UWS Series
Sixteenth-brick DOSA-Compatible,
Wide Input Isolated DC-DC Converters
TECHNICAL NOTES
Input Fusing
I/O Filtering, Input Ripple Current, and Output Noise
Certain applications and/or safety agencies may require the installation of
fuses at the inputs of power conversion components. Fuses should also be
used if the possibility of sustained, non-current-limited, input-voltage polarity
reversals exists. For Murata Power Solutions UWS series DC-DC converters, we
recommend the use of a fast blow fuse, installed in the ungrounded input sup-
ply line with a typical value about twice the maximum input current, calculated
at low line with the converter’s minimum efficiency.
All models in the UWS Series are tested/specified for input reflected ripple
current and output noise using the specified external input/output components/
circuits and layout as shown in the following two figures. External input capaci-
tors (CIN in Figure 2) serve primarily as energy-storage elements, minimiz-
ing line voltage variations caused by transient IR drops in conductors from
backplane to the DC-DC. Input caps should be selected for bulk capacitance
(at appropriate frequencies), low ESR, and high rms-ripple-current ratings. The
switching nature of DC-DC converters requires that dc voltage sources have
low ac impedance as highly inductive source impedance can affect system sta-
bility. In Figure 2, CBUS and LBUS simulate a typical dc voltage bus. Your specific
system configuration may necessitate additional considerations.
All relevant national and international safety standards and regulations must
be observed by the installer. For system safety agency approvals, the convert-
ers must be installed in compliance with the requirements of the end- use
safety standard, i.e. IEC/EN/UL60950-1.
TO
Input Reverse-Polarity Protection
CURRENT
PROBE
OSCILLOSCOPE
If the input voltage polarity is accidentally reversed, an internal diode will be-
come 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 permanent damage to the converter.
+VIN
–VIN
LBUS
+
V
IN
CBUS
CIN
–
Input Under-Voltage Shutdown and Start-Up Threshold
Under normal start-up conditions, devices will not begin to regulate properly
until the ramping-up input voltage exceeds the Start-Up Threshold Voltage.
Once operating, devices will not turn off until the input voltage drops below the
Under-Voltage Shutdown limit. Subsequent re-start will not occur until the input
is brought back up to the Start-Up Threshold. This built in hysteresis prevents
any unstable on/off situations from occurring at a single input voltage.
C
IN = 33μF, ESR < 700mΩ @ 100kHz
BUS = 220μF, ESR < 100mΩ @ 100kHz
C
L
BUS = 12μH
Figure 2. Measuring Input Ripple Current
In critical applications, output ripple/noise (also referred to as periodic and
random deviations or PARD) may be reduced below specified limits using filter-
ing techniques, the simplest of which is the installation of additional external
output capacitors. They function as true filter elements and should be selected
for bulk capacitance, low ESR and appropriate frequency response.
Start-Up Time
The VIN to VOUT Start-Up Time is the time interval between the point at which
the ramping input voltage crosses the Start-Up Threshold and the fully loaded
output voltage enters and remains within its specified accuracy band. Actual
measured times will vary with input source impedance, external input capaci-
tance, and the slew rate and final value of the input voltage as it appears at the
converter. The UWS Series implements a soft start circuit to limit the duty cycle
of its PWM controller at power up, thereby limiting the input inrush current.
All external capacitors should have appropriate voltage ratings and be
located as close to the converter as possible. Temperature variations for all
relevant parameters should also be taken carefully into consideration. The most
effective combination of external I/O capacitors will be a function of line voltage
and source impedance, as well as particular load and layout conditions.
The On/Off Control to VOUT start-up time assumes the converter has its
nominal input voltage applied but is turned off via the On/Off Control pin. The
specification defines the interval between the point at which the converter is
turned on (released) and the fully loaded output voltage enters and remains
within its specified accuracy band. Similar to the VIN to VOUT start-up, the On/Off
Control to VOUT start-up time is also governed by the internal soft start circuitry
and external load capacitance. The difference in start up time from VIN to VOUT
and from On/Off Control to VOUT is therefore insignificant.
+SENSE
+VOUT
R
LOAD
SCOPE
C1
C2
Input Source Impedance
–VOUT
The input of UWS converters must be driven from a low ac-impedance source.
The DC-DC’s performance and stability can be compromised by the use of
highly inductive source impedances. The input circuit shown in Figure 2 is a
practical solution that can be used to minimize the effects of inductance in the
input traces. For optimum performance, components should be mounted close
to the DC-DC converter.
–SENSE
C1 = 1μF
C2 = 10μF
LOAD 2-3 INCHES (51-76mm) FROM MODULE
Figure 3. Measuring Output Ripple/Noise (PARD)
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