Requirements for Transformer Isolation in DC-DC Converter

[biker.josh07]

I am building a 2.4 kW mppt buck converter to step down from 450V to 200 V to connect an array of solar panels to a resistive load. I aim to build the converter to meet IEC standards and UL safety requirements. I was thinking that I could build a converter without an isolation transformer, but after reading the wiki page on switch mode power supplies I came across this sentence.

"When equipment is human-accessible, voltage and power limits of <=42.4 V peak/60 V dc and 250 VA apply for Safety Certification (UL, CSA, VDE approval)."

Although Wikipedia shouldn't be the definitive reference in this matter it seems that I will require an isolation transformer.

Please elaborate on the transformer isolation requirements for DC-DC converters and if you know, what would be the relevant IEC standard and or UL standard where this information could be found.

Thanks

 

[FvM]

But you don't have ELV (extra low voltage < 42V) in your design, you are referring to the wrong regulations.

Isolation requirements partly depend on the insulation of the 200V supply and grounding. But generally, isolation isn't necessarily required.

 

[biker.josh07]

The sentence that I quoted was in the section titled "Non-Isolated Topologies" of this wiki article. I took it to mean that a non-isolated switched-mode power supply (buck converter in this case) must have an input voltage of less than 60 V DC in order for it to be applicable for UL safety certification. I have also read elsewhere regarding the 60 V DC limit for non-isolated converters, e.g. in The Power Supply Cookbook by Marty Brown he states that "Transformer isolation is mandatory for all switching power supplies having a DC input voltage of 60 VDC or higher". The book is twenty years old however so standards might have changed since it was published. He gave no reference for where he got this information unfortunately.

I think I may have found the answer to my question though. The relevant standard seems to be IEC60950.

This technical paper by Ericsson seems to sum up the requirements for modern day DC-DC converters nicely.

"General market requirements dictate that if the DC/DC converter’s input voltage might exceed 60 VDC, it must pass a 1500 VDC electrical strength test for basic insulation. The 2250 VDC isolation voltage level is only necessary for certain PoE power devices, but de-facto standards or regional market demands may also require it."

Any thoughts/comments?

Thanks

 

[FvM]

What's your specific question?

Non-isolated converters can be used. It's a matter of the system design, safety requirements for the 200 V circuit etc.

 

[biker.josh07]

My question specifically was "Is an isolation transformer necessary for a 450V to 200V 2.4 kW buck converter so as to meet any UL/IEC safety requirements/standards pertaining to such equipment"

I think the answer to the question is no, but it is still required to pass the necessary electrical strength tests for adequate insulation in the converter. I believe IEC60950-1 outlines such tests.

 

[FvM]

An isolation transformer is not generally required, but possibly a higher insulation rating for the 200 V circuit.