LT8300
applicaTions inForMaTion
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
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.
inductance will cause a voltage spike (V
) on top of
LEAKAGE
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
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.
ratio, N , for a given application. Choose a turns ratio
PS
low enough to ensure:
150V − VIN(MAX) − VLEAKAGE
NPS <
Leakage Inductance and Snubbers
VOUT + V
F
Transformer leakage inductance on either the primary or
secondarycausesavoltagespiketoappearontheprimary
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.
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
internalpowerswitch, themultipliedparasiticcapacitance
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.
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 V
IN
shouldbekeptbelow120V. Thisleavesatleast30Vmargin
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.
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%.
In addition to the voltage spikes, the leakage inductance
also causes the SW pin ringing for a while after the power
switchturnsoff. Topreventthevoltageringingfalselytrig-
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.
8300f
13
Linear [ Linear ]
