‘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 NKA 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 NKA 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 NKA series has toroidal isolation transformers, 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.
1. 12V input types have typically 3% less load regulation.
The voltage tolerance envelope shows typical load regulation characteristics for this product series. The tolerance envelope is the maximum output voltage variation due to
The minimum load to meet datasheet specification is 10% of the full rated load across the specified input voltage range. Lower than 10% minimum loading will result in
an increase in output voltage, which may rise to typically double the specified output voltage if the output load falls to less than 5%.
Capacitive loading and start up
Typical start up times for this series, with a typical input voltage rise time of 2.2μs and output capacitance of 10μF, are shown in the table below. The product
series will start into a capacitance of 47μF with an increased start time, however, the maximum recommended output capacitance is 10μF.
NKA0303SC
NKA0305SC
NKA0309SC
NKA0312SC
NKA0315SC
NKA0503SC
NKA0505SC
NKA0505SEC
Start-up time
ms
1.35
3.35
9.30
22.13
25.04
0.80
2.32
2.03
NKA0509SC
NKA0512SC
NKA0515SC
NKA1205SC
NKA1209SC
NKA1212SC
NKA1215SC
Start-up time
ms
8.01
14.63
28.38
2.11
7.62
9.08
14.39
Typical Start-Up Wave Form
Ripple & Noise Characterisation Method
Ripple and noise measurements are performed with the following test configuration.
C1
1μF X7R multilayer ceramic capacitor, voltage rating to be a minimum of 3 times the output voltage of the DC-DC converter
10μF tantalum capacitor, voltage rating to be a minimum of 1.5 times the output voltage of the DC-DC converter with an ESR of less
C2
than 100mΩ at 100 kHz
C3
100nF multilayer ceramic capacitor, general purpose
R1
450Ω resistor, carbon film, ±1% tolerance
R2
50Ω BNC termination
T1
3T of the coax cable through a ferrite toroid
RLOAD
Resistive load to the maximum power rating of the DC-DC converter. Connections should be made via twisted wires
Measured values are multiplied by 10 to obtain the specified values.
By using the values of inductance and capacitance stated, the output ripple at the rated load is lowered to 5mV p-p max.
Component selection
Capacitor: It is required that the ESR (Equivalent Series Resistance) should be as low as possible, ceramic types are recommended.
The voltage rating should be at least twice (except for 15V output), the rated output voltage of the DC-DC converter.
Inductor: The rated current of the inductor should not be less than that of the output of the DC-DC converter. At the rated current, the DC resistance of the inductor
should be such that the voltage drop across the inductor is <2% of the rated voltage of the DC-DC converter. The SRF (Self Resonant Frequency) should be
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