The present invention relates to a single-output resonant switched power converter, in which the transfer of energy between the input and the output thereof is governed by a switching element located on the primary side of the resonant switched power converter.
The secondary side of the converter includes a resonant circuit that carries out the demagnetisation of the core of a transformer, which electrically isolates the input from the output of the resonant switched power converter.
The resonant switched power converter is of special application, but not exclusively, in distributed power supply systems in which switched power converters with low voltage and power consumption are required, since they are mounted on printed circuit boards as one more electronic component thereof, and consequently a high integration density and reduced dimensions are required.
A switched power supply converter is known from the U.S. Pat. No. 5,886,881 granted to Xia et al., incorporated in the present patent application by reference. It describes a single-output forward converter that comprises on the primary side a serial combination of a primary winding of a transformer and a first switching element, which is connected to an input voltage supplied by a direct current (DC) power source.
On the secondary side, a secondary winding of the transformer is connected in cascade to a self-excited synchronous rectifier and to a filter that applies an output voltage to a load. The output of the filter constitutes the output of the switched power converter.
A serial combination consisting of a capacitor and a second switching element is connected in parallel with a third switching element, which forms the rectifier branch of the self-excited synchronous rectifier.
When the first switching element is in a conducting state, the input voltage is applied to the input of the self-excited synchronous rectifier through the transformer. During this period there is direct energy transfer between the input and the output.
When the first switching element is in a non-conducting state, the second switching element is in conduction, a magnetising current flowing from the secondary winding to the capacitor, which is charged to a voltage proportional to the input voltage. The result being that the voltage on the secondary winding is maintained constant, or clamped, and the core of the transformer is demagnetised, or reset.
A drawback of the forward switched power converter is that the non-conducting period of the first switching element is constant and, as a consequence, the demagnetisation of the core of the transformer does not take place during the entire non-conducting period of the first switching element, the result being that in the event of major variations of the input voltage, undesirable dead times appear in the switching of the self-excited synchronous rectifier.
For this reason, it is necessary to develop a switched forward power converter which accepts a broad range of input voltages and guarantees, for all of them, that the demagnetisation of the core of the transformer takes place throughout the entire non-conducting period of the first switching element, in such a manner that in the operation of the self-excited synchronous rectifier no dead times appear over the whole range of input voltages. All this leading to an enhancement of the forward converter performance, while maintaining both simplicity of design and of operation of the forward converter.
To overcome the problems described above, a resonant switched power converter is proposed that is capable of making the end of the resonant period coincide with the start of the conducting period of a first switching element, thereby avoiding undesirable dead times during the operation of a self-excited synchronous rectifier.
To achieve said objective a first capacitor is added which has the characteristic that its capacitance is a function of the duty cycle of the first switching element. To achieve said objective the first capacitor is connected in parallel with a second winding of a transformer.
The variation of the value of the capacitance of the first capacitor is carried out by adding in parallel with said first capacitor a serial combination of a second capacitor and a second switching element. The value of the capacitance varies between a first value and a second value.
By controlling the value of the capacitance of the first capacitor, the end of the resonant period is made to coincide with the end of the nonconducting period of the first switching element.