[Standard circuit design] Isolated SPI solution standard circuit reference

Technical background and core concepts

Isolating SPI communications

SPI (Serial Peripheral Interface) adopts a master-slave synchronous, full-duplex architecture, and uses chip select CS, clock SCLK and MOSI/MISO four-wire communication to complete communication. It has simple timing and high speed. It is a mainstream interface for ADC/DAC, sensors, AFE and other devices. However, standard SPI defaults to a common ground between the transmitting and receiving ends and working in close proximity on the same board. Once there is a high potential difference across boards, across power domains, or between both ends, ground loops and common-mode interference will directly threaten communication security. While retaining the high-speed synchronous transmission characteristics of SPI, isolated SPI establishes an electrical isolation barrier between the two ends through a digital isolator, transformer or capacitive coupling, so that both sides of the transmitter and receiver have independent power sources and grounds, which can withstand voltage differences of hundreds of volts or even kilovolts, fundamentally blocking ground loops, suppressing common-mode noise, and extending SPI safety to high-voltage and strong interference scenarios such as industry, automobiles, and energy storage.

Why is this technology needed?

In scenarios such as BMS battery management, energy storage systems, servo drives, industrial control, and automotive electronics, there are often significant potential differences and strong electromagnetic interference between the high-voltage side and the low-voltage side, and between boards. There are extremely high requirements for electrical isolation, common-mode suppression, sampling accuracy, and personal and device safety of communications.

The SPI protocol itself has the natural advantages of high speed, full duplex, and fixed timing, but it does not define an isolation and protection mechanism. Without an adapted physical layer circuit, ground potential differences, common mode noise, surge impact, and electrostatic discharge will erode signal integrity, ranging from bit errors to chip breakdown, making it difficult for SPI's high-speed advantages to be used in harsh environments.

A verified reference for isolated SPI standard circuit design can help developers reasonably select isolation methods (digital isolator/transformer/capacitive coupling) on ​​the signal link, complete impedance matching and decoupling filtering, provide complete overvoltage and electrostatic protection for the port, and ensure physical layer consistency through standardized power domain division and grounding processing, thereby fully unleashing the potential of the SPI protocol, significantly shortening the development cycle, reducing trial and error costs, and making the isolated communication system truly stable and reliable.

Wohu Electronics' SPI solution abandons the bloated wiring harness of traditional parallel buses and adopts an isolated SPI topology to precisely connect each SPI slave board with adjacent boards in series to form a "digital neural chain" from the battery module to the terminal BMU.

Isolated SPI circuit design

Primary side wiring:

SPI signal connection:

The SPI signals (SDO, SDI, CLK, GPIO) on the controller side are connected to the primary side input of the BMS isolation transformer through the driver.

Decoupling capacitor:

The primary-side power pin should be connected to GND via a 100nF capacitor to ensure driver power supply stability and signal integrity.

Secondary side wiring

Differential signal connections:

The secondary side differential output (RDTX+/RDTX−) should be connected to the corresponding pins of the isolation connector (Connector) respectively, and then communicate with the outside through the D-SUB connector.

TVS Protection:

TVS protection devices should be connected in front of all RDTX+/RDTX− pins to suppress surge impacts, and connected to the secondary ground through bypass capacitors of appropriate value. It is recommended to use chip ceramic capacitors in 0402 or 0603 packages and place them as close to the connector pins as possible.

Circuit function description

Signal isolation:

The BMS isolation transformer transmits SPI differential signals through magnetic coupling to achieve complete electrical isolation between the controller side and the battery side, effectively blocking the high-voltage side current path, protecting the main control MCU from high-voltage impact, and suppressing common-mode interference.

Decoupling capacitor:

The 100nF capacitor is used to filter out high-frequency noise from the driver side power supply to ensure stable operation and signal integrity on the primary side of the isolation transformer.

TVS Protection:

The clamping voltage of the TVS device should match the system operating voltage and provide bidirectional surge and ESD protection for the RDTX differential pin to ensure long-term reliable operation in a high-voltage isolation environment.

Recommended supporting products for VOOHU Wohu solution:

Battery Management System (BMS)

Scenario:Electric vehicle power battery pack, energy storage power station, cell sampling AFE daisy chain

New energy power generation and energy storage

Scenario:Photovoltaic inverters, wind power converters, PCS energy storage converters

Industrial Automation and Motor Drives

Scenario:Servo drive, frequency converter, PLC analog quantity acquisition, robot joint

Automotive electronics

Scenario:Electric drive controller, OBC vehicle charger, DC-DC converter, high-voltage power distribution box (PDU)

Note: The above solutions are standard designs for reference only. The final circuit design is subject to on-site requirements. Call us for free in-depth support.

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