Dear Boštjan:
Page 7 is mostly wrong. Comments in blue.
Best regards,
Rich
DC/DC converter isolation requirements
Functional insulation between the DC/DC
converter's
input and output is allowed if:
>
The AC/DC power supply has
reinforced or
double insulation between the AC
line supply and
its DC output.
... if the output is less than 60 volts d.c.
(The DC/DC converter must pass
fault condition testing and
withstand an electrical
strength test for basic insulation
if its normal input
voltage exceeds 60 VDC.)
Well. no electric strength is specified for a
secondary circuit to a secondary circuit. No
transients get through the AC/DC power supply; if
they did, the secondary semiconductors would be
damaged or destroyed.
If the input voltage exceeds 60 volts d.c., then
the input to output insulation must be comprised
of basic plus supplementary (double). Or, basic
plus grounding circuit sufficiently robust to
carry the input d.c. fault current.
>
The AC/DC power supply has basic
or
supplementary insulation between
the AC line
supply and its DC output, and the
output of the
DC/DC converter connects to
protective
Earth. (Again, the DC/DC converter
must pass fault condition testing
and
withstand an electrical strength
test
for basic insulation if its normal
input
voltage exceeds 60 VDC.)
The AC/DC power supply cannot have supplementary
insulation alone. Supplementary insulation is
supplementary to basic insulation.
For any voltage greater than 42.4 volts peak or 60
volts d.c., the input to output insulation must be
comprised of basic plus supplementary (double).
Or, basic plus grounding circuit sufficiently
robust to carry the input a.c. and d.c. fault
current. Alternatively, the converter can be
subjected (in its application) to input-to-output
fault tests, where the output is to remain less
than 42.4 volts peak or 60 volts d.c.
>
The AC/DC power supply has basic
or
supplementary insulation between
the
AC line supply and its DC output,
and the
input of the DC/DC converter
connects to
protective Earth. (In this case,
the normal
input voltage must not exceed 60
VDC.)
The AC/DC power supply cannot have supplementary
insulation alone. Supplementary insulation is
supplementary to basic insulation.
The d.c. grounding circuit (output of the AC/DC
power supply) must be sufficiently robust to carry
the input a.c. fault current.
In summary, this means that
functional insulation is
adequate for the DC/DC converter
in almost all practical
system implementations. If the
converter's normal input
voltage exceeds 60 VDC, it must
additionally withstand
an electrical strength test for
basic insulation and pass
fault condition testing.
Assuming the DC/DC converter input is SELV, then I
agree with the first sentence. I disagree with
the second sentence; see previous comments
regarding isolation requirements.
As Boštjan has pointed out, if the DC/DC converter
creates voltages exceeding the limits of SELV,
then fault-condition tests from the non-SELV to
the output must be done. Chances are, such tests
will cause the inverter circuit to stop, and the
output will be zero (SELV) or the input D.C.
-
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