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The Designer’s Guide Community downloaded from www.designers-guide.org
Copyright © 2006, Kenneth S. Kundert – All Rights Reserved 1 of 12
Version 4, January 2004 An introduction into the basics of power supply noise reduction using supply bypassing
and decoupling.
Originally written in 1979, with modest updates in 1981 and 2002.
Last updated on May 11, 2006. You can find the most recent version at www.designers-guide.org.
Contact the author via e-mail at ken@designers-guide.com.
Permission to make copies, either paper or electronic, of this work for personal or classroom use
is granted without fee provided that the copies are not made or distributed for profit or commer-
cial advantage and that the copies are complete and unmodified. To distribute otherwise, to pub-
lish, to post on servers, or to distribute to lists, requires prior written permission.
Power Supply Noise Reduction
Ken Kundert
Designer’s Guide Consulting, Inc.
Power Supply Noise Reduction Introduction
2 of 12
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www.designers-guide.org
1 Introduction
Many of the problems that appear out of Murphy’s box upon transforming a design from
the mythical world of textbooks and S
PICE to the real world emanate from the non-ideal
power supply. Real power supplies can cause noise and spurious oscillations that can
force the designer into a frustrating glitch hunt. Rules of thumb can usually be applied
successfully to simple problems, but a little understanding and forethought will usually
provide clean solutions to even the more obscure problems. With this paper, I hope to
provide the understanding of some of the dynamics of power distribution. The fore-
thought is up to you.
2 Definitions
Bypassing and decoupling are often poorly understood and poorly applied. Many
designers believe bypassing and decoupling are synonymous. They are not; they are dis-
tinct concepts and each is a solution to a different problem (see Figure 1).
Bypassing is the reduction of high frequency current flow in a high impedance path by
shunting that path with a bypass, usually a capacitor (in this case, C
byp
). Bypassing is
used to reduce the noise current on power supply lines.
Decoupling is the isolation of two circuits on a common line. The decoupling network is
usually a low pass filter and the isolation is rarely equal in both directions. Decoupling
is used to prevent transmission of noise from one circuit to another. In the figure a
bypass capacitor, C
byp
, is shown along with the decoupling circuit, L
dec
and C
dec
. This is
because in practice bypassing is always used when decoupling.
Most circuits require bypassing, not decoupling. Using decoupling techniques to
accomplish bypassing will give disappointing, if not disastrous, results. Complete
understanding of both concepts is vital. We begin with bypassing.
FIGURE 1 Bypassing and decoupling.
Power
Supply
Load
C
byp
Power
Supply
Bypassing
Decoupling
Load
C
byp
C
dec
L
dec
to sensitive circuits
Bypassing Power Supply Noise Reduction
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3 Bypassing
Due to the finite bandwidth of all voltage regulators, their output impedance increases
with frequency. This can be modeled as an inductor in series with the output. Typical
values lie between 1 μH and 2 μH for a linear three terminal regulator. The output
impedance of switching regulators varies widely and should be measured for each case.
The interconnecting leads add about 20 nH per inch. When an active load is connected,
the time varying current demand creates a noise voltage across these inductors. This
noise voltage can be reduced in only two ways: reduce the rate of change of the current
(di/dt) passing through the inductor, or reduce the inductance. Bypassing reduces the
rate of change of the current through the inductor.
In bypassing, a secondary, high frequency low impedance path (a capacitor) is provided
for the varying currents from the load that shares as little inductance as possible with the
power supply leads. The key to successful bypassing is to properly determine the flow
of current from a load and to supply a return path that is not common with any other part
of the circuit (see Figure 2). The bypass path must be a significantly lower impedance at
the frequency of interest than the power supply leads. It is always better to use many
small parallel capacitors than one large one. This is because the equivalent series induc-
tance does not vary significantly with capacitance. The parallel bypass paths achieved
with the small capacitors results in a much lower total inductance.
Care must be taken when determining the return current path. It is often not obvious at
first glance as demonstrated in Figure 3. In this case the standard bypassing is applied,
which causes a current to flow in the ground and supply leads, which generates a noise
voltage. In Figure 4, this situation is remedied.
4 Reducing Inductance
As mentioned above, one way to reduce the noise voltage developed in the power supply
inductance is to reduce that inductance. To reduce the inductance of a linear regulator,
you can either increase its bandwidth or decrease its open loop output impedance. Both
are really not options unless you design your own regulator. There are also two methods
for decreasing the inductance of the power supply bus. One is to decrease its self induc-
tance, and the other is to increase the mutual coupling to its return path. A wire’s self
FIGURE 2 Proper bypassing.
V+
V–
C
byp
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