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发表于 2008-5-14 22:41:06
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Layout Guidelines for
Switching Power Supplies
Introduction
When designing a high frequency switching regulated power
supply, layout is very important. Using a good layout can
solve many problems associated with these types of supplies.
The problems due to a bad layout are often seen at
high current levels and are usually more obvious at large
input to output voltage differentials. Some of the main problems
are loss of regulation at high output current and/or large
input to output voltage differentials, excessive noise on the
output and switch waveforms, and instability. Using the
simple guidelines that follow will help minimize these problems.
Inductor
Always try to use a low EMI inductor with a ferrite type closed
core. Some examples would be toroid and encased E core
inductors. Open core can be used if they have low EMI
characteristics and are located a bit more away from the low
power traces and components. It would also be a good idea
to make the poles perpendicular to the PCB as well if using
an open core. Stick cores usually emit the most unwanted
noise.
Feedback
Try to run the feedback trace as far from the inductor and
noisy power traces as possible. You would also like the
feedback trace to be as direct as possible and somewhat
thick. These two sometimes involve a trade-off, but keeping
it away from inductor EMI and other noise sources is the
more critical of the two. It is often a good idea to run the
feedback trace on the side of the PCB opposite of the
inductor with a ground plane separating the two.
Filter Capacitors
When using a low value ceramic input filter capacitor, it
should be located as close to the VIN pin of the IC as
possible. This will eliminate as much trace inductance effects
as possible and give the internal IC rail a cleaner voltage
supply. Some designs require the use of a feed-forward
capacitor connected from the output to the feedback pin as
well, usually for stability reasons. In this case it should also
be positioned as close to the IC as possible. Using surface
mount capacitors also reduces lead length and lessens the
chance of noise coupling into the effective antenna created
by through-hole components.
Compensation
If external compensation components are needed for stability,
they should also be placed closed to the IC. Surface
mount components are recommended here as well for the
same reasons discussed for the filter capacitors. These
should not be located very close to the inductor as well.
Traces and Ground Plane
Make all of the power (high current) traces as short, direct,
and thick as possible. It is a good practice on a standard
PCB board to make the traces an absolute minimum of 15
mils (0.381mm) per Ampere. The inductor, output capacitors,
and output diode should be as close to each other possible.
This helps reduce the EMI radiated by the power traces due
to the high switching currents through them. This will also
reduce lead inductance and resistance as well which in turn
reduces noise spikes, ringing, and resistive losses which
produce voltage errors. The grounds of the IC, input capacitors,
output capacitors, and output diode (if applicable)
should be connected close together directly to a ground
plane. It would also be a good idea to have a ground plane
on both sides of the PCB. This will reduce noise as well by
reducing ground loop errors as well as by absorbing more of
the EMI radiated by the inductor. For multi-layer boards with
more than two layers, a ground plane can be used to separate
the power plane (where the power traces and components
are) and the signal plane (where the feedback and
compensation and components are) for improved performance.
On multi-layer boards the use of vias will be required
to connect traces and different planes. It is good practice to
use one standard via per 200mA of current if the trace will
need to conduct a significant amount of current from one
plane to the other.
Arrange the components so that the switching current loops
curl in the same direction. Due to the way switching regulators
operate, there are two power states. One state when the
switch is on and one when the switch is off. During each
state there will be a current loop made by the power components
that are currently conducting. Place the power components
so that during each of the two states the current
loop is conducting in the same direction. This prevents magnetic
field reversal caused by the traces between the two
half-cycles and reduces radiated EMI.
Heat Sinking
When using a surface mount power IC or external power
switches, the PCB can often be used as the heatsink. This is
done by simply using the copper area of the PCB to transfer
heat from the device. Refer to the device datasheet for
information on using the PCB as a heatsink for that particular
device. This can often eliminate the need for an externally
attached heatsink.
These guidelines apply for any inductive switching power
supply. These include Step-down (Buck), Step-up (Boost),
Flyback, inverting Buck/Boost, and SEPIC among others.
The guidelines are also useful for linear regulators, which
also use a feedback control scheme, that are used in conjunction
with switching regulators or switched capacitor converters.
Some layout pictures are included: Figure 1 shows
Step-up switching regulator schematic to be used for some
layout examples. Figure 2 is an example of a bad layout that
violates many of the suggestions given. Figure 3 and Figure
4 show an example of a good layout that incorporates most
of the suggestion given. |
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