MAX5942A/MAX5942B
IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
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V
OUT
= Output voltage (5V).
V
D1
= Voltage drop across D1 (typically 0.5V for
power Schottky diodes).
D
MAX
= Minimum value of maximum operating duty
cycle (44%).
V
IN_MIN
= Minimum input voltage (30V).
In this example:
Choose N
P
based on core losses and DC resis-
tance. Use the turns ratio to calculate N
S
, rounding
up to the nearest integer. In this example, N
P
= 14
and N
S
= 6.
For a forward converter, choose a transformer with a
magnetizing inductance in the neighborhood of
200µH. Energy stored in the magnetizing inductance
of a forward converter is not delivered to the load
and must be returned back to the input; this is
accomplished with the reset winding.
The transformer primary to secondary leakage
inductance should be less than 1µH. Note that all
leakage energy is dissipated across the MOSFET.
Snubber circuits may be used to direct some or all
of the leakage energy to be dissipated across a
resistor.
To calculate the minimum duty cycle (D
MIN
), use the
following equation:
=
where V
IN_MAX
is the maximum input voltage (67V).
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
) determines
the peak voltage across the N-channel MOSFET.
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 supply voltage (30V).
V
IN_MAX
is the maximum input supply 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.
