參數(shù)資料
型號(hào)： DC1561B
廠商： Linear Technology
文件頁(yè)數(shù)： 20/42頁(yè)
文件大小： 0K
描述： EVAL BOARD FOR LTC4278
標(biāo)準(zhǔn)包裝： 1
主要目的： 電源管理，以太網(wǎng)供電（POE）
嵌入式： 否
已用 IC / 零件： LTC4278
已供物品： 板
LTC4278
27
4278fc
APPLICATIONS INFORMATION
A final note—the susceptibility of the system to bistable
behavior is somewhat a function of the load current/
voltage characteristics. A load with resistive—i.e., I =
V/R behavior—is the most apt to be bistable. Capacitive
loads that exhibit I = V2/R behavior are less susceptible.
Secondary Leakage Inductance
Leakage inductance on the secondary forms an inductive
divider on the transformer secondary, reducing the size
of the flyback pulse. This increases the output voltage
target by a similar percentage. Note that unlike leakage
spike behavior, this phenomenon is independent of load.
Since the secondary leakage inductance is a constant
percentage of mutual inductance (within manufacturing
variations), the solution is to adjust the feedback resistive
divider ratio to compensate.
Winding Resistance Effects
Primary or secondary winding resistance acts to reduce
overallefficiency(POUT/PIN).Secondarywindingresistance
increases effective output impedance, degrading load
regulation. Load compensation can mitigate this to some
extent but a good design keeps parasitic resistances low.
Bifilar Winding
A bifilar, or similar winding, is a good way to minimize
troublesome leakage inductances. Bifilar windings also
improve coupling coefficients, and thus improve cross
regulation in multiple winding transformers. However,
tight coupling usually increases primary-to-secondary
capacitance and limits the primary-to-secondary
breakdown voltage, so is not always practical.
Primary Inductance
The transformer primary inductance, LP, is selected
based on the peak-to-peak ripple current ratio (X) in the
transformer relative to its maximum value. As a general
rule, keep X in the range of 20% to 40% (i.e., X = 0.2 to
0.4).Highervaluesofripplewillincreaseconductionlosses,
while lower values will require larger cores.
Ripplecurrentandpercentagerippleislargestatminimum
duty cycle; in other words, at the highest input voltage.
LP is calculated from the following equation.
LP =
VIN(MAX) DCMIN
(
)2
fOSC XMAX PIN
=
VIN(MAX) DCMIN
(
)2 Eff
fOSC XMAX POUT
where:
fOSC is the oscillator frequency
DCMIN is the DC at maximum input voltage
XMAX is ripple current ratio at maximum input voltage
Using common high power PoE values, a 48V (41V < VIN
< 57V) to 5V/5.3A converter with 90% efficiency, POUT=
26.5W and PIN = 29.5W. Using X = 0.4 N = 1/8 and fOSC
= 200kHz:
DCMIN =
1
+
N VIN(MAX)
VOUT
=
1
+ 1
8
57
5
= 41.2%
LP =
57V 0.412
(
)
2
200kHz 0.4 26.5W
=260H
Optimization might show that a more efficient solution
is obtained at higher peak current but lower inductance
and the associated winding series resistance. A simple
spreadsheet program is useful for looking at tradeoffs.
Transformer Core Selection
Once LP is known, the type of transformer is selected.
High efficiency converters use ferrite cores to minimize
core loss. Actual core loss is independent of core size for
afixedinductance,butdecreasesasinductanceincreases.
Sinceincreasedinductanceisaccomplishedthroughmore
turns of wire, copper losses increase. Thus, transformer
design balances core and copper losses. Remember that
increasedwindingresistancewilldegradecrossregulation
and increase the amount of load compensation required.
The main design goals for core selection are reducing
copper losses and preventing saturation. Ferrite core
material saturates hard, rapidly reducing inductance
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