LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 35

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
For each secondary-side power MOSFET, the BV
be greater than:
Choose the primary-side MOSFET R
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:
For each secondary-side power MOSFET RMS current is
given by:
Calculate MOSFET power dissipation next. Because the
primary-side power MOSFET operates at high V
transition power loss term is included for accuracy. C
is the most critical parameter in determining the transition
loss, but is not directly specifi ed on the data sheets.
C
on most MOSFET data sheets (Figure 16).
The fl 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:
The curve is done for a given V
for different V
computed C
the curve specifi ed V
MILLER
BV
I
I
C
RMS PRI
RMS SEC
MILLER
DSS
(
(
is calculated from the gate charge curve included
≥ V
)
)
=
=
MILLER
=
OUT
DS
Q
V
V
Figure 16. Gate Charge Curve
B
IN MIN
GS
V
voltages are estimated by multiplying the
− 1
−
+ V
DS
(
I
Q
by the ratio of the application V
OUT
DC
Q
IN(MAX)
A
a
DS
A
GATE CHARGE (Q
P
)
MILLER EFFECT
IN
MAX
.
DC
MAX
• N
DS
SP
G
Q
b
. The Miller capacitance
)
B
DS(ON)
42691 F16
at the nominal
DSS
should
MILLER
DS
DS
, a
to
With C
MOSFET power dissipation:
where:
(1 + δ) is generally given for a MOSFET in the form of a
normalized R
have a curve, use δ = 0.005/°C • ΔT for low voltage
MOSFETs.
The secondary-side power MOSFETs typically operate
at substantially lower V
losses. The dissipation is calculated using:
With power dissipation known, the MOSFETs’ junction
temperatures are obtained from the equation:
where T
junction to ambient thermal resistance.
Once you have T
δ and 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 effi ciency,
but the high frequency components of these signals can
cause problems. Keep the traces short and wide to reduce
parasitic inductance.
The parasitic inductance creates an LC tank with the
MOSFET gate capacitance. In less than ideal layouts, a
series resistance of 5Ω or more may help to dampen the
ringing at the expense of slightly slower rise and fall times
R
V
f
V
P
T
P
V
OSC
TH
J
GATE(MAX)
DIS(SEC)
D PRI
DR
IN MAX
(
= T
(
MILLER
is the MOSFET gate threshold voltage
is the gate driver resistance (≈10Ω)
is the operating frequency
A
A
)
is the ambient temperature and θ
=
+ P
)
I
•
= I
RMS PRI
determined, calculate the primary-side power
P
DS(ON)
DIS
DC
= 7.5V for this part
I
N N MAX
RMS(SEC)
(
(
J
MIN
• θ
iterate your calculations recomputing
)
JA
vs temperature curve. If you don’t
2
)
•
•
R
R
DS
2
DR
DS ON
• R
, so you can neglect transition
(
•
DS(ON)
V
)
GATE MAX
(
1 δ
+
C
(1 + δ)
LTC4269-1
(
MILLER
)
+
)
JA
−
is the MOSFET
V
TH
•
35
f
OSC
42691f