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UEI25 Series
Single Output Isolated 25-Watt DC/DC Converters
TECHNICAL 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. 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 Speci?cations). Once operating, converters will not turn off until
the input voltage drops below the Under-Voltage Shutdown Limit. Subsequent
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 speci?es 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 speci?ed for input re?ected
ripple current and output noise using designated external input/output compo-
nents, circuits and layout as shown in the ?gures below. External input capaci-
tors (C IN in the ?gure) 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 ?gure below,
the C BUS and L BUS components simulate a typical DC voltage bus. Your speci?c
system con?guration may require additional considerations. Please note that the
values of C IN , L BUS and C BUS may vary according to the speci?c converter model.
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.
TO
OSCILLOSCOPE
CURRENT
PROBE
1
+VIN
Users should be aware however of input sources near the Under-Voltage
Shutdown whose voltage decays as input current is consumed (such as capacitor
V IN
+
–
+
C BUS
L BUS
C IN
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
2
?VIN
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 Speci?cations) 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 speci?ed regulation band. Actual measured
times will vary with input source impedance, external input capacitance, input volt-
age slew rate and ?nal 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.
The On/Off Remote Control interval from inception to V OUT regulated as-
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 speci?ed regulation
band. The speci?cation assumes that the output is fully loaded at maximum
rated current.
Input Source Impedance
These converters will operate to speci?cations without external components,
assuming that the source voltage has very low impedance and reasonable in-
C IN = 33μF, ESR < 700mΩ @ 100kHz
C BUS = 220μF, ESR < 100mΩ @ 100kHz
L BUS = 12μH
Figure 2. Measuring Input Ripple Current
In critical applications, output ripple and noise (also referred to as periodic
and random deviations or PARD) may be reduced by adding ?lter elements
such as multiple external capacitors. Be sure to calculate component tempera-
ture rise from re?ected AC current dissipated inside capacitor ESR.
Floating Outputs
Since these are isolated DC/DC converters, their outputs are “?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 Speci?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 speci?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.
put voltage regulation. Since real-world voltage sources have ?nite impedance,
performance is improved by adding external ?lter components. Sometimes only
a small ceramic capacitor is suf?cient. Since it is dif?cult to totally characterize
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