🚀 This week’s newsletter is all about testing the transformer I designed in the previous newsletters (missed one? Check here), as well as discussing various topics during the sample manufacturing and measurement process. And, at the end, I will reveal you a small (not that small...) secret. 🤐
Let’s start with a picture of the manufactured sample.
⚠️ When winding the sample, the first thing that came to my mind was the windings height warning. The actual sample barely fitted the transformer bobbin😅. Remember the online toolhad warned me for this, as you see in the picture below!
Had I used more insulating tape in between the layers, the windings wouldn’t fit!
✅Lesson learned.
⚡Apart from the height problem, given that the fill factor was about 75% the winding process went smoothly. Well, I must say that I’ve wounded the same transformer a couple of times, but still the patient engineer wouldn’t get into much trouble 👍.
Having the transformer wounded it’s time for a varnish bath, but I skipped this step because I wanted to go into testing right away.
⚡Varnishing is not just a luxury to make things look nice. The most noticeable effect an unvarnished transformer will exhibit is audible noise when put under test. There are low frequency harmonics in the audible spectrum that cause screeching noises. Varnishing will prevent the windings-core mechanical oscillations thus eliminating most of the noise. It also strengthens voltage isolation between windings when it impregnates deep into the construction, but we shouldn’t rely on this positive effect, when designing a transformer.
Before testing the design, its basic specs were measured at 100kHz frequency:
Looking at the table above one would argue that the predicted values are far off. This is however not true❗
🤔❓
This is a no-gap transformer right ❓
So why did I put a 0.05mm gap in the tool ❓
👉 That’s because zero gap distance is not exactly zero unless the two cores were welded together somehow…
In reality, 20-50μm of minimum distance is about right.
Let’s see how much of a difference it makes on the AL value and consequently on the inductance of each winding 20μm gap and a 50μm gap scenario.
(Remember L=N2*AL)
🔑The difference between these 2 scenarios is 30% in the AL value thus the magnetizing inductance (or any other inductance) as you see. The gap really depends on the core surfaces and matching. No one can control this…
🔑Also if you take a look into the core material (TDK N87) the AL value with zero gap may vary up to +30/-20% !!! It depends on the batch of cores that you’ll receive from the factory.
If you combine these two facts you can be off by 50-60% if you’re unlucky enough.
🔑Luckily in the phase shifted full bridge we really care about the minimum magnetizing inductance, so more is good less is bad, to put it in simple terms.
✅An important spec in this topology is the primary leakage inductance! Take a look on the table above.
The prediction VS reality here is off by 10% which pretty good for such a difficult spec to calculate-predict, if you ask me, that’s success❗ 🚀
💥PSFB transformer under test now! 💥
👉 After 1hr+ temperature stabilized at approx. 88.2°C.
As the table below suggests the error is well within the target. 3% of difference is a success 🚀, by all measures. Just to give you an idea, moving the camera a bit can change the temperature by 1°C. Also, the emissivity value we have talked before plays the biggest role in these measurements.
🤐 SECRET TIME 🤐
Frenetic just launched a new release of its online tool. Among several new features (you´ll learn more about that in the next days) there´s the brand new Simba Circuit Simulator😎 It´s a powerful simulator with an intuitive design and ultra fast Predictive Time-Step solver. It can be run online, without any need for installation, directly accessible from our website.Give it a try!! 🚀
And if you want to learn more and get a personalized demo, just clickhereto book a meeting. 👈
WHAT’S COMING NEXT WEEK?
👉 Next week I start designing another transformer for the 1kW PSFB topology. This time the mistakes of the past will be corrected, targeting a better result both with temperature rise and power losses.
