參數(shù)資料
型號(hào)： LTC3766MPUFD#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 316 kHz SWITCHING FREQ-MAX, PQCC28
封裝： 4 X 5 MM, LEAD FREE, PLASTIC, MO-220, QFN-28
文件頁(yè)數(shù)： 16/60頁(yè)
文件大?。?/td> 607K
代理商： LTC3766MPUFD#TRPBF
LTC3766
23
3766f
APPLICATIONS INFORMATION
transients when operating at minimum input voltage. A
value for DMAX of 0.65 to 0.70 is appropriate for most
applications.
Having selected a particular transformer, calculate the
copper losses associated with the transformer winding.
These losses are highest when operating at maximum
duty cycle and full load. However, it is better to evaluate
copper losses at the nominal operating point of 50% duty
cycle, where the losses are approximately:
PCU =
IMAX
( )2
2
RSEC +
NS
NP
2
RPRI
where RPRI and RSEC are the primary and secondary
winding resistances respectively, and IMAX is the maxi-
mum output current. An optimal transformer design has
a reasonable balance between copper and core losses. If
they are significantly different, then adjust the number of
secondary turns (and recalculate the needed turns ratio)
to achieve such a balance.
Inductor Value Calculation
The selection of an output inductor is essentially the same
as for a buck converter. For a given input and output volt-
age, the inductor value and operating frequency determine
the ripple current. The ripple current
IL increases with
higher VIN and decreases with higher inductance:
IL =
VOUT
fSWL
1–
VOUT
VIN
NP
NS
Accepting larger values of
IL allows the use of low in-
ductances, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
the ripple current is
IL = 0.3(IOUT(MAX)) for nominal VIN.
The maximum
IL occurs at the maximum input voltage.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of the more expensive ferrite cores. Actual
core loss is essentially independent of core size for a fixed
inductor value but it is very dependent on the inductance
selected.Astheinductanceincreases,corelossesdecrease.
Unfortunately, increased inductance requires more turns
of wire and therefore copper losses will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductancecollapsesabruptlywhenthepeakdesigncurrent
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Active Clamp Capacitor
The active clamp capacitor, CCL, stores the average reset
voltage of the transformer over many cycles. The voltage
on the clamp capacitor is generated by the transformer
coreresetcurrent,andwillintrinsicallyadjusttotheoptimal
reset voltage regardless of other parameters. The voltage
across the capacitor at full load is approximately given by:
VCL =
VIN2
VIN –1.15 VOUT
NP
NS
NP/NS is the main transformer turns ratio. The factor of
1.15 accounts for typical losses and delays. When PG and
AG on the LTC3765 are low, the bottom side of the clamp
capacitor is grounded, placing the reset voltage, VCL, on
the SWP node. When PG and AG are high, the top side of
the capacitor is grounded, and the voltage on the bottom
side of the capacitor is –VCL. Therefore the voltage seen
on the capacitor is also the voltage seen at the drains of
the PG and AG MOSFETs.
AsshowninFigure8,theVCLvoltagehasaminimumwhen
the converter is operating at 50%. For a given range on
VIN,therefore,themaximumclampvoltage(VCL(MAX))will
occur either at the minimum or maximum VIN, depending
on which input voltage causes the converter to operate
furthest from 50% duty cycle. The maximum VCL voltage
can be determined by substituting the maximum and
minimum values of VIN into this equation and selecting
the larger of the two. In order to leave room for overshoot,
相關(guān)PDF資料
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