LT3837EFE-PBF LINER [Linear Technology], LT3837EFE-PBF Datasheet - Page 22

• Manufacturer Part Number: LT3837EFE-PBF
• Description: Isolated No-Opto Synchronous Flyback Controller
• Manufacturer: LINER [Linear Technology]

LT3837
APPLICATIONS INFORMATION
where N
mary winding. L
and C
C
may be added to reduce the leakage inductance spike as
discussed earlier.
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 may operate 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 6).
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:
22
OSS
MILLER
BV
I
I
C
RMSPRI
RMSSEC
MILLER
DSS
P
of the primary-side power MOSFET). A snubber
is the primary-side capacitance (mostly from the
SP
is calculated from the gate charge curve included
≥ V
refl ects the turns ratio of that secondary-to-pri-
=
=
=
OUT
V
Q
IN MIN
B
LKG
1–
Figure 6. Gate Charge Curve
(
V
V
GS
+ V
–
DS
I
OUT
DC
Q
is the primary-side leakage inductance
P
)
IN(MAX)
A
Q
IN
MAX
a
A
GATE CHARGE (Q
DC
MILLER EFFECT
MAX
• N
SP
G
Q
b
)
B
DS(ON)
3825 F06
at the nominal
DSS
should
MILLER
DS
, a
The curve is done for a given V
for different V
computed C
the curve specifi ed V
With C
MOSFET power dissipation:
where:
(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 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, 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.
R
V
f
P
T
P
V
OSC
TH
J
D(SEC)
DPRI
IN MAX
DR
= T
(
MILLER
is the MOSFET gate threshold voltage
is the gate driver resistance approximately 10Ω
A
is the operating frequency.
A
is the ambient temperature and θ
=
+ P
= I
I
)
RMS PRI
•
MILLER
determined, calculate the primary-side power
RMS(SEC)
DS
P
D
IN
(
DC
• θ
voltages are estimated by multiplying the
(
J
M M AX
, iterate your calculations recomputing
IN
JA
)
by the ratio of the application V
2
DS
•
)
2
R
•
.
DS
• R
DS ON
R
DR
, so you can neglect transition
(
DS(ON)
•
)
DS
V
(
1 δ
GATE MAX
(1 + δ)
. The Miller capacitance
+
C
)
(
MILLER
+
JA
)
–
is the MOSFET
V
TH
•
f
OSC
DS
3837fa
to