Contents
- 1Introduction
- 1.1Packaging and accessories
- 2Connectors & cabling
- 2.1Casing & cooling
- 3Input filtering
- 4Primary side
- 4.1+5 V stand-by rail
- 5Secondary side
- 5.1Build quality
- 6Load testing
- 6.1Loading +5 V SB
- 6.2Voltage hold-up time
- 6.3Combined loading
- 6.4Combined loading ripple
- 6.5Crossloading, overloading
- 6.6Crossloading, overloading ripple
- 7Conclusion and evaluation
- 7.1Thanks
- 7.2Discussion
Input filtering
The input filtration – as usual – starts directly on the AC inlet with two ceramic Y capacitors and one film X capacitor. No ferrite bead on the wires though. The inlet itself is partially shielded.
Then there are two more Y caps on the main board (and fifth between primary and secondary sides) together with two X film caps and two common-mode chokes. The varistor (MOV) is next to input rectifier, first thermistor too. Yes, there are indeed two thermistors in this unit, second is next to input capacitor (with no heatshrink).
We also have the Power Integrations CAP004DG chip for intelligent discharging of X capacitors to avoid constant waste of power, on solder side of the small board soldered directly on the AC inlet. There is also a second one on the main board – completely unnecessary, this is just a waste of money. If the fuse is OK, then the first one discharges everything. If the fuse blows, the current cannot flow back to the inlet if somebody tried to stick fingers in there just after unplugging AC cord.
X capacitors (between phase and neutral) and Y (between phase and ground/neutral and ground, often also between one side of the high-voltage DC and ground) are used to filter out high-frequency ripple from power grid (often from devices which lack filtration because of cost cutting, but also from devices where filtration is very difficult to implement) and also keeps ripple from this unit from entering the grid. Chokes are used for the same reason, together they form an input filter. These components may also (partially) help to filter smaller voltages spikes in the power grid while the MOV is used to suppress more serious spikes (for example from lightning hitting the power grid at a distance). These days, more Y capacitors are used even between rectified ground (ground after an input rectifier) and earth ground to suppress internal interference and keep it from getting to secondary side, because really high-frequency ripple goes everywhere it can to some extent (including coupling through the insulation, metal casing etc.). That is also why the AC wires themselves are often inserted through the ferrite core.
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