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
3
L6574 ballast driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Device block description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1
3.2
3.3
Preheating and ignition section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Control section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Bootstrap section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.1
C
boot
selection and charging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4
Description of the demonstration application . . . . . . . . . . . . . . . . . . . 15
4.1
4.2
4.3
4.4
4.5
4.6
Power factor section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Ballast section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Preheating and ignition sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Current feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Start-up and supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
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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Doc ID 5656 Rev 10
AN993
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Half bridge topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Internal block diagram of the L6574 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Connection of a typical application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Startup timing diagram and EN2 function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Timing block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Timing oscillator block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Cpre voltage and frequency shifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Operating frequency at Cf = 470 pF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Controls timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Startup timing diagram and EN2 function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
External bootstrap diode connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
L6574 integrated bootstrap diode connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Demonstration application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
PCB and components layouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Current feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Cpre waveform (Ch1) and amplifier output (Ch2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Open load safety circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Extra voltage safety circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Simplified schematic of the lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Preheating transfer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Operating transfer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Iterative process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Doc ID 5656 Rev 10
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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
HV
DRIVER
L
RES
C
RES
AM01309v1
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.
4/27
Doc ID 5656 Rev 10
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.
Internal block diagram of the L6574
H.V.
Bus
16 Vboot
BOOTSTRAP
DRIVER
15 HVG
Figure 2.
VS
OPOUT
OPIN-
OPIN+
5
6
OP AMP
+
-
12
UV
DETECTION
HVG
DRIVER
Cboot
LOAD
7
Imin
4
V
REF
DEAD
TIME
Ifs
Imax
2
CONTROL
LOGIC
3
VCO
Cf
1
Cpre
V
REF
+
Rpre
-
+
-
+
-
V
THE
Vthpre
+
-
V
THE
Ipre
LVG
DRIVER
LEVEL
SHIFTER
V
S
14 OUT
DRIVING
LOGIC
Rign
11
LVG
10
GND
8
EN1
9
EN2
AM01310v1
Doc ID 5656 Rev 10
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