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EETE JUN 2014

ENERGY STORAGE Low cost isoSPI coupling circuitry for high voltage high capacity battery systems By Jon Munson The isoSPI feature built into the LTC6804 battery stack monitor, when combined 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 minimize hazards to personnel. In a typical isoSPI application - see figure - pulse transformers provide the dielectric isolation and reject commonmode interference that can be impressed on the wiring. The isoSPI function operates with readily available and inexpensive Ethernet LAN magnetics, which typically include a Fig. 1: Generalized isoSPI point-to-point link. common-mode-choke section to improve common-mode line noise, along with the usual 100Ω line termination resistors and common-mode decoupling capacitors. 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 Fig. 2: AC-coupled isoSPI point-to-point link for increased voltages. 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 center-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 10MΩ, so that fault currents are within the fine wire rating of the transformers and the shock hazard to personnel is minimal. Eliminating the high voltage requirement from the transformer magnetic design enables a number of relatively low cost options. One is to simply use appropriately approved Ethernet transformers. Another is to use other off-the-shelf low profile magnetics to reduce component height and part mass (reducing solder fatigue issues). These can be installed via surface-mount automated assembly methods like any other part, reducing production costs. A good candidate with these features is the discrete commonmode choke (CMC), a transformer structure that is ordinarily used as a filtering element. Such parts are available up to 100μH and carry approvals for use with Fig. 3: Using two common-modechokes as a center-tapped isoSPI transformer. automotive systems, making them desirable for isoSPI configurations as well. Suitable CMCs are inexpensive. They can be quickly and easily produced as a machine-wound wire pair on a chip-sized ferrite form. Although isoSPI designs require somewhat higher inductance to effectively pass the longer pulse waveforms, adequate inductance can be achieved by using two of the chokes with windings in series to produce 200μH. This has the additional benefit of forming virtual center-tap connections, which are useful for common-mode biasing and decoupling functions. Figure 3 shows an equivalent transformer model realized with two CMCs. The chokes indicated have an 1812 SMT footprint and bifilar windings (wires paired in construction), so primary and secondary are intimately matched—minimizing the leakage inductance and thus preserving high frequency performance. Jon Munson is Applications Engineer at Linear Technology Corp. - www.linear.com 48 Electronic Engineering Times Europe June 2014 www.electronics-eetimes.com


EETE JUN 2014
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