ADDC02808PB

REV. A

–13–

SHUTDOWN CASE TEMPERATURE – C

1400

0

120

150

125

130

135

140

145

1200

1000

800

400

200

600

Figure 36. External Resistor Value for Raising

Temperature Shutdown Point

INPUT VOLTAGE RANGE

The steady state operating input voltage range for the converter

is defined as 18 V to 40 V. The abnormal operating input volt-

age range is defined as 16 V to 50 V. In accordance with MIL-

STD-704D, the converter can operate up to 50 V dc input for

transient conditions as long as 50 milliseconds, and it can oper-

ate down to 16 V dc input for continuous operation during

emergency conditions. Figure 3 (typical low line dropout vs.

load) shows that the converter can work continuously down to

and below 16 V dc under reduced load conditions.

The ADDC02808PB can be modified to survive, but not work

through, the upper limit input voltages defined in MIL-STD-

704A (aircraft) and MIL-STD-1275A (military vehicles). MIL-

STD-704A defines an 80 V surge that lasts for 1 second before

it falls below 50 V, while MIL-STD-1275A defines a 100 V

surge that lasts for 200 milliseconds before it falls below 50 V.

In both cases, the ADDC02808PB can be modified to operate

to specification up to the 50 V input voltage limit and to shut

down and protect itself during the time the input voltage ex-

ceeds 50 V. When the input voltage falls below 50 V as the

surge ends, the converter will automatically initiate a soft start.

In order to survive these higher input voltage surges, the modified

converter will no longer have input transient protection, how-

ever, as described below.

Contact the factory for information on units surviving high

input voltage surges.

Input Voltage Transient Protection: The converter has a

transient voltage suppressor connected across its input leads to

protect the unit against high voltage pulses (both positive and

negative) of short duration. With the power supply connected

in the typical system setup shown in Figure 15, a transient volt-

age pulse is created across the converter in the following man-

ner. A 20

µF capacitor is first charged to 400 V. It is then

connected directly across the converter’s end of the two meter

power lead cable through a 2

Ω on-state resistance MOSFET.

The duration of this connection is 10

µs. The pulse is repeated

every second for 30 minutes. This test is repeated with the

connection of the 20

µF capacitor reversed to create a negative

pulse on the supply leads. (If continuous reverse voltage protec-

tion is required, a diode can be added externally in series at the

expense of lower efficiency for the power system.)

The converter responds to this input transient voltage test by

shutting down due to its input overvoltage protection feature.

Once the pulse is over, the converter initiates a soft-start, which

is completed before the next pulse. No degradation of converter

performance occurs.

THERMAL CHARACTERISTICS

Junction and Case Temperatures: It is important for the

user to know how hot the hottest semiconductor junctions

within the converter get and to understand the relationship

between junction, case, and ambient temperatures. The hottest

semiconductors in the 100 W product line of Analog Devices’

high density power supplies are the switching MOSFETs and

the output rectifiers. There is an area inside the main power

transformers that is hotter than these semiconductors, but it is

within NAVMAT guidelines and well below the Curie tempera-

ture of the ferrite. (The Curie temperature is the point at which

the ferrite begins to lose its magnetic properties.)

Since NAVMAT guidelines require that the maximum junction

temperature be 110

°C, the power supply manufacturer must

specify the temperature rise above the case for the hottest semi-

conductors so the user can determine what case temperature

is required to meet NAVMAT guidelines. The thermal charac-

teristics section of the specification table states the hottest junction

temperature for maximum output power at a specified case

temperature. The unit can operate to higher case temperatures

than 90

°C, but 90°C is the maximum temperature that permits

NAVMAT guidelines to be met.

Case and Ambient Temperatures: It is the user’s responsi-

bility to properly heat sink the power supply in order to main-

tain the appropriate case temperature and, in turn, the maximum

junction temperature. Maintaining the appropriate case tem-

perature is a function of the ambient temperature and the me-

chanical heat removal system. The static relationship of these

variables is established by the following formula:

T

θ

CA

)

where

= case temperature measured at the center of the package

bottom,

= ambient temperature of the air available for cooling,

= the power, in watts, dissipated in the power supply,

CA

= the thermal resistance from the center of the package

to free air, or case to ambient.

from the efficiency, , given in the data sheets and the actual

formula:

P

1

η –1









For example, at 80 W of output power and 80% efficiency, the

power dissipated in the power supply is 20 W. If under these

conditions, the user wants to maintain NAVMAT deratings

(i.e., a case temperature of approximately 90

°C) with an ambi-

ent temperature of 75

°C, the required thermal resistance, case

to ambient, can be calculated as

90 = 75 + (20

CA

)or

CA

= 0.75

°C/W

This thermal resistance, case to ambient, will determine what

kind of heat sink and whether convection cooling or forced air

cooling is required to meet the constraints of the system.