This week’s Newsletter is an introduction to the basic concepts of isolation in a transformer/inductor designs. We often talk about power losses, efficiency and temperature, but we hardly mention isolation. Depending on the voltages that the windings experience, isolation can be the number 1 design parameter.
A design that can’t pass the isolation standard should be seen as a failed design. Design for safety standards is not trivial and can increase both R&D costs and complexity.
🤓 Before starting to implement safety standards, we need to understand the basic definitions that are used in this context. And that’s exactly what todays Newsletter is about. So, sit back, relax, and enjoy the reading!
🔎 Clearance and creepage
Let’s get the definitions straight, before moving forward:
Clearance is "line of sight" distance or the shortest air path between 2 conductors that experience a voltage difference.
Creepage is the shortest distance between two conductors along the insulating surface that experience a voltage difference.
😉 You’re probably reading this on an early morning, I know… Have a look at Figure 1 to make your life easier!
Figure 1. Clearance vs creepage
Another way I see clearance and creepage is by thinking of high voltage as electrons that go from point A to point B, either by plane (clearance) or by car (creepage). In my opinion, the silliest analogies are usually the easiest ones to remember! Have you ever heard of the memory palace?
🤔 How do clearance and creepage apply to magnetics?
Figure 2. Clearance and creepage between primary-secondary windings
In Figure 2, we can see that the clearance between the primary (blue wires) and the secondary (orange wires) is the direct distance between them. However, since there is insulating tape, the two windings never face each other directly. The dielectric strength of a single layer of insulating tape normally used is about 4.5kV (e.g. Tecroll 11B), way above the voltage difference between the primary and secondary winding.
⚡ If the windings were touching the margin tape, the creepage and clearance distance would be the same and the double of the width of the margin tape. Just look at the tracking path, in black colour. So, if for example the requirement is 6mm creepage distance, then we would use 3mm of margin tape in the primary layers and 3mm margin on the secondary layer, as shown in the picture.
Following a safety standard protocol is going to dictate how much creepage and clearance is needed for a particular transformer. A good rule of thumb for mains powered transformers like flybacks in USB PD chargers is 3 & 6mm of creepage required for basic and reinforced isolation, respectively. However, depending on the standard and some other operating conditions, those numbers can be different.
💡 Margin tape and insulated wires
As mentioned in the description of Figure 2, the margin tape is inserted just to keep the windings a specific distance away from the side of the bobbin. You can think of it as an insulating spacer. The drawback of the margin tape is that it shortens the bobbin width, thus reducing the space available for the windings. For smaller cores, the margin tape could potentially take up most bobbin space. There is a way to avoid margin tapes by using fully & triple insulated wires, and that’s the approach we’ve used in today’s library design.
Table 1. Generic 25W flyback – universal input voltage range
🤌 How did I chose the RM8 core (see Table 1) for this design? I checked the power density achieved in the previous flyback designs! Let’s see some numbers:
Table 2. Power densities comparison for similarly speced designs – different power levels
➡️ The first design was an EQ25 60W compatible for USB PD 3.0 charger applications, and that design managed to achieve the highest power density. But problems with the bobbin availability made that design hard to manufacture. Use of flying leads and core insulation requirements led to the next iteration.
➡️ The second design (ETD29 - iteration of EQ25 design) achieved a lower power density. Essentially, power density was sacrificed to make sure that a reinforced isolation, standard core, no flying leads, no core grounding considerations were needed, this way addressing all the drawbacks of the first design.
➡️ The third design is a 25W similarly speced flyback transformer that employs an RM8/I core. The transformer passes basic insulation, avoiding the need for flying leads, but needs core grounding. It’s in a sense somewhere in between the first two designs, as far as drawbacks go. Power density-wise could probably be better, but the difference is actually minimal.
💭 Using a simplistic approach, and realizing that the creepage requirements are similar in that design, I’ve made the decision to go for a power density somewhere between 3.5-5kW/L. The idea is that the higher the power density, the more compact the design is, leaving little wiggle room for isolation needs.
😎 Remember that choosing a power density goal is entirely up to you. You may disagree with my approach here and suggest a different way to choose the power density goal, but please remember to include isolation needs in your approach. Not just textbook equations…
I would be more than happy to receive some feedback on that!
💥 About the windings
Figure 4. Windings 2D/3D cross section
⚠️ There is no margin tape in this design, as I mentioned earlier. Notice that only the secondary winding is chosen to be a triple insulated wire (TIW). That way, creepage and clearance specs are not applicable between windings. However, they are applicable between the bobbin pins and core! Remember the core is made of ferrite, which is considered a conductive material in compliance protocols. The distances of the pins to the core allow for a Class I transformer with basic insulation.
🙆♂️ More on these complex subjects in future newsletters!
The winding arrangement is always a crucial step for any designer. The solution that Frenetic provides is simple enough, though. I’ve copied my original project two times and changed the winding arrangement with just a few clicks. Using the comparator option of the platform we can find the best solution:
Figure 5. Comparing iterations with different winding arrangements
✅ Although power losses and hotspot temperature are similar out of the three iterations, V0 is the best achieving 3.91uH of leakage inductance*. I wanted the leakage to be below 5uH to minimize power losses in the RCD snubber filter of the flyback, increasing the overall flyback efficiency.
*Leakage inductance here is the primary reflected leakage inductance of the secondary winding.
🚀 Discover how to achieve the best winding arrangement with Frenetic Online!
Schedule a demo and explore all the possibilities that our platform offers.
😎 Hope you enjoyed it. Stay tuned!
References
[1] Sudberg, 2016, “Clearance, creepage and other safety aspects in "MySensors" PCBs.” Available at: forum.mysensors.org/topic/4175/clearance-creepage-and-other-safety-aspects-in-mysensors-pcbs
