with switching power supply; the input EMI low-pass filter is placed at the input end to the problem of EMI-conduction of the system, and even the design of EMS is also very critical!
Transient interference (EMS) will pose a threat to the electronic products or equipment of the switching power supply system, and there will be problems with product functions and performance!
This kind of transient EMS interference is the injection test of differential mode interference & common mode interference to the system;
Common mode interference (common mode current): In the same phase, the current between the L and N wires (or signal wires) and the ground wire;
Differential mode interference (differential mode current): a current that travels between the L and N lines (or the signal line and the return line) and has opposite amplitude and phase;
Common mode current and differential mode current can exist in a pair of wires at the same time; carry out the following theoretical analysis:
The peak noise voltage of differential mode interference (EMS) will directly pose a threat to electronic products & equipment, and there will be problems with product functions and performance!
The peak noise voltage of common mode interference (EMS) will not directly pose a threat to electronic products & equipment; common mode interference does not directly affect the equipment, but affects the equipment by converting it into a differential mode voltage! Since the system uses AC power supply and requires small size and efficiency, the application of switching power supply is essential!
Note: Electronic products & equipment are concerned with switching power supply systems! If you leave out the input filter of the switching power supply
1. The suppression effect of the switching power supply line itself on the burst interference is very low. The reason is that the essence of the burst interference is high-frequency common-mode interference.
2. The filter capacitors in the switching power supply line are all set to suppress low-frequency differential mode interference, and the electrolytic capacitors in it are not enough for the ripple suppression of the switching power supply itself, let alone for the pulse group with harmonic components above 60MHz Interference has a suppressive effect,
3. When using an oscilloscope to observe the pulse group waveforms of the input and output terminals of the switching power supply, there is no obvious interference attenuation effect.
From this point of view, the input filter of the switching power supply system of the Internet of Things and smart products & equipment is an important measure in terms of suppressing the pulse train interference received by the switching power supply.
Pay attention to the problems of EMS and pay attention to the problems of PCB design!
1. The quality of the design of the high-frequency transformer in the circuit of the switching power supply system has a certain inhibitory effect on the pulse group interference;
2. The jumper capacitance between the primary circuit and the secondary circuit of the switching power supply system can provide a path for the common mode interference entering the secondary circuit from the primary circuit to return to the primary circuit, so it also has a certain inhibitory effect on the burst interference;
3. The setting of the common mode filter circuit at the output end of the switching power supply system can suppress the pulse group interference to a certain extent.
4. The circuit of the switching power supply system itself has no inhibitory effect on the pulse group interference, but if the circuit layout of the switching power supply is not good, it will aggravate the invasion of the switching power supply by the pulse group interference.
In particular, the essence of burst interference is the combination of conduction and radiation interference. Even if the input filter is used to suppress the conduction interference, the radiation interference around the transmission line still exists and can still pass through the switching power supply. bad layout of
(If the primary or secondary circuit layout distance of the switching power supply is too long, a "big loop antenna" will be formed),
The radiation component in the induction burst interference affects the anti-interference performance of the entire device.
