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Part: IRPLCFL4
Category:
Power Management
Description: a 3-Way CFL Dimming Ballast
Company: International Rectifier Corp.
Datasheet: Download IRPLCFL4 datasheet File size : 52 kB
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Datasheet text preview:
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 7075kHz) 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
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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
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