In modern electronic systems, switch-mode power supplies are highly favored due to their efficiency. However, a significant drawback of these supplies is the noise they produce, which can interfere with the performance of sensitive analog or digital circuits. This guide aims to simplify the process of EMI filter design for DC-DC and AC-DC power supplies, providing practical insights and tips.
Understanding Types Of EMI
There are two main types of EMI: radiated EMI and conducted EMI.
1. Radiated EMI
Radiated EMI from switch-mode power supplies are introduced at switching nodes, generating high-frequency voltage ringing and spikes. This noise depends heavily on the PCB board layout, making it difficult to predict on paper.
Source: High dV/dt noise at switching nodes.
Frequency Range: 30 MHz to 1 GHz.
Impact: Generated by voltage ringing and spikes, influenced by PCB layout.
2. Conducted EMI
Source: Input current of the switching regulator.
Frequency Range: 150 kHz to 30 MHz.
Modes: Common mode (CM) and differential mode (DM) noise.
Conducted EMI is typically measured using a Line Impedance Stabilization Network (LISN). It includes
Common Mode (CM) Noise: This occurs when fast-switching parts disturb the device, returning to the power supply through inherent capacitance.
Differential Mode (DM) Noise: Results from fast pulsating currents in the power lines, predictable using software tools like LTpowerCAD. To download LTpowerCAD and LTpowerPlanner software Click Here, It provides recommendations for analog devices and power products based on the user’s application.
Here Are Some Key Features Of LTpowerCAD Software
Help users find parts according to their supply specifications.
Guide users in designing and optimizing circuit components.
Provide insights into efficiency, feedback loops, and transient performance.
Give a detailed summary of the circuit design.
Export the design to LTspice® for further simulation and analysis.
Use LTpowerPlanner for system-level power tree design and optimization.
EMI Filter Design Considerations
To mitigate EMI, proper filtering must be added. Here are some key components and their roles:
Fuse
Function: Protects the power supply by breaking the circuit during overcurrent conditions.
Placement: In series with the non-grounded input terminal.
Selection Criteria: Consider voltage, current, response time, and operating temperature.
Metal Oxide Varistor (MOV)
Function: Protects against sudden voltage spikes.
Placement: Connected to input terminals.
Selection Criteria: Based on normal operating voltage and maximum shock level.
Input Surge Current Limiting Resistor
Function: Controls the initial current burst when power is turned on.
Selection Criteria: Wire wound resistors are preferred due to their energy-handling capacity.
Output Transient Voltage Suppression (TVS) Diode
Function: Shunts induced voltage spikes on output terminals.
Selection Criteria: Choose based on the voltage and energy levels at the output.
Practical Implementation Tips
Proximity Of Filters To Converters
Place EMI filters close to the DC-DC converter to minimize noise propagation.
PCB Design Considerations
Minimize trace lengths and reduce overall inductance to improve EMI performance.
Environmental Factors
Be aware of how temperature and applied bias affect the performance of filtering components, as shown in the capacitance variation with temperature.
Practical Example
Case Study: Noise Reduction In A Basic Power Converter
Imagine a graph showing noise from a power converter without any EMI filters. The largest noise spike occurs at the switching frequency, with smaller spikes at multiples of this frequency. These spikes exceed the CISPR 22 standards, necessitating a noise filter to reduce interference.
Conclusion
Designing effective EMI filters for DC-DC and AC-DC power supplies is crucial to ensure the optimal performance of electronic systems. By understanding the types of EMI, selecting appropriate components, and following best practices in PCB design, engineers can significantly reduce EMI and improve the reliability of their power supplies.
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