Evaluates: MAX5051
4) Connect a 100µF, 100V bulk storage capacitor to
the top of the +V
IN
and -V
IN
pins.
5) Connect the positive terminal of a 36V to 72V power
supply to the +VIN terminal. Connect the power sup-
ply’s ground to the -VIN terminal.
6) Turn on the power supply above 36V and verify that
the voltmeter reads +3.3V.
Detailed Description
The MAX5051 EV kit is a 50W isolated forward convert-
er that provides +3.3V at up to 15A output. The circuit
can be powered from a ±36V to ±72V DC source. The
user should supply an additional 100µF bulk stor-
age capacitor between the input terminals (+VIN,
-VIN). This capacitor should be rated for 100V and be
able to carry 1.5A of ripple current. Lower ripple-cur-
rent-rated capacitors should be fine for short-term
operation.
The 50W forward converter achieves high efficiency by
using a clamped two-transistor power topology at the
input power stage. The PC board footprint is minimized
by using two external surface-mount, 8-pin SO N-chan-
nel, 100V-rated MOSFETs. Cycle-by-cycle current limit-
ing protects the converter against short circuits at the
output. For a continuous short circuit at the output, the
MAX5051’s fault integration feature provides hiccup
fault protection, thus greatly minimizing destructive
temperature rise. Current-sense resistor R17 senses
the current through the primary of transformer T1 and
turns off both external transistors N1 and N2 when the
trip level of 154mV (typ) is reached. The programmable
integrating fault protection allows transient overload
conditions to be ignored and is configured by resistor
R4 and capacitor C7.
The planar surface-mount transformer features a bias
winding that (along with diode D5, current-limiting resis-
tor R18, and reservoir capacitor C21) power the
MAX5051 once the input voltage is stable. Upon initial
input voltage application, bootstrap resistor R22 and
capacitor C21 enable the MAX5051 to start up within
approximately 70ms. No reset windings are required on
the transformer with a clamped two-transistor power
topology simplifying transformer design and maximizing
the available copper window in the transformer. When
both external primary-side transistors turn off, Schottky
diodes D2 and D3 recover the magnetic energy stored
in the core and feed it back to the input supply. The
transformer provides galvanic isolation up to 500V.
On the transformer’s secondary side, a 0.6V shunt reg-
ulator (MAX8515, U3) along with feedback resistors R1
and R2 provide voltage feedback to the primary side
through optocoupler U2. Remote output voltage sens-
ing is provided by the SENSE(+) and SENSE(-) pins for
accurate output voltage regulation across the load. The
MAX5051 receives the voltage feedback signal on the
primary side from biasing resistors R15, R16, and com-
pensation resistor-capacitor network R11/C17 and C24
connected to optocoupler U2.
Optocoupler U6 receives the MAX5051 synchronous
rectifier drive signal from the primary side and provides
the MAX5048 secondary-side high-speed MOSFET dri-
vers, U4 and U7, with a galvanically isolated signal.
MOSFET N4 forms a synchronous rectifier for free-
wheeling-diode D4 and MOSFET N3 forms the synchro-
nous rectifier for rectifier-diode D7. Voltage regulation
for U4, U6, and U7 is provided by a MAX5023 linear
regulator on the secondary side.
The MAX5051 controller switches at 250kHz frequency
set by resistor R21 and capacitor C1. The duty cycle is
varied to control energy transfer to the output. The
maximum duty cycle is 50% for the EV kit's forward
converter design.
The MAX5051 features output-voltage soft-start, thus
eliminating any output-voltage overshoots. Soft-start
allows the output voltage to slowly ramp up in a con-
trolled manner within approximately 3ms. Capacitor C5
sets the soft-start time.
The brownout UVLO threshold voltage is set by resis-
tors R5 and R6. This prevents the power supply from
operating below the programmed input supply voltage.
The four-layer PC board layout and component place-
ment have been designed to have an industry-standard
1/8th brick pinout. The actual PC board dimensions
(58.42mm x 41.65mm) of the power-supply board are
somewhat larger than that of 1/8th brick power supplies.
Evaluating Other Output Voltages, Current
Limits, Soft-Starts, and UVLOs
VOUT Output Voltage
The MAX5051 EV kit’s output (VOUT) is set to +3.3V by
feedback resistors (R1, R2). To generate output volt-
ages other than +3.3V (from +2.6V to +4.0V, limited by
the output-capacitor voltage rating), select different
voltage-divider resistors (R1, R2). Resistor R1 is typical-
ly chosen to be less than 25kΩ. Using the desired out-
put voltage, resistor R2 is then found by the following
equation:
where V
REF
= 0.6V
