AN-0971 Datasheet, PDF (1/20 Pages) Analog Devices – Recommendations for Control of Radiated Emissions with isoPower Devices

AN-0971
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: [phone] • Fax: [phone] • www.analog.com
Recommendations for Control of Radiated Emissions with isoPower Devices
by Mark Cantrell
INTRODUCTION
iCoupler® digital isolators with integrated isolated power
(isoPower®) employ isolated dc-to-dc converters that switch
currents of ~700 mA at frequencies as high as 300 MHz.
Operation at these high frequencies raises concerns about
radiated emissions and conducted noise. PCB layout and
construction is a very important tool for controlling radiated
emissions and noise from applications containing isoPower
components. This application note identifies the radiation
mechanisms and offers specific guidance on addressing them.
Several standards for radiated emissions exist. In the U.S.,
the Federal Communications Commission (FCC) controls
the standards and test methods. In Europe, the International
Electrotechnical Commission (IEC) generates standards, and
CISPR test methods are used for evaluating emissions. The
methods and pass/fail limits are slightly different under the
two standards. Although this application note is written with
reference to CISPR standards, all results are applicable to both
standards.
With proper design choices, isoPower devices can easily meet
CISPR Class A (FCC Class A) emissions standards. With care,
these products can also meet CISPR Class B and FCC Part 15,
Subpart B, Class B (FCC Class B) standards in an unshielded
environment. This application note examines PCB-related EMI
mitigation techniques, including board layout and stack-up issues.
The techniques, example layouts, and measurements described in
this application note are highly dependent on the options available
in PCB construction. A 4-layer PCB is required to implement the
evaluated EMI control techniques. Availability of more internal
PCB layers offers the same EMI reductions within smaller
geometry PCBs. For the purposes of this application note, a 4-layer
board was designed and manufactured using materials and
structures well within industry norms.
OSC
Control of emissions from signal cables and chassis shielding
techniques are outside of the scope of this application note.
isoPower OVERVIEW
The ADuM5xxx and ADuM6xxx product families represent a
significant step forward in isolation technology. Analog Devices,
Inc., has leveraged their experience in microtransformer design
to create chip scale dc-to-dc power converters. These power
converters are incorporated into Analog Devices signal isolation
products. Power levels of up to one-half watt are available, at
output voltages ranging from 3.3 V to 15 V. isoPower is used to
power the secondary side of the iCoupler data channels as well
as to provide power to off-chip loads.
Because the Analog Devices standard data couplers use a similar
magnetic technology, EMI can be an issue in data-only iCouplers
and the data channels of isoPower devices. Emissions from data
channels are addressed in the AN-1109 Application Note, Control
of Radiated Emissions in iCoupler Devices.
Analog Devices uses several power architectures to achieve desired
design goals, such as efficiency, small size, and high output voltage
(see Figure 1). These architectures have three common ele-
ments: a transformer to couple power to the secondary side of
the iCoupler, an oscillator tank circuit that switches current into
the transformer at an optimum frequency for efficient power
transfer, and a rectifier that recreates a dc level on the secondary
side. Several regulation methods are used in these products.
The physics of the transformer requires that the oscillator
circuit switch current into the transformer at a rate ranging
from between 180 MHz and 300 MHz. The rectifier circuit on
the secondary side effectively doubles this frequency during the
rectification process. These functions are common in switching
power supplies; however, the operating frequency is three
orders of magnitude higher than a standard dc-to-dc converter.
Noise generated by the operation of the converter into the
30 MHz to 1 GHz range is of concern for radiated emissions.
RECT REG
OPEN LOOP WITH LINEAR REGULATOR
ADuM524x
OSC
RECT REG
OSC
RECT
PWM
FULL FEEDBACK PWM CONTROL
ADuM540x, ADuM520x, ADuM5000
ADuM640x, ADuM620x, ADuM6000
OPEN LOOP WITH PWM CONTROL
ADuM5230, ADuM6132
Figure 1. isoPower Architectures
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