Datasheet

RBE-12/20-D48 Series
Eighth-Brick 240-Watt Isolated DC/DC Converters
MDC_RBE-12-20-D48.B05Δ Page 12 of 16
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Thermal Shutdown
Extended operation at excessive temperature will initiate overtemperature
shutdown triggered by a temperature sensor inside the PWM controller. This
operates similarly to overcurrent and short circuit mode. The inception point
of the overtemperature condition depends on the average power delivered,
the ambient temperature and the extent of forced cooling airfl ow. Thermal
shutdown uses only the hiccup mode (autorestart).
Start Up Considerations
When power is fi rst applied to the DC/DC converter, there is some risk of start up
diffi culties if you do not have both low AC and DC impedance and adequate regula-
tion of the input source. Make sure that your source supply does not allow the
instantaneous input voltage to go below the minimum voltage at all times.
Use a moderate size capacitor very close to the input terminals. You may
need two or more parallel capacitors. A larger electrolytic or ceramic cap sup-
plies the surge current and a smaller parallel low-ESR ceramic cap gives low
AC impedance.
Remember that the input current is carried both by the wiring and the
ground plane return. Make sure the ground plane uses adequate thickness
copper. Run additional bus wire if necessary.
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.
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.
Start-Up Time
Assuming that the output current is set at the rated maximum, the Vin to Vout
Start-Up Time (see Specifi cations) is the time interval between the point when
the rising input voltage crosses the Start-Up Threshold and the fully loaded
output voltage enters and remains within its specifi ed accuracy band. Actual
measured times will vary with input source impedance, external input capaci-
tance, input voltage 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 its
PWM controller at power up, thereby limiting the input inrush current.
The On/Off Remote Control interval from On command to Vout (fi nal ±5%)
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
TECHNICAL NOTES
band. The specifi cation assumes that the output is fully loaded at maximum
rated current. Similar conditions apply to the On to Vout regulated specifi cation
such as external load capacitance and soft start circuitry.
Recommended Input Filtering
The user must assure that the input source has low AC impedance to provide
dynamic stability and that the input supply has little or no inductive content,
including long distributed wiring to a remote power supply. The converter will
operate with no additional external capacitance if these conditions are met.
For best performance, we recommend installing a low-ESR capacitor imme-
diately adjacent to the converter’s input terminals. The capacitor should be a
ceramic type such as the Murata GRM32 series or a polymer type. Make sure
that the input terminals do not go below the undervoltage shutdown voltage at
all times. More input bulk capacitance may be added in parallel if needed.
Recommended Output Filtering
The converter will achieve its rated output ripple and noise with no additional
external capacitor. However, the user may install more external output capacitance
to reduce the ripple even further or for improved dynamic response. Again, use
low-ESR ceramic (Murata GRM32 series) or polymer capacitors. Mount these
close to the converter. Measure the output ripple under your load conditions.
Use only as much capacitance as required to achieve your ripple and noise
objectives. Excessive capacitance can make step load recovery sluggish or
possibly introduce instability. Do not exceed the maximum rated output capaci-
tance listed in the specifi cations.
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 com-
ponents, circuits and layout as shown in the fi gures below. The Cbus and Lbus
components simulate a typical DC voltage bus.
Minimum Output Loading Requirements
All models regulate within specifi cation and are stable under no load to full
load conditions. Operation under no load might however slightly increase
output ripple and noise.
C IN
V IN
C BUS
L BUS
C IN = 300µF, ESR < 700mΩ @ 100kHz
C
BUS = TBDµF, ESR < 100mΩ @ 100kHz
L
BUS = <500µH
+VIN
-VIN
CURRENT
PROBE
TO
OSCILLOSCOPE
+
–
+
–
Figure 2. Measuring Input Ripple Current