A METHOD AND DEVICE FOR POWER CONVERSION

Abstract: As a front end power factor correction circuit in a rectifier a boost conversion topology is often used. A great problem with this topology is the rectifier diode. This diode must be of a high voltage type in a boost application and will therefore have a large reverse-recovery current before it will block the voltage. This current may cause large losses. With the solution according to the invention care will be taken to the recovery current in a more effective way than any earlier solution has obtained. The proposed circuit comprises essentially a boost circuit, where a main and first inductor L1 (3) and a main and first diode D1 (4) are connected in series from an input to an output and a main and first capacitor C1 (5) from an output to ground. A main and first transistor M1 (1) is connected from between the first inductor L1 (3) and the first diode D1 (4) to ground. An auxiliary and second inductor L11 (6), a second diode D11 (7) and a second transistor M11 (2) are connected in series from between the first inductor L1 (3) and the first diode D1 (4) to ground. Two other diodes a third and fourth diode D12 (10), D13 (9) are connected in series from between the second diode D11 (7) and the second transistor M11 (2) to the output.

Description: A method and device for power conversion TECHNICAL FIELD The present invention relates to a method for power conversion using boost topology with a loss-less snubber circuit and a device for power conversion using boost topology with a loss-less snubber circuit. BACKGROUND OF THE INVENTION A boost DC/DC converter is usually chosen to be the front end power stage in an AC/DC converter. The output -voltage is always higher than the peak value of the main voltage, which means that the output voltage of the boost converter is more than 300 V. A typical value would be about 400 V in European applications. At these high voltage levels the boost diode may exhibit large reverse recovery currents in the precise moment, when the active switch, the transistor, is turned on. The reverse recovery current will cause high losses at high frequency switching, which in practice will be a limiting factor for the choice of switching frequency. In for example US, A, 5 543 704 a boost converter circuit is described, wherein the switching losses will be minimized by the boost converter by using zero voltage switching techniques. SUMMARY OF THE INVENTION As a front end power factor correction circuit in a rectifier a boost conversion topology is often used. A great problem with this topology is the rectifier diode. This diode must be of a high voltage type in a boost application and will therefore have a large reverse-recovery current before it will block the voltage. This current may cause large losses. With the solution according to the invention care will be taken to the recovery current in a more effective way than any earlier solution has obtained. With a loss-less snubber circuit according to the invention a boost converting is obtained, where the proposed circuit will: -minimize the reverse recovery current in a diode of a boost converter -eliminate the turn-on losses of a main switch -minimize the turn-on losses of an auxiliary switch -minimize the turn-off losses of an auxiliary switch -minimize the reverse recovery current in auxiliary diodes -eliminate the oscillations when the auxiliary diodes turn off -minimize the turn-off losses of the main switch -minimize the current in an auxiliary inductor, when the main transistor is turned off. The proposed circuit comprises a boost circuit, where a main and first inductor LI and a main and first rectifying element as a diode Dl are connected in series from an input to an output and a main and first capacitor Cl from output to ground. A main and first electronic switch as a transistor Ml is connected from between the first inductor LI and the first diode Dl to ground. An auxiliary and second inductor Lll, a second rectifying element as a diode Dll and a second electronic switch as a transistor Mil are connected in series from between the first inductor Ll and the first diode Dl to ground. Two other rectifying elements as diodes, a third and fourth diode D12, D13 are connected in series from between the second diode Dll and the second transistor Mil to the output. A second capacitor Cll is connected from between the other two diodes D12, D13 to between the first inductor Ll and the first diode Dl . A further rectifying element as a diode, a fifth diode D14 in series with a first resistor R14 is connected in parallel with the second capacitor Cll. A last rectifying element as a diode, a sixth diode D15 in series with a second resistor R15 is connected from ground to between the auxiliary and second inductor Lll and the second diode Dll. DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit of a boost converter with loss-less snubber according to the invention. Figure 2 shows the circuit in figure 1 with inserted zener diodes. Figure 3 A-G show voltage and current waveforms, which illustrate the switching cycle of the circuit of figure 1. DESCRIPTION OF THE PREFERRED EMBODIMENT In the figure 1 is shown the circuit according to the invention, where a main and first transistor Ml/1 is turned on a constant delay time after a second transistor Mll/2 is turned on. The on- time of the second transistor Mll/2 is constant and the on-time of the first transistor Ml/1 is regulated. Initially a current will flow through a main and first inductor Ll/3 and a main and first diode Dl/4 from an input to an output. A main and first capacitor Cl/5 is connected from output to ground. The second transistor Mll/2 is then turned on. The rise of the current through the second transistor Mll/2 is limited by an auxiliary and second inductor Lll/6, therefore the turn-on losses of the second transistor Mll/2 will be limited. The first inductor Ll/3 is large and will