EE 5741
Design of Electronic Ballasts
Eric Persson
Exec. Director, WW Field Applications Engineering
April 17, 2009
COMPANY CONFIDENTIAL
1
Agenda
• Gas-Discharge lamps
• ZVS Resonant Half-Bridge output stage
• Typical ballast designs
• PFC
• Protection features
• Summary
COMPANY CONFIDENTIAL
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Gas Discharge Lamp Basics
Glass Envelope Blocks UV
Argon and Mercury Vapor
Fluorescent Coating
Coated Tungsten Filaments
“Low Pressure” Fluorescent Lamp
• Current flows through plasma between electrodes
• Must be AC or mercury migrates to one end
• Plasma emits mostly UV radiation
• UV excites coating to fluoresce and produce visible light
• Heated electrodes improve lamp life and lower strike voltage
COMPANY CONFIDENTIAL
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High Intensity Discharge (HID) Lamps
Argon
Mercury Vapor
Metallic Salts
Quartz
Arc-Tube
Coated
Tungsten Electrodes
Glass Envelope
Blocks UV
Medium Pressure Lamp
• AC current flows through plasma between electrodes
• Plasma emits UV radiation AND visible light directly
COMPANY CONFIDENTIAL
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Why Gas Discharge Lamps?
Advantages
• Lower total cost of
ownership
• Higher efficacy
(Lumens/watt)
• Broad-spectrum, high CRI
(HID)
• Long lifetime (15k hours)
• Dimmable (fluorescent)
Disadvantages
• Higher initial cost
• Requires ballast for
operation
• Narrow spectrum, lower
CRI (most fluorescent)
• Long warmup time (HID)
• Disposal/recycling issues
COMPANY CONFIDENTIAL
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Fluorescent Lamp Operating Modes
V lamp
time
I = IPH, V = VPH
Start
PH
V = VIGN
IGN
V = VRUN
RUN
COMPANY CONFIDENTIAL
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Simple Ballast Block Diagram
EMI Filter
Rectifier
DC Bus
Output Stage
Line
Input
Lamp
UVLO
Control
Circuit
Half-Bridge
Driver
Lamp Fault
COMPANY CONFIDENTIAL
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Power stage - resonant LC Circuit
COMPANY CONFIDENTIAL
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Parallel Resonant, Above Resonance
COMPANY CONFIDENTIAL
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Frequency Controls V, I
Design Constraints
•
f RUN ≥ 42 KH Z
•
f PH − f IGN ≥ 10 KH z (tolerances)
•
VPH ≤ VPH MAX
(ignition during preheat)
•
I IGN ≤ I MAX
(component ratings)
(infrared)
COMPANY CONFIDENTIAL
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CFL Ballast Typical Schematic
RSUPPLY
SPIRAL
CFL
LAMP
DCP2
BR1
MHS
VCC
1
LF
CVCC
L1
CBUS
CF
COM
2
FMIN
3
RFMIN
L2
CVCO
VCO
4
VB
8
IRS2520D
F1
LRES
HO
7
VS
6
CBS
CSNUB
CRES
LO
5
MLS
CDC
DCP1
COMPANY CONFIDENTIAL
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Typical Design Procedure for Ballast
Begin with lamp requirements
Lamp Type: 36W/T8
• Preheat Current:
0.6
[A]
• Preheat Time:
2
[sec]
• Max Preheat Voltage:
600
[Vpp]
• Ignition Voltage:
1500
[Vpp]
• Running Lamp Power:
34
[W]
• Running Lamp Voltage:
141
[Vpk]
COMPANY CONFIDENTIAL
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Select Resonant L and C
Define
Lamp
Requirements
Select L and C
Starting
Values
Calculate
Operating
Points
Iterate
L and C
No
Constraints
Fulfilled?
Yes
Design
Ballast
COMPANY CONFIDENTIAL
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Spreadsheet Method
L and C selection example for linear lamp:
Lamp Type = T8/36W, PRUN = 32W, VRUN = 141 Vpk, VPHMAX = 300 Vpk,
IPH = 0.6 Arms, and VIGN = 600 Vpk.
Inputs
Outputs
L
C
IIGN
fPH
fIGN fRUN
(H)
(F)
(Apk)
0.002
3.3E -9
0.9
75
74
43
545
0.002
6.8E -9
1.3
57
52
45
350
0.002
1.0E -8
1.6
49
42
43
273
0.002
1.5E -8
2.0
43
35
39
208
(KHz) (KHz)
(KHz)
VPH
(Vpk)
COMPANY CONFIDENTIAL
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Ballast Design Assistant Software
COMPANY CONFIDENTIAL
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Adding Power Factor Correction
COMPANY CONFIDENTIAL
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CrCM boost-mode PFC commonly used
LPFC
DPFC
DC Bus
(+)
IL
+
CBUS
MPFC
(-)
Boost Converter
V, I
ton
toff
t
Critical-conduction Mode (CrCM)
Constant on-time
control
t
COMPANY CONFIDENTIAL
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Advanced protection – lamp out circuit
SD pin (upper) and half-bridge voltage (lower) during
lamp out/re-insert condition
COMPANY CONFIDENTIAL
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End-of-life detection - protection
Vspec
Vspec + 30%
0V
0V
0V
- Vspec- 30%
LAMP OK
DC End Of
Lamp Life
DC End Of
Lamp Life
Vspec = VpK in the spec of the lamp
The SD pin is internally biased at 2V. During end of life the lamp voltage will increase asymmetrically
(DC end of life). The voltage offset sensed at the top of REOL4 will increase and one of the Zeners will
turn on, therefore triggering the shutdown.
COMPANY CONFIDENTIAL
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Example Ballasts
COMPANY CONFIDENTIAL
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Summary
• Gas-discharge (Fluorescent) lamps offer:
• Improved efficacy (lumens/watt)
• Lower total operating cost
• Better CRI lowers light output
• Ballast is necessary to operate lamp
• Ballast operates ZVS to minimize switching loss
• Ballast controls frequency to adjust lamp current
• Ballast self protects by shutdown when lamp misbehaves
COMPANY CONFIDENTIAL
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