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Note that the temperatures are of the ambient airfl ow, not the converter
itself which is obviously running at higher temperature than the outside air.
Also note that very low fl ow rates (below about 25 LFM) are similar to “natural
convection,” that is, not using fan-forced airfl ow.
Murata Power Solutions makes Characterization measurements in a closed
cycle wind tunnel with calibrated airfl ow. We use both thermocouples and an
infrared camera system to observe thermal performance. As a practical matter,
it is quite diffi cult to insert an anemometer to precisely measure airfl ow in
most applications. Sometimes it is possible to estimate the effective airfl ow if
you thoroughly understand the enclosure geometry, entry/exit orifi ce areas and
the fan fl owrate specifi cations.
CAUTION: If you exceed these Derating guidelines, the converter may have
an unplanned Over Temperature shut down. Also, these graphs are all collected
near Sea Level altitude. Be sure to reduce the derating for higher altitude.
Output Overvoltage Protection (OVP)
This converter monitors its output voltage for an over-voltage condition. If
the output exceeds OVP limits, the sensing circuit will power down the unit,
and the output voltage will decrease. After a time-out period, the PWM will
automatically attempt to restart, causing the output voltage to ramp up to its
rated value. It is not necessary to power down and reset the converter for the
automatic OVP-recovery restart.
If the fault condition persists and the output voltage climbs to excessive
levels, the OVP circuitry will initiate another shutdown cycle. This on/off cycling
is referred to as “hiccup” mode.
Output Fusing
The converter is extensively protected against current, voltage and temperature
extremes. However your application circuit may need additional protection. In
the extremely unlikely event of output circuit failure, excessive voltage could be
applied to your circuit. Consider using appropriate external protection.
Output Current Limiting
As soon as the output current increases to approximately 125% to 150% of
its maximum rated value, the DC-DC converter will enter a current-limiting
mode. The output voltage will decrease proportionally with increases in output
current, thereby maintaining a somewhat constant power output. This is also
commonly referred to as power limiting.
Current limiting inception is defi ned as the point at which full power falls
below the rated tolerance. See the Performance/Functional Specifi cations.
Note particularly that the output current may briefl y rise above its rated value
in normal operation as long as the average output power is not exceeded. This
enhances reliability and continued operation of your application. If the output
current is too high, the converter will enter the short circuit condition.
Output Short Circuit Condition
When a converter is in current-limit mode, the output voltage will drop as the
output current demand increases. If the output voltage drops too low (approxi-
mately 98% of nominal output voltage for most models), the magnetically
coupled voltage used to develop the PWM bias voltage will also drop, thereby
shutting down the PWM controller. Following a time-out period, the PWM will
restart, causing the output voltage to begin rising to its appropriate value.
If the short-circuit condition persists, another shutdown cycle will initiate. This
rapid on/off cycling is called “hiccup mode.” The hiccup cycling reduces the
average output current, thereby preventing excessive internal temperatures
and/or component damage.
The “hiccup” system differs from older latching short circuit systems
because you do not have to power down the converter to make it restart. The
system will automatically restore operation as soon as the short circuit condi-
tion is removed.
Remote Sense Input (Models UEE-3.3/30-D48 only)
Use the Sense inputs with caution. Sense is normally connected at the load.
Sense inputs compensate for output voltage inaccuracy delivered at the load.
This is done by correcting IR voltage drops along the output wiring and the
current carrying capacity of PC board etch. This output drop (the difference
between Sense and Vout when measured at the converter) should not exceed
0.5V. Consider using heavier wire if this drop is excessive. Sense inputs also
improve the stability of the converter and load system by optimizing the control
loop phase margin.
Note: The Sense input and power Vout lines are internally connected through
low value resistors to their respective polarities so that the converter can
operate without external connection to the Sense. Nevertheless, if the Sense
function is not used for remote regulation, the user should connect +Sense to
+Vout and –Sense to –Vout at the converter pins.
The remote Sense lines carry very little current. They are also capacitively
coupled to the output lines and therefore are in the feedback control loop to
regulate and stabilize the output. As such, they are not low impedance inputs
and must be treated with care in PC board layouts. Sense lines on the PCB
should run adjacent to DC signals, preferably Ground. In cables and discrete
wiring, use twisted pair, shielded tubing or similar techniques.
Any long, distributed wiring and/or signifi cant inductance introduced into the
Sense control loop can adversely affect overall system stability. If in doubt, test
your applications by observing the converter’s output transient response during
step loads. There should not be any appreciable ringing or oscillation. You
may also adjust the output trim slightly to compensate for voltage loss in any
external fi lter elements. Do not exceed maximum power ratings.
Figure 9. Remote Sense Circuit Confi guration
LOAD
Contact and PCB resistance
losses due to IR drops
Contact and PCB resistance
losses due to IR drops
+VOUT
+SENSE
TRIM
−
SENSE
-VOUT
+
VIN
ON/OFF
CONTROL
–VIN
Sense Current
IOUT
Sense Return
IOUT Return
UEE Series
Isolated, High-Density, Eighth-Brick
DOSA Low Profi le DC-DC Converters
MDC_UEE_Series.C01 Page 26 of 29