MQFL-28E-15S Datasheet, PDF (12/17 Pages) SynQor Worldwide Headquarters – HIGH RELIABILITY DC-DC CONVERTER

Technical Specification
MQFL-28E-15S
Output: ^15V
Current: ^8A
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, input voltage and the maximum desired power
MOSFET junction temperature. All other components within the
converter are cooler than the hottest 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 main-
tained. 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 recommended 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-28E-15S
Phone [phone]
www.synqor.com
Doc.# 005-0005105 Rev. 2
04/30/09
Page 12