Stuff we learned:
max duty cycle should be between 0.4 - 0.6 (we’ll spec it to be 0.5)
operating switching frequency should be between 50 to 140 kHz
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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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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
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According to Table 4-9 in chapter 4, we the MINIMUM wire size we can use is 23-AWG
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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
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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)
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Spreadsheet: https://docs.google.com/spreadsheets/d/1O1wjWdbnPksTQmW_tAbk-rAPtSNWPjp_Yc3J71V07ag/edit#gid=1019363053
https://www.digikey.ca/en/products/detail/epcos-tdk-electronics/B65875A0000R087/3914436
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