TECHNICAL NOTES
Input Fusing
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 reversal
exists. For MPS BWR 15-17 Watt DC/DC Converters, you should use slow-blow
type fuses with values no greater than the following:
Model Fuse Value
All D12A Models 4 Amp
BWR-5/1500-D24A 2 Amp
BWR-12/725-D24A, BWR-15/575-D24A 2.5 Amp
All D48A Models 1 Amp
Start-Up Time
The VIN to VOUT start-up time is the interval of time where the input voltage
crosses the turn-on threshold point, and the fully loaded output voltage enters
and remains within its specified accuracy band. Actual measured times will vary
with external output capacitance and load. The BWR 15-17W Series implements
a soft start circuit that limits the duty cycle of the PWM controller at power up,
thereby limiting the Input Inrush current.
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 time at which the converter is turned on 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.
Input Overvoltage/Undervoltage Shutdown and Start-Up Threshold
Under normal start-up conditions, devices will not begin to regulate until the ramping-up
input voltage exceeds the Start-Up Threshold Voltage (35V for D48 models). Once
operating, devices will not turn off until the input voltage drops below the Undervoltage
Shutdown limit (33.5V for D48 models). 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.
Input voltages exceeding the input overvoltage shutdown specification listed in
the Performance/Functional Specifications will cause the device to shutdown. A
built-in hysteresis of 0.6 to 1.6 Volts for all models will not allow the converter to
restart until the input voltage is sufficiently reduced.
Input Source Impedance
The converters must be driven from a low ac-impedance input source. The DC/
DC's performance and stability can be compromised by the use of highly induc-
tive source impedances. The input circuit shown in Figure 2 is a practical solu-
tion 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. If the application has a high source impedance, low VIN models can
benefit of increased external input capacitance.
I/O Filtering, Input Ripple Current, and Noise Reduction
All BWR 15-17W DC/DC Converters achieve their rated ripple and noise
specifications without the use of external input/output capacitors. In critical
applications, input/output ripple and noise may be further reduced by installing
additional external I/O caps.
Figure 2. Measuring Input Ripple Current
C
IN
V
IN
C
BUS
L
BUS
C
IN
= 33µF, ESR < 700mΩ @ 100kHz
C
BUS
= 220µF, ESR < 100mΩ @ 100kHz
L
BUS
= 12µH
+INPUT
–INPUT
CURRENT
PROBE
TO
OSCILLOSCOPE
+
–
External input capacitors (CIN in Figure 2) serve primarily as energy-storage
elements, minimizing 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 imped-
ance can affect system stability. In Figure 2, CBUS and LBUS simulate a typical
dc voltage bus. Your specific system configuration may necessitate additional
considerations.
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. These output caps function as true filter elements and
should be selected for bulk capacitance, low ESR and appropriate frequency
response. 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.
Floating Outputs
Since these are isolated DC/DC converters, their outputs are "floating," with
respect to the input. As such, it is possible to use +Output, –Output or Output
Return as the system ground thereby allowing the flexibility to generate a
variety of output voltage combinations.
Regulation for BWR 15-17W bipolar converters is monitored between
–Output and +Output (as opposed to Output to Return).
Minimum Loading Requirements
BWR 15-17W converters employ a classical diode-rectification design topology
and require a minimum 10% loading to achieve their listed regulation specifica-
tions and a stable operating condition.
Load Regulation
Regulation for the BWR 15-17W bipolar converters is monitored between
–Output and +Output (as opposed to Output to Return). As such regulation will
assure that voltage between –Output and +Output pins remains within the VOUT
accuracy listed in the Performance/Functional Specifications table.
BWR Series
15-17W, Dual Output DC/DC Converters
MDC_BWR15-17W.D01 Page 4 of 9
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