SWITCH MODE POWER SUPPLY WITH REDUCED INPUT CURRENT DISTORTION

Abstract: A full wave rectified AC main suppply voltage (V1) is produced from an AC mains supply voltage (VM) without low-pass filtering. The rectified voltage is developed in a winding (W1) of a flyback transformer (T). The winding of the flyback transformer is also coupled to a switching transistor (Q1) that generates in the winding a first plurality of current pulses (ip) at a frequency (20 KHz-50 KHz) that is higher than the frequency of the mains supply voltage (50 Hz). The current pulses energize a load circuit (LOAD). A capacitor voltage (V4) is coupled via a second switching transistor (Q2) to the winding to produce in the winding a second plurality of current pulses (ip2) that energize the load circuit during a portion of the period of the mains supply voltage that does not occur in the vicinity of the peak of the mains supply voltage. When the second plurality of current pulses are generated, the mains supply voltage is decoupled from the first winding.

Description: SWITCH MODE POWER SUPPLY WITH REDUCED INPUT CURRENT DISTORTION The invention relates to a switch mode power supply (SMPS). In particular, the invention relates to a SMPS that derives a rectified input supply voltage from an AC, mains supply voltage to produce an output supply voltage. Typically, such SMPS employs a full wave rectifier bridge that rectifies a sinusoidal mains supply voltage and charges an input filter capacitor to produce the rectified input supply voltage. The filter capacitor has a sufficiently large value for reducing, in the input supply voltage, the magnitude of a ripple at the fundamental frequency and at the harmonics of the mains supply voltage. In this way, ripple voltage is prevented from occurring in the output supply voltage. The filter capacitor draws narrow pulses of input current having a high peak value from the mains supply voltage source. The current pulses occur only at the peaks of the sinusoidal mains supply voltage. Therefore, the waveform of the input current contains undesirable low frequency harmonics of the mains supply voltage frequency. The result is an undesirable decrease in the power factor to about 0.65 and an undesirable increase in distortions of the waveform of the mains supply voltage. Recently, an interest has evolved for revising European Standard EN60 555-2 for the purpose of reducing the permissible low frequency harmonic content of the waveform of the input current. It may be desirable to reduce the low frequency harmonic content of the waveform of the input current without significantly increasing a ripple voltage in the output supply voltage. In an SMPS, embodying an aspect of the invention, an AC mains supply voltage produced in a mains supply voltage source is rectified to form a rectified mains supply voltage. The rectification operation occurs without low-pass filtering with respect to low frequency voltage components. A winding of a flyback transformer is coupled to the rectified, mains supply voltage, and to a switching transistor. Pulses of current at high frequency that are produced in the transformer by switching operation of the transistor produce an output supply voltage that is applied to a load. The peak amplitudes of the current pulses have an envelope that is similar to a waveform of the rectified voltage. Consequently, a ratio between the mains supply voltage and a mains supply current remains constant, during at least a substantial portion of the mains supply voltage. Pulses of current that are produced in the transformer are stored in a capacitor. Pulses of current are produced from the stored charge in the capacitor and re-circulated through the transformer in subsequent cycles, during a portion of the period of the mains supply voltage when the mains supply voltage is low. The re¬ circulated current is applied to the load so as to reduce a ripple voltage in the output supply voltage. A switch mode power supply, embodying a further aspect of the invention, includes a source of an AC, mains supply voltage, an inductance and a first switching arrangement coupled to the mains source and responsive to a switching signal. A first plurality of current pulses are generated in the inductance at a frequency that is substantially higher than that of the mains supply voltage. During a first portion of a given period remote from a peak of the mains supply voltage, when the mains supply voltage decreases, the decrease causes a concurrent decrease in amplitudes of the first plurality of current pulses and in a level of a mains supply current supplied from the mains source. Second switching arrangement is coupled to the inductance for generating a second plurality of current pulses in the inductance at a frequency that is substantially higher than that of the mains supply voltage. The second plurality of current pulses are generated at least during the first portion of the given period of the mains supply voltage. An output supply voltage and an output supply current are generated in a load circuit from the first and second pluralities of current pulses. FIGURE 1 illustrates an SMPS, embodying an aspect of the invention, that utilizes a current interleaving technique; FIGURES 2a-2k illustrate waveforms useful for explaining the operation of the SMPS of FIGURE 1; FIGURES 3a and 3b illustrate additional waveforms useful for explaining the operation of the SMPS of FIGURE 1; FIGURE 4 illustrates an SMPS, embodying another aspect of the invention,that utilizes a current superposition technique; FIGURES 5a-5f illustrate waveforms useful for explaining the operation of the SMPS of FIGURE 4; FIGURES 6a and 6b illustrate additional waveforms for the SMPS of FIGURE 4; FIGURE 7 illustrates an SMPS, embodying a further aspect of the invention; and FIGURES 8a and 8b are waveforms useful for explaining the operation of the SMPS of FIGURE 7. FIGURE 1 illustrates an SMPS 100, embodying an aspect of the invention, that utilizes a technique, referred to herein as current interleaving technique. A mains supply voltage source 107 that produced an AC voltage VM is coupled to a full- wave bridge rectifier 102 that produces an AC full-wave rectified input supply voltage VI in a relatively small filter capacitor Cl. AC voltage VM of source 107 is sinusoidal at, for example, a frequency of 50 Hz. Because of the small capacitance of capacitor Cl, voltage VM is rectified without significant low-pass filtering with respect to the frequency of voltage VM. Therefore, low frequency harmonics of voltage VI are not filtered out. Voltage VI, that is a fullwave rectified voltage, is coupled via a switching diode Dl to a primary winding Wl of a flyback transformer T. A switching signal VO, having a controllable duty cycle, is coupled via an "OR" gate 113 to produce a switching signal V16. Signal V16 is coupled via a conventional driver stage 103 to a base of a switching transistor Ql to turn on transistor Ql only when signal VO is at a HIGH level. The frequency of signal VO is selected to be within a range of 20- 50 KHz, as determined by an oscillator, not shown, of a control circuit 101. In a given cycle of signal V0, after signal V0 attains the HIGH level, magnetic energy is stored in transformer T and an upramping current ipl or ip is produced in winding Wl. In such given cycle, immediately after signal V0 becomes LOW, transistor Ql becomes non- conductive and a flyback operation occurs to produce a flyback pulse current isl in a given secondary winding of transformer T. The collector voltage of transistor Ql is limited by means of a snubber circuit that includes a capacitor C8, a resistor R6 and a diode D8. As a result of flyback current pulses isl that are produced from pulses ipl, an output supply voltage V2 and an output supply voltage V6 are produced in capacitors C2 and C6. Voltages V2 and V6 are produced by rectifying, in diodes D2 and D6, flyback pulse voltages of secondary windings W2 and W6, respectively, of transformer T when transistor Ql becomes nonconductive. Voltages V2 and V6 may be used for energizing loads such as a horizontal output stage and an audio stage, respectively, of a television receiver, not shown. Transformer T may be used for isolating, with respect to electrical shock hazard, source 107 and a "hot" ground conductor, H, from a "cold" ground conductor G. Secondary windings W3, W4 and W5 produce flyback pulse voltages that are rectified by diodes D3, D4 and D5 to produce DC, rectified voltages V3, V4 and V5 with low ripple in filter capacitors C3, C4 and C5, respectively. Voltages V3, V4 and V5 are referenced to hot ground conductor H. Whereas, voltages V2 and V6 are referenced to cold ground conductor G. DC sense voltage V3 that is indicative of voltages V2 and V6 is applied to a differential amplifier, not shown, of a regulator control stage 105 of circuit 101. Voltage V3 is compared with a reference voltage, not shown. An error voltage, not shown, is produced from a difference between voltage V3 and the reference voltage. The error voltage is used to control, in a conventional manner, a pulse-width or duty cycle of signal VO to regulate the levels of output voltages V2 and V6. When signal VO is HIGH, transistor Ql is conductive, and a pulse of a current ipl flows via diode Dl to provide current ip in winding Wl. A ratio between the peak amplitudes of the pulses of current ipl and between the instantaneous value of voltage VM or of full-wave rectified voltage VI is maintained constant throughout the period of voltage VM or VI. Since voltage VI has a full- wave rectified sinusoidal waveform, the peaks or envelope of current ipl also varies in full- wave rectified sinusoidal manner. Consequently, an AC current iM that is supplied from source 107 varies, advantageously, in a sinusoidal manner at the frequency and phase of voltage VM, and with relatively small amount of low order harmonics distortion. The amplitudes of the pulses of current ipl significantly decrease, during a given period of voltage VI, when voltage VI decreases. Therefore, had only current ipl been produced in winding Wl, voltage V2 would tend to have a significant ripple voltage. It may be desirable to reduce the ripple voltage. FIGURES 2a-2k illustrate waveforms useful for explaining the operation of SMPS 100 of FIGURE 1. Similar symbols and numerals in FIGURES 1 and 2a-2k indicate similar items or functions. The falling edge of signal VO of FIGURE 2b triggers a monostable multivibrator (MMV) 104 of FIGURE 1 that produces a signal VI 4 at a LOW level with a predetermined pulse width. Signal V14 is coupled to a base of a switching transistor Q4 via a pulse shaping parallel arrangement of a resistor R5 and a capacitor C7. As a result of the falling edge of signal VO, signal V14 of MMV 104 attains the LOW level and transistor Q4 becomes nonconductive. The collector of transistor Q4 is coupled to a base of a series pass switching transistor Q2 to control the switching operation of transistor Q2. Transistor Q2 is switched on by a drive current that is supplied via an arrangement formed by a transistor Q3 and resistors Rl, R2, R3 and R4. Transistors Q3 and Q2 are turned on when transistor Q4 becomes nonconductive, immediately following the falling edge of signal VO. Forward- biased transistor Q2 couples DC voltage V4 to an anode of a switching diode D7 having a cathode that is coupled to a junction terminal Ta, between a cathode of diode Dl and winding Wl. As long as transistor Ql is nonconductive, following the falling edge of signal VO, no current is supplied to winding Wl via diode D7 and transistor Q2. The falling edge of signal VO of FIGURE 2b also triggers a sawtooth generator, not shown, in stage 105 that produces a sawtooth signal V13 of FIGURE 2e. Sawtooth signal V13 is compared in a comparator COMP2 with a controllable slicing voltage V10. Voltage V10 has a level that is shown as a broken line in FIGURE 2e. For a given cycle of voltage VM or VI of FIGURE 1, voltage V10 is constant. Immediately after the falling edge of signal V0, when signal VI 3 is smaller than voltage V10, an output signal VI 1 of comparator COMP2 is at a LOW level. Signal VI 1 is applied to a reset input of a flip-flop (F-F) 106. F-F 106 is not affected by signal VI 1, as long as signal VI 1 is at the LOW level. Prior to the falling edge of signal VO, signal VI 1 is at the HIGH level. Consequently, signal VI 1 maintains an output signal V7 of F-F 106 at the LOW level. An unrectified, flyback voltage V15, across winding W3, is indicative of an instant when the magnetic energy in transformer T, or a secondary current is, becomes zero. Current is or isl becomes zero following the falling edge of signal VO. Circuit 105 generates a narrow pulse signal VI 2 of FIGURE 2d when current is of FIGURE 2c becomes zero. The leading edge of pulse signal VI 2 of FIGURE 2d triggers F-F 106 of FIGURE 1 to a "set" state. As a result, output signal V7 of FIGURE 2g of F-F 106 of FIGURE 1 attains a HIGH level immediately after the magnetic energy has decayed in transformer T. Transistor Ql is also controlled by signal V7 that is coupled via "OR" gate 113. Signals VO and V7 cannot occur simultaneously at the active HIGH levels. When, in a given cycle, signal V7 attains the HIGH level, transistor Ql begins conducting. In accordance with an inventive feature, transistor Q2, that is already forward biased, as explained before, begins conducting a pulse of a current ip2 in winding Wl, and diode Dl becomes back-biased. In a given cycle of signal V0, for example, primary current ip2 increases in an upramping manner until F-F 106 is reset by signal VI 1. As a result, a pulse of current is2 is produced in a given secondary winding of transformer T in a flyback mode of operation. The pulses of currents ip2 and is2 are shown in broken lines in FIGURES 2a and 2c. When current is2 ceases, the pulse of signal VI 2 of FIGURE 2d is produced but has no effect on F-F 106 since F-F 106 is already reset. Signal V14 of FIGURE 2i, at the LOW level, changes to the HIGH level, prior to the occurrence of a rising edge of signal V0 of FIGURE 2b, in accordance with pulse-width determining parameters of MMV 104. When signal VI 4 becomes HIGH, transistor Q4 becomes conductive that causes transistor Q2 to become nonconductive. A new cycle begins at the rising edge of the following pulse of signal VO of FIGURE 2b. Signal VI 4 remains at the HIGH level until signal VO attains the LOW level, so as to maintain transistor Q2 nonconductive. The time difference between the falling edge of signal VO and the leading edge of signal V12 or V7 varies as a function of the instantaneous value of voltage VI or VM or the peak of current ipl or isl within a given period of voltage VI or VM. The smaller is voltage VI, the earlier is the occurrence of the leading edge of signal V7. Voltage V10 is constant for a given period of voltage VI. Therefore, signal VI 1 attains the HIGH level and signal V7 attains the LOW level at the same time, with respect to signal VO, in each cycle throughout the period voltage VI. Thus, the pulse-width of signal V7 is modulated or varies during the period of voltage VI. The pulse-width of signal V7 decreases when voltage VI or current ipl increases, and vice versa. When, for example, voltage VI is zero, the pulse- width of signal V7 is at a maximum. During the given period of voltage VI of FIGURE 1, as voltage VI, for example, decreases, the peaks of the pulses of current ipl or current isl of FIGURE 2a and FIGURE 2c, respectively, also decrease, as shown in solid lines. In accordance with