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ADDC02812DA датащи(PDF) 11 Page - Analog Devices

номер детали ADDC02812DA
подробное описание детали  28 V/100 W DC/DC Converters with Integral EMI Filter
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производитель  AD [Analog Devices]
домашняя страница  http://www.analog.com
Logo AD - Analog Devices

ADDC02812DA датащи(HTML) 11 Page - Analog Devices

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OBSOLETE
ADDC02812DA/ADDC02815DA
REV. 0
–11–
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 volt-
age protection 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 junc-
tion 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 maintain
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
mechanical heat removal system. The static relationship of
these variables is established by the following formula:
TC = TA + (PD × Rθ
CA
)
where
TC = case temperature measured at the center of the package
bottom,
TA = ambient temperature of the air available for cooling,
PD = the power, in watts, dissipated in the power supply,
Rθ
CA =
the thermal resistance from the center of the package
to free air, or case to ambient.
The power dissipated in the power supply, PD, can be calculated
from the efficiency, h, given in the data sheets and the actual
output power, PO, in the user’s application by the following
formula:
P
D = PO
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
× Rθ
CA
) or Rθ
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.
SYSTEM INSTABILITY CONSIDERATIONS
In a distributed power supply architecture, a power source
provides power to many “point-of-load” (POL) converters. At
low frequencies, the POL converters appear incrementally as
negative resistance loads. This negative resistance could cause
system instability problems.
Incremental Negative Resistance: A POL converter is designed
to hold its output voltage constant no matter how its input volt-
age varies. Given a constant load current, the power drawn from
the input bus is therefore also a constant. If the input voltage
increases by some factor, the input current must decrease by the
same factor to keep the power level constant. In incremental
terms, a positive incremental change in the input voltage results
in a negative incremental change in the input current. The POL
converter therefore looks, incrementally, as a negative resistor.
The value of this negative resistor at a particular operating
point, VIN, IIN, is:
R
N =
–V
IN
I
IN
Note that this resistance is a function of the operating point. At
full load and low input line, the resistance is its smallest, while
at light load and high input line, it is its largest.
Potential System Instability: The preceding analysis assumes
dc voltages and currents. For ac waveforms the incremental input
model for the POL converter must also include the effects of its
input filter and control loop dynamics. When the POL con-
verter is connected to a power source, modeled as a voltage
source, VS, in series with an inductor, LS, and some positive
resistor, RS, the network of Figure 25 results.
LP
CP
–|RN|
ADI DC/DC CONVERTER
LS
RS
VS
INPUT
TERMINALS
Figure 25. Model of Power Source and POL Converter
Connection
The network shown in Figure 25 is second order and has the
following characteristic equation:
s2(LS + LP )C + s
(LS + LP )
–|RN|
+ R
SCP


+1 = 0


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