AN017 Datasheet, PDF (1/6 Pages) Analog Intergrations Corporation – Switching Mode

AN017
The analysis and layout of a Switching Mode
Power Supply
The more knowledge you have about a switching
mode power supply, the better chances your job
works on layout.
Introductions
The implementation of layout would be different in
both analog and digital fields. Digital designers
usually line up the components in neat, logical rows,
and the trace on the PCB is mostly as narrow as 8 mil
(1 mil=0.001 inch) or less. It would be good to see an
aesthetically digital design, but a similarly designed
power supply probably doesn’t work very well.
However, many analog designers start by dealing
with the portions within a circuit, which is most likely
to be interfered, and then take care of the output
node. Again, it might work well for an analog design,
but mostly not suitable for power supply layout
design.
Following paragraph is to discuss how to build a
correct layout regulation from knowing the operation
of a circuit.
Relationships
Every switching power supply has four current loops
(figure 1). Keep them separate from each other in
March 2001
various degrees of their interfering capability, they
are:_[1]
1. The output load loop
2. The input source loop
3. The output rectifier loop
4. The power switch loop
Fig 1. The Major Current Loop of a Switching Mode
Power Supply
The current flows in both input and output loops are
composed of DC elements with some AC ripple
current. That means these two loops do not result in
problems most of the time. It’s easy to get rid of the
high frequency switching noise due to some special
filter attached. On the other hand, the power switch
and rectifier current loops are entirely AC. They have
trapezoidal current waveforms with high peak
currents and sharp edges (di/dt). We know that
those waveforms contain excellent high order
harmonic after some mathematical expansion,
meanwhile, the EMI takes place.
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