The present invention relates to DC to DC converters and where particularly to multiresonant DC to DC converters.
Multiresonant DC to DC converters employ reactive networks to shape the voltage waveforms across their switching transistors in order to allow these devices to turn on with either zero voltage or zero current. Switching losses are thereby substantially reduced allowing switching frequencies to be pushed into the megahertz range. High-frequency operation in turn allows smaller filter components to be used thereby reducing power supply volume.
Referring now to FIG. 1, a typical zero-voltage-switched multiresonant converter 10 is shown for receiving an input voltage V.sub.s from a voltage supply source 12 and supplying a regulated output voltage V.sub.0 to a load 20. The switch S.sub.1 ordinarily comprises metal oxide semiconductor field effect transistor (MOSFET) which includes an intrinsic drain-to-source diode that allows for control only when the current i.sub.l is positive. The diode Dl operates as a passive switch. The capacitors 14 and 16 operate in combination with the inductor 18 to form a reactive network which may assume different resonant circuit combinations depending on the conditions of the switch S1 and the diode Dl. The inductor 22 and capacitor 24 form an output filter. This type of converter is regulated against output load and input line variations by adjustment of the switching frequency of the switch S1. Unfortunately, frequency variations on the order of two-to-one are ordinarily necessary to accommodate changes from zero load to full load.
Recently, a constant or fixed frequency version of the multiresonant converter was proposed by D. Maksimovic and S. Cuk in a paper entitled "Constant Frequency Control of Quasi-resonant Converters" presented at the fourth annual High Frequency Power Conversion Conference in 1989. Constant frequency operation is greatly preferred for use in conjunction with radio frequency equipment because the noise spectrum generated during fixed frequency operations is more predictable and because the inductors employed in the circuit can be optimally designed for the operation at a single frequency. Referring now to FIG. 2, a fixed frequency zero-voltage-switched multiresonant converter 30 is illustrated in which the diode Dl, shown in the circuit of FIG. 1, has been replaced by a second transistor switch S2. In operation the switch S1 is regulated by the switching control circuit 26 to have both fixed on and fixed off times while the switch S2 is regulated by the switching control circuit 26 to have a duty factor which is a function of output voltage V.sub.0. The switches S1 and S2 are synchronized for operation at a single fixed frequency. The second transistor switch S2 adds another degree of freedom to the control of the circuit thereby allowing the converter to be regulated while maintaining a constant frequency of operation. Unfortunately, fixed frequency zero-voltage-switched multiresonant converters such as the converter 30 are characterized by substantially decreased efficiency at higher voltage levels within their input voltage ranges. These efficiency losses occur because the power losses in the converter 30 primarily comprise conduction losses such as I.sup.2 R losses in the transistor switches S1 and S2 and core and winding losses in the resonant inductor 18 which are all a function of the peak current in the inductor 18 which is in turn a strong function of the supply voltage V.sub.s since the supply voltage V.sub.s is applied directly to the inductor 18 when the switches S1 and S2 are both on.
It is therefore an object of the present invention to Provide a multiresonant converter which operates at a substantially constant frequency yet is characterized by stable efficiency levels as a function of input voltage.
It is another object of the present invention to provide a multiresonant converter having a substantially fixed frequency of operation in which the peak resonant current in the resonant inductor is maintained at a constant level despite variations in input voltage in order to control conduction losses and reduce voltage and current stresses on the circuit components.
It is a further object of the present invention to provide a multiresonant converter which operates at a substantially fixed frequency at high efficiency levels despite variations in input voltage by controlling the switching action of the transistor switches in the converter.