The influence of DC/DC noise is mainly: duty cycle Duty; switching frequency Fs; rise time Tr. The influence of the switching frequency is actually very large, not only the effect of EMC, but different influences can be obtained in different cases.
1) DCDC noise source characteristics
The three parameters of DCDC noise are mainly:
Duty Duty: The duty cycle rises and the noise amplitude increases.
Switching frequency Fs: Yes, the noise attenuation changes in the spectrum. The switching frequency can be divided into several major categories.
20 ~ 100Khz: The cost and size caused by the large inductance basically make the low frequency design slowly not an option.
100～550Khz: the main choice
The switching frequency mainly affects the inductor size, efficiency, and RE/CE characteristics.
Rise time Tr
Remarks: The effect of switching frequency is actually very large, not only the effect of EMC, but different influences can be obtained in different cases, as shown in "Choosing the opTImum switching frequency of your DC/DC converter":
2) as shown in the case below
2.1) This is a typical BuckBoost power circuit. If you are not sick, you must have CISPR25 Class 3
2.2 Initial test results
2.3 Corrective measures
2.3.1 Principle analysis
The non-continuous high current loop in the two switching loops is the most important noise source
Increase Vin and Vss capacitance to reduce current in high frequency loops
2.3.2 Frequency selection
Noise component and current magnitude, the loop area is proportional to the frequency, and inversely proportional to the distance
The following are the effect comparisons of different frequencies (120, 240 and 400Khz), and the deep purple is the effect of frequency hopping.
2.3.2 Optimization of switching speed
2.3.3 Layout Optimization Impact
Place C14 (Vin-Vss capacitor) on the freewheeling diode and MOSFET side as much as possible to reduce the high frequency loop area
The following is a physical photo
In addition to some related materials, in addition to the efforts of chip manufacturers, the design methods we can join include:
1) Input filter optimization
RSIL filter 5μH 100nF 50Ω
2) Output filter optimization
3) Snubber circuit
summary:
1) Thanks to Jian Yuxiong's case summary, we can quantitatively and qualitatively look at the choice of these parameters for the actual test results.
2) The design of this needs to open the loop of principle, calculation, functional test and actual verification.
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