Using bullet-proof isoSPI data links to boost battery-management systems

Introduction
For reliability, performance and longevity of battery packs being engineered into HEV, PHEV and EV drive-trains, a key factor is the electronics employed in the battery management subsystem (BMS). To date, most pack designs are constrained to large, singular assemblies by the centralized nature of practical BMS hardware. In particular, the electrically noisy working environment of the battery and associated equipment places harsh limitations on data communication links that are required to transport vital information within the vehicle. The ubiquitous CANbus was designed to handle this level of noise-rejection, but the data throughput demands of raw BMS data and component costs associated with it tend to preclude modularization and distribution of cell modules into structurally attractive designs, especially to provide good weight distribution. Enter the isoSPI™ physical layer adaptation of the standard chip-level Serial Peripheral Interface (SPI), unleashing the full potential of cost-effective distributed-pack architectures.

How the isoSPI Interface Works
To handle high levels of interference, the primary technique to employ is differential signaling on a “balanced” wire pair (neither wire grounded). This permits noise to ride on the wires, but since the noise on both wires (the common-mode) will be nearly identical, the transmitted difference-mode signal remains relatively unaffected. To handle really large common-mode noise ingress, an isolation method is required as well, the simplest being a magnetic coupling as provided by a tiny transformer. The transformer windings couple the important difference information across the dielectric barrier, but being electrically insulated, does not strongly couple the common-mode noise. These are the same approaches used in the highly successful Ethernet twisted-pair standards. The last aspect is to tailor the signaling scheme to provide a full-duplex SPI activity map that supports up to 1Mbps signaling yet only requires a single twisted-pair for transmission. Figure 1 shows an idealized isoSPI differential waveform that depicts the DC-free pulse generation that can be transformer coupled without information loss. The width, polarity, and timing of the pulses encode the various state changes of the conventional SPI signals.