ST AN993 Application note

AN993

Application note

Electronic ballast with PFC using L6574 and L6561

Introduction

Dedicated ICs for lamp ballast applications are now replacing the old solutions based on bipolar transistors driven by a saturable pulse transformer.

The L6574 is a high-performance ballast driver, designed using 600-V BCD offline technology, which ensures all the features needed to properly drive and control a fluorescent bulb. It is provided with built-in VCO start-up sequence circuitry, protections, and an operation amplifier for implementing a closed-loop control of the lamp current.

July 2009 Doc ID 5656 Rev 10 1/27

www.st.com

Contents AN993

Contents

1 Half bridge converter for electronic lamp ballast . . . . . . . . . . . . . . . . . . 4

1.1 Lamp requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 L6574 ballast driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Device block description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Preheating and ignition section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Control section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.3 Bootstrap section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3.1 C

selection and charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

boot

4 Description of the demonstration application . . . . . . . . . . . . . . . . . . . 15

4.1 Power factor section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.2 Ballast section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.3 Preheating and ignition sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.4 Current feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.5 Start-up and supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.6 Safety circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5 Design tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5.1 Inductance and capacitor evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

6 Dimming the lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.1 Dimming level and lamp turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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AN993 List of figures

List of figures

Figure 1. Half bridge topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 2. Internal block diagram of the L6574 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. Connection of a typical application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 4. Startup timing diagram and EN2 function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. Timing block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 6. Timing oscillator block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 7. Cpre voltage and frequency shifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 8. Operating frequency at Cf = 470 pF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 9. Controls timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 10. Startup timing diagram and EN2 function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 11. External bootstrap diode connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 12. L6574 integrated bootstrap diode connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 13. Demonstration application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 14. PCB and components layouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 15. Current feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 16. Cpre waveform (Ch1) and amplifier output (Ch2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 17. Open load safety circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 18. Extra voltage safety circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 19. Simplified schematic of the lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 20. Preheating transfer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 21. Operating transfer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 22. Iterative process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Doc ID 5656 Rev 10 3/27

Half bridge converter for electronic lamp ballast AN993

1 Half bridge converter for electronic lamp ballast

Voltage-fed, series-resonant half-bridge inverters are currently used for fluorescent lamps (Figure 1). This topology facilitates operation in zero voltage switching (ZVS) resonant mode, dramatically reducing the transistor switching losses and the electromagnetic interference.

To design a cost-effective, compact and smart electronic lamp ballast, a dedicated IC could be used to drive directly the power MOSFETs of the half bridge. Such controllers require a high voltage capability for the high-side floating transistor driver.

Figure 1. Half bridge topology

DRIVER

1.1 Lamp requirements

To prolong lamp life and to ensure efficient ignition of the lamp, the cathodes must be preheated. In fact, the preheating of the filaments allows an easy strike of the lamp, reducing the ignition voltage. During the preheating time, the lamp is characterized by a high impedance and the current flows only in the filaments. The resistance value of the filaments strictly depends on the type of lamp. Typically, these filaments present an initial low value (a few Ohms) that will increase by four to five times during the preheating phase.

After the preheating phase, the lamp must be ignited by increasing the voltage across it. The ignition voltage value also depends on the type of lamp, and it increases with the aging of the lamp. For a typical TL 58 W, the ignition voltage value is not much less than 1000 V. When a simple inverter with a constant switching frequency is used, external circuitry is also necessary (for example, a PTC or discrete timer). However, with ST’s L6574 smart controller, both the preheating and ignition functions are achieved by using simple resistors and a capacitor, which set all the start-up procedures.

RES

AM01309v1

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AN993 L6574 ballast driver

2 L6574 ballast driver

The L6574, whose internal block diagram is shown in Figure 2, is an IC intended to drive two power MOSFETs or IGBTs in half-bridge topology, ensuring all the features needed to properly drive and control a fluorescent bulb. Moreover, by varying the switching frequency, it is possible to modulate the current in the lamp and as a consequence, the output power as well. The device is available in DIP16 and SO16N packages.

The L6574 has the following distinctive features.

● High voltage rail up to 600 V

● dV/dt immunity ± 50 V/ns in full temperature range

● Driver current capability (250 mA source and 450 mA sink)

● Switching times 80/40 ns rise fall with 1 nF load

● CMOS shutdown input

● Under-voltage lock-out

● Preheat and frequency shifting timing

● Sense operational amplifier for closed-loop control or protection features

● High-accuracy current-controlled oscillator

● Integrated bootstrap diode

● Clamping on VS

● SO16, DIP16 package.

Figure 2. Internal block diagram of the L6574

H.V.Bus

Cboot

LOAD

OPOUT

OPIN-

OPIN+

Rign

Rpre

OP AMP

Imin

Imax

VCO

REF

CONTROL

LOGIC

DETECTION

DEAD

TIME

Ifs

Vthpre

Ipre

BOOTSTRAP

Cpre

DRIVER

DRIVING

LOGIC

HVG DRIVER

LEVEL

SHIFTER

DRIVER

LVG

VTHE

THE

Vboot

HVG

OUT

LVG

GND

EN1

EN2

AM01310v1

Doc ID 5656 Rev 10 5/27

L6574 ballast driver AN993

Figure 3. Connection of a typical application

HVBus

CSupply

Ref.

