UEI15 データシートの表示（PDF） - Murata Manufacturing

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UEI15

Isolated Wide Input Range 15-Watt DC/DC Converters

Murata Manufacturing 

UEI15 Series

Isolated Wide Input Range 15-Watt DC/DC Converters

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 com-

monly 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 primary side voltages will also

drop, thereby shutting down the PWM controller. Following a time-out period,

the PWM will restart, causing the output voltage to begin ramping up to its ap-

propriate value. If the short-circuit condition persists, another shutdown cycle

will initiate. This on/off cycling is called “hiccup mode”. The hiccup cycling

reduces the average output current, thereby preventing excessive internal

temperatures. A short circuit can be tolerated indefinitely.

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 equa-

tions and circuit diagrams that follow, trim adjustments use either a trimpot or

a single fixed resistor connected between the Trim input and either the positive

or negative output terminals. (On some converters, an external user-supplied

precision DC voltage may also be used for trimming). Trimming resistors should

have a low temperature coefficient (±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 specified

output voltage accuracy.

There are two CAUTIONs to be aware for the Trim input:

CAUTION: To avoid unplanned power down cycles, do not exceed EITHER the

maximum output voltage OR the maximum output power when setting the trim.

Be particularly careful with a trimpot. If the output voltage is excessive, the

OVP circuit may inadvertantly shut down the converter. If the maximum power

is exceeded, the converter may enter current limiting. If the power is exceeded

for an extended period, the converter may overheat and encounter overtem-

perature shut down.

CAUTION: Be careful of external electrical noise. The Trim input is a senstive

input to the converter’s feedback control loop. Excessive electrical noise may

cause instability or oscillation. Keep external connections short to the Trim

input. Use shielding if needed.

−INPUT

+OUTPUT

ON/OFF

TRIM

CONTROL

7 5-22

TURNS

LOAD

+INPUT

−OUTPUT

Figure 4 – Trim adjustments using a trimpot

−INPUT

+OUTPUT

ON/OFF

CONTROL

TRIM

RTRIM DOWN

LOAD

+INPUT

−OUTPUT

Figure 5 – Trim adjustments to decrease Output Voltage using a Fixed Resistor

−INPUT

+OUTPUT

ON/OFF

CONTROL

TRIM

R TRIM UP

LOAD

+INPUT

−OUTPUT

Figure 6 – Trim adjustments to increase Output Voltage using a Fixed Resistor

www.murata-ps.com

Technical enquiries email: sales@murata-ps.com, tel: +1 508 339 3000

MDC_UEI15W.B21 Page 8 of 13

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