Benign Environment DC-DC Converters <40 W IMR 15-Family
Edition 2/96 - © Melcher AG 6 - 85
MELCHER
The Power Partners.
6.3
Safety and Installation Instructions
Safety
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 60 V DC).
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 we ver , 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.
Filter recommendations for compliance with
EN 55022, class B
Electromagnetic emission requirements according to EN
55022, class B can be easely achieved by adding an exter-
nal input filter consisting of two additional capacitors and
one current compensated circular double choke.
The filter components should be placed as close as possi-
ble to the input of the converter.
Fig. 4
Input filter arrangement for 48 V DC types
Table 3: Input filter components
Input voltage
C
1
C
2Type
L
1Type
C
iType
12, 24, 40 V DC 2.2 µF 2.2 µF Siemens 2.2 mH Siemens 2.2 µF Siemens
100 V 100 V B 32522 B 82722 100 V B 32522
C1225-K A2202-N1 C1225-K
48 V DC 2.2 µF 1 2.2 µF 1 Siemens 2.2 mH Siemens –
100 V 100 V B 32522 B 82722
C1225-K A2202-N1
1Only valid for input voltages up to 60 V DC.
C
1
C
2
L
1IMR 15
Vi+
Vi–
Fig. 3
Input filter arrangement for 12, 24 and 40 V DC types
C
1
C
2
L
1IMR 15
Vi+
Vi–
C
i