AN-2052 Datasheet, PDF (1/4 Pages) National Semiconductor (TI) – National Semiconductors Simple Switcher® Power Modules and EMI

National Semiconductor's
Simple Switcher® Power
Modules and EMI
National Semiconductor
Application Note 2052
Don Rhodes
May 5, 2011
Power supply design, even the design of common DC to DC
switching convertors can present a number of challenges; this
is especially true with higher power designs. Beyond the func-
tional issues, an engineer must make sure the design is
robust, meets the required cost targets and thermal and
space constraints all while staying on schedule. Additionally,
and hopefully not as an afterthought, the design must produce
sufficiently low Electromagnetic Interference (EMI) both for
reasons of product compliance and system performance.
However, a power supply’s EMI level is one of the more dif-
ficult aspects of the design to accurately predict. Some might
even argue that it’s simply impossible and the best a designer
can do is take sufficient care in the design especially in the
layout.
While the principles discussed in this article apply more
broadly to power design, we’re going to focus on DC to DC
convertor design given its broad application. It affects virtually
every hardware engineer who at some point has to design a
power convertor. In this article we’ll consider two common
trade-offs related to low EMI design; thermal performance
and EMI and also solution size related to PCB layout and EMI.
We’ll use a simplified buck convertor as our example, shown
in Figure 1.
FIGURE 1.
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Both radiated and conducted EMI are measured in the fre-
quency domain and that is really nothing more than a Fourier
series of a given waveform. We’ll focus our attention on radi-
ated EMI for this article. The dominant switching waveforms
generating EMI in a synchronous buck convertor are gener-
ated by Q1 and Q2; namely the d_i/d_t flowing drain to source
in each MOSFET during their respective on time. The current
waveforms (Q1_on & Q2_on), as shown in Figure 2, are not clas-
sically trapezoidal in shape, however, we can take a few
liberties since the inductor current transitions are relatively
slow allowing the application of Equation 1 from Henry Ott’s
classic Noise Reduction Techniques in Electronic Systems.
We see that the rise and fall time of a waveform like these
directly affects the harmonic amplitude or the Fourier coeffi-
cient (I_n).
I_n = 2Id Sin (nπd)/nπd x Sin (nπt_r/T)/nπt_r/T
(1)
Where n is the harmonic number, T is the period, I is the p-p
current amplitude of the waveform, d, the duty cycle and t_r is
the shortest of either t_r or t_f.
FIGURE 2.
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In reality you will most likely have odd and even order har-
monic emissions. A wave form must have a precise 50% duty
cycle to generate only odd order harmonics. Real world wave-
forms rarely, if ever, have that kind of dutycycle precision.
The EMI amplitude of the harmonics series is affected by the
turn-on and turn-off of Q1 and Q2. This can be seen in mea-
suring the V_DS t_r and t_f across or d_i/d_t through Q1 and Q2. This
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