AN4026 datasheet(6/40 Pages) STMICROELECTRONICS

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Main characteristics and circuit description

AN4026

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Doc ID 022603 Rev 1

Main characteristics and circuit description

The main features of the SMPS are listed here below:

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Universal input mains range: from 90 to 264 Vac - frequency from 45 to 65 Hz

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Output voltage: 19 V at 4.75 A continuous operation

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Mains harmonics: meets EN61000-3-2 Class-D and JEITA-MITI Class-D

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No load mains consumption: according to ENERGY STAR 2.0 for external power

supplies

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Average efficiency: according to ENERGY STAR 2.0 for external power supplies

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Light load efficiency: according to ErP Lot 6 Tier2 requirements

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EMI: within EN55022-Class-B limits

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Safety: meets EN60950

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Dimensions: 65 x 151 mm, 25 mm components maximum height

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PCB: double side, 70 µm, FR-4, mixed PTH/SMT.

The circuit is made up of two stages: a front-end PFC using the L6563H and an LLC

resonant converter featuring the L6699.

The PFC stage works as pre-regulator and powers the resonant stage with a constant

voltage of 400 V. The downstream converter operates only if the PFC is working and

regulating its output voltage. In this way, the resonant stage can be optimized for a narrow

input voltage range improving the efficiency of the primary side power components.

1.1

Startup sequence

As previously indicated, the PFC acts as master and the resonant stage can operate only if

the PFC output is delivering its rated output voltage. Therefore the circuit is designed so that

at startup the PFC starts first, the downstream converter then turns on by means of the

LINE pin (#7). At the beginning, the L6563H is supplied by the integrated high-voltage

startup circuit; once the PFC starts switching, a charging pump connected to the PFC

inductor supplies both PFC and resonant controllers. Once both stages are working, the

controllers are supplied also by the auxiliary winding of the resonant transformer, assuring

correct supply voltage even during standby operation.

After reaching the turn-on threshold on the VCC pin the L6563H integrated HV startup

circuit is turned off and it is therefore not dissipative during normal operation, significantly

contributing to the reduction of input power consumption once the power supply operates at

light load, and meeting the standby worldwide efficiency standards that are currently

required.

1.2

Brownout protection

Brownout protection prevents the circuit from working with abnormal mains levels. It is easily

achieved using the pin RUN (#12) of the L6563H: this pin is connected through a resistor

divider to the pin VFF (#5) which provides a DC voltage the same as the peak of the MULT

pin (#3) signal which is a partition of the rectified mains input voltage. An L6563H internal

comparator allows IC operations only if the mains level is correct, within the nominal limits,

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