How compact fluorescent lamps work--and how to dim them

Fluorescence versus incandescence
Incandescence is the conversion of heat to light, which requires the filament inside an incandescent lamp to burn at a high temperature (350° F or 176° C). This conversion is very simple but the disadvantages are that only 5% of the total energy consumed by the lamp is used to generate light (95% is wasted as heat!) and the lifetime is limited to about 2,000 hours.

Fluorescence is the conversion of ultraviolet (UV) light to visible light. Electrons flow through the fluorescent lamp and collide with mercury atoms, causing photons of UV light to be released. The UV light is then converted into visible light as it passes through the phosphor coating on the inside of the glass tube.

This two-stage conversion process is much more efficient than incandescent lamp process, resulting in 25% of the total energy consumed used to generate light, lower lamp temperatures (40° C) and longer lifetime (10,000 hours). The lamp load itself is resistive, but the electronic ballast that is connected between the AC line voltage and the lamp for controlling the lamp current is a capacitive load. The complete CFL (Figure 1) includes the Edison screwbase and plastic housing, the electronic ballast, and the fluorescent lamp formed into a compact spiral shape.

Figure 1: CFL components and assembly
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CFL Operation
The electronic ballast circuit block diagram (Figure 2) includes the AC line input voltage (typically 120 VAC/60 Hz), an EMI filter to block circuit-generated switching noise, a rectifier and smoothing capacitor, a control IC and half-bridge inverter for DC to AC conversion, and the resonant tank circuit to ignite and run the lamp. The additional circuit block required for dimming is also shown; it includes a feedback circuit for controlling the lamp current.

Figure 2: CFL electronic ballast block diagram
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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-to-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.

During pre-ignition, the resonant tank is a series-LC circuit with a high Q-factor. After ignition and during running, the tank is a series-L, parallel-RC circuit, with a Q-factor somewhere between a high and low value, depending on the lamp dimming level.

When the CFL is first turned on, the control IC sweeps the half-bridge frequency from the maximum frequency down towards the resonance frequency of the high-Q ballast output stage. The lamp filaments are preheated as the frequency decreases and the lamp voltage and load current increase (Figure 3).

Figure 3: CFL operation timing diagram
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The frequency keeps decreasing until the lamp voltage exceeds the lamp ignition voltage threshold and the lamp ignites. Once the lamp ignites, the lamp current is controlled such that the lamp runs at the desired power and brightness level.

To dim the fluorescent lamp, the frequency of the half-bridge is increased, causing the gain of the resonant tank circuit to decrease and therefore lamp current to decrease. A closed-loop feedback circuit is then used to measure the lamp current and regulate the current to the dimming reference level by continuously adjusting the half-bridge operating frequency.

The IRS2530D Dimming Control IC from International Rectifier includes such a feedback control circuit, as well as all of the necessary functions to preheat and ignite the lamp,and to protect against fault conditions such as open filament failures, lamp non-strike and mains brown-out. The dimming function is realized by combining the AC lamp current measurement (Figure 4) with the DC reference voltage at a single node. The AC lamp current measurement across the sensing resistor RCS is coupled onto the DC dimming reference through a feedback capacitor CFB and resistor RFB.

Figure 4: IRS2530D AC+DC dimming control method..
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The feedback circuit regulates the valley of the AC+DC signal to COM as the DC dimming level is increased or decreased by continuously adjusting the half-bridge frequency. This causes the amplitude of the lamp current to then increase or decrease for dimming. If the DC reference is increased, the valley of the AC+DC signal will increase above COM and the feedback circuit will decrease the frequency to increase the gain of the resonant tank.

This will increase the lamp current, as well as the amplitude of the AC+DC signal at the DIM pin, until the valley reaches COM again. If the DC reference is decreased, the valley will decrease below COM. The feedback circuit will then increase the frequency to decrease the gain of the resonant tank until the valley reaches COM again.

3-Way Dimming
One popular dimming application is for 3-way lamp sockets. The 3-way dimming incandescent lamps include two filaments and two connections on the lamp screw base. A 4-position switch in gthe socket (off, low, medium, high) is then used to switch between different filament connections, to step through three dimming levels (Figure 5).

Figure 5: 3-way dimming CFL circuit schematic
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The first socket switch position is the off setting, where no filaments are connected, the second position connects the first filament across the AC line for the lowest-brightness setting, the third position connects the second filament for the medium-brightness setting, and the fourth position connects both filaments in parallel for the highest-brightness setting. To achieve the equivalent functionality for a CFL, a dimming electronic ballast circuit is used to control the lamp current for each brightness level.

The circuit includes a rectifier and voltage doubler circuit at each input (D1, D2, D3, D4, C3 and C4), the half-bridge control circuit and MOSFETs (IRS2530D, Q1 and Q2), the resonant tank (LRES and CRES), the lamp-current sensing and feedback circuit (RCS, RFB and CFB), and the 3-way interface circuit (R3, R4, R5, R6, R7, RPU, Q3, Q4, DZ1 and C5). As the switch position is changed for each dim setting, the circuit detects the change in voltage at the two screwbase input connections (PL1 and PL2) with the voltage divider formed by resistors R5, R6 and R7.

Resistors R5 and R6 pull up the DC dimming reference across resistor R7 and capacitor C5 to the appropriate level to set the minimum and medium brightness levels. To set the maximum brightness level, transistors Q1 and Q2 are both turned on and the DC dimming reference is then pulled up high enough to ensure the circuit will reach the maximum brightness level.

The IRS2530D controls the preheat and ignition timing with capacitor CPH, and controls the dimming loop speed with capacitor CVCO. If the lamp does not ignite, or one or both of the filaments open up, then the IRS2530D will disable the complete circuit safely to prevent excessive voltages or currents from damaging components.

The waveforms from the circuit (Figures 6A, 6B, and 6C) show the lamp current and voltage at each brightness setting. For the 32 W lamp load, the measured lamp current is approximately 240 mA at the maximum brightness level, 94 mA at the medium level, and 31 mA at the minimum level. The operating frequencies at each level are 43 kHz at maximum, 62 kHz at medium, and 67 kHz at minimum.

Figure 6A, B, C: Lamp voltage (upper, 100 V/div) and current (lower, 200 mA/div) at each 3-way dim setting (time=10 μsec/div); (A) Maximum, (B) Medium, and (C) Minimum
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Conclusion
The dimming function opens up a completely new family of CFL applications. Each dimming application presents a different set of challenges, especially with the interface circuit required. The dimming control loop required to regulate the lamp current is basically the same for each application.

The challenge is to design each different interface circuit that converts the user dimming method to the necessary DC dimming reference. The new IRS2530D greatly simplifies dimming designs and helps close the gap between dimming and non-dimming designs. This will enable CFL products to compete with incandescent ones, while maintaining a small form factor and a low cost. Additional dimming circuits to consider to further enhance the performance of CFLs include triac dimming, powerline communication and wireless applications.

About the author
Tom Ribarich is the Director, Lighting IC Design Center, at International Rectifier Corporation, El Segundo, CA, where he is responsible for developing control ICs for the global lighting market, including fluorescent, halogen, HID, LED and LCD backlighting applications. He has a BSEE degree from California State University, Northridge, and a master's degree in ASIC design from University of Rapperswil, Switzerland. He has also designed ASICS for new generation of high-performance electronic ballast products