CAN FD Transceiver Emission Reduction via Dynamic Impedance Balancing
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Solution Overview
Problem
CAN FD bus systems face challenges in adhering to conducted emission limits due to increased bit rates, which result in excessive electromagnetic interference, particularly at data transmission rates of 2 Mbit/s and 5 Mbit/s, exceeding the specified emission limits.
Innovation Solution
A transceiver device with an emission reduction unit that controls the switch-on path of a stand-off device based on the dominant state of the transmit signal, ensuring reduced interference radiation and improved electromagnetic compatibility by balancing impedance symmetry between bus wires, using a P-channel stand-off transistor and an N-channel transistor in the transmitting stage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the bit rate is increased to achieve higher data transmission rates (2 Mbit/s, 5 Mbit/s), then the productivity of the bus system is improved, but the conducted emissions exceed the specified limiting values
Solution Approach 1:
The stand-off device is dynamically controlled to switch between connected and disconnected states based on the transmit signal state. During dominant transmit states, the stand-off device is connected to provide additional impedance balancing and reduce emission asymmetry. During recessive states, it is disconnected to maintain normal differential signaling. This dynamic switching resolves the contradiction by adapting the impedance balancing only when needed for emission reduction.
Solution Approach 2:
The invention changes the impedance parameters of the bus system by introducing a controllable stand-off device that modifies the common-mode impedance balance. The stand-off device alters the electrical characteristics of the transmitting stage, specifically adjusting the impedance symmetry between CAN_H and CAN_L lines during dominant states, thereby reducing conducted emissions without affecting recessive state operation.
2Reliability
If a stand-off device is connected in the transmitting stage to reduce emission asymmetry, then the electromagnetic compatibility is improved, but the device complexity increases
Solution Approach 1:
The stand-off device acts as an intermediary element between the differential signaling path and ground. It provides a controlled impedance path that balances the common-mode currents without interfering with the differential signal transmission. The emission reduction unit serves as a mediator that intelligently controls when this intermediary element should be active, adding minimal complexity while achieving emission reduction.
Solution Approach 2:
The emission reduction function is extracted as a separate controllable unit rather than being integrated into the core transmitting stage. The stand-off device is selectively activated only when needed for emission reduction during dominant states, rather than being permanently connected. This extraction approach minimizes the impact on overall device complexity while achieving the EMC improvement.
3Object-generated harmful factors
If the stand-off device is continuously connected to balance impedance, then the conducted emissions are reduced, but the capacity of the bus wires increases
Solution Approach 1:
The stand-off device is activated periodically or intermittently based on the transmit signal state rather than continuously. It is connected during dominant transmit states when emission asymmetry occurs and disconnected during recessive states. This periodic activation reduces the average capacity added to the bus wires while maintaining emission reduction effectiveness during critical transmit periods.
Data Source
AI summary
A transceiver device for a bus system and a method for reducing conducted emissions. The transceiver device has a transmitting stage for transmitting a transmit signal to a first bus wire of a bus of the bus system, in which bus system an exclusive, collision-free access of a user station to the bus of the bus system is at least temporarily ensured, and for transmitting the transmit signal to a second bus wire of the bus, a receiving stage for receiving the bus signal transmitted on the bus wires, and an emission reduction unit for controlling a switch-on path of a first stand-off device in the transmitting stage as a function of whether or not a dominant stage of the transmit signal occurs.