MAX5941A/MAX5941B
IEEE 802.3af-Compliant Power-Over-Ethernet
Interface/PWM Controller for Power Devices
18 ______________________________________________________________________________________
where k = 0.75 to 1, and N
S
and N
P
are the number of
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
SENSE
equal to the slope compensa-
tion. The controller responds to transients within one
cycle when this condition is met.
N-Channel MOSFET Gate Driver
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
CC
with a ceramic capacitor. The
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.
Applications Information
Design Example
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
winding turns ratio.
4) Calculate the reset to primary winding turns ratio.
5) Calculate the tertiary to primary winding turns
ratio.
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
IN
≤ 67V, V
OUT
= 5V, I
OUT
= 10A, V
RIPPLE
≤
50mV. Turn-on threshold is set at 38.6V.
2) To set the output voltage, calculate the values of
resistors R1 and R2 according to the following
equation:
where V
REF
is the reference voltage of the shunt
regulator, and R
1
and R
2
are the resistors shown in
Figures 5 and 6.
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:
where:
N
S
/N
P
= Turns ratio (N
S
is the number of secondary
turns and N
P
is the number of primary turns).
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 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
MIN
), use the
following equation:
=
where V
IN_MAX
is the maximum input voltage (67V).
