1. FIELD OF THE INVENTION
This invention relates to a power supply circuit, and more particularly to a power supply circuit which is incorporated in a power IC obtained by monolithic integration of power semiconductor elements having a high blocking voltage capability and a large current handling capability and drive circuits for driving the power semiconductor elements and which is suitable for use as a means for supplying power required for driving the power semiconductor elements.
2. DESCRIPTION OF THE RELATED ART
A power IC obtained by monolithic integration of output semiconductor elements having a high blocking voltage capability and a large current handling capability and drive circuits driving these semiconductor elements requires generally two input power sources, that is a first power source for suppling a required voltage to the output semiconductor elements and a second power source for supplying a voltage lower than that of the first power source to the drive circuits. When the output semiconductor elements are, for example, N-channel MOS transistors constituting a three-phase inverter, four voltage sources having different reference levels of the drive voltage are required as the second power source, and it has been a prior art practice to supply such voltages insulated by insulating transformers respectively. However, because such a method is inconvenient, it has been proposed to incorporate the second power source of the kind described above in the IC itself.
As prior art disclosures proposing incorporation of such power supplying means in an IC, methods described in, for example, Japanese patent unexamined publication JP-A-59-15331, "Nikkei Electronics, January 26, 1987, page 154" and GE Application Note entitled "GES mart TM MODULE SIMPLIFIES MOTOR DRIVE DESIGN" November, 1985 pages 1-12 are known. In the circuit described in the latter, IGBT's (insulated gate bipolar transistors) are connected in a half bridge circuit as output semiconductor elements to be controlled by a control IC, and voltage sources having different reference voltages levels are required for the IGBT's disposed in the upper and lower stages of the output part respectively.
Externally applied input voltages include a d.c. voltage Vdc applied to the IGBT's in the output part and a power supply voltage Vcc required for the control IC.
The power supply voltage Vcc is directly used to drive the lower-stage IGBT's because the level of the reference voltage is the ground potential. On the other hand, the reference voltage level of the voltage source driving the upper-stage IGBT's changes depending on the operating state of the IGBT's in the output part. That is, this reference voltage is Vdc/2 when both the upper-stage and lower-stage IGBT's are in their off state, but the reference voltage is Vdc when the upper-stage IGBT is in its on state and the lower-stage IGBT is in its off state.
According to the latter prior art method, a capacitor is connected between the collector terminal of the lower-stage IGBT and the gate terminal of the upper-stage IGBT, and the lower-stage IGBT is used as switch means to establish a circuit state in which the voltage source applying the power supply voltage Vcc to the control IC is connected in series with the capacitor, so that the control IC has the function of supplying charges to the capacitor to charge the capacitor.
Although such a method is advantageous in that Vcc only is required as a drive input, no consideration is given to produce Vcc itself from the d.c. voltage Vdc.
Thus, in the prior art power supply circuit, no consideration is given to an idea in which the d.c. voltage input Vdc applied to the output semiconductor elements is lowered by means of a low-loss voltage lowering circuit so as to produce the power supply voltage input Vcc applied to the control IC. Therefore, the prior art power supply circuit has had the problem that two kinds of input voltage sources are still required.
In addition to the primary function of controlling the load current, the lower-stage IGBT in the output part has also the function of switch means for charging the capacitor as described above. Because the latter function has a priority over the former function, the prior art power supply circuit has had such another problem that the required control of the load current is temporarily interrupted during the period of charging the capacitor.