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SMPS Transformer build up satisfying IEC600079-11 standard.

Posted by: Kiran.Surve on 10/18/2021

Hi Power Integrations Team,

I would require your suggestion and advice for intrinsically safe transformer construction as required to pass the explosive environment IEC 600079-11 standard. This standard is applicable to systems used in the Oil and Gas industry in an explosive environment.

I have designed a SMPS power supply using Power Integrations INNO switch (INN3164) with the following specifications.

Input : Universal

Output: 1) 5V@2.0A

             2) 15V @ 0.3A (Both are are floating outputs)

I have design transformer using PI expert tool with following no of turns.

Primary : 66 turns

Primary bias: 8 turns

Sec 1 (5V) : 3 turns

Sec 2 (15V) : 7 turns

The transformer is designed in EF20 bobbin, but it seems impossible to build a transformer to meet the creepage and clearance distance between primary and secondary which requires creepage distance of 10mm and clearance distance of 6mm as per Table attached with the mail.

I have attached creepage and clearance requirements as per IEC 600079 (level of protection ib, voltage peak value = 375V).

I have also selected one EF20 transformer bobbin having two section separate Primary and Secondary which is attached in the mail.

Please go through it and suggest the transformer build up to pass the creepage and clearance test as per the required by standard.


Purwang Patel



Submitted by PI-Yoda on 10/19/2021

Hi Purwang,

Dual section bobbins are not realistic solution for transformer in a fly-back topology. 

One feasible solution with small cores is to implement the output windings with triple insulated wire (TIW).

The primary is a standard enameled wire connected to the pins and the core is considered 'primary side'. The outputs (TIW) wires ends are terminated on the secondary side -the the other side of the safety barrier.

For long, high current wires it is critical to twist the leads together as much as possible through the length between the transformer and the solving points.

Best Regards,







Submitted by Kiran.Surve on 10/19/2021


Thanks for the feedback.

I am still not clear regarding how to achieve required creepage and clearance between Primary and Secondary circuit in transformer.

What I have understand I have mentioned below.

The attached file "clearence.pdf" for which the table 5 (level of protection ib, voltage peak value = 375V).

1) Clearance - (6mm) - By using horizontal bobbin

1) Separation distance through solid insulation (1mm). What does it mean? 

2) Creepage distance mm - (10mm) ----  If I use magnet wire then Creepage distance I have to maintain between primary and secondary is 10mm,  5mm on both the sides of the winding.

3) Distance under coating in mm -(3.3mm). ---- If I use TIW for primary and Sec then creepage distance between primary and secondary shall be minimum 3.3mm in the transformer.

4) Please go through the attached pdf file Milesplatts-3783 section 6 page 15, where you will find EF20 Horizontal bobbin which have separate primary and secondary bobbin and can be clubbed together, the whole primary winding layer is covered with secondary winding. Can this be a realistic solution to achieve required creepage an clearance solution.

Please guide and correct me if I am wrong.




Attachment Size
Clearence.pdf 101.77 KB
MilesPlatts-3783.pdf 8.24 MB
Submitted by PI-Yoda on 10/19/2021

Hi Kiran,

You cannot achieve creepage and clearance inside a small core transformer simply because the space is not enough.

It is a well known problem, and the solution is to take the creepage/clearance issue outside the transformer where is more space available. 

To do this:

1. Inside the transformer - completely isolate one of the sides (most likely secondary because is less turns) via using TIW which by design complies with  safety under certain conditions. Please check the link above.

2. Take the leads soldering end points to the secondary side PCB side, where creepage and clearance is provided on the PCB.


Picture attached. 

Best Regards,

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CC_0.png 44.59 KB