Quasi-resonant Converters

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. 

The advantages of a quasi-resonant converter over a classic PWM converter are smaller size and typically higher efficiencies. Although when a smaller size is pursued by increasing its operating frequency, the improvement in efficiency is sacrificed due to other frequency dependent losses. 

The disadvantage of the quasi-resonant converter compared to the newer lossless snubber and active clamp techniques in addition to the basic PWM converters, is the voltage or current stresses placed upon the power components. The peak voltage or current values that exist within quasi-resonant converters can be two to three times higher than in PWM converters. This forces the designer to use higher-rated power switches and rectifiers which may not have as good conduction characteristics. 

Quasi-resonant converters force the voltage or current waveform into a haver-sine waveshape. If the power switch(es) are switched at the right moments, then there are no switching losses experienced. Also because of the controlled rates of change for the voltage or current waveforms, much better RFI/EMI performance is realized. Most of the basic topologies that exist within the PWM family are also in the quasi-resonant family. 

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. 

The Zero-current Switching Quasi-resonant Converter... 

A second type of quasi-resonant converter is the zero voltage switching (ZVS) quasi-resonant family. A ZVS QR buck converter and its waveforms are shown in Figure 4-11. Here the power switch remains on most of the time and performs resonant off periods to decrease the output power. Actually, the ZCS and the ZVS families mirror one another. If you were to compare the switch voltage and current waveforms between the two families, and if one inverts both the voltage and current waveforms in order to reference them to the power switch, the waveforms would have a striking resemblance to one another. 

The simulation results from Micro-Cap and PSpice for the control loop characteristics of the quasi-resonant converter are shown in Figure 4.80 and 4.81, respectively. 

The zero current switching (ZCS) quasi-resonant (QR) switching power supply forces the current through the power switch to be sinusoidal. The transistor is always switched when the current through the power switch is zero. To understand the operation of a ZCS QR switching power supply, it is best to study in detail the operation of its most elementary topology—the ZCS QR buck converter (and its waveforms) as seen in Figure 4-10. 

Figure 4-10 The schematic and waveforms of a ZCS quasi-resonant buck converter.

A15 Watt, ZVS Quasi-resonant, Current-mode Controlled Flyback Converter... 

A Zero Voltage Switched Quasi-resonant Off-line Half-bridge Converter... 

This converter is intended to function as a bulk power supply for a distributed system. It has only one +28VDC output at 10 A. This is going to be a classic ZVS quasi-resonant half-bridge converter that is, variable frequency, voltagemode controlled with averaging overcurrent protection. It is representative of the designs using the available control ICs on the market today. 

Quasi-resonant and resonant transition switching power supplies have a much more attractive radiated spectral shape. This is because the transitions are forced to be at a lower frequency by the resonant elements, hence only the low frequency spectral components are exhibited (below 30MHz). The lower rate of change during the transitions are responsible for behavior. The higher frequency spectral components are almost non existent. The near-held radiated spectrum of a quasi-resonant, hyback converter are shown in Figure E-2. The quasi-resonant and soft switching families of converters are much quieter and easier to hlter. 

In applications where high power density or thermal management is of prime importance, hard- switched converters are not feasible using conventional Si components. In these cases, resonant or quasi-resonant (also termed soft-switching ) topologies can be used. The electrical resonance is obtained through parasitic... 

The following circuit (Fig. 4.72) shows the implementation of a quasi-resonant flyback converter featuring the STR6600. 

Figure 4.72 Circuit schematic for quasi-resonant flyback converter.

The discovery of the epi alkaloid made possible a useful NMR-comparison of 16-substituted derivatives (Table I) in which the down-field resonance of the protons of the quasi-equatorial substituents can be seen and for which structural assignments have been made. Deoxy vin-camine and deoxyepivincamine were produced by the hydrogenation of apovincamine in a ratio of about 1 9. Deoxyepivincamine after hydrolysis and reesterification was converted into deoxyvincamine (23). 

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