High-Frequency Input Decoupling

One of the things the preceding impedance calculations tell us is that if we are trying to draw a sharp current waveform from a capacitor, that capacitor had better be good. 

Don t Forget to Place that 0.1 pF Ceramic Capacitor Really Close to the IC 

However, as you will see a little later, it is not a bad idea to always include this O.ljiF input decoupling capacitor. The reasons may be different on different occasions, and for different types of switchers, but this component is generally always nice to have. 

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High-Frequency Effects and the Importance of Input Decoupling... 

In Figure 2-10, we Anally break up the input capacitance into a high-frequency capacitor and a relaAvely low-frequency bulk capacitor. The current distribuAons are shown, as well as how they all add up eventually. The mystery is clear now, and in the process we also understand how the decoupling capacitors are supposed to behave. Now we can also start to understand how this delicate balance can be easily shattered by lack of proper decoupling ... 

The entire loop of the PCB traces (up to the input side) as shown in Figure 11-4 needs to be thick and short. Unfortunately, this often tends to be necessarily long, considering board layout constraints, and all the other components that need to be mounted on it. So in that case we can provide a high frequency decoupling capacitor from the HVDC to primary ground, very close to the mosfet. 

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