keep its current constant during commutation, therefore as the current in the second inductor Lll/6 increases the current in the first diode Dl/4 will decrease with the same amount. When the current in the second inductor Lll/6 equals the current in the first inductor Ll/3 the current in the first diode Dl/4 will be zero. Due to reverse recovery the current in the first diode Dl/4 will go negative and the current in the second inductor Lll/6 will continue to rise until the first diode Dl/4 is turned off. When the first diode Dl/4 is turned off the current in the second inductor Lll/6 is larger than the current in the first inductor Ll/3, therefore the excess current is forced to go in the reverse direction in the first transistor Ml/1. At first this current will discharge the drain-source capacitance of the first transistor Ml/1, after that the current will free-wheel in the body-diode of the first transistor Ml/1. Now the first transistor Ml/1 is turned on. The turn on will be free of losses because the capacitance of the first transistor Ml/1 is already discharged and the voltage over it is zero. At the same moment the second transistor Mll/2 is turned off. This will force the current in the second inductor Lll/6 to commutate from the second transistor Mll/2 to a third diode D12/10 and a second capacitor Cll/8. The second capacitor Cll/8 is initally discharged. During this period the voltage over the second transistor Mll/2 equals the voltage of the second capacitor Cll/8, therefore the turn-off losses of the second transistor Ml1/2 will be minimal. After that two possibilities will arise: 1. The second capacitor Cll/8 is now charged to the output voltage by the current in the second inductor Lll/6. When the second capacitor Cll/8 is fully charged a further diode a fourth diode D13/9 starts to conduct. The current in the second inductor Lll/6 will now decrease with a constant rate depending on the output voltage and the value of the second inductor Lll/6. At the same time the current in the first transistor Ml/1 will increase so that the total current is constant, equal to the current in the first inductor Ll/3. 2. During charging of the second capacitor Cll/8 the current in the second inductor Lll/6 decreases. Eventually the current is zero and the second capacitor Cll/8 keeps its charge to a voltage between zero and the output voltage. Both possibilities may continue in the same manner: Because of the reverse recovery in the third diode D12/10 the current in the second inductor Lll/6 will be negative before the third diode D12/10 (and the fourth diode D13/9) are turned off. When the third diode D12/10 turns off, the current in the second inductor Lll/6 will commutate to a further a sixth diode D15/11 and a second resistor R15/12. The current in the second inductor Lll/6 is now damped by the second resistor R15/12 and will rapidly decrease to zero . The current now flows through the first inductor Ll/3 and the first transistor Ml/1, there is no current in the so called snubber circuit, and the second capacitor Cll/8 is charged to a voltage between zero and the output voltage. After that the first transistor Ml/1 turns off. The current in the first inductor Ll/3 is commutated from the first transistor Ml/1 to the second capacitor Cll/8 and the fourth diode D13/9 to the output. The voltage over the first transistor Ml/1 equals the output voltage minus the voltage over the second capacitor Cll/8. This voltage is usually less than the output voltage and the turn off losses therefore are small. The current in the first inductor Ll/3 will discharge the second capacitor Cll/8 through the fourth diode D13/9. A further diode, a fifth diode D14/13 and a first resistor R14/14 prevent the second capacitor Cll/8 from being overdischarged due to inductance and slow turn on of the first diode Dl/4. The commutation from the fourth diode D13/9 to the first diode Dl/4 will be slow and without oscillations and also without reverse recovery in the fourth diode D13/9. The first resistor R14/14 will control the speed of the commutation. When the voltage over the first transistor Ml/1 rises, the voltage over the second transistor Mll/2 is also forced to rise by a current through the second inductor Lll/6. The link of the fifth diode D14/13 and the first resistor R14/14 has earlier prevented the voltage over the second capacitor Cll/8 to become negative. This has the effect that no extra current is needed in the second inductor Lll/6 to charge the second capacitor Cll/8 and the current in the second inductor Lll/6 is decided only by the output capacitance of the second inductor Lll/6. The second inductor Lll/6 is discharged through three diodes, which makes the current in the second inductor Lll/6 to quickly decrease to zero. This prevents any further diode reverse recovery, when the second transistor is turned on again and the switching period is completed. With a first zener diode Z14/16 inserted a more rapid and definite commutation is achieved from the fourth diode D13/9 to the first diode Dl/4. With a second zener diode Z15/17 a more rapid damping of the current in inductor Lll/6 is achieved after turn off of the second transistor Mll/2. With a third zener diode Z12/15 a more rapid discharge is achieved of the second inductor Lll/6 after turn off of the first transistor Ml/1. In figure 2 the zener diodes Z12, Z14 and Z15/15,16,17 are inserted. In figures 3 A-G different waveforms are shown illustrating U D s drain source for Ml, U G s gate source for Ml, U DS for Mil, U G s for While the foregoing description includes numerous details and specificities, it is to be understood that these are merely illustrative of the present invention, and are not to be construed as limitations. Many modifications will be readily apparent to those skilled in the art which do not depart from the spirit and scope of the invention, as defined by the appended claims and their legal equivalents.

