Datasheet

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UEI30 Series
30W Isolated Wide-Range DC-DC Converters
MDC_UEI Series 30W.B02 Page 13 of 18
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 Specifi cations). 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 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 operat-
ing 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 Specifi 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 specifi ed regulation band. Actual measured
times will vary with input source impedance, external input capacitance, input volt-
age slew rate and fi 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
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 specifi ed accuracy
band. The specifi cation assumes that the output is fully loaded at maximum
rated current.
Input Source Impedance
These converters will operate to specifi cations without external components,
assuming that the source voltage has very low impedance and reason-
able input voltage regulation. Since real-world voltage sources have fi nite
impedance, performance is improved by adding external fi lter components.
TECHNICAL NOTES
Sometimes only a small ceramic capacitor is suffi cient. Since it is diffi cult 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 specifi 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 specifi ed for input refl ected
ripple current and output noise using designated external input/output compo-
nents, circuits and layout as shown in the fi gures below. External input capaci-
tors (C
IN in the fi 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 fi gure below,
the C
BUS and LBUS components simulate a typical DC voltage bus. Your specifi c
system confi guration may require additional considerations. Please note that the
values of C
IN, LBUS and CBUS will vary according to the specifi c converter model.
C IN
V IN
C BUS
L BUS
C IN = 33μF, ESR < 700mΩ @ 100kHz
C
BUS = 220μF, ESR < 100mΩ @ 100kHz
L
BUS = 12μH
1
2
+VIN
−VIN
CURRENT
PROBE
TO
OSCILLOSCOPE
+
–
+
–
Figure 2. Measuring Input Ripple Current
Figure 3. Measuring Output Ripple and Noise (PARD)
C1
C1 = 1μF
C2 = 10μF LOW ES
LOAD 2-3 INCHES (51-76mm) FROM MODULE
C2
R
LOAD
SCOPE
+VOUT
−VOUT