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LTC3872-1 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3872-1 Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page LTC3872-1 11 38721f For more information www.linear.com/LTC3872-1 Calculating Power MOSFET Switching and Conduction Losses and Junction Temperatures In order to calculate the junction temperature of the power MOSFET,thepowerdissipatedbythedevicemustbeknown. This power dissipation is a function of the duty cycle, the load current and the junction temperature itself (due to the positive temperature coefficient of its RDS(ON)). As a result, some iterative calculation is normally required to determineareasonablyaccuratevalue.Sincethecontroller is using the MOSFET as both a switching and a sensing element, care should be taken to ensure that the converter is capable of delivering the required load current over all operating conditions (line voltage and temperature), and for the worst-case specifications for VSENSE(MAX) and the RDS(ON) of the MOSFET listed in the manufacturer’s data sheet. ThepowerdissipatedbytheMOSFETinaboostconverteris: PFET = IO(MAX) 1– DMAX 2 • RDS(ON) •DMAX • ρT +k • VO 1.85 • IO(MAX) 1– DMAX ( ) • CRSS • f The first term in the equation above represents the I2R losses in the device, and the second term, the switching losses.Theconstant,k=1.7,isanempiricalfactorinversely related to the gate drive current and has the dimension of 1/current. From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following formula: TJ = TA + PFET • RTH(JA) The RTH(JA) to be used in this equation normally includes the RTH(JC) for the device plus the thermal resistance from the case to the ambient temperature (RTH(CA)). This value of TJ can then be compared to the original, assumed value used in the iterative calculation process. Output Diode Selection To maximize efficiency, a fast switching diode with low forwarddropandlowreverseleakageisdesired.Theoutput diode in a boost converter conducts current during the switch off-time. The peak reverse voltage that the diode must withstand is equal to the regulator output voltage. The average forward current in normal operation is equal to the output current, and the peak current is equal to the peak inductor current. ID(PEAK) =IL(PEAK) = 1+ χ 2 • IO(MAX) 1–DMAX The power dissipated by the diode is: PD = IO(MAX) • VD and the diode junction temperature is: TJ = TA + PD • RTH(JA) The RTH(JA) to be used in this equation normally includes the RTH(JC) for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. Remember to keep the diode lead lengths short and to observe proper switch-node layout (see Board Layout Checklist) to avoid excessive ringing and increased dis- sipation. Output Capacitor Selection Contributions of ESR (equivalent series resistance), ESL (equivalent series inductance) and the bulk capacitance mustbeconsideredwhenchoosingthecorrectcomponent for a given output ripple voltage. The effects of these three parameters (ESR, ESL and bulk C) on the output voltage ripple waveform are illustrated in Figure 6e for a typical boost converter. The choice of component(s) begins with the maximum acceptable ripple voltage (expressed as a percentage of the output voltage), and how this ripple should be divided between the ESR step and the charging/discharging DV. For the purpose of simplicity we will choose 2% for the maximum output ripple, to be divided equally between the ESRstepandthecharging/discharging DV.Thispercentage ripple will change, depending on the requirements of the application, and the equations provided below can easily be modified. For a 1% contribution to the total ripple voltage, the ESR of the output capacitor can be determined using the fol- lowing equation: ESRCOUT ≤ 0.01• VO IIN(PEAK) applicaTions inForMaTion |
Similar Part No. - LTC3872-1_15 |
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Similar Description - LTC3872-1_15 |
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