Analog Dimming: A Brighter Way to Design Lamp Ballasts

Oct 1, 2008 12:00 PM
By Peter B. Green, Senior Lighting Systems Engineer, International Rectifier, Lighting Group, El Segundo, Calif.

When considering alternative approaches to the design of dimmable electronic ballasts for fluorescent lamps, the lamp power regulation method employed is the critical factor. It determines the linearity (i.e., the linear response of the light output with the dimming control input), the smoothness of dimming and, most importantly, the stability of the light output. Both digital and analog approaches are possible.

The method most often used in dimming ballasts involves controlling the lamp power by modulating the frequency of the switched dc bus. This is done through a series inductor and a parallel resonant output capacitor network. Increasing the resonant circuit's frequency reduces the lamp current and vice versa (Fig. 1). Pulse-width modulation of the half-bridge voltage also is used in some dimming designs to widen the dimming range. However, this is not usually necessary.

Analog Versus Digital

Analog dimming control is generally simpler to implement than a digital approach in this application. This is mainly because microcontrollers are limited to adjustment of the ballast frequency in steps, as the output frequency is divided down from the clock frequency. Even with a clock frequency of 20 MHz, a single-step frequency change in the lamp is generally visible by the human eye. It then becomes a complicated problem to disguise these step changes in light output.

Advanced microcontrollers are available that contain additional functionality specially designed to disguise frequency steps. However, such microcontrollers are relatively complex and expensive. Microcontroller designs also require additional high-side/low-side driver ICs to supply the half-bridge MOSFET switches, as well as circuitry to provide a 5-V supply capable of sourcing several milliamperes of current (Fig. 2).

In analog designs, linear adjustment is possible at any frequency, which allows smooth dimming with no steps. Several different choices of analog ballast-control application-specific ICs (ASICs) are available for dimming control. They also include an integrated high-side and low-side driver, as well as a variety of fault-protection and ballast-management functions.

Two alternative approaches of analog dimming regulation will be considered here. Both employ frequency modulation as a means of power control, but use two different techniques for providing the feedback information from the lamp that the system uses to regulate. Frequency modulation enables effective dimming down to less than 5% of the Lumen output for fluorescent lamps. Below this level, and depending on the specific lamp characteristics, the frequency modulation method can run into difficulties maintaining very low arc currents, because the frequency is substantially above the resonant frequency of the output circuit.

Maintaining the arc discharge in the lamp is difficult at very low dimming levels, regardless of which control method is being used. This limitation is generally more problematic with an increasing lamp length and a smaller tube diameter.

The Arc-Current Method

The first feedback method considered here senses the arc current discharged through the lamp, which includes neither the cathode currents nor the current in the resonant output capacitor. This current is converted to a voltage through a shunt resistor and fed to one input of an integrating error amplifier. The other input of this error amplifier is connected to the dimming control reference input to provide the target dimming level, and the output is connected to a voltage-controlled oscillator (VCO), which drives the power switching stage. In this way, the loop is closed and the lamp arc current is directly controlled (Fig. 3).

It is possible to obtain a very stable output response over the range of dimming by optimization of the integrating capacitor and resistor in the error amplifier circuit. The best results are achieved when the loop-frequency response is optimized for the particular lamp being driven, because the characteristics of different lamps vary. Lamps with smaller tube diameters, such as T5 types, are generally more susceptible to instability than those with larger diameters, such as T8 and T12 types.

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