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LTC4278

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) =

VIN(MIN) •DCMAX

• 1+ XMIN













APPLICATIONS INFORMATION

where XMINispeak-to-peakcurrentratioasdefinedearlier.

For each secondary-side power MOSFET, the peak cur-

rent is:

IPK(SEC) =

IOUT

1−DCMAX

• 1+ XMIN













Select a primary-side power MOSFET with a BVDSS

greater than:

BVDSS ≥IPK

LLKG

+ 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) =

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.

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