AN-2052 [NSC]
National Semiconductors Simple Switcher? Power Modules and EMI; 美国国家半导体SIMPLE SWITCHER ？功率模块和EMI
| 型号： | AN-2052 |
| 厂家： | 
National Semiconductor |
| 描述： | National Semiconductors Simple Switcher? Power Modules and EMI |
| 文件： | 总4页 (文件大小：156K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
National Semiconductor
Application Note 2052
Don Rhodes
National Semiconductor's
Simple Switcher® Power
Modules and EMI
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.
30122301
FIGURE 1.
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 di/dt flowing drain to source
in each MOSFET during their respective on time. The current
waveforms (Q1on & Q2on), 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 (In).
30122302
FIGURE 2.
In = 2Id Sin (nπd)/nπd x Sin (nπtr/T)/nπtr/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 tr is
the shortest of either tr or tf.
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 VDS tr and tf across or di/dt through Q1 and Q2. This
© 2011 National Semiconductor Corporation
301223
www.national.com
would imply that we could reduce the EMI levels by simply
slowing down the turn-on and turn-off of Q1 and or Q2. This
in fact is true and slowing down these switching times has a
greater affect on harmonics above f = 1/πtr. The trade-off,
however, is increased heat dissipation as the transition losses
increase. Nonetheless, adding some control to these param-
eters is a good idea in order to strike a balance between EMI
and thermal performance. This can be done by adding a small
amount of resistance, typically < 5Ω, in series with the gate of
Q1 and Q2 to control tr and tf. You can also add a “turnoff
diode” in parallel with the gate resistor to independently con-
trol the tr or tf transitions, Figure 3. This effort is an iterative
process and even the most seasoned power designers go
through it. The end goal is to reduce the EMI level to an ac-
ceptable level by slowing down the transistors while at the
same time keeping them cool enough to be reliable.
The physical area of the high di/dt current loops also plays a
significant role in the resulting EMI levels. Typically a designer
wants to make the design as compact as possible given that
PCB real estate is rarely a luxury. However, many designers
don’t know which portion of the layout is EMI critical. Going
back to our buck regulator example there are two loops, (Fig-
ure 4 & Figure 5) whose size will have a direct affect on EMI
levels.
Ott’s equation (Equation 2) for differential mode EMI level
shows the direct and linear affect that loop area has on a
circuit’s EMI level.
E = 263 x 10-16 (f2AI)(1/r)
(2)
The radiated field is proportional to the frequency (f ) of the
harmonic of interest in Hz, loop area (A) of the net in m2, the
current (I) and the measured distance (r) in m.
This concept has application to all circuit designs employing
trapezoidal waveforms, though we will limit our discussion
here to power design. Let’s consider the AC model, in Figure
4, for current flowing through the loop starting at the input ca-
pacitor then flowing through Q1 while it’s on, through L1 into
the output capacitor and returning to the input capacitor.
The second loop is formed when Q1 turns off and Q2 turns
on. The energy stored in L1 then flows through the output
capacitor and Q2 as seen in Figure 5. Control of these loop
areas is essential to controlling EMI and begins with careful
parts placement with forethought to the routing of your PCB
traces. But, of course, there’s a practical limit as to just how
small a loop area can be obtained.
30122303
FIGURE 3.
30122304
FIGURE 4.
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2
30122305
FIGURE 5.
From Equation 2 we can see that reducing the area of the
noted current loops can translate into large improvements in
the measured EMI level. Reducing the loop area by a factor
of 3 will result in an EMI reduction of 9.5dB, a factor of 10
results in 20dB of improvement. There are additional PCB
layout guidelines for DC to DC convertors in application note
AN-1229 available at www.national.com . It’s best to start with
the goal of minimizing the area of the two loops noted in Fig-
ure 4 & Figure 5. Begin with careful parts placement with an
eye to routing copper. Avoid the temptation to use both sides
of the board as vias add significant inductance and thus other
problems.
module’s high di/dt loops will be much smaller than a compa-
rable discreet regulator or controller design. Power modules
aren’t new and have been around for some time. But until
now, a couple of the things plaguing modules have been their
inability to effectively remove heat from the package and the
difficulty in reworking a module once it’s mounted.
National Semiconductor’s new SIMPLE SWITCHER® Power
Modules provide the designer a low EMI alternative while also
addressing traditional issues with power modules. This is ac-
complished in part by packages more often associated with
power components than modules. As an example the
LMZ10505, a 5A module with a Vin Max of 5V, comes in a
10mm x 10mm package with a relatively large 8.5 x 6.4mm
thermal die attach pad from which the heat can be easily
transferred to ground plane making the need for cooling air-
flow optional. Thermal and EMI problems no longer have to
be the inherent trade-off in solving your power problem. So
while the EMI solutions discussed in this article will work for
any power design, it takes time to find the correct balance.
Even seasoned power designers spend considerable time
placing components, let alone routing traces for a power de-
sign and will never achieve the compact size afforded by
these power modules. The advanced packaging techniques
used on the SIMPLE SWITCHER® Power Modules allow for
critical loop areas that are far smaller than otherwise possible.
The benefit found from decreased loop area, of course, scales
beneficially as the output current increases.
Often overlooked is the importance of proper placement of the
high frequency input and output capacitors. Some years ago
the company I worked for transferred the design of our prod-
ucts to an off-shore manufacturer. As a result my role
changed to largely that of a consultant which included helping
the new power designer through the trade-offs mentioned
here as well as a few more. This was an off-line switcher de-
sign with integrated lamp ballast and it was in this final power
stage that he struggled to reduce the EMI. I simply moved a
high frequency output capacitor closer to the output stage re-
ducing the loop area by about half and we realized an EMI
reduction of about 6dB. The designer, not really understand-
ing the principles at work, called that capacitor the “magic
cap,” yet all that really happened was we shrunk the area of
critical loop.
The obvious issue related to what we have discussed so far
is that the cure may be worse than the illness. In other words
the thermal problems resulting from slowing down the transi-
tion times in order to pass EMI could now be the big problem.
One solution to controlling EMI as described is to use fully
integrated power modules in place of traditional DC to DC
convertors. Power modules are complete switching regula-
tors with fully integrated power transistors and inductor and
are as easy to design in as a linear regulator. The area of
National has also addressed the rework issue common in
other power modules, by utilizing rework friendly packaging.
Additionally, parts are pin compatible within a family;
LMZ10503/04/05, LMZ12001/02/03 and LMZ14201/02/03 al-
lowing the designer to replace parts as needed. The part
numbers are intuitive with the last two digits denoting the rated
output current and the two previous digits denoting the rated
input voltage.
3
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Notes
For more National Semiconductor product information and proven design tools, visit the following Web sites at:
www.national.com
Products
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www.national.com/refdesigns
www.national.com/samples
www.national.com/evalboards
www.national.com/packaging
www.national.com/quality/green
www.national.com/contacts
www.national.com/quality
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Clock and Timing
Data Converters
Interface
Reference Designs
Samples
Eval Boards
LVDS
Packaging
Power Management
Green Compliance
Distributors
Switching Regulators www.national.com/switchers
LDOs
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www.national.com/led
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www.national.com/powerwise
Quality and Reliability
Feedback/Support
Design Made Easy
Applications & Markets
Mil/Aero
LED Lighting
Voltage References
PowerWise® Solutions
www.national.com/solutions
www.national.com/milaero
www.national.com/solarmagic
www.national.com/training
Serial Digital Interface (SDI) www.national.com/sdi
Temperature Sensors
PLL/VCO
www.national.com/tempsensors SolarMagic™
www.national.com/wireless
PowerWise® Design
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