Dielectric Heating

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field which reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axis aligns to the field.
The most obvious advantage to using such a dielectric material is that it prevents the conducting plates on which the charges are stored from coming into direct electrical contact. More significant, however, a high permittivity allows a greater charge to be stored at a given voltage.

Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric.RF dielectric heating at intermediate frequencies, due to its greater penetration over microwave heating, shows greater promise than microwave systems as a method of very rapidly heating and uniformly preparing certain food items, and also killing parasites and pests in certain harvested crops.

Molecular rotation occurs in materials containing polar molecules having an electrical dipole moment, with the consequence that they will align themselves in an electromagnetic field. If the field is oscillating, as it is in an electromagnetic wave or in a rapidly-oscillating electric field, these molecules rotate to continuously align with it. This is called dipole rotation. As the field alternates, the molecules reverse direction. Rotating molecules push, pull, and collide with other molecules (through electrical forces), distributing the energy to adjacent molecules and atoms in the material. Once distributed, this energy appears as heat.
Temperature is the average kinetic energy (energy of motion) of the atoms or molecules in a material, so agitating the molecules in this way increases the temperature of the material. Thus, dipole rotation is a mechanism by which energy in the form of electromagnetic radiation can raise the temperature of an object. Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most efficiently on liquid water, and much less so on fats and sugars. This is because fats and sugar molecules are far less polar  than water molecules, and thus less affected by the forces generated by the alternating electromagnetic fields.

Flyback Transformers

                                    

Flyback transformers, popularly known as the Line Output Transformers, is a special mechanism of converting the energy supply, both voltage and current, into electronic circuits. Although it is termed as a transformer, it works against the typical functions of a conventional transformer, and is exploited more as energy storage equipment. When the primary switch is on, the energy is stored on ferrite core that has the air gap in it. However, when the primary switch is off, the energy is not stored but transferred to the outputs. The current will flow either in the primary winding, or the secondary one, but not both at the same time. Thus, a flyback transformer is often misguided to be an inductor having the secondary windings.

                              

                              

The primary  winding of the flyback transformer  is wound first around a ferrite rod, and then the secondary is wound around the primary. This arrangement minimizes the leakage inductance of the primary. A ferrite frame is wrapped around the primary/secondary assembly, closing the magnetic field lines. Between the rod and the frame is an air gap, which increases the reluctance. The secondary is wound layer by layer with enameled wire.

The primary winding of the flyback transformer is driven by a switch from a DC supply (usually a transistor). During the switch on time, there wont be any power conversion from primary to secondary side, since secondary side diode will be reverse biased, hence energy is stored in the inductor itself. In order to store the energy in magnetic field we use airgap in the inductor. but if it is too large, leakage inductance problem will occur. So provide airgap with minmum as per calculation obtained. 
When switch is off, the stored energy in the inductor(primary) will be transfered to the secondary. Through diode the capacitor will get charged. and this stored energy in the capacitor will be discharged when switch is in ON. The cycle then can be repeated. If the secondary current is allowed to discharge completely to zero (no energy stored in the core) then it is said that the transformer works in discontinuous mode. When some energy is always stored in the core then this is continuous mode.

Once the voltage reaches such level as to allow the secondary current to flow, then the current in the secondary winding begins to flow in a form of a descending ramp signal.

The current does not flow simultaneously in primary and secondary (output) windings. Because of this the flyback transformer is really a loosely coupled inductor rather than classical transformer, in which currents do flow simultaneously in all magnetically coupled windings.

Cuk Converter

The Cuk converter is a step-down/step-up converter based on a switching boost-buck topology. Essentially, the converter is composed of two sections, an input stage and an output stage.

The input voltage vg is fed into the circuit via inductor L1. When transistor Q1 is on, current i1 builds the

magnetic  field of the inductor in the input stage. The diode CR1 is reverse biased, and energy dissipates

from the storage elements in the output stage. When Q1 turns off, inductor L1 tries to maintain the current flowing through it by reversing polarity and sourcing current as its magnetic field collapses. It thus provides energy to the output stage of the circuit via capacitor C1. R1 and R2 are parasitic or stray resistances of inductor.

The inductor currents are the input and output currents, therefore, if the principle of conservation of energy is applied:

where Ds is the duty cycle of the switch :

The voltage ratio of a Cuk converter is the same as that of a buck-boost converter, but its main advantage over other converters is that the input and output inductors result in a filtered current on both sides of the converter, while buck, boost, and buck-boost converters have a pulsating current that occurs on at least one side of the circuit i.e either on input side or output side.

This pulsation will increase the ripple in the circuit and due to this ripple , the efficiency of battery gets lowered. To ensure good efficiency ripple should be reduced.

By controlling the duty cycle of the switch , the output voltage vo  can be controlled and can be higher or lower than the input voltage vg. By using a controller to vary the duty cycle during operation, the circuit can also be made to reject disturbances ,as second part of circuit consists of parallel resonance circuit and it work as a tank circuit for specific frequency (resonant frequency) , and during resonance current will not be allowed to enter in the circuit.