Evaluates: MAX5042
The 40W forward converter achieves high efficiency by
using a clamped two-transistor power topology with both
power transistors integrated on the MAX5042 IC. Cycle-
by-cycle current limiting protects the converter against
short circuits at the output. Current-sense resistor R10
senses the current through the primary of transformer T1
and then turns off both internal transistors when the
156mV trip level is reached. For a continuous short cir-
cuit at the output, the MAX5042’s fault integration feature
provides hiccup fault protection, thus greatly minimizing
destructive temperature rise.
The planar surface-mount transformer features a bias
winding, which, along with diode D4, resistor R9, and
capacitor C7, powers the MAX5042 IC after PWM start-
up is complete. A reset winding is not required with a
clamped two-transistor power topology. Schottky
diodes D1 and D2 recover the magnetic energy stored
in the core and feed it back to the +VIN input when
both internal transistors turn off. The transformer pro-
vides galvanic isolation.
On the transformer’s secondary side, optically isolated
error amplifier U2 along with feedback resistors R1 and
R2 provide voltage feedback to the primary side. The
MAX5042 receives the voltage feedback signal on the
primary side. Biasing resistor R21 provides biasing for
the optocoupler transistor while the resistor/capacitor
network R6/C8 provides compensation.
Remote output-voltage sensing is provided by the
+SENSE and -SENSE for accurate output-voltage regu-
lation across the load. The soft-start feature allows the
output voltage to slowly ramp up in a controlled manner
within 4ms. The MAX5042 switches at a preconfigured
250kHz frequency set by resistor R15 and capacitor
C14. The output provides up to 8A of continuous cur-
rent when a cooling fan with at least 100LFM airflow is
used. Dual-diode D3’s heatsink is connected to SGND.
The 6-layer PC board layout and component placement
has been designed for the industry-standard half-brick
footprint and pinout. Resistor/capacitor network R14 and
C2 prevent voltage overshoot as a result of the ±VIN
input line inductance when hot plugging the EV kit.
Shutdown
Shutdown Mode
The MAX5042 EV kit features a jumper pad (JU1) and a
hole-pad (SHDN) to remotely shut down the hot-plug-
gable, 40W forward DC-DC converter. Once the
MAX5042 EV kit is shut down by either jumper method,
the power to the EV kit must be cycled on/off before the
MAX5042 starts switching again. Jumper pad JU1 can be
used to manually shut down. An isolated optocoupler with
an open-collector/drain transistor or relay contact can be
connected across jumper JU1 to remotely shut down the
EV kit.
Evaluating Other Output Voltages, Current
Limits, Soft-Starts and UVLOs
VOUT Output Voltage
The MAX5042 EV kit’s output (VOUT) is set to +5.0V by
feedback resistors R1 and R2. To generate output volt-
ages other than +5.0V (from +3.2V to +5.0V), select dif-
ferent voltage-divider resistors (R1, R2). Resistor R1 is
typically chosen to be less than 25kΩ. Using the
desired output voltage, resistor R2 is then found by the
following equation:
where VREF is 1.24V and VOUT is the desired output
voltage.
The maximum output current should be limited to less
than 8A. The usable output voltage range for the EV kit
is +3.2V to +5.0V. U2 and resistor R3 limit the minimum
output voltage (VOUT) to +3.2V.