Clamped Quasi-Resonant Step-Up Inverter

Abstract

A device which, through its self-oscillation, generates a stable high voltage DC or AC output from a low voltage DC input. The device automatically maintains a desired voltage on an output capacitor, despite changes in output load or input voltage. The device is capable of dead-short operation, capacitor charging, high voltage step-up, high efficiency, and high power density. The capability to step up low voltage to high voltage in such a manner paves the way for advancement in battery-to-grid inverter technology, portable welding devices, portable medical devices, aircraft and spacecraft propulsion devices among many other areas.

Claims

1. An apparatus which accepts direct current and outputs direct current, by means of one or more transformer(s) operating at a high frequency and: a) an oscillating mechanism on the primary side of the transformer(s), the frequency of which is positively correlated with the voltage of the input, and b) at a secondary side of the transformer(s), coupled magnetically with and electrically insulated from the primary side, the current alternates in direction, and c) is then rectified into direct current, and d) a capacitor is connected to the output wherein its voltage, correlated with a known reference, is fed back into the oscillatory mechanism on the primary side in order to maintain a stable output voltage regardless of the input voltage to the apparatus.

2. The apparatus of claim 1, wherein the alternating current flowing out of the transformer passes through a non-polarized capacitor before passing into a bridge rectifier which is coupled to a common ground with the primary side.

3. The apparatus of claim 1, wherein the conversion from direct to alternating current is accomplished by the means of a series of electrical switches whose switched paths and control signals have the capability to operate at high frequency and are electrically coupled to a similar but not identical set of one or more electrical switches on an opposing side of a transformer's primary winding.

4. The apparatus of claim 1, wherein the inductors, capacitors and resistors of the system force the switches to operate at a frequency similar to the transformer's maximum efficiency frequency.

5. The apparatus of claim 1, wherein the system contains an inductor in the path of its low voltage input whose saturation current rating is comparable or higher than that of the primary winding of the transformer.

6. The apparatus of claim 1, wherein the primary side of the transformer has one or more taps.

7. The transformer of claim 1, wherein such a transformer is designed to operate at a frequency between 1-750 kHz.

8. The switching elements of claim 3, wherein diodes are placed between the current path leading from one set of switches' control paths to the switched paths of an opposing set of switches, and at least one capacitor is placed between the cathodes of such opposing diodes.

9. At the diodes of claim 8, wherein such diodes are of standard PN type, or of type Schottky, Zener, Tunnel, Step, PIN, LED or Varactor.

10. The capacitor(s) of claim 8, wherein the capacitors are of multi-layer ceramic, mica, single layer ceramic, PTFE, metallized film, oil-filled, tantalum, or electrolytic.

11. The capacitors of claim 10, wherein such capacitors have a capacitance value which is strongly dependent upon applied voltage.

12. A system for processing low voltage direct current into other forms of electrical energy comprising: a) One or more switched mode power supplies operating at high frequency, and b) An energy storage device which is rated to the maximum output voltage of the switched mode power supplies, and c) One or more secondary switching stages which act upon the stored energy within the system.

13. The system of claim 12, wherein the maximum output voltage is at least 100V.

14. The system of claim 12, wherein the system of power supplies are connected in parallel in such a way as to share a common energy storage device driven by the combined direct current outputs of all power supplies set to similar output voltage levels.

15. The system of claim 12, wherein the energy storage device is a capacitor, flywheel, inductor, series of capacitors, series of electric double layer capacitors, or series of batteries.

16. The system of claim 12, wherein one or more of the switched mode power supplies are supervised by a microcontroller.

17. The microcontroller of claim 16, controlling the operation of a secondary switching stage and its connection to the apparatus in such a way that: a) The microcontroller may adjust, if needed, either the frequency of a secondary switching stage, or the output voltage of one or more power supplies, or both, in order to suit the needs of the final load, and b) The microcontroller may sense other variables such as input voltage, temperature, etc. in order to make more complex decisions regarding the voltage, frequency and operational state of the system.

18. The system of claim 12, wherein a microcontroller controls a secondary switching stage which is a H-bridge or multi-phase switching circuit.

19. The secondary switching stage of claim 18 wherein the stage operates at a frequency between 49-250 Hz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1This schematic depicts the layout connections within a Clamped Quasi-Resonant Inverter as built and demonstrated. This schematic depicts various circuit elements and nets including (1) High Voltage DC output, (2) Gate drive signal, (3) Low Voltage DC input, (4) Primary Inductor, (5) Transformer, (6) and (7) Isolated AC output.

[0013] FIG. 2This drawing depicts a notional layout of a higher level structure which can be constructed using the Clamped Quasi-Resonant (CQR) Inverter shown in FIG. 1, shown as (8). This figure shows the (3) Low voltage DC input, (1) High voltage DC output, connected via a capacitor (9) to a common ground.

[0014] FIG. 3This drawing depicts an alternate embodiment of the invention as shown first in FIG. 2, where two or more CQR Inverters (8) are connected in parallel to drive a larger load (9).

[0015] FIG. 4This drawing depicts an alternate embodiment showing the CQR Inverter (8) outputting isolated AC from (6) and (7) with a capacitor connected between High Voltage DC output (1) and common ground.

[0016] FIG. 5This drawing depicts a higher level structure which builds upon the configuration shown in FIG. 2 and features the addition of a microcontroller (12) which controls the action of an H-Bridge (10) to generate variable frequency AC or DC 2 or 3 phase output.

[0017] FIG. 6This drawing depicts an alternate embodiment of the system depicted in FIG. 4 where the Low voltage DC input (3) is supplied by a battery (14) also connected to the common ground.

[0018] FIG. 7This photograph shows an embodiment of the present invention

[0019] FIG. 8This photograph shows an alternate embodiment of the present invention which was improved upon to develop the current embodiment shown in FIG. 6.

[0020] FIG. 9This photograph shows an oscilloscope graph of the input amperage to the embodiment shown in FIG. 6 versus time.

[0021] FIG. 10This photograