07-May-2004
EPR-54 – LinkSwitch 2.75 W Low Cost Charger / Adapter
4 Circuit Description
The circuit schematic shown in Figure 3 shows a design that provides a constant voltage
/ constant current (CV/CC) output characteristic from a universal input voltage range of
85 VAC-265 VAC. This design delivers 2.75 W with nominal peak power point voltage of
5.5 V and a current of 500 mA. The overall operating envelope is shown in Figure 10.
The unit provides a CC operating range of ±25% over a case internal temperature range
of 25 °C to 65 °C, and a transformer primary inductance tolerance of ±10%.
Appendix A details the performance of a simplified EE16 transformer without using foil in
the construction. This may be a more attractive design for some magnetics vendors but
does result in a slight degradation in the CV regulation. See Appendix A for schematic,
transformer construction and performance curves.
The PCB layout includes provisions to allow a transformer based on an EE13 bobbin to
be fitted. An EE16 core size was selected for this design based on feedback that this is
generally the lowest cost core size.
4.1 Input Stage and EMI Filtering
The bridge rectifier, D1-D4, rectifies the AC input and is smoothed by C1 and C2, with
inductor L1 forming a π-filter to attenuate differential mode conducted EMI. Resistor RF1
is a fusible, flame proof type, providing protection from primary-side short circuits and line
surges and provides additional differential EMI filtering. The switching frequency of
42 kHz allows such a simple EMI filter to be used without the need for a Y capacitor while
still meeting international EMI standards.
It is recommended that RF1 be of wire wound construction to withstand input current
surges while the input capacitor charges (metal film type are not recommended), and be
compliant with safety flammability hazard requirements. Please consult your safety
agency representative for requirements specific to your end-use application.
Capacitors C1 and C2 are sized to maintain a minimum DC voltage of around 90 V at the
minimum AC input voltage. Their ESR should also be as low as possible to reduce
differential mode EMI generation. The value of L1 is selected to give acceptable
differential mode EMI attenuation with a current rating to meet the RMS input current at
low line (or acceptable temperature rise). Conducted emissions in this design are
compliant with EN55022B / CISPR 22B and FCC B limits with no input Y1 safety
capacitor.
4.2 LinkSwitch Primary and Output Feedback
The LNK520P contains the necessary functions to implement start-up and auto-restart
(output protection) operation, output constant voltage (CV) and constant-current (CC)
control.
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