MQFL-270-28S Datasheet, PDF (12/16 Pages) SynQor Worldwide Headquarters – HIGH RELIABILITY DC-DC CONVERTER

Technical Specification
MQFL-270-28S
Output: 28.0 V
Current: 4 A
INPUT OVER-VOLTAGE SHUTDOWN: The MQFL converter
also has an over-voltage feature that ensures the converter will be
off if the input voltage is too high. It also has a hysteresis and
time delay to ensure proper operation.
CURRENT LIMIT: The converter will reduce its output voltage
in response to an overload condition, as shown in Figure 6. If
the output voltage drops to below approximately 50% of the
nominal setpoint for longer than 1 second, the auto-restart feature
will engage. The auto-restart feature will stop the converter from
delivering load current, in order to protect the converter and the
load from thermal damage. After four seconds have elapsed, the
converter will automatically restart.
In a system with multiple converters configured for load sharing
using the SHARE pin, if the auto-restart feature engages, the con-
verters will synchronize their restart using signals communicated
on the SHARE pin.
BACK-DRIVE CURRENT LIMIT: Converters that use MOSFETs as
synchronous rectifiers are capable of drawing a negative current
from the load if the load is a source of short- or long-term energy.
This negative current is referred to as a “back-drive current”.
Conditions where back-drive current might occur include paral-
leled converters that do not employ current sharing, or where the
current share feature does not adequately ensure sharing during
the startup or shutdown transitions. It can also occur when con-
verters having different output voltages are connected together
through either explicit or parasitic diodes that, while normally
off, become conductive during startup or shutdown. Finally, some
loads, such as motors, can return energy to their power rail. Even
a load capacitor is a source of back-drive energy for some period
of time during a shutdown transient.
To avoid any problems that might arise due to back-drive current,
the MQFL converters limit the negative current that the converter
can draw from its output terminals. The threshold for this back-
drive current limit is placed sufficiently below zero so that the con-
verter may operate properly down to zero load, but its absolute
value (see the Electrical Characteristics page) is small compared
to the converter’s rated output current.
THERMAL CONSIDERATIONS: Figure 5 shows the suggested
Power Derating Curves for this converter as a function of the case
temperature and the maximum desired power MOSFET junction
temperature. All other components within the converter are
cooler than its hottest MOSFET, which at full power is no more
than 20ºC higher than the case temperature directly below this
MOSFET. The Mil-HDBK-1547A component derating guideline
calls for a maximum component temperature of 105ºC. Figure
5 therefore has one power derating curve that ensures this limit
is maintained. It has been SynQor’s extensive experience that
reliable long-term converter operation can be achieved with a
maximum component temperature of 125ºC. In extreme cases,
a maximum temperature of 145ºC is permissible, but not recom-
mended for long-term operation where high reliability is required.
Derating curves for these higher temperature limits are also
included in Figure 5. The maximum case temperature at which
the converter should be operated is 135ºC.
When the converter is mounted on a metal plate, the plate will
help to make the converter’s case bottom a uniform temperature.
How well it does so depends on the thickness of the plate and
on the thermal conductance of the interface layer (e.g. thermal
grease, thermal pad, etc.) between the case and the plate. Unless
this is done very well, it is important not to mistake the plate’s
temperature for the maximum case temperature. It is easy for
them to be as much as 5-10ºC different at full power and at high
temperatures. It is suggested that a thermocouple be attached
directly to the converter’s case through a small hole in the plate
when investigating how hot the converter is getting. Care must
also be made to ensure that there is not a large thermal resistance
between the thermocouple and the case due to whatever adhe-
sive might be used to hold the thermocouple in place.
INPUT SYSTEM INSTABILITY: This condition can occur
because any DC-DC converter appears incrementally as a
negative resistance load. A detailed application note titled
“Input System Instability” is available on the SynQor website
which provides an understanding of why this instability arises,
and shows the preferred solution for correcting it.
Product # MQFL-270-28S
Phone [phone]
www.synqor.com
Doc.# 005-MQ2728S Rev. B 09/16/08
Page 12