IRPLCFL3
R
EFERENCE
D
ESIGN
By Peter Green
TOPICS COVERED
Introduction
Dimmer Circuit
Proposed Solution
Block Diagram
Functional Description
Schematic Diagram
Design Considerations
Bill of Materials
IR2156
IRPLCFL3
rev.b
International Rectifier
•
233 Kansas Street, El Segundo, CA 90245 USA
IRPLCFL3: A ballast that can be dimmed
from a domestic (phase cut) dimmer
IRPLCFL3
INTRODUCTION
It has often been a disadvantage with electronic ballasts that they have been unable to
be dimmed with a standard (phase cut) type of dimmer, particularly in the case of small
integral ballast-lamp combinations commonly used in the home to save energy. This is
due to the fact that, where there is no power factor correction, the ballast circuit input
consists of a rectification stage followed by a large storage capacitor connected to the
AC mains supply that provides the DC bus from which the high frequency half bridge
and output section is supplied. In such a system current is drawn from the mains only
near the peak of the mains voltage where the storage capacitor charges and not during
the remainder of the mains half-cycle.
Virtually all domestic and professional dimming systems are based on
triacs.
These
devices will conduct once they have been fired, only while current flows in excess of the
holding current
of the device. These dimmers work very well with a
resistive
load such
as an ordinary Tungsten filament light bulb as the triac can be fired at any point during
the mains half-cycle and will continue to conduct until very close to the end of the half-
cycle as current is drawn continuously over this period. In this way the lamp current can
be adjusted from maximum to zero.
1
IRPLCFL3
A BASIC 120VAC DIMMER CIRCUIT
AC INPUT
16uH
100nF
15K
TRIAC
500K
47nF
33nF
AC OUTPUT
When a compact ballast is connected to a circuit containing such a dimmer, the triac will
only conduct if it is fired at a point during the mains half-cycle where the rectified mains
voltage is greater that the storage capacitor voltage. In this instance the capacitor will
be charged to the same voltage and the triac will then switch off. In this way it would be
possible to adjust the DC bus voltage of the ballast to some extent by adjusting the triac
firing point from 90º to 180º however this will not provide a satisfactory means of
controlling the light output.
There is also an additional problem encountered due to the fact that a dimmer of this
kind requires an inductor in series with the triac to limit the rise time of the current when
the device is fired. Without this inductor, mains current harmonics would be produced at
frequencies high enough to cause considerable radiated and conducted interference
problems. Since the load presented by a ballast circuit is effectively capacitive, when
the triac is fired there will be
ringing
caused by the resonance of the suppression
inductor of the dimmer and the capacitive load. This can cause the triac to fire and then
switch off as the ringing output voltage swings above and then below the input voltage
causing the current to fall below the holding current. This can occur several times during
each half cycle, resulting in severe lamp flicker and loss of control of the output.
PROPOSED SOLUTION
A system has now been developed, based around the IR2156, where the ballast is able
to operate with minimal flicker over a considerable portion of the adjustment range of a
dimmer and the light output may be controlled over this range from maximum output
down to around 10%.
2
IRPLCFL3
In this system the front end of the ballast has been designed so that when the triac in
the dimmer has fired, it will remain on continuously until almost the end of the mains
half-cycle. In addition to this there is circuitry that detects the firing angle of the triac and
adjusts the lamp current by increasing the switching frequency hence the controlling
light output depending on the level set by the dimmer.
It should be noted that if the dimmer is set too low the triac will never fire when a
capacitive load is connected and also when the ballast is running and the dimmer is
turned too low there will be insufficient bus voltage for the ballast to be able to operate.
Because of these factors it is impossible for the ballast to operate over the complete
range of adjustment of the dimmer. There will also be some hysteresis so when the
ballast is being dimmed down and reaches the point where the lamp goes out, the
dimmer has to be turned back up some way before the lamp will strike again.
BLOCK DIAGRAM
Triac Based
Domestic Dimmer
Doubler and Charge Pump
Control Stage
Output Stage
Lamp
(with auxilary filament
heating windings)
NC
VCC
VDC
RT
RPH
CT
CPH
1
2
14
13
VB
HO
VS
LO
CS
SD
IR2156
3
4
5
6
7
12
11
10
9
8
COM
Triac firing point sense
Lamp Arc Current Sense
FUNCTIONAL DESCRIPTION
When the dimmer triac is fired the current that flows must remain above the holding
current until a point close to the end of the mains half cycle at which the voltage is very
low. This is achieved by ensuring that the ballast draws current exceeding the holding
current of a standard power triac as used in most dimmers with a
charge pump
circuit
consisting of C5 and C6 in conjunction with D1 to D4 (not to be confused with the
charge pump that supplies the Vcc). When the ballast is operating the point between L2
and C15 swings low when Q2 is on, this charges C5 through D2 during the positive half-
cycle of the mains voltage. When Q2 switches off and Q1 switches on the voltage
between L2 and C15 swings high causing C6 to be discharged through D1 into the
storage capacitor C3. During the negative half-cycle of the mains voltage, the opposite
happens with C6 and C4. The result is that a continuous series of current pulses are
drawn from the input during the period when the triac has been fired until close to the
end of the mains half-cycle. The inductor L1 ensures that a continuous current is drawn
from the input and that the triac does not switch off between pulses. In order to do this
the inductor must store energy when current is being drawn via the charge pump and
release this energy during the period when C5 or C6 is discharging. When the ballast is
dimmed the frequency increases and so the current in C5 or C6 also increases,
ensuring that there will be sufficient to keep the triac switched at all dim levels.
