Datasheet

UWQ-12/20-T48 Series
Wide Input, Isolated DOSA Quarter Brick DC-DC Converters
MDC_UWQ-12/20-T48 Series.A02 Page 25 of 29
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Schottky power diodes with approximately 0.3V drops or “OR”ing MOSFETs
may be suitable in the loop whereas 0.7 V silicon power diodes may not be
advisable. In the event of an internal device fault or failure of the mains power
modules on the primary side, the other devices automatically take over the
entire supply of the loads. In the basic N+1 power system, the “N” equals the
number of modules required to fully power the system and “+1” equals one
back-up module that will take over for a failed module. If the system consists
of two power modules, each providing 50% of the total load power under
normal operation and one module fails, another one delivers full power to the
load. This means you can use smaller and less expensive power converters as
the redundant elements, while achieving the goal of increased availability.
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 Shut-
down whose voltage decays as input current is consumed (such as capacitor
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
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 com-
mand until the output enters and remains within its specifi ed regulation 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 reasonable in-
put voltage regulation. Since real-world voltage sources have fi nite impedance,
performance is improved by adding external fi lter components. 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. Us-
ers should select input capacitors for bulk capacitance (at appropriate frequen-
cies), low ESR and high RMS ripple current ratings. In the fi gure below, the C
BUS
and L
BUS 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 may vary according to the specifi c converter model.
In critical applications, output ripple and noise (also referred to as periodic and
random deviations or PARD) may be reduced by adding fi lter elements such as
multiple external capacitors. Be sure to calculate component temperature rise
from refl ected AC current dissipated inside capacitor ESR.
Figure 6. Measuring Output Ripple and Noise (PARD)
C1
C1 = 1µF
C2 = 10µF
LOAD 2-3 INCHES (51-76mm) FROM MODULE
C2
R
LOAD
SCOPE
+VOUT
−VOUT
C IN
V IN
C BUS
L BUS
C IN = 33µF, ESR < 200mΩ @ 100kHz
C
BUS = 220µF, 100V
L
BUS = 12µH
+VIN
−VIN
CURRENT
PROBE
TO
OSCILLOSCOPE
+
–
+
–
Figure 5. Measuring Input Ripple Current