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LTC3873ETS8-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3873ETS8-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LTC3873 11 3873fa plus the secondary-to-primary referred voltage of the flyback pulse (including leakage spike) must not exceed the allowed external MOSFET breakdown rating. Leakage Inductance Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the output switch (Q1) turn-off. This is increasingly prominent at higher load currents where more stored energy must be dissipated. In some cases a “snubber” circuit will be required to avoid overvoltage breakdown at the MOSFET’s drain node. Application Note 19 is a good reference on snubber design. A bifilar or similar winding technique is a good way to minimize troublesome leakage inductances. However, remember that this will limit the primary-to- secondary breakdown voltage, so bifilar winding is not always practical. Power MOSFET Selection The power MOSFET serves two purposes in the LTC3873: it represents the main switching element in the power path and its RDS(ON) represents the current sensing element for the control loop. Important parameters for the power MOSFET include the drain-to-source breakdown voltage (BVDSS), the threshold voltage (VGS(TH)), the on-resistance (RDS(ON))versusgate-to-sourcevoltage,thegate-to-source and gate-to-drain charges (QGS and QGD, respectively), the maximum drain current (ID(MAX)) and the MOSFET’s thermal resistances (RTH(JC) and RTH(JA)). For boost applications with RDS(ON) sensing, refer to the LTC3872 data sheet for the selection of MOSFET RDS(ON). MOSFETs have conduction losses (I2R) and switching losses. For VDS < 20V, high current efficiency generally improves with large MOSFETs with low RDS(ON), while for VDS > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower reverse transfer capacitance, CRSS, actually provides higher efficiency. Output Capacitors The output capacitor is normally chosen by its effective series resistance (ESR), which determines output ripple voltage and affects efficiency. Low ESR ceramic capaci- tors are often used to minimize the output ripple. Boost regulators have large RMS ripple current in the output capacitor that must be rated to handle the current. The output ripple current (RMS) is: II VV V RMS COUT OUT MAX OUT IN MIN IN MIN () ( ) () () • – ≈ Output ripple is then simply: VOUT = RESR(ΔIL(RMS)) The output capacitor for flyback converter should have a ripple current rating greater than: II D D RMS OUT MAX MAX = • – 1 Input Capacitors The input capacitor of a boost converter is less critical due to the fact that the input current waveform is triangular, and does not contain large square wave currents as found in the output capacitor. The input voltage source impedance determines the size of the capacitor that is typically 10μF to 100μF. A low ESR is recommended although not as critical as the output capacitor can be on the order of 0.3 Ω. The RMS input ripple current for a boost converter is: I V Lf D RMS CIN IN MIN MAX () () .• • • = 03 Please note that the input capacitor can see a very high surge current when a battery is suddenly connected to the input of the converter and solid tantalum capacitors can fail catastrophically under these conditions. APPLICATIONS INFORMATION |
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