Patent Publication Number: US-6219263-B1

Title: Electronic power supply device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an electronic supply device. More particularly, it relates to a device with a power factor correction circuit. 
     2. Description of the Prior Art 
     There is a known circuit for the correction of power factor without line inductance. The circuit is connected to the output of a bridge rectifier and uses rectifier diodes for the series charging and parallel discharging of two filtering capacitors. An electrical diagram of such a circuit is shown in FIG. 1. A bridge rectifier  1  receives a periodically varying voltage V AC  at a pair of input terminals. A power factor correction circuit  3  is connected between the rectifier  1  and a load  2  at a pair of output terminals of the rectifier. This circuit  3  uses two filtering capacitors, C 1  and C 2 , having the same capacitance. 
     A first rectifier diode D 1  is connected directly between the two capacitors C 1  and C 2 . The assembly is connected between two output terminals of the bridge rectifier. 
     A second rectifier diode D 2  is reverse-connected in parallel with the combination of the first capacitor C 1  in series with the first diode D 1 . 
     A third rectifier diode D 3  is reverse-connected in parallel with the combination of the second capacitor in series with the first diode D 1 . 
     The working of such a circuit shall now be explained with reference to the curves shown in FIG.  2 . 
     In a steady operating state, at a start of a half-wave of the line voltage V in , the diode D 1  is off. The diodes D 2  and D 3  are on. This corresponds to the end of the period of the discharging of the capacitors C 1  and C 2 . When the line voltage V in  exceeds the charging voltage of the capacitors, the diodes D 2  and D 3  go to the off state. Then, when the line voltage V in  exceeds the sum of the charging voltages of the two capacitors (Vc1+Vc2), the diode D 1  becomes conductive (T 1 ) and the two capacitors are charged in series until the line voltage reaches its peak value Vc (T 2 ). The diode D 1  then goes back to the off state. The two capacitors are each charged at Vc/2 (being identical capacitors). 
     The line voltage, which then decreases, becomes lower than this charging voltage Vc/2: the diodes D 2  and D 3  therefore come on, while D 1  remains off (T 3 ). The capacitors are again parallel-connected. The capacitor C 1  supplies the load through the diode D 3  and the capacitor C 2  supplies the load through the diode D 2 . This process stops as soon as the line voltage V in  again starts increasing (at the next half-wave) and becomes greater than the voltage of each capacitor: the diodes D 2  and D 3  go back to the off state, the diode D 1  remains off. The system is then at T 0 , and the cycle then repeats. The current waveform I in  shown in FIG. 2 is obtained. 
     Between T 0  and T 1 , it is the mains supply system (V AC ) that directly supplies the load (with D 1 , D 2  and D 3  off). The shape of the current waveform for a value of power P out  consumed in the load  2  is given by the relationship: 
     
       
         
           I 
           in(t) 
           =P 
           out 
           /V 
           in(t)  
         
       
     
     For Pout constant, between T 0  and T 1 , V in  increases and I in  decreases. 
     Between T 1  and T 2 , the capacitors are charged. On top of the current consumed in the load  2  (shown in dashes), there is superimposed the charging current for the capacitors. 
     Between T 2  and T 3 , the charging of the capacitors, each at half of the peak voltage Vc, is over. The current I in  is only the current consumed in the load  2  and the waveform of the current is given by the relationship: 
     
       
           I   in(t)   =P   out   /V   in(t) .  
       
     
     The line voltage decreases and I in  decreases (with P out  constant). 
     Finally, between T 3  and T 0 , it is the capacitors C 1  and C 2  that supply the load  2 . The current I in  drawn from the rectifier is zero. 
     The circuit  3  therefore makes it possible to increase the angle of flow of the bridge rectifier. The waveform of the current I in  is spread over the voltage half-wave with three phases of conduction: [T 0 -T 1 ], [T 1 -T 2 ] and [T 2 -T 3 ]. In this way, the power factor of the device (namely the ratio of the actual power to the apparent total power) is improved since the line is forced to consume current during the most significant part of the voltage wave, namely when the instantaneous value of the line voltage exceeds half of the peak value Vc. 
     However, for the charging of the capacitors, there is a drawing of charging current which gives rise to a steep leading edge of the line current. There is therefore a current peak. This corresponds to non-negligible low frequency harmonic contents that limit the value of the power factor (with a supply of power at harmonic frequencies different from the line frequency). 
     SUMMARY OF THE INVENTION 
     An object of the invention is to improve the afore-mentioned power factor correction circuit. 
     An object of the invention is to reduce the low frequency harmonic contents of the waveform of the current drawn from the rectifier. 
     As characterized, the invention relates to an electronic supply device for a load comprising a bridge rectifier receiving a periodic voltage at a pair of input terminals and a power factor correction circuit connected to a pair of output terminals of the rectifier. The power correction circuit includes two capacitors, a rectifier diode to charge them in series and two rectifier diodes to discharge them in parallel. According to the invention, the correction circuit further includes a resistor that is series-connected to the first rectifier diode to limit the current drawn in the capacitors and reduce the low frequency harmonic contents of the current conducted by the rectifier. 
     The addition of a resistor in series with the diode that enables the control of the charging of the capacitors in series makes it possible to attenuate the charging current. This results in a more rounded-out waveform of the line current: the low frequency harmonic contents of such a waveform are highly attenuated. The power factor of this device is thus appreciably improved. 
     Furthermore, when the voltage is turned on, the capacitors are charged immediately. However, the resistor, in addition to attenuating low frequency harmonic contents, will limit the drawn current which, if excessively high, damages the diodes and the capacitors. 
     In one improvement of the invention, a particular three-diode structure of the power factor correction circuit according to the invention is used to protect the circuitry downline with respect to the rectifier against overvoltages on the mains supply system. 
     According to the invention, zener diodes are used as rectifier diodes. In the event of overvoltage in the mains supply system, the three zener diodes are series-connected. The circuitry is therefore protected against overvoltages greater than three zener voltages. Each capacitor is protected against overvoltages greater than two zener voltages. 
     One variant uses a current-controlled power switch parallel-connected with the resistor and the diode which controls the series charging of the capacitors. A zener diode is used for each of the two diodes that controls the discharging of the capacitors. In this way, it is possible to protect the circuitry against overvoltages greater than two zener voltages and the capacitors against overvoltages greater than one zener voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention are described in detail in the following description made with reference to the appended drawings, in which: 
     FIG. 1 is an electrical drawing of a power factor correction circuit already described, 
     FIG. 2 shows corresponding waveforms of current and line voltage, 
     FIG. 3 shows a diagram of a power factor correction circuit according to the invention, and 
     FIG. 4 shows another diagram of a power factor correction circuit according to the invention. 
    
    
     MORE DETAILED DESCRIPTION 
     FIG. 3 shows a diagram of a power factor correction circuit  3  according to the invention, connected to a pair of output terminals of a bridge rectifier supplied by a mains voltage V AC . 
     The rectifier  1  and the power factor correction circuit  3  supply a load  2 . 
     The circuit  3  has two filtering capacitors Cl and C 2  series-connected with the output terminals of the rectifier  1 . A resistor R and a first directly mounted diode D 4  are series-connected between the two capacitors C 1 , C 2 . 
     A second diode D 5  is reverse-connected and parallel-connected with the series-connected assembly formed by the first capacitor C 1 , the resistor R and the first diode D 4 . 
     A third diode D 6  is reverse-connected and parallel-connected to the series-connected assembly of the second capacitor C 2 , the resistor R and the first diode D 4 . 
     The assembly formed by the resistor R and the first diode D 4  controls the series charging of the capacitors C 1  and C 2 . The resistor R according to the invention makes it possible to limit the current drawn in the capacitors when the voltage is first turned on. In the worst possible case, when the equipment is turned on at a time corresponding to a half-wave peak, the value of the drawn current will thus be limited to Vc/R, Vc being the peak voltage of the line voltage. The resistor R according to the invention further enables the charging current to be attenuated. This results in a rounding out of the waveform portion of the corresponding line current as shown in dashes in FIG. 2 (interval [T 1 -T 2 ]). 
     The trade-off here is that the resistor consumes current. However, it has been determined in practice that for a resistor consuming only 1% of the power available, the power factor of the device is very appreciably improved. 
     In one example, for 100 watts, with: 
     V AC =230 volts AC, 
     C 1 =C 2 =40 microfarads, 
     R=20 ohms, P Rmax =1 watt, 
     there is obtained a power factor PF=0.88. 
     Finally, the unit formed by the device for rectifying and power factor correction may advantageously take the form of an integrated circuit. 
     In one improvement shown in FIG. 3, zener diodes are used as rectifier diodes D 4 , D 5  and D 6 . 
     Indeed, the structure of the power factor correction circuit  3  has three rectifier diodes series-connected with the pair of output terminals of the rectifier. By using three voltage limitation rectifier diodes, it is then possible to shield the downline circuitry (the load  2 ) against mains overvoltages greater than three times the limitation voltage (three zener voltages). Furthermore, the structure has two rectifier diodes parallel connected with each of the capacitors. 
     In the same way, by using two voltage limiting rectifier diodes, each capacitor is shielded against overvoltages greater than twice the limiting voltage (namely two zener voltages). This limiting voltage should be chosen as to be greater than half the peak value Vc of the line voltage. 
     One variant of the power factor correction circuit used for the protection against overvoltages according to the invention is shown in FIG.  4 . The diodes D 5  and D 6  which control the discharging of the capacitors in parallel are voltage limiting rectifier diodes. 
     However, in parallel with the first rectifier diode D 4  that is in series with the resistor R, there is placed an electronic switch T 1  current-controlled in reverse by one of the zener diodes, D 5  in the example. This electronic switch is, for example, a triac, and a resistor r 1  connected to the zener diode D 5  gives the negative trigger current needed to activate the triac. 
     In the event of overvoltage, the assembly D 6 , T 1 , D 5  allows the passage of the reverse current and enables protection against overvoltages greater than only twice the zener voltage. 
     As we have seen here above, the diodes D 5  and D 6  must be such that they remain off when the line voltage reaches the peak value Vc. This leads in practice to taking the following value as a zener voltage: 
     
       
           Vz&gt;Vc/ 2  
       
     
     For example, for 220 VAC, the peak voltage Vc of the rectified line voltage V in  is equal to 310 volts. If the zener diodes are chosen with a zener voltage of 180 volts, the circuitry is protected for overvoltages of over 360 volts. 
     This circuit with current-controlled switch requires another zener diode D 7  connected between the zener diode D 5  and between the series connection of D 4 ′ and C 2  so that the switch is accurately controlled during the overvoltage. In practice, a 5-volt zener diode is sufficient for this task. 
     Finally, in the two examples of a power factor correction circuit with protection against the overvoltages, the rectifier device and the correction circuit may be made in one and the same integrated circuit.