UWQ-12/17-Q48 Series
Wide Input, Isolated DOSA Quarter Brick DC/DC Converters
most applications. Sometimes it is possible to estimate the effective airflow if
you thoroughly understand the enclosure geometry, entry/exit orifice areas and
the fan flowrate specifications.
COPPER STRIP
+OUTPUT
−OUTPUT
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.
RLOAD
SCOPE
C1
C2
Output Overvoltage Protection (OVP)
COPPER STRIP
This converter monitors its output voltage for an over-voltage condition using
an on-board electronic comparator. The signal is optically coupled to the pri-
mary side PWM controller. 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 volt-
age to ramp up to its rated value. It is not necessary to power down and reset
the converter for this automatic OVP-recovery restart.
C1 = 1μF CERAMIC
C2 = 10μF LOW ESR
LOAD 2-3 INCHES (51-76mm) FROM MODULE
Figure 3. Measuring Output Ripple and Noise (PARD)
be damaged. Designers will normally use the negative output (-Output) as
the ground return of the load circuit. You can however use the positive output
(+Output) as the ground return to effectively reverse the output polarity.
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.
Minimum Output Loading Requirements
Output Fusing
These converters employ a synchronous rectifier design topology. All models
regulate within specification and are stable under no load to full load conditions.
Operation under no load might however slightly increase output ripple and noise.
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 an appropriate external protection.
Thermal Shutdown
To protect against thermal over-stress, these converters include thermal shut-
down circuitry. If environmental conditions cause the temperature of the DC/
DC’s to rise above the Operating Temperature Range up to the shutdown tem-
perature, an on-board electronic temperature sensor will power down the unit.
When the temperature decreases below the turn-on threshold, the converter
will automatically restart. There is a small amount of hysteresis to prevent
rapid on/off cycling. CAUTION: If you operate too close to the thermal limits, the
converter may shut down suddenly without warning. Be sure to thoroughly test
your application to avoid unplanned thermal shutdown.
Current Limiting (Power limit with current mode control)
As power demand increases on the output and enters the specified “limit
inception range” (current in voltage mode and power in current mode) limiting
circuitry activates in the DC-DC converter to limit/restrict the maximum current
or total power available. In voltage mode, current limit can have a “constant or
foldback” characteristic. In current mode, once the current reaches a certain
range the output voltage will start to decrease while the output current con-
tinues to increase, thereby maintaining constant power, until a maximum peak
current is reached and the converter enters a “hiccup” (on off cycling) mode of
operation until the load is reduced below the threshold level, whereupon it will
return to a normal mode of operation. Current limit inception is defined as the
point where the output voltage has decreased by a pre-specified percentage
(usually a 2ꢀ decrease from nominal).
Temperature Derating Curves
The graphs in this data sheet illustrate typical operation under a variety of condi-
tions. The Derating curves show the maximum continuous ambient air temperature
and decreasing maximum output current which is acceptable under increasing
forced airflow measured in Linear Feet per Minute (“LFM”). Note that these are
AVERAGE measurements. The converter will accept brief increases in temperature
and/or current or reduced airflow as long as the average is not exceeded.
Short Circuit Condition (Current mode control)
The short circuit condition is an extension of the “Current Limiting” condition.
When the monitored peak current signal reaches a certain range, the PWM
controller’s outputs are shut off thereby turning the converter “off.” This is
followed by an extended time out period. This period can vary depending on
other conditions such as the input voltage level. Following this time out period,
the PWM controller will attempt to re-start the converter by initiating a “normal
start cycle” which includes softstart. If the “fault condition” persists, another
“hiccup” cycle is initiated. This “cycle” can and will continue indefinitely until
such time as the “fault condition” is removed, at which time the converter will
resume “normal operation.” Operating in the “hiccup” mode during a fault
condition is advantageous in that average input and output power levels are
held low preventing excessive internal increases in temperature.
Note that the temperatures are of the ambient airflow, not the converter itself
which is obviously running at higher temperature than the outside air. Also note
that “natural convection” is defined as very low flow rates which are not using
fan-forced airflow. Depending on the application, “natural convection” is usu-
ally about 30-65 LFM but is not equal to still air (0 LFM).
Murata Power Solutions makes Characterization measurements in a closed
cycle wind tunnel with calibrated airflow. We use both thermocouples and an
infrared camera system to observe thermal performance. As a practical matter,
it is quite difficult to insert an anemometer to precisely measure airflow in
www.murata-ps.com/support
MDC_UWQ-12/17-Q48 Series.A01.D7 Page 14 of 17