an aspect of the invention, during a portion of the period of voltage VI of FIGURE 1, when the peaks of the pulses of current ipl of FIGURE 2a or current isl of FIGURE 2c, for example, decrease, the peak and pulse-width of each of the pulses of current ip2 or is2 increase, as shown in broken lines in FIGURES 2a and 2c. When the pulse-width and amplitude of the pulses of, for example, current is2 of FIGURE 2c increase, additional energy is supplied to the load. The additional energy is supplied to the load via current is2 of FIGURE 1 when the energy that is supplied to the load by the pulses of current ipl of isl is diminished. Therefore, the reduction in the amplitude of the pulses of current ipl or isl that tends to produce the ripple voltage in, for example, voltage V2 is, advantageously, reduced by the current pulses of current ip2 or is2. It may be desirable to, for example, increase the pulse-width of the pulses of signal V7 in each cycle of signal V7 when the loading of capacitor C2 increases so as to control current ip2. To that end, current ip2 is controlled by controlling the pulse-width of signal V7 in a close loop manner. The pulse-width of signal V7 is determined by set and reset signals VI 2 and VI 1, as explained before. Signal VI 2 ensures that the pulse of current ip2 starts to flow after the pulse of secondary current isl has decayed to zero. Signal VI 1 determines the amplitude of current ip2. In the closed-loop operation, signal V7 is demodulated by applying signal V7 via a low pass filter LPF1. A resulting low-pass filtered signal V8 has a waveform that is illustrated in FIGURE 2j. During a first portion of the cycle of voltage VM, signal V8 varies as a function of cosωt, where "t" denotes time and "ω " denotes the radian velocity; whereas, voltage VM of FIGURE 1 varies as a function of sin ωt. Thus, pulse-width modulated signal V7 is transformed to an equivalent signal, signal V8, having an amplitude and a DC, average level that are determined in accordance with the pulse- width of signal V7. Signal V8 provides a relative measure for the power required to compensate for a reduction in the input power derived from mains supply voltage source 107, during a given period of voltage VI. For example, at mains voltage VM zero crossing, the input power from source 107 is zero. When the input power from source 107 is zero, a maximum pulse-width of signal V7 and a peak of signal V8 occur, indicating that SMPS 100 requires maximum power compensation via the pulses of current ip2. On the other hand, in a second portion of the cycle of voltage VI or VM, in the vicinity of an instant when the input power is at a maximum, signal V8 is a minimum, a shown in broken lines, and no power compensation is required. Signal V8 is compared with a reference voltage VR in a comparator COMP1. The level of voltage VR is shown in FIGURE 2] as a broken line. An output signal V9 of comparator COMP1 is produced when signal V8 is smaller than voltage VR. Signal V9 is applied to an input of a second low pass filter LPF2. Low pass filter LPF2 produces slicing voltage V10 that is used as a slicing level for comparator COMP2. Assume, for example, that an increase in loading of capacitor C2 occurs. Therefore the pulse of signal V12 and the leading edge of signal V7 occur later in a given cycle of signal VO. Therefore, the pulse-width of each of the pulses of signal V7 tends to decrease, throughout the period of voltage VM. The result is that the average value of signal V8 becomes less positive. The decrease in the average value of signal V8 of FIGURE 2j tends to increase the pulse-width of signal V9 of FIGURE 2k. As a result, the level of voltage V10 becomes more positive. Consequently, the leading edges of the pulses of signal VI 1 of FIGURE 2f are more delayed or occur later within a given period of, for example, signal VO. Therefore, advantageously, the pulse-width of signal V7 in each cycle, when signal V7 occurs, increases. The increase in the pulse-width of signal V7 causes the average value of signal V8 to increase in a manner to attain steady state operation. In this way, steady-state operation is maintained when a change in loading occurs in a manner to vary the amplitude of current is2 as a function of loading. Signal VI 1 of FIGURE 2f causes FF 106 to be at the "reset" state throughout an interval, Tl, of FIGURE 2g of voltage VM of FIGURE 1. The value of voltage VR of FIGURE 1 determines the length of interval Tl of FIGURE 2g. Throughout interval Tl, signal V7 is maintained continuously at the LOW level. Thus, signal VI 1 prevents the generation of the pulses of current ip2 or is2, throughout interval Tl of FIGURE 2g. Interval Tl occurs in the vicinity of the peak of voltage VI or VM, when the pulses of current ipl or isl are sufficiently large to energize the load. Maintaining interval Tl for an entire range of loading and input voltage VM conditions provides protection feature against an undesirable mode of operation of SMPS 100 of FIGURE 1. For example, had interval Tl not been maintained, capacitor C4 might have been overloaded. FIGURE 3b illustrates schematically the peaks or envelope of the pulses of currents ipl and ip2 in a given