‘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 NDTD 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 NDTD 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 NDTD 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.
The percentage change in output voltage between low intput voltage and high intput voltage, measured with fixed output load i.e. a 5V part with an output voltage of 5.05V @ high
input voltage and 5.03V @ low input voltage would have a line regulation of 0.4%.
Line regulation =
V
OUT
(Low Input V) - V
OUT
(High Input V)
V
OUT
(Nominal Input V)
Where V
OUT
(Nominal Input V) is 5V.
x100%
APPLICATION NOTES
Recommended Input & Output Capacitors
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 characterized using the following values and test circuit.
Input Voltage
5V, 12V
24V, 48V
C
IN
100μF, 25V (0.25Ω at 100kHz)
10μF, 100V (1.5Ω at 100kHz)
Output Voltage
5V
12V, 15V
C
OUT
100μF, 25V (0.25Ω at 100kHz)
47μF, 25V (0.4Ω at 100kHz)
Test circuit
+ V
OUT
Supply + V
IN
+ V
IN
C
OUT
C
IN
Supply - V
IN
- V
IN
NDTD
OV
C
OUT
- V
OUT
Reflected Ripple Current Measurement
12μH
Supply +V
IN
Current
Probe
+V
IN
+V
OUT
C
OUT
Load
C
1
C
IN
NDTD
OV
C
OUT
Load
Supply -V
IN
C
1
= 220μF, ESR < 0.1Ω at 100kHz
-V
IN
-V
OUT
Cross Regulation
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 out-
put. To meet ripple specification a total minimum load of 25% full load is required, however, the NDTD can be used with much lighter loading at the expense of increased ripple. A
small load is required on the negative output of 150mW to ensure the maximum negative output voltage is not exceeded.
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.
The following graphs show the typical required minimum load required for stable operation in mA verses input voltage. Some variants are not included as they do not typically
require a minimum load for stable operation: NDTD0512C, NDTD0515C, NDTD1212C, and NDTD1215C.
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