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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?

Another good practice to avoid leakage current across the HV barrier is to suppress soldermask in the area of the HV components (parts over the gap between grounds). This facilitates effective rinsing of flux residue under the parts, and avoids moisture retention in the porous soldermask layer.

Special considerations for an isoSPI bus
The previous circuits apply to point-to-point isoSPI links, but one of the important cases for providing a high voltage solution is the bus-connected addressable LTC6804-2 with the twisted-pair link passing through each tap connection, as shown in figure 8. The bus application places a high voltage requirement on every transformer since the same twisted-pair potential must interface with any voltage on the floating cell-stack.

Figure 8: Complete high voltage isoSPI bus with echo control.

The use of the CMC and AC-coupling capacitors for added insulation is the same as previously described, but we suggest slightly different coupling circuitry to damp the multitude of reflections and provide a consistent wave shape for communicating devices irrespective of their physical position in the network. There are three differences:

The LTC6820 termination is changed to a 100pF capacitor (CT).

Far-end termination is only applied to the live bus (RT) and set to 68 (no termination at any of the LTC6804-2s).

22 coupling resistors (RC) are used for all bus connections to decouple stray capacitive loading.

These are shown in the figure 8 circuit, which again assumes the LTC6820 is operating at a safe earth potential. The modified waveforms are band-limited to control distortion from reflections, so the received pulses at the IC pins appear more rounded as in figure 9, but the isoSPI pulse discriminator circuit works fine with this filtered shaping and supports a full sixteen address bus.

Figure 9: Modified pulse shaping for echo control in isoSPI bus applications.

Depending on actual losses encountered in a given system, it may be necessary to lower the pulse-detection thresholds for optimal operation (configure thresholds to be 40%50% of the differential signal peak).

Note that for networks of five or less addresses, the reflections are generally not a significant problem, so standard resistive end-terminations can be retained (namely 100 at the CTERM and RTERM positions of figure 8, with the RCs omitted).

Conclusion
Use an AC-coupling method to mitigate the cost impact of high voltage isoSPI systems, eliminating the double insulation requirement on magnetics. Cost can be further reduced by replacing speciality toroidal transformer magnetics with inexpensive bobbin-wound common-mode-choke (CMC) components. Both the capacitors and CMCs are relatively low profile surface-mount chip components that are competitively priced and available with automotive approvals for high reliability. The biasing resistors for the AC coupling offer a useful means of monitoring the dielectric integrity of the system.

About the author
Jon Munson is an applications engineer at Linear Technology Corp.

To download the PDF version of this article, click here.


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