
Product # MQHL-28-2R5S
Phone 1-888-567-9596
www.synqor.com
Doc.# 005-0005898 Rev. 1
05/19/11
Page 8
Output:
Current:
2.5V
20A
MQHL-28-2R5S
Application Section
CURRENT LIMIT: In the event of excess load, the MQHL
converter will quickly reduce its output voltage to keep
the load current within safe limits (see Figure 6). If the
overload persists for more than 14 milliseconds, the converter
will shut off, wait a restart delay, and then automatically
attempt to re-start. The timeout is internally implemented
with an integrator: counting up whenever current limit is
active, and counting down at 1/5th the rate whenever current
limit becomes inactive. In this way a series of short-duration
overloads will not cause the converter to shut down, while it
will shut down in response to sustained overloads.
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 5-10C 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 adhesive 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.
THERMAL CONSIDERTAIONS: 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 20C
higher than the case temperature directly below this MOSFET.
The Mil-HDBK-1547A component derating guideline calls
for a maximum component temperature of 105C. 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 125C. In extreme
cases, a maximum temperature of 145C 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 135C.
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