EPR-16 - LinkSwitch 2.75 W Charger/Adapter
17-May-04
11 Appendix B – LNK520P in the High-Side Configuration
11.1 Introduction
In applications with looser CV/CC limits, the lower cost LinkSwitch LNK520P can be used
in the high-side configuration, which may be attractive since the internal MOSFET drive
has been optimized, typically reducing radiated EMI by 6 dB to 10 dB. This appendix
details the changes necessary to replace U1 from a LNK501 to a LNK520 device. In
addition, resultant changes in the output characteristic, efficiency, no-load consumption
and radiated EMI are presented.
11.2 Comparison of LNK501 and LNK520
The LNK501 was designed for use in a high-side configuration, using the VOR across the
primary winding to provide output sensing and feedback. Since the feedback signals
differ, the internal control characteristics of the LNK501/LNK520 have therefore been
specifically optimized for high-side/low-side operation, respectively. The LNK520 was
designed for use in a low-side configuration, using an auxiliary or bias winding to sense
the output and provide feedback. In addition, the LNK520 has a wider I2f data sheet
tolerance than the LNK501.
When using the LNK520 device in the high-side configuration, the output voltage tends to
be higher at no-load and the CC region of the output characteristic is less linear, the
output current increasing as the output voltage reduces. The wider I2f specification of the
LNK520 increases the CC tolerance from approximately ±20% to at least ±25%.
In many applications, for example replacing a linear supply, the wider output tolerances
and linearity are acceptable.
The LNK520 has one key advantage over the LNK501. The internal MOSFET drive has
been optimized to reduce high frequency radiated EMI. Typically a 6 dB to 10 dB
improvement in radiated (>30 MHz) EMI performance is seen compared with the
LNK501.
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