ST AN1060 Application note

AN1060

®

APPLICATION NOTE

FLYBACK CONVERTERS WITH THE L6561 PFC

CONTROLLER

by C. Adragna & G. Gattavari

The L6561, controller specifically designed for Power Factor Correction (PFC) circuits, may be suc­cessfully used in flyback converters as well.

The excellent performance of the device, along with its characteristics in terms of low current con­sumption, makes L6561-based flyback converters really attractive in medium-low power applications.

There are basically three different configurations that an L6561- bas ed flyback converter can assume,
each of them with its own characteristics, bene fits and peculiarities. This paper describes these con­figurations and highlights advantages/drawbacks with the aim of identifying the most suitable appli­cations they can fit.

INTRODUCTION

Common practice bounds their use in conventional boost PFC stages, yet Transition Mode (TM) Power
Factor Corrector IC’s can be used in applications different from those they are primarily intended for.

This is particularly true for the L6561, PFC controller for medium-low power applications, because of its
peculiar characteristics.

Reference [2] presents a special example showing how to extend the use of this device to Mag Amp ap­plications.

Figure 1 - L6561 Internal Block Diagram

COMP MULT CS
23 4

1

INV

V

-

2.5V
+

VOLTAGE

REGULATOR

8

CC

20V

R2

2.1V

1.6V

6

GND

INTERNAL

SUPPLY 7V

R1

V

REF2

OVER-VOLTAGE

+

-

5

ZCD

DETECTION

UVLO

ZERO CURRENT

+

-

DETECTOR

MULTIPLIER

+-

RSQ

DISABLE

5pF

STARTER

40K

DRIVER

D97IN547D

V

CC

7

GD

The outperforming L6561 offers a number of unique advantages that make the device an interesting al­ternative to the t raditionally used PWM controllers where quite a good performance is required at low
cost:

January 2003

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AN1060 APPLICATION NOTE

disable function for power management and/or protection schemes;

•

true micropower start-up current, 50µA typ., for cost-effective start-up circuits;

•

very low quiescent current, 3mA typ., for high efficiency at light load;

•

two-level (static and dynamic) overvoltage protection (OVP);

•

on-chip RC filter on current sense pin for improved noise immunity;

•

pulse-by-pulse current limiting. In conjunction with TM operation, this ensures a safe operation under

•

short circuit conditions.

Refer to [1] for a detailed explanation of the internal architecture (shown in fig. 1) and the functionality of
the device.

L6561-based flyback converters can be realised as schematically illustrated in fig. 2a, 2b, 2c, and which
will be referred to as "TM", "Synchronised" and "High-PF" respectively.

Figure 2. L6561-based flyback converter configurations

DISABLE

OPTO
TL431

Vout

+

Vac

VCCZCD

L6561

GD

BULK

C

a) TM Flyback

Vac

DISABLE

VCCZCD

MULT

IN

C

SYNCH

DISABLE

Vac

C

VCCZCD

L6561

GD

b) Sy nchronised Flyb ack

Vout

BULK

OPTO
TL431

Vout

+

L6561

GD

INV

COMP

OPTO

+

TL431

(BW<100 Hz)

c) High-PF Flyback

Each of them has its own peculiarities but they all share some key points:

low parts count, which helps reduce total cost and space;

•

high efficiency at very light load: an L6561-based flyback can be easily compliant with Blue Angel

•

standards;
standby function: the internal start-up timer may be used to make the system work at a (fixed) low

•

frequency under light load conditions, so as to minimise losses;
disable function: pin ZCD, if grounded, turns off the L6561 and reduces its consumption at a couple

•

of mA; this can be used either for power management or protection.

2/11

AN1060 APPLICATION NOTE

In the following, the three basic configurations will be taken into consideration and their advantages,
benefits and drawbacks will be highlighted so as to identify their most appr opriate field of application.
This will be made easier by some application examples.

TM Flyback

This configuration, very similar to a free-running flyback, always works on (actually, very close to) the
boundary between Continuous and Discontinuous Mode (i.e. Transition Mode, or TM), t heref ore at a fre­quency dependent on the input voltage and on the output current.

This type of operation requires a low induc tance and therefore a small-size magnetics but on t he other
hand, involves high peak current. Therefore it can be reasonably used for power levels up to 50-60 W in
110 V or wide-range mains applications, and up to 100 W with 220/240 V mains.

