Noise and Ripple

One common use for wirewound resistors seems to be as the load for a converter. I also use that configuration when doing thermals to simulate the customer s system and for noise and ripple measurements. But rarely do I use it for anything else. I would strongly suggest you get yourself a good electronic load. But do remember to set it to CC mode (constant current mode). Because a resistor (or an electronic load set to CR mode) is just too benign. For example, rarely does it reveal any fundamental start-up issues. 

In general, the high-frequency components are usually referred to as noise and the low-frequency component is called the ripple. Together they constitute the converter s Noise and Ripple (N R), which is also sometimes called by rather weird names such as PARD (I still don t see any need to remember what that stands for). 

Figure 2-8 Typical Input Noise and Ripple Waveform of a Buck (AC Component Only)...

Single Ceramic Capacitor for Both Noise and Ripple ... 

Ultimately, the power supply is only part of a larger system. Therefore, besides being concerned about the effect of noise and ripple on the converter itself, we need to worry about its effect on the rest of the system. The good news is that if the system were excessively noise sensitive, no one would have touched switchers with a ten-foot pole (or a lOdB zero) in the first place. They would have been using those low-noise, power-guzzling LDOs (linear regulators) instead ... 

Engineers are known to beef up the input capacitor for various reasons. At least some are hoping to reduce the output noise and ripple That actually does work in some cases, but the arguments are subtle and need to be qualified, depending on the type of converter and its application. Let us go through the reasoning. 

Recommendation 2 (Figure 6-9) As previously discussed, a small ceramic capacitor of about 0.1 pF is recommended at the output, for carrying out more meaningful noise and ripple measurements. That measurement is tricky and we must do our best to avoid any pickup by removing the ground lead of the probe. The scope probe must be used correctly as shown. Also disconnect all other scope probes completely from the board during this measurement. 

Figure 6-9 Putting a Small Ceramic Capacitor Between Closely Spaced Output Prongs Allows for a Proper Measurement of Noise and Ripple... 

Figure 11-1 One Possible Way of Conducting a Noise and Ripple Measurement for an AC-DC Power Supply... 

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