Design considerations

AN2811

6/25

Doc ID 14904 Rev 1

2

Design considerations

2.1

Selected topology

This is a 500 mA constant-current source conversion from 176 VAC ~ 264 VAC line input.

The specifications shown in Table 1 are for refrigerator lighting usage.

According to the specifications the maximum operating power is 3.5 watts. No power factor

correction circuit is required. Therefore, both buck and flyback topologies are suitable for this

application. Figure 2 shows the conventional buck converter while Figure 4 illustrates the

flyback converter. To convert high voltage to low voltage, a conventional buck converter just

requires a few components. Output current ripple is small due to Vout obtained from

inherent filter L1 and C1, thus the voltage and current stresses on these power components

are small. In order to properly drive the MOSFET (Q1), a controller and an additional

transformer are required. Additional winding with L1 to bias Q1 as well as a feedback current

to manage output in constant-current mode are needed.

Figure 2.

Conventional buck converter

Table 1.

Specifications

Parameter

Value

AC input

220 VAC ± 20%

Output current

500 mA

Output voltage

7 V max

Dimensions

30 mm x 30 mm

Isolation

Not required

Topology

Constant-current source