Title of Invention

"AN IMPROVED EXTERNALLY DRIVEN FLYBACK TYPE DC-TO-DC CONVERTER"

Abstract

This invention relates to an improved externally driven flyback type DC-to-DC converter for charging an energy storage capacitor used for charge/discharge applications such as source of electrical power in flash lamp pumped solidstate lasers. The converter has a switching device connected to a waveform generating circuit for driving said switching device. A converter transformer connected to said switching device for storage of energy during conduction of the switching device. A diode and an energy storage capacitor is connected to the transformer and a feed back circuit is connected to the waveform generating circuit.

Full Text

FIELD OF INVENTION: This invention relates to an improved externally driven flyback type DC-to-DC converter as a source of electrical power for charging energy storage capacitors. The invention has a beneficial application as a source of electrical power to the flash lamp in the flash lamp pumped solidstate lasers., in particular, but without implying any limitation to the scope of the invention. PRIOR ART : DC-to-DC Converters and DC-to-AC Inverters belong to the broad category of Switched Mode Power Supplies (SMPS). The SMPS is undoubtedly the contemporary power conversion option when compared to the other category of power supplies called Linear Power Supplies. The SMPS has the distinct edge when efficiency and size/weight are the important selection criteria. There are three basic types of DC-to-DC converters namely (i) Flyback Converters (ii) Forward Converters and (iii) Push-Pull Converters. In a flyback converter in general, the energy is stored in the transformer primary during the conduction time of the active switching device which could be a Bipolar transistor or a MOSFET or an Insulated Gate Bipolar Transistor (IGBT) or any other switching device. The energy is transferred to the output and the load during the time when the switching device is non-conducting or OFF. Regulation is achieved by controlling the duty cycle of the switching drive waveform. In case of Forward Converters, most of the energy is stored in the output inductor rather than the transformer primary and also the load is receiving the energy both during the conduction as well as the non-conduction time of the active switching device. In Push-Pull Converters, the energy is getting stored and transferred at all times, The basic Push-Pull converter uses two active switching devices driven in push pull so that when one is conducting, the other is OFF and vice versa. When the energy is getting stored due to conduction of one switching device, then the energy stored in the immediate past due to the conduction of the other switching device is getting transferred to the output side and the load. Further, in the flyback converter category, there are (a) Externally driven flyback converters (b) Self oscillating or the Ringing choke type converters. In the class of Forward Converters, the common configurations include (a) Single ended isolating type converter and (b) Step down or Buck converter. In the Push-Pull Converter category, there are (a) Self oscillating type converters (b) Externally driven converters (c) Half bridge converters and (d) Full bridge converters. Different push-pull converter configurations differ only in the mode in which the transformer primary is driven. This invention relates to an Externally Driven Flyback type DC-to-DC Converter. The converters belonging to the category of Forward Converters and Push-Pull Converters are suitable only for those types of loads that are predominantly resistive in nature such as those encountered in the power supplies designed for TVs, VCRs, Music Systems, Personal Computers (PCs), Test equipment and so on. These types can not be used in solidstate lasers where the load is highly capacitive in the form of an energy storage capacitor. The voltage across the energy storage capacitor builds up only in voltage packets of continuously varying sizes with the first voltage packet being the largest and the last packet which takes it to desired voltage being the smallest, the energy in each packet remaining the same. Such a load necessitates an independent control on the energy transfer time in accordance with the pattern of voltage build-up across the capacitor for an optimum power conversion . It is theoretically feasible to have such a control in a flyback converter where the energy is stored during the conduction time of the switching device and transferred to the load during the non-conduction time of the switching device in addition to its suitability for the above said conventional applications. While in case of resistive loads, it is the voltage regulation that is of prime concern and is achieved by some form of duty cycle control [usually Pulse Width Modulation (PWM)] in all types of DC-to-DC converters listed above, in case of load being an energy storage capacitor, the energy transfer process is the critical factor. In view of the description above, an externally driven flyback converter is therefore the heart of the electronics that goes with a solidstate laser system. Such a converter typically, provides more than 90 percent of electrical power required by the laser system. Its conversion efficiency therefore is the primary determinant factor for the overall electrical efficiency and consequently the size and weight of the