MAX5942BCSE Maxim Integrated, MAX5942BCSE Datasheet - Page 19

• Manufacturer Part Number: MAX5942BCSE
• Description: Power Switch ICs - POE / LAN
• Manufacturer: Maxim Integrated

=
4) The reset winding turns ratio (N
Choose N
tance. Use the turns ratio to calculate N
up to the nearest integer. In this example, N
and N
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
following equation:
where V
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
D
D
cycle (44%).
V
In this example:
V
V
power Schottky diodes).
IN_MIN
R
MAX
OUT
MAX
D1
/N
P
= Voltage drop across D1 (typically 0.5V for
’ = Maximum value of maximum duty cycle:
S
= Output voltage (5V).
= Minimum value of maximum operating duty
= Reset winding turns ratio.
= 6.
IN_MAX
Interface/PWM Controller for Power Devices
= Minimum input voltage (30V).
N
N
D
P
P
S
MIN
based on core losses and DC resis-
≥
______________________________________________________________________________________
5V + 0.5V 0.44
is the maximum input voltage (67V).
=
N
R
⎡
⎢
⎣
V
0 44 30
≤
IN_MAX
(
.
N
P
×
×
V
1-D
×
OUT
D
×
V
MAX
MAX
N
N
S
P
)
IEEE 802.3af Power-Over-Ethernet
′
⎤
⎥
⎦
′
=
R
- V
/N
0 395
D1
.
P
) needs to be
MIN
S
, rounding
), use the
P
= 14
5) Choose the tertiary winding turns ratio (N
Round N
The turns ratio of the reset winding (N
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
V
Choose MOSFETs with appropriate avalanche
power ratings to absorb any leakage energy.
the minimum input voltage provides the minimum
operating voltage at V
equation to calculate the tertiary winding turns ratio:
Choose N
where:
V
V
V
V
in this design example).
N
N
DSMAX
IN_MAX
DDMIN
DDMAX
IN_MIN
IN_MAX
P
T
is the number of turns of the tertiary winding:
is the number of turns of the primary winding.
is the minimum V
V
is the minimum input supply voltage (30V).
R
= Maximum MOSFET drain-source voltage.
= Maximum input voltage:
is the maximum V
is the maximum input supply voltage (67V
DSMAX
V
T
to the nearest smallest integer.
DSMAX
= 7.
6 39
13 7
30
.
V
V
N
.
DDMIN
DDMAX
R
≥
V
V
≤
×
≥
IN_MIN
IN_MAX
≤
67
N
1
V
1
4
T
IN_MAX
V
4
+
≤
≤
+
×
×
DD
0 7
N
7 67
0 7
⎛
⎜
⎝
1- 0.5
.
T
.
1 +
DD
.
0.5
DD
≤
(13V). Use the following
×
×
×
supply voltage (13V).
N
36 7
N
67
supply voltage (30V).
⎛
⎜
⎝
14
14
P
=
P
1 +
.
≤
14
⎞
⎟ =
⎠
N
×
T
14
R
134
N
N
≤
/N
P
R
P
⎞
⎟
⎠
V
T
) determines
/N
P
) so that
19