Claims: CLAIMS 1. A method for power conversion using boost topology with lossless snubber circuit comprising a first inductive element, a first electronic switch for controlling the • flow of a forward current from said first inductive element to a first rectifying element and charging a first capacitive element, said first switch having a parasitic capacitance and being cyclically turned on and off and discharging the parasitic capacitance of said first switch, while minimizing turn on losses in said first switch and losses caused by reverse recovery current from said first rectifying element, wherein the method further comprises the steps of: -directing the flow of forward current from said first inductive element, and the flow of reverse recovery current from said first rectifying element, to a second inductive element in series with a second rectifying element to allow for the discharge of said parasitic capacitance, when said first switch is turned off by turning on a second switch, -directing the flow of current from said second inductive element to a third rectifying element connected to a second capacitive element, which acts to limit the voltage across said first and said second electronic switches, when each of said electronic switches, respectively, is turned off. -directing current from ground to said inductive element through a sixth rectifying element to absorb the reverse recovery current from said second rectifying element, when said second switch is turned off, -directing the flow of current from said first inductive element to a fifth rectifying element when said first switch is turned off and said second capacitive element is discharged, thereby preventing the voltage across said second capacitive element to become negative, -directing the flow of current from said first inductive element to a first rectifying element when said first switch is turned off and the voltage across said first switch is equal to the voltage across said first capacitive element thereby assuring that the current in a fourth rectifying element is zero, and -directing the flow of current from said second inductive element to at least three rectifying elements to assure that the current in said second inductive element decreases rapidly to zero, when parasitic capacitance of said second switching element has been charged. 2. A method according to claim 1, wherein said fifth rectifying element comprises a series combination of a diode and a resistor. 3. The method according to claim 1, wherein said fifth rectifying element comprises a series combination of diode and a zener diode. 4. The method according to claim 1, wherein said fifth rectifying element comprises a series combination of a diode, a zener diode, and a resistor. 5. The method according to claim 1, wherein said third rectifying element comprises a series combination of a diode and a zener diode. 6. The method according to claim 1, wherein said sixth rectifying element comprises a series combination of a diode and a resistor. 7. The method according to claim 1, wherein said sixth rectifying element comprises a series combination of a diode and a zener diode. 8. The method according to claim 1, wherein said sixth rectifying element comprises a series combinatoin of a diode, a zener diode and a resistor. 9. A device for power conversion using boost topology with lossless snubber circuit comprising: a main and first inductor Ll(3) and a main and first diode Dl(4) connected in series from an input to an output, a main and first transistor Ml(l) connected from between the first inductor Ll(3) and the first diode Dl(4) to ground, and a auxiliary and second inductor Lll (6), a second diode Dll (7) and a second transistor Mil (2) connected in series from between the first inductor Ll(3) and the first diode Dl(4) to ground, characterized in that two further diodes, a third and fourth diode, D12, D13 (10, 9) are connected in series from between the second diode Dll (7) and the second transistor Mil (2) to the output . 10. The device according to claim 9, characterized in that a second capacitor Cll (8) is connected from between the further diodes D12,D13 (10, 9) to between the first inductor Ll(3) and the first diode Dl(4) . 11.The device according to claim 10, characterized in that a fifth diode D14(13) is connected in parallel with the second capacitor Cll (8) . 12. The device according to claim 10, characterized in that a fifth diode D14(13) in series with a first resistor R14(14) are connected in parallel with the second capacitor Cll (8). 13. The device according to claim 10, characterized in that a fifth diode D14(13) in series with a first zener diode Z14(15) are connected in parallel with the second capacitor Cll (8). 14. The device according to claim 10, characterized in that a fifth diode D14(13) in series with a first resistor R14(14) and a first zener diode Z14(16) are connected in parallel with the second capacitor Cll (8). 15. The device according to any of claim 10-14, characterized in that a sixth diode D15(ll) is connected from ground to between the second inductor Lll (6) and the second diode Dll (7) . 16. The device according to any of claim 10-14, characterized in that a sixth diode D15(ll) in series with a second resistor R15(12) are connected from ground to between the second inductor Lll (6) and the second diode Dll (7). 17. The device according to any of claim 10-14, characterized in that a sixth diode D15(ll) in series with a second zener diode Z15(17) are connected from ground to between the second inductor Lll (6) and the second diode Dll (7). 18. The device according to any of claim 10-14, characterized in that a sixth diode D15(ll) in series with a second resistor R15(12) and a second zener diode Z15(17) are connected from ground to between the second inductor Lll (6) and the second diode Dll (7) . 19. The device according to any of claims 10-18, characterized in that a third zener diode Z12(15) is inserted in series with the third diode D12(10) .

Inventors: SAHLSTROEM THOMAS, THOREN CHRISTER, SAHLSTROEM, THOMAS, THOREN, CHRISTER
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1998
Main classification: H02M3
IPC codes: H02M1, H02M3, H03K17