Tank circuit

Quasi-resonant converters are a separate class of switching power supplies that tune the ac power waveforms to reduce or eliminate the switching loss within the supply. This is done by placing resonant tank circuits within the ac current paths to create pseudo-sinusoidal voltage or current waveforms. Because the tank circuits have one resonant frequency, the method of control needs to be modified to a variable frequency control where the resonant period is fixed and the control varies the period of the non-resonant period. The quasi-resonant converters usually operate in the 300 kHz to 2 MHz frequency range. 

Quasi-resonant converters utilize an T-C tank circuit, which rings at its natural resonance frequency in response to a step change in its terminal voltage or current. The tank circuit is placed between the power switch and the transformer and/or the transformer and the output filter. 

Frequency limiting is also provided on all the resonant controllers on the market today. The resonant frequency of the tank circuit is given by... 

One-half of the reciprocal of this equation is the positive half period of the tank circuit waveform. Since the tank circuit has been partially emptied of its energy, the ringbaek period is shorter than a half period. On the average, an additional 75 percent of the above period should be reserved for the ringbaek period. 

Let us examine how one determines the values of the inductor and capacitor. Several assumptions have to be made at the beginning of the design process since several of the tank circuit s characteristics are variable within the application. The first is to assume a value for the Q of the tank circuit. In the application, the Q varies greatly with the amount of load placed on the output of the supply. So, a good value to start with is... 

Next, decide the natural resonance frequency of the tank circuit. For the available quasi-resonant controller ICs on the market, the range is between 1 and 2 MHz. This limit should be considered the maximum limit within conventional QR designs. So 1 to 1.5 MHz is a typical choice. Lower frequencies can be used and some efficiencies can be gained. The equation for the resonance frequency is... 

Figure 4-15 The two methods of loading a resonant tank circuit.

Identifying and harnessing these parasitic losses into the operation of the QR power supply is an interesting challenge that should be undertaken. So, carefully analyze its operation and play with the arrangement of the tank circuits. 

It is desired that the resonanee frequency of the tank circuit be 1 MHz. In ZVS QR converters, the tank circuit is not responsible for storing and passing on energy as it is in ZCS QR converters. The tank circuit can be seen more as an off-time transition shaper similar to a snubber when used in PWM converters. Here a wide range of values for both the inductor and the capacitor will work as long as their combined resonant frequency is 1 MHz or... 

Fig. 2.5.3 Typical NMR resonant tank circuit, showing coil loss mechanisms. This LC circuit is then placed in series with two matching capacitors (Cmatch). The resistance of the circuit is represented by Rco], the inductive losses by Rm and the dielectric losses by Ci, Cd and Rd.

The tank circuit may be in the form of discrete capacitors or inductance coils but more usually it is of aluminium cavity construction, with the valve enclosed by inherent inductive and capacitative elements. The last-mentioned arrangement has the advantages of being partly self-screening and of minimizing the possibility of parasitic oscillations. 

For signal enhancement we use superconducting tank circuits attached to the correction electrodes (some are splitted to allow detection of the radial motion). The resonance frequencies of the circuits are tuned to the corresponding ion oscillation frequencies. 

If R is small, then Q0 is large. Equation (9.2.51) also applies to radio receivers, whose "tank" circuit which can be analyzed as a single RLC circuit. 

Centuries ago, time was measured by the gnomon, the clepshydra, weights and gears (eventually controlled by an escapement), incense sticks, hourglasses, and then finally mechanical clocks, pendula, and self-winding watches (mechanically wound, self-winding, or by now controlled by a quartz crystal oscillator tank circuit). 

The RF SQUID is based on the AC Josephson effect, uses only one Josephson junction, and is less sensitive than the DC SQUID, but is cheaper and easier to manufacture its SQUID is inductively coupled to a resonant tank circuit. Depending on the external magnetic field, as the SQUID operates in the resistive mode, the effective inductance of the tank circuit changes, thus changing the resonant frequency of the tank circuit. These frequency measurements can be easily done, and thus the losses that appear as the voltage across the load resistor in the circuit are a periodic function of the applied magnetic flux with a period of 0. 

Figure 11 Description of dual microcoil probe. (A) Two coils wrapped around a polyimide sleeve (B) Dual-coil probe mounted on top of the probe head. (C) Schematic of balanced tank circuit used for each microcoil. Coil (L), series capacitors (Cs) 3.3 pF, tuning capacitors (CT) 0.6-4.5 pF, matching capacitor (CM) 0.6-4.5 pF, bridge capacitors (CB) 24 pF. (Reproduced with permission from Ref. 41. Copyright 2002 American Chemical Society.)...

Several traditional methods are based on use of an electrical oscillator incorporating an inductance L and a capacitance C in parallel this combination is known as a tank circuit. The frequency of such an oscillator is given by... 

The recommended procedure for determining the dielectric constant of an unknown gas with the cell described here depends on the relationship of changes in the capacitance of the cell to changes produced in the effective capacitance in the tank circuit. These are not the same because other capacitances are present besides that of the capacitor contained in the cell, Cceii. These always include the capacitance of the shielded cable connecting the cell to the oscillator and stray capacitances in the oscillator tank these are in parallel with the cell, and lumped together may be called In addition there is in the circuit described... 

In a similar way also the cyclotron motion can be cooled and detected when the tank circuit connecting two segments of the splitted correction electrode is kept in resonance with the ions cyclotron frequency. Fig. 5 shows an example for resistive cooling of the cyclotron motion of a single 12( 5+ calibration... 

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