laser system. The flyback converters used for the purpose in the prior art for charging energy storage .capacitors .use a fixed frequency switching drive waveform. Such a drive waveform has the drawback that it does not take into account the varying energy transfer time requirement of the energy storage capacitor for different cycles of energy storage and transfer as the capacitor charges from zero to its final voltage. This not only seriously compromises the converter efficiency and the size / weight of the power pack feeding the converter input, but also leads to a hardware that uses highly over-specified components. Another disadvantage of the known flyback converter is that in the converter circuits used in these converters, an attempt is made to store energy much large'r than would be necessary to compensate for the inefficient energy transfer in the initial transfer cycles if one chooses to work at a relatively higher switching frequency or to compensate for the time to compensate for the time wasted in the latter part of storage/transfer cycles if one opts for a relatively lower frequency. An externally driven flyback converter known in the prior art for charging an energy storage capacitor has the typical hardware contiguration as shown in figure 1 of the accompanying drayings. The drive waveform generator (1) is an astable niu I t i v i brator (free running oscillator) producing a fixed frequency drive waveform at its output. The switching device shown here is a bipolar" transistor (3), it could be a MDSEFT or an IGBT also. The converter trang former (2) is so wired that when the energy is being stolen! in the transformer primary during the conduction time of the switching device the djode (-1) in the second-is v circuit is reverse biased and as a result the energy i^ not being transferred to the energy storage capar i. tor (5) . , When the switching device (3) is switched OFF, the magnetic field in the primary collapses, the polarities of induced voltages reverse? and the diotlt? (4) gets forward biased and the stored energy is transferred to the capacitor . The process of storage and transfer continues till the capacitor is charged to the desired voltage. At that time, the Voltage Sense ! onp (6), which is basically a comparator, undergoes a change of state at its output and resets or disables the f's iver. One of the major drawbacks of the known externally driven flyback type DC-to-DC converters is that its operation does not lake into consideration the continuous changes required in the OFF t.im^s of the drive waveform as the capacitor" charges towards its final desired voltage. If the hardware uses a relatively lower frequency drive waveform (7) as shown in figure 2a of the accompanying drawings to mef-t the larger energy transfer time requirement in th™ initial stages of energy transfer to ensure a camplf'te or a near complete energy transfer, then the converter would be sitting idle for larger part of the duration of the time (9) alloted for energy transfer in the latter part of capacitor charging as shown in figure 2c of the accompanying drawi.ngs. The waveform (8) as shown in " figure 2b of the accompanying drawings is the current ramp waveform through the primary winding of the converter transformer (2). If the hardware uses a relatively higher frequency drive waveform (10) as shown in figure 3a of the accompanying drawings to avoid such a situation in the latter part of the capacitor charging, then the energy Transfer would be highly incomplete in the initial cycles as shown at (12) in figure 3c of the accompanying drawings. Figure 3b of the accompanying drawings shows the current ramp waveform through the transformer primary winding for the case when the hardware uses a relatively h.iijher drive frequency. The result in both cases is a significant loss of conversion efficiency. The conversion efficiency typically varies between 40% to 60%. Another drawback of the hardware configuration of the externally driven flyback DC—toDC converters known in the prior art is that is TRriously compromises the reliability if an attempt is made to use a relatively higher drive frequency. A higher frequency leads to highly incomplete energy transfer in the initial few cycles with the result that the untransferred energy is reflee ted back to the primary circuit. This puts an increased stress on thr? switching device, converter transformer and the iJ.i.odo .in the secondary. One or more titan one of these components may fail thus jeopardising the reliability of the circuit. ' Yet another drawback of the hardi-jarr of the known externally driven flyback DC—to-DC converters is that these do not lead to an optimum size and weight due to the fact that non-optimum operation forces the designer to choose overspecified components and poorer efficiency means a larger capacity battery pack for a given operational life which is particularly important when such a converter is used in a portable system. Still another drawback of the hardware of the known externally driven flyback DC-to-DC converters is that the converter transformer which is one of the vital components of the converter is designed using mathematical expressions that are valid only for a known fixed frequency design and which again do not take into consideration the varying frequency nature of circuit operation. As a result, the transformer operation even as an individual component is far from optimum. There is a need