參數(shù)資料
型號(hào)： LLT3837EFE
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 110 kHz SWITCHING FREQ-MAX, PDSO16
封裝： 4.40 MM, PLASTIC, TSSOP-16
文件頁數(shù)： 15/28頁
文件大小： 255K
代理商： LLT3837EFE
LT3837
22
3837fa
where NSP reects the turns ratio of that secondary-to-pri-
mary winding. LLKG is the primary-side leakage inductance
and CP is the primary-side capacitance (mostly from the
COSS of the primary-side power MOSFET). A snubber
may be added to reduce the leakage inductance spike as
discussed earlier.
For each secondary-side power MOSFET, the BVDSS should
be greater than:
BVDSS ≥ VOUT + VIN(MAX) NSP
Choose the primary side MOSFET RDS(ON) at the nominal
gate drive voltage (7.5V). The secondary side MOSFET
gate drive voltage depends on the gate drive method.
Primary side power MOSFET RMS current is given by:
I
P
VDC
RMSPRI
IN
IN MIN
MAX
=
()
For each secondary-side power MOSFET RMS current is
given by:
I
DC
RMSSEC
OUT
MAX
=
1–
Calculate MOSFET power dissipation next. Because the
primary-side power MOSFET may operate at high VDS, a
transition power loss term is included for accuracy. CMILLER
is the most critical parameter in determining the transition
loss, but is not directly specied on the data sheets.
CMILLER is calculated from the gate charge curve included
on most MOSFET data sheets (Figure 6).
APPLICATIONS INFORMATION
QA
VGS
ab
3825 F06
QB
MILLER EFFECT
GATE CHARGE (QG)
Figure 6. Gate Charge Curve
The at portion of the curve is the result of the Miller
(gate-to-drain) capacitance as the drain voltage drops.
The Miller capacitance is computed as:
C
QQ
V
MILLER
BA
DS
=
–
The curve is done for a given VDS. The Miller capacitance
for different VDS voltages are estimated by multiplying the
computed CMILLER by the ratio of the application VDS to
the curve specied VDS.
With CMILLERdetermined,calculatetheprimary-sidepower
MOSFET power dissipation:
PI
R
V
P
DPRI
RMS PRI
DS ON
IN MAX
IN
=+
()+
()
(
)
()
(
2
1
δ
M
MAX
IN
DR
MILLER
GATE MAX
TH
OSC
DC
R
C
VV
f
)
()
–
where:
RDR is the gate driver resistance approximately 10Ω
VTH is the MOSFET gate threshold voltage
fOSC is the operating frequency.
(1 +
δ) is generally given for a MOSFET in the form of a
normalized RDS(ON) vs temperature curve. If you don’t
have a curve, use
δ = 0.005/°C as an estimate.
The secondary-side power MOSFETs typically operate
at substantially lower VDS, so you can neglect transition
losses. The dissipation is calculated using:
PD(SEC) = IRMS(SEC)2 RDS(ON)(1 + δ)
With power dissipation known, the MOSFETs’ junction
temperatures are obtained from the equation:
TJ = TA + PD θJA
where TAistheambienttemperatureandθJAistheMOSFET
junction to ambient thermal resistance.
Once you have TJ, iterate your calculations recomputing
δ, power dissipations until convergence.
Gate Drive Node Consideration
The PG and SG gate drivers are strong drives to minimize
gate drive rise and fall times. This improves efciency
but the high frequency components of these signals can
cause problems. Keep the traces short and wide to reduce
parasitic inductance.
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