The converter is powered by the high-voltage DC bus, ranging from 90 to 400 VDC, generated by the

front-end AC-DC stage (bridge rectifier + input capacitor) shared with the main SMPS (power factor cor-

rected or not).

The output is post-regulated in order to provide a better regulation and supplies the

µP as well as the

logic circuit needed to wake up the system.

The auxiliary winding will be properly designed so as to supply the controller(s) of the main SMPS be-

sides powering the L6561. To minimise component count, a primary sensing feedback technique is

used.

The auxiliary winding is used also by the ZCD circuit for detecting transformer’s full demagnetisation and

turning on the MOSFET to start a new switching cycle (TM operation). The resistor driving the ZCD pin

is in the ten k

Ω but can be optimised so as to achieve a "quasi zero-voltage turn-on" as described in Ref.

[1]. The optimum value depends mainly on the inductance of transformer’s primary winding and on the

With the component values shown in fig. 3 the wake-up time of the converter, that is the time the system

In fig. 4, the circuit of fig. 3 is proposed with a different power rating: 15W output power so as to be able

to support USB function in computer equipment. The modifications concern the MOSFET, the trans-

former and the sense resistor on the primary side, the catch diode and the filter capacitors on the secon-

dary side. They all have been increased in size.

Fig. 5 shows another example of low-power TM flyback application, an AC-DC adapter for battery

charger of cellular phones. The system looks very simple and very few parts are required.

The feedback uses a popular arrangement making use of a TL431 as secondary reference/error ampli-

fier and of an optocoupler for transferring the control signal to the primary side. This provides very good

regulation of the output voltage and galvanic isolation from the primary side at the same time.

The self-supply winding both powers the L6561and provides transformer’s demagnetisation signal to the

ZCD pin. The start-up circuit arrangement and its component values ensures that the wake-up time of

In fig. 6 an example of multi-output SMPS for inkjet printer is presented. The converter accepts input

voltages from 85 to 270 Vac and is rated for 40W output power. The 28V output is used for motors, the

12V output for the printhead and the 5V bus supplies the logic circuitry.

2

1

L6561

4

7

3

5

6

8

5 Vdc / 3A

Input bulk capacitor

of the main SMPS

470 k

Ω

BZW04-154

STTA106

47 µF

47 k

Ω

100 nF

4.7 nF

STP3NB60FP

STPS560B

22

Ω 1N4148

1

Ω

Vin=90 to 400 Vdc

7.5 k

Ω

33 k

Ω

22

Ω

to L4990A or L5991A

UC3843A/B or UC3845A/B

or L4981A

L4955V5.1

13

2

TRANSFORMER SPECS:

CORE: E20x10x6, 3C85 material or equivalent

≈0.5 mm air gap for a primary inductance of 0.8 mH

N1: 2 series windings 48 T each, AWG30 (

∅ 0.30 mm)

N2: 8 T, 2xAWG22 (

∅ 0.71 mm)

N3: 15 T, AWG32 (

∅ 0.24 mm)

N1

N2

N3

1N4148

47 µF

Figure 4. 15W, Wide-range, Auxiliary Power Supply supporting USB function

AN1060 APPLICATION NOTE

4/11