ADDC02808PB Analog Devices, Inc., ADDC02808PB Datasheet - Page 13

• Manufacturer Part Number: ADDC02808PB
• Description: 28 V, 200 W Pulsed Dc/dc Converter With Integral Emi Filter
• Manufacturer: Analog Devices, Inc.

REV. A
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 F 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
The duration of this connection is 10 s. The pulse is repeated
every second for 30 minutes. This test is repeated with the
connection of the 20 F 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.
Figure 36. External Resistor Value for Raising
Temperature Shutdown Point
1400
1200
1000
800
600
400
200
0
120
125
SHUTDOWN CASE TEMPERATURE – C
130
on-state resistance MOSFET.
135
140
145
150
–13–
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:
where
T
T
P
R
The power dissipated in the power supply, P
from the efficiency, , given in the data sheets and the actual
output power, P
formula:
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
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.
D
C
A
CA
= case temperature measured at the center of the package
= ambient temperature of the air available for cooling,
= the power, in watts, dissipated in the power supply,
= the thermal resistance from the center of the package
90 = 75 + (20 R
bottom,
to free air, or case to ambient.
O
, in the user’s application by the following
T
C
P
D
T
CA
A
P
)
( P
O
or R
D
1 – 1
ADDC02808PB
R
CA
CA
)
= 0.75 C/W
D
, can be calculated