MAX5941BCSE-T Maxim Integrated, MAX5941BCSE-T Datasheet - Page 18

• Manufacturer Part Number: MAX5941BCSE-T
• Description: Power Switch ICs - POE / LAN
• Manufacturer: Maxim Integrated

where k = 0.75 to 1, and N
turns on the secondary and primary side of the trans-
former, respectively. L is the output filter inductor. This
makes the output inductor current downslope as refer-
enced across R
tion. The controller responds to transients within one
cycle when this condition is met.
NDRV drives an N-channel MOSFET. NDRV sources
and sinks large transient currents to charge and dis-
charge the MOSFET gate. To support such switching
transients, bypass V
average current as a result of switching the MOSFET is
the product of the total gate charge and the operating
frequency. It is this current plus the DC quiescent cur-
rent that determines the total operating current.
The following is a general procedure for designing a
forward converter (Figure 5) using the MAX5941B:
1) Determine the requirements.
2) Set the output voltage.
3) Calculate the transformer primary to secondary
4) Calculate the reset to primary winding turns ratio.
5) Calculate the tertiary to primary winding turns
6) Calculate the current-sense resistor value.
7) Calculate the output inductor value.
8) Select the output capacitor.
The circuit in Figure 5 was designed as follows:
1) 30V ≤ V
2) To set the output voltage, calculate the values of
IEEE 802.3af-Compliant Power-Over-Ethernet
Interface/PWM Controller for Power Devices
18
winding turns ratio.
ratio.
50mV. Turn-on threshold is set at 38.6V.
resistors R1 and R2 according to the following
equation:
where V
regulator, and R
Figures 5 and 6.
______________________________________________________________________________________
IN
N
N
REF
S
P
≤ 67V, V
×
N-Channel MOSFET Gate Driver
Applications Information
k R
is the reference voltage of the shunt
SENSE
×
1
SENSE
CC
and R
OUT
V
V
OUT
REF
equal to the slope compensa-
with a ceramic capacitor. The
L
= 5V, I
S
2
×
=
V
and N
are the resistors shown in
R
OUT
1
R
+
OUT
2
R
Design Example
=
P
2
26
are the number of
= 10A, V
mV
/
µ
s
RIPPLE
≤
3) The turns ratio of the transformer is calculated based
=
on the minimum input voltage and the lower limit of
the maximum duty cycle for the MAX5941B (44%).
To enable the use of MOSFETs with drain-source
breakdown voltages of less than 200V, use the
MAX5941B with the 50% maximum duty cycle.
Calculate the turns ratio according to the following
equation:
D
cycle (44%).
V
In this example:
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 will be dissipated across the MOS-
FET. 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
where:
N
turns and N
V
V
power Schottky diodes).
IN_MIN
OUT
MAX
D1
S
/N
= Voltage drop across D1 (typically 0.5V for
P
S
= Output voltage (5V).
= Minimum value of maximum operating duty
= Turns ratio (N
= 6.
IN_MAX
= Minimum Input voltage (30V).
D
P
MIN
N
N
P
N
N
based on core losses and DC resis-
P
S
is the number of primary turns).
P
S
=
≥
is the maximum input voltage (67V).
≥
V
V
5V + 0.5V 0.44
IN_MAX
OUT
D
0 44 30
MAX
(
.
S
+
V
(
is the number of secondary
OUT
V
×
×
×
D1
V
N
N
×
IN_MIN
P
S
×
V
D
- V
MAX
D1
)
=
= 17 7 .
)
0 395
.
MIN
S
, rounding
), use the
P
= 14