Table 1. Description of device pins

Number Name Function

Preheat timing capacitor. The capacitor C shift time, according to the relations: t

1.5 s/ ∝F, KFS= 0.15s/∝F). This feature is obtained by charging C

1 C

pre

is charged up to 3.5 V (preheat timing comparator threshold). During tSH, the current

currents. During t

depends on the value of R way t

Figure 5).

Maximum oscillation frequency setting. The resistance connected between this pin

2 R

pre

and ground sets the f the end of the start-up procedure, the effect current drown from R voltage at this pin is fixed at V

3 CF

4 R

ign

5 OPout

Oscillator frequency setting. The capacitor C and f

Minimum oscillation frequency setting. The resistance connected between this pin and ground sets the f

Out of the operational amplifier. To implement a feedback control loop this pin can be connected to the R

6 OPin- Inverting input of the operational amplifier.

7 OPin+ Non-inverting Input of the operational amplifier.

Rpre.

pre

is always set at 0.1t

. In normal operation this pin shows a triangular wave.

ING

CSnub

L6574

410

Rign.

CBoot

Cpre

pre

= K

pre

Rgh

Rgl

Rcs

sets the preheating and the frequency

· C

pre

and tSH= KFS· C

pre

, this current is independent of the external components, so C

(that is, on the difference between f

pre

. In steady state the voltage at pin 1 is 5 V (see

pre

value, fixing the difference between f

pre

=2 V.

REF

, along with to R

value. The voltage at this pin is fixed at V

ign

pin by means of appropriate circuitry.

ign

L_ballast

LAMP

pre

Cres

pre

with two different

pre

and f

pre

and f

ign(fpre

is over. The

pre

and R

pre

=2 V.

REF

(typ. K

ign

, sets f

ign

AM01311v1

pre

). In this

> f

). At

ign

pre

6/27 Doc ID 5656 Rev 10

AN993 L6574 ballast driver

Table 1. Description of device pins (continued)

Number Name Function

Enable 1. This pin (active high), forces the device into a latched shutdown state (like in undervoltage conditions). There are two ways of resuming normal operation: the

8 EN1

9 EN2

10 GND Ground.

11 LVG

12 VS Supply voltage. This pin is connected to the supply filter capacitor (15.6 V typical).

13 N.C.

14 OUT

first is by reducing the supply voltage below the undervoltage threshold and then increasing it again until the valid supply is recognized; the second is by activating the EN2 input (see Figure 9). The Enable 1 is specifically designed for strong faults (for example, in case of lamp disconnection).

Enable 2. EN2 input (active high) restarts the start-up procedure (preheating and ignition sequence). This feature is useful if the lamp does not turn on after the first ignition sequence (see Figure 10).

Low-side driver output. This pin must be connected to the low-side power MOSFET gate of the half bridge. A resistor connected between this pin and the power MOSFET gate can be used to reduce the peak current.

Not connected. This pin sets a distance between the pins related to the high-voltage side and those related to the low-voltage side.

High-side driver floating reference. This pin must be connected close to the source of the high-side power MOSFET or IGBT.

High-side driver output. This pin must be connected to the high-side power MOSFET

15 HVG

gate of the half bridge. A resistor connected between this pin and the power MOSFET gate can be used to reduce the peak current.

Bootstrapped supply voltage. The bootstrap capacitor must be connected between

16 VBOOT

this pin and OUT. A patented integrated circuit replaces the external bootstrap diode by means of a high-voltage DMOS, synchronously driven with the low-side power MOSFET.

Doc ID 5656 Rev 10 7/27

Device block description AN993

3 Device block description

The preheating control section and the bootstrap section are tightly linked to the application’s design. This chapter describes their workings and usage.

3.1 Preheating and ignition section

The L6574’s turn-on sequence is divided into three phases: the preheating phase, the ignition phase and the normal operation phase (Figure 4). The preheating phase is characterized by the highest oscillation frequency (f phase, the frequency shifts from f period T

Figure 4. Startup timing diagram and EN2 function

Power-O.K.

VSupply

max

to f

(which is the normal operating frequency) in a

min

) for a period T

max

. During the ignition

pre

V(Cpre)

Osc. freq.

K= C

●

pre

●

IGN

⋅

0.1= T

⋅

pre

preheating

MAX

PRE

steady state operation

ignition

fMIN

Time

All the above-mentioned parameters are set by carefully selecting a few external components. T

During the preheating phase (T current I

pre,

The voltage across C

and Tsh are set by means of the capacitor C

pre

the capacitor C

pre)

is charged by means of a constant

pre

that is connected to pin 1.

pre

which is generated internally and does not depend on any external components.

increases linearly up to the "preheating threshold" at which the

pre

preheating phase terminates.

Equation 1

AM01312v1

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AN993 Device block description

That is to say:

Equation 2

Figure 5. Timing block

Ifs Ipre

Cpre

Figure 6. Timing oscillator block

TIMING

DISCHARGE

IminImax

Iosc

AM01313v1

CSOmg

Cpre

AM01314v1

After the preheating time, the capacitor C by the current I

, generating a second voltage ramp which feeds a transconductance

is first quickly discharged and then recharged

pre

amplifier, as shown in Figure 6 (the switch is closed). Thus, this voltage signal is converted into a growing current which is subtracted from I f

to f

max

. The current that drives the oscillator to set the frequency during this shifting is

min

to produce the frequency shifting from

max

equal to:

Equation 3

Doc ID 5656 Rev 10 9/27

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