Why prefer CoolMOS™ S7 over electro-mechanical (EM) solutions?
The main arguments against the use of CoolMOS™ S7 as alternative of an electro-mechanical solution are:
price, EM switches are cheap
losses, EM switches have very low resistance
Safety, EM switches provide contact gap so galvanic insulation of the output
However all these objections can be overcome:
1. Price. Relay prices are generally low but pretty constant in time, while SJ MOSFETs solid state solutions can still benefit from years of price down roadmap. On top of this, the on-resistance of an EM switch, very small at the beginning, degrades with use due to arching, so an equal loss comparison with solid state alternatives appears more and more unfavorable with use towards an EM relay. Furthermore, the solid state solutions show a better TCO – total cost of ownership - than the EM counteparts.
2. Losses. The argument that EM losses are much lower than a solid state alternative is generally true when the solid alternative is bases on components like TRIACs, SCRs or HV planar MOSFETs. The superjunction structure of CoolMOS™ S7 can help reduce these losses, at a competitive price.
3. Output contact gap (galvanic insulation). MOSFETs do not provide per se an air insulation of the output poles, as it happens in an electro-mechanical relay for instance, while the galvanic insulation between input and output circuits is normally obtainable with a reinforced insulation Infineon gate driver IC, based on the innovative coreless transformer technology. The galvanic insulation of the output poles is generally achieved by placing in series to the CoolMOS™ S7 a cheap EM relay, which is used uniquely to provide a contact gap, it does not alter the BOM and, since it opens at zero voltage, keeps the advantages of a solid state versus an EM switch. In addition, it has to be considered that the galvanic insulation of the output contacts is not always required.