Datasheet

MAX5941A/MAX5941B
IEEE 802.3af-Compliant Power-Over-Ethernet
Interface/PWM Controller for Power Devices
______________________________________________________________________________________ 19
4) The reset winding turns ratio (N
R
/N
P
) needs to be
low enough to guarantee that the entire energy in
the transformer is returned to V+ within the off cycle
at the maximum duty cycle. Use the following equa-
tion to determine the reset winding turns ratio:
where:
N
R
/N
P
= Reset winding turns ratio.
D
MAX
’ = Maximum value of maximum duty cycle:
Round N
R
to the nearest smallest integer.
The turns ratio of the reset winding (N
R
/N
P
) deter-
mines the peak voltage across the N-channel MOS-
FET.
Use the following equation to determine the maxi-
mum drain-source voltage across the N-channel
MOSFET:
V
DSMAX
= Maximum MOSFET drain-source voltage.
V
IN_MAX
= Maximum input voltage:
Choose MOSFETs with appropriate avalanche
power ratings to absorb any leakage energy.
5) Choose the tertiary winding turns ratio (N
T
/N
P
) so that
the minimum input voltage provides the minimum
operating voltage at V
DD
(13V). Use the following
equation to calculate the tertiary winding turns ratio:
where:
V
DDMIN
is the minimum V
DD
supply voltage (13V).
V
DDMAX
is the maximum V
DD
supply voltage (30V).
V
IN_MIN
is the minimum input voltage (30V).
V
IN_MAX
is the maximum input voltage (67V in this
design example).
N
P
is the number of turns of the primary winding.
N
T
is the number of turns of the tertiary winding:
Choose N
T
= 7.
6) Choose R
SENSE
according to the following equation:
where:
V
ILIM
is the current-sense comparator trip threshold
voltage (0.465V).
N
S
/N
P
is the secondary side turns ratio (5/14 in this
example).
I
OUTMAX
is the maximum DC output current (10A in
this example):
7) Choose the inductor value so that the peak ripple
current (LIR) in the inductor is between 10% and
20% of the maximum output current:
where V
D
is the output Schottky diode forward volt-
age drop (0.5V) and LIR is the ratio of inductor rip-
ple current to DC output current:
8) The size and ESR of the output filter capacitor deter-
mine the output ripple. Choose a capacitor with a
low ESR to yield the required ripple voltage.
Use the following equations to calculate the peak-to-
peak output ripple:
VV V
RIPPLE
RIPPLEESR RIPPLE C
=+
,,
22
L
-
≥
()
×
()
××
=µ
5 5 1 0 198
0 4 275 10
401
..
.
.
kHz A
H
L
V-
OUT
≥
+
()
×
()
×× ×
VD
LIR kHz I
DMIN
OUTMAX
1
2 275
R
SENSE
≤
××
=Ω
0 465
6
14
12 10
90 4
.
.
.
V
m
R
V
N
N
SENSE
ILIM
S
P
≤
××12.I
OUTMAX
13 7
4
36 7
14
639 767
..
..
30
1N
67
N
T
T
×≤ ≤ ×
≤≤
V
V
NN
V
V
N
DDMIN
IN_MIN
PT
DDMAX
IN_MAX
P
+
×≤≤
+
×
07
07
.
.
V1 +
14
14
DSMAX
≥×






=67 134VV
VV 1 +
N
N
DSMAX IN_MAX
P
R
≥×






N1
1- 0.5
0.5
R
≤× =414
NN
1-D
D
RP
MAX
MAX
≤×
′
′