Ripple represents the AC fluctuations (periodic) and noise (random) found in the DC rails of a PSU. Ripple significantly decreases the life span of capacitors since it increases their temperature; a 10 °C increase can cut into a capacitor's life span by 50 percent. Ripple also plays an important role in overall system stability, especially when it is overclocked.
The ripple limits, according to the ATX specification, are 120mV for the +12V and -12V rails, and 50mV for the remaining rails (5V, 3.3V and 5VSB). Nonetheless, in modern PSUs, we expect to find much lower ripple. It should be just a small fraction in high-end platforms with quality components and the proper amount of filtering capacitors. Below, you will find a schematic that analyzes a ripple waveform.
In the above schematic, four AC components can be identified:
- Low-frequency ripple associated with AC mains frequency.
- High-frequency ripple due to PWM of the main switches.
- Switching noise that has the same frequency with switching PWM.
- Non-periodic random noise that is not related to any of the above.
Here are a few examples of the typical ripple test results you'll find in our PSU reviews: