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Per Table 2N of IEC 60950, the minimum creepage distance for material group I, pollution degree 2 of functional insulation for a working voltage of 1600V is 8mm. The Bourns® Model SM91501AL transformer data sheet shows a minimum 10mm creepage distance. This is because the actual tracking distance over the surface of the transformer and chokes has been calculated at 10.4mm in the samples measured. The replacement test for IEC 60950 (IEC 62368-1), which became mandatory in June, 2019 for audio/video, information technology, and communication equipment will recognize FIW in the future. The use of FIW may qualify the device as having reinforced insulation with a lower working voltage (depending on the standard) of approximately 800V. This may allow the device to meet UL Listing requirements and may enable its use in additional applications such as consumer energy storage, which mandate reinforced insulation. Recommended Electrical Characteristics The recommended primary inductance values by some IC manufacturers will depend on the voltage of the communication signals, the pulse widths and the frequency. Bourns designed its Model SM91501AL transformer with a primary inductance span between 150µH and 450µH over an operating temperature range of -40°C to +125°C. The inductance is directly proportional to the permeability of the core. The permeability of the ferrite core of a transformer is temperature dependent and tends to increase with temperature. Therefore, the primary inductance in the Bourns model will drift up towards 450µH at the upper end of the temperature range. This is the reason for the large variation in the inductance value as specified on the data sheet. Figure 5: Bourns ® Model SM91501AL Transformer on Bourns ' BMS Demonstration Board The noise immunity of the BMS IC and transformer can be evaluated using a bulk current injection (BCI) test. The BCI test injects current into the twisted pair lines at set levels over a frequency range of 1MHz to 400MHz with the bit error rate | 18 | Serial Peripheral Interface (SPI) is an interface bus commonly used to send data where one device or "master" transmits a clock pulse and control bit to a series of slaves. On each clock pulse, the slave either reads a command from the master or if the control bit is inverse, transmits its data on the data line. In this way, a central battery controller IC (master) can interrogate each monitoring IC (slave) in turn and retrieve necessary voltage and temperature information from the whole pack. In addition, the transformer and integrated common mode choke filter out common mode noise from the daisy-chained network. Figure 4: BMS Transformer with Center Tap Capacitor and Resistor. Bottom Image of SPI Signal Although BMS ICs have balanced currents on their I/O pins, most manufacturers recommend a center-tapped transformer. These have been found to improve Common Mode Noise Rejection (CMNR) if a filter capacitor and termination resistor are used, as shown in Figure 4. Bourns ® BMS Transformer Safety Features The windings inside the Bourns ® Model SM91501AL transformer use enameled fully-insulated wire (FIW) that passes the dielectric strength (Hi-POT) test of 4.3kV (1mA, 60s).

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