At high input voltage and especially at light load, the switch ON-time becomes very short and the switch­ing frequency tends to become quite high. There is, however, a minimum ON-time (0.4-0.5µs) below
which it is not possible to go. This is due to the internal delay of the L6561 as well as the turn-off delay of
the MOSFET.

When this minimum is reached, TM operation can no longer be kept. The energy drawn each cycle ex­ceeds the short-term demand from the load and the control loop delays MOSFET’s turn-on so as to
maintain the long-term energy balance. Switching becomes asynchronous, and this can be seen as a
"ghosting" of the waveform on the scope.

If the load is decreased furt her on, so many cycles need to be skipped that the amplitude of the drain
voltage ringing becomes very small, and the ZCD can no longer be triggered. In this case the internal
starter of the IC will start a new switching cycles sequence. Under this condition, the system will operate
in "burst" mode: there will be short periods of switching spaced out by long intervals where L6561’s OVP
keeps the switch in OFF state.

Fig. 3 shows a 7W power s upply, r ealis ed in TM flyback. It is intended as an auxiliary power supply suit­able for systems provided with power management, such as monitor displays, printers, servers, photo­copiers, fax machines, etc.

According to an approach that is becoming mor e and more popular, when the system is requested to go
into some low-consumption mode, a µP switches off the main SMPS. A small auxiliary supply, optimised
for a low power level, keeps alive the µP itself and the circuits needed for waking up the system again.
This approach allows to minimise the power consumption from the mains, in compliance with regulations
coming into force (such as Blue Angel and others).

Figure 3. 7W, Wide-range, Auxiliary Power Supply.

Vin=90 to 400 Vdc

BZW04- 154

Ω

Inpu t bulk capacitor

of the mai n S M PS

to L4990 A or L5991A

UC3843A/B or UC3 845A/B

or L4981A

100 nF

Ω

33 k

Ω

7.5 k

470 k

3

L6561

2

1

6

8

4

47 µF

5

7

1N4148

STD1NB60

STTA106

47 µF

47 k

Ω

22

22

Ω

Ω

1N4148

Ω

2

STPS360B

N1 N2

N3

TRANSFORMER SPECS:

CORE: E20x10x6, 3C85 material or equivalent

≈

0.5 mm air gap for a primary inductance of 1.7 mH
N1: 2 series windings 66 T each, AWG32
N2: 11 T, AWG24
N3: 21 T, AWG32

2x330

4.7 nF

µF

13

L4955V5.1

2

(∅0.57 mm)

5 Vdc / 1A

100

µF

(∅0.24 mm)

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AN1060 APPLICATION NOTE

The converter is powered by the high-voltage DC bus, rangin g from 90 to 400 VDC, generated by the
front-end AC-DC stage (bridge rectifier + input capacitor) shared with the main SMPS (power factor cor­rected 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 be­sides 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 kΩ 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

of the power MOSFET, thus it can be found empirically after bench tests.

C

oss

With the component values shown in fig. 3 the wake-up time of the converter, that is the time the system
takes to start operating after being powered, does not exceed 3 s at 90 V

In fig. 4, the circuit of f ig. 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 trans­former and the sense resistor on the primary side, the catch diode and the filter capacitors on the secon­dary side. They all have been increased in size.

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

supply and 1 s at 400 VDC.

DC

Vin=90 t o 400 Vd c

Input bulk capacitor

of the main SMPS

to L4990A or L5 991A

UC3843A/B or UC3845A/B

100 nF

Ω

33 k

Ω

7.5 k

or L4981A

BZW04-154

Ω

470 k

3

L6561

2

1

6

8

4

47 µF

5

7

STTA106

1N4148

22

STP3NB60FP

Ω

47 µF

47 k

Ω

22

1N4148

Ω

Ω

1

STPS560B

N1 N2

N3

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
N2: 8 T, 2xAWG22
N3: 15 T, AWG32

2x1000

4.7 nF

µF

(∅0.24 mm)

13

L4955V5.1

2

(∅0.71 mm)

5 Vdc / 3A

220

µF

(∅0.30 mm)

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 ampli­fier and of an optocoupler for transferring the control sign al 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 cir cuit arrangement and its component values ensures that the wake-up time of
the converter does not exceed 3 s at 90 VAC supply (it will be less than 1 s at 270 V

AC

).

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. T he 28V output is used f or motors, the
12V output for the printhead and the 5V bus supplies the logic circuitry.

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