UEI25-120 Series

Single Output Isolated 25-Watt DC/DC Converters

www.murata-ps.com

email: sales@murata-ps.com

17 Aug 2010

MDC_UEI25-120.A02 Page 8 of 11

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. For

greatest safety, we recommend a fast blow fuse installed in the ungrounded

input supply line.

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.

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 rising 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 operation 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 capacitor

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

charges. Such situations could oscillate. To prevent this, make sure the operating

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

Start-Up Delay

Assuming that the output current is set at the rated maximum, the Vin to Vout Start-

Up Delay (see Specifications) is the time interval between the point when the rising

input voltage crosses the Start-Up Threshold and the fully loaded regulated output

voltage enters and remains within its specified regulation band. Actual measured

times will vary with input source impedance, external input capacitance, input volt-

age 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 the

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

sumes 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 regulation

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

rated current.

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

APPLICATION NOTES

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-

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,

system configuration may require additional considerations. Please note that the

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 tempera-

ture rise from reflected AC current dissipated inside capacitor ESR. In figure 3,

the two copper strips simulate real-world printed circuit impedances between

the power supply and its load. In order to minimize circuit errors and standard-

ize tests between units, scope measurements should be made using BNC

connectors or the probe ground should not exceed one half inch and soldered

directly to the fixture.

Floating Outputs

Since these are isolated DC/DC converters, their outputs are “floating” 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 Specifications).

These leakages consist of both an AC stray capacitance coupling component

1

2

+INPUT

−INPUT

CURRENT

PROBE

TO

OSCILLOSCOPE

+

–

+

–

Figure 2. Measuring Input Ripple Current