1. No category

Frequency Dither Circuit for Electronic Ballast EMI Reduction
Peter Bredemeier and Tom Ribarich, International Rectifier, USA, [email protected]
Abstract
Electronic ballasts include switched-mode circuits that can generate a high amount of electro-magnetic interference (EMI). This EMI can conduct through various paths within the electronic ballast circuit and eventually reach the AC mains voltage as conducted noise. To block
this noise, an L-C filter is typically placed at the input of the circuit. These filters can be difficult to design due to their second-order nature, and, can be a significant overall cost increase due to the size and the multi-winding requirements of the filter inductor. This paper
describes a novel frequency dithering circuit used to spread and reduce the amount of EMI
and therefore reduce the size and cost of the EMI filter. This paper introduces the designer
to the main circuit blocks of an electronic ballast, presents the new frequency dither EMI reduction circuit, and provides the complete electronic ballast circuit driving a single 26W compact fluorescent lamp with experimental results.
1.
OVERVIEW
The functions performed by present day electronic ballasts include electromagnetic interference (EMI) filtering to block ballast generated noise, rectification, and a half-bridge resonant
output stage for high-frequency AC control of the lamp (Figure 1). This is presently one of
the most popular approaches to powering fluorescent lamps with power levels below 26W.
Controlling fluorescent lamps also requires additional circuitry necessary for preheating the
lamp filaments, constant ignition voltage control, and end-of-life (EOL) protection. The focus
of this paper is on the oscillator circuit used to control the operating frequency of the halfbridge resonant output circuit and the new frequency dithering feature to reduce EMI.
EMI Filter
Rectifier
DC Bus
Capacitor
Half-Bridge
Resonant Tank
AC
Mains
Input
UVLO
Control
Circuit
Resonant Tank
Control
Lamp Fault
Fig. 1. Electronic ballast block diagram.
Lamp
2.
Control Circuit
The resonant output stage is controlled with a voltage-controlled oscillator (VCO) input and a
high- and low-side half-bridge driver output (Figure 2). The half-bridge driver operates at a
given frequency with a 50% duty cycle and a fixed non-overlapping dead-time. This produces a high-voltage square-wave that feeds the resonant tank and lamp. The frequency of the
square-wave first starts at a higher frequency to preheat the lamp filaments, and is then decreased through resonance to ignite the lamp to a final lower frequency for running. The frequency is controlled by the VCO input, which starts at a higher voltage during preheat and is
then decreased smoothly to a lower voltage for ignition and running.
High-Side
Supply
SET
RST
Gate
Driver
Logic
High-Side
Gate Drive
High-Voltage Well
High-Side
Return
VCO
Vph
VCC
Vrun
Driver
Logic
VCO
Level
Shift
Logic
Level-Shift
FETs
VCC
Low-Side
Gate Drive
tph
tign
trun
Fig. 2. Voltage-controlled oscillator (VCO) and half-bridge driver circuit.
The VCO circuit includes an additional frequency dithering function that varies the
frequency linearly above and below a nominal frequency level continuously at a given
dithering rate. The frequency dither circuit (Figure 3) includes a current source and current
sink for charging and discharging the VCO voltage above and below the nominal level. The
source and sink currents are turned on and off from a second oscillator (dither oscillator)
running at 50% duty-cycle and at a given frequency. The frequency of the dither oscillator
directly sets both the desired rate and amount of dithering necessary for spreading the operating frequency to reduce EMI.
VCO’
Dither
Osc.
Isource
High-side
gate drive
VCO’
VCO
Input
Ballast
Osc.
Gate
Drive
Logic
Low-side
gate drive
Isink
Max
Nominal
Min
Dither
Oscillator
Output
Source-Time
Fig. 3. Frequency dither circuit
Sink-Time
3.
Ballast Design
A fully functional 26W ballast is designed around the IRS2526DS “Mini8” Ballast Control IC. The
circuit includes (Figure 4) the complete control for the half-bridge resonant output stage and the lamp.
