Resonance converter

Resonant converters are available in the electrical industry to resonance based circuit topologies of an inverter . Resonance converters typically work with an almost constant load and deliver more or less sinusoidal output voltages.

Quasi-resonant converters are also used as switched-mode power supplies and welding current inverters and generate direct voltage or small sinusoidal output voltage.

Inverter to supply a fluorescent tube

species

Depending on the application, there are different types of resonance converters with different topologies. What they all have in common is that the energy-transmitting path is operated in the area of ​​its resonance point and, in some converter types, is also part of the frequency-determining oscillator . The resonance transformer used can, depending on the application, for example for galvanic isolation , also contain or be supplemented with a transformer .

• For power applications from 1 kW, the goal is achieved of minimizing the power losses during the switching processes in the switching transistors. These resonance converters come in two variants: Either switching is always carried out at the zero crossing of the voltage ( ZVS for Zero Voltage Switching ) or always at the zero crossing of the current ( ZCS or Zero Current Switching ). The power-transmitting path including the transformer forms an oscillating circuit with additional capacitances and inductances, which help determine the range of the switching frequency .
• Another type are very compact power supplies with low power in the range of a few 10 W, which, for reasons of cost, have to make do with a minimal number of discrete components. The main feature is that you do not need your own oscillating circuit and control with additional electronic components.

Applications

lighting

Inverter from the base of an energy-saving lamp

The resonance converter with powers in the range of a few 10 W is used as an electronic ballast for fluorescent lamps in order to generate the high voltage required to operate the fluorescent lamp. In compact fluorescent lamps ("energy saving lamps "), the inverter is usually integrated permanently into the lamp base. When it comes to the electronic disposal of defective energy-saving lamps, it is a bigger problem than conventional incandescent lamps without built-in electronics.

Another large area of ​​application for these inverters is the power supply of fluorescent tubes (cold cathode fluorescent lamp, CCFL), which are often used as background lighting for TFT flat screens . In English, these inverters are also referred to as display inverters , CCFL inverters or backlight inverters . Inverters are also used in the area of case modding and for battery or rechargeable battery-operated fluorescent lamps .

Inductive heating

The induction heating for hardening, melting and tempering used as induction hotplates resonant converter. The coil used to excite the heating eddy currents forms the resonance circuit together with correspondingly resilient capacitors. The load is the part to be heated in the immediate vicinity (component made of iron or iron pot) - there is no transformer in the narrower sense. The coils are often water-cooled, which means that they consist of copper pipes through which water flows. Induction hotplates, however, are only air-cooled.

Realization variants

Compact fluorescent lamps

Circuit of a resonance converter for a compact fluorescent lamp

The illustration opposite shows a resonance converter as it is used in the base of compact fluorescent lamps. The two transistors switch alternately, typical switching frequencies are around 40 kHz. Due to the series resonant circuit C ₃ and L ₂ , an approximately sinusoidal alternating current begins to “rock” in these components when the lamp is still not ignited. The voltage reaches the ignition voltage of the tube through resonance increase. The transformer L ₁ is used for feedback to the transistors, the diac is used to start the resonance converter.

The high-voltage-resistant capacitor C ₄ connected in parallel to the fluorescent tube (peak voltages of up to 1 kV when starting) is connected on the other side of the heating wires in order to conduct a heating current through the directly heated hot cathodes when starting .

Once the lamp has ignited, only a small amount of current flows through C ₄ . The converter frequency is now determined by the saturation of the core of L1 and the desaturation of the transistors. C ₃ is used to separate the DC voltage.

CCFL inverter

Above: Schematic circuit with switches. Below: Simplified, self-oscillating principle circuit with bipolar transistors.

Inverter circuits for the power supply of cold cathode tubes (CCFL), as they are used in the background lighting of flat screens or in flat bed scanners, are also designed as self-oscillating inverters, like those described above for compact fluorescent lamps. They convert direct voltages in the range from 10 V to 300 V into higher alternating voltages in the range from 600 V to 700 V with a frequency of approx. 30 to 100  kHz . Another typical feature of these power supplies is that the load is known and is usually permanently connected to the inverter.

The first figure shows the basic circuit of an inverter, implemented with a changeover switch. In this configuration the converter corresponds to a half-bridge converter. Below is a basic circuit diagram with bipolar transistors . The voltage source for the supply is designated with Vcc , the output on the far right . Due to the simplification, this circuit is not self-starting, but is intended to clarify the principle of positive feedback of the two bipolar transistors via induced currents. This principle invented by Royer is based on the saturation of the transformer core. When this occurs, the conducting phase of the respective transistor ends because the voltage induced in the auxiliary winding collapses. In English, this basic circuit is also called Royer's Circuit or Royer Converter according to George H. Royer , who patented this circuit in 1957. The induced current in the auxiliary windings, with the appropriate winding direction, blocks one transistor each and lets the opposite bipolar transistor become conductive, whereby a continuous switching between the two switching states is achieved.

