UEI25 查看數據表（PDF） - Murata Manufacturing

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UEI25

Single Output Isolated 25-Watt DC/DC Converters

Murata Manufacturing 

UEI25 Series

Single Output Isolated 25-Watt DC/DC Converters

+OUTPUT

COPPER STRIP

C1

C2

SCOPE

RLOAD

−OUTPUT

COPPER STRIP

C1 = 1μF CERAMIC

C2 = 10μF LOW ES

LOAD 2-3 INCHES (51-76mm) FROM MODULE

Figure 3. Measuring Output Ripple and Noise (PARD)

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 rectifier design topology. All models

regulate within specification and are stable from 0% load to full load conditions,

unless otherwise specified. Operation under no load will not damage the con-

verter but might, however, slightly increase regulation, 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 the performance data section illustrate typical operation under a

variety of conditions. 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 tem-

perature and/or current or reduced airflow as long as the average is not exceeded.

Note that the temperatures are of the ambient airflow, not the converter it-

self 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

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 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

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.

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 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 defined as the point at which full power falls

below the rated tolerance. See the Performance/Functional Specifications.

Note particularly that the output current may briefly 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 Specifications). In the trim equations

and circuit diagrams that follow, trim adjustments use a single fixed resistor

connected between the Trim input and either Vout pin. Trimming resistors should

have a low temperature coefficient (±100 ppm/°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 specified output voltage

accuracy.

There are two CAUTIONs to observe for the Trim input:

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MDC_UEI25W.B03 Page 20 of 23

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