02-May-2005

EP48 – 12 V, 1.44 W, Non-Isolated Buck Converter

3

Schematic

Figure 2 – EP48 Schematic.

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Circuit Description

4.1

Input Stage and EMI Filtering

The input stage is comprised of fusible resistor RF1, diodes D3 and D4, capacitors C4

and C5, and inductor L2. Two diodes are used to both increase the surge withstand to

2 kV and provide EMI gating (noise current only flows when the diodes conduct). Placing

D3 and D4 directly in series would meet the surge requirements, but without EMI gating,

conducted EMI on the neutral line would be higher.

Resistor RF1 is a flameproof, fusible, wire wound resistor.

It accomplishes several

functions: (a) limits inrush current to safe levels for rectifiers D3 and D4, (b) provides

differential mode noise attenuation and (c) acts as an input fuse in the event any other

component fails short circuit. As this component is used as a fuse, it should fail safely

open circuit without emitting smoke, fire or incandescent material to meet typical safety

requirements. To withstand the instantaneous inrush power dissipation, wire wound types

are recommended. Metal film resistors are not recommended.

4.2

LinkSwitch-TN

LinkSwitch-TN integrates a 700 V power MOSFET and control circuitry into a single low

cost IC. The internal fixed switching frequency of 66 kHz was selected to allow up to

120 mA of output current using a standard 1 mH inductor. Lower frequencies require

higher value, more costly inductors while higher frequencies increase EMI and cause

undesirable high di/dt values as the inductor value reduces.

The device is completely self-powered from the DRAIN pin with local supply decoupling

provided by a small 100 nF capacitor connected to the BYPASS pin.

Page 5 of 20

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