Good morning there!

Sotiris here! 👋

I hope you all had an amazing weekend! And what’s better than starting off the new week with some interesting Power Electronics content?

🚀 This is an in-depth newsletter about isolation and safety requirements. This time we will get into detail about how to select the appropriate distances in a transformer design. If you missed my previous newsletter with the basic definitions, go back and read it in our repository here.

⚠️ The goal of this newsletter is to decide exactly how much creepage and clearance is needed for the flyback design that I want to build for our library designs.

Specs

👉 Long story short, I needed a flyback auxiliary psu a while back for a big project of mine. The input voltage was about 40-50V and the output 12/8.4V. I had no idea back then about safety concerns so, I built the transformer, and everything has worked great until today. The reason why it’s still working is the low input-output voltage requirements. Years later I found another application where my little flyback card would work just fine, if I changed the input voltage to about 450V. Owning the rights for this design I decided to freely share aspects of the actual design with all of you, starting with the spec sheet shown in Table 1.

Table 1. Generic 10W auxiliary PSU design

Figure 1. 3D render of the 10W flyback transformer

Insulation type

The main insulation categories are functional, basic, and reinforced (or double) insulation. Functional insulation only accounts for a bare minimum insulation that can only guaranty functionality of the transformer. Basic insulation provides protection against fault conditions, such as overvoltage conditions at the primary transformer side. Reinforced isolation is the strictest option, providing maximum protection against fault conditions, making sure that isolation doesn’t break down from the primary to the secondary side.

✅   For this design reinforced isolation was chosen.

Working voltage

✅   That is the maximum voltage that the insulation of the transformer can experience under normal operating conditions. In our case that’s about 450V between the primary winding and the secondaries.

Pollution degree

The standard specifies different pollution degrees depending on the actual machine/product the transformer is used in. For example an epoxy potted transformer will always be clean, away from humidity and dust. But the most common thing is unpotted transformers that are subjected to dust humidity and other “pollutants”.

IEC 61558-1 defines pollution degree as below:

Pollution degree 1: “Pollution degree in which no pollution or only dry, non-conductive pollution occurs.”

Pollution degree 2: “pollution degree in which only non-conductive pollution occurs, except that occasionally a temporary conductivity caused by condensation is to be expected.”

Pollution degree 3: “pollution degree in which conductive pollution occurs, or dry non-conductive pollution occurs which becomes conductive due to the condensation which is to be expected.”

✅ Most times pollution degree 2 is chosen, as is the case with this design as well.

Overvoltage category

That is a probabilistic categorization that hasn’t got to do with the transient voltages that a transformer will experience necessarily, but it’s rather a categorization depending on the application the transformer is used in.

We have 4 overvoltage categories (OVCs) named I to IV. Take a look at Table 2 to understand the applications and description of each overvoltage category.

Table 2. OVCs as described in IEC61558-1

✅ Overvoltage category II was appropriate here.

CTI index

CTI index is the voltage which causes tracking after 50 drops of 0.1% ammonium chloride solution have fallen on the material. The results of testing at 3 mm thickness are considered representative of the material's performance in any thickness. Tracking is the breakdown of insulation on top of an insulator created from a voltage potential which gradually creates a carbonized conductive path along the surface, creating a leakage-conductive path.

Table 3. CTI-material group category based on Annex G of IEC 61558-1

When we talk about insulating materials we consider the bobbin, insulating tape between layers, margin tape and wire ending sheeves like heat shrink tubes.

✅ The cost effective/good compromise is to select material group II (400V ≤ CTI < 600V ) for all of the aforementioned components.

Finding the minimum clearance distance

⚙️ We have previously selected:

• OVC II
• Reinforced isolation
• Pollution degree 2
• Working voltage 450V

From IEC61558-1 in Table 4 we see that the necessary clearance is between 3-5.5mm. Although most standards allow linear interpolation between values in tables, for this table the standard forbids this. So the minimum clearance is 5.5mm.

Table 4. IEC 61558-1 clearance selection table

Finding the minimum creepage distance

⚙️ We have previously selected:

• Material category II
• Reinforced isolation
• Pollution degree 2
• Working voltage 450V

From IEC61558-1 in Table 5 the necessary creepage is between 4.3-8.6mm. In this case linear interpolation between table values is permitted, thus 6.5mm of creepage distance is calculated for 450V.

Table 5. IEC 61558-1 creepage selection table

Margin tape and windings picture

This design was built with margin tape. Using 3.5mm of margin tape on each layer the final creepage is set at 3.5x2=7mm, as shown in Figure 2.

Figure 2. Margin tape of the 10W flyback transformer

🔑 Now you’re in the position to understand in more detail the way we comply with a safety standard. To be honest there are a lot more things that need to be taken care of to make sure the transformer complies, but this is a mayor step in this direction.

😎​ Stay tuned! 

🚀​ New Automatic Manufacturing Drawing in Frenetic Online!

Do you want to learn more how Frenetic can help you with your HF magnetics? You can schedule a demo with a member of our team!

References

1. (2018) Transformer Design Considerations. tech. Wurth Electronik. Available at: www.st.com/content/ccc/resource/sales_and_marketing/promotional_material/newsletter/group0/6b/10/c1/aa/ba/76/41/cf/Wurth_Design_Considerations_Munich/files/Wurth_Design_Considerations_Munich.pdf/jcr:content/translations/en.Wurth_Design_Considerations_Munich.pdf (Accessed: February 13, 2023).

2. IEC 61558-1, Safety of transformers, reactors, power supply units and combinations thereof. part 1, General Requirements and tests (IEC 61558-1:2017) (2019).

Sotiris Zorbas, MSc 

Power Εlectronics Εngineer