UL requirements for 600Volts

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:

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.

cheers, Jamie

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

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

cheers, Jamie

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

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