Corsair HX750i: the basis for RMi series

Input filtering

The first stage of the input filtering seems familiar, doesn’t it? Once again we see the partially shielded receptacle, two ceramic Y capacitors and double-pole switch.


Than there are four more Y capacitors, two film X capacitors, two common mode chokes, a varistor (by the fuse, almost hidden in silicone) and also a thermistor. It is hiding between the PFC coil and the bulk capacitors, this time there is also the bypass relay present to improve efficiency. Once again, instead of the X cap discharge IC, two rectification diodes are used which than feed some control ICs.


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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