🔥 I’m back this week focusing my attention on a Texas Instruments application note for a 500W LLC Half bridge (PMP40379) power supply. I started off searching through the app note to see if I could find some info about the magnetic components. Without having a clear image about the transformer materials or the exact construction, I tried to simulate a compatible transformer to get similar results and see if we can improve the design in the next Newsletter.
🗒️Specs
In Table 1 you can have a look at the most relevant specs for the construction of the transformer.
Table 1. PMP40379 500W transformer specs
This is enough information for us to start the simulation of the design. This time we will use Frenetic’s SUZUKACircuit Simulator™ to create the excitation waveforms for this transformer design, as shown in Figure 1.
⚙️ Simulation Time
Figure 1. Simulating the LLC design of Ti app note PMP40379
Since we have a thermal image of the transformer in the application note with the converter working under 500W/60kHz and at nominal input voltage 310-320V, we will target this steady state condition in our simulation, to make a few comparisons at the end.
In Figure 1 we can see that the simulation converged at 6.6s. Now we are ready to export the transformer voltages/currents to Frenetic Online and begin with the design process.
🔎 The search for the core
Figure 2. Using the Core Optimizer™ to find a similar core to that of the app note.
Looking for the transformer used in app note, in the BOM file we can find a Wurth custom transformer (Mfr Part No: 750343844). That’s where I collected the transformer dimensions that are listed in Table 1. Analyzing the images of the actual pcb and the pdf spec sheet, I figured out it was an ETD type core.
📈At this point, I used the Core Optimizer™ to plot all ETD cores setups that, for a certain number of primary turns, satisfy the requirement to be below the peak flux density of 250mT. From the results, the ETD44/22/15 has very similar dimensions.
Let’s see the losses and the Bpeak for 24T in the primary in Figure 3 (the exact number of primary turns is known from the app note). Core losses are approximately 2.85W for 24 turns, and the peak flux density is just below 200mT.
Figure 3. Using the Core Optimizer™ to see power core losses versus primary turns
➿ The Windings
As I’ve mentioned previously, in the files of this app note the number of turns is known for all windings. Taking into account that we need 59uH of leakage inductance, which is a lot, a two-chamber approach is the only way to achieve this. The two-chamber approach is also confirmed from the transformer data available in the Wurth datasheet, as well as from some actual transformer photos.
Figure 4. Windings
🎯 In Figure 4 you can have a look at the windings. Using a 5mm distance window between primary and secondary windings, we can get close to the 59uH leakage inductance needed. The Wurth transformer had a 50uH leakage inductance. The reality is that achieving tight tolerances and repeatability for such a high number of leakage inductance is a very difficult task.
However, Frenetic Online provided us with a very good starting point to achieve the goal. In reality, adjusting the switching frequency closer to the actual LC tank resonant frequency is the way to solve inconsistencies with the resonant frequency of an actual LLC board. Notice that the two secondaries are grouped together and are wound in a bifilar approach, to minimize potential inconsistencies with the leakage inductance of each secondary.
💡 Performance Comparisons
Figure 5. Performance of the simulated transformer VS app note thermal image
Figure 5 shows the Frenetic simulation and an actual thermal picture of the transformer for the same operating conditions.
We predict that the maximum hotspot temperature is in the center leg of the transformer at 94°C (we can’t see that in the image). The windings temperature is predicted at 88°C, while they appear to be at 94-99°C and the core surface temperature is really close to the actual one, around 78-80°C.
⚠️ DISCLAIMER: The exact Litz wire dimensions and winding arrangements were unknown from the online material, so any differences in temperature can be easily explained in this case. Despite this fact, the overall temperature behavior of this design is in line with the actual transformer behavior.
🙌 Conclusions
The main idea of this simulation was to get as close as possible to the actual transformer built for the Texas Instruments app note, and not to optimize it! What I needed was a starting point to make iterations later.
In the next Newsletter, I will focus on the weak points of this transformer, trying to improve the design and to realize a more efficient component!
😎 Stay tuned!
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References
Texas Instruments, September 2018, “500-W, Single Stage LLC Power Supply Reference Design for Audio Amplifier”, PMP40379
