AN2708 datasheet(33/42 Pages) STMICROELECTRONICS

33 / 42 page

AN2708

DALI master AC-DC adapter

33/42

power IC is powered by the auxiliary winding of the transformer via the diode D31. No spike

killer for the auxiliary voltage fluctuations is needed thanks to the wide range of the Vdd pin

(9-38 V). The primary current is measured using the integrated current sensing for current

mode operation.

The output rectifier D29 has been chosen in accordance with the maximum reverse voltage

and power dissipation. In particular a 1 A - 150 V power Schottky, type STPS1150, has

been selected.

The output voltage regulation is performed by secondary feedback on the 20 V output while

the 5 V output, is linearly post-regulated from the 20 V output. This operation is performed

by a low drop voltage regulator, L78L05CZ, in the TO92 package. The feedback network

consists of a programmable voltage reference, TL431, driving an optocoupler which ensures

the required insulation between the primary and secondary sections. The optotransistor

drives directly the VIPer12A-E feedback pin which controls the operation of the IC.

A small LC filter has been added on the 20 V output in order to reduce the high frequency

ripple with reasonable output capacitor value.

The flyback transformer is a layer type based on the EF13 core and Fi 324 ferrite,

manufactured by Vogt, and ensures safety insulation in accordance with the EN60950.

Figure 26 shows the main features of the transformer. The power supply has been

implemented on a double-sided 35 µm PCB in FR-4, sizing 81 x 37 mm.

Figure 23.

Adapter schematic

J17

Input 250V Con

R98

150K

R96

33K

C65

10nF

U17

PC817

U19

TL431

C64

100nF

R99

4.7K

50V

VIN

VOUT

U16

L78L05CZ

SL 060 918 11 01

C62

2.2uF

C58

470pF

J10

Supply Voltage

R92

56K

400V

D30

STTH1L06

100uH 600mA

35V

NTC2

10R @ 25°

FUSE2

0.5A

C56

2.2uF

C57

2.2uF

1mH 130mA

R94

10R

35V

D29

STPS1150

C59

150uF

R97

10K

1kV

C60

22uF

C63

10uF

U18

VIPer12A

20V@100mA

C61

2.2nF Y 1

5V@10mA

D31

1N4148

T1: SMT

- EF12.6/3.7core, Fi324 f errite

- 0.16mm gap f or 2mH primary inductance

- Primary : 135 turns (0.14mm-AWG35)

- Secondary : 28 turns (0.36mm-AWG27)

- Auxiliary : 21 turns (0.05mm-AWG44)

BRIDGE2

DF06

R93

560R

R95

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