A final object of the invention is to disclose one specific PRC which operates under zero voltage switching conditions in the continuous conduction mode. Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention, the embodiments described therein, from the claims, and from the accompanying drawings.
I nLOW ;. While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one specific embodiment of the invention. It should be understood, however, that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.
See ref. Before describing a specific converter, the basic principles of the invention will be described. Turning to the drawings, FIG. The transistor switches Q1 through Q4 of the H-bridge are gated alternately in pairs to provide a square wave current drive to the resonant circuit comprising a fixed capacitor C and fixed inductor L.
Subsequently, the inductor current free wheels through diodes i. During this period time, the capacitor voltage is clamped to zero until the next pair of switches i. This circuit has the disadvantage of not operating in a continuous conduction mode.
The time period when the diodes conduct i. Another disadvantage is that as the dead time increases, a larger output filter is needed. Input capacitors C 00 have values much greater than resonant capacitor C 0. The input capacitors C 00 function as voltage dividers. The resonant inductor L 0 is schematically represented to be a variable inductance, much unlike that of the prior art.
As will be explained later, the resonant capacitor may be made variable and the inductor held fixed. Here bi-directional switches S1 and S2 are formed by the parallel combination of a transistor and a diode, so as to operate complementarily. The output DC voltage is obtained by rectifying the voltage across the resonant capacitor C 0. A low pass filter to smooth out the output voltage and current to the load R is obtained by capacitor Cf and inductor Lf.
The output current I 0 can be considered constant during the switching period, since the switching frequency is usually much higher than the corner frequency of the output filter. In each of these figures, the voltage across the capacitor and the current through the inductor are sinusoidal.
This is in contrast to the discontinuous conduction mode where there are discontinuities see FIG. The problem of turn-off switching loss can be over come by switching at a zero voltage across the resonant capacitor. When load current is varied, the output voltage cannot remain at the rated value under the zero voltage switching condition. However, if the characteristic impedance Z 0 is changed so that the quality factor Q b is maintained constant, the zero voltage switching condition can be maintained.
In other words, the output voltage can be maintained at the rated value under the zero voltage switching constraint by changing the characteristic impedance Z 0 so that the quality factor becomes constant. There are two ways adjust the characteristic impedance Z 0 :.
To control by adjustable inductance, an extra winding N c around the inductor core 10 may be used to provide a DC bias current so that the dynamic permeability can be varied. This concept is illustrated in FIG.
To control by adjustable or variable capacitance C 0 , a varactor diode VAR may be used with a proper adjustable bias, much as that shown in FIG. In either of these two ways, the gain or DC conversion ratio M b can be adjusted by varying or adjusting the quality factor Q b. A block diagram of this concept is illustrated in FIG.
This method of control may be termed "quality factor control" or "Q-control". In high frequency operation, Q-control can be well applied due to the ease in the regulation of quality factor to cover a wide load range. Note however, a practical converter would need a Switching Start-up Circuit from the initial or at rest condition so that the zero voltage switching circuit can function.
Referring now to FIG. A simplified block diagram of the circuit in FIG. The following parameters are assumed to be given or specified:. If the quality factor Q b is given, then the voltage gain M b , by definition of Q-control, is equal to Q b. As a result, V 0 at the primary side of the transformer becomes:. If the transformer turn ratio is given, then Q b can be calculated as:.
Now, the parameter "gamma" g a is the product of pi i. For a known or given value of Q b , the value of gamma g a can be obtained from a plot of gamma versus Q b. Therefore, the full load resonant angular frequency can be determined from the full load switching frequency F s by the formula:.
The characteristic impedance Zo at full load and light load can be determined from:. From equations 1 and 2 , the resonant capacitance C 0 and the resonant inductance L 0 can be found by:. The source inductance Lg can be much bigger than the resonant inductance at light load. It can assumed that:. The peak current through the resonant inductor can be determined from:.
The outline of the design just presented provides the necessary information to proceed. Cr can reverse into negative because there is a series diode D with the transistor T. During v. Cr is negative, the transistor T should be turned off. When v. Cr resonates back from negative to zero, since T is EE stops. Inductor recovering stage: Resonance stops, Lr begins to be charged by the input voltage Vin The solution: EE Finish of this stage Finish when i.
Lr reaches the value of output current IO. DF no longer conducts because its current is now all conducted by Lr. This stage finishes when the transistor turns off again at t 4 is the same as t 0 in next cycle. EE In this case, the condition is: This condition is the same as 12 : EE Zero-voltage switching When to turn on and off The instant to turn on the switch is also important.
For half-wave mode, the transistor must be turned on after v. Cr reaches zero at t 2 and before isw increases back to positive. For full-wave mode, the transistor must be turned on when v. Cr is negative. When T is turned on, the series diode D is still in reverse bias and the voltage across T is virtually zero. Why full-wave is no good The charge stored in the junction capacitor of T cannot be conducted to outside, therefore when T is turned on this energy is dissipated internally.
Therefore the switching loss is not zero. In order to determine two current zero-cross points for switch-on and switch-off, the quasi-resonant state is employed. The quasi-resonant state is performed in full-waveform mode. This makes output voltage independent of output load variation. This paper proposed a new control method to track the resonant frequency of the unregulated LLC series resonant converter. Theoretically, the time of zero diode current in secondary is zero when the working frequency is equal or greater than the resonance frequency, and it is changed as the frequency varies when the working frequency is less than the resonance frequency.
The proposed control algorithm is based on the time measurement of zero diode current to realize the resonance frequency tracking.
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