ADDC02803SC/ADDC02805SA

REV. A

–13–

supply. A lower voltage rating capacitor (500 V) was therefore

chosen to fit more capacitance in the same space in order to

better meet the conducted emissions requirement of MIL-STD-

461D (CE102). For those applications requiring 250 V or less

of isolation from input to output, the present designs would

meet NAVMAT guidelines.

Switching Transistors: 100 V MOSFETs are used in the

standard unit to switch the primary side of the transformers.

Their nominal off-state voltage meets the NAVMAT derating

guidelines. When the MOSFETs are turned off, however, mo-

mentary spikes occur that reach 100 V. The present generation

of MOSFETs are rated for repetitive avalanche, a condition that

was not considered by the NAVMAT deratings. In the worst

case condition, the energy dissipated during avalanche is 1% of

the device’s rated repetitive avalanche energy. To meet the

NAVMAT derating, 200 V MOSFETs could be used. The

100 V MOSFETs are used instead for their lower on-state resis-

tance, resulting in higher efficiency for the power supply.

NAVMAT Junction Temperatures: The two types of power

deratings (current and temperature) can be independent of one

another. For instance, a switching diode can meet its derating

of 70% of its maximum current, but its junction temperature

can be higher than 110

°C if the case temperature of the con-

verter, which is not controlled by the manufacturer, is allowed

to go higher. Since some users may choose to operate the power

supply at a case temperature higher than 90

°C, it then becomes

important to know the temperature rise of the hottest semicon-

ductors. This is covered in the specification table in the section

entitled Thermal Characteristics.

EMI CONSIDERATIONS

The ADDC02803SC and ADDC02805SA have an integral

differential- and common-mode EMI filter designed to meet all

applicable requirements in MIL-STD-461D when the power

converters are installed in a typical system setup (described

below). The converters also contain transient protection circuitry

that permit the units to survive short, high voltage transients across

their input power leads. The purpose of this section is to

describe the various MIL-STD-461D tests and the converters’

corresponding performance. Consult factory for additional

information.

The figures and tests referenced herein were obtained from

measurements on the ADDC02805SA, a single 5 V dc output

converter. Since the construction and topology of the 3.3 V

output converter is almost identical to the 5 V dc output con-

verter, and the component values of the EMI differential- and

common-mode filter in the 3.3 V output converter are identical

to the 5 V output converter, the text references these figures and

tests as typical of the ADDC02803SC converter as well.

Electromagnetic interference (EMI) is governed by MIL-STD-461D,

which establishes design requirements, and MIL-STD-462D,

which defines test methods. EMI requirements are categorized

as follows (xxx designates a three digit number):

• CExxx: conducted emissions (EMI produced internal to the

power supply which is conducted externally through its input

power leads)

• CSxxx: conducted susceptibility (EMI produced external to the

power supply which is conducted internally through the input

power leads and may interfere with the supply’s operation)

• RExxx: radiated emissions (EMI produced internal to the

power supply which is radiated into the surrounding space)

• RSxxx: radiated susceptibility (EMI produced external to the

power supply which radiates into or through the power supply

and may interfere with its proper operation)

It should be noted that there are several areas of ambiguity with

respect to CE102 measurements that may concern the systems

engineer. One area of ambiguity in this measurement is the

nature of the load. If it is constant, the ripple voltage on the

converter’s input leads is due only to the operation of the con-

verter. If, on the other hand, the load is changing over time,

this variation causes an additional input current and voltage

ripple to be drawn at the same frequency. If the frequency is

high enough, the converter’s filter will help attenuate this second

source of ripple, but if it is below approximately 100 kHz, it will

not. The system may then not meet the CE102 requirement,

even though the converter is not the source of the EMI. If this

is the case, additional capacitance may be needed across the

load or across the input to the converter.

Another ambiguity in the CE102 measurement concerns common-

mode voltage. If the load is left unconnected from the ground

plane (even though the case is grounded), the common-mode

ripple voltages will be smaller than if the load is grounded. The

test specifications do not state which procedure should be used.

However, in neither case (load grounded or floating) will the

typical EMI test setup described below be exactly representative

of the final system configuration EMI test. For the following

reasons, the same is true if separately packaged EMI filters are

used.

In almost all systems the output ground of the converter is ulti-

mately connected to the input ground of the system. The para-

sitic capacitances and inductances in this connection will affect

the common-mode voltage and the CE102 measurement. In

addition, the inductive impedance of this ground connection

can cause resonances, thereby affecting the performance of the

common-mode filter in the power supply.

In response to these ambiguities, the Analog Devices’ converter

has been tested for CE102 under a constant load and with the

output ground floating. While these measurements are a good

indication of how the converter will operate in the final system

configuration, the user should confirm CE102 testing in the

final system configuration.

CE101: This test measures emissions on the input leads in the

frequency range between 30 Hz and 10 kHz. The intent of this

requirement is to ensure that the dc/dc converter does not cor-

rupt the power quality (allowable voltage distortion) on the

power buses present on the platform. There are several CE101

limit curves in MIL-STD-461D. The most stringent one app-

licable for the converter is the one for submarine applications.

Figure 19 shows that the converter easily meets this requirement

(the return line measurement is similar). The components at

60 Hz and its harmonics are a result of ripple in the output of

the power source used to supply the converter.