Electromagnetic Interference (EMI) is a critical concern in power supply design, impacting system performance, reliability, and regulatory compliance. This guide provides practical tips and best practices for minimizing EMI in power supply circuits designed with Power Integrations' ICs, helping you achieve robust and compliant designs.
1. Understanding EMI in Switching Power Supplies
Switching power supplies inherently generate high-frequency noise due to rapid switching transitions of voltage and current. This noise can propagate through conduction (conducted EMI) or radiation (radiated EMI). Effective EMI reduction involves minimizing the generation of noise and preventing its propagation.
Figure 1: Common sources of EMI in switching power supplies.
2. PCB Layout Optimization
PCB layout is the single most critical factor in EMI reduction. A well-designed layout can significantly reduce EMI without adding costly components.
2.1. Minimize High-Current Loop Areas
Identify and minimize the area of high-frequency current loops, especially the primary switching loop (input capacitor, primary winding, MOSFET/IC). Smaller loop areas reduce radiated EMI.
2.2. Proper Grounding
Implement a solid ground plane or a star-grounding scheme. Separate noisy power grounds from quiet signal grounds. Ensure low-impedance connections to ground for all components.
2.3. Short and Wide Traces
Use short and wide traces for high-current and high-frequency paths to minimize parasitic inductance and resistance. Avoid sharp 90-degree bends in traces.
2.4. Component Placement
Place critical components (e.g., input capacitors, snubber components) as close as possible to the switching IC to minimize trace lengths and loop areas.
3. Snubber Circuits
Snubber circuits are used to suppress voltage spikes and ringing, which are significant sources of EMI.
3.1. Primary-Side Snubber (RCD Snubber)
An RCD snubber across the primary switch (MOSFET/IC) can absorb energy from leakage inductance, reducing voltage spikes and associated EMI. Optimize R, C, and D values for effective damping.
3.2. Secondary-Side Snubber (RC Snubber)
An RC snubber across the secondary rectifier diode can reduce high-frequency ringing caused by parasitic capacitance and inductance, improving EMI performance and reducing diode stress.
4. Filtering Components
External filters can be used to attenuate conducted EMI.
4.1. Common Mode Chokes (CMC)
Place a common mode choke on the AC input lines to suppress common mode noise. Select a CMC with appropriate inductance and current rating.
4.2. X and Y Capacitors
X-capacitors (line-to-line) and Y-capacitors (line-to-ground) are essential for filtering differential and common mode noise, respectively. Ensure Y-capacitors are safety-rated.
5. Power Integrations' Integrated Features
Many Power Integrations' ICs include features that inherently help with EMI reduction:
- **Frequency Jittering:** Spreads the switching noise spectrum, making it easier to filter.
- **Soft Switching:** Reduces switching losses and EMI by minimizing voltage and current overlap during transitions.
- **Integrated MOSFET:** Reduces parasitic inductances associated with external MOSFETs.
Conclusion
Effective EMI reduction in power supply designs is a multi-faceted task that combines careful PCB layout, strategic use of snubber circuits, appropriate filtering components, and leveraging the inherent EMI-reducing features of Power Integrations' ICs. By implementing these techniques, you can achieve designs that are not only efficient and reliable but also compliant with stringent EMI/EMC regulations. For advanced EMI challenges, our FAE team can provide expert analysis and guidance.