ADDC02812DAKV 查看數據表（PDF） - Analog Devices
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ADDC02812DAKV Datasheet PDF : 20 Pages
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ADDC02812DA/ADDC02815DA
It is then connected directly across the converter’s end of the
For example, at 80 W of output power and 80% efficiency, the
two meter power lead cable through a 2 Ω on-state resistance
power dissipated in the power supply is 20 W. If under these
MOSFET. The duration of this connection is 10 µs. The pulse conditions, the user wants to maintain NAVMAT deratings
is repeated every second for 30 minutes. This test is repeated
(i.e., a case temperature of approximately 90°C) with an ambi-
with the connection of the 20 µF capacitor reversed to create a
ent temperature of 75°C, the required thermal resistance, case
negative pulse on the supply leads. (If continuous reverse volt-
to ambient, can be calculated as
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
90 = 75 + (20 × RθCA) or RθCA = 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.
is completed before the next pulse. No degradation of converter SYSTEM INSTABILITY CONSIDERATIONS
performance occurs.
In a distributed power supply architecture, a power source
OBSOLETE 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-
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:
RN
=
–VIN
I IN
Note that this resistance is a function of the operating point. At
tion temperature for maximum output power at a specified case full load and low input line, the resistance is its smallest, while
temperature. The unit can operate to higher case temperatures
than 90°C, but 90°C is the maximum temperature that permits
at light load and high input line, it is its largest.
NAVMAT guidelines to be met.
Potential System Instability: The preceding analysis assumes
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
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.
these variables is established by the following formula:
where
TC = TA + (PD × RθCA)
TC = case temperature measured at the center of the package
bottom,
RS
LS
VS
INPUT
TERMINALS
LP
CP
–|RN|
TA = ambient temperature of the air available for cooling,
ADI DC/DC CONVERTER
PD = the power, in watts, dissipated in the power supply,
RθCA = 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:
PD
=
PO
1
η
– 1
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|
)
+
RSCP
+1
=
0
REV. 0
–11–
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