The DC source can come from rectified AC mains (as seen on standard CFLs) or from a low-voltage bus or battery (shown on camper interior lights, laptop screen taillights, or emergency lights).
How are DC-to-AC circuits built?
The book, Practical Home Electrical Power Electronics, published by Elektor, contains a chapter on CFL inverters with some circuit diagrams of reverse engineered inverters and an engineering explanation of how they work. See http://www.elektor.com/products/books/home/practical-eco-electrical-home-power-electronics.35.1079402.lynkx”>Practical Eco-Electrical Home Power Electronics published by Elektor.
The fluorescent tube has different circuit models when lit and unlit, and they correspond to two different resonance modes that the reflector must adapt to in its design. After ripping off multiple CFLs, I find the design is well standardized as mentioned in the previous answer for battery powered lighting, and as a half bridge (sometimes preceded by a voltage multiplier) for line powered CFLs.
All of these transformers are buzzing and when the lamp is not lit, it depends on its capacitance to adjust the buzzing frequency. Once lit, the lamp has a low impedance value and a series capacitor with the lamp sets the serial buzzing frequency.
The vast majority of circuits in use are resonant transformers (eg Royer transformers; see Bright, Pittman, and Royer, “Transistors as on-off switches in saturated core circuits”, Electrical Manufacturing, December 1954.). The pulse current is fed through a retrograde transformer to the primary connections of the drive transistors through the auxiliary windings on the same transformer.
This answer to a question about the special transformers used in these resonant transformers provides plenty of links to good sources for further reading. Compact fluorescent lamps (CFLs) use a very simple but elegant type of such circuit, the basic saturation properties of which determine the power output of the lamp, while most backlight circuits for computer monitors or laptops use this circuit by electronic means of pre-regulation, as designed by Jim Williams (1948). -2011) and is documented as US Patent No. 5,408,162 and 6,127,785 and Linear Technology Application Notes AN49, AN55 and AN65. This concept was developed into the use of piezoelectric transducers, cf. AN81.
There are also circuits that use a constant frequency oscillator and a transformer to increase the voltage according to the requirements of the lamp. Often, a 555 (timer IC) is used as a primitive low-frequency oscillator, providing a pulse train of transistors that switch the primary transformer, giving you AC output from the secondary. An example of the kind of circuit liked here.
In addition, circuits exist between the concepts of a fixed and resonant frequency oscillator. Considering the panel of commercially available emergency lamp,…
Emergency lamp panel picture
… I tried extracting this schematic. Please note that it is incomplete and covers only the components between the oscillator IC (555 timer) and the converter:
Schematic extracted from reflector for fluorescent lamp
The output stage would look simpler if a complementary transistor pair (npn and pnp) were used, or if one rectangular driving voltage was going to one npn power transistor, and reversed by another small transistor, to the second npn power transistor, but it seems the designers decided to stick with one From the transistor only or not to use an additional inverting phase transistor – at the expense of using an additional winding on the transformer. Here’s what the circuit does:
The open collector output of the IC drives transistor Q6 through the 2k4 resistor. I’m assuming the Q6’s collector voltage is designed to be perfectly rectangular, i.e. the transitions from high to low and back to high should not be slow. While the transistor inside the IC is still off, Q6 is turned off because its base is high. Once the transistor in the IC is turned on, Q6 is also turned on and feeds the base current to Q8. This causes two things to happen: current flows through the first winding of the transformer (S1 becomes low relative to F1), and Q7 is kept in the off state because S1 is lower than F1, and S3 is lower than F3. So, at the same time that Q8 base goes up, Q7 base goes down.
If, after all this, the output of the IC rises again, Q6 is turned off, and the collector current through Q8 will also turn off. However, the energy stored in the transformer wants to go somewhere, and this will cause all windings (!) to reverse their polarity: S1 starts high with respect to F1, S3 will also start high with respect to F3, Q7 is turned on because the base is pushed to High level by S3-F3, F2 will drop below S2, of course, the output coil (S4-F4) will also reflect its voltage, thus creating the AC output of the lamp.
This case appears to be related to the energy stored in the transformer and in the inductor above and the capacitors below the primary windings.
From there, the process starts again as the timer IC starts the next cycle of the AC output signal; It seems that the frequency at the IC’s output has to be tailored to what the switch and its surrounding components are designed to do.
The circuit appears to operate somewhere between purely pulse-width-drive mode, where the timer IC is the only part that shows when the Q7 and Q8 power transistors are on or off, and purely resonant mode, where the switch and surrounding capacitors have the power to drive the Q7 and Q8 , because then we’ll need another coil driving the Q8 base. My understanding is that the 555 starts each cycle and that the components of the resonance (L, C, transformer) determine when the cycle should stop if the IC is not fast anyway. Using LT Spice, I found that this circuit could operate at a frequency of maybe 500Hz…3kHz.
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