Simple and cost effective dimming solutions

S ecial Re crt - Li htin S stems
Simple and cost effective dimming solutions
Dimming of fluorescent lamps will normally require the incorporation of a complex, high pin count
controller IC into the circuit design. As a result, dimming circuits are more difficult to create than nondimming alternatives, needing a larger number of components, taking up more board space, and
raising the system's total price tag.
By Tom Ribarich, Director, Lighting Systems, International Rectifier, EI Segundo, California
T
hese issues are
compounded
still further when
Bcotwap
vee
11--.----------------------,FET.----,
considering compact
fluorescent lamps, due
15.6'1
eOM
2
to the small form factors involved and the
need to be highly cost
effective. Here we look
at a non-dimming ballast control based on an
8-pin controller Ie and
5 LO
explain how dimming
functionality can be added without needing to
increase the pin count.
With an existing
J
'!N
8-pin, non-dimming
lighting ballast control-
5V
'FIotI""~
n.RrI.UN
FMIN 3
ler (for example, the
IR2520D from Interna-
All values are typical
Figure 1: Block diagram of the IR2520D non-dimming ballast IC
tional Rectifier, shown
in Figures 1 and 2) only two pins are
cuit, current charges the vce until it
for the IC, maintaining the VCC at
preheat, ignition and running require-
reaches the internal UVLO+ threshold.
the internal clamp voltage. A small
ments of the fluorescent lamp. The re-
When vee goes past this threshold,
internal current source at the VCO
maining pins perform standard func-
the IR2520D enters frequency sweep
pin slowly charges up an external
used (FMIN and VeO) to deal with
tions such as Ie supply and ground
mode, the gate driver outputs (LO
capacitor causing the voltage on the
(pins vee and COM), plus high- and
and HO) and the half-bridge circuit
VCO pin to ramp up linearly. This in
low-side gate drive for the half-bridge
then starts oscillating at the maximum
turn ramp downs the frequency of the
(pins LO, VS, HO and VB).
frequency. The charge-pump circuit
gate driver outputs (LO and HO), and
then becomes the main supply circuit
the half-bridge switching circuit from
Within a non-dimming ballast cirwww.powersystemsdesign.com
27
S ecial Re art - Li htin S stems
Symbol
Description
vee
Supply voltage
COM
Ie power and signal ground
FMIN
Minimum frequency setting
veo
Voltage controlled oscillator input
LO
Low-side gate driver output
VS
High-side floating return
HO
High-side gate driver output
VB
High-side gate driver floating supply
vee [[ ~ [[J
:::0
I\)
II
COM
-
UI
VB
mHO
I\)
FMIN
[I
veo ~
-0
C
en
till
VS
m
LO
Figure 2: Lead assignment for IR2520D
its maximum starting value. The lamp
can be achieved by using operating
of the AC+DC signal at the DIM pin,
voltage increases as the frequency
frequency to control the current being
until the valley reaches COM again.
ramps down towards the resonance
applied to the lamp. As the frequency
If the DC reference is decreased, the
frequency of the high-Q, under-
of the half-bridge is increased, the
valley will decrease below COM. The
damped output stage. The VCO pin
gain of the resonant tank circuit de-
feedback circuit will then increase the
voltage continues to increase and the
creases and the lamp current lowers.
frequency to lower the resonant tank
frequency keeps decreasing until the
It is possible to regulate the lamp
gain until the valley reaches COM
lamp ignites. The output circuit then
current to a dimming reference level
again. The IR2520D's FMIN pin, used
becomes an over-damped, low-Q
by continuously adjusting the half-
to program a single running frequen-
circuit. The VCO voltage increases,
bridge frequency through closed-loop
cy, has now been replaced with a
causing the IC to enter run mode.
feedback circuit. Dimming is enabled
DIM pin, which measures the AC+DC
The frequency level stops decreasing
by combining the AC lamp current
signal for dimming.
once the VCO pin surpasses 5V and
measurement with a DC reference
stays at the minimum frequency as
voltage at a single node. The AC lamp
programmed by an external resistor
current is measured across sensing
offers a complete 8-pin solution that
on the FMIN pin.
resistor RCS and coupled onto the
contains all dimming ballast functions.
