Field of the Invention
The present invention relates to an AC/DC converter and a driving circuit.
Description of Related Art
<First Related Art>
Recent years, there is proposed an AC/DC converter that can directly convert an AC input voltage into a DC output voltage (see, for example, Japanese Patent No. 5007966 and JP-A-2011-193709).
<Second Related Art>
In addition, recent years, there is proposed an insulation type AC/DC converter that realizes software switching function using a resonant capacitor (see, for example, JP-A-2011-193709).
<Third Related Art>
In addition, conventionally, there is known a driving circuit that sends a control signal from a primary side to a control IC on a secondary side via a pulse transformer, so as to control the control IC to turn on and off a switching element on the secondary side (see, for example, JP-A-2011-259673).
<Problem about the First Related Art>
However, the conventional AC/DC converter described above has a structure in which an ON/OFF ratio is selected so that an absolute value of a flyback voltage becomes higher than an absolute value of a peak value of a forward voltage, and thus only the flyback voltage is output. Therefore a disadvantage of a flyback method, which is that peak current generated in a secondary winding of an insulation transformer is increased, is not improved, and therefore it is not suitable for a power supply for use of handling middle or high power (for example, 100 W or higher) in particular.
In addition, in the conventional AC/DC converter described above, when it is activated or a load is short-circuited, an absolute value of a flyback voltage can be lower than an absolute value of a peak value of a forward voltage. In this state, unintentional forward operation may be generated so that excessive current may flow in the secondary side, which may cause abnormal heating or breakdown of an element.
In view of the above-mentioned problem found by the inventors, it is a first object of the invention disclosed in this specification to provide an AC/DC converter that can directly convert an AC input voltage into a DC output voltage with high efficiency also when being applied to a middle or high power.
<Problem about the Second Related Art>
When excitation energy of the transformer becomes zero, a voltage across both ends of the resonant capacitor starts to drop along with oscillation. Therefore, the conventional AC/DC converter described above has a structure in which a point at which the excitation energy of the transformer becomes zero is detected, and the detected signal is delayed by a constant delay time so that a switching drive signal is generated.
However, the necessary time after the excitation energy of the transformer becomes zero until the voltage across both ends of the resonant capacitor becomes 0 V depends on an input and output state in each period and is not always constant.
Therefore, the conventional AC/DC converter described above with the constant delay time has a problem that an error occurs in the switching drive timing, and hence the software switching cannot be correctly performed, resulting in deterioration of conversion efficiency.
In addition, the conventional AC/DC converter described above has another problem that a delay circuit, an external resistor, and the like are necessary, so that a circuit scale is increased.
In view of the above-mentioned problem found by the inventors, it is a second object of the invention disclosed in this specification to provide an AC/DC converter that can realize the software switching function in a simple and precise manner.
<Problem about the Third Related Art>
In the conventional driving circuit described above, in a period while an ON pulse or an OFF pulse is output to the secondary winding of the pulse transformer, it is required to continuously apply a drive voltage to a primary winding of the pulse transformer. Therefore, there is a problem that drive current flowing in the primary winding continuously increases over time, and hence current consumption is large.
In addition, in order to decrease rising time and falling time of an induced voltage generated in the secondary winding so as to increase responsiveness when driving the switching element on the secondary side, it is necessary to rapidly charge and discharge a parasitic capacitor between gate and source of the switching element (for example, MOSFET). For this purpose, it is necessary to decrease inductance of the primary winding, and as a result, the drive current is increased. In this way, the conventional driving circuit described above has another problem that a decrease of current consumption and an improvement of responsiveness are incompatible.
In addition, in an application using a high voltage withstand switching element (such as a SiC power device) on the secondary side, a gate voltage of the switching element is required to be higher than power supply voltage on the primary side in many cases. Therefore the inductance of the primary winding must be increased more, and hence the problem described above becomes more conspicuous.
In view of the above-mentioned problem found by the inventors, it is a third object of the invention disclosed in this specification to provide a driving circuit that can realize both the decrease of current consumption and the improvement of responsiveness.