UHP28_2_12A Datasheet, PDF (5/9 Pages) Murata Manufacturing Co., Ltd. – Isolated, 12 Amp Half Brick, 48VIN/28.2VOUT DC/DC Converters

UHP-28.2/12-D48
Isolated, 12 Amp Half Brick, 48VIN/28.2VOUT DC/DC Converters
I/O Filtering, Input Ripple Current, and Output Noise
All models in the UHP Series are tested/specified for input reflected ripple cur-
rent and output noise using the specified external input/output components/
circuits and layout as shown in the following two figures.
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 L simulate a typical dc
voltage bus. Your specific system configuration may necessitate additional
considerations.
TO
OSCILLOSCOPE
+
VIN
–
LBUS
CBUS
CURRENT
PROBE ₄
CIN
1
CIN = 33μF, ESR < 700m
@ 100kHz
CBUS = 220μF, ESR < 100m
@ 100kHz
LBUS = 12μH
+INPUT
–INPUT
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
filtering 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.
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. Our Applications Engineers can recommend potential solutions and
discuss the possibility of our modifying a given device's internal filtering to
meet your specific requirements. Contact our Applications Engineering Group
for additional details.
In Figure 3, the two copper strips simulate real-world PCB impedances
between the power supply and its load. In order to minimize measurement
errors, scope measurements should be made using BNC connectors, or
the probe ground should be as short as possible (i.e. less than ½ 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. Designers will normally use the –Output (pin 9) as the
ground/return of the load circuit. You can however, use the +Output (pin 5) as
ground/return to effectively reverse the output polarity.
+SENSE ⁶
+OUTPUT ⁵
COPPER STRIP
C1
C2
9
–OUTPUT
8
–SENSE
COPPER STRIP
SCOPE
RLOAD
C1 = 0.1μF CERAMIC
C2 = 10μF TANTALUM
LOAD 2-3 INCHES (51-76mm) FROM MODULE
Figure 3. Measuring Output Ripple/Noise (PARD)
Minimum Output Loading Requirements
UHP converters employ a synchronous-rectifier design topology and all
models regulate within spec and are stable under no-load to full load condi-
tions. Operation under no-load conditions however might slightly increase the
output ripple and noise.
Thermal Shutdown
The UHP converters are equipped with thermal-shutdown circuitry. If envi-
ronmental conditions cause the temperature of the DC/DC converter to rise
above the designed operating temperature, a precision temperature sensor
will power down the unit. When the internal temperature decreases below the
threshold of the temperature sensor, the unit will self start. See Performance/
Functional Specifications.
Output Overvoltage Protection
The UHP output voltage is monitored for an overvoltage condition using a
comparator. The signal is optically coupled to the primary side and if the
output voltage rises to a level which could be damaging to the load, the sens-
ing circuitry will power down the PWM controller causing the output voltage to
decrease. Following a time-out period the PWM will restart, causing the output
voltage to ramp to its appropriate value. If the fault condition persists, and the
output voltage again climbs to excessive levels, the overvoltage circuitry will
initiate another shutdown cycle. This on/off cycling is referred to as "hiccup"
mode.
Current Limiting
As soon as the output current increases to approximately 125% of its rated
value, the DC/DC converter will go into a current-limiting mode. In this condi-
tion, the output voltage will decrease proportionately with increases in output
current, thereby maintaining somewhat constant power dissipation. This is
commonly referred to as power limiting. Current limit inception is defined as
the point at which the full-power output voltage falls below the specified toler-
ance. See Performance/Functional Specifications. If the load current, being
drawn from the converter, is significant enough, the unit will go into a short
circuit condition as described below.
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