Understanding Modern Power MOSFETs - Fairchild Semiconductor

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31 32 Selection Steps: Power Dissipation Junction Temperature = Maximum Ambient Temperature + Thermal Resistance x Power Dissipation Thermal Resistance = 100 K/W Junction Temp Max = 175 o C Ambient Temp Max = 85 o C Power Dissipation Max = (175 – 85)/100 = 0.9W = I load 2 x RDS(ON) (Switching losses have been ignored) Selection Steps: Determining R DS(ON) I load 2 x RDS(ON) = 0.9W (in the absence of switching losses) As I load = 4A, R DS(ON) needs to be less than 56 milliohms The R DS(ON) specification must be met at 175 o C Choose the smallest device (device with the highest R DS(ON) ) to meet this requirement: here the FDD16AN080A 16
PCB Package 33 34 Simple Thermal Models Silicon die PCB Package Dissipated power Silicon die R th + C th junction-case R th + C th case-ambient Dissipated power The upper thermal model considers just thermal resistance It implicitly assumes that the PCB and the package heat up instantly. In reality, all materials have a specific heat capacity. This is modeled in the lower thermal model with a thermal capacitance. Calculation of Die Temperature at 4A Continuous Current R th junction-case R th case-ambient • The calculation of die temperature using the previous formula will give a very conservative result • 100 x I 2 load x RDS(ON) + 85 = 100 x 16 x 0.037 + 85 = 144 o C • Reason: the RDS(ON) rating used is the 175 o C rating • So we calculate the temperature at 4A using iteration • The RDS(ON) versus temperature curve is approximated as a line • RDS(ON) = 1, T = 25 o C • RDS(ON) = 2, T = 160 o C • Here RDS(ON) is normalized with respect to the 25 o C value • The iteration spreadsheet is shown on the following page • The die temperature is around 125 o C 17
Page 1 and 2: 1 2 Understanding Modern Power MOSF
Page 3 and 4: 5 Introducing the P-channel MOSFET
Page 5 and 6: Design challenges •Size • Power
Page 7 and 8: V gs FDS6688S 13 14 V ds FDS6688S E
Page 9 and 10: 17 18 Example: FDD16AN080AN R DS(ON
Page 11 and 12: 21 22 Source Technology Trends: R D
Page 13 and 14: 25 DC Characteristics • The trans
Page 15: 29 30 Selection and Thermal Calcula
Page 19 and 20: 37 38 FDD26AN06A0 26 mΩ, 2 o C/W
Page 21 and 22: 41 42 The Importance of Avalanche C
Page 23 and 24: 45 46 FDD26AN06A0 26 mΩ, I as = 2
Page 25 and 26: 49 Second Stage of Switching • Wh
Page 27 and 28: 53 Final Stage of Switching • The
Page 29 and 30: 57 58 FDD26AN06A0 26 mΩ, Q g = 17
Page 31 and 32: 61 Synchronous Buck Topology • Ou
Page 33 and 34: 65 V IN Synchronous Buck Switches
Page 35 and 36: 69 Selecting Power MOSFETs for Sync

Understanding Modern Power MOSFETs - Fairchild Semiconductor

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