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Posted on 17 July 2019

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A Simple yet Versatile Dimming-Ballast Solution for Low-Energy Lighting

EU-wide legislation promoting use of Compact Fluorescent lamps (CFLs) in place of incandescent bulbs could save as much as 1.2 million tonnes of greenhouse gas emissions per year, according to the European Efficient Residential Lighting Initiative (EnERLIn).

By Andre Tjokrorahardjo, Applications Engineer, Lighting Systems and Applications, International Rectifier

 

Lamp dimming is also known to be effective in saving energy, but CFLs have historically been unable to operate with the phase-cut (triac-based) dimmers most commonly installed in house and commercial premises. To maximise the energy-saving benefits of CFLs, ballasts compatible with phase-cut dimmers are required. However, these must be delivered at extremely low cost, and must also be consistent with the established form factors for incandescent-replacement CFL units.

Bright Ideas for Dimming

International Rectifier’s IRS2530D DIM8™ is an example of this new type of dimming-ballast control IC. This device leverages highvoltage technology and advanced ballastdesign IP allowing the high-side and lowside half-bridge drivers, and closed-loop lamp current dimming control to be fabricated on the same chip as the functions to control preheat, ignition and dimming. Halfbridge voltage-sensing circuitry for protection is also built in. This enables a higher level of integration than other triac-dimmable ballast ICs, leading to lower parts count and lower costs.

In addition, the DC-dimming reference input and AC lamp current feedback are combined so that a single pin can be used for dimming. This allows the IC to be delivered in a compact and low-cost 8-pin package.

The device can control analogue or digital dimming for CFL or linear fluorescent lamps. Moreover, with minimal modifications to the external output components the IC can also be used to dim LEDs, thereby presenting a versatile solution that will realise additional cost advantages through economies of scale.

Phase-Cut Dimmer Interface

Phase-cut dimmers are built using triacs, which are fired once in each half cycle at a phase angle corresponding to the desired dimming level. After firing, the triac will conduct for as long as the current remains in excess of the device’s holding current. These dimmers work very well with a resistive load such as an incandescent light bulb as the triac conducts continuously from firing until very close to the end of the half-cycle. In contrast, traditional CFL ballast, without power factor correction, operates from a DC level provided by a storage capacitor. Current is drawn from the mains near the peak of the mains voltage, to charge the storage capacitor, but not during the remainder of the mains half-cycle. Hence the traditional ballast prevents proper conduction of the dimmer triac, resulting in severe flickering of the lamp.

Figure 1 shows the schematic of a triac-dimmable CFL ballast using the IRS2530D, suitable for driving a single 15W spiral CFL from an AC line input. Capacitors C2, C3, C4 and C5 interface with the triac in the dimmer, allowing the ballast to maintain triac conduction until almost the end of the mains halfcycle. In addition, there is circuitry that detects the firing angle of the triac and adjusts DC reference voltage to set the lamp current.

Triac-dimmable CFL ballast

The voltage waveform at the junction of D1 and D4 is equivalent to the output voltage of the dimmer. This approximates to a phasecut sine wave with a DC offset such that the negative peak is at ground. This is reduced by the voltage divider network of R2 and R3 which is then fed into D5 and D6. Only the signal representing the positive half-cycle of the mains is left at the anode of D6. The filter of R4 and C6 converts this to a DC level. 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. The DC level is further reduced with the voltage divider network of R5 and R6, and used as the DC dimming reference voltage.

The AC lamp current is sensed by the resistor RCS, and the resulting AC voltage is coupled with the DC dimming reference voltage. This DC + AC signal is then fed into the DIM pin of IRS2530D. The dimming control loop of the IRS2530D regulates the amplitude of the lamp current by continuously adjusting the frequency of the halfbridge switching circuit. In this way, the dimming control circuit keeps the AC lamp current peak-to-peak amplitude regulated to the desired value at all firing angle of the dimmer.

Analogue and Digital Dimming

Because of the simplicity of its dimming control method, the IRS2530D can be easily utilised for other dimming systems, including analogue and digital techniques (figure 2). The ballast designer needs to determine how to generate the proper DC voltage reference from the dimming control method being used.

Support for multiple dimming techniques and low-energy lighting technologies

Typical analogue dimmers include 1- 10VDC dimmers, three-wire phase control, and dimmers controlled by photo-sensor, motion-sensor or wireless infrared. In these, the DC reference voltage is typically set using a voltage divider network. A low reference voltage is desirable, to limit power losses in the current-sense resistor. On the other hand, setting the voltage too low can incur noise problems at the lower dimming levels.

Digital dimming delivers advantages including simplified wiring and precise control of the dimming level, as well as the potential for twoway communication. Logarithmic control of the dimming level is possible, which produces apparently linear dimming since the human eye is much more sensitive to lower light levels than high levels. The most prominent digital dimming method is the open-standard twowire Digital Addressable Lighting Interface (DALI).

A DALI dimming ballast designed around the IRS2530D requires a microcontroller to communicate with the digital protocols. The microcontroller is used to interpret the data from the digital control, and generates the square-wave signal with fixed frequency but varying duty-cycle which corresponds to the desired dimming level.

Dimmable LED Driver Using the IRS2530D

The IRS2530D can also be used to control and dim the current of LED, using a resonant-mode circuit architecture similar to that of a dimming CFL ballast. An example of an off-line LED driver schematic using the IRS2530D is shown in Figure 3. Unlike a fluorescent lamp, an LED requires constant current control and does not need to be preheated or ignited. With this in mind, a modified output stage is used. This replaces the preheat/ignition tank circuit with a series L-CLED configuration and a full-bridge rectifier to convert the resulting square-wave AC output to a positive full-wave rectified voltage. The AC current sensing is still performed by the current-sensing resistor (RCS). This gives a direct AC measurement of the full-wave rectified LED current amplitude. This AC signal is then coupled with the DC voltage from the current reference setting onto the DIM pin of the IRS2530D.

Dimmable LED driver using IRS2530D

The dimming control loop of the IRS2530D regulates the amplitude of the LED current by continuously adjusting the frequency of the halfbridge switching circuit such that the nominal r.m.s LED current is maintained within the manufacturer’s specifications. When the DC reference voltage is decreased for dimming, the switching frequency is increased to decrease the gain of the resonant circuit and thus decrease the LED current. This control scheme keeps the LED current constant over line, load and temperature variations for any given dimming reference input.

The IRS2530D’s dimming control loop allows the circuit to be scaled to any number of LEDs in series. In order to work with LEDs with different current rating, the current-sense resistor and the DC reference setting need to be adjusted accordingly.

Conclusion: Solution for Lower-Energy Lighting

The IRS2530D can also be used in dimming ballasts for standard fluorescent tubes. A number of reference designs with evaluation hardware are available, including a 1-10VDC analogue dimmer, a quadlevel switch dimmer, and a three-way dimmer for CFL replacement of double-filament incandescent bulbs; these are widely adopted in the US. Each of the reference designs can also be adapted for LED dimming, helping to maximise the environmental benefits of today’s most commonly used low-energy lighting technologies.

 

 

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