Electronic apparatus, AC/DC conversion device and power factor correction circuit thereof

This application claims the benefit of Taiwan application Serial No. 95131034, filed Aug. 23, 2006, the subject matter of which is incorporated herein by reference.

1. Field of the Invention

The invention relates in general to an electronic apparatus, AC/DC conversion device and power factor correction (PFC) circuit thereof, and more particularly to an electronic apparatus, AC/DC conversion device and power factor correction circuit thereof, which can reduce electromagnetic interference (EMI) by using a spread spectrum signal.

2. Description of the Related Art

Referring to FIG. 1 , a block diagram of a conventional projector is shown. A projector 100 includes a power factor correction circuit 110 , a flyback converter 120 , a lighting circuit 130 and a lamp 140 . The power factor correction circuit 110 is a power supply for the projector 100 . The power factor correction circuit 110 supplies a high DC voltage about 380V to the lighting circuit 130 and the flyback converter 120 . The lighting circuit 130 is, for example, a ballast for lighting up the lamp 140 . The flyback converter 120 supplies a DC power of 12V, 5V and 3.3V, for example.

However, there usually occurs electromagnetic interference between electronic apparatuses. Therefore, intensity of the electromagnetic waves generated by the electronic apparatuses should be suitably limited. For example, American federal communication commission (FCC) sets up a standard for limiting radiation quantity of electronic products. However, the issue that the power factor correction circuit easily causes electromagnetic interference to other electronic apparatus is still not resolved.

The invention is directed to an electronic apparatus, AC/DC conversion device and power factor correction circuit thereof. A spread spectrum signal is provided to the power factor correction circuit to reduce the electromagnetic interference.

According to a first aspect of the present invention, a power factor correction (PFC) circuit is provided. The power factor correction circuit comprises a switch and a power control chip. The power control chip is for receiving a spread spectrum synchronization signal, wherein the power control chip controls the switch by a control signal according to frequency of the spread spectrum synchronization signal.

According to a second aspect of the present invention, an AC/DC conversion device is provided. The AC/DC conversion device comprises a power factor correction circuit and a flyback converter. The power factor correction circuit comprises a switch and a power control chip. The power control chip is for receiving a spread spectrum synchronization signal, wherein the power control chip controls the switch by a control signal according to frequency of the spread spectrum synchronization signal. The flyback converter is for receiving a second voltage and accordingly generating an output voltage and outputting the spread spectrum synchronization signal.

According to a third aspect of the present invention, an electronic apparatus is provided. The electronic apparatus comprises a power factor correction circuit, a flyback converter, and a display module. The power factor correction circuit comprises a switch and a power control chip. The power control chip is for receiving a spread spectrum synchronization signal, wherein the power control chip controls the switch by a control signal according to frequency of the spread spectrum synchronization signal. The flyback converter is for receiving the second voltage for generating an output voltage and outputting the spread spectrum synchronization signal. The display module operates according to the output voltage.

The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.

FIG. 1 (Prior Art) is a block diagram of a conventional projector.

FIG. 2 is a block diagram of an electronic apparatus according to a preferred embodiment of the invention.

FIG. 3 is a circuit diagram of a power factor correction circuit according to the invention.

FIG. 4 is a circuit diagram of a flyback converter according to a first example of the invention.

FIG. 5 is a circuit diagram of a flyback converter according to a second example of the invention.

FIG. 6 is a structure diagram of a power module chip.

Referring to FIG. 2 , a block diagram of an electronic apparatus according to a preferred embodiment of the invention is shown. An electronic apparatus 200 includes an AC/DC conversion device 210 and an electrical module, such as a display module 220 as the electronic apparatus 200 is a liquid crystal display. The AC/DC conversion device 210 includes a power factor correction circuit 211 and flyback converter 212 . The power factor correction circuit 211 supplies, such as, a high DC voltage about 380V to the display module 220 and the flyback converter 212 . The flyback converter 212 supplies a DC power, such as 12V, 5V and 3.3V. The electronic apparatus 200 is, such as, a liquid crystal display or a projector. When the electronic apparatus is a projector, the electrical module is a spatial light modulation module.

The power factor correction circuit 211 receives a spread spectrum synchronization signal Sy 1 for reducing its electromagnetic interference. In the embodiment, the spread spectrum synchronization signal Sy 1 is provided by the flyback converter 212 .

The application of the principle of spread spectrum enables a clock signal operating at a constant frequency to have regular fluctuations within a specific range of the constant frequency in order to generate a frequency waveform similar to a triangular wave. Therefore, the energy originally accumulated at a frequency point can be scattered over a frequency band surrounding the frequency point such that the radiation energy at the frequency point will not exceed a standard value.

