White LED power supply

AN2067

6/25

DocID10973 Rev 2

2

White LED power supply

2.1

Schematic

The power topology is an offline flyback, working at a fixed frequency of 66 kHz. The overall

schematic is presented in Figure 2.

2.1.1

Primary section

On the left side of the schematic, the fuse F1, inrush current limiter CTN1, input filter T1,

followed by the rectifier BR1 and its bulk capacitance C3 can be found. R4, C4 and D4 build

the RCD clamper, to discharge the leakage inductance of the transformer.

D2, R2 and C5 are the rectifiers and filters of the primary auxiliary winding, used in forward

mode (refer to Section 3). This generates a voltage supply from 21 V up to 80 V,

proportional to turn ratio between main primary winding and auxiliary primary winding, and

A serial voltage regulator is required to supply the VIPer53-E with the correct voltage

higher than 80 V. This transistor may also dissipate 0.7 W when the input voltage is 250

The COMP pin filter is made of C8, R9 and C9.

2.1.2

Transformer

A current generator may have a large output voltage variation, according to the output load.

If the auxiliary winding is used in flyback mode, a wide voltage variation on auxiliary winding

may happen, as it is proportional to the reflected voltage. So, it is used in forward mode in

order to limit the voltage variation to supply the VIPer53-E.

In order to guarantee the functionality even with low output voltage load, an auxiliary

winding at secondary side has been added. This auxiliary winding is also used in forward

mode.