AN1523 APPLICATION NOTE

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Electrical diagram

The SMPS topology is the standard Fly-back, working in continuous mode at low input voltage. Core of this

SMPS is the L6590, a monolithic device integrating the controller and a 700V MOSFET, available in Minidip or

SO-16 popular packages. In this design, the Minidip has been used. The switching frequency is fixed by an

internal oscillator at 65KHz during normal operation. When a light load is detected, the oscillator switches auto-

matically to 22KHz, thus increasing the stand-by performance of the circuit. At start-up, the L6590 is activated

by an internal current source that draws current from the DC bus and charges the capacitor C2. Thanks to this

circuit, the wake-up time is shorter than the conventional resistor solution and independent from the input mains

voltage. The current source is internally disconnected after that the Vcc voltage has reached the VccON value,

to prevent power dissipation during light load operation. During normal operation, the device is powered by the

transformer, via the diode D3. The network Q1, Q2, C6, R9, R10, R11 improves the circuit performance during

faults. The components C3 and R2 belong to the feedback loop. The power dissipation of the L6590 is ensured

by a copper area on the bottom side of the printed circuit board.

The transformer is a layer type, using Triple Insulation Wire for the secondary windings, manufactured by EL-

DOR in accordance with the EN60065. The transformer reflected voltage is ~105V and the ferrite core size is

a small, standard E20. The Transil D1 and the diode D2 clamp the peak of the leakage inductance voltage spike

at a safe level for the operation of the L6590, providing enough room for the leakage inductance voltage spike

with still margin for reliability.

The output rectifiers have been chosen in accordance with the maximum reverse voltage and their power dis-

sipation. Standard, low-cost, axial, fast recovery rectifiers have been selected in order to avoid transformer frac-

tional number of turns and to obtain the output voltage values as close as possible to the nominal ones. Of

course, using High-voltage Schottky rectifier the efficiency at full load would be higher but the cost and the out-

put voltage precision would be adversely affected. A small LC filter has been added on the +5V in order to filter

the high frequency ripple without increasing the output capacitors size.

D5

BYW100-200

C7

1000uF-25VYXF

C4

2N2-2KV(Y1)

C5

100N-250VacX2

C1

22uF-400V

D3

1N4148

C2

22uF-25V

3

2

14

L1

2*27MHB82731

T1

2362.0019rev. C

R3

560R

R4

2K4-1%

R6

2K4-1%

IC2

TL431ACZ

C9

100NF

C11

470uF-25VYXF

JP1

12V @0.3A

5V @1.4A

R1

12R

D4

BYW98-200

C3

2N2

OPT1

PC817

VIN: 88-265 Vrms

D2

STTA106

C8

220uF-10V-ZL

L2

4u7

R8

2K7

R2

6K8

1

2

3

45

6

7

8

GND

1

3

D6

DF04G

D1

BZW06-188

VCC

3

VFB

5

IC1

L6590 _MINIDIP

Q1

BC548

Q2

BC548

R9

1K0

R10

33K

R11

10K

C6

2u2-50V

R5

1K0

C10

330PF

R7

560R

R12

NTC_10R

F1

FUSE1

R13

4K7