Avoidance of ripple in switch mode power supplies
Following the switching of SWITCH, the current in the inductor L is also fluctuating up and down in the RMS value of the output current. Therefore, a ripple with the same frequency as SWITCH will also be generated at the output, and this is what is generally referred to as ripple. It has a relationship with the capacity of the output capacitor and ESR.
How to press the switching power supply ripple generation, switching power supply ripple generation we ** purpose is to reduce the output ripple to a tolerable level, to achieve this purpose * fundamental solution is:
Switching power supply ripple generation
Our purpose is to reduce the output ripple to a tolerable level, to achieve this purpose the most fundamental solution is to try to avoid the generation of ripple, first of all, we must be clear about the type of switching power supply ripple and the reasons for the generation.
Following the switching of the SWITCH, the current in the inductor L also fluctuates up and down in the RMS value of the output current. So the output will also be flooded with a ripple with the same frequency as the SWITCH, generally referred to as ripple is this. It has a relationship with the capacity of the output capacitor and ESR. The frequency of this ripple is the same as the switching power supply, tens to hundreds of KHz.
In addition, SWITCH generally choose bipolar transistors or MOSFETs, either one, in its on and off, there will be a rise time and fall time. At this time in the circuit will be flooded with a SWITCH rise and fall time of the same frequency or an odd multiple of the frequency of the noise, generally tens of MHz. the same diode D in the reverse recovery moment, the equivalent circuit for the resistance capacitance and inductance of the series connection, will cause resonance, resulting in the noise frequency of several tens of MHz. these two types of noise is generally called high-frequency noise, amplitude is usually much larger than the ripple.
If the AC/DC converter, in addition to the above two ripple (noise), there is AC noise, the frequency is the frequency of the input AC power supply, for about 50 to 60Hz. There is also a common-mode noise, caused by the equivalent capacitance generated by the power devices of many switching power supplies using the case as a heat sink. As I am doing automotive electronics research and development, for the latter two noise contact less, so do not consider for the time being.
Switching power supply ripple measurement
Basic requirements: the use of oscilloscope AC coupling, 20MHz bandwidth limitation, unplug the probe ground
1, AC coupling is to remove the superimposed DC voltage, to get the correct waveform.
2, Open the 20MHz bandwidth limit is to prevent the interference of high-frequency noise, preventing the measurement of the wrong results. Due to the high-frequency component amplitude is large, the measurement should be removed.
3, unplug the oscilloscope probe grounding clamp, use the grounding ring measurement, in order to reduce interference. Many parts do not have a grounding ring, if the error promises to directly use the probe grounding clip measurement. However, this factor should be taken into account when determining whether it is qualified.
