Not recommended for new designs - please refer to IRPLCFL8U
IRPLCFL4
International Rectifier
•
233 Kansas Street, El Segundo, CA 90245 USA
Rev.C
A 3 Way Dimming CFL Ballast
By
Peter Green
TOPICS COVERED
Introduction
Existing Ballast Solutions
Functional Description
Protection Circuits
Bill of Materials
Introduction
The 3 way dimming system widely adopted in the US with conventional filament lamps consists of a
light bulb that has a modified Edison screw type base which allows 3 connections to be made to a
special lamp socket that also has 3 connections.
Layout Issues
Component Selection
Output Inductor Design
Lamp Preheating
Standard Edison Screw Base
3 Way Dimming Edison Screw
Base
Live
Neutral
Live 1 Live 2 Neutral
IRPLCFL4
The 3 way dimming light bulb has two filaments inside which produce different light outputs when
connected to the AC line. These filaments are connected in series such that the mid point goes to the
line common and the two ends can be connected to the live either independently or both together.
Thus with an external switch that has four positions, it is possible to obtain 3 different light levels or to
switch off.
3 Way Dimming Switch
3 Way Dimming Light Bulb
Live
120V AC Line
Neutral
40W Filament
60W Filament
Existing Ballast Solutions
There are in existence CFL ballast designs that perform the same task. A common approach is a
system whereby the line voltage is full wave rectified when one live input is connected and a voltage
doubler circuit comes into operation when the other live input is connected or both are connected
together thereby having two DC bus voltages in the ballast during dim level settings. This type of
design also operates at two different frequencies, a low frequency (typically 40-45kHz) when both
live inputs are connected providing a high lamp current and a higher frequency (for example 70-
75kHz) when either of the two lives is connected alone which will produce a lower lamp current. In this
way the following combinations are achieved:
1. Low DC bus (150V) / high frequency ….. minimum output
2. High DC bus (300V) / high frequency …... medium output
3. High DC bus (300V) / low frequency …… maximum output
2
www.irf.com
IRPLCFL4
This approach has some serious drawbacks:
Firstly, since the ballast must be designed to give 100% light output for the lamp when the bus voltage
is 300V and the frequency is 40kHz, it is not easy to achieve satisfactory preheat and ignition when
the bus voltage is at 150V because of the limitations in the peak voltage that the output circuit is able
to produce from a 150Vpp half bridge voltage.
One strategy that has been used is to omit the preheating phase and steer the oscillator frequency to
resonance during ignition using feedback from the output circuit. This ensures that at switch on the
highest possible ignition voltage will be applied to the lamp. In this way the lamp will ignite in whichever
position the 3 way switch is set.
Such a scheme could reliably ignite the lamp when the DC bus is at 300V, however without correct
preheating the ignition voltage of the lamp and consequently the peak current in the MOSFET half
bridge during ignition will be higher. Also the life of the lamp is substantially reduced when there is no
preheat due to far greater stress occurring on the cathodes at the point of ignition.
Ignition when the DC bus voltage is at 150V is very difficult. Tests indicated that sweeping the
frequency down through resonance failed to produce sufficient ignition voltage leaving the ballast in
open circuit running mode with inevitable hard switching at the half bridge. The conclusion from this
is that the ballast needs to oscillate at resonance for an extended period of time in order for the lamp
to ignite at 150V considering that the output inductor and capacitor have been designed to produce
100% lamp power at 300VDC bus when the frequency is 40-45kHz.
Many CFL ballast designs do not incorporate a current sense and shutdown function to protect the
circuit in the case of ignition failure and so the ballast would eventually fail if left switched on due to
the high MOSFET switching losses causing thermal destruction. This would not matter with and
integrated ballast / lamp type product when the lamp has failed.
It has also been observed that hard switching occurs at the MOSFET half bridge when the DC bus
voltage is low in position (1) since when the ballast is running it will be close to resonance, bearing in
mind that the resonant frequency shifts downwards in run mode. Hard switching is very undesirable
because of the high peak currents that occur when each MOSFET switches on. This has been shown
to result in a higher rate of field failures in ballasts due to MOSFET failure.
The conclusion is that the approach to design described above is unable to provide a reliable ballast.
Solution
A completely new approach has been developed that overcomes all of the above limitations based
around the popular and versatile IR2156 ballast control I.C.
www.irf.com
3
IRPLCFL4
Functional Description
This solution adopts a completely different approach to the problem. Here we incorporate a voltage
doubler at the front end in all modes of operation giving a fixed 300VDC bus. Consequently by
choosing correctly the dead time and value of the snubber capacitor it is not difficult to achieve soft
switching in all modes of operation.
By simply changing the frequency between 3 defined settings, however, it was found to be extremely
difficult to set a point where the dim level is 50%. The problem with this is that the lamp current against
ballast frequency characteristic of the system exhibits a very sharp knee such that as the frequency
increases the lamp current is gradually reduced up to a point at which a small increase of frequency
will result in a very large reduction in the lamp current.
Ballast / Lamp Operating Characteristic
Lamp
Current
Ballast Running
Frequency
4
www.irf.com
IRPLCFL4
To obtain 50% output the frequency would have to be very precisely set. This is not practical since
the tolerances of the output inductor, capacitor and oscillator timing components do not allow this.
Even if each ballast was individually adjusted in production variations in lamp behavior over tempera-
ture would mean that under some conditions the lamp arc would extinguish at this setting leaving the
system in permanent preheat which would burn out the cathodes eventually.
This explains why the 150VDC bus solution has been adopted as this allows 50% output to be
achieved without this problem. However the disadvantages discussed in the previous section demon-
strate that this approach is not without some major disadvantages.
Consequently it becomes necessary to include a closed loop feedback system that controls the lamp
current by adjusting the ballast frequency from a VCO (voltage controlled oscillator) driven by the
output of an error amplifier that senses the lamp arc current directly and compares it with a refer-
ence. This same strategy has been used in the IRPLCFL3 reference design “A Ballast that can be
dimmed from a domestic (phase cut) dimmer” and has been demonstrated to be capable of control-
ling the lamp output down to levels below 10% with very good stability. This also compensates for any
tolerances in the components of the circuit or the lamp.
Regulated Lamp Current Control System
VCO
&
DRIVER
∑
Reference
+
Lamp Arc Current
-
www.irf.com
5
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
