In a typical isoSPI application (Figure 1) pulse transformers provide the dielectric isolation and reject common-mode interference that can be impressed on the wiring. The isoSPI function operates with readily available and inexpensive Ethernet LAN magnetics, which typically include a common-mode-choke section (as shown in Figure 1) to improve common-mode line noise, along with the usual 100Ω line termination resistors and common-mode decoupling capacitors.
Figure 1. Generalized isoSPI point-to-point link
Ordinary signal transformers, including Ethernet and gate-driver types, are wound with enameled wire that can have pin-hole sized insulation defects, which expose the copper to the atmosphere, inherently limiting the inter-winding bias that for which such transformers are certified. Such units are tested in production with high potential (called hi-pot screening) to identify gross insulation problems, typically with 1.5kV. This is established as a safe design margin for long-term bias of 60V, since the tiny corrosion sites tend to require more than 60V to form conductive paths between windings.