Referring to FIG. 3 , a circuit diagram of a power factor correction circuit according to the invention is shown. The power factor correction circuit 211 includes an inductance L 1 , a switch SW 1 , a power control chip 216 , a first diode D 1 and a first capacitor C 1 . The inductance L 1 has a first end and a second end. The first end is for receiving a first voltage V 1 . The switch SW 1 has one end coupled to the second end of the inductance L 1 and has the other end coupled to a low voltage Vs, such as a ground voltage. The power control chip 216 is for receiving the spread spectrum synchronization signal Sy 1 and accordingly controlling the switch SW 1 to be turned on/off by using a control signal Ct 1 in order to improve the power factor. The first diode D 1 has an anode coupled to the second end of the inductance L 1 . The first capacitor C 1 has one end coupled to a cathode of the first diode D 1 for generating a second voltage V 2 and has the other end coupled to the low voltage Vs. The switch SW 1 is, such as, a metal oxide semiconductor field effect transistor (MOSFET) with a gate for receiving the control signal Ct 1 , a drain coupled to the second end of the inductance L 1 , and a source coupled to the low voltage Vs.

Referring to FIG. 4 , a circuit diagram of a flyback converter according to a first example of the invention is shown. The flyback converter 212 includes a power module chip 217 , a transformer T 1 , a second diode D 2 , and a second capacitor C 2 . The transformer T 1 includes a first primary coil T 11 , a secondary coil T 21 and a second primary coil T 12 . The first primary coil T 11 has one end for receiving the second voltage V 2 and has the other end for receiving a control signal C 2 of the power module chip 217 . The secondary coil T 21 generates a third voltage V 3 according to the second voltage V 2 . The second primary coil T 12 generates a fourth voltage, that is, the spread spectrum synchronization signal Syl, according to the second voltage V 2 .

The second diode D 2 has an anode coupled to one end of the secondary coil T 21 while the other end of the secondary coil T 21 is coupled to the low voltage Vs 1 . The second capacitor C 2 has one end coupled to a cathode of the second diode D 2 for generating an output voltage Vo and has the other end coupled to the low voltage Vs 1 . The display module 220 operates according to the output voltage Vo.

The flyback converter 212 further includes a feedback circuit 218 , a photo-coupler 219 , a third diode D 3 and a third capacitor C 3 . The second primary coil T 12 has one end coupled to an anode of the third diode D 3 and has the other end coupled to the low voltage Vs.

The photo-coupler 219 includes a light emitting device 219 ( a ), such as a light emitting diode (LED), and a light receiving device 219 ( b ), such as a photo-transistor.

The third capacitor C 3 has one end coupled to a cathode of the third diode D 3 and one end b of the light receiving device 219 ( b ) and has the other end coupled to the low voltage Vs. The feedback circuit 218 is coupled to the output voltage Vo and a low voltage Vs 1 , and coupled to the ends d and c of the light emitting device 219 ( a ). The light receiving device 219 ( b ) has one end a for providing a reference signal to the power module chip 217 .

Referring to FIG. 5 , a circuit diagram of a flyback converter according to a second example of the invention is shown. Unlike the flyback converter 212 of the first example, the flyback converter 213 of the second example further includes an auxiliary coil T 13 , wherein the auxiliary coil T 13 has one end coupled to the one end of the second primary coil T 12 being coupled to the low voltage Vs and has the other end for generating the spread spectrum synchronization signal Sy 1 . Moreover, the spread spectrum synchronization signals respectively generated by the flyback converters 213 and 212 have a phase difference of 180 degrees. The flyback converter 212 of FIG. 2 can also be replaced by the flyback converter 213 to achieve the same effect.

Therefore, when the flyback converter 213 or 212 uses spread spectrum signals, the power factor correction circuit 211 also can achieve a spread spectrum effect. For example, if the frequency of the flyback converter 213 or 212 is 66 KHz, the power factor correction 211 can also be synchronized to have a frequency 66 KHz.

Referring to FIG. 6 , a structure diagram of a power module chip is shown. The power module chip 217 includes a frequency modulation unit 620 , an oscillator 610 and a transistor switch 630 . The frequency modulation unit 620 is for modulating an oscillation frequency of the oscillator 610 and the oscillator 610 is for controlling turning on/off of the transistor switch 630 . The transistor switch 630 has a source coupled to the low voltage Vs and has a drain coupled to the end of the first primary coil T 11 that is the end not for receiving the second voltage V 2 .

In the electronic apparatus, AC/DC conversion device and power factor correction circuit thereof disclosed by the above embodiment of the invention, a spread spectrum signal is provided to the power factor correction circuit to reduce the electromagnetic interference caused by its radiation.

While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.