Resonant transformer

Resonant transformer 

                                

For more details on this topic, see Resonant inductive coupling.

For more details on this topic, see Tesla coil § Resonant transformer.

A resonant transformer is a transformer in which one or both winding has a capacitor across it and functions as a tuned circuit. Used at radio frequencies, resonant transformers can function as high Q factor bandmpass filters. The transformer winding have either air or ferrite cores and the bandwidth can be adjusted by varying the coupling (mutual inductance). One common form is the IF (intermediate frequency) transformer, used in super heterodyne radio receivers. They are also used in radio transmitters.

Resonant transformers are also used in electronic ballasts for gas discharge lamps, and high voltage power supplies. They are also used in some types of switching power supplies. Here the short-circuit inductance value is an important parameter that determines the resonance frequency of the resonant transformer. Often only secondary winding has a resonant capacitor (or stray capacitance) and acts as a serial resonant tank circuit. When the short-circuit inductance of the secondary side of the transformer is L sc and the resonant capacitor (or stray capacitance) of the secondary side is C_r' The resonance frequency ω_s of 1' is as follows

{\displaystyle \omega _{s}={\frac {1}{\sqrt {L_{sc}C_{r}}}}={\frac {1}{\sqrt {(1-k^{2})L_{s}C_{r}}}}}

The transformer is driven by a pulse or square wave for efficiency, generated by an electronic oscillator circuit. Each pulse serves to drive resonant sinusoidal oscillations in the tuned winding, and due to resonance a high voltage can be developed across the secondary.

Applications:

·        Intermediate frequency (IF) transformer in super heterodyne radio receiver

·        Tank transformers in radio transmitters

·        Tesla coil

·        CCFL inverter

·        Oudin coil (or Oudin resonator; named after its inventor Paul Oudin)

·        D'Arsonval apparatus

·        Ignition coil or induction coil used in the ignition system of a petrol engine

·        Electrical breakdown and insulation testing of high voltage equipment and cables. In the latter case, the transformer's secondary is resonated with the cable's capacitance.