DER-56
UPS Battery Charger - LNK520
April 20, 2005
4 Circuit Description
The circuit schematic shown in Figure 2 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 4.8 W with nominal peak power point voltage of
12 V and a current of 400mA.
4.1 Input 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, flameproof type, providing protection from primary-side short circuits and line
surges and provides additional differential EMI filtering. The switching frequency of
42kHz allows a simple EMI filter to be used without the need for a Y capacitor while still
meeting international EMI standards.
Capacitors C1 and C2 are sized to maintain a minimum DC voltage of around 127 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.
When power is applied, high voltage DC appears at the DRAIN pin of LinkSwitch (U1).
The CONTROL pin capacitor C5 is then charged through a switched high voltage current
source connected internally between the DRAIN and CONTROL pins. When the
CONTROL pin reaches approximately 5.6 V relative to the SOURCE pin, the internal
current source is turned off. The internal control circuitry is activated and the high voltage
MOSFET starts to switch, using the energy in C5 to power the IC.
Diode D6 rectifies the output of the bias winding, which is then smoothed by C3 to
provide a DC voltage to be fed to the CONTROL pin via R4. Resistor R3 is added to filter
noise due to leakage inductance. The value of R4 is set such that, at the peak power
point, where the output is still in CV regulation, the CONTROL pin current is
approximately 2.2mA.
As the output load is increased, the peak power point (defined by 0.5 × L × I2 × f) is
exceeded. The output voltage and therefore primary side bias voltage reduce. The
reduction in the bias voltage results in a proportional reduction of CONTROL pin current,
which lowers the internal LinkSwitch current limit (current limit control).
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