for building the hardware that would simulate a changing energy transfer time from cycle to cycle in accordance with the voltage packet variation pattern and the present invention meets this need. OBJECTS OF INVENTION : The primary object of the present invention is to provide an improved externally driven flyback type DC-to-DC converter for charging an energy storage capacitor, a situation typical of a flash lamp pumped solidstate laser system. Another object of the present invention is to propose an improved externally driven flyback type DC-to-DC converter which would overcome the disadvantages, some of them being very serious, of a fixed frequency drive flyback type DC-to-DC converters known in the prior art when used with capacitive loads as those encountered in solidstate laser systems. Still another object of the present invention is to provide a novel hardware for an externally driven flyback type DC-to-DC converter that fully meets the requirements of changing energy transfer timings during different energy storage/transfer cycles when the load is an energy storage capacitor, a situation typical of a flash lamp pumped solidstate laser system. Further-object of the present invention is to provide an externally driven flyback type DC-to-DC converter with a drive waveform generator circuit that produces at its output a drive waveform whose OFF-time ( which is the same as the energy transfer time ) changes from cycle to cycle according to the energy transfer requirements of the load. Yet further object of the present invention is to provide an externally driven flyback type DC-to-DC converter with feedback circuit that can sense the status of the load in real time and generate an equivalent electrical information to control the OFF time of the ,drive waveform, and the OFF-time when controlled in this fashion leads to a significant improvement in the. conversion efficiency and circuit reliability and reduction in hardware size and weight. Still further object of invention is td provide an externally driven flyback type DC-to-DC converter with converter transformer that fully takes into consideration the variable frequency nature of the energy storage and transfer and does not assume a known fixed frequency of operahian. This leads to enhancement of conversion efficiency and reduction in size and weight of the transformer. STATEMENT HF INVENTION This invention discloses a novel hardware of an externally driven flyback type DC-to-OC converter optimum circuit hardware with significantly improved conversion efficiency and consequent reduced drain on powder pack and considerably reduced stress on active components with consequent improved reliability of operation. The proposer! converter is meant for highly capacitive loads and incorporate improved hardware components which include the converter transformer, the drive waveform generator and its associated closet! loop circuit. A flash lamp pumped solidstate laser system is a major example where the converter is required to charge an energy storage capacitor to it voltage depending upon the quantum of energy it is supposed to deliver to the flash lamp when made to discharge through the lamp. The converter here provides mme than 90 percent of the electrical power input required by the laser system and therefore converter efficiency is the primary determinant factor for the overall electrical efficiency of the system. According to this invention there: is provided an improved externally driven flyback type DC-to-DC converter for charging an energy storage capacitor for charge/discharge applications such as source of electrical power in flash lamp pumped solidstate lasers comprising a switching device (16) connected to a waveform generating circuit (25) for dr.i ving said switching device, a converter transformer (17) connected to said switching device (16) for storage of energy during conduction of the switching device, a diode (18) and an energy storage capacitor (19) connected to transformer (17) and a feed back circuit connected to waveform generating circuit (25), Fhe converter transformer used in the proposed hardware is built using design equations that reflect the actual conversion process in case of capacitive loads and not the ones used for designing the transformers meant for resistive loads The transformer construction takes into account the varying voltage packet size from cycle to cycle. The drive waveform generator used in the proposed hardware- is not (tie astable multivibrator configuration with the duty cycle of the drive waveform controlled by the output regulation requirements as is the case in the converter hardware of the prior art. Instead, it is so designed as to generate a drive waveform for the switching device where the OFF-time in any given cycle is just the one required to optimally transfer the energy stored during the ON-time of that very cycle. The shape of the drive waveform is therefore strictly controlled by the pattern of voltage build-up across the capacitor. The heart of the drive waveform generator in the proposed hardware is a Voltage Controlled Oscillator (VCO) whose control voltage is generated by the feedback circuit that is monitoring the status of the capacitive load in real time. The feedback circuit translates the output status to a control voltage in such a manner that when this control voltage is fed to the control input of the VCO, it produces the desired lowest output frequency when the capacitor is fully discharged and the desired highest output frequency when the capacitor has charged to the desired final voltage The feedback circuit also senses the output status to generate a command to disable the drive circuit when the capacitor has charged to the desired final voltage. The drive waveform when controlled by a feedback circuit has a constant ON-time depending upon the quantum of energy to be stored which further depends upon the desired power output capability of the converter and an OFF-time that varies from the largest in tin; first cycle to the smallest in the last cycle of energy storage / transfer. DETAILS OF THE FIGURES ENCLOSED The proposed invention wiM now be illustrated with the help of drawings whirh are not intended to be taken restrictive!