DER-42
Portable CD Player – LNK520P
November 18, 2004
4 Circuit Description
LinkSwitch LNK520P is used in a Flyback converter shown in Figure 3 to obtain 4.5V,
0.5A output from 90-265 Vac input. The bias winding output voltage is used provide
primary side feedback to LinkSwitch. Therefore, the bias winding is wound over the
secondary winding to obtain very good regulation. Special construction methods are used
for the transformer to obtain low No-Load input power, and at the same time to pass EMI
testing and have good AM (530 kHz – 1.6 MHz) reception quality.
4.1 Input EMI Filtering
Fusible resistor RF1 acts as fuse and also limits inrush current during power ON of the
power supply. Bridge diodes D1, D3, D5, D6 rectify AC voltage into DC voltage and
charges capacitors C3 and C4. Inductor L1, Ferrite bead L2, and capacitors C3 and C4
form a π filter and attenuate EMI noise.
4.2 LinkSwitch Primary
This design uses RCD (C1, D4, R1, and R2) clamping across primary winding to limit the
drain voltage below 700V, when the MOSFET inside U1 turns OFF. The capacitor C5
connected to C (control) pin of U1 stores energy and provide power to the internal circuit
of U1 and also to turn ON the U1’s Mosfet, during power-up and steady state operation.
C6, D7, and R4 form bias supply components, which provide feedback current to the
Control pin during steady state operation. The bias winding voltage is designed for about
6.4V to keep the No-Load input power to minimum.
4.3 Output Rectification
When U1 Mosfet is turned ON, current flows through transformer primary and stores
energy. When U1 is ON, output diode D2 is OFF. When the U1 Mosfet is OFF, D2 is
forward biased, and the stored energy is transferred to the secondary and stores in C2.
The snubber C7 and R5 across D2 will slow down D2 switching and improve EMI. L3 and
C8 form post filter to reduce output ripple voltage and also attenuate EMI noise.
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