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AN4149 Scheda tecnica(PDF) 21 Page - STMicroelectronics |
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AN4149 Scheda tecnica(HTML) 21 Page - STMicroelectronics |
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21 / 43 page ![]() DocID023523 Rev 2 21/43 AN4149 Designing a CCM FOT-controlled PFC The MOSFET total losses at minimum input voltage (VACmin) is the sum of the three previous losses from equation 54, equation 55, and equation 60: Equation 61 From equation 61, using the data relevant to the MOSFET selected, it can be observed that the maximum total losses, occurring at VACmin and full load, is around 5 W. From this number and the given maximum ambient temperature, the total maximum thermal resistance required to keep the MOSFET's junction temperature below 125 °C is: Equation 62 As the result of equation 62 is much lower than the junction-ambient thermal resistance given in the MOSFET datasheet for the selected device package (62.5 °C/W), a heatsink must be used. Figure 12 shows the trend of the total losses on the line voltage for the two selected STF21N65M5 MOSFETs. Figure 12. Total MOSFETs losses in the 350 W FOT PFC ( ) () () W W W W V P VAC P VAC P VAC P VAC P loss cap sw cond loss 23 . 5 1 2 . 2 02 . 2 ) 90 ( ) ( = + + = + + = ) ( 125 VAC P T C R loss ambx th − ° = W C W C C R th ° = ° − ° = 3 . 14 23 . 5 50 125 AM13311v1 80 100 120 140 160 180 200 220 240 260 0 1 2 3 4 5 6 Conduction losses Switching losses Capacitive losses Total losses Power losses vs Input voltage (full load) Input voltage (Vac) |
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