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34 page 
LTC4278 34 4278fc design evaluates the switcher for short-circuit protection and adds any additional circuitry to prevent destruction. Output Voltage Error Sources The LTC4278’s feedback sensing introduces additional minor sources of errors. The following is a summary list: • Theinternalbandgapvoltagereferencesetsthereference voltage for the feedback amplifier. The specifications detail its variation. • The external feedback resistive divider ratio directly affects regulated voltage. Use 1% components. • Leakage inductance on the transformer secondary reduces the effective secondary-to-feedback winding turns ratio (NS/NF) from its ideal value. This increases the output voltage target by a similar percentage. Since secondary leakage inductance is constant from part to part (within a tolerance) adjust the feedback resistor ratio to compensate. • The transformer secondary current flows through the impedances of the winding resistance, synchronous MOSFET RDS(ON) and output capacitor ESR. The DC equivalent current for these errors is higher than the load current because conduction occurs only during the converter’s off-time. So, divide the load current by (1 – DC). Iftheoutputloadcurrentisrelativelyconstant,thefeedback resistive divider is used to compensate for these losses. Otherwise, use the LTC4278 load compensation circuitry (see Load Compensation). If multiple output windings are used, the flyback winding will have a signal that represents an amalgamation of all these windings impedances. Take carethatyouexamineworst-caseloadingconditionswhen tweaking the voltages. Power MOSFET Selection ThepowerMOSFETsareselectedprimarilyonthecriteriaof on-resistance RDS(ON),inputcapacitance,drain-to-source breakdown voltage (BVDSS), maximum gate voltage (VGS) and maximum drain current (ID(MAX)). For the primary-side power MOSFET, the peak current is: IPK(PRI) = PIN VIN(MIN) •DCMAX • 1+ XMIN 2 APPLICATIONS INFORMATION where XMINispeak-to-peakcurrentratioasdefinedearlier. For each secondary-side power MOSFET, the peak cur- rent is: IPK(SEC) = IOUT 1−DCMAX • 1+ XMIN 2 Select a primary-side power MOSFET with a BVDSS greater than: BVDSS ≥IPK LLKG CP + VIN(MAX) + VOUT(MAX) NSP where NSP reflects the turns ratio of that secondary-to primary winding. LLKG is the primary-side leakage induc- tanceandCPistheprimary-sidecapacitance(mostlyfrom the drain capacitance (COSS) of the primary-side power MOSFET). A clamp may be added to reduce the leakage inductance as discussed. Foreachsecondary-sidepowerMOSFET,theBVDSSshould be greater than: BVDSS ≥ VOUT + VIN(MAX) • NSP Choose the primary-side MOSFET RDS(ON) at the nominal gatedrivevoltage(7.5V).Thesecondary-sideMOSFETgate drive voltage depends on the gate drive method. Primary-side power MOSFET RMS current is given by: IRMS(PRI) = PIN VIN(MIN) DCMAX For each secondary-side power MOSFET RMS current is given by: IRMS(SEC) = IOUT 1– DCMAX Calculate MOSFET power dissipation next. Because the primary-side power MOSFET operates at high VDS, a transitionpowerlosstermisincludedforaccuracy.CMILLER is the most critical parameter in determining the transition loss, but is not directly specified on the data sheets. |
