MAX5003-50WEVKIT Maxim Integrated Products, MAX5003-50WEVKIT Datasheet - Page 4

• Manufacturer Part Number: MAX5003-50WEVKIT
• Description: MAX5003-50WEVKIT Evaluation Kit For The MAX5003-50W
• Manufacturer: Maxim Integrated Products

Among the several power topologies available, the sin-
gle-transistor forward topology offers a simple and low-
cost solution and provides very good efficiency
throughout the operating power range. However, this
topology requires a transformer reset winding connect-
ed to pins T1–3 and T1–4 (Figure 7). The forward con-
verter was chosen because it offers higher power den-
sity and higher efficiency than a flyback converter at
these power levels. Transformer T1 provides 1500V iso-
lation between primary and secondary. Efficiency is fur-
ther improved by powering the control circuit from a
primary bias winding (T1–5, T1–6, Figure 7) after initial
startup. A 250kHz switching frequency was selected to
allow small form-factor transformer, inductor, and out-
put capacitors.
Key operating waveforms are always useful in under-
standing the operation of switching power supplies. A
10× oscilloscope probe is necessary for effective prob-
ing. A digital scope is very useful in capturing startup
sequences. However, extreme caution should be exer-
cised when probing live power supplies. For example,
shorting the drain-source terminals of Q1 while power is
applied is sure to produce a big spark and may dam-
age the EV kit.
Figure 5 shows the drain-to-source waveform of Q1.
Notice the leading-edge voltage spike. This is a result
of the energy stored in transformer T1’s leakage induc-
tance.
Figure 6 shows the voltage at the output of the sec-
ondary rectifier (cathode of D4).
MAX5003-50W Evaluation Kit
Figure 5. Drain-Source Voltage Waveform
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50V/div
The Power Circuit Topology
Key Operating Waveforms
400ns/div
MAX5003 fig05
As with any other switching power supply, component
placement is very important. Because of the primary-to-
secondary isolation, the primary and secondary
grounds are separated. Figure 10 clearly shows the
separation on both sides of the PC board. The layout of
the board can be changed to accommodate different
footprints. Also, the power MOSFET and output rectifier
should be mounted on a heatsink for best thermal man-
agement. In this implementation, both of these compo-
nents are on the noncomponent side of the board, with
their tabs mounted to the heatsink plate.
The critical layout considerations are as follows:
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Figure 6. Waveform at Cathode of D4
Distance from the secondary transformer leads to
diode D4 should be kept to a minimum. This will
improve EMI as well as the effective available
power transfer.
Bypass capacitors C4, C5, and C6 should be as
close as possible to T1–1.
The PC board trace connecting T1–2 to the drain of
Q1 should be as short as possible.
The current-sense resistor R6 should be as close as
possible to the source of Q1 and should return with a
very short trace either to the ground plane or to the
negative lead of bypass capacitors C4, C5, and C6.
The gate-drive loop, consisting of pin 14 of
MAX5003, R16, Q1, R6, and pin 13 of the
MAX5003, must be kept as short as possible and
preferably routed over a ground plane.
Relevant trace spacing (relating to trace creepage)
must be observed according to applicable safety
agency guidelines.
5V/div
Component Placement
PC Board Layout and
200ns/div
MAX5003 fig06