Stuff we learned:
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Nominal duty cycle of 0.23 is reasonable, so L = 240uH is reasonable!
Turns Ratio Calculation
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Therefore, for an N:1 turns-ratio transformer, N must be less than 7.2. Conveniently, we will pick N=7, so our transformer should have a 7:1 primary to secondary turns ratio
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So we will assume there is no 2 involved:
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Ipk = 3.4A, which is very high….
Looking at the InnoSwitch datasheet, these are the rated I_limit values
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With this in mind, we should use INN4076 (or higher) with the 4.7uF BPP configuration
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According to Table 4-9 in chapter 4, we the MINIMUM wire size we can use is 23-AWG. Since that is not a common AWG size, we will instead use22-AWG as our minimum wire size. 22 AWG will be used for the primary coils, since they carry low current and require many turns.
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We will semi-blindly trust the area product method shown in this video: https://www.youtube.com/watch?v=45kTQwZSLHc Ac is the area of the core [mm^2] AW is the area of the winding space [mm^2] E = max energy the inductor needs to store = 0.5*L*Ipk^2 =0.5 * 240uH * 3.4A = 1.3872 mJ Bpk = saturation flux density of the core, which is determined by the material of the core. We will use a ferrite core (since it low losses), at which the common saturation limit is 300mT J = current density in winding, which as a rule of thumb is given as 3A/mm for copper wire (from the video), so we will use that as well PF = packing factor, which accounts for what percentage of the available wiring space is actually fillable by wire, since things like insulation take up space. The video estimates 50%, or 0.5, so we’ll use that too  Then 'A' (which is Aw*Ac) is 6200 mm^4, so we try to find a transformer where Ac (area of core) multiplied by Area of Winding (Aw) is similar  |
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We need these parameters:  Then calculate Irms, which they give as:  We can simplify the equation to just:  Then, follow their steps for determining the right transformer to use
Assuming alpha (proportion of power loss due to copper resistance) is also their value of 1% (which is just 1 in their equation for some reason), then we find Kg = 0.0213 cm^5  So then go to chapter 3 of their guide, and find a core with a similar Kg (but slightly larger, just so that there’s more area to work with) For EE cores, nothing is within good range  Also nothing good for EC:  Nothing for ETD:  Nothing for ER:  For EFD ferrite cores, best is EFD-25 = 0.01911 or EFD-30 - 0.03047  For EPC ferrite cores, EPC-27 = 0.024036 is very close too!  For EP ferrit cores, EP-20 = 0.02892 works  For PQ ferrite cores (what they use in examples), we see PQ20/20 = 0.0227  So the ‘valid’ cores, in order of ascending Ku, are: PQ20/20, EPC-27, EP-20, EFD-30 Looked on Digikey and found the below options, then downloaded a spreadsheet format for all the valid options so it’s easier to filter and calculate stuff.
 https://www.digikey.ca/en/products/detail/epcos-tdk-electronics/B65875A0000R087/3914436 EPC-27: (does not exist on digikey)  https://www.tdk-electronics.tdk.com/inf/80/db/fer/ep_20.pdf  https://www.digikey.ca/en/products/detail/epcos-tdk-electronics/B66423G0000X187/3914948 |
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So, I will assume it is always better to have over-voltage than under-voltage. So we will pick bias windings of Bias 1 = 4 5 turnsand Bias 2 = 3 4 turns