/ to imply and limitation but are intended to illustrate typical embodiments, wherein: Fig. 4 proposed converter, shows extermally the schematic arrangement of the driven flyback type DC-to-DC Fig. 5 shows the voltage waveform at the output of the driver (figure 5a) the transformer primary current waveform (figure 5b) and the waveform of voltage build-up across the energy storage capacitor from cycle to cycle (figure 5c) (Figure 1 to 3 relate to the prior art and have already been described there-under) DESCRIPTION OF THE INVENTION WITH REFERENCE TO DRAWINGS: Referring to figure 4, the DRIVE PORTION of the converter hardware comprises a waveform generating circuit (25) of a cascaded arrangement of a Voltage Controlled Oscillator (VCO) (13), a monosho! (14) and a driver (15). The switching device (16) is either a Bipolar transistor or a Power MOSFET or an IGBT as shown within the bloack. The output of the Voltage Controlled Oscillator feeds the trigger input of the monoshot. The frequency of the VCO output as controlled by a control voltage applied at the control input terminaJ therefore determines the frequency of the monoshot output. The monoshot output feeds the driver inptu. The' driver provides the required drive current and.or voltage depending upon the type and specifications of the switching device chosen for the application and does not alter the shape of the? drive waveform present at the monoshot output. The F?iic?rgy is stored in the primary of the converter transformer (17) during the conduction time of the switching device and transferred to the secondary side and the energy storage capacitor (17) during the non-conduction titm:? of the? switching device. The FEEDBACK CIRCUIT comprisvs of two independent potential divider arrangement (73,74), a subtracter circuit (21) and a comparator circuit (70). The potential divider R1.-R7 (24) and the compatator (20) constitute the VOLTAGE SENSE LOOP whereas the potential divider R3-R4 (23) and the subtracter (71) l.rtu*? ther form the ENERGY TRANSFER SENSE I OOP . The vo 1 t.ujt: i.l i v i der in the Voltage divider in the Voltage* Sense loop provides a sample of the output voltage to the compare tot where it is compared with a refer F-nco voltage marked Vref 1 in figure 4. The comparator output feeds the reset terminal of the monoshot and disables the drive? wveform generator the moment the voltage across the i apacitor reaches the desired magnitude. The voltage divier in the Energy Transfer Sense Loop also samples tin?? output and the sampled voltage ic subtracted f-i.m another reference voltage marked Vief 7 in figure '1. The subtracter output fpr?drs the trtntrfil terminal of t!ie VCO. The Voltage Sense Loop and the? Energy Trvsnsfi/t* Sense Loop and I IIP? Energy Transfer Sense Loop together constitute the Feedback Circuit. The monoshot output is a waveform having a fixed ON-time and an OFF-time that depends upon the VCO output frequency. As the VCO output frequency changes, the OFF-time in the rr.onoshot output changes accordingly. The converter transformer (17) and the switching device (16) have been wired in the flyback converter topology. The 'dots' shown on the transformer winding:, in the converter transformer (17) indicate the winding polarity. The winding polarity is such that the diode D (18) remains reverse biased during the conduction time of the switching device thus disallowing transfer of energy as it is being stored. The diode is forward biased only when the switching device (16) is switched OFF and consequently the energy stored in (he transformer primary collapses When the circuit is initially switched on, the capacitor is fuily discharged so that one of the inputs to the subtracter is zero, the other being Vref2. Vre(2 and the R5, C1(22) are so chose:1, as to produce a VCO output frequency that is the one required to produce the desired OFF-time for the first cycle for a given ON-time R5, C1 (22} are a part of the VCO (13) and together with VCO control input voltage decide the VCO output frequency. While ON-time is a transformer design parameter and together with transformer primary inductance decides output power capability of the converter, the OFF-time required for the first cycle can be computed from : T1(off) = 2nJTp~C where Lp is the primary inductance, C is the capacitance of the energy storage capacitor and n is the transformer turns ratio (Ns/Np). The OFF-time required for the transfer of the first voltage packet is the largest leading to the VCO output frequency being the least The energy storage/transfer cycfe that takes the capacitor to the desired ojtpul voltage Vo requires the least OFF-time (or the transfer