SWITCHED-MODE POWER SUPPLY WITH SYNCHRONOUS PRECONVERTER

Abstract: A power supply (100) comprises: a rectifier (10); a power preconverter (20) coupled to the rectifier (10); a switched-mode power supply circuit (30) coupled to the power preconverter (20); and, a phase control circuit (21, 22) for synchronizing operation of the power preconverter (20) with operation of the switched-mode power supply circuit (30). An energy storage device (C1) may be coupled to the power preconverter (20) and to the switched-mode power supply circuit (30). A preconverter switch (Q2) conducts during each time interval that a power switch (Q1) in the switched-mode power supply circuit (30) conducts in order, for example, to charge the energy storage device (C1). The preconverter switch (Q2) begins conducting only after each time interval begins and always stops conducting before each time interval ends. In a portion of the time interval after the preconverter switch (Q2) has stopped conducting, a current can flow directly from the power preconverter (20) to the switched-mode power supply circuit (30) without, for example, charging the energy storage device (C1).

Description: SWTTCHED-MODE POWER SUPPLY WITH SYNCHRONOUS PRECONVERTER BACKGROUND OF THE INVENTION This invention relates generally to the field of switched- mode power supplies, and, in particular, to switched-mode power supplies employing preconverter circuits to effect power factor correction. An off-line switched-mode power supply may consist of a rectifier stage cascaded with a DC-DC converter stage. The rectifier stage itself consists of a rectifier section and a filter section. The rectifier section uses semiconductor power rectifiers to convert the mains voltage into a pulsating DC voltage. This DC voltage is then filtered by capacitors in the filter section to obtain a DC voltage that has relatively low ripple. One consequence of using capacitors in the filter section is that the resulting input current waveform consists of current pulses corresponding to the peaks of the mains voltage. This happens because the rectifier diodes cannot conduct current until the mains voltage exceeds the filter capacitor's voltage. As a result, the power supply only draws power at the peaks of the mains voltage, which results in a poor power factor at the input to the power supply. One method for improving the power factor at the input to the power supply is to place a boost-type DC-DC preconverter between the rectifier section and the filter section. The preconverter effects a power factor correction by allowing the input rectifiers to conduct sooner and conditions the input current to be sinusoidal and in phase with the mains voltage. A prior art switched-mode power supply with a boost-type DC-DC preconverter is shown in FIGURE 8. Power from the mains is rectified and fed to the preconverter. In the preconverter, a power factor controller, such as a Siemens TDA4815, controls preconverter switch Q2. Energy is stored in inductor LI during the "on" time of preconverter switch Q2. This energy is transferred to a storage capacitor Cl by a charge current icharge through a diode D l during the "off time of preconverter switch Q2, and is ultimately provided to the input of the switched-mode power supply as an input current i i . A problem with the prior art arises from the fact that preconverter switch Q2 switches asynchronously with respect to power switch Ql of the switched-mode power supply. This asynchronous switching results in a random phase relationship between charge current icharge through diode Dl and input current i i to the switched-mode power supply. SUMMARY OF THE INVENTION Establishing a more optimal phase relationship between charge current icharge and input current i ] by synchronizing switching of preconverter switch Q2 and power switch Ql would reduce the total rms current through storage capacitor C l , which is desirable for several reasons. The use of a synchronous preconverter eases the specifications for storage capacitor C l , thus allowing for the use of standard, low cost components. The life expectancy of a given storage capacitor Cl can be extended or a lower cost capacitor can be used instead. The voltage rating for storage capacitor Cl can also be reduced. The harmonic components of a mains current imains can be further attenuated from present levels. A ripple voltage at the output of the switched-mode power supply can be reduced. Finally, when used with a flyback-topology switched-mode power supply in a television, the synchronous preconverter switches "off when the television is operating in "standby" mode. Briefly stated, a power supply according to an inventive arrangement taught herein results in a reduction of the total rms current flowing through the filter section of the power supply. The operation of a switched-mode power supply circuit is synchronized with the operation of a preconverter circuit so that an input current flows directly from the preconverter circuit to the switched-mode power supply circuit during a portion of each time interval that the switched-mode power supply circuit draws an input current. Accordingly, a power supply according to an inventive arrangement taught herein comprises: rectifying means; power preconverting means coupled to the rectifying means; a switched- mode power supply circuit coupled to the power preconverting means; and, means for synchronizing operation of the power preconverting means with operation of the switched-mode power supply circuit. A power supply according to a further inventive arrangement taught herein comprises: rectifying means; power preconverting means coupled to the rectifying means; energy storage means coupled to the power preconverting means; a switched-mode power supply circuit coupled to the power preconverting means and to the energy storage means; and, means for synchronizing operation of the power preconverting means with operation of the switched-mode power supply circuit. A power supply according to another inventive arrangement taught herein comprises: rectifying means; power preconverting means coupled to the rectifying means; energy storage means coupled to the power preconverting means; a switched-mode power supply circuit coupled to the power preconverting means and to the energy storage means; and, means for supplying a current from the power preconverting means directly to the switched-mode power supply circuit during a portion of each time interval that the switched-mode power supply circuit draws an input current. The switched-mode power supply circuit and the power preconverting means may comprise first and second switch means, respectively. A control circuit in the power preconverting means may compare the amplitude of a ramp signal, keyed to operation of the switched-mode power supply circuit, to lower and upper reference voltages. Conduction of the second switch means in the power preconverting means may be enabled whenever the ramp signal is between the lower and upper reference voltages. The second switch means in the power preconverting means may conduct during each time interval that the first switch means in the switched-mode power supply circuit conducts, beginning only after each time interval begins and always stopping before each time interval ends. In a portion of the time interval after the second switch means has stopped conducting, a current may flow directly from the power preconverting means to the switched-mode power supply circuit. The above, and other features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a diagram, in block and schematic form, of a switched-mode power supply having an inventive arrangement. FIGURE 2 is a block diagram of a control circuit used in a switched-mode power supply having an inventive arrangement. FIGURE 3 is a block diagram of a phase-offset circuit used in a switched-mode power supply having an inventive arrangement. FIGURE 4 shows particular voltage and current waveforms associated with a switched-mode power supply having an inventive arrangement. FIGURE 5 is a schematic diagram of a presently preferred embodiment of a switched-mode power supply having an inventive arrangement. FIGURE 6 is a diagram of the mains current drawn by a presently preferred embodiment of a switched-mode power supply having an inventive arrangement. FIGURE 7 is a diagram of the harmonic content of the mains current drawn by a presently preferred embodiment of a switched-mode power supply having an inventive arrangement. FIGURE 8 is a diagram, in block and schematic form, of a prior art switched-mode power supply. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A power supply 100 that provides a more optimal phase relationship between a charge current icharge and an input current ii is shown in FIGURES 1 , 2 and 3. Voltage and current waveforms for power supply 100 are shown in FIGURE 4. Power supply 100 includes a switched-mode power supply circuit 30 coupled to a full-wave rectifier 10 by a preconverter 20. for use in a television. A control IC 31 of switched-mode power supply circuit 30 provides a drive signal Vi , shown in FIGURE 4(a), to turn a power switch Ql "on" and "off . The "on" time of power switch Ql is determined by a load coupled to an output of switched-mode power supply circuit 30. A phase-offset circuit 22 uses drive signal V i to synchronize switching of a preconverter switch Q2 of preconverter 20 with power switch Ql of switched-mode power supply circuit 30. By so synchronizing the switching of preconverter switch Q2 with the switching of power switch Ql , the charge current icharge supplied to storage capacitor C l by preconverter circuit 20 is modulated in response to the load coupled to an output of switched-mode power supply circuit 30. Drive signal V ] for power switch Ql determines a duration for a ramp signal V ra mp- which is generated by ramp generator 25 and is shown in FIGURE 4(b), and a voltage Vc i of storage capacitor C l determines a slope for ramp signal Vramp - Ramp signal V ram p is compared to two reference voltages, an upper reference voltage V r ef2 and a lower reference voltage Vref l , both shown in FIGURE 4(b), in a window comparator circuit 23. When ramp signal V ra mp is between upper and lower reference voltages V r ef2 and Vref l , respectively, a drive signal V2 for preconverter switch Q2 goes "high", as shown in FIGURE 4(c), and preconverter switch Q2 turns "on". Upper reference voltage V re f2 is lower than ramp voltage V ramp by the forward voltage drop of a diode (not shown) to achieve a phase offset between preconverter switch Q2 and power switch Ql , meaning that preconverter switch Q2 turns "off" before power switch Ql turns "off". While power switch Ql is "on" but preconverter switch Q2 is still "off, that part of charge current icharge that flows directly into a transformer Tl through an inductor LI and a diode Dl of preconverter 20 without being stored in a storage capacitor Cl is represented by the shaded areas in FIGURES 4(e) and 4(f). Upper and lower reference voltages V r ef2 and V re f i are modulated in response to line and load conditions, as shown in FIGURE 4(b). In order to maximize that part of charge current icharge flowing directly into transformer Tl without being stored in storage capacitor C l , upper reference voltage V r ef2 is modulated proportionally to the peak of ramp voltage V r amp so that upper reference voltage V re f2 is lower than ramp voltage V ramp by the forward voltage drop of a diode (not shown). In this way, the slope of ramp signal V r amp is modulated by storage capacitor voltage Vc 1 such that preconverter switch Q2 turns "off" sooner as storage capacitor voltage VcT increases and stays "on" longer as storage capacitor voltage Vc i decreases. Thus, as the "on" time of preconverter switch Q2 varies according to variations in line and load conditions, the rms value of a storage capacitor current ic i , shown in FIGURE 4(g), is reduced, and the power factor and storage capacitor voltage NC I become more independent of the load. Lower reference voltage V r efl is modulated proportionally to a rectified mains voltage V r ec so that the time during which preconverter switch Q2 is "on" is reduced as rectified mains voltage Vrec is increased. This is done by delaying the time at which preconverter switch Q2 turns "on" by deriving lower reference voltage V r ef l from rectified mains voltage Vrec, and it allows for a tradeoff between storage capacitor voltage Vc i and a desired power factor. If storage capacitor voltage Vc i increases above a predetermined limit because of a fault in the circuit or because the television is operating in "standby" mode, an overvoltage protection circuit 24 pulls lower reference voltage V r efl above upper reference voltage V r ef2- thus turning preconverter 20 "off . A more detailed implementation of a synchronous preconverter 20 combined with a switched-mode power supply circuit 30 for use in a television is shown in FIGURE 5. Switched- mode power supply circuit 30 includes a pulse-width modulator U2, represented in FIGURE 5 as a Siemens TDA4605-2, coupled to power switch Ql , and transformer Tl , which comprises a primary winding and a plurality of secondary windings. The primary winding of transformer Tl is coupled in series with power switch Q l and each of the plurality of secondary windings of transformer Tl is coupled in series with either one of diodes D3, D9, DIO, Dl l , D12 or with resistor R3. Pulse-width modulator U2 provides power switch drive signal Vi to a gate terminal of power switch Ql through resistor R2. When power switch drive signal Vi goes "high", power switch Ql turns "on" and input current ii begins to flow through the primary winding of transformer Tl and through power switch Ql to a reference voltage potential. As input current ii flows through the primary winding of transformer Tl , energy is stored therein. When power switch drive signal Vi goes "low", power switch Ql turns "off", so that the flow of input current i i through the primary winding of transformer Tl is interrupted. A magnetic field that built up in the primary winding of transformer Tl while power switch Ql was "on" now collapses, so that the polarity of the primary winding reverses. All the energy stored in the primary winding is now delivered to a plurality of secondary windings of transformer Tl . Each one of diodes D3, D9, DIO, Dl l, D12 now becomes forward biased, and the secondary windings deliver energy to their respective loads through diodes D3, D9, DIO, Dl l, D12 and through resistor R3. Switched-mode power supply circuit 30 is coupled to preconverter 20 through phase offset circuit 22, which includes comparator Ula, resistors R12, R13, R14, diode D5 and capacitors C8, C9. Comparator Ula might be one of a plurality of comparators found in an integrated circuit Ul, such as the National Semiconductor LM339, as shown in FIGURE 5. Using storage capacitor voltage Vc i and power switch drive voltage Vi , phase-offset circuit 22 determines the shape of ramp signal V ramp anc *' hence, the phase offset between preconverter switch Q2 and power switch Ql . As power switch drive signal Vi of switched-mode power supply circuit 30 goes "high", the output of comparator Ula goes "high", allowing storage capacitor voltage Vc i to charge capacitor C8 through resistor R 14, thus generating the positively-sloped portion of ramp signal V r amp - Diode D5 limits the voltage across capacitor C8, and thus the peak voltage of ramp signal V ra mp- to + 12 volts. When the output of comparator U l a goes "low", capacitor C8 discharges through the open collector output of comparator U l a. Phase-offset circuit 22 further includes a peak-detector circuit 26, comprising diode a D6, a capacitor C9 and a resistor R15. Peak-detector circuit 26 limits ramp signal V ra mp and produces upper reference voltage V re f2 - A forward voltage drop of diode D6 sets the level of upper reference voltage V r ef2 to 0.7 volts below a peak of ramp signal V r amp- Window comparator circuit 23 comprises comparators Ulb, U l c; resistors R17, R18, R24, R25, R26; and gate-discharge transistor Q3. A voltage divider formed by resistors R17, R18 divides rectified mains voltage V re c to provide lower reference voltage V re f l to an inverting input of comparator Ulb. Upper reference voltage V r ef2 is coupled from peak-detector circuit 26 to a non-inverting input of comparator Ulc. Ramp signal V r amp is provided by phase-offset circuit 22 to both the non-inverting input of comparator Ulb and the inverting input of comparator Ulc. The outputs of comparators Ulb, Ulc are coupled to each other. Window comparator output voltage V4 is pulled "high" by resistor R25 if ramp signal V r amp is between lower reference voltage Vref l and upper reference voltage V r ef2- Capacitor Cl l suppresses small spikes in window comparator output voltage V4. When window comparator output voltage V4 is "high", gate- discharge transistor Q3 is "off, and +12 volts is coupled to the gate terminal of preconverter switch Q2 through resistor R26, thereby turning "on" preconverter switch Q2. When ramp signal V r amp is below lower reference voltage Vref l or above upper reference voltage Vref2> window comparator output voltage V4 goes "low" and allows gate-discharge transistor Q3 to turn "on", thereby discharging the gate terminal of preconverter switch Q2, which then turns "off". Using gate-discharge transistor Q3 to open preconverter switch Q2 allows a fast turn-off time for preconverter switch Q2. Overvoltage protection circuit 24 includes comparator U ld and resistors R19, R20, R21, R22, R23. A voltage divider formed by resistors R20, R21 provides an overvoltage threshold voltage at the inverting input of comparator Uld. Storage capacitor voltage V c i is divided by a voltage divider formed by resistors R22, R23, and the resulting voltage is provided to the non-inverting input of comparator Uld. If storage capacitor voltage Vc i exceeds an overvoltage limit as determined by the overvoltage threshold voltage present at the inverting input of comparator Uld, the output of comparator Uld goes "high" and resistor R19 pulls up lower reference voltage Vrefl through diode D8. Window comparator output voltage V4 then goes "low", thereby allowing gate-discharge transistor Q3 to turn "on" and discharge the gate terminal of preconverter switch Q2, thereby opening preconverter switch Q2 and turning "off preconverter 20.

Claims: WHAT IS CLAIMED IS: 1. A power supply ( 100), comprising: rectifying means ( 10); and, a switched-mode power supply circuit (30); characterized by: power preconverting means (20) coupling said rectifying means ( 10) to said switched-mode power supply circuit (30); and, means (21 ,22) for synchronizing operation of said power preconverting means (20) with operation of said switched-mode power supply circuit (30). 2. The power supply ( 100) of claim 1 , characterized in that said switched-mode power supply circuit (30) and said power preconverting means (20) have first (Ql) and second (Q2) switch means, respectively, said synchronizing means (21 ,22) controlling a phase relationship in operation of said second switch means (Q2) in said power preconverting means (20) relative to operation of said first switch means (Q l ) in said switched-mode power supply circuit (30). 3. The power supply (100) of claim 1 , characterized in that said power preconverting means (20) supplies a current directly to said switched-mode power supply circuit (30) during a portion of each time interval that said switched-mode power supply circuit (30) draws an input current (i i ) . 4. A power supply ( 100) according to claim 1 , wherein said synchronizing means (21 ,22) is characterized by: means for generating a ramp signal (Vramp); and, means for generating an upper reference voltage (V r ef2) having a value below a peak amplitude of said ramp signal (Vramp) - 5. A power supply ( 100) according to claim 4, wherein said synchronizing means (21 ,22) is further characterized by: means for generating a lower reference voltage (V r efl ); and, a comparator circuit having inputs coupled to said lower ( V re f l ) and upper (V re f2) reference voltages. 6. A power supply ( 100) according to claim 5, characterized in that said means for generating said lower reference voltage (Vref l ) is coupled to said rectifying means ( 10). 7. A power supply ( 100) according to claim 6, characterized in that said comparator circuit enables conduction of a preconverter switch (Q2) while said ramp signal (V ra mp) is between said lower (V r efl ) and upper (V r ef2) reference voltages. 8. A power supply ( 100), comprising: rectifying means ( 10); and, a switched-mode power supply circuit (30); characterized by: power preconverting means (20) coupling said rectifying means ( 10) to said switched-mode power supply circuit (30); energy storage means (Cl ) coupled to said power preconverting means (20) and to said energy storage means (Cl ); and, means for synchronizing (21,22) operation of said power preconverting means (20) with operation of said switched-mode power supply circuit (30). 9. The power supply ( 100) of claim 8, characterized in that a charge current (icharge) supplied to said energy storage means (Cl) from said power preconverting means (20) is responsive to a load coupled to an output of said switched-mode power supply circuit (30). 10. The power supply ( 100) of claim 8, characterized in that said switched-mode power supply circuit (30) and said power preconverting means (20) have first (Ql ) and second (Q2) switch means, respectively, said synchronizing means (21 ,22) controlling a phase relationship in operation of said second switch means (Q2) in said power preconverting means (20) relative to operation of said first switch means (Ql ) in said switched-mode power supply circuit (30). 1 1. The power supply ( 100) of claim 10, characterized in that said second switch means (Q2) in said power preconverting means (20) conducts during each time interval that said first switch means (Ql ) in said switched-mode power supply circuit (30) conducts. 12. The power supply ( 100) of claim 1 1 , characterized in that said second switch means (Q2) in said power preconverting means (20) conducts for a period of time shorter than said time interval . 13. The power supply ( 100) of claim 12, characterized in that said second switch means (Q2) in said power preconverting means (20) begins conducting only after said time interval begins and always stops conducting before said time interval ends. 14. A power supply ( 100) according to claim 8, wherein said synchronizing means (21 ,22) is characterized by: means for generating a ramp signal (V r amp); and, means for generating an upper reference voltage (Vref2) having a value below a peak amplitude of said ramp signal (Vramp) - 15. A power supply (100) according to claim 14, wherein said synchronizing means (21 ,22) is further characterized by: means for generating a lower reference voltage (Vrefl ); and, a comparator circuit having inputs coupled to said lower (Vref l ) and upper (V r ef2) reference voltages. 16. A power supply ( 100) according to claim 15, characterized in that said means for generating said lower reference voltage (V r ef l ) is coupled to said rectifying means ( 10). 17. A power supply ( 100) according to claim 16, characterized in that said comparator circuit initiates conduction of a preconverter switch (Q2) while said ramp signal (V r amp) is between said lower (Vrefl ) and upper (V r ef2) reference voltages. 18. A power supply (100), comprising: rectifying means ( 10); and, a switched-mode power supply circuit (30); characterized by: power preconverting means (20) coupling said rectifying means ( 10) to said switched-mode power supply circuit (30); energy storage means (Cl) coupled to said power preconverting means (20) and to said energy storage means (Cl ); and, means for supplying a current from said power preconverting means (20) directly to said switched-mode power supply circuit (30) during a portion of each time interval that said switched-mode power supply circuit (30) draws an input current (i l ) . 19. The power supply (100) of claim 18, characterized in that said switched-mode power supply circuit (30) and said power preconverting means (20) have first (Ql ) and second (Q2) switch means, respectively. 20. The power supply ( 100) of claim 19, characterized in that said second switch means (Q2) in said power preconverting means (20) begins conducting only after said time interval begins and always stops conducting before said time interval ends. 21. The power supply (100) of claim 20, characterized in that said portion of said time interval comprises a remainder of said time interval after said second switch means (Q2) in said power preconverting means (20) stops conducting.

Inventors: MUCHENBERGER MANFRED, MUCHENBERGER, MANFRED
Applicants: RCA THOMSON LICENSING CORP, RCA THOMSON LICENSING CORPORATION
Application year: 1996
Main classification: H02M7
IPC codes: H02M1, H02M3