ST AN2459 Application note

AN2459

Application 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 blocks are still the same. They include a power factor corrector stage 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 ballast scheme

Input Filter
8- Bit	Power
Microcontroller	Management Unit
ST7FLITE19B	L6382D5
January 2007	Rev 1	1/35
		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 voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	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

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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

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List of figures	AN2459 - Application note

List of figures

Figure 1.	Digital ballast scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 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-ILB002V1) 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 design . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	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.	Input voltage sensing circuit output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
Figure 11.	The mains turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
Figure 12.	Output voltage sensing circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
Figure 13.	Output voltage control loop flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
Figure 14.	Application start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
Figure 15.	Lamp restart - behavior of the control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
Figure 16.	Zero current crossing detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22
Figure 17.	PFC MOSFET overcurrent detection circuit and zero coil current detection circuit with
	indicated testing connection and microcontroller inner structure . . . . . . . . . . . . . . . . . . . .	24
Figure 18.	Maximum MOSFET's TON protection routine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	24
Figure 19.	Overcurrent reaction demonstration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25

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AN2459 - Application note	Power Factor Correction (PFC)

2 Power Factor Correction (PFC)

Theoretically, any switching topology can be used to achieve a high 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:

●The 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 Frequency 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 currents, 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.

2.1Transition 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.

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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

On

MOSFET

On

AI12647

2.2Digital 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 event occurs the microcontroller will reset the timer and turn-on the PFC MOSFET. 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.

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AN2459 - Application note	Power Factor Correction (PFC)

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

	Timer reset caused by	Timer reset caused by
	ZCD	autoreload value match
Compare event
Timer
Events ignored, because	Event	Event	No event
MOSFET is turned-on			occured
			}

ZCD

On

MOSFET

Off

AI12651

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Power circuits design	AN2459 - Application note

3 Power circuits design

3.1Power 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.

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AN2459 - Application note	Power circuits design

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.

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10/35

F1

D12

Figure

23.

Power

1

4

2

-STEVAL of Schematics .6

design circuits

FUSE

R1

D7

1M

3

T1

4

+

1N4007

350V

J1

BRIDGE RB156

Not assembled

DC400V

C1

L

1

3

D2

100n

TRANSFORMER

1

2

1

2

N

275VAC

1 CM C hoke 2

Schematics

R2

T2

8

3

PE

1M

C29

Vcap

350V

100n

–

C3

R3

STTH1R06

NTC1

AC

275VAC

2

750k

1

5

10

R11

750k

100n 275VAC

C2

R6

R37

C7

240k

PFC Zero Current Detect

R12

R4

+

750k

750k

27k

uFµ22

450V

1n

Q1 2

R14

PFC VOUT Sense

275VAC

STP5NK60Z

PFC nWaveformVi

PFC Mosfet Gate

R7

1

R38

3

1k

10

240k

R10 1k

DC5V

R5

C4

PFC OC

20k

10n

AverageCurrent

R24

RsenseCurrent

R8

R9

0.5

C6

R13

R41

R39

47k

10k

2k4

240k

C22

10k

C5

4n7

470n

2n7

AverageLampVoltage

C18

R40

470n

2k4

PeakCurrent

R25

D5

RsenseCurrent

PFC Gate Driver

ILB002V1

C27

R26

C21

4k7

BAT46

10p

DC5V

R18

4k7

470n

C25

C26

C8

U2

10p

10p

0.6W

0

Out pin

1

PFI

VREF

20

Q2

Low Side Input

2

19

CSI

R19 10

LSI

CSI

STP5NK60Z

2.2nF

High Side Input

3

HSI

CSO

18

CSO

1000V

PFC Mosfet Gate

4

HEI

HEG

17

D3

5

PFG

NC

16

reference

R16

R46

Out pin

Vcap

6

NC

HVSU

15

7

TPR

NC

14

1N4148 SMD R20 33

R27

8

GND

OUT

13

0.6W

18

0.6W

18

9

LSG

HSG

12

Not assembled

L1

C15

CSI

RsenseCurrent

1

10

VCC

BOOT

11

C11

J2

3k9

D13

+ 47µF

C12

L6382

1.8mH

R28

C20

STTH1R06A

25V

100nF

C13

1

1k5

1n

50V 100nF

100nF

2

2

Q3

400V

C16

3

STP5NK60Z

10n

4

1600V

R21 10

D4

58W T8 lamp

design

DC5V

DC5V

1N4148 SMD

R22 33

R29

Not assembled

DC5V

RsenseCurrent

1M

R42

R43

R44

R45

PFC Zero Current Detect

R23

R30

10k

10k

10k

10k

1

10k

C9

C28

2W1%,

220nF

10p

LampDetection

U1

CSO

Vcap

1

V

OSC1/CLKIN/PC0

20

2

19

VSS

OSC2/PC1

C14

C17

3

DD

PA0(HS)/LTIC

18

10n

C10

4

RESET

17

PA1(HS)/ATIC

10n

5

16

High Side Input

R31

COMPIN+/SS/AIN0/PB0

PA2(HS)/ATPWM0

AN2459

10nF

6

SCK/AIN1/PB1

15

Low Side Input

300k

RESET

7

MISO/AIN2/PB2

PA3(HS)/ATPWM1

14

8

MOSI/AIN3/PB3

PA4(HS)/ATPWM2

13

PFC Gate Driver

9

COMP-/CLKIN/AIN4/PB4

PA5(HS)/ATPWM3/ICCDATA

12

PFC OC

10

AIN5/PB5

PA6/MCO/ICCCLK/BREAK

11

LampDetection

AIN6/PB6

R32

PFC Vout Sense

PA7(HS)/COMPOUT

300k

PFC VinWaveform

ST7LITE1B 20pin

AverageCurrent

R33

300k

-

R35

D6

PeakCurrent

note Application

PeakLampVoltage

PeakLampVoltage

AverageLampVoltage

C23

R36

75k

1N4148 SMD

R34

68n

24k

C19

100k

4n7

100V

AI12648

AN2459 - Application note								Power circuits design
Figure 7.	Modified EMI filter (not included in STEVAL-ILB002V1 reference design
	F1
	FUSE	R1					+ 4	D7
				T1
	C1	1M	3		4			BRIDGERB156
J1	100n	350V
						1		3
L
	275VAC		1		2
N		R2				H
PE
		1M		CM				C3
AC		350V		Choke			2
								100n 275VAC
				45m
	C2
	1n
	275VAC
								AI12646

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