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UEI30 Series
30W Isolated Wide-Range DC-DC Converters
MDC_UEI Series 30W.B02 Page 14 of 18
In critical applications, output ripple and noise (also referred to as periodic
and random deviations or PARD) may be reduced by adding fi lter elements
such as multiple external capacitors. Be sure to calculate component tempera-
ture rise from refl ected AC current dissipated inside capacitor ESR.
Floating Outputs
Since these are isolated DC/DC converters, their outputs are “fl oating” with
respect to their input. The essential feature of such isolation is ideal ZERO
CURRENT FLOW between input and output. Real-world converters however do
exhibit tiny leakage currents between input and output (see Specifi cations).
These leakages consist of both an AC stray capacitance coupling component
and a DC leakage resistance. When using the isolation feature, do not allow
the isolation voltage to exceed specifi cations. Otherwise the converter may
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.
Minimum Output Loading Requirements
These converters employ a synchronous rectifi er design topology. All models
regulate within specifi cation and are stable under no load to full load conditions.
Operation under no load might however slightly increase output ripple and noise.
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.
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 airfl ow 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 airfl ow as long as the average is not exceeded.
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 “natural convection” is defi ned as very fl ow rates which are not
using fan-forced airfl ow. Depending on the application, “natural convection” is
usually 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 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 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 the this 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 an appropriate external protection.
Output Current Limiting
As soon as the output current increases to approximately its overcurrent limit,
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 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. 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, the
magnetically coupled voltage used to develop 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 initi-
ate. This on/off cycling is called “hiccup mode.” The hiccup cycling reduces the
average output current, thereby preventing excessive internal temperatures.
Trimming the Output Voltage
The Trim input to the converter allows the user to adjust the output voltage over
the rated trim range (please refer to the Specifi cations). In the trim equations and
circuit diagrams that follow, trim adjustments use either a trimpot or a single
fi xed resistor connected between the Trim input and either the +Vout or –Vout
terminals. (On some converters, an external user-supplied precision DC voltage
may also be used for trimming). Trimming resistors should have a low tempera-
ture coeffi cient (±100 ppm/deg.C or less) and be mounted close to the converter.
Keep leads short. If the trim function is not used, leave the trim unconnected.
With no trim, the converter will exhibit its specifi ed output voltage accuracy.
There are two CAUTIONs to observe for the Trim input:
