Means of eliminating electrolytic capacitor as the energy storage component in the single phase AD/DC two-stage converter

A power factor correction (PFC) circuit includes an inductor, a diode, a storage capacitor, a switch and a control unit. The input power has a voltage fluctuation V1. The storage component absorbs a first voltage fluctuation and a switch regulation circuit absorbs a second voltage fluctuation V2. Thus output voltage from the PFC circuit is not a conventional constant voltage but a voltage of a great ripple. The PFC circuit further has a harmonic regulation unit. The harmonic regulation unit generates a voltage signal containing 3rd harmonic. The control unit receives a feedback signal and the voltage signal containing 3rd harmonic to generate a reference to the inductor current. Therefore, the inductor current contains 3rd harmonic. Thus power fluctuation absorbed and released by the capacitor is smaller. As a result energy storage capacitance can be reduced significantly.

FIELDS OF THE INVENTION

The present invention relates to a power factor correction circuit and particularly to a control circuit for using in active power factor correction circuit.

BACKGROUNDS OF THE INVENTION

Power supply is widely used in various types of electronic devices nowadays. As technology advances constantly, the output power of power supply also increases continuously. To further boost the output power, its power efficiency has to be enhanced. There are many factors impacting efficiency, such as the input power factor and loss of power conversion. Power factor correction (PFC in short hereinafter) converter is used to improve the input power factor in the power supply. An active PFC circuit can improve the power factor to 95% or more. Referring toFIG. 1, the input power goes through a rectification circuit8and is regulated by the PFC circuit1to correct the input power factor. Then the output voltage of the PFC circuit1is further regulated by a Power conversion unit9to become a constant output voltage. The Power conversion unit9may be a converter or a switch type conversion circuit. The PFC circuit1and the Power conversion unit9are respectively controlled by a first control unit11and a second control unit91. A typical PFC circuit1includes a switch12, an inductor13, a storage capacitor14, a diode15and the first control unit11. The first control unit11receives a feedback signal from the output voltage of the PFC circuit1and incorporates with a zero current detection (ZCD) circuit and a RS flip-flop to generate a period control signal to drive the switch12, thereby regulate the input power factor. The operation principle of the PFC circuit1is known in the art, thus details are omitted herein. To balance the pulsating input power and the constant output power, the storage capacitor14must have a greater capacitance and usually use electrolytic capacitor. The rated lifetime of conventional electrolytic capacitor is only a few thousand hours. So the conventional PFC circuit1has a limited lifetime due to the usage of electrolytic capacitor. Please refer toFIGS. 2 and 3, where vgis the voltage after rectification, vCBis the storage capacitor voltage in the PFC circuit, and vois the output voltage of the power supply. The multiplication product of vinand iin. pinis the instantaneous input power (referring toFIG. 3). As shown inFIGS. 2 and 3, the input power pinis pulsating and the output power of PFC circuit is constant owing to its constant power load which is usually a DC/DC converter. The storage capacitor voltage vCBin the conventional PFC circuit is constant with small voltage ripple, saying about 10˜20V peak to peak. Therefore, electrolytic capacitor which has large capacitance is required. Therefore, incorporating the PFC circuit1in a long life electronic element results in a waste. For instance, adopting the conventional PFC circuit1in a driving circuit to drive LEDs whose lifetime is about 100,000 hours, the PFC circuit1could last only up to about 20 thousand hours. Then the circuit board with the PFC circuit1mounted thereon and LEDs soldered thereon have to be replaced. The LEDs which are being used for less than one half of their lifetime also have to be discarded with the entire circuit board. This is a waste and makes the cost higher. Therefore, the lifetime of the PFC circuit1must be prolonged to deserve long life LEDs.

SUMMARY OF THE INVENTION

In view of the problem of the conventional power factor correction (PFC) converter that is limited by the lifetime of electrolytic capacitor, it is an object of the present invention to provide a circuit structure to overcome the aforesaid shortcoming of the conventional techniques and prolong the lifetime of PFC circuit.

The present invention provides a PFC circuit which includes an inductor, a storage capacitor, a switch, a diode and a control unit. The control unit generates a period control signal to control the switch, thereby to control the inductor current. The present PFC circuit has a harmonic regulation unit and a multiplier3. The harmonic regulation unit is used to obtain harmonic wave from the voltage after rectification and the amplitude of the harmonic wave can be regulated by it. The multiplier3receives a feedback signal of output voltage and the voltage signal containing 3rdharmonic, and its output is the reference to the inductor current. Therefore, the input current of the PFC circuit is regulated to the desired wave. Such an approach can reduce the fluctuation of the input power. And the storage capacitor with less capacitance can be selected (such as a film capacitor to replace the electrolytic capacitor in the conventional circuit) to prolong the lifetime of the PFC circuit and improve the power density.

In addition, the power supply includes a PFC circuit and a power conversion unit. The PFC circuit has a storage capacitor. The power conversion unit has a switching regulation circuit. The input power of PFC circuit is pulsating, while the power conversion unit is constant power load for PFC circuit. By means of the technique set forth above, output voltage of the PFC circuit is not the constant voltage output from the conventional circuit, but a voltage which has a larger ripple, saying hundreds of voltage peak-to-peak, which is against the conventional wisdom. Then the voltage can be regulated to a constant output voltage by the power conversion unit located after the PFC circuit. Hence the energy storage capacitance can be greatly reduced to increase the lifetime of the PFC circuit. Compared with the conventional PFC circuit which has a larger storage capacitance to stabilize the output of the PFC circuit at constant voltage with very small amount of voltage ripple, the invention uses a storage capacitor with less capacitance; as a result, the output voltage of the PFC circuit is not a constant voltage as that of the conventional PFC circuit, but a output voltage with a larger ripple. Therefore, the energy storage capacitance can be reduced significantly, and the lifetime of the PFC circuit can be extended.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Please refer toFIG. 4for a circuit block diagram of an embodiment of the invention. The power supply according to the invention includes a rectification unit8, a power factor correction (PFC) circuit1and a power conversion unit9. The power conversion unit9may be a flyback circuit. The PFC circuit1includes an inductor13, a storage capacitor14, a switch12, a diode15and a first control unit11. The first control unit11generates a period control signal to drive switching of the switch12, thereby controls an inductor current passing through the inductor13so that the input power factor is regulated. The PFC circuit1is driven by the first control unit11, and the power conversion unit9is driven by a second control unit91. The PFC circuit1is set on by a switch12switched by the first control unit11to regulate input power factor. This is a technique known in the art, thus details are omitted herein. InFIG. 4when the input power factor is unity, the input voltage (rectified input voltage vg) and current are sine waveform. Multiplying these two generates a greater power fluctuation (referring toFIG. 10for the power waveform of a conventional one). Also refer toFIGS. 5,6and7, withFIG. 5showing the waveform of the input power voltage vg. A voltage fluctuation V1is defined between a peak value Vband a valley value Vaof the input power voltage vg. The PFC circuit1has a storage capacitor14. The power conversion unit9has a switching regulation circuit92. The input power passes through the storage capacitor14which absorbs a first voltage fluctuation. As a result, the input power passing through the PFC circuit1is regulated to become power having the second voltage fluctuation V2(shown as vCBinFIG. 6). Power curve pcshown inFIG. 6is output power of the PFC circuit1. Although the PFC circuit1absorbs only a portion of voltage fluctuation, due to the power conversion unit9at the rear stage is a constant power load, the output power of the PFC circuit1is constant. But the voltage vCBstill has the second voltage fluctuation V2which is not being absorbed. Referring toFIG. 7, the output voltage vCBfrom the PFC circuit1is regulated by the switching regulation circuit92to become a constant output voltage VO. After the PFC circuit1is regulated, the input power vCBis sent to the switching regulation circuit92which performs high frequency regulation to make the voltage vCBcontaining the second voltage fluctuation V2to become the constant output voltage Vo. Compared with the conventional technique which has a capacitor with large capacitance in the PFC circuit1, and the output voltage of the conventional PFC circuit1is eventually constant, the storage capacitor14provided by the invention can be a capacitor with less capacitance to absorb the first voltage fluctuation, while the input power still has fluctuation of the second voltage fluctuation V2as shown inFIG. 8. Compared the waveforms inFIGS. 8 and 2, the voltage after being regulated by the PFC circuit1of the invention is no longer the constant voltage output from the conventional PFC circuit, but has a greater ripple that passes through the switching regulation circuit92to become the constant output voltage Vo.

By means of the technique of the invention previously discussed, the PFC circuit1of the invention has a capacitor with less capacitance (such as a film capacitor) to serve as the storage capacitor14. Thus the cost of the storage capacitor14can be reduced and power fluctuation absorbed and released by the storage capacitor14also is smaller. As a result, the lifetime of the PFC circuit1is extended. The power supply thus formed also can be made at a lower cost and smaller size.

Please refer toFIG. 4. In order to increase the lifetime of the storage capacitor14, the PFC circuit1further has a harmonic regulation unit2and a waveform regulation unit3. The harmonic regulation unit2receives the rectified input voltage and generates a voltage signal containing 3rdharmonic by processing. The waveform regulation unit3receives a feedback signal from the output of the PFC circuit1and regulates the feedback signal according to the voltage signal containing 3rdharmonic to generate a current reference signal. Namely the feedback signal is regulated to adjust the amplitude of the voltage signal containing 3rdharmonic to form the current reference signal. The current reference signal is sent to the first control unit11to regulate the period width of the period control signal according to the current reference signal. The period control signal sets ON/OFF of the switch12to regulate the inductor current. Hence by regulating the period control signal through the current reference signal, the inductor current has the same phase as the voltage signal containing 3rdharmonic (namely the inductor current also contains harmonic wave). By means of the circuit structure set forth above, the period control signal contains the harmonic wave so that the inductor current is regulated to have the same phase as the voltage signal containing 3rdharmonic. When the harmonic wave is added to the inductor current, the inductor current fluctuation changes, the multiplication product of the inductor current and input voltage also changes. Hence the input power fluctuation is smaller. Consequently, the input power fluctuation absorbed by the storage capacitor14also decreases. Hence accelerated attenuation of the storage capacitor14can be prevented. The life span of the storage capacitor14increases and the storage capacitor14with less capacitance can be selected.

In order to add the harmonic wave in the period regulation circuit, the harmonic regulation circuit2contains two multipliers and one subtracter. The multiplier gets a rectified input voltage signal from the rectified input voltage. The rectified input voltage signal is multiplied to get a harmonic wave. Refer toFIG. 9for an embodiment of the aforesaid circuit. The harmonic regulation unit2has two multipliers21and22to receive a same rectified input voltage signal va. One multiplier21multiplies the rectified input voltage signal vatwice to get v2a. Another multiplier22multiplies the harmonic wave v2aand the rectified input voltage signal vato getv3a.v3ais sent to the subtracter which consists of a process amplifier U1and resistors R1, R2, R3and R4. The output of the subtracter vccontains 3rdharmonic. The harmonic regulation unit2may also be a multiplier to multiply the feedback signal and the voltage signal containing 3rdharmonic vcto generate the current reference signal. Hence the current reference signal also contains the 3rdharmonic. In the circuit shown inFIG. 9, the first control unit11gets the current reference signal and the current of the switch12. The two signals are compared to determine OFF time series of the switch12. In the event that the inductor current detected is greater than the current reference signal, the comparator sends a higher level output to reset the RS flip-flop so that the driving unit sets off the switch12. Thus the inductor current is confined within the waveform formed by the current reference signal, and the inductor current has a waveform containing the 3rdharmonic.

Refer toFIG. 10for the current waveform of a conventional PFC circuit andFIG. 11for the current waveform of circuit of the invention previously discussed.FIG. 10shows the waveforms of a conventional inductor current71, a conventional current peak envelope curve72and a conventional current average value73. The conventional inductor current71is controlled by ON/OFF of the switch12. The conventional circuit generates the conventional current peak envelope curve72. Namely by forcing the maximum current of the switch12at the OFF time, the inductor current71is limited smaller than the conventional current peak envelope curve72. Through processing the conventional current average value73is obtained. As shown inFIG. 11, by adding the harmonic wave in a current peak envelope curve75, the peak value of inductor current74and a current average value76also contain harmonic waveforms. Refer toFIG. 12for waveforms at various nodes of the conventional circuit andFIG. 13for waveforms at various nodes of the invention, where vgis being defined as the voltage waveform of the input power after rectification, vois being defined as the output voltage of the PFC circuit1, va3is being defined as the harmonic wave generated by the harmonic regulation unit2, and iLis being defined as the inductor current.FIG. 12shows the conventional circuit capable of generating the harmonic wave; hence va3is a straight line to indicate its absence. The rectified input voltage vgis a sinusoidal wave. The conventional inductor current iLalso is a sinusoidal wave. When the phases of the input voltage waveform vgand the conventional inductor current iLare regulated to be coincided with each other, the peak power generated by multiplying the two is greater. It forms a greater difference with the minimum power. Hence the power absorbed and released by the storage capacitor14also is greater. Referring toFIG. 13, after adding the harmonic wave va3, the phases of the peak value of the inductor current iLand the peak value of the input voltage waveform vgare shifted away from each other. As a result, the peak power generated by multiplying the two is reduced significantly. The power absorbed and released by the storage capacitor14also is smaller. Thus accelerated attenuation of the storage capacitor14that might otherwise take place due to charging and discharging for a prolonged period of time can be prevented, and the life span increases.

The harmonic regulation unit2mentioned above is a based embodiment of the present invention, the harmonic regulation unit2can further install a plurality of multipliers to continuously generate 5thharmonic or 7thharmonic or the combination of the harmonic thereof, so as the voltage signal containing harmonic is formed. By means of the circuit set forth above, the peak value of input power is lower, and the power fluctuation of the storage capacitor14caused by power absorption and release can be reduced. The storage capacitor14with less capacitance can be selected and the lifetime is longer. The PFC circuit1is less likely to be damaged due to the attenuation of the storage capacitor14. And a film capacitor can be selected to replace the electrolytic capacitor used on the conventional circuit, and the PFC circuit1mentioned above can work in continuous current mode (CCM) or in discontinuous current mode (DCM). It is not limited to generate the harmonic wave by the multiplier, but still can use other circuits to realize the technique of generating the harmonic wave, and the purpose of generating the harmonic wave is to reduce the fluctuation of the input power.