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UCC28600 mosfet design

Francesco Fraccaroli1
Prodigy 40 points
Other Parts Discussed in Thread: UCC28600

Hi,

I'm using the UCC28600 to design a 24V 40W DCDC converter, that works with input voltage of 70-250Vdc.

I undertood quite well how to use the excel calculator, but I have already some question:

- why the reflected voltage is so high? It is calculated from the MOS breakdown voltage, that is the opposite of what I used to do. Generally I fix a maximum input duty cycle and calculate the reflected voltage by Dmax*Vin_min, and I calculate all other values from there. In general the results are pretty the same, but if I oversize the breakdown voltage of the MOS the results are very different.

- designing the mosfet I'm trying to calculate the dissipated power. Generally I use the maximum current (at lower input voltage), but here I see that the frequency variation get the switching loss increase at higher frequency. In the "QR simulator" tab of the excel spreadsheet I see that at row 113-114 there are the minimum and maximum input voltage values. In my case, at maximum voltage the actual frequency is 168kHz. In my opinion this vaue is saturated to 130kHz, but how about other values? Are they valid? Have you got some advices for mosfet thermal designing with this controller?

Thanks,

Francesco

  • 0
    TI__Expert 7395 points
    Francesco,
    thank you for your interest in TI and our UCC28600. I've contacted the engineer for this part to comment here so please lookout for a response from them soon.
    Regards,
    John
  • 0 in reply to John Stevens
    TI__Genius 15910 points
    Hi Francesco,
    -What's referred to as "VXFMR,reflect" voltage, as written in this spreadsheet is really equal to the reflected voltage + the allowed leakage spike (determined by what is called "Snubber overshoot% of reflected Vout"). I believe the author of this spreadsheet did it this way because he wanted the MOSFET chosen and characterized before calculating the required on-time because the MOSFET characteristics play such an important role in the resonant design of this controller. In the flow of these calculations, the User is able to state what the breakdown rating of the FET is, choose the allowable derating, relative size of snubber, and then the transformer is designed to meet the resultant timing. So if you select a FET that is much larger, the calculations should adjust to utilize the new FET, numbers will change.

    -You are absolutely correct in that the maximum switching frequency is 130kHz and will not switch above this level. Admittedly, the spreadsheet does not convey this well at all. The values shown in rows 91 through 111, 113, 114 are used in the macros that are calculating the modes of the converter.
    For Low line conditions, the peak primary current, switching frequency, and maximum on-time for full load conditions is shown on the QR_Sim page , row 138. The High line condition, full load is shown on row 180.