EDNE July 2012 - (Page 22)

baker’s best By Bonnie Baker off switches in these power supplies generate large, discontinuous currents. These discontinuities are present at the input of buck converters, the output of boost converters, and at both the input and the output ports of flyback and buck-boost topologies. The discontinuous currents from the switching action generate a voltage ripple, which travels through the PCB traces to other parts of the system. Inputvoltage ripple, output-voltage ripple, or both from an SMPS can compromise the operation of the load circuit. Figure 2, available here shows an example of the frequency content of a dc/dc buck SMPS input running at 2 MHz. The fundamental frequency content of SMPS-conducted noise is 90 to 100 MHz. Conducted emissions are either differential-mode or common-mode interference. A differential-mode-interference signal appears between the circuit’s input terminals, such as signal and ground. The current flows through both inputs with the same phase. However, the direction of one current input is equal to and opposite from the direction of the other current input. The load at the end of these two inputs creates a voltage that changes with the current magnitude. This change in voltage between line one and the differential reference creates noise or communication errors in the system. Common-mode interference occurs as you add a ground loop or an undesirable current path to the circuit. If an interfering source exists, common-mode currents and a common-mode voltage develop on both lines, with the ground loop acting as the common-mode reference. Both differential-mode and common-mode interference require filters to combat the detrimental effects of EMI. In next month’s column, we will address circuit approaches to solving EMInoise problems.EDN Read the first parts of this series; www.edn-europe.com/article. asp?articleid=5477 2 www.edn-europe.com/article. asp?articleid=5508 3 www.edn-europe.com/article. asp?articleid=5547 1 R EMI-SOURCE ELECTROSTATIC COUPLING EMI problems? Part four: It could be conduction adiated electromagnetic-interference noise can originate from an unintended source and an unintentional antenna (see parts 1, 2, and 3 of this series, available at available through the links below). Conductive EMI noise also can originate from a radiating EMI-noise source or from board components. Once your board receives conductive noise, it resides in your application circuit’s PCB traces. Common radiated EMI-noise sources include the elements that the previous articles discuss as well as onboard SMPSs (switch-mode power supplies), connection wires, and switching or clocked networks. Conducted EMI noise is a consequence of the normal operation of switching circuits in combination with parasitic capacitances and inductances. Figure 1 illustrates the origin of EMI-noise sources that can couple into PCB traces. Vemi1 originates with switching networks, such as clocking signals or digital-signal traces. The mode of coupling for these noise sources is through the parasitic capacitance between traces. These signals transmit current spikes into a neighboring PCB trace. Vemi2 again originates with switching networks or from an antenna on the VEMI1 + 2 pF M1 L2 10k + VIN CONDUCTOR-CONNECTION WIRE TO PCB-TRACE-INTERCONNECT CABLE − PCB. The coupling mode for these noise sources is through the parasitic inductance between traces. This signal transmits voltage disturbances into the neighboring PCB trace. The third EMI-noise source is a consequence of neighboring wires in a cable. Signals traveling down these wires can produce a crosstalk effect. SMPSs generate Vemi4. The noise from the SMPS rides on the power-supply traces and appears as Vemi4 signals. During normal operation, SMPS circuits create conducted EMI opportunities. The on and EMI-SOURCE MAGNETIC COUPLING VEMI2 + L1 10k OP AMP + VOUT + SWITCHING-SUPPLY EMI 100 kHz TO 1 MHz VEMI4 VSUPPLY 10 nF Figure 1 Vemi1 noise originates with switching networks, such as clocking signals or digital-signal traces. Vemi2 noise originates with switching networks or from an antenna on the PCB. The third EMI-noise source is a consequence of neighboring wires in a cable. Switch-mode power supplies generate Vemi4 noise. 22 EDN EUROPE | JULY 2012 www.edn-europe.com http://www.edn-europe.com/cmsimages/bb_emi4_fig2.png http://www.edn-europe.com/article.asp?articleid=5477 http://www.edn-europe.com/article.asp?articleid=5508 http://www.edn-europe.com/article.asp?articleid=5547 http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE July 2012

Cover
Contents
International Rectifer
Microchip
FTDI
Masthead
Comment
International Rectifer
Pulse
Analog Devices
Digi-Key
Analog Devices
Test & Measurement
Agilent Technologies
Baker's Best
Messe München
Understand and characterize envelope-tracking power amplifiers
Digi-Key
Cover Story
Coilcraft
Digi-Key
Signal Integrity
Altera
USB 3.0 : Bringing SuperSpeed connectivity to mobile devices
Digi-Key
Developing high-frequency integrated circuits for test and measurement
Design idea
Product Roundup
Tales from the cube

EDNE July 2012