Switching power supply ripple generation
Our ultimate goal is to reduce the output ripple to a level that can be tolerated. The most fundamental solution to this goal is to avoid ripple generation. First, we must understand the type and cause of switching power supply ripple.
The picture above is the simplest topology in a switching power supply - the buck step-down power supply.
With the SWITCH switch, the current in the inductor L also fluctuates around the effective value of the output current. Therefore, there will also be a ripple at the same frequency as the SWITCH at the output. Generally speaking, the ripple refers to this. It is related to the capacity of the output capacitor and the ESR. This ripple has the same frequency as the switching power supply and is tens to hundreds of KHz.
In addition, SWITCH generally uses bipolar transistors or MOSFETs, whichever is the case, there will be a rise and fall time when it is turned on and off. At this time, there will be a noise in the circuit that is the same as the frequency of the SWITCH rise and fall time or an odd multiple, usually tens of MHz. Similarly, in the reverse recovery moment of diode D, the equivalent circuit is a series connection of resistors and capacitors, which causes resonance and generates a noise frequency of several tens of MHz. These two kinds of noise are generally called high frequency noise, and the amplitude is usually much larger than the ripple.
In the case of an AC/DC converter, in addition to the above two kinds of ripple (noise), there is AC noise, and the frequency is the frequency of the input AC power source, which is about 50 to 60 Hz. There is also a common mode noise due to the equivalent capacitance generated by the power devices of many switching power supplies using the case as a heat sink.
Because I am doing automotive electronics research and development, there is less contact with the latter two types of noise, so I will not consider it for the time being.
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