UEI15 Series

Isolated Wide Input Range 15-Watt DC/DC Converters

www.murata-ps.com

MDC_UEI15W.B24 Page 6 of 13

APPLICATION NOTES

Input Fusing

Certain applications and/or safety agencies may require fuses at the inputs of

power conversion components. Fuses should also be used when there is the

possibility of sustained input voltage reversal which is not current-limited. We

recommend a time delay fuse installed in the ungrounded input supply line

with a value which is approximately twice the maximum line current, calcu-

lated at the lowest input voltage.

The installer must observe all relevant safety standards and regulations. For

safety agency approvals, install the converter in compliance with the end-user

safety standard, i.e. IEC/EN/UL 60950-1.

Input Reverse-Polarity Protection

If the input voltage polarity is reversed, an internal diode will become forward

biased and likely draw excessive current from the power source. If this source

is not current-limited or the circuit appropriately fused, it could cause perma-

nent damage to the converter.

Input Under-Voltage Shutdown and Start-Up Threshold

Under normal start-up conditions, converters will not begin to regulate properly

until the ramping-up input voltage exceeds and remains at the Start-Up

Threshold Voltage (see Specifications). Once operating, converters will not

turn off until the input voltage drops below the Under-Voltage Shutdown Limit.

Subsequent restart will not occur until the input voltage rises again above the

Start-Up Threshold. This built-in hysteresis prevents any unstable on/off opera-

tion at a single input voltage.

Users should be aware however of input sources near the Under-Voltage

Shutdown whose voltage decays as input current is consumed (such as capaci-

tor inputs), the converter shuts off and then restarts as the external capacitor

recharges. Such situations could oscillate. To prevent this, make sure the oper-

ating input voltage is well above the UV Shutdown voltage AT ALL TIMES.

Start-Up Time

Start-Up Time (see Specifications) is the time interval between the point when

the ramping input voltage crosses the Start-Up Threshold and the fully loaded

regulated output voltage enters and remains within its specified accuracy band.

Actual measured times will vary with input source impedance, external input

capacitance, input voltage slew rate and final value of the input voltage as it

appears at the converter.

These converters include a soft start circuit to moderate the duty cycle of its

PWM controller at power up, thereby limiting the input inrush current.

assumes that the converter already has its input voltage stabilized above the

Start-Up Threshold before the On command. The interval is measured from the

On command until the output enters and remains within its specified accuracy

band. The specification assumes that the output is fully loaded at maximum

such as external load capacitance and soft start circuitry.

Input Source Impedance

These converters will operate to specifications without external components,

assuming that the source voltage has very low impedance and reasonable in-

put voltage regulation. Since real-world voltage sources have finite impedance,

performance is improved by adding external filter components. Sometimes only

a small ceramic capacitor is sufficient. Since it is difficult to totally characterize

all applications, some experimentation may be needed. Note that external input

capacitors must accept high speed switching currents.

Because of the switching nature of DC/DC converters, the input of these

converters must be driven from a source with both low AC impedance and

adequate DC input regulation. Performance will degrade with increasing input

inductance. Excessive input inductance may inhibit operation. The DC input

regulation specifies that the input voltage, once operating, must never degrade

below the Shut-Down Threshold under all load conditions. Be sure to use

adequate trace sizes and mount components close to the converter.

I/O Filtering, Input Ripple Current and Output Noise

All models in this converter series are tested and specified for input reflected

ripple current and output noise using designated external input/output compo-

nents, circuits and layout as shown in the figures below. External input capaci-

tors (Cin in the figure) serve primarily as energy storage elements, minimizing

line voltage variations caused by transient IR drops in the input conductors.

Users should select input capacitors for bulk capacitance (at appropriate

frequencies), low ESR and high RMS ripple current ratings. In the figure below,

the Cbus and Lbus components simulate a typical DC voltage bus. Your specific

system configuration may require additional considerations. Please note that the

values of Cin, Lbus and Cbus will vary according to the specific converter model.

In critical applications, output ripple and noise (also referred to as periodic and

random deviations or PARD) may be reduced by adding filter elements such as

multiple external capacitors. Be sure to calculate component temperature rise from

reflected AC current dissipated inside capacitor ESR.

+INPUT

-INPUT

CURRENT

PROBE

TO

OSCILLOSCOPE

+

–

+

–

Figure 2. Measuring Input Ripple Current