AN2950 datasheet(6/37 Pages) STMICROELECTRONICS

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Adapter features

AN2950

6/37

Doc ID 15494 Rev 1

Adapter features

Table 1 lists the electrical specifications of the demonstration board.

1.1

Circuit description

The power supply is set-up in a flyback topology. Its schematic is shown in Figure 2. The

input section includes the protection elements (fuse and NTC for inrush current limiting), a

filter for EMC suppression (C1, T2, C13), a diode bridge (BR1) and an electrolytic bulk

capacitor (C3) as the front-end AC-DC converter. The transformer uses a standard E25

ferrite core. A transil clamp network is used to demagnetize the leakage inductance.

At power-up, the DRAIN pin supplies the internal HV start-up current generator that charges

the C4 capacitor up to VDDon. At this point, the power MOSFET starts switching, the

generator is turned off and the IC is powered by the energy stored in C4 until the auxiliary

winding voltage becomes high enough to sustain the operation through D1 and R1.

The value of the resistor R3 between the CONT and GND pins is high enough that the

VIPer28’s current limit does not change with respect to the datasheet’s default value IDlim.

This resistor, in conjunction with D2, R14 and R15, is used to realize the overvoltage

protection and the feedforward correction function, as described further in this document.

The output rectifier D4 has been selected according to the calculated maximum reverse

voltage, forward voltage drop and power dissipation, and is a power Schottky type.

The output voltage regulation is performed by a secondary feedback with a TS431 driving

an optocoupler (in this case a PC817) ensuring the required insulation between the primary

and secondary. The opto-transistor drives directly the FB pin of the VIPer28, which is

connected to the compensation network made up by C6, C7 and R12.

A small LC filter has been added at the output in order to filter the high-frequency ripple

without increasing the size of the output capacitors, and a 100 nF capacitor has been placed

very close to the solder points of the output connector to limit the spike amplitude.

Table 1.

Electrical specification

Parameter

Symbol

Value

Input voltage range

VIN

[90 VRMS; 265 VRMS]

Output voltage

VOUT

5 V

Maximum output current

IOUTmax

2 A

Peak output current

IOUTpk

3 A

Precision of output regulation

VOUT_LF

±5%

High-frequency output voltage

ripple

VOUT_HF

50 mV

Maximum ambient operating

temperature

°C

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