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.
The installer must observe all relevant safety standards and regulations. For
safety agency approvals, install the converter in compliance with the end-user
safety standard, i.e. IEC/EN/UL 60950-1.
Input Reverse-Polarity Protection
If the input voltage polarity is reversed, an internal body diode will become
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. Please be sure to install a properly-
rated external input fuse (see Specifi cations).
Input Under-Voltage Shutdown and Start-Up Threshold
Under normal start-up conditions, converters will not begin to regulate properly
until the ramping-up 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 opera-
tion at a single input voltage.
Users should be aware however of input sources near the Under-Voltage
Shutdown whose voltage decays as input current is consumed (such as ca-
pacitor inputs), the converter shuts off and then restarts as the external capaci-
tor recharges. 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 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 ramping input voltage crosses the Start-Up Threshold and the fully loaded
regulated output voltage enters and remains within its specifi ed accuracy band.
Actual measured times will vary with input source impedance, external input
capacitance, 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 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 command until the output enters and remains within its specifi ed accuracy
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.
TECHNICAL NOTES
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 (Cin in the fi gure) serve primarily as energy storage elements, minimizing
line voltage variations caused by transient IR drops in the input conductors.
Users should select input capacitors for bulk capacitance (at appropriate
frequencies), low ESR and high RMS ripple current ratings. In the fi gure below,
the Cbus and Lbus components simulate a typical DC voltage bus. Your specifi c
system confi guration may require additional considerations. Please note that
the values of Cin, Lbus and Cbus will 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 tempera-
ture rise from refl ected AC current dissipated inside capacitor ESR.
In the fi gure, the two copper strips simulate real-world printed circuit
impedances between the power supply and its load. In order to minimize circuit
errors and standardize tests between units, scope measurements should be
made using BNC connectors or the probe ground should not exceed one half
inch and soldered directly to the fi xture.
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
+VIN
−VIN
CURRENT
PROBE
TO
OSCILLOSCOPE
+
–
+
–
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Single Output UCH Models
Isolated, “Half-Brick”
1.8−15V Output DC/DC Converters
MDC_UCH Models.C01 Page 14 of 18
