OverviewAll applicationsCategoryData LinesInterfacesIsolated SPI
OverviewAll applicationsCategoryData LinesInterfacesIsolated SPI

Isolated SPI

Component selectionDesign tips

Characteristics

To transmit data between ICs within a system, on-board interfaces like I2C or SPI are well suited. However, if the communication is conducted over several boards, or even over longer distances, a galvanically isolated, differential solution is required. Isolated SPI offers a simple and low effort solution for this.

In case there is a high potential difference between transmitter and receiver, i.e. in battery management systems, then there are higher requirements for the galvanic isolation between the devices. For these use cases we can offer special signal transformers.

Products of interest

I²C

Transformer for Battery Management Systems with integrated Common Mode Choke

The WE-BMS series isolates the serial-connected components of the battery pack and the following BMS controllers from each other, suppressing any voltage differences and EMI that may occur. The design from Würth Elektronik provides a much larger physical creepage distance as opposed to other competitors, following market requirements for this kind of applications.

By maintaining the physical distance through the innovative design regardless of coated traces, high system safety and reliability can be achieved through special robustness against dryness or heat. Fully insulated wires are used for flat design and even better isolation properties.

Thanks to the innovative construction, a larger physical creepage distance can be achieved with the same size as with the conventional design.
Thanks to the innovative construction, a larger physical creepage distance can be achieved with the same size as with the conventional design.
The included transformers and common mode chokes ensure BMS isolation and suppresselectromagnetic interference for reliable signal transmission.
The included transformers and common mode chokes ensure BMS isolation and suppresselectromagnetic interference for reliable signal transmission.
The WE-BMS series supports serial Daisy Chain, isoSPI, SPI and other signal isolations. Single and dual transformer for one or two channels are available.
The WE-BMS series supports serial Daisy Chain, isoSPI, SPI and other signal isolations. Single and dual transformer for one or two channels are available.

BMS Layout Guidelines

The signal lines should be kept as short as possible and the transformer should be as close as possible to the isoSPI cable connector. A distance of <2 cm is suggested.
The signal lines should be kept as short as possible and the transformer should be as close as possible to the isoSPI cable connector. A distance of <2 cm is suggested.
IC should be placed between 1-2 cm from the transformer to avoid magnetic coupling.
IC should be placed between 1-2 cm from the transformer to avoid magnetic coupling.
For four-layer or multilayer PCBs, the area around the BMS traces, the BMS socket and the transformer should be exposed in all layers. The distance around this area to copper surfaces and traces should be at least 4 mm, depending on the system working voltage.
For four-layer or multilayer PCBs, the area around the BMS traces, the BMS socket and the transformer should be exposed in all layers. The distance around this area to copper surfaces and traces should be at least 4 mm, depending on the system working voltage.
What is a battery management system (BMS)?

A BMS is an electronic system that manages a rechargeable battery with the goal to make it safe and reliable.

Functions of a BMS:

  • Protecting the battery from operating outside its safe operation area
  • Monitoring its state of charge and state of health
  • Calculating secondary data and reporting that data
  • Controlling its environment
  • Authenticating it and/or balancing it
Why is a transformer needed in a BMS?

The individual cells in a battery pack are connected in series as well as the following BMS controllers. Between the components or PCBs connected in series, voltage differences and electromagnetic interference can occur. A transformer can be used to insolate the components from each other and suppress EMI interference. Therefore, WE-BMS transformer supports the BMS isolation and EMI noise suppression for a safe and reliable operation.

What is the partial discharge test?

Partial discharges are localized dielectric breakdowns. Fixed insulation in which partial discharges occur during operation is not permitted in the long term.

To ensure the long term reliability of the insulation, Würth Elektronik carries out the partial discharge test in accordance with IEC 60664-1.

The test is passed if the quantity of partial discharges is below a limit value of 10 pC (pico coulomb).

Transformer with or without center tap?

The Würth Elektronik product portfolio offers transformers with and without a center pin. In addition to the integrated common mode choke in the transformer module, the center pin contributes to the suppression of common mode interference. For this purpose, a bypass capacitor is connected to the center pin, which provides a low impedance ground connection for common mode interference and thus diverts it towards earth. This leads to an improvement in isoSPI performance. Please refer to the IC manufacturer's data sheet for the selection of capacitors.