Inductance and capacitance
Loops in a PCB design can cause switching noise from a buck converter to couple into high-speed signal lines.
By Martin Rowe, Senior Technical Editor -- Test & Measurement World, 10/1/2010 12:00:00 AM
One technical track at the 2010 EMC Symposium included three papers on EMI (electromagnetic interference) that results from switching noise in DC-DC buck converters. The papers explained how the converters' noise can couple into circuit traces, which can disrupt components or let a product emit excessive EMI. All papers showed how PCB (printed-circuit board) design greatly affects EMI and signal integrity.
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Engineers from Intel explained how loops in a PCB design can cause switching noise from a buck converter to couple into high-speed signal lines (Ref. 1). Measurements showed how placement of high-frequency decoupling capacitors affected emissions that coupled into high-speed signal lines in a server. The figure shows a typical PWM (pulse-width modulation) circuit. The capacitors were first mounted on the opposite side of the PCB from the two MOSFETS. That produced a signal path whose inductance was too large. Moving the capacitors to the top of the board shortened the path, which reduced loop inductance. That lowered emissions and improved signal integrity.
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A typical PWM circuit for a switching regulator needs filter capacitors and short loops with low inductance to minimize switching noise. |
Students and faculty from the Missouri University of Science and Technology's EMC lab showed how capacitive coupling between a buck converter and signal lines lets high-frequency noise interfere with signals (Ref. 2). In their example, ringing noise in the converter's switching signals coupled into signal traces in a PCB. Using simulation, the researchers showed that capacitive coupling decreased as the distance between a signal trace and PCB vias carrying noisy signals increased.
Researchers from the Korea Advanced Institute of Science and Technology also evaluated how three types of ground-plane designs affect noise from a buck converter (Ref. 3). They investigated how the size and shape of a ground plane affected emissions. They measured impedance on the PCB to predict performance, then used a TEM (transverse electromagnetic) cell, a spectrum analyzer, and an oscilloscope to measure the emissions. Nodes between MOSFETS in a PWM circuit such as those in the figure are potential emissions sources. A solid ground plane under the node minimizes loop inductance and thus minimizes interference.
References
1. Ouyang, G., et al., "Switching Voltage Regulator Noise Coupling to Signal Lines in a Server System." Proceedings of the 2010 IEEE Symposium on Electromagnetic Compatibility. www.emcs.org.
2. Wu, S. et al., "Investigation of Noise Coupling from Switching Power Supply to Signal Nets," Proceedings of the 2010 IEEE Symposium on Electromagnetic Compatibility. www.emcs.org.
3. Koo, K., et al. "Impact of PCB Design on Switching Noise and EMI of Synchronous DC-DC Buck Converter, Proceedings of the 2010 IEEE Symposium on Electromagnetic Compatibility. www.emcs.org.
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