AN-0971

APPLICATION NOTE

Recommendations for Control of Radiated Emissions with isoPower Devices

by Mark Cantrell

Rev. B | Page 1 of 20

INTRODUCTION

i

Coupler® 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.

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.

OSC

REG

RECT

OPEN LOOP WITH LINEAR REGULATOR

ADuM524x

OSC

REG

RECT

FULL FEEDBACK PWM CONTROL

ADuM540x, ADuM520x, ADuM5000

ADuM640x, ADuM620x, ADuM6000

OSC

PWM

RECT

OPEN LOOP WITH PWM CONTROL

ADuM5230, ADuM6132

Figure 1. isoPower Architectures