DC dimming reference via feedback
The VCO pin includes the frequency
capacitor CFB and resistor RFB.
sweep timing control for preheat and
With demand for dimming functionality becoming ever greater, but
engineers not wanting to sacrifice
ignition, and also programs the loop
The feedback circuit regulates the
speed for the dimming feedback cir-
the advantages of compact low pin
valley of the AC+DC signal to COM
count devices like the IR2520D, a way
as the DC dimming level is raised or
to control dimming through the pins
lowered by continuously adjusting the
When a voltage is first applied to
cuit during dim mode.
already available needed to be found.
half-bridge frequency. This causes the
VCC (14V, typical) the IC exits UVLO
As the VCO pin is required to perform
amplitude of the lamp current to then
mode and enters preheat/ignition
the necessary frequency sweep for
increase or decrease so that dimming
mode. The half-bridge begins oscil-
preheat and ignition, the FMIN pin was
can be carried out. If the DC reference
lating at the maximum frequency and
left as the only viable option through
is increased, the valley of the AC+DC
the internal current source at the VCO
which this could be accomplished.
signal will rise above COM and the
pin begins charging up an external
feedback circuit will lower the fre-
capacitor (CVCO) linearly from COM.
Dimming control through a single
quency in order to enlarge the gain of
The output frequency decreases as
pin
the resonant tank. This will raise the
the VCO voltage increases and the
lamp current, as well as the amplitude
lamp filaments are preheated by sec-
The dimming of a fluorescent lamp
28
The IRS2530D dimming control IC
Power Systems Design Europe
May 2010
S ecial Re crt - Li htin S stems
220V
If
AC Line
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Input
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CRUS
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DIM
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~~~ +-~
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Potentiometer
POIM
Dimming
10K POI ~ ..t+---____f----+-t"""--""'--<t----ll-...A,f-,M
SlnogleTl/tI'l
Input
CfB
RFB
RMIN
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I~
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ALMPI
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Figure 3: Dimming mini-ballast circuit using the IRS2530D
ondary windings from the resonant
NUB, DCP1 and DCP2) takes over as
tank inductor. As the VCO voltage
the IC supply once the half-bridge
charges up, the frequency decreases
begins to oscillate. The resonant tank
nent count.
There is a clear need for simple
towards the resonance frequency
circuit (LRES and CRES) provides
and cost effective dimming solu-
of the resonant tank circuit and the
the transfer function for generating
tions which take up the minimum of
output voltage across the lamp in-
the high voltages needed for lamp
real estate and do not require a large
creases. The lamp ignites when the
ignition and low-pass filtering for
number of components. The 8-pin
output voltage exceeds the lamp igni-
dimming. Secondary windings from
IRS2530D offers the means to de-
tion threshold voltage, lamp current
the resonant inductor (LRES: A, B)
velop dimming circuits in a quick and
begins to flow, and the IC enters dim
are used to heat the filaments of the
unproblematic manner. Furthermore,
mode.
lamp during preheat and dimming,
it has the potential to bring dimming
and also separate the lamp current
features to a broader spectrum of
from the filament current allowing
applications, thus allowing marked
dimming mini-ballast circuit is de-
for a single current-sensing resis-
energy savings to be realized.
scribed in Figure 3. It is designed
tor (RCS) to be utilized for sens-
to run from a 220VAC line and to
ing the lamp current. The AC lamp
drive a 25W compact fluorescent
current measurement across RCS
A schematic showing a complete
lamp. The 220VAC/50Hz line input
is coupled to the DIM pin through a
voltage is full-wave rectified (BR1)
feedback capacitor and resistor (CFB
and then goes through the EMI filter
and RFB). A potentiometer dimming
(CF and LF) before being smoothed
input circuit is used (PDIM, RMIN,
by the DC bus capacitor (CBUS).
RMAX) to convert the potentiometer
The half-bridge switches (MHS and
resistance to the dimming reference
MLS), which are controlled by the
voltage for the IRS2530D through
IRS2530D, allow preheating, ignit-
the DIM pin. Protection against bal-
ing and dimming of the lamp. RVCC1
last fault conditions (failure to strike,
and RVCC2 provide the micro-power
open filament, and low AC linel
start-up current for the IC's VCC
brown-out) are incorporated into the
supply, and the charge pump (CS-
IRS2530D to further reduce compo-
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29