AN1523 datasheet, PDF - EEWORLD Datasheet

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AN1523: Description11W FLYBACK CONVERTER FOR AUXILIARY POWER SUPPLY APPLICATION USING THE L6590; File Size374KB，24 Pages; ManufacturerSTMicroelectronics; Websitehttp://www.st.com/

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11W FLYBACK CONVERTER FOR AUXILIARY POWER SUPPLY APPLICATION USING THE L6590

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AN1523

APPLICATION NOTE

11W FLYBACK CONVERTER FOR

AUXILIARY POWER

SUPPLY APPLICATION USING THE L6590

by Claudio Spini

This document describes an 11W Switch Mode Power Supply reference design, dedicated to Consumer

Applications, e.g. TV chassis auxiliary power supply, low cost Set-top box or digital equipment. The

board accepts full range input voltage (90 to 265Vrms) and delivers 2 output voltages. It is based on the

monolithic controller L6590, integrating the controller and a POWERMOS and working at fixed frequen-

cy, PWM mode and including a stand-by function to minimize the power consumption during light load

operation. It incorporates also all the protections, offering a complete and very compact solution for low

power SMPS.

Introduction

Low power SMPS are today very popular in consumer applications for example like low-cost cable, terrestrial

decoders or high end TV chassis and the manufacturers need to design circuits with good performance, small

size with high cost effectiveness. An integrated monolithic solution controlling the SMPS like the L6590 makes

it a very suitable device, able to satisfy all the requirements of a compact and flexible solution, integrating all the

necessary functions to obtain a robust design just adding few external components. In this proposed reference

design, the board is thru-hole technology, without any heat sink. A specific application circuit fully tested is pro-

posed and the test results, including thermal and EMI, are enclosed in this document. The transformer data are

included too, making it a good way to achieve a very short time to market solution.

SMPS Main characteristics

INPUT AND OUTPUT VOLTAGES:

INPUT VOLTAGE:

Vout

Vin:

264 Vrms

45÷ 66 Hz

[V]

OUTPUT VOLTAGES AT FULL LOAD:

Iout

[A]

1.4

0.3

OUT

(W) = 11.1

Pout

[W]

7.5

3.6

STABILITY

±2%

±5%

STAND BY:

During the stand-by operation the power consumption from the mains has to be

≤1W,

when the circuit

delivers 50mA from the 5V output and the 12V is unloaded.

PROTECTIONS:

Overload ad short circuit on both outputs, with auto-restart at short removal. An OVP circuit for open-

loop protection.

SAFETY:

In acc. with EN60065, creepage and clearance minimum distance is 4.8mm

EMI:

In acc. with EN50022 Class B

March 2002

1/24

AN1523 APPLICATION NOTE

The output voltage regulation is performed by the secondary feedback on the 5V output. The feedback network

is the typical using a TL431 driving an optocoupler, in this case a PC817, and insuring the insulation required

by the safety regulation between primary and secondary. The opto-transistor drives directly the COMP pin of

the L6590 modulating the PWM internal block of the L6590. The stability of the 12V is guaranteed by the trans-

former coupling.

The input EMI filter is a classical LC-filter, 1-cell for differential and common mode noise. A NTC has been in-

serted in series with the bulk capacitor to prevent very high peak current at plug insertion, while a standard 5*20

fuse protects in case of catastrophic failures. The PCB type is single layer, FR-4, 2 oz (70

m) thickness. The

L6590 power dissipation is ensured by a copper area of 4 cm2 connected to primary return.

Here following some waveforms during the normal operation at full load are depicted:

Figure 1. Vds & Id @FULL LOAD

Vin = 115 Vrms - 50 Hz

Figure 2. Vds & Id @FULL LOAD

Vin = 220 Vrms - 50 Hz

CH1:

CH4:

PIN1

– DRAIN VOLTAGE

DRAIN CURRENT

CH1:

CH4:

PIN1

– DRAIN VOLTAGE

DRAIN CURRENT

The pictures of figure 1 and 2 show the drain voltage and current at the peak of the nominal input mains voltage

during normal operation at full load. The circuit works in continuous mode for the effect of the voltage ripple

across the input bulk capacitor at 115V while it goes in a depth discontinuous mode at 220V. Here are captured

the trace at the peak of the input voltage sine wave.

Figure 3 gives the measurement of the drain peak voltage at full load and maximum input mains voltage. The

voltage peak, which is 604V, guarantees a reliable operation of the L6590 thanks to a good margin against the

maximum BVDSS of the device, which is 700V. Hence, a derating of 86% is achieved in the worst mains line

condition. The maximum PIV of the diodes (on figure 4) has been measured during the worst operating condi-

tion at 265Vac and it is indicated on the right of each picture. The margin, with respect to the maximum voltage

sustained by the diodes, assures a safe operating condition for the devices, contributing to obtain a high MTBF

of the circuit, using the MIL-HDBK217 calculation method.

In figure 5 and 6 the most salient controller IC signals are represented. In both pictures, it is possible to distin-

guish clean waveforms free of hard spikes or noise that could affect the controller correct operation

3/24

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