Series Resonance

In a series RLC circuit there becomes a frequency point were the inductive reactance of the inductor becomes equal in value to the capacitive reactance of the capacitor. In other words, X_L = X_C. The point at which this occurs is called the Resonant Frequency point, ( ƒ_r ) and as we are analysing a series RLC circuit this resonance frequency produces a Series Resonance circuit.

As the frequency approaches infinity the inductors reactance would also increase towards infinity with the circuit element acting like an open circuit. However, as the frequency approaches zero or DC, the inductors reactance would decrease to zero, causing the opposite effect acting like a short circuit. This means then that inductive reactance is "Proportional" to frequency and is small at low frequencies and high at higher frequencies.

The major difference between series and parallel resonance is that due to the formation of Tank Circuit , large amount of circulating current exists and it will be exactly oppositely following series resonance current curve.   

As the frequency approaches infinity the capacitors reactance would reduce to zero causing the circuit element to act like a perfect conductor of 0Ω's. However, as the frequency approaches zero or DC level, the capacitors reactance would rapidly increase up to infinity causing it to act like a very large resistance acting like an open circuit condition. This means then that capacitive reactance is "Inversely proportional" to frequency for any given value of capacitance.

Electrical resonance occurs in an AC circuit when the two reactances which are opposite and equal cancel each other out as X_L = X_C and the point on the graph at which this happens is were the two reactance curves cross each other. 

Note that when the capacitive reactance dominates the circuit the impedance curve has a hyperbolic shape to itself, but when the inductive reactance dominates the circuit the curve is non-symmetrical due to the linear response of X_L. If the circuits impedance is at its minimum at resonance then consequently, the circuits  admittance must be at its maximum and one of the characteristics of a series resonance circuit is that admittance is very high. But this can be a bad thing because a very low value of resistance at resonance means that the circuits current may be dangerously high.

The frequency response curve of a series resonance circuit shows that the magnitude of the current is a function of frequency and plotting this onto a graph shows us that the response starts at near to zero, reaches maximum value at the resonance frequency when I_MAX = I_R and then drops again to nearly zero as ƒ becomes infinite. The result of this is that the magnitudes of the voltages across the inductor, L and the capacitor, C can become many times larger than the supply voltage, even at resonance but as they are equal and at opposition they cancel each other out. As a series resonance circuit only functions on resonant frequency, this type of circuit is also known as an Acceptor Circuit because at resonance, the impedance of the circuit is at its minimum so easily accepts the current whose frequency is equal to its resonant frequency. The effect of resonance in a series circuit is also called "voltage resonance"

Boost Converter

A boost converter also known as step-up converter is a power converter with an output DC voltage greater than its input DC voltage. It is a class of switching-mode power supply containing at least two semiconductor switches which includes a diode and a transistor, and at least one energy storage element. Filters made of capacitors (sometimes in combination with inductors) are normally added to the output of the converter to reduce output voltage ripple. Output voltage ripple can be defined as difference between maximum and minimum output current. Large ripple introduces large loses e.g. harmonics, negative sequence current etc..

 A boost converter is sometimes called a step-up converter since it “steps up” the source voltage. Since power  must be conserved, the output current is lower than the source current. The key principle that drives the boost converter is the tendency of an inductor to resist changes in current. When the switch is turned-ON, the current flows through the inductor and energy is stored in it. When the switch is turned-OFF, the stored energy in the inductor tends to collapse and its polarity changes such that it adds to the input voltage. Thus, the voltage across the inductor and the input voltage are in series and together charge the output capacitor to a voltage higher than the input voltage.

The other function of inductor is to avoid any sudden change in current due to the fact that as these converters are generally attached to battery therefore to  ensure the good efficiency of battery , current should be uniform for most of the time. The other function of capacitor is to absorb AC component of supply which generally originates due to switching.

Photodiode And Photovoltaic Mode (PV mode)

Silicon photodiodes are constructed from single crystal silicon wafers . The purity of silicon is directly related to its resistivity, with higher resistivity indicating higher purity silicon.  A cross section of a typical silicon photodiode is shown in the figure. N type silicon is the starting material. A thin "p" layer is formed on the front surface of the device by thermal diffusion or ion implantation of the appropriate doping material (usually boron). The interface between the "p" layer and the "n" silicon is known as a pn junction. Small metal contacts are applied to the front surface of the device and the entire back is coated with a contact metal. The back contact is the cathode, the front contact is the anode. The active area is coated with either silicon nitride, silicon monoxide or silicon dioxide for protection and to serve as an anti-reflection coating. The thickness of this coating is optimized for particular irradiation wavelengths. As an example, a Centro Vision Series 5-T photodiode has a coating which enhances its response to the blue part of the spectrum.

Photodiode junctions are unusual because the top "p" layer is very thin. The thickness of this layer is determined by the wavelength of radiation to be detected. Near the pn junction the silicon becomes depleted of electrical charges. This is known as the "depletion region". The depth of the depletion region can be varied by applying a reverse bias voltage across the junction. When the depletion region reaches the back of the diode the photodiode is said to be "fully depleted". The depletion region is important to photodiode performance since most of the sensitivity to radiation originates there. The capacitance of the pn junction depends on the thickness of this variable depletion region. Increasing the bias voltage increases the depth of this region and lowers capacitance until the fully depleted condition is achieved. Junction capacitance is also a function of the resistivity of silicon used and active area size.        