The „VCO‟ pin sets the frequency of the half-bridge gate driver outputs, „HO‟ and „LO‟ pins. A resistor voltage divider at the „VCO‟ pin programs the desired VCO voltage levels. These voltage levels
control the frequency of the internal voltage-controlled oscillator (Figure 3). The internal oscillator
signal then feeds into the high- and low-side gate driver logic circuitry to generate the correct preheat,
ignition, and running frequencies for the half-bridge and resonant output stage. The EMI inductor now
consists of a single-winding differential-mode inductor (LF) instead of the multi-winding commonmode inductor that is typically required to adequately filter out ballast generated noise.
F1
L
AC
LINE
INPUT
LF
BR1
RVCC1
N
RVCC2
RLIM1
RVCC3
VB
VCC
CF
CBUS
1
8
COM
R2
2
C1
VCO
R1
C2
3
CPH/EOL
4
IRS2526D
CVCC
RHO
MHS
HO
LRES:A
7
VS
CDC
CBS
LRES:B
CSNUB
6
REOL1
LO
RLO
CRES
MLS
5
CPH
CH1
REOL2
DCP2
RLMP1
CIGN2
DIGN
DEOL-
RIGN2
REOL
DEOL+
CEOL
RLMP2
REOL3
DR2
CIGN1
LAMP
CH2
DCP1
DR1
RIGN1
LRES:C
Fig. 4. 26W electronic ballast circuit schematic
4.
Experimental Results
The evaluation results from the functional ballast show that the half-bridge resonant stage
and lamp are both working properly. The waveforms include (Figure 5) the half-bridge
switching voltage, the AC lamp voltage and AC lamp current during running at maximum (left
waveforms) and minimum (right waveforms) dithering frequencies. The conducted EMI
measurements are also shown (Figure 6) for a wide range of measured frequencies (9kHz to
300MHz). The circuit demonstrates proper fluorescent lamp control with conducted EMI below the allowed limits using only a single-winding filter inductor.
Fig. 5. Half-bridge voltage (red trace), lamp current (green trace) and lamp voltage (yellow trace) during running with maximum dithering (left waveforms) and minimum dithering (right waveforms)
120 LISN:ENV216-N
Scan6: 150.0 kHz, 5.0 kHz, 30.0 MHz; IF:9kHz, 100.0 ms, Average, TDF on, Att AutodB,
Scan4: 150.0 kHz, 5.0 kHz, 30.0 MHz; IF:9kHz, 1.0 s, QP, TDF on, Att AutodB,
Scan2: 9.0 kHz, 100.0 Hz, 150.0 kHz; IF:200Hz, 1.0 s, QP, TDF on, Att AutodB,
80 LISN:ENV216-N
Scan2: 30.0 MHz, 60.0 kHz, 300.0 MHz; IF:120kHz, 1.0 s, QP, TDF on, Att AutodB,
70
100
EN55015,QP
60
50
60
EN55015,QP
EN55015,AV
40
Magnitude [dBµV]
Magnitude [dBµV]
80
40
30
20
10
20
0
0
-10
-2
10
Protocol Number:
Serial Number:
Operation State:
Notes:
-1
10
0
10
Frequency [MHz]
2
10
Frequency [MHz]
1
10
Protocol Number:
Serial Number:
Operation State:
Notes:
Fig. 6. Conducted EMI measurement results for 9kHz to 30MHz (left graph) and for 30MHz to
300MHz (right graph).
5.
Literature
[1]
E.E. Hammer and T.K McGowan, “Characterization of F40 Fluorescent Systems at 60Hz and
High Frequency,” IEEE Trans Ind. Appl., vol. IA-21, no. 1, pp. 11-16, Jan/Feb. 1985.
[2]
T. Ribarich, J. Ribarich, “A New Model for High-Frequency Ballast Design,” IEEE-IAS Conf.
Rec., 1997, pp. 2334-2339.
[3]
International Rectifier, IRS2526DS Mini8 Ballast Control IC Datasheet, www.irf.com, 2011.
[4]
International Rectifier, IRPLLMB7E 26W Reference Design Kit, www.irf.com, 2011.