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LT8301MP 查看數據表(PDF) - Analog Devices

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LT8301MP Datasheet PDF : 24 Pages
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LT8301
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 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 65V 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
<
65V
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 may cause the switch turn-on cur-
rent spike ringing beyond 170ns leading-edge blanking,
thereby producing light load instability in certain applica-
tions. So any 1:N turns ratio should be fully evaluated
before its use with the LT8301.
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%.
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 LT8301, 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 LT8301.
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 50V. This leaves at least 15V
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-
gering the boundary mode detector, the LT8301 internally
blanks the boundary mode detector for approximately
350ns. Any remaining voltage ringing after 350ns may
turn the power switch back on again before the second-
ary current falls to zero. So the leakage inductance spike
ringing should be limited to less than 350ns.
Rev. B
12
For more information www.analog.com

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