Noise peak interference suppression measures for PWM switching regulated power supply
1. Extend the instantaneous conduction duration of the power switch tube
2. Reduce transformer leakage inductance
3. Reasonably select the switching speed of the switching transistor and the freewheeling diode
4. Use P-type LC balanced filter
5. Use high frequency electrolytic capacitors
6. Shorten the output wire length
Influence of Power Switch Transient On Duration on Spike Disturbance
The turn-on and turn-off times of a transistor are inversely proportional to its cutoff frequency. The shorter the turn-on and turn-off times, the faster the switching speed. The duration of simultaneous conduction depends on the switching speed of the devices used for KQ and KD. Comparing switching devices with different speeds, the faster the speed of the switching device and the shorter the duration of conduction at the same time, the narrower the width and the larger the amplitude of the spike interference.
Reducing Voltage Spikes Caused by Transformer Leakage Inductance The greater the leakage inductance of the transformer, the higher the voltage spike and the greater the RFI. Especially after the transformer is shielded, the leakage inductance is correspondingly larger due to poor coupling. Generally speaking, the leakage inductance of the transformer wound with toroidal core is smaller than that of the E type. In addition, the winding process is also very important. A better winding method is to first wind half of the total number of turns of the primary, then wind all the turns of the secondary, and finally wind the remaining half of the primary, that is, the secondary coil is in the primary coil. middle. In this way, the primary coil remains well coupled, so that the transformer has less leakage inductance.
The Influence and Suppression of the Switching Waveform of the Power Tube on the Spike Interference The squareness of the switching waveform Usr(t) affects the spike interference. The harmonic amplitudes of rectangular waves decrease at a rate of 20dB decade with increasing frequency, and for trapezoid waves it is 40dB decade. Consciously changing the steepness of the rectangular wave and the bluntness of the two corners can suppress high-frequency components and reduce spike interference. Therefore, the switching speed of the switching transistor and the freewheeling diode should be selected reasonably.
In the input power grid, the noise suppression method in the switching regulated power supply switches on and off quickly, the didt is very large, and a large transient voltage drop is generated on the leakage inductance of the power supply system, so that the input voltage source has a very short time. The instantaneous drop of the power grid will destroy the normal waveform of the power grid and cause interference. Interference in the input power supply can also affect the switching regulated power supply. The input filter has a certain isolation effect, usually a P-type LC balanced filter is used, which can attenuate 20dB for pulsating interference and 6dB for peak interference.
Suppression of peak interference by switching regulated power supply wiring When the switching regulated power supply works, it radiates interference into space. Radiated noise level is inversely proportional to the distance from the radiation source. Generally, the wiring can be done away from 5cm. If the structure does not allow it, it should be shielded. A strong electromagnetic field is generated around the power input line. In order to reduce the electromagnetic coupling between the input line and the output line, the two must be kept away.
