MAX5941A/MAX5941B
IEEE 802.3af-Compliant Power-Over-Ethernet
Interface/PWM Controller for Power Devices
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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:
