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AN3027 датащи(PDF) 17 Page - STMicroelectronics |
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AN3027 датащи(HTML) 17 Page - STMicroelectronics |
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17 / 41 page ![]() AN3027 Designing a TM PFC Doc ID 16134 Rev 4 17/41 Figure 7. Conduction losses and total losses in the STF7NM50N MOSFET for the L6563S TM PFC From Equation 33 using the data relevant to the MOSFET selected, and calculating the losses at VACmin and VACmax, we observe that the maximum total loss occurs at VACmin and is 2.61 W. From this number and the maximum ambient temperature (12), the total maximum thermal resistance required to keep the junction temperature below 125°C is: Equation 34 If the result of Equation 34 is lower than the junction-ambient thermal resistance given in the MOSFET datasheet for the selected device package, a heatsink must be used. For the STF7NM50N the junction - ambient thermal resistance is 62 °C/W, so a heatsink has been used. 3.3.6 Boost diode selection Following a similar criterion as that used for the MOSFET, the output rectifier can be selected. A minimum breakdown voltage of 1.2·(Vout + ∆VOVP) and current rating higher than 3·Iout from Equation 1 can be chosen for a rough initial selection of the rectifier. The correct choice is then confirmed by the thermal calculation. If the diode junction temperature operates within 125°C, the device has been selected correctly, otherwise a bigger device must be selected. In this 100 W application an STTH2L06, (600 V, 2 A) has been selected. The rectifier AVG Equation 1 and RMS Equation 8 current values and the parameter V th (rectifier threshold voltage) and Rd (dynamic resistance) given in the datasheet allow calculating the rectifier losses. From the STTH2L06 datasheet, Vth is 0.89 V, Rd is 0.08 Ω. Equation 35 ) VAC ( P T C 125 R loss ambx th − ° = W C 29 W 58 . 2 C 50 C 125 R th ° = ° − ° = rms 2 d out th diode ID R I V P ⋅ + ⋅ = () W 26 . 0 A 72 . 0 08 . 0 A 25 . 0 V 89 . 0 P 2 diode = ⋅ Ω + ⋅ = |
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