3
IRPLCFL3
A
snubber
has also been included consisting of C2 and R2. The purpose of this is to
reduce ringing caused by the resonance of the series inductors and load capacitances
when the dimmer triac fires. R1 is also essential in achieving this and it is important to
select L1 such that it has a DC resistance of about 5 Ohms to provide additional
damping. If the series resistance is too low the ballast will not be stable at low dimming
levels.
SCHEMATIC DIAGRAM
PHASE CUT
LINE VOLTAGE
R3
120VAC
THROUGH
DIMMER
D1
R1
C3
L1
C5
5
6
R7
R11
IC1a
7
IC2
NC
VB
D8
Q1
L2
R22
C15
L2a
D2
1
VCC
14
HO
C2
C1
R2
R6
R8
D5
R10 C7
2
13
VS
R15
R16
VDC
C20
C13
Q2
D9
C17
C14
IR2156
3
RT
12
LO
4
11
CS
120VAC
LINE
R12
R9
R17
RPH
R19
D10
R22
C16
5
CT
10
SD
6
CPH
9
COM
D3
R4
C4
C6
D4
R5
D6
2
8
C8
R18
D7
1
IC1b
C9
7
8
R21
C18
D11
C10
C21
C11
R20
C19
L2b
D12
R13
3 4
C12
R14
Phase Cut Voltage and Current
Voltage at Ballast Input
Current at Ballast Input
4
IRPLCFL3
The voltage waveform at the junction of D2 and D3, ignoring high frequency
components is equivalent to the output voltage of the dimmer. With respect to the
negative rail of the bus this will be a phase cut approximate sine wave with a DC offset
such that the negative peak is at 0V. This is reduced by the voltage divider network of
R8 and R13 which is then fed into D6 and D15. Only the signal representing the positive
half-cycle of the mains is left at the anode of D5 which is then converted to a DC level
via the filter network of R7, R10 and C10. Because the minimum dimming level occurs
at a point where the dimmer is still capable of providing enough output for the ballast to
operate, this voltage will never actually be zero. Also when the dimmer is at maximum
this voltage will only be approximately 3 times the level at minimum dimming level.
Since we require a reference voltage to control the lamp current in a closed loop system
it is clear that this would only allow dimming down to one third of the lamp current which
is not sufficient range. For this reason an offset stage has been added. The inverting
input of op amp IC1a is connected to a reference voltage determined by R4 and R5
divided down from the IC supply voltage which is clamped at around 15.6V by the
IR2156. The gain is positive and set by the ratio of R11 and R6.
If the voltage at the non-inverting input is Vin and the voltage at the junction of R4 and
R5 is Vref, then Vout will be;
(1 + R11/R6).Vin – (R11/R6).Vref
In this circuit R6 = R11 therefore;
Vout = 2.Vin – Vref
The values of R4 and R5 have been selected so that when the dimmer is set to
minimum the output of IC1a will be zero and as the dimmer output is increased the
output of IC1a will increase to approximately 0.5V. The current sense resistor R14 is
selected to give 0.5V when the lamp current is at maximum.
The error amplifier is implemented in IC1b. The lamp current feedback is fed into the
non-inverting input and the control voltage is fed into the inverting input. The input
resistors R12 and R14 have been chosen with an integrating capacitor of 10nF to give
the best response and elimination of flicker but to avoid generating RF interference. The
output of IC1b works as a current source. The more positive the output swings the more
current is supplied to the CT pin of the IR2156 which is added to the current provided by
RT causing CT to charge more quickly and the frequency to increase. If IC1b output is
zero the frequency is at its minimum determined by the value of RT only. During preheat
the frequency is set by RPH only and IC1b has no effect.
R9 ensures that even when the dimming reference voltage from the output of IC1a is
zero, the error amplifier has a small voltage at the inverting input which means that it is
impossible to dim the lamp below a particular point. This point is selected as the
minimum lamp level at which the lamp will remain lit and will not flicker. The resistor
R22 can be added if necessary to remove
striations
(dark rings) in the lamp that may
occur at low dimming levels.
5
stm32/stm8
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