IC fluorescent dimming considerations

Here’s what you need to know about dimming fluorescent lighting

BY T. RIBARICH
International Rectifier, El Segundo, CA
http://www.irf.com

This article provides an overview of fluorescent dimming and an application circuit for low-end small fixture applications ICs where dimming levels below 10% are required and more comprehensive protection features are needed.

The electronic ballast circuit block diagram (see Fig. 1 ) includes the ac-line input voltage (typically 220 Vac/50 Hz or 120 Vac/60 Hz), an EMI filter to block circuit generated switching noise, a rectifier and smoothing capacitor for ac to dc conversion, a control IC and half-bridge inverter for dc to high-frequency ac conversion, and the resonant tank circuit to ignite and run the lamp. The additional circuit block required for dimming is also shown that includes a dimming reference signal, a lamp current sensing and feedback signal, and a summing circuit for closed-loop control of the lamp current.

Fig. 1. Dimming electronic ballast block diagram.

The lamp requires a current to preheat the filaments, a high-voltage for ignition, and a high-frequency ac current during running. To fulfill these requirements, the electronic ballast circuit first performs a low-frequency ac/dc conversion at the input, followed by a high-frequency dc-to-ac conversion at the output. The ac mains voltage is full-wave rectified and then peak-charges a capacitor to produce a smooth dc bus voltage. The dc bus voltage is then converted into a high-frequency, 50% duty-cycle, ac square-wave voltage using a standard half-bridge switching circuit. The high-frequency ac square-wave voltage then drives the resonant tank circuit and becomes filtered to produce a sinusoidal current and voltage at the lamp.

Dimming mini-ballast design

The complete schematic is shown in Fig. 2 . The 220-Vac/50-Hz line input voltage is full-wave rectified (BR1) and then goes through the EMI filter (CF and LF) before being smoothed by the dc bus capacitor (CBUS).

Fig. 2. IRS2530D dimming mini-ballast circuit schematic.

The half-bridge switches (MHS and MLS) are controlled by the IRS2530D DIM8 IC for preheating, igniting and dimming the lamp. RVCC1 and RVCC2 provide the micro-power start-up current for the VCC supply of the IC, and the charge pump (CSNUB, DCP1, and DCP2) takes over as the IC supply once the half-bridge begins to oscillate.

The resonant tank circuit (LRES and CRES) provides the necessary transfer function for generating high voltages for lamp ignition and low-pass filtering for dimming. A dc-blocking capacitor (CDC) ensures that the lamp current is always ac to prevent mercury migration which can cause lamp end blackening and a shortened lamp life.

Secondary windings from the resonant inductor (LRES:A,B) are used to heat the lamp filaments during preheat and dimming, and also separate the lamp current from the filament current allowing for a single current-sensing resistor (RCS) to be used to sense the lamp current. The ac lamp current measurement across RCS is coupled to the DIM pin through a feedback capacitor and resistor (CFB and RFB). A potentiometer dimming input circuit is used (PDIM, RMIN, RMAX) to convert the potentiometer resistance to the necessary dimming reference voltage for the IC that is connected to the DIM pin.

Finally, resistors RLMP1 and RLMP2 are used to detect if the lamp has been removed and to automatically restart the ballast when the lamp is re-inserted. Protection against all other ballast fault conditions such as failure to strike, open filament, and low ac line/brown-out, are included internally to the IRS2530D to further reduce component count and increase reliability.

Fig. 3. Mini-ballast measured waveforms.

The measured ballast waveforms are shown in Fig. 3 . Figure 3a shows the VCO pin voltage (upper trace), lamp voltage (middle trace) and lamp current (lower trace) during normal preheat, ignition and dimming modes. The VCO pin and lamp voltage ramp up during preheat and ignition to preheat the lamp filaments and then to ignite the lamp when the lamp ignition voltage threshold is reached.

Lamp current starts to flow immediately after ignition at the start of dimming. Figures 3b and 3c show the half-bridge output voltage (VS pin, lower trace) together with the DIM pin voltage (upper trace) during 100% and 10% dimming conditions. The DIM pin voltage amplitude decreases (together with the lamp current) from 100% down to 10% and the operating frequency is continuously adjusted to keep the valley of the sinusoid regulated at COM.

Linear dimming ballast

A linear dimming ballast for a single 28W/T5 fluorescent lamp is shown (see Fig. 4 ) that is designed around the IRS2158D ballast control IC. This part incorporates fixed frequency preheat, regulated ignition, lamp fault and end of life protection circuitry that covers all failure modes, as well as auto-restart when the lamp is removed and replaced.

Fig. 4. IRS2158D dimming linear ballast circuit schematic.

In addition a low-offset operational amplifier is included, which is used in this design to implement lamp dimming by frequency modulation. The lamp arc current is fed back and compared with a control voltage derived from a potentiometer (RDIM).

The upper and lower dimming limits are bounded by the resistors RDIV1 and RDIV2. The op-amp output produces an error voltage, which is connected to the FMIN pin through a diode and by sinking more current from the FMIN pin, it causes the frequency to be increased, which in turn reduces the lamp power.

Stability is achieved by optimizing the response time of the control loop by correct selection of capacitor CDIM. The value of this is likely to vary depending on the particular lamp that is being dimmed.

Fig. 5. T5/28W linear ballast measured waveforms.

The measured ballast waveforms are shown in Fig. 5 . Figure 5a shows the lamp voltage (upper trace) and CPH pin (lower trace) during normal preheat, ignition and running modes. Fig. 5b shows the lamp current (upper trace, black), lamp voltage (upper trace, blue) and the half-bridge output node (lower trace, green) during 100% dimming. Figure 5c shows the lamp current (upper trace, black), lamp voltage (upper trace, blue) and the half-bridge output node (lower trace, green) during 5% dimming.

The IRS2530D DIM8 IC includes all of the necessary functions for smooth dimming and protection against fault conditions. The dimming mini-ballast circuit provides excellent performance in fewer than 35 components. The small 8-pin package together with low component count allows for the complete design to be realized in a small mini-ballast form factor. The IRS2158D offers additional programmability to the designer such as the oscillator frequency range and dead-time, and includes a non-dedicated, low-offset op amp. The IRS2158D also includes an additional end-of-life protection feature that is mandatory for T5 lamps.

To adjust either circuit for different lamp types, power levels or line input voltages, International Rectifier offers a Ballast Design Assistant (BDA) software program to help designers get their ballast designs working on the bench quickly. This program allows the user to select from different line input voltage ranges, lamp types and lamp configurations, and then generates necessary ballast output data, IC programmable component values, inductor specifications, and the complete schematics and bill of materials. ■