The circuit has been significantly further developed by switching on when the transistors are de-energized and switching off when they are de-energized. This is where LC resonance circuits come into play, which determine the working frequency independent of the saturation of the core. This feature of resonance topologies lowers the switching losses of the transistors to ideally zero. The circuit principle became the basis for CCFL inverter circuits. This principle is also known as collector resonance .

Classic CCFL inverter circuit

The oscillator starts automatically by first connecting the base connections of the two transistors in parallel via the control coil, as shown in the adjacent figure of the classic CCFL inverter circuit. As with any other oscillator, small disturbances and noise lead to oscillation. As soon as vibrations occur, the two transistors are always activated in phase opposition and can never conduct at the same time. The power is supplied via a choke Lc, which enables the transistors to always switch through despite the sinusoidal shape of the transformer voltage; this reduces losses considerably.

In this circuit, the switching frequency f _o is only determined by the primary-side main inductance L _{p of} the transformer and the capacitor C _o as an oscillating circuit:

${\displaystyle f_{o}={\frac {1}{2\pi {\sqrt {L_{p}\cdot C_{o}}}}}}$

The secondary-side series resonant circuit consisting of load capacitor C _b and short-circuit inductance L _k plays a subordinate role here - the resonance frequency of the secondary side is well above the switching frequency. C _b serves as a capacitive series resistor, i.e. to stabilize the lamp current.

Optimized CCFL inverter circuit with matched secondary resonance and resonance transformer

CCFL inverter with resonance transformer (left)

A disadvantage of this classic circuit is the disadvantageous influence of the transformer's high short-circuit inductance due to the insulation. It has to be comparatively large because it has to generate the high ignition voltage through the transmission ratio.

By including the secondary-side resonance circuit , with the formation of a resonance transformer in the resonance circuit and for impedance matching of the tube, it is possible and even desirable to use transformers with high short-circuit inductance and to reduce the size of the inverter circuit while improving the efficiency by generating the ignition voltage through resonance overshoot. Depending on the circuit, the short-circuit inductance L _{k is also increased} by an additional coil on the secondary side. This is used for stability and reproducibility in series production. It is essential that the resonance frequency f _{o of} the secondary-side resonant circuit corresponds approximately to the resonance frequency of the primary-side resonant circuit:

${\displaystyle f_{o}={\frac {1}{2\pi {\sqrt {L_{p}\cdot C_{o}}}}}\approx {\frac {1}{2\pi {\sqrt {L_{\mathrm {k} }\cdot (C_{w}+C_{a}+C_{s})}}}}}$

The disadvantage of this optimized shape is that the electrical parameters of the fluorescent tube ( impedance ) have a significant impact on the circuit dimensions of the inverter and its efficiency. As a rule, the tube type cannot simply be changed without circuit adaptations.

The circuits shown are not regulated. The lamp current can be regulated by connecting a step-down converter that uses the input choke Lc. Special integrated circuits can control all 3 transistors (2 from the inverter, 1 from the buck converter), detect the zero crossing of the resonance circuit and measure the lamp current.

If the tube fails or is interrupted, a high voltage is generated due to the increased resonance, just like during ignition, which many of the components cannot tolerate over the long term.

The CCFL inverters based on piezoelectric transformers represent a special design of CCFL inverters. The resonance circuit is formed by the piezoelectric transformer which supplies the high sinusoidal alternating voltage for the fluorescent tube.

See also

• Piezoelectric transformer
• Joule thief

literature

• Ulrich Schlienz: Switching power supplies and their peripherals . Vieweg, 2007, ISBN 978-3-8348-0239-2 .
• BD Bedford, Richard G. Hoft: Principles of Inverter Circuits . John Wiley & Sons Inc., 1964, ISBN 0-471-06134-4 .

Individual evidence

1. ^ Resonance converter by Jörg Rehrmann: The power supply and converter manual
2. ^ Royer oscillator circuit United States Patent 2783384
3. ↑ Collector resonance oscillator circuit United States Patent 3818314
4. ↑ Short-circuit inductance in power electronic circuits: function, design and application Author: Dierk Schröder.
5. ↑ Short-circuit inductance ( memento of the original from March 5, 2017 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Prof. Dr.-Ing. Dieter Gerling: Lecture Electrical Machines and Drives, University of Munich, 2007, p.169 
6. ↑ Comparing magnetic and piezoelectric transformer approaches in CCFL applications Application Note Texas Instruments, 2005.