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Enable low-cost isoSPI coupling for battery systems

Posted: 02 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:isoSPI? LTC6804? battery stack monitor? communications interface? energy storage?

The isoSPI feature built into the LTC6804 battery stack monitor, when used in conjunction with an LTC6820 isoSPI communications interface, enables safe and robust information transfer across a high voltage barrier. isoSPI is particularly useful in energy storage systems that produce hundreds of volts via series-connected cells, which require full dielectric isolation to minimise hazards to personnel.

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 100 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.

Problem: High Voltage = High Cost
For battery-stack voltages in the 400V range, good design practice is to specify transformers with reinforced (double) insulation and hi-pot testing to 3750V or higher. Such transformers are difficult to find as small parts due to the creepage (surface distance) and clearance (air spacing) dimensions required, and they are relatively expensive. isoSPI is applied in battery systems up to 1kV, which requires transformers with hi-pot testing to 5kV for conservative design margin. At this level, isolation components can become bulky, costly, and compromise pulse fidelity.

Solution: Divide and Conquer
One alternative to using reinforced transformers is to separate the bias requirement from the magnetics by moving the extra insulation to coupling capacitors instead. While capacitors alone could provide a seemingly complete isolation option, they offer neither common-mode rejection nor the shock-resistant isolation characteristics that transformers offer, so an L-C approach is actually optimal. In this way capacitors charge to the nominal DC bias and leave the transformer to handle transients, for which even ordinary units are well suited.

The coupling capacitors are biased by high value resistors, generally tied to the transformer centre-tap connection, as shown in figure 2. As a bonus, if the DC current of the biasing resistors is monitored, then any dielectric breakdown becomes a detectable fault. The resistance is chosen to be a high value, like 10M, so that fault currents are within the fine wire rating of the transformers and the shock hazard to personnel is minimal.

Figure 1: AC-coupled isoSPI point-to-point link for increased voltages.

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