NDX Series
Isolated 7.5W Wide Input Single & Dual Output DC/DC Converters
KDC_NDXC.G01 Page 4 of 6
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APPLICATION NOTES
External Capacitance
Although these converters will work without external capacitors, they are
necessary in order to guarantee the full parametric performance over the full
line and load range. All parts have been tested and characterised using the
following values and test circuit.
Value
Input Voltage CIN COUT1 COUT2
5V & 12V 100µF, 100V
0.1µF, 25V multi-layer
ceramic
100µF, 25V (low ESR)
24V & 48V 10µF, 200V
Recommended Input & Output Capacitors
Control Pin Cross Regulation
Dual Output: This provides an OFF function, which puts the converter into a low
power mode. When the pin is high the converter is OFF. Standard TTL levels can
be used but the maximum high level must not exceed 7.0V. The pin can be left
open for normal operation or at voltage below 0.8V with respect to the -V
IN pin.
Single Output: As per dual output, however a series diode must be fitted when
the control pin is used on the single output variants. Recommended diode
1N4148 or similar (direction of fitting is cathode to control pin).
Load regulation is at its best when the positive and negative loads are balanced.
When the loads are asymmetric, the negative output is not as tightly regulated as the
positive output. To meet ripple specification a total minimum load of 25% full load
is required, however, the NDX can be used with much lighter loading at the expense
of increased ripple. A small load of 150mW is required on the negative output to
ensure the maximum negative output voltage is not exceeded. NDX cross regulation
is defined on page 2.
Minimum load
The minimum load for correct operation is 25% of the full rated load across the specified input voltage range. Lower loads may cause a significant increase in output ripple
and may cause the output voltage to exceed its specification transiently during power-down when the input voltage also falls below its rated minimum. A minimum loading
of 30% load is required on NDXD4805 to prevent output voltage rise above specification during power-down.
TECHNICAL NOTES
ISOLATION VOLTAGE
‘Hi Pot Test’, ‘Flash Tested’, ‘Withstand Voltage’, ‘Proof Voltage’, ‘Dielectric Withstand Voltage’ & ‘Isolation Test Voltage’ are all terms that relate to the same thing, a test voltage,
applied for a specified time, across a component designed to provide electrical isolation, to verify the integrity of that isolation.
Murata Power Solutions NDX series of DC/DC converters are all 100% production tested at their stated isolation voltage. This is 1kVDC for 1 second.
A question commonly asked is, “What is the continuous voltage that can be applied across the part in normal operation?”
For a part holding no specific agency approvals, such as the NDX series, both input and output should normally be maintained within SELV limits i.e. less than 42.4V peak, or
60VDC. The isolation test voltage represents a measure of immunity to transient voltages and the part should never be used as an element of a safety isolation system. The part
could be expected to function correctly with several hundred volts offset applied continuously across the isolation barrier; but then the circuitry on both sides of the barrier must
be regarded as operating at an unsafe voltage and further isolation/insulation systems must form a barrier between these circuits and any user-accessible circuitry according to
safety standard requirements.
REPEATED HIGH-VOLTAGE ISOLATION TESTING
It is well known that repeated high-voltage isolation testing of a barrier component can actually degrade isolation capability, to a lesser or greater degree depending on materials,
construction and environment. The NDX series has an EI ferrite core, with no additional insulation between primary and secondary windings of enameled wire. While parts can be
expected to withstand several times the stated test voltage, the isolation capability does depend on the wire insulation. Any material, including this enamel (typically polyurethane)
is susceptible to eventual chemical degradation when subject to very high applied voltages thus implying that the number of tests should be strictly limited. We therefore strongly
advise against repeated high voltage isolation testing, but if it is absolutely required, that the voltage be reduced by 20% from specified test voltage.
This consideration equally applies to agency recognized parts rated for better than functional isolation where the wire enamel insulation is always supplemented by a further
insulation system of physical spacing or barriers.
NDXDC
CIN
+VOUT
-VIN
+VIN
-VOUT
OV
COUT2
COUT2
COUT1
COUT1
NDXSC
CIN
+VOUT
-VIN
+VIN
OV
C
OUT2COUT1
