LT3752 datasheet(39/52 Pages) AD

39 / 52 page

LT3752/LT3752-1

Rev. C

For more information www.analog.com

The selection of the main transformer will depend on the

applications requirements : isolation voltage, power level,

maximumvolt-seconds,turnsratio,componentsize,power

losses and switching frequency.

Transformerconstructionusingtheplanarwindingtechnol-

ogy is typically chosen for minimizing leakage inductance

and reducing component height. Transformer core type is

usually a ferrite material for high frequency applications.

Find a family of transformers that meet both the isolation

and power level requirements of the application. The next

step is to find a transformer within that family which is

suitable for the application. The subsequent thought pro-

cess for the transformer design will include :

(1) Secondary turns (NS), core losses, temperature

rise, flux density, switching frequency

(2) Primary turns (NP), maximum duty cycle and reset

voltages

(3) Copper losses

The expression for secondary turns (NS) is given by,

NS = 108 VOUT/(fOSC • AC • BM)

where,

AC = cross-sectional area of the core in cm2

BM = maximum AC flux density desired

For flux density, choose a level which achieves an accept-

ablelevelofcoreloss/temperatureriseatagivenswitching

frequency. The transformer data sheet will provide curves

of core loss versus flux density at various switching fre-

quencies.Thedatasheetwillalsoprovidetemperaturerise

versus core loss. While choosing a value for BM to avoid

excessive core losses will usually allow enough headroom

for flux swing during input / load transients, still make

sure to stay well below the saturation flux density of the

transformer core. If needed, increasing NS will reduce flux

density. After calculating NS, the number of primary turns

(NP) can be calculated from,

NP = NS • DMAX VIN(MIN)/VOUT

where,

VIN(MIN) = minimum system input voltage

DMAX=maximumswitchdutycycleatVIN(MIN)(typically

chosen between 0.6 and 0.7)

At minimum input voltage the converter will run at a maxi-

mum duty cycle DMAX. A higher transformer turns ratio

(NP/NS) will create a higher DMAX but it will also require

higher voltages at the drain of the primary side switch to

resetthetransformer(seeprevioussectionsLosideActive

ClampTopologyandHisideActiveClampTopology).DMAX

values are typically chosen between 0.6 and 0.7. Even for

a given DMAX value, the loop must also provide protection

against duty cycles that may excessively exceed DMAX

during transients or faults. While most converters only

provide a fixed duty cycle clamp, the LT3752/LT3752-1

provideaprogrammablemaximumdutycycleclampDVSEC

that also moves inversely with input voltage.

The resulting function is that of a programmable volt-

secondclamp.Thisallowstheusertochooseatransformer

turns ratio for DMAX and then customize a maximum duty

cycle clamp DVSEC above DMAX for safety. DVSEC then

follows the natural duty cycle of the converter as a safety

guardrail (see previous section Programming Duty Cycle

Clamp).

Afterdecidingontheparticulartransformerandturnsratio,

the copper losses can then be approximated by,

PCU = D • I(Load)(MAX)2 (RSEC + (NS/NP)2 RPRI)

where,

D = switch duty cycle (choose nominal 0.5)

I(Load)(MAX) = maximum load current

APPLICATIONS INFORMATION

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