LCS700-708
Bootstrap Circuit and HB Node Layout
and system-level ESD immunity. The resulting increase in thermal
resistance must be considered in the thermal design.
Please refer to Figure 10. Note the location of the bootstrap diode,
capacitor, resistor, and the HB trace routing. The objective is to
keep them away from the small signal components and traces,
such as the feedback optocoupler. Do not unnecessarily
increase the area of the PCB traces on this node, because it will
increase the dv/dt (capacitive) coupling to low-voltage circuits.
Transformer Secondary
The transformer secondary pins, output diodes, and main output
capacitors should be positioned close together and routed with
short thick traces. This is critical for secondary current symmetry
and to minimize output diode inverse voltage stress. The use of
ceramic capacitors allows placement between the transformer
secondary pins and the output rectifier, producing a very tight
layout. See Figure 11. The secondary winding halves should be
inter-twined together before they are wound on the bobbin. This
minimizes the leakage inductance between them and greatly
improves current symmetry and minimizes output diode inverse
voltage stress. For a 2-output design the half-windings of a given
output need to be intertwined.
Heat Sink Grounding
The exposed metal in the back of the HiperLCS package is
internally connected to the GROUND pin. If the HiperLCS has a
dedicated heat sink and there is no electrical insulator between
the device and the heat sink, the heat sink should be floating and
not electrically connected anywhere else. If the heat sink is
shared with other devices in the system, and the heat sink
requires grounding to minimize EMI, a thin insulator is strongly
recommended under the HiperLCS, for improved noise, surge,
DRAIN
SOURCE
Figure 9. Placement of B+ and B- High-Voltage Bypass Capacitors.
9
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