MAX5942A/MAX5942B
IEEE 802.3af Power-Over-Ethernet
Interface/PWM Controller for Power Devices
18 ______________________________________________________________________________________
PWM Comparator and Slope Compensation
An internal 275kHz oscillator determines the switching
frequency of the controller. At the beginning of each
cycle, NDRV switches the N-channel MOSFET on.
NDRV switches the external MOSFET off after the maxi-
mum duty cycle has been reached, regardless of the
feedback.
The MAX5942B uses an internal ramp generator for
slope compensation. The internal ramp signal is reset
at the beginning of each cycle and slews at 26mV/µs.
The PWM comparator uses the instantaneous current,
the error voltage, the internal reference, and the slope
compensation (MAX5942A only) to determine when to
switch the N-channel MOSFET off. In normal operation,
the N-channel MOSFET turns off when:
where I
PRIMARY
is the current through the N-channel
MOSFET, V
REF
is the 2.4V internal reference, V
EA
is the
output voltage of the internal amplifier, and V
SCOMP
is a
ramp function starting at zero and slewing at 26mV/µs
(MAX5942A only). When using the MAX5942A in a for-
ward-converter configuration, the following condition
must be met to avoid control-loop subharmonic oscilla-
tions:
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 using the MAX5942B:
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 reg-
ulator, 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 MAX5942B (44%).
To enable the use of MOSFETs with drain-source
breakdown voltages of less than 200V, use the
MAX5942B 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).
