EDNE May 2012 - (Page 47)

By J e f f Ba r r ow • I n t e g r at e d d e v I c e tec h no lo g y I nc Understand and reduce dc/dc-switching-converter ground noise D Once yOU Understand the twO primary nOise sOUrces, yOU can minimize them. Whenever charge moves, a magnetic field develops. Current in a wire, a resistor, a transistor, a superconductor, or a capacitor’s plate-to-plate displacement creates a magnetic field. Magnetic flux is a magnetic field passing through a current-loop area and equals the product of the field cutting the loop surface at a right angle: φB=B×A, where φB is the magnetic flux, B is the magnetic field, and A is the currentloop area. The magnetic field at a distance encircling a wire is directly proportional to the wire’s electrical current, B=μoI/2πr, where r is the magnetic field at a distance. Electrical components have length, and charge must flow from one device to the next in the various wire segS1 T=S1+S2 LBUCK VOUT c/dc-switching power converters are notorious for physically disrupting otherwise carefully designed systems and circuit schematics. These converters drive unwanted charge onto electrical ground, causing false digital signals, flip-flop double clocking, electromagnetic interference, analog-voltage errors, and potentially harmful high voltages. As the complexity of these designs increases and applications become more densely populated, the physical-circuit implementation begins to play a critical role in the electrical integrity of the system. To address these issues, you need to learn how to reduce two major sources of ground noise. Ground noise: Problem number 1 Figure 1 shows an ideal buck converter with a constant load current. Switches S1 and S2 toggle back and forth, chopping the input voltage across the buck inductance and the buck capacitance. Neither inductor current nor capacitor voltage can change instantaneously, and the load current is constant. All switching voltages and currents should successfully span buck inductance or pass through the buck capacitance, respectively, because an ideal buck converter produces no ground noise. But experienced designers know that a buck converter is a notorious noise source. This fact means that the circuit in Figure 1 is missing some important physical elements. S1 T=S1+S2 LBUCK ≃ICONSTANT VOUT=VINS1/T + VIN – B FIELD B FIELD S2 I1 CBUCK (a) 0V S1 LOOP AREA CHANGES + SO MAGNETIC FLUX ALSO VIN CHANGES – AND INDUCES GROUND BOUNCE dA dϕ =B VGB= dt dt 0V LBUCK VOUT+VGB B FIELD S2 I2 CBUCK + VIN – S2 CBUCK LOAD VGB GROUND≠0V GROUND=0V (b) –VGROUNDBOUNCE+ Figure 1 in a buck-converter circuit, inductor current cannot change instantaneously, so identifying a source of ground bounce in an ideal buck converter is difficult. Figure 2 changing flux induces voltage (a). as a buck switches, the changing current-loop path causes a changing flux and induces ground bounce (b). www.edn-europe.com MAY 2012 | EDN EuropE 47 http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE May 2012

Cover
Contents
International Rectifier
RS Components
Masthead
Comment
International Rectifier
Pulse
Analog Devices
Toshiba
Digi-key
Bergquist
Baker's best
Test & Measurement
Agilent Technologies
Digi-key
Audio-converter-subsystem design challenges in the 21st century
Coilcraft
Silicon Labs
Digi-key
Cover Story
Digi-key
Sensor & Test
Mechatronics in design
Vicor's insert
Understand and reduce dc/dc-switching-converter ground noise
Power MOSFETs continue to evolve
Design Idea
Product Roundup
Tales from the cube

EDNE May 2012