Technical Specification
BASIC OPERATION AND FEATURES
The MQFL DC/DC converter uses a two-stage power conversion
topology. The first, or regulation, stage is a buck-converter that
keeps the output voltage constant over variations in line, load,
and temperature. The second, or isolation, stage uses transformers
to provide the functions of input/output isolation and voltage
transformation to achieve the output voltage required.
In the MQFL-28V series of converters the regulation stage is
preceeded by a boost-converter that permits these converters
to operate through various Military and Aircraft under-voltage
transients. Further discussion of this feature can be found later in
these notes.
MQFL-28V-06S
Output:
6V
Current:
17A
The MQFL converter’s control circuit does not implement an output
over-voltage limit or an over-temperature shutdown.
The following sections describe the use and operation of additional
control features provided by the MQFL converter.
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UNDER-VOLTAGE TRANSIENTS
Both the regulation and the isolation stages switch at a fixed
frequency for predictable EMI performance. The isolation stage
switches at one half the frequency of the regulation stage, but due
to the push-pull nature of this stage it creates a ripple at double its
switching frequency. As a result, both the input and the output of
the converter have a fundamental ripple frequency of about 550
kHz in the free-running mode.
Rectification of the isolation stage’s output is accomplished with
synchronous rectifiers. These devices, which are MOSFETs with a
very low resistance, dissipate far less energy than would Schottky
diodes. This is the primary reason why the MQFL converters have
such high efficiency, particularly at low output voltages.
Besides improving efficiency, the synchronous rectifiers permit
operation down to zero load current. There is no longer a need
for a minimum load, as is typical for converters that use diodes
for rectification. The synchronous rectifiers actually permit a
negative load current to flow back into the converter’s output
terminals if the load is a source of short or long term energy. The
MQFL converters employ a “back-drive current limit” to keep this
negative output terminal current small.
There is a control circuit on both the input and output sides of the
MQFL converter that determines the conduction state of the power
switches. These circuits communicate with each other across the
isolation barrier through a magnetically coupled device. No opto-
isolators are used.
A separate bias supply provides power to both the input and
output control circuits. Among other things, this bias supply
permits the converter to operate indefinitely into a short circuit and
to avoid a hiccup mode, even under a tough start-up condition.
An input under-voltage lockout feature with hysteresis is provided,
as well as an input over-voltage shutdown. There is also
an output current limit that is nearly constant as the load
impedance decreases to a short circuit (i.e., there is not fold-
back or fold-forward characteristic to the output current under this
condition). When a load fault is removed, the output voltage rises
exponentially to its nominal value without an overshoot.
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The MQFL-28V series of DC/DC converters incorporate a special
“boost-converter” stage that permits the converters to deliver full
power through transients where its input voltage falls to as low as
5.5V. Normally, the boost-converter is non-operational, and the
converter’s input voltage is passed directly to its pre-regulation
stage (see the Block Diagram). When an under-voltage transient
occurs, the boost-converter becomes operational, and it steps-up
the input voltage to a value greater than 16V so that the nominal
output voltage can be sustained.
It is important to note that the boost-converter stage must first
become “armed” before it can become operational. This “arming”
occurs when the converter’s input voltage exceeds approximately
18V. The boost-converter then becomes operational whenever the
input voltage drops below the arming voltage, and it will remain
operational as long as the input voltage remains within the region
shown in the Under-Voltage Transient Profile. If the input voltage
drops below this transient profile, the boost-converter stage is not
guaranteed to continue operating (it may, but it will protect itself
from excessive stresses). Once the boost-converter stops operating,
the converter’s input voltage will be reconnected directly to the
input of the pre-regulator stage. The output voltage will therefore
collapse unless the input voltage is 16V, or greater.
Note: the boost-converter will not become re-armed for the
next transient unless the input voltage once again exceeds
approximately 18V.
The transient profile shown in the Under-Voltage Transient Profile
is designed to comply (with appropriate margins) with all initial-
engagement surges, starting or cranking voltage transients, and
under-voltage surges specified in:
•
•
•
•
MIL-STD-704-8 (A through F)
RTCA/DO-160E
MIL-STD-1275B
DEF-STAN 61-5 (Part 6)/5 (operational portions)
Any input voltage transient that fits within the Under-Voltage
Transient Profile can be repeated after a delay that is at least four
times longer than the duration of the previous transient.
During the time when the boost-converter stage is operational, the
converter’s efficiency is reduced and the input ripple current is
increased. The lower the input voltage, the more these parameters
are affected.
Product # MQFL-28V-06S
www.synqor.com
Doc.# 005-0005182 Rev. 1
04/16/09
Page 10
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