            

                                           

Due to concentration gradient, the diffusion of electrons from the N- type region to the P-type region and the diffusion of holes from the P- type region to the N-type region, develops a built-in voltage across

the junction. The inter-diffusion of electrons and holes between the N and P regions across the junction results in a region with no free carri- ers. This is the depletion region. The built-in voltage across the deple- tion region results in an electric field with maximum at the junction and no field outside of the depletion region. Any applied reverse bias adds to the built in voltage and results in a wider depletion region.The electron-hole pairs generated by light are swept away by drift inthe depletion region and are collected by diffusion from the undepleted region. The current generated is proportional to the incident light or radiation power. The light is absorbed exponentially with distance and is proportional to the absorption coefficient. The absorption coefficient is very high for shorter wavelengths in the UV region and is small for longer wavelengths . Hence, short wavelength photons such as UV, are absorbed in a thin top surface layer while silicon becomes transparent to light wavelengths longer than 1200 nm. Moreover, photons with energies smaller than the band gap are not absorbed at all. The boundaries of the depletion region act as the plates of a parallel plate capacitor. The junction capacitance is directly proportional to the diffused area and inversely proportional to the width of the depletion region. In addition, higher resistivity substrates have lower junction capacitance. In photoconductive mode (reverse biased), however, the drift current becomes the dominant current (dark current) and varies directly with temperature.

As time constant is directly proportional to capacitance, therefore, lower the capacitance, lesser will br time constant and hence lesser will be response time.

Tunnel Diode

In a conventional semiconductor diode, conduction takes place while the p–n junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the “reverse breakdown voltage” when conduction begins (often accompanied by destruction of the device). In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction.

This reverse resistance occurs because as doping is increased, reverse voltage will decrease and a time will come when there will be reverse  breakdown voltage in forward bias condition. The application of a reverse voltage to the p-n junction will cause a transient current to flow as both electrons and holes are pulled away from the junction. When the potential formed by the widened depletion layer equals the applied voltage, the current will cease except for the small thermal current i.e. as voltage will increase , current will  decrease.

                                                     

However, when forward-biased, an odd effect occurs called “quantum mechanical tunnelling” which gives rise to a region where an increase in forward voltage is accompanied by a decrease in forward current due to change in conduction band position. Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels  i.e. transmitted through a barrier that it classically could not be able to cross.Barrier is the depletion region of p-n junction.

                                                              ^{dynatron oscillator}

This negative resistance region can be used  in  the dynatron oscillator .A dynatron oscillator is an electronic circuit that uses negative resistance to keep an LC tank circuit oscillating .If an ideal capacitor is connected in parallel with an ideal inductor, they form a resonant circuit that, once it begins oscillating, will oscillate forever as the energy is transferred back and forth between the capacitor and the inductor.In practice, however, the two components are not ideal. Real inductors and capacitors are equivalent to an ideal component in parallel (or in series) with a resistance; a real resonant circuit is equivalent to an ideal capacitor, inductor, and resistor connected in parallel. If a negative resistance equal in magnitude to this positive resistance can be connected in parallel with the above circuit, then the two resistances will cancel and the circuit will oscillate forever .

Gas Discharge Tube

A gas discharge tube (GDT) is a sealed glass-enclosed surge protector device containing a special gas mixture trapped between two electrodes, which conducts electric current after becoming ionized by a high voltage spike.  Also referred to as "spikes," electrical surges are sudden, brief rises in voltage and/or current to a connected load. 

GDTs can conduct more current for their size than other components .They can handle a few very large transients or a greater number of smaller transients. The typical failure mode occurs when the triggering voltage rises so high that the device becomes ineffective, although lightning surges can occasionally cause a dead short.

When the voltage is at a certain level, the gas acts as a poor conductor. When the voltage surges above that level, the electrical power is strong enough to ionize the gas, making it a very effective conductor. It passes on current to the ground line until the voltage reaches normal levels, and then becomes a poor conductor again. During this period, this gas acts as a negative resistance i.e. resistivity decreases as current increases.

This methods has a parallel circuit design i.e. the extra voltage is fed away from the standard path to another circuit. A few surge protector have a series circuit design i.e. the extra electricity isn't shunted to another line, but instead is slowed on its way through the main line. Basically, these suppressors detect when there is high voltage and then store the electricity, releasing it gradually. This  method offers better protection because it reacts more quickly and doesn't dump electricity in the ground line, possibly disrupting the building's electrical system.

As a backup, some surge protectors also have a built-in fuse. A fuse is a resistor that can easily conduct current as long as the current is below a certain level. If the current increases above the acceptable level, the heat caused by the resistance burns the fuse, thereby cutting off the circuit. If the  gas discharge arrestor doesn't stop the power surge, the extra current will burn the fuse, saving the connected machine. This fuse only works once, as it is destroyed in the process.

Some surge protectors have a line-conditioning system for filtering out "line noise," smaller fluctuations in electrical current. Basic surge protectors with line-conditioning use a fairly simple system. On its way to the power strip outlet, the main wire passes through a toroidal choke coil. The choke is a just ring of magnetic material, wrapped with wire i.e. a basic electromagnet. The ups and downs of the passing current in the main wire charge the electromagnet, causing it to emit electromagnetic forces that smooth out the small increases and decreases in current. This conditioned current is more stable, and so easier to handle.