time) which is computed from : TN (off) = 2rWLp.C [/N~ - /NM ] where N is the total number of energy storage/transfer cycles required to charge the capacitor to the desired voltage and is given by : N = {( C.Vo )/( Lp.lp2)] where Ip is the peak transformer primary current given by lp=(Vin.ton/Lp). R3, R4 are chosen in such a way that they alongwith already chosen value of Vref2 produce the required control input voltage for the VCO so that its output frequency leads to the desired OFF-time for the last energy storage/transfer cycle. R1, R2 are chosen in such a way that for the desired output voltage Vo, the voltage at R1-R2 junction equals the chosen VreM. The transition at the comparator output resets the monoshot thus disallowing the energy storage capacitor to charge beyond Ihe desired value The converter transformer is constructed using a set of equations that take into account the changing voltage packet size during the capacitor charging process The transformer parameters are computed from . 1 The core size is determined from known ( Window Area x Core Cross-section ) produCt versus power relationship. 2. The Primary Inductance Lp is computed from : (Formula Removed) where ( /\) = conversion efficiency, Vin = Input Voltage, Po = Output Power VCBO(max) = Acceptable collector-base reverse voltage of the switching transistor (Usually 60 to 70 percent of the rated maximum). It is VGDO(max) in case of MOSFETS and VCGO(max) in case of IGBTs. 3. Number of Primary turns (Np) and number of Secondary turns (Ns) are computed from : (Formula Removed) 4. Size of the air gap (Ig) is computed from : (Formula Removed) where lg = size of air gap / = magnetic path length of the core \\r = Relative permeability of the core material HO = Free space permeability The diode D is a fast recovery rectifier with a PIV of greater than twice the desired output voltage and a reverse recovery time that is a small fraction of the drive waveform ON-time, typically 1/50 of the ON-time. • Referring to Figure-5, Figure-5a shows the drive waveform generator output waveform (25). The changing OFF time decreasing with increase in voltage build-up across the energy storage capacitor as desired is visible from the waveform shown. Figure-5b shows the current ramp waveform (26) through the transformer primary in this case. The waveform of the voltage build-up across the energy storage capacitor (27) as shown in figure 5c clearly demonstrates the superiority of performance of the invented hardware when compared with the one well known in the prior art. As IB clear, the harware conceptually ensures that each storage/transfer cycle is allotHtf an energy transfer time according to its requirement with the result that not only the energy transfer in different cycles is complete, the converter does not sit idle throughout the capacitor charging process. This leads to the most optimum conversion process thus providing the highest possible conversion efficiency and consequently reduced size/weight reduced stress on components and significantly improved circuit reliability. WE CLAIM 1. An improved externally driven flyback type DC-to—DC converter for charging an energy storage capacitor for charge/discharge applications such as source of electrical power in flash lamp pumped solidstate lasers comprising a switching device (1.6) connected to a waveform generating circuit (25) for driving said switching device, a converter transformer (17) connected to said switching device (16) for storage of energy during conduction of the switching device, a diode (18) and an energy storage capacitor (19) connected to transformer (17) and a feed back circuit connected to waveform generating circuit (25). 2. An improved externally driven flyback type DC—to-DC converter as claimed in claim 1 wherein said waveform generating circuit (25) comprises of a Voltage Controlled Oscillator (VCO) (13) alongwith R5.C1(22), a monoshot with RESET facility (14) and a driver (15) 3. An improved externally driven flyback type DC—to-DC converter as claimed in claim 1 wherein said feedback circuit comprises of R1-R2 potential divider arrangement (24), a comparator (20) constituting the Voltage Sense Loop part of the feedback circuit and potential divider arrangement R3—R4 (23) and a subtracter circuit (21) constituting the energy transfer sense part of the feedback circuit, 4. An improved externally driven flyback type DC-to-DC converter as claimed in claim 1 wherein said switching device (16) is either a Bipolar transistor or a Power MOSFET or an insulated Gate Bipolar Transistor (IGBT). 5. An improved externally driven flyback type DC—to—DC converter as claimed in claims 1 ho 4 wherein the control input voltage to the Voltage Controlled Oscillator (13) is given by (Kl Vo - K2) where Vo is the output voltage of the converter, Kl is hhe divider ratio (R4/(R3+R4) and K2 =Vref2 and Vref2 is reference voltage to be subtracted from a fraction of the output voltage produced by R3-R4 divider arrangement (23). 6. An improved externally driven flyback type DC—to-DC converter substantially as herein described and illustrated.

Documents:

1599-del-1997-abstract.pdf

1599-del-1997-claims.pdf

1599-del-1997-correspondence-others.pdf

1599-del-1997-correspondence-po.pdf

1599-del-1997-description (complete).pdf

1599-del-1997-drawings.pdf

1599-del-1997-form-1.pdf

1599-del-1997-form-19.pdf

1599-del-1997-form-2.pdf

1599-del-1997-form-3.pdf

1599-del-1997-gpa.pdf