SiC GaN Switching Module CM EMI Reduction
Solution Overview
Problem
Wide bandgap semiconductor switches, such as SiC and GaN, generate significant common mode (CM) electromagnetic interference (EMI) noise due to their high slew rates, which is difficult to filter and can cause electrical noise and interference in power modules, especially at high frequencies.
Innovation Solution
A switching module design incorporating an insulating substrate with a conductive layer between two substrates, forming a series parasitic capacitance to increase high-frequency impedance and divert CM current to the input voltage power rails, thereby maintaining a substantial portion of the CM current within the module.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If wide bandgap switching devices (SiC, GaN) are used to increase switching frequency and reduce power consumption, then switching performance and electron mobility are improved, but common mode EMI noise and electromagnetic interference increase significantly
Solution Approach 1:
A conductive layer is introduced as an intermediary element between the insulating substrate and the heat sink. This conductive layer acts as a mediator that provides a controlled path for CM current while maintaining electrical isolation. The layer has controlled impedance characteristics that help manage high-frequency noise without compromising thermal performance.
Solution Approach 2:
The patent modifies the electrical parameters of the packaging structure by introducing a conductive layer with specific impedance characteristics. This changes the CM current path impedance from very low (direct heat sink connection) to a controlled value that reflects CM current back to the switching device, thereby reducing radiated EMI while maintaining adequate heat dissipation.
2Temperature
If the heat sink is directly connected to ground for safety, then safety and thermal management are improved, but CM noise currents are conducted through electrical circuits and radiated as electrical fields
Solution Approach 1:
The conductive layer serves as an intermediary between the heat sink and the electrical circuits. It provides thermal conduction path while presenting controlled electrical impedance to CM currents. This mediator allows heat to flow to ground while preventing CM noise currents from easily conducting through the circuit.
Solution Approach 2:
The direct connection between heat sink and ground is segmented by inserting the conductive layer. This segmentation creates a controlled impedance interface that separates the thermal management function from the electrical ground reference, allowing independent optimization of both functions.
3Object-generated harmful factors
If filtering and shielding are used to reduce CM EMI currents, then EMI noise is reduced, but device complexity and packaging constraints are increased
Solution Approach 1:
The EMI mitigation function is extracted from external filtering and shielding components and integrated directly into the packaging structure itself. The conductive layer within the package provides the EMI reduction function without requiring additional external filters or shields, thereby reducing overall system complexity.
Solution Approach 2:
The conductive layer performs multiple functions simultaneously: it provides a thermal conduction path from the switching device to the heat sink, provides electrical isolation, and manages CM current impedance. This multi-functionality eliminates the need for separate EMI filtering components.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design effectively reduces CM EMI noise and maintains CM currents within the switching module, reducing electromagnetic interference and peak electric fields, while maintaining adequate heat dissipation and thermal performance.
Implementation Method 1
a conductive layer between said insulating substrate and said further insulating substrate, and a connection arrangement for diverting current capacitively coupled to said conductive layer
Implementation Method 2
forming a series parasitic capacitance to increase high-frequency impedance
Data Source
AI summary
In a switching module structure that includes a low-impedance path to ground, such as a parasitic capacitance of an insulating substrate, a further insulating substrate presenting a parasitic capacitance placed in series with the low impedance current path and a connection of a conductive layer to input voltage rails using a single decoupling capacitor or, preferably, a midpoint of the voltage rails formed by a series connection of decoupling capacitors maintains a large portion of common mode (CM) currents which are due to high dV/dt slew rates of SiC and GaN transistors within the switching module.