3 Watt-Family DC-DC Converters <40 W Rugged Environment
6 - 12 Edition 2/96 - © Melcher AG
MELCHER
The Power Partners.
6.1
Safety Instructions
If the output circuit of a DC-DC converter is operator-acces-
sible according to the IEC 950 related safety standards, it
shall be an SELV circuit (Safety Extra Low Voltage circuit,
i.e. a circuit, separated from mains by at least basic insula-
tion, that is so designed and protected that under normal
and single fault conditions, the voltage between any two
conductors and between any conductor and earth does not
exceed DC 60 V).
In the following section an interpretation is provided of the
IEC 950 safety standard with respect to the safety status of
the output circuit. Ho w ev er, it is the sole responsibility of the
installer or user to assure the compliance with the relevant
and applicable safety standards.
If the following table is observed, the output of any DC-DC
converter is considered to be an SELV.
Table 2: Insulation concept for SELV circuits
Nominal Minimum required grade Maximum Minimum required Minimum required grade of Resulting safety
mains of isolation, to be output safety status of the isolation between the input status of the
supply provided by the AC-DC voltage front end output circuit and the output of the DC-DC DC-DC converter
voltage front end, including mains from the converter, provided by output circuit
(AC) supplied battery charger front end the converter
≤250 V Basic ≤60 V Earthed SELV circuit 1 Operational SELV circuit 2
≤72 V Unearthed hazardous Operational Earthed SELV
voltage secondary circuit 3 circuit 1 2
Double or reinforced ≤60 V SELV circuit Operational SELV circuit 2
≤72 V Double or reinforced insu- Operational
lated unearthed hazardous
voltage secondary circuit,
supplying an SELV circuit 4
1The earth connection has to be provided by the installer according to the relevant safety standard, e.g. IEC 950.
2The output has to be protected against overvoltages higher than 60 V by external means, e.g. an overvoltage suppressor diode.
3Has to be insulated from earth by at least basic insulation according to the relevant safety standard, based on the maximum input
voltage of the DC-DC converter.
4Has to be insulated from earth by double or reinforced insulation according to the relevant safety standard, based on the maximum in-
put voltage of the DC-DC converter.
Immunity to Environmental Conditions
Thermal Considerations
Table 3: Temp. specification values given are valid for air pressures in the range 800...1200 hPa (800...1200 mbar)
Characteristics Conditions Standard -T Option -S Unit
min max min max
T
AAmbient temperature
U
i min...
U
i max –25 71 –40 85 °C
T
CCase temperature
I
o = 0...
I
o nom –25 95 –40 105
T
SStorage temperature not operational –55 105 –55 105
The case temperature
T
C should not exceed the maximum value. In applications with limited air circulation, additional
measures should be taken (either larger spacing or a fan) to avoid case temperatures higher than
T
C max!
Table 4: MTBF
Values at specified Modules Types Ground Benign Ground Fixed Ground Mobile Unit
Case Temperature 40°C40°C70°C40°C70°C
MTBF according to Group 01 2'900'000 1'400'000 310'000 430'000 150'000 h
MIL-HDBK-217F Group 02/03 2'300'000 1'000'000 270'000 310'000 110'000