LT3757 datasheet(12/24 Pages) AD

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LT8300

Rev. A

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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 pri-

mary 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 current spike ringing beyond 160ns leading-edge

blanking, thereby producing light load instability in cer-

tain applications. 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

The current in the transformer windings should not exceed

its rated saturation current. Energy injected once the core

is 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 volt-

age regulation will be maintained independent of winding

resistance 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 energy must be dissipated. It is very impor-

tant 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 condi-

tions. 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

the boundary mode detector for approximately 250ns.

Any remaining voltage ringing after 250ns may turn the

power switch back on again before the secondary cur-

rent falls to zero. So the leakage inductance spike ringing

should be limited to less than 250ns.

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