AN2708 datasheet(32/42 Pages) STMICROELECTRONICS

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DALI master AC-DC adapter

AN2708

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DALI master AC-DC adapter

This is an offline wide-range double-output SMPS based on the VIPer12A-E. The first

output, 20 V at 100 mA, is dedicated to the bus communication allowing to address up to 64

slaves, while the second one delivers 5 V at 10 mA to the MASTER DALI microcontroller

thanks to a linear post-regulator.

The VIPer12A-E combines on the same silicon chip a dedicated current mode PWM

controller, a high voltage power MOSFET and the protection features (thermal, overcurrent,

and overvoltage) which increases the converter reliability and saves size, parts count and

cost.

The converter topology is an isolated flyback designed to work in discontinuous current

mode according to the following specifications:

5.1

Adapter description

The schematic of the board is shown in Figure 23.

The AC input is rectified by the diodes bridge and then filtered by the bulk capacitor C1, and

C2 to generate the high voltage DC.

The input EMI filter is a simple CLC PI filter for both differential and common mode noise

suppression.

An NTC limits the inrush current and ensures a reliable operation of the bridge at startup.

The switching frequency is fixed at 60 kHz by the IC internal oscillator allowing optimization

of the transformer size and cost. An RCD snubber circuit (R92, C59, D30) reduces the

leakage inductance voltage spike and the voltage ringing on the drain pin of VIPer12A-E.

As soon as the voltage is applied on the input of the converter the high voltage startup

current source connected to the drain pin is activated and starts to charge the Vdd capacitor

C8 by a constant current of 1mA. When the voltage across this capacitor reaches the Vddon

threshold (about 14 V) the VIPer12AS-E starts to switch. During normal operation the smart

Table 11.

SMPS operating conditions

Parameter

Value

Input voltage range

90 – 265 Vac

Input frequency range

50/60 Hz

Output voltage 1

V1=20 V

Output voltage 2

V2=5 V

Output current 1

I1=100 mA

Output current 2

I2=10 mA

Output power (peak)

2.2 W

Line regulation

+/- 1%

Load regulation

+/- 1%

EMI

EN55015

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