1. Field of the Invention
The present invention relates to a power supply circuit which can be made to supply a relatively large electric power and to an electronic apparatus which uses a relatively large electric power.
2. Description of the Related Art
Heretofore, a circuit such as shown in FIG. 1 has been proposed as a switching type power supply circuit which can supply a relatively large electric power, for example, 100 W (see patent reference 1). The power supply circuit of FIG. 1 is a circuit added with a power factor improving circuit (PFC circuit) in accordance with a higher harmonic wave regulation.
In FIG. 1, a reference numeral 1 designates a commercial power supply of, for example, 100V and 50 Hz, and one terminal and the other terminal of the commercial power supply 1 are connected to one terminal and the other terminal on the input side of a rectifying circuit 3 of a diode bridge constitution respectively.
A pulsating flow of a positive direction corresponding to the frequency of the commercial power supply 1 is obtained at the positive-polarity terminal and negative-polarity terminal on the output side of the rectifying circuit 3. The positive-polarity terminal on the output side of the rectifying circuit 3 is connected to one terminal of a primary winding 5a of a converter transformer 5 by means of a series circuit of a choke coil 4a and a diode 4b which forms a power factor improving circuit 4, the other terminal of the primary winding 5a is connected to the drain of a field effect transistor 6 which constitutes a switching device, and the source of the field effect transistor 6 is connected to the negative-polarity terminal on the output side of the rectifying circuit 3.
It is constituted such that a connection mid point of the choke coil 4a and the diode 4b is connected to the drain of the field effect transistor 4c which forms the power factor improving circuit 4, the source of the field effect transistor 4c is connected to the negative-polarity terminal of the rectifying circuit 3, and a switching signal from a control circuit 4d is supplied to the gate of the field effect transistor 4c. In addition, a connection mid point of the diode 4b and one terminal of the primary winding 5a is connected to the negative-polarity terminal of the rectifying circuit 3 through a capacitor 4e which forms the power factor improving circuit 4.
The power factor improving circuit 4 is to make the pulsating flow obtained on the output side of the rectifying circuit 3 be sine-wave shaped and supplies it to the primary winding 5a of the converter transformer 5.
Also, one terminal of a secondary winding 5b wound in counter phase with respect to the primary winding 5a of the converter transformer 5 is connected to one DC voltage output terminal 8a through a diode 7a forming a rectifying circuit 7, a connection mid point of the diode 7a and the one DC voltage output terminal 8a is connected the other terminal of the secondary winding 5b through a smoothing capacitor 7b forming the rectifying circuit 7, and the other terminal of the secondary winding 5b is connected to the other DC voltage output terminal 8b. 
The one DC voltage output terminal 8a is connected to the input side of a pulse width modulation control circuit 9 composed of a semiconductor integrated circuit, a switching signal of a pulse width modulation signal obtained on the output side of the pulse width modulation control circuit 9 is supplied to the gate of a field effect transistor 6 and the field effect transistor 6 is switched by the switching signal of the pulse width modulation signal such that a constant DC voltage V0 is obtained at the one and the other DC voltage output terminals 8a and 8b. 
In such a power supply circuit shown in FIG. 1, the power factor is improved by carrying out control for making the input pulsating flow current from the rectifying circuit 3 be sine-wave shaped by means of the power factor improving circuit 4. In this case, the power factor is a power factor cos φ and expressed as a following equation when the input power is made to be |W|, the input current to be |A| and the input voltage to be |V|:cos φ=|W|/(|A|×|V|)
When the power factor improving circuit 4 such as shown in FIG. 1 is provided, the power factor cos φ can be improved as much as 0.8 to 0.99 and the input current waveform is made approximated to the input voltage waveform.
[Cited Patent Reference 1] Jap. Laid-open Patent No. 11-164555
However, when the power factor improving circuit 4 is provided in a conventional switching type power supply circuit, the efficiency of the power factor improving circuit 4 is to be integrated or multiplied to the efficiency of the power supply circuit, so that the efficiency decreases. For example, even if the efficiency of the conventional switching type power supply circuit is 90% and the converting efficiency of the power factor improving circuit 4 is 90%, the total efficiency will come to be 81%.
In addition, the power factor improving circuit 4 switches a large current by the field effect transistor 4c, so that there is a disadvantage that it becomes a noise generating source.
Further, since the power factor improving circuit 4 is provided, the circuit becomes complicated as much as the power factor improving circuit 4 and at the same time a space for mounting-the power factor improving circuit 4 becomes necessary, so that there was a disadvantage that the cost became higher as much.