Surge protection devices may include a filtering stage to help condition the waveshape, thereby providing superior protection for sensitive electronics. This said, it is important to realize that a number of different topologies of filter circuit exist, each providing significantly different performance. At its simplest, a manufacturer may include a capacitor in parallel with the output. This may serve to reduce any fast ringing voltages and will also help absorb the energy in a small transient thereby providing a limited level of attenuation.
A far more effective approach is the series LC filter. This type of filter is connected between the surge limiting components and is in series with the supply powering the equipment. It consists of series inductors and parallel capacitors. Surge protection devices of this nature are often referred to as “two port” devices since they have a distinct input and output side.
The SRF N-Series use true series-connected LC filtering. Since the load current feeding the equipment to be protected has to go through the SRF, the inductors have to be rated to carry this load current. As a consequence, the range consists of a number of models to suit increasing load currents, with the larger current models being physically larger. In addition to the heavy-duty inductors, the capacitors used in this filter stage are of a special high-reliability type, having received safety approvals from a number of ratings agencies.
Benefits of SPDs with Filters
- They reduce the transient voltage reaching the equipment.
- They reduce the rate-of-rise of the leading edge of the impulse. The residual leading edge spike after a standard SPD, although it may only be 500 V to 900 V in amplitude, can cripple electronics due to its extremely high rate-of-voltage rise of 3,000-12,000 V/μs. Effective filtering reduces this rate-of-rise to less than 20 V/μs. This slower change in voltage is better withstood by electronic equipment using switched-mode power supplies. The filter also helps to attenuate small-signal RFI/EMI noise problems.