ADDC02808PB
BASIC OPERATION
Each unit has an INHIBIT pin that can be used to turn off the
converter. This feature can be used to sequence the turn-on of
multiple converters and to reduce input power draw during
extended time in a no load condition.
The ADDC02808PB converter uses a flyback topology with
dual interleaved power trains operating 180° out of phase. Each
power train switches at a fixed frequency of 500 kHz, resulting
in a 1 MHz fixed switching frequency as seen at the input and
output of the converter. In a flyback topology, energy is stored
in the inductor during one half portion of the switching cycle
and is then transferred to the output filter during the next half
portion. With two interleaved power trains, energy is transferred
to the output filter during both halves of the switching cycle,
resulting in smaller filters to meet the required ripple.
A SYNC pin, referenced to the input return line (Pin 10), is
available to synchronize multiple units to one switching fre-
quency. This feature is particularly useful in eliminating beat
frequencies which may cause increased output ripple on paral-
leled units. A current share pin (ISHARE) is available which
permits paralleled units to share current typically within 5% at
full load.
A five pole differential input EMI filter, along with a common-
mode EMI capacitor and careful attention to layout parasitics, is
designed to meet all applicable requirements in MIL-STD-461D
when installed in a typical system setup. Due to the higher output
level in this product compared to the 100 W continuous output
products, input stability is more of a concern. As a result, the two
inductors in the internal input EMI filter have smaller values of
inductance to mitigate input stability concerns. The effectiveness of
the internal input EMI filter is, therefore, slightly diminished
compared to these other products. A more detailed discussion
of CE102 and other EMI issues is included in the section
entitled “EMI Considerations.”
A low level dc auxiliary voltage supply referenced to the input
return line is provided for miscellaneous system use.
PULSED OUTPUT POWER VS. PULSE LENGTH
The maximum specified pulsed output power in the standard
configuration of the ADDC02808PB is 200 W. This limit is
based on issues of working down to the minimum input volt-
age, of providing a reasonable short circuit current limit, and so
on. However, this power level assumes that the junction tem-
peratures of the converter’s power semiconductor devices have
not exceeded 110°C. For short pulse lengths and low duty
cycles, this condition will be met. Otherwise, the pulsed output
power will have to be reduced to keep the junction tempera-
tures below 110°C if NAVMAT guidelines are to be followed.
The maximum available peak power out is 200 W and is based
on a combination of maximum junction temperatures, maximum
pulsewidth, and maximum duty cycle. Refer to section entitled,
“Pulsed Output Power vs. Pulse Length,” for peak power derat-
ing curves for varying conditions.
Figures 16 and 17 show the tradeoff that must be made be-
tween the highest allowable pulsed output power and the pulse
length. Notice that for each curve, as the pulse length is made
longer, the pulsed power that causes a 110°C junction tem-
perature to be reached is lower. The curves are provided for
two baseplate temperatures (25°C and 90°C) and three average
output powers. The duty cycle that corresponds to any point
on a curve can be calculated by dividing the average power by
the pulsed power for that point. The curves represent typical
upper limits; operation anywhere below the curves is accept-
able and will result in cooler junctions.
The unit is compensated for ultrafast transient response with
minimum output voltage deviation. The compensation has been
optimized and output stability insured for an external load
capacitance in the range of 500 µF, 20 mΩ ESR to 4,000 µF,
2.5 mΩ ESR. Peak performance and output stability are depen-
dent on minimizing parasitic inductance and resistance in the con-
nection from the converter to the load.
The converter uses current mode control and employs a high
performance opto-isolator in its feedback path to maintain isola-
tion between input and output. The control circuit is designed
to give a nearly constant output current as the output voltage
drops from VO nom to VSC during a short circuit condition. It
does not let the current fold back below the maximum rated
output current. The output overvoltage protection circuitry,
which is independent from the normal feedback loop, protects
the load against a break in the remote sense leads. Remote
sense connections, which can be made at the load, can adjust
for voltage drops of as much as 0.25 V dc between the converter
and the load, thereby maintaining an accurate voltage level at
the load.
The ADDC02808PB is designed to deliver a continuous 100
watts to its output while keeping its hottest junction tempera-
ture below 110°C with a baseplate temperature of 90°C.
250
MAXIMUM PEAK
POWER LIMIT
200
10W ave
100W ave
150
50W ave
100
An input overvoltage protection feature shuts down the con-
verter when the input voltage exceeds (nominally) 52.5 V dc.
MAXIMUM CONTINUOUS
POWER LIMIT
50
0
An internal temperature sensor shuts down the unit and pre-
vents it from becoming too hot if the heat removal system fails.
The temperature sensed is the case temperature and is factory
set to trip at a nominal case temperature of 110°C to 115°C.
The shut down temperature setting can be raised externally or
disabled by the user.
0
100
200
300
400
500
600
700
PULSEWIDTH – ms
Figure 16. Largest On-State Power vs. Pulsewidth that
Maintains TJMAX ≤ 110°C at 25°C Baseplate
REV. A
–7–
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