LT8303 datasheet(13/24 Pages) LINER

13 / 24 page

LT8303

8303f

For more information www.linear.com/LT8303

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

primary windings relative to the secondary to maximize

the 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. Together 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 LT8303.

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 LT8303, 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 LT8303.

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.

Whendesigninganapplication,adequatemarginshouldbe

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 trans-

formers 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 LT8303 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.

LT8303 Datasheet(PDF) 13 Page - Linear Technology

LT8303 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL