Reliability Above All: Caps And Fan
Capacitors play an important role in determining a power supply's long-term reliability. The APFC (bulk) cap must be of high quality. Moreover, the electrolytic caps on the secondary side must come from a reputable vendor and have a temperature rating of 105 °C, not 85 °C.
Besides decoupling, capacitors in PSUs are used to smooth out voltage and reduce ripple. They can also be used to store charge (voltage) and to block DC current (coupling). Electrolytic capacitors are used in the APFC converter as reservoir caps, and you will also find electrolytic caps on the secondary side of most PSUs. As their name indicates, these caps contain a conductive liquid called electrolyte. If this fluid is of low quality, the capacitor will quickly lose its characteristics and in a worst-case scenario it can even explode. Many of you will still remember the capacitor plague problem that affected many electronic devices between 1999 and 2007.
During a cap’s operating life, the amount of electrolyte decreases due to evaporation. In addition, the same material can be used to heal a damaged plate inside the cap, a procedure which leads again to its depletion. As the electrolyte’s quantity drops, the cap's electrical parameters naturally change, cutting into its useful lifespan. Polymer aluminum caps use a solid electrolyte as dielectric between the plates, so they don’t have the problems described above.
So why not use polymer caps exclusively and forget about electrolytic caps and their problems? The main reason is that the increased ESR values of electrolytic caps actually help avoid unwanted oscillations, which can cause instability. Although low ESR is critical in ripple filtering, minimizing ESR in PSUs isn’t a good idea either. This is why some re-cap jobs don’t go as expected. On top of that, polymer caps have a limited voltage rating. This doesn’t affect their use on the secondary side much, but in the APFC converter, an electrolytic capacitor has to be used.
Like capacitors, fans are also critical for ensuring your PSU delivers reliable service for many years. There are a number of popular fan bearing types, but three are most common to modern PSUs:
- Sleeve bearing fans (30,000h lifetime)
- Double ball-bearing fans (40-50,000h lifetime)
- Fluid dynamic bearing (FDB) and hydro dynamic bearing (HDB) fans (50-150,000h lifetime)
The best bearing type is considered to be FDB since it combines low noise output and an increased lifetime. Double ball-bearing fans are also popular in high-end PSUs for their quality. Meanwhile, sleeve bearing fans are mostly used in budget PSUs. They just aren't designed for horizontal mounting since oil inside the bearing moves to one of the shaft's sides, providing non-uniform friction protection.