Dimming operation
Dimming of fluorescent lighting offers significant benefits – giving users control of their own lighting, and realizing energy savings. In certain environments dimming is a must, for instance in a lecture theater to modify the lighting level when beaming a presentation. In other applications the flexibility it gives to people has significant value, allowing them to match the luminance on the working plane to their personal needs. The control systems surrounding dimming of fluorescent lamps are becoming increasingly complex, for instance when linked to a daylight harvesting system to provide artificial lighting only at times of reduced natural illumination. Dimming creates a rich visual experience and adds flexibility to any room, providing the right lighting environment for a variety of activities. An added value is the fact that modern electronic ballasts permit a reduction in power consumption in parallel with the reduced light level, and this presents a valuable energy saving opportunity. A further advantage is the use of dimming as an alternative to repeated on/off switching. In this way substantial energy savings can be realized but without negatively impacting lamp life.
A light dimmer works by essentially chopping parts out of the ac-voltage. This allows only parts of the waveform to pass to the lamp. The brightness of the lamp is determined by the power transferred to it, so the more the waveform is chopped, the more it dims. At the zero crossing point of the ac, a dimmer is electronically synchronized to turn the power on or off. By chopping the waveform at the zero-crossing point, smooth dimming can be achieved without the lamp flickering. Traditionally light dimmers are manufactured using a triac or thyristor as the power control device. A thyristor is a uni-directional device and hence, because ac-power flows in both directions, two devices are needed. A triac is a bidirectional device and therefore only one device is needed. An electronic circuit determines the point in time at which they turn-on. The on-state continues until the next zero-crossing point, at which point the device turns itself off. Fig. 1(a) shows dimming operation with a triac or thyristor. However, this leads to high EMI and noisy light bulbs, as the current spike during the turn-on causes vibrations of the filament. The IGBTs (insulated gate bipolar transistors) have the ability of being turned off anytime during the phase. Therefore, by using IGBTs to control the lights, the dimming can start on the leading edge of the power cycle as shown in Fig. 1(b) . This allows for a smooth turn on and off, thereby eliminating the rapid rise times that produce noise. This design eliminates costly noise filtering and extends the life of the lights by reducing thermal shock.
Fig. 1a: Dimming with a triac/thyristor can lead to EMI; Fig 1b: IGBT dimming starts on the leading edge for smooth turn-on.
MOSFET as power switch
The MOSFETs (metal oxide semiconductor field effect transistors) are also switching power device like IGBTs. They typically characterized as faster switching but higher conduction loss during on-state compared to IGBTs. The MOSFET is majority carrier device and therefore it is not suffering from tail current that happens in bipolar power devices such as IGBT or BJT (Bipolar Junction Transistors). Consequently MOSFETs have less switching loss and are capable of higher switching frequency than IGBTs. On the contrary bipolar devices are having benefits from conductivity modulation during on-state. The conductivity of IGBT is getting lowered during on-state thanks to hole injection from additional p-type layer, and it results in lower conduction loss of IGBT compared to MOSFETs especially at high current range. A drawback of bipolar action is certain amount of voltage drop – Vce(sat) across the IGBT even with small current level.
The operating frequency of power switch in light dimmer is line frequency. Since the switching frequency is low, IGBTs are often considered as appropriate power switch for dimmers. However, operating current is less than 4 A even with the case of 400-W dimmer as shown in Fig. 2 . The average current through the power switch is 1.52 A when on-duty is 0.8. Typical IGBT rating for this 400-W rated dimmer is 600 V, 15 A, and Vce(sat) of IGBT is around 1.4 V at 3 A, 100°C. In this current range, MOSFET can achieve lower conduction loss when on-resistance is below 920 mΩ at 100°C. There is almost no switching loss as shown in Fig. 2. Therefore conduction loss is primary power loss and MOSFET can be selected based on on-resistance characteristics. With the latest high voltage MOSFET technologies, low on-resistance MOSFETs are widely available at affordable price. Two candidates are selected and evaluated. One device is 600 V, 550 mΩ planar MOSFET (FDPF17N60NT), and the other is 600 V, 300 mΩ super-junction MOSFET (FCPF190N60). All on-resistance values are rated at 100°C.
Fig. 2: Operating Waveforms
With simple math, 550-mΩ 600-V MOSFET will have 1.27-W conduction loss while 15-A 600-V IGBT has conduction loss of 2.13 W. The MOSFET solution will definitely show better efficiency and thermal performance. Fig. 3 shows operating temperature of devices under test. The 15-A 600-V IGBT runs hotter than MOSFETs as expected. The difference in package should be noted here. The 15-A 600-V IGBT is packaged in TO-3P but two MOSFETs are in TO-220F, smaller and fully isolated package.
Fig. 3: Device operating temperature
The MOSFETs are evaluated as alternative power switch for light dimmer application. The planar MOSFET, FDPF17N60NT performance is equivalent to 15-A 600-V IGBT, and the super-junction MOSFET, FCP190N60 performs much better. They provide smaller footprint, easier assembly, and better performance.
About the author
Won-Seok Kang has worked for Fairchild Semiconductor since 2006 and is currently an Industrial Senior Application Engineer for Fairchild’s Switch Mode Power Supply in the Power Conversion Division located in Korea. His research interests are in functional power module, LED drivers, resonant and soft switching dc/dc converters, and electronic ballast. He has a B.S. and M.S. degree in electronics engineering.
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