Page 21 - EngineerIT August 2022 Digital

ELECTRONICS

low voltage is converted to a higher switch is conducting current. The second colour shows the current flow during the off-
voltage. Once again, the current paths time – that is, when the power switch is switched off. And finally, the third colour is used
in which the current flow changes for all paths marked either only in the first colour or only in the second colour. The critical
with switching of the power switches path, in which the current flow changes with the switching of the power switches, can
are shown in red. Interestingly, the then be clearly identified.
placement of the input capacitor is Inexperienced circuit designers often consider the board layout for a switching
not critical at all. The most crucial is regulator to be black magic. The most important rule is to design the traces, in which the
the placement of the output capacitor. current flow changes with the switch transitions, to be as short and compact as possible.
It must be as close as possible to the This can be explained easily, follows logical relationships, and is the basis for an optimised
flyback diode (or the high-side switch) board layout in a switch-mode power supply design.
as well as to the ground connection of
the low-side switch. About the author
After that, any other switching Frederik Dostal is a power management expert with more than 20 years of experience in
regulator topology can be examined to this industry. After his studies in microelectronics at the University of Erlangen, Germany,
yield information on how the current he joined National Semiconductor in 2001, where he worked as a field applications
flow changes when the power switches engineer, gaining a lot of experience in implementing power management solutions
are switched. The classic method in customer projects. During his time at National, he also spent four years in Phoenix,
involves printing out the circuit and Arizona (USA), working on switch mode power supplies as an applications engineer. In
drawing the current flow, using three 2009, he joined Analog Devices, and since then has held a variety of positions working
differently coloured pens. One colour is for the product line and European technical support. He currently contributes his broad
used to indicate the current flow during design and application knowledge as a power management expert. Frederik works in the
the on-time – that is, when the power ADI office in Munich, Germany. n