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LT8300

8300f

Typically, choose the transformer turns ratio to maximize

available output power. For low output voltages (3.3V or

5V), a larger N:1 turns ratio can be used with multiple

primarywindingsrelativetothesecondarytomaximizethe

transformer’s current gain (and output power). However,

remember that the SW pin sees a voltage that is equal

to the maximum input supply voltage plus the output

voltage multiplied by the turns ratio. In addition, leakage

inductance will cause a voltage spike (VLEAKAGE) on top of

this reflected voltage. This total quantity needs to remain

below the 150V absolute maximum rating of the SW pin

to prevent breakdown of the internal power switch. To-

gether these conditions place an upper limit on the turns

ratio, NPS, for a given application. Choose a turns ratio

low enough to ensure:

NPS <

150V

− VIN(MAX) − VLEAKAGE

VOUT + VF

For lower output power levels, choose a smaller N:1 turns

ratio to alleviate the SW pin voltage stress. Although a

1:N turns ratio makes it possible to have very high output

voltages without exceeding the breakdown voltage of the

internal power switch, the multiplied parasitic capacitance

through turns ratio coupled with the relatively resistive

150V internal power switch may cause the switch turn-on

currentspikeringingbeyond160nsleading-edgeblanking,

thereby producing light load instability in certain applica-

tions. So any 1:N turns ratio should be fully evaluated

before its use with the LT8300.

The turns ratio is an important element in the isolated

feedback scheme, and directly affects the output voltage

accuracy. Make sure the transformer manufacturer speci-

fies turns ratio accuracy within ±1%.

applicaTions inForMaTion

Saturation Current

Thecurrentinthetransformerwindingsshouldnotexceed

itsratedsaturationcurrent.Energyinjectedoncethecoreis

saturated will not be transferred to the secondary and will

instead be dissipated in the core. When designing custom

transformers to be used with the LT8300, the saturation

current should always be specified by the transformer

manufacturers.

Winding Resistance

Resistance in either the primary or secondary windings

will reduce overall power efficiency. Good output voltage

regulation will be maintained independent of winding re-

sistance due to the boundary/discontinuous conduction

mode operation of the LT8300.

Leakage Inductance and Snubbers

Transformer leakage inductance on either the primary or

secondary causes a voltage spike to appear on the primary

after the power switch turns off. This spike is increasingly

prominent at higher load currents where more stored en-

ergy must be dissipated. It is very important to minimize

transformer leakage inductance.

When designing an application, adequate margin should

be kept for the worst-case leakage voltage spikes even

under overload conditions. In most cases shown in Figure

5, the reflected output voltage on the primary plus VIN

should be kept below 120V. This leaves at least 30V margin

for the leakage spike across line and load conditions. A

larger voltage margin will be required for poorly wound

transformers or for excessive leakage inductance.

In addition to the voltage spikes, the leakage inductance

also causes the SW pin ringing for a while after the power

switch turns off. To prevent the voltage ringing falsely trig-

ger boundary mode detector, the LT8300 internally blanks

theboundarymodedetectorforapproximately250ns.Any

remaining voltage ringing after 250ns may turn the power

switch back on again before the secondary current falls

to zero. So the leakage inductance spike ringing should

be limited to less than 250ns.

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