ADDC02808PB datasheet(15/20 Pages) AD

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ADDC02808PB

REV. A

–15–

Diodes (Switching, General Purpose, Rectifiers)

70% current (surge and continuous) derating

65% peak inverse voltage derating

110

°C maximum junction temperature

Diodes (Zeners)

70% surge current derating

60% continuous current derating

50% power derating

110

°C maximum junction temperature

Microcircuits (Linears)

70% continuous current derating

75% signal voltage derating

110

°C maximum junction temperature

The ADDC02808PB can meet all the derating criteria listed

above. However, there are a few areas of the NAVMAT

deratings where meeting the guidelines unduly sacrifices perfor-

mance of the circuit. Therefore, the standard unit makes the

following exceptions.

Common-Mode EMI Filter Capacitors: The standard sup-

ply uses 500 V capacitors to filter common-mode EMI.

NAVMAT guidelines would require 1000 V capacitors to meet

the 50% voltage derating (500 V dc input to output isolation),

resulting in less common-mode capacitance for the same space.

In typical electrical power supply systems, where the load

ground is eventually connected to the source ground, common-

mode voltages never get near the 500 V dc rating of the standard

supply. Therefore, a lower voltage rating capacitor (500 V)

was 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 which require

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

Figures 11 through 14 show the results of EMI measurements

conducted in accordance with MIL-STD-461D/462D for the

ADDC02805SA dc/dc converter (28 VIN, 5 VOUT, 100 W)

using the test setup shown in Figure 15. The EMI performance

of the ADDC02808PB dc/dc converter will be different for

several reasons. The purpose of this section is to describe the

various MIL-STD-461D baseline tests and the ADDC02805SA

converter’s corresponding performance and then explain how

the EMI performance of the ADDC02808PB will differ from

this baseline.

28 VIN, 100 W Out, Baseline Performance: The

ADDC02805SA has an integral differential- and common-

mode EMI filter that is designed to meet all applicable require-

ments in MIL-STD-461D when the power converter is

installed in a typical system setup (described below). The

converter also contains transient protection circuitry that per-

mits the unit to survive short, high voltage transients across its

input power leads.

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 opera-

tion)

• 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, then the ripple voltage on

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

converter. 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 fre-

quency 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 com-

mon-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 represen-

tative of the final system configuration EMI test. For the fol-

lowing reasons, the same is true if separately packaged EMI

filters are used.

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