CCFL inverter

A resonant converter is in the electrical industry a special circuitry form of a DC-DC converter, which operates to transfer energy to a resonant circuit. The resonance converter converts a DC voltage into a single or multiphase alternating voltage and is typically operated for optimum operation with a nearly constant load. In addition, resonant converters operate, as all switching power supplies, and in contrast to converters, without mechanically moving parts.

If no rectification at the output, then from the English name derived inverter ( inverter dt ) is also common.

  • 3.1 CFLs
  • 3.2 CCFL Inverters

Species

Depending on the application, there are different types of resonant converters having different topologies. Is common to all, that the energy transmitting range is operated in the vicinity of its resonance point and in some types of converters is also part of the frequency-determining oscillator. The resonant transformer used may vary depending on application, such as for electrical isolation, include a transformer as part or completed it.

  • For power applications from 1 kW to the target is reached to minimize the power losses during the switching operations in the switching transistors. This resonant converters come in two variants: either always switched (ZCS or Zero Current Switching) at the zero crossing of the voltage ( ZVS Zero Voltage Switching ) or always at the zero crossing of the current. The power-transmitting path, including the transformer forming a resonant circuit with additional capacitance and inductance, which also determined the range of the switching frequency.
  • Another type is a very compact power supplies less power in the range of some 10 W, which have to make do for reasons of cost with a minimal number of discrete components. The essential feature is to not need a separate resonant circuit, and control of additional electronic components.

Applications

Lighting

Applies the resonant converter with power of a few 10 W as an electronic ballast for fluorescent lamps, in order to produce a necessary for operating the fluorescent lamp, high voltage. In compact fluorescent lamps ( " CFLs ") of the inverter is usually permanently integrated into the lamp socket. He is at the disposal of defective electronics energy saving lamps more of a problem than conventional incandescent lamps without integrated electronics.

Another major area of ​​application of this inverter is the power supply of fluorescent tubes ( engl. cold cathode fluorescent lamp, CCFL ), which are often used as a backlight for TFT flat screens. In English, these inverters are also referred to as the display inverter, CCFL inverter or backlight inverter. In the area of case modding and for battery or battery-powered fluorescent find inverter application.

Inductive heating

Induction heating for hardening, melting and tempering as well as induction hotplates used resonant converter. The excitation of the heated coil eddy currents serving together with corresponding duty capacitors the resonant circuit. The load is located directly next to it to be heated part ( component made of iron or iron pot) - a transformer is not present in the strict sense. The coils are often water-cooled, that is, they consist of water -carrying copper pipes. However, induction hotplates are only air cooled.

Implementation variants

CFLs

The figure shows a resonant converter, the kind found in the base of CFLs. The two transistors turn on alternately, typical switching frequencies are 40 kHz. Through the series resonant circuit C3 and L2, an approximately sinusoidal alternating current to begin in these devices " Rock". The transformer L1 is the feedback to the transistors, the diac is used to oscillation of the resonant converter.

The parallel with the fluorescent tube connected and high voltage resistant capacitor C4 ( at startup are peak voltages up to 1 kV ) is used to ensure a low current through the filament of the two electrodes during operation. For quick start and rapid heating can parallel with this capacitor, a varistor are connected, which by then occurring peak voltage allows a higher heating in cold and not yet ignited tube.

CCFL Inverters

Inverter circuits for the power supply of cold cathode fluorescent lamps (CCFL ) as in the backlight of flat screens or even with energy saving lamps are used, are built are usually classified as self-oscillating inverter. Convert dc voltages up to 300 V in order to higher AC voltages ranging from 600 V to 700 V at a relatively high frequency of about 30 to 100 kHz in the range of 10 volts. Typical of this power supply is on, the load is known and generally rigidly connected to the inverter. The inverter circuits shown below will not work optimally without tube at idle.

Opposite, the basic circuit of an inverter is shown in the first image, realized by a switch. In this configuration of the transducer corresponds to a half-bridge converter. This includes a basic circuit diagram with bipolar transistors. Vcc voltage source is referred to supply, on the right outside of the output (Output) to the connection of the arc tube. This circuit is due to the simplification does not self-starting, but is intended to illustrate the principle of feedback to the vibration generating means of the two bipolar transistors via induced currents.

The induced current in the two primary-side auxiliary windings on the corresponding winding direction, respectively a transistor and can be locked to the opposite bipolar transistor conductive, thereby switching a current between the two switched states is achieved. In English, this basic circuit is also referred to as the Royer 's circuit or a Royer converter to George H. Royer which this circuit 1957 patented. The circuit principle has been further improved in the following decades and became the basis for CCFL inverter circuits. This principle is also called collector resonance.

The independent starting of the oscillator is achieved by the base terminals of the two transistors are connected in parallel on the drive coil, first, as shown in the illustration of the conventional CCFL inverter circuit. Small interference and noise lead now just like any other oscillator for oscillation. Once vibrations occur, the two transistors are always driven in opposite phase and can never conduct at the same time. The power is supplied via an inductor Lc, which makes it possible that the transistors in spite of the sinusoidal shape of the voltage transformer can always be carried off completely; thus the losses are significantly reduced.

The switching frequency f is determined in this circuit only by the primary-side magnetizing inductance Lp of the transformer and capacitor Co as a resonant circuit:

The transformer should possess a high degree of efficiency the smallest possible leakage inductance Lsc, so as to maintain the resonant frequency of the secondary side significantly higher than the switching frequency. The secondary-side resonant circuit plays only a minor role in a small parasitic leakage inductance. The load capacitor Cb in this case is primarily a series resistor and to the stabilization of the lamp current through the tube.

The circuit shown is unregulated. By connecting a step-down converter, which uses the input choke Lc, the power conversion can be controlled or the lamp current can be controlled. Special integrated circuits, all three transistors ( two from the inverter, one of the buck converter ) drive and purpose detect the zero crossing of the resonant circuit.

A disadvantage of this conventional circuit, the influence of the insulation due to the high inductance of the transformer, which is therefore comparatively large.

By including the secondary-side resonant circuit, a resonant transformer to form the resonant circuit and for impedance matching of the tube, it is possible to use exchangers with high leakage inductance and effective to reduce the inverter circuit in improving the efficiency. Depending on the circuit and the leakage inductance Lsc is thereby increased by an additional coil on the secondary side. Which serves for stability and reproducibility in series production. It is essential that the resonance frequency fo of the secondary-side resonant circuit is approximately equal to the resonant frequency of the primary-side resonant circuit:

The disadvantage of this form is optimized is that the electrical parameters of the CCFL tube, in particular the impedance with substantially included in the dimensioning of the inverter circuit and its efficiency. Thus, the tube type can not be changed easily without circuit adjustments in the rule.

A special design of CCFL Inverters provide those based on piezoelectric transformers CCFL inverter dar. Here, the resonant circuit is formed by the based on piezoelectricity transformer which supplies the high sinusoidal AC voltage for the fluorescent tube.

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