ST AN2459 Application note

AN2459

Applica t ion note

Digital Power Factor Correction for Tube Lamp Ballasts and

other digital power supplies controlled by an 8-bit microcontroller

1 Introduction

The electronic ballast market has undergone dramatic changes over the last few years. It has moved from full analog, very differentiated applications made by a collection of drivers and controllers, where use of custom ASICs was widespread, to a couple of standard platforms. The basic building bloc ks are st i ll the same. They include a power factor corrector s tage and an inverting high voltage stage (Figure 1). On the one hand, analog platforms are targeting the low cost/basic performance applications. Their main drivers and controllers are widely used and well known ICs such as Power Factor Correctors (L6561/2/3) and High Voltage Ballast Controllers (L6569x/ L6571x/ L6574). On the other hand, a new digital platform concept has gained more interest and acceptance. A microcontroller with a simple Half Bridge Driver (L638x) has replaced the ballast controller. The Half Bridge Driver is used mainly for high-end applications, especially where the microcontroller has to deal with communication tasks (e.g. using the Dali protocol). STMicroelectronics' digital ballast reference design STEVAL-ILB002V1 introduces a safe operating Power Factor Controller (PFC) and Ballast Controller. Even with relatively simple microcontroller firmware routines, the results for power control and ballast protection are in line with advanced analog controlled ballasts, while adding flexibility, for example, the possibility to drive a wide variety of lamps, or to easily introduce different protection schemes. This application note deals in detail with the first block of the digital ballast, which provides stable DC bus voltage for the halfbridge in all load conditions, as well as controlling the input current shape which fulfills IEC standards (6.: IEC 61000-3-2 "Electromagnetic

compatibility".).

The final description of the digital ballast - the lamp control block - will be described in detail in a separate application note.

Figure 1. Digital ba llast scheme

Input Filter

8-Bit

Microcontroller ST7FLITE19B

January 2007 Rev 1 1/35

Power

Management

L6382D5

Unit

www.st.com

Contents AN2459 - Application note

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Power Factor Correction (PFC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 Transition Mode operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Digital implementation - Enhanced One Pulse Mode . . . . . . . . . . . . . . . . . 6

3 Power circuits design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Power components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3 Bill of material (STEVAL-ILB002V1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Signals measurement, processing & control . . . . . . . . . . . . . . . . . . . . 15

4.1 Input vo ltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.2 Output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.3 Zero Current Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.4 MOSFET current measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5 Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

6 References and related materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Appendix A Components calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

A.1 Input capacitor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

A.2 Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

A.3 Boost inductor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

A.4 Power MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

A.5 Boost Diode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2/35

AN2459 - Application note List of tables

List of tables

Table 1. Bill of material - PFC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 2. Bill of material - Lamp Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Table 3. Bill of material - general . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3/35

List of figures AN2459 - Application note

List of figures

Figure 1. Digital ballast sch eme. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. PFC Transition Mode principle (frequency is not to scale) . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 3. Principle of the Enhanced One Pulse Mode, inside the ST7Lite1B . . . . . . . . . . . . . . . . . . . 7

Figure 4. Input voltage & current with modified EMI filter

(compared to STEVAL-ILB002V 1) PF = 0.994 THD = 10.3% . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 5. Input voltage & current measured on STEVAL-ILB002V1 (old EMI filter)

PF = 0.991 THD = 10.4% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 6. Schematics of STEVAL-ILB002V1 reference des i gn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 7. Modified EMI filter (not included in STEVAL-ILB002V1 reference design . . . . . . . . . . . . . 11

Figure 8. General flowchart of PFC software. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 9. Input voltage sensing circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 10. Inpu t voltage sensing circuit output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 11. The mains turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 12. Out put voltage sensing circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 13. Out put v oltage control loop flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 14. Application start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figu r e 1 5 . Lamp re s tart - behavi o r o f the con t r o l lo o p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 16. Zero current crossing dete ction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 17. PFC M OSFET overcurrent detection circuit and zero coil current detection circuit with

indicated testing connection and microcontroller inner structure . . . . . . . . . . . . . . . . . . . . 24

Figu r e 1 8 . Maxim u m MOSFET's TON p r o tection routine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 19. Overcurrent reaction demonstration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4/35

AN2459 - Application note Power Factor Correction (PFC)

2 Power Factor Correction (PFC)

Theoretically, any switching topology can be used to achieve a hi gh power factor but, in

practice, the boost topology has become the most popular because of the advantages it

offers. These include:

● Circuit requires the least external parts, thus it is the cheapest available.

● Boost inductor, located between the bridge and the switch, lowers the input di/dt, thus

minimizing noise generated at the input and consequently reducing the EMI filter input requirements.

● Switch is source-grounded and therefore easy to drive.

Three methods of controlling the PFC preregulator are currently widely used. They are:

● T he Fixed Frequency Average Current Mode PWM.

● The Transition Mode (TM) PWM (fixed on-time, variable frequency).

● The peak current mode with fixed off-time.

Control of the first method is complicated and requires a sophisticated IC controller (e.g.

either ST's L4981A or ST’s L4981B which offers frequency modulation) and a considerable

component count.

Control of the second method is simpler (e.g. ST's L6561/2/3 family) and requires fewer

external parts. It is therefore much less expensive.

With the Fixed Fre quency Average Current Mode method, the boost inductor operates in

continuous conduction mode, while the TM method causes the inductor to work on the

boundary between continuous and discontinuous modes. Thus, for a given throughput

power, TM operation involves higher peak c urrents, suggesting it is more efficient at lower

power ranges (typically below 200W). In contrast, the Fixed Frequency Average Current

Mode is recommended for higher power levels.

A third method of control, that of applying constant. Toff control, results in continuous

conduction mode. The same simple TM-controllers may be used, as may a small RC

network to set the off-time. This method is described in AN1792 (7) It is optimal for an input

power of between 200 and 400W.

2.1 Transition Mode operation

As mentioned above, the typical PFC topology used in electronic ballasts is a step-up

(boost) regulator (Figure 1) working in transition conduction mode. Figure 2 outlines the

Transition Mode principles. When the MOSFET is turned on, the inductor is charged from

the input voltage source. When the MOSFET is turned off, the boost inductor discharges its

energy into the load until its current falls to zero. When the latter occurs, the boost inductor

has no energy and a zero current (ZCD) signal is detected, due to a demagnetization

change on the auxiliary winding. This drives the MOSFET on again, whereby another

conversion cycle starts. As the drain voltage drops before turn-on, the turn-on switching

losses are minimized. Figure 2 indicates the geometric relationship of average and peak

currents. Due to the triangular shape of the inductor current, the peak current is twice the

average current.

5/35

Power Factor Correction (PFC) AN2459 - Application note

Figure 2. PFC Transition Mode principle (frequency is not to scale)

Peak current

enveloppe

Inductor current

Average current

MOSFET

2.2 Digital implementation - Enhanced One Pulse Mode

To provide good switch control, as described in Chapter 2.1 above, a simple 8-bit

microcontroller may be used and a special PWM timer mode has been introduced. The

timer mode, called "Enhanced One Pulse Mode" of the PWM generator (12-bit autoreload

timer) is found inside the ST7FLITE19B microcontroller. It is explained in Figure 3 and in

datasheet ST7Lite1xB (4). In principle, when a zero current e v ent occurs the microcontroller

will reset the timer and turn -on the PFC MOSFE T. If there is no signal from ZCD, the timer

will overflow and turn-on the MOSFET anyway (it means a minimum switching frequency is

secured). The on-time of the MOSFET is set by a software control routine and is constant

during the mains half-cycle (this is detailed below in Chapter 4). The control routine

executed by the MCU alters the on-time depending on the input voltage level and the load

current.

AI12647

6/35

AN2459 - Application note Power Factor Correction (PFC)

Figure 3. Principle of the Enhanced One Pulse Mode, inside the ST7Lite1B

Compare event

Timer

Events ignored, because

MOSFET is turned-on

ZCD

MOSFET

Off

Timer reset caused by

ZCD

Event

Timer reset caused by

autoreload value match

Event

No event

occured

}

AI12651

7/35

Power circuits design AN2459 - Application note

3 Po wer circuits design

3.1 Power components

All components have been calculated following application note AN966 (3). A full description

of the design and selection of each component, based on the analog TM PFC controller

L6561, is also given in Appendix A. At the moment, input voltage is limited for European

mains. Future Software updates will include wide range input capability.

Besides the passive and discrete components of the microcontroller, the most important

part is the power management unit, L6382D5, which helps control the power. It provides a

stable (±2%) 5V supply for the microcontroller during the whole operation. It also supplies a

high voltage start-up. In addition, one of the general purpose gate drivers integrated inside

L6382D5 is used to translate TTL PWM signals from the microcontroller to the boost

converter gate of the MOSFET.

Figure 4. Input voltage & current with modified EMI filter

(compared to STEVAL-ILB002V1) PF = 0.994 THD = 10.3%

Note: Brown = Mains voltage, Blue = Input current.

8/35

AN2459 - Application note Pow er cir cuits d esign

Figure 5. Input voltage & current measured on STEVAL-ILB002V1 (old EMI filter)

PF = 0.991 THD = 10.4%

Note: Brown = Mains voltage, Blue = Input current.

Reference board design measurements of STEVAL-ILB002V1(Figure 5) show a THD value

of 10.4% and a PF value of 0.991. Between the manufacturing of the STEVAL-ILB002V1

reference design and publication of this application note, design work has continued and

some improvements have been made. For example, EMI filter parameters have been

changed from C-L-C to C-L filters, which give better results for waveform, power factor, and

THD .This optimized version is given in Figure 7 and result in the measured waveforms

shown in Figure 4 with THD = 10.3% and PF = 0.994.

9/35

Power circuits design AN2459 - Application note

3.2 Schematics

Figure 6. Schematics of STEVAL-ILB002V1 reference design

C11

123

1.8mH

C13

L6382

C12

100nF

47µF

25V

D13

STTH1R06A

3k9

C20

R28

100nF

50V

1k5

10n

C16

400V

STP5NK60Z

1600V

58W T8 lamp

DC5V

R29

mpDetection

C17

10n

R30

10k

R2312W,1%

Vcap

R31

220nF

300k

CSO

C14

10n

10p

High Side Input

Low Side Input

OSC2/PC1

/CLKIN/PC0

PA0(HS)/LTIC

PA1(HS)/ATIC

OSC1

RESET3COMPIN+/SS/AIN0/PB04SCK/AIN1/PB15MISO/AIN2/PB26MOSI/AIN3/PB37COMP-/CLKIN/AIN4/PB48AIN5/PB59AIN6/PB6

10nF

C10

RESET

R21 100

R22 33

Not assembled

RsenseCurrent

1N4148 SMD

PFC Zero Current Detect

C28

10k

R45

DC5V

R44

10k

R43

R42

10k

DC5V

R33

300k

R32

300k

R35

PeakLampVoltage

LampDetection

PFC Gate Driv er

DATA

CLK/BREAK

ST7LITE1B 20pinU1ST7LITE1B 20pin

PA4(HS)/ATPWM214PA3(HS)/ATPWM115PA2(HS)/ATPWM0

PA7(HS)/COMPOUT

PA6/MCO/I

PA5(HS)/ATPWM3/I

AverageCurrent

PeakCurrent

PFC OC

PeakLampVoltage

PFC Vout Sense

PFC VinWaveform

R34

100k

1N4148 SMD

C194n7

75k

R36

24k

C23

68n

AverageLampVoltage

AI12648

100V

Vcap

R37240k

R38

240k

R39

240k

R40

2k4

R41

2k4

DC5V

22uF 450V+22µF 450V

CM Choke

R21M350V

275VAC

C29

R11R11

750k

NTC1

STTH1R06

750k

–

100n

275VAC

R12

PFC Zero Current Detect

100n

100n 275VAC

750k

R141kR14

Sense

PFC V

27k

OUT

STP5NK60Z

R710R7

PFC Mosfet Gate

PFC VinW

PFC Vi aveform

275VAC

PFC OC

R101kR10

10n

20k

DC400V

1 2

D12

1N4007

Not assembled

2 1

TRANSFORMER

BRIDGE RB156

R11M350V

100n

FUSE

C18

470n

AverageLampVoltage

R13

10k

R13

4n7

2n7

0.5

47k

C27

10p

PFC Gate Driv er

RsenseCurrentPeakCurrent

BAT46

R25

4k7

RsenseCurrentAverageCurrent

R24

10k

2 C22

470n

C21

R26

R18

DC5V

Vcap

C15

Out pin

STP5NK60Z

R19 100

R20 33

00.6W

10p

C25

C26

10p

Out pin

470n

7 4k7

CSI

VREF

PFI1LSI2HS

Low Side Input

CSI

O CS

HEG17CS

HEI4PFG5NC6TPR7GND8LS

PFC Mosfet Gate

gh Side Input Hi

R46

R16

2.2nF

1000V

HVSU

1N4148 SMD

OUT

R27

Not assembled

G HS

BOOT

180.6W

RsenseCurrentCSI

10/35

AN2459 - Application note Pow er cir cuits d esign

Figure 7. Modified EMI filter (not included in STEVAL-ILB002V1 reference design

D7 BRIDGE RB156

C3 100n 275VAC

100n 275VAC

AI12646

FUSEF1FUSE

L N

275VAC

1n 275VAC

275VAC

100n

R1 1M 350V

R2 1M 350V

T1T1

CM Choke 45m

11/35

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