period of voltage VI of FIGURE 2a. Similar symbols and numerals in FIGURES 1, 2a-2k and 3a-3b indicate similar items or functions. During interval Tl of FIGURE 3b that occurs in the vicinity of the peak of voltage VI of FIGURE 3a, the pulses of current ip2 are disabled. Whereas, during the rest of the period, an interval T2, they are enabled. During interval T2, voltage V4 is larger than voltage VI, for proper operation. FIGURE 4 illustrates an SMPS 100', embodying another aspect of the invention, that utilizes a technique referred to herein as a current superposition technique. FIGURES 5a-5f illustrate waveforms useful for explaining the operation of SMPS 100 * of FIGURE 4. Similar symbols and numerals in FIGURES 1, 4 and 5a-5f indicate similar items or functions, except that in FIGURES 4 and 5a-5f the symbol (') is added. In FIGURE 4, transistors Ql' and Q2' are, illustratively, MOS transistors. Transistor Q2' is turned on by a voltage V5' that is more positive by about 15V than voltage V4'. Transistor Q2' is turned off by a transistor Q4'. A signal VI 4' at a HIGH level that is produced in an inverting output Q of a flip-flop (FF) 200 causes transistor Q4' to turn on and transistor Q2' to turn off. A protection zener diode D9 limits a gate-source voltage excursions of transistor Q2'. Transistor Ql' is controlled by signals VO' and V7'. Only one of signals VO' and V7' can be at a HIGH level at any one time. Signal VO' controls a primary current component ipl' and signal V7' controls a primary current ip2'. A primary current ip' in a winding Wl' is sampled by means of a very low value resistor R31 coupled between the source electrode of transistor Ql' and ground H'. A voltage V31, developed across resistor R31, is proportional to current ipl. Voltage V31 is applied via a low pass filter 202 to remove any existing spikes. A resulting low-pass filtered signal V91 is shown in FIGURE 5a. Signal V91 is compared with a slicing voltage V10' that varies in a closed-loop manner in a comparator 201. Voltage V10' is indicated in FIGURE 5a as a broken line. The slicing level of voltage V10' relative to signal V91 determines the lengths of intervals Tl' and T2'. A resulting signal Vl l' at an output of comparator 201 is illustrated in FIGURE 5c. A flip-flop (F-F) 200 is set by the falling edge of signal V0' that is coupled via an inverter 200a and reset by the HIGH level of signal Vll'. Output signal V7' developed in a non-inverting output Q of F-F 200 is applied to an input ___[ an OR-gate 113'. Signal V14' developed at inverting output Q of F-F 200 is coupled to a resistor R5' and to a capacitor C7'. The parallel combination of resistor R5' and capacitor C7 improves the switching characteristics of transistor Q4'. Control signal V0' is applied to the other input OR-gate 113'. During a portion of a given cycle of, for example, signal V0', F-F 200 is reset, non-inverting output signal V7' is at the LOW level and inverting output signal VI 4' is HIGH. Therefore, transistors Ql' and Q2' are turned off and transistor Q4' is turned on. Signal VO 1 changes, during its cycle, from the LOW to the HIGH level. The change in signal V0' causes transistor Ql' to conduct and each of primary current ipl' and signal V91 to increase in an upramping manner. Should signal V91 reach the level of voltage VI 0' when signal VO' is at the HIGH level, comparator 201 output signal Vl l' would attain the HIGH level. After a predetermined conduction time of transistor Ql', signal VO' changes again to the LOW level that causes signal V91 to be zero. Since signal V91 is also zero, comparator 201 output signal Vl l' is at the LOW level. Throughout an interval Tl', in the vicinity of the peak of voltage VM', signal Vl l' attains the HIGH level prior to the instant when signal VO' changes to the LOW level. As a result, F-F 200 cannot be set. Thus, throughout interval Tl', control signal V7 1 remains at the LOW level. Throughout interval Tl', after signal V0' changes to the LOW level, primary current ipl' becomes zero. The result is that current ip2' is disabled, throughout interval Tl'. In contrast to the situation during interval Tl', during an interval T2' of voltage VM', signal V91 does not reach the level of voltage V10' during the period of signal V0' in which both signal V0' is at the HIGH level and transistor Ql' is conductive. This is so because, during interval T2', the rate of change of current ipl' is smaller than during interval Tl' due to the decrease in input voltage VM' or VI'. During interval T2\ when signal V0' attains the LOW level, the reset signal Vl l * is still at the LOW level. Therefore, F-F 200 is set by the falling edge of signal V0'. Thus, when signal V0' becomes LOW, signal V7' changes to the HIGH level. Transistor Ql' conduction time is now determined by signal V7' which keeps transistor Ql' conducting after signal V0' becomes LOW. The current in transistor Ql' is not disturbed by the fast transition from being controlled by signal V0' to being controlled by signal V7'. Simultaneously, transistor Q4' is turned off by signal VI 4' which attains the LOW level. Transistor Q2 immediately conducts, a diode Dl' becomes nonconductive and a diode D7' becomes conductive. Input supply voltage VI' is decoupled from a winding Wl' and a voltage V4' in a capacitor C4' is coupled to winding Wl' via transistor Q2'. In cycles that occur during interval T2', primary current ip2' continues to flow without a gap of time relative to current ipl', as shown in broken lines in FIGURE 5a. Voltage V91 also increases until voltage V91 becomes equal to voltage VI 0'. Then, comparator 201 output signal VI 1' becomes HIGH that causes F-F 200 to reset. Transistor Ql' is turned off, transistor Q4' is turned on and transistor Q2' is turned off. Thereafter, a new cycle begins. The result is that the current pulses of current ip2' are generated from the charge stored in capacitor C4' and are re-circulated via winding Wl', during interval T2 * . FIGURE 6b illustrates schematically the peaks or envelope of the pulses of currents ipl' and ip2' in a given period of voltage VI' of FIGURE 5a. Similar symbols and numerals in FIGURES 4, 5a-5f and 6a-6b indicate similar items or functions. During interval Tl' of FIGURE 6b, that occurs in the vicinity of the peak of voltage VI' of FIGURE 5a, the pulses of current ip2' are disabled. Whereas, during the rest of the period, interval T2', they are enabled. It may be desirable to vary current ip2' or is2' as a function of current loading of, for example, capacitor C2'. This is achieved by controlling signal Vll' in a closed-loop manner. To that end, signal Vl l' is coupled via a low pass filter 203 to an inverting input terminal of a comparator 205. A reference voltage 204 is developed at a noninverting input terminal of comparator 205. An output signal 205a of comparator 205 is coupled via a resistor R9 to a base of a current sinking switching transistor Q5. A collector of transistor Q5 is coupled to a junction terminal between a resistor R6 and a resistor R7. Resistor R6 and R7 are coupled in series between a voltage V3' and a capacitor C9 to generate a current that charges capacitor C9. The average value of a collector current of transistor Q5 determines the level of voltage V10' that is developed in capacitor C9. Thus, capacitor C9 is discharged via transistor Q5 and is charged via resistor R6. In steady state operation, voltage V10' is constant. Assume that an increase in current loading in capacitor C2' occurs. Therefore, the peak amplitude of current ipl' and of voltage V31 in resistor R31 increase. Therefore, the pulse-width of signal Vl l' increases. When the pulse- width of signal Vll' increases, an output voltage 203a of low- pass filter 203 becomes larger. When voltage 203a exceeds voltage 204 output signal 205a of comparator 205 attains the LOW level. The result is that transistor Q5 is turned off for a longer interval during a given period of, for example, signal V0'. Thus, the average current in transistor Q5 decreases. Therefore, the level of voltage V10' increases. In this way, the peak level of each pulse of current ip2' increases in a manner to transfer more energy to the load. Upon turning on the SMPS 100' and prior to steady state operation, voltage V10' is undefined and could be 2 or 3 volts higher than in steady state which might allow excessive current to flow. Therefore capacitor C9 is coupled to a voltage divider that includes a resistor Rl and a resistor R2 through a diode D10. Upon switching on the power supply voltage V4' is zero and diode D10 is coupled to ground potential. As a result, voltage V10' can not be greater than 0.6V. As the power supply starts to operate, voltage V4' increases. The voltage level across resistor R2 also increases, thereby allowing voltage V10' to increase. The voltage divider that includes resistors Rl and R2 causes diode D10 to be in cut off when voltage V4' reaches its nominal value. FIGURE 7 illustrates an SMPS 100", embodying another aspect of the invention, that utilizes a compromise technique. FIGURES 8a-8b illustrate waveforms useful for explaining the operation of SMPS 100" of FIGURE 7. Similar symbols and numerals in FIGURES 1, 4, 7 and 8a-8b indicate similar items or functions, except that in FIGURES 7 and 8a-8b the symbol (") is added. In FIGURE 7, switch SI performs the function of transistors Ql of FIGURE 1, for example, and switch S2 of FIGURE 7 performs the function of transistor Q2 of FIGURE 1. Capacitor C4" of FIGURE 7 is charged directly from mains supply voltage VM", and not via the transformer, as in FIGURE 1 or 4. Therefore, a mains current iM" of FIGURE 7 is equal to the sum of the two currents, ipl" and i4. Current ipl" is sinusoidal at the fundamental frequency. Capacitor " C4" is charged only during the peaks of mains voltage VM". Thus, current i4 includes narrow, non-sinusoidal current pulses. The peak value of current i4 is low, because the energy stored in capacitor C4" is used to supply only a portion of a primary current ip" of winding Wl". About 65% of the power consumption is obtained by means of current ipl" and 35% by means of current ip2". The resultant current waveform of current iM" is shown in FIGURE 8b. Advantageously, the third harmonic of this current waveform is only about 30% of what it would have been had capacitor Cl" been a very large capacitor. The fifth harmonic is also reduced. A control circuit 101" in FIGURE 7 could be of the type, "current interleaving" of FIGURE 1 or of the type, "current superposition" of FIGURE 4. A comparison between the performances of arrangements of FIGURES 1 and 7, when the mains supply voltage is 220 VAC, the input power is 150W and the output voltage, for example, voltage V2 or V2", is 150 VDC, provides the following results. In the arrangement of FIGURE 1, the harmonic content of current iM in A rms is 0.72 at 50 Hz, 0.05 at 150 Hz, 0.038 at 250 Hz and 0.024 at 350 Hz. The power factor is 0.96, the efficiency is 0.72 and the ripple voltage is IV. In the arrangement of FIGURE 7, the harmonic content of current iM in A rms is 0.72 at 50 Hz, 0.18 at 150 Hz, 0.126 at 250 Hz and 0.09 at 350 Hz. The power factor is 0.92, the efficiency is 0.78 and the ripple voltage is 0.8V. The current interleaving technique of FIGURE 1 is particularly suitable for an SMPS with a power rating below, for example, 100W. Whereas, the current super-position technique is suitable for an SMPS with a higher power rating.

Claims: CLAIMS: 1. A switch mode power supply, comprising: a source (107) of an AC, mains supply voltage(VM); an inductance (Wl); first switching means (Ql) coupled to said mains source and responsive to a switching signal (VI 6) for generating a first plurality of current pulses (ipl, isl) in said inductance at a frequency (20-50 KHz) that is substantially higher than that (50 Hz) of said mains supply voltage such that, during a first portion (T2,Fig. 2g) of a given period remote from a peak of said mains supply voltage, when said mains supply voltage decreases, the decrease causes a concurrent decrease in amplitudes of said first plurality of current pulses and in a level of a mains supply current (iM) supplied from said mains source; second switching means (Q2) synchronized to said first switching means and coupled to said inductance for generating a second plurality of current pulses (ip2,is2) in said inductance at a frequency that is substantially higher than that of said mains supply voltage, said second plurality of current pulses being generated at least during said first portion of said given period of said mains supply voltage; a load circuit (LOAD); and means (W2) responsive to said first and second pluralities of current pulses for developing an output supply voltage (V2) and an output supply current (iL) in said load circuit. 2. A power supply according to Claim 1 further comprising, a rectifier (D2) for rectifying said first (iLl) and second (iL2) pluralities of current pulses to develop said output supply voltage (V2) and said output supply current (iL), such that said second plurality of current pulses compensate for the decrease in said amplitudes of said first plurality of current pulses in a manner to reduce a ripple component at a frequency (100 Hz) that is related to that (50 Hz) of said mains supply voltage. 3. A power supply according to Claim 1 wherein said second plurality of current pulses (ip2) are generated in said inductance (Wl) and wherein said output supply voltage (V2) is produced in a flyback manner from said first (ipl) and second (ip2) pluralities of current pulses. 4. A power supply according to Claim 1 wherein said current pulses of said first (ipl) and second (ip2) pluralities of current pulses occur in mutually exclusive intervals within a given period of said switching signal (V16). 5. A power supply according to Claim 1 wherein said second plurality of current pulses (ip2) are generated in said inductance (Wl) and wherein a given current pulse of one of said first plurality of current pulses (ipl) occurs adjacent to a given current pulse of said second plurality of current pulses such that there is no time gap between the adjacent pulses. 6. A power supply according to Claim 1 further comprising, means (COMP 1, COMP 2) for disabling the generation of said second plurality of current pulses (ip2) throughout a second portion (Tl, Fig. 2g) of said period of said mains supply voltage (VM), in a vicinity of said peak voltage of said mains supply voltage. 7. A power supply according to Claim 1 further comprising, a pulse-width modulator (101) for pulse- width modulating the current pulses of at least one (ipl, ip2) of said first (ipl) and said second (ip2) plurality of current pulses in accordance with said one of said output supply voltage (V2) and current (iL) in a manner to provide regulation. 8. A power supply according to Claim 7 wherein said pulse-width modulator (101) modulates each of said first (ipl) and second (ip2) pluralities of current pulses in a manner to provide the regulation. 9. A power supply according to Claim 1 wherein said second plurality of current pulses (ip2) are generated in said inductance and wherein said second plurality of current pulses comprise re-circulated current pulses with respect to said inductance (Wl). 10. A power supply according to Claim 1 wherein said second plurality of current pulses (ip2) are generated in said inductance (Wl) and wherein said inductance is included in a first winding (Wl) of a transformer (T). 11. A power supply according to Claim 10 further comprising, a second rectifier (D4) and a capacitor (C4) for rectifying current pulses in a second winding (W4) of said transformer (T) that are produced from said first plurality of current pulses (ipl) to develop a second supply voltage (V4) in said capacitor, said second supply voltage being coupled to said second switching means (Q2) for generating said second plurality of current pulses (ip2) from said second supply voltage. 12. A power supply according to Claim 10 wherein said second switching means (Q2) couples a second supply voltage (V4) to said first winding (Wl) of said transformer (T) to generate a given pulse of said second plurality of current pulses (ip2) and wherein said source (107) of mains supply voltage (VM) is simultaneously decoupled from said first winding, during a portion of a given period of said switching signal. 13. A power supply according to Claim 10 wherein said second plurality of current pulses (ip2) are produced in a current inter-leaving manner (Fig. 1) with respect to said first plurality of current pulses (ipl). 14. A power supply according to Claim 10 wherein said second plurality of current pulses (ip2) are produced in a current super-position manner (Fig. 4) with respect to said first plurality of current pulses (ipl). 15. A power supply according to Claim 10 further comprising, means (D4", Fig. 7) for generating a second supply voltage (in C4") from said mains supply voltage (VM") via a current path (D4") that excludes said transformer (W1 ",W2") wherein said second supply voltage is coupled via said second switching means (Q2") to said transformer (W1",W2") for generating said second plurality of current pulses (ip2"). 16. A power supply according to Claim 10 further comprising, a rectifier (102) coupled to said mains source (107) for rectifying said mains supply voltage (VM) to generate a rectified supply voltage (VI) that is coupled to said transformer (T) wherein said rectified supply voltage is developed from said mains supply voltage without significant low-pass filtering with respect to a frequency (50 Hz) of said mains supply. 17. A power supply according to Claim 10 wherein, during said given period of said mains supply voltage (VM), a ratio between an amplitude of a given current pulse of said first plurality of current pulses (ipl) and a level of said mains supply voltage (VM) that prevails during said given current pulse of said first plurality of current pulses is substantially constant for each current pulse. 18. A switch mode power supply, comprising: a source (107) of an AC, mains supply voltage (VM); a flyback transformer (T); a rectifier (102) coupled to said mains source for rectifying said mains supply voltage to develop in a first winding (Wl) of said transformer a first rectified supply voltage (VI) such that said rectified supply voltage is developed without low-pass filtering with respect to a frequency (50 Hz) and low order harmonics of said mains supply voltage; first switching means (Ql) responsive to a switching signal (VI 6) at a frequency (20-50 KHz) that is substantially higher than a frequency (50 Hz) of said mains supply voltage for generating a first plurality of current pulses (ipl) in said first winding; means (W4,D4) generating a second supply voltage (V4); and second switching means (Q2) coupled to said transformer and synchronized to said first switching means for coupling said second rectified supply voltage to said transformer and for decoupling said first rectified supply voltage from said transformer to generate in said transformer from said second rectified supply voltage a second plurality of current pulses (ip2), such that a given current pulse of said first plurality of current pulses and a given current pulse of said second plurality of current pulses occur, during a given period of said switching signal, said first and second pluralities of current pulses being transformer-coupled via said transformer to a load circuit (LOAD) to develop in said load circuit an output supply voltage (V2) and an output supply current (iL). 19. A power supply according to Claim 18 further comprising, means (COMP l,COMP 2) for disabling the generation of said second plurality of current pulses (ip2) throughout a portion (Tl) of a period of said mains supply voltage (VM), in a vicinity of a peak voltage of said mains supply voltage. 20. A power supply according to Claim 18 wherein said second plurality of current pulses (ip2) are generated during a portion (T2) of a period of said main supply voltage (VM) when amplitudes of said first plurality of current pulses substantially decrease. 21. A power supply according to Claim 18 wherein said second plurality of current pulses (IP2) are produced in a current inter-leaving manner (Fig. 1) with respect to said first plurality of current pulses (ipl). 22. A power supply according to Claim 18 wherein said second plurality of current pulses (ip2) are produced in a current super-position manner (Fig. 4) with respect to said first plurality of current pulses (ipl).

Inventors: MEGEID MAGDY, MEGEID, MAGDY
Applicants: RCA THOMSON LICENSING CORP, RCA THOMSON LICENSING CORPORATION
Application year: 1992
Main classification: H02M1
IPC codes: H02M1, H02M3, H02M7