Fortron Source Power Group FSP200-50GSV-5K (85): a 200W dwarf

Input filtering

The input filtering in the FSP200-50GSV-5K (85) has two stages. The first one is directly at the inlet, as usually, two ceramic Y capacitors directly on the partially-shielded input receptacle. The conductors leading to the PSU board then also go through a ferrite bead.

On the main board are two more Y caps (and fifth between the primary and secondary side ground/return), one film X capacitor, two common-mode chokes and single differential choke. The chokes have quite thin wire for winding but consider this unit will only draw 2 A @115 V. There are also spark gaps printed on the PCB just beneath the common-mode coils. We can also see a varistor next to one of them. The thermistor, located near the input bulk capacitor, has however been replaced with wire jumper.

The X capacitors (between the live and neutral) and Y capacitors (between live and ground/neutral and ground) are used to filter out high-frequency ripple that emanates from the power grid. That is the noise of which manifests in the form of feedback from electronic devices which lack adequate filtering due to cost cutting. But also from devices where filtering was very difficult to implement (powerful devices, e.g. microwave ovens). It also prevents ripple from this unit itself from feeding back into the grid.

Chokes are used for the same reason, and together with the X/Y capacitors they form an input filter. Such filters are often made as one component, they may also be integrated together with AC receptacle. These components may also (partially) help to filter smaller voltage spikes in the power grid. To suppress more serious spikes (for example from distant lightning strikes hitting the power grid), the MOV (metal-oxide varistor) is used. Thermistor is then used to suppress current spikes when first connecting the unit to power (i.e. flipping the power switch).

The Y capacitors are also often situated between the high-voltage primary and the low-voltage secondary sides. These days, more Y capacitors are used even between primary common (ground after an input rectifier) and earth ground to suppress internal interference and keep it from getting to the secondary side. It is 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 toroid inductor (to suppress such coupling).

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