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UL requirements for 600Volts

J

Jamie Morken

Jan 1, 1970
0
Hi,

I am trying to pick out gate drive transformers for a 600Volt SMPS,
and am wondering what the required clearances are between PCB pads
for the transformer primary to secondary for this to meet UL
requirements for 600Volts operation. I remember reading 0.1" for
500Volts was enough, but I've also read 8mm (0.31") somewhere.

Anyone have experience on this?

cheers,
Jamie
 
P

Paul Mathews

Jan 1, 1970
0
Hi,

I am trying to pick out gate drive transformers for a 600Volt SMPS,
and am wondering what the required clearances are between PCB pads
for the transformer primary to secondary for this to meet UL
requirements for 600Volts operation.  I remember reading 0.1" for
500Volts was enough, but I've also read 8mm (0.31") somewhere.

Anyone have experience on this?

cheers,
Jamie

The requirements are not based on the nominal voltage difference in
potentials across the insulation barrier. Instead, they are based on
peak voltage measurements, which can be very much influenced by how
much ringing exists in your circuitry. This is because the peak
voltage determines the distance that corona discharge can form
'tracks' through contamination on the circuit surfaces. In my
experience, different certification labs interpret the standards
(e.g., UL908 and the newer 'harmonized' standards) differently. In any
case, the relevant standard generally includes a graph, equation, or
table that you can use to determined required creepage and clearance
dimensions. In general, isolating transformers, whether linear or
switchmode, must meet 'reinforced' or 'double' insulation standards.
Very likely, this would put you somewhere upwards of 3.1 mm.
Paul Mathews
 
J

Jamie Morken

Jan 1, 1970
0
Paul said:
The requirements are not based on the nominal voltage difference in
potentials across the insulation barrier. Instead, they are based on
peak voltage measurements, which can be very much influenced by how
much ringing exists in your circuitry. This is because the peak
voltage determines the distance that corona discharge can form
'tracks' through contamination on the circuit surfaces. In my
experience, different certification labs interpret the standards
(e.g., UL908 and the newer 'harmonized' standards) differently. In any
case, the relevant standard generally includes a graph, equation, or
table that you can use to determined required creepage and clearance
dimensions. In general, isolating transformers, whether linear or
switchmode, must meet 'reinforced' or 'double' insulation standards.
Very likely, this would put you somewhere upwards of 3.1 mm.
Paul Mathews

Thanks,

Does this 3.1mm spec apply to spacing between pins/pads of the actual
transformers themselves?

I am looking at a couple possible gate drive transformers:

this one would seem to meet all the UL requirements but is huge
and expensive:
http://www.coilcraft.com/pdfs/pwrbase.pdf



this one is nice and tiny and has 4mm+ clearance between primary
and secondary PCB pads. I'm not sure about the transformer winding
insulation itself though if it can meet the UL specs, but it specifies
a 2250VDC primary to secondary isolation.

http://www.coilcraft.com/pdfs/fa2659.pdf


cheers,
Jamie
 
P

Paul Mathews

Jan 1, 1970
0
Thanks,

Does this 3.1mm spec apply to spacing between pins/pads of the actual
transformers themselves?

I am looking at a couple possible gate drive transformers:

this one would seem to meet all the UL requirements but is huge
and expensive:http://www.coilcraft.com/pdfs/pwrbase.pdf

this one is nice and tiny and has 4mm+ clearance between primary
and secondary PCB pads.  I'm not sure about the transformer winding
insulation itself though if it can meet the UL specs, but it specifies
a 2250VDC primary to secondary isolation.

http://www.coilcraft.com/pdfs/fa2659.pdf

cheers,
Jamie- Hide quoted text -

- Show quoted text -

What kind of circuit do you have where the gate drive transformers are
mains isolating? In most topologies, the primary switches and gate
transformers are entirely on the mains side of the isolation barrier.
Their construction is therefore not based on safety, per se, but is
based on good engineering practices. I'm aware that some types of
synchronous rectifiers and some types of feedback circuits, for
example, use transformers across the insulation barrier. In
particular, many types of high rel standards prohibit the use of
optocouplers, so you end up using transformers. In those cases, the
same standards apply to all transformers that bridge the isolation
barrier. To make them smaller, you end up using insulating barriers to
increase creepage distances, encapsulation, special pinouts, and so
on.
Paul Mathews
Paul Mathews
 
L

legg

Jan 1, 1970
0
Hi,

I am trying to pick out gate drive transformers for a 600Volt SMPS,
and am wondering what the required clearances are between PCB pads
for the transformer primary to secondary for this to meet UL
requirements for 600Volts operation. I remember reading 0.1" for
500Volts was enough, but I've also read 8mm (0.31") somewhere.

Anyone have experience on this?

How the hell have you ended up with 600V as your supply rail
..........or is this still on paper?

This is a class of service that isn't normally adopted for domestic or
consumer applications, Again, you have to determine what is a safety
isolation issue and what is a functional isolation issue. For safety
isolation, consult the standard - this is not something you want to
rely on anecdotes to achieve.

RL
 
J

Jamie Morken

Jan 1, 1970
0
legg said:
How the hell have you ended up with 600V as your supply rail
.........or is this still on paper?

600VDC is the maximum voltage going into the SMPS, its from a brushless
DC generator.

cheers,
Jamie
 
L

legg

Jan 1, 1970
0
600VDC is the maximum voltage going into the SMPS, its from a brushless
DC generator.
I think that's still UL1012 (if at one output is not marked as
class2). Class 2 circuits are UL1310.

Internal transformer spacings are table 19.2 on page 41 - 6.5mm
between safety-isolated conductors or conductive structures.

External and printed wiring spacings are listed in table 28A.1 on page
50 - 9.5mm cl / 12.7mm cr for safety isolation at 600V or less.
The latter refers to UL840 - which is a standard that only covers
creepage and clearance - as guiding functional distances.

If your output is hazardous (ie greater than 42V), isolation between
the two hazardous sections could be considered as requiring functional
isolation.

The standards I'm referencing are not coordinated. I'm not sure if
there has been an effort to do this, for industrial devices, since
Y2K. You should inquire, if your intended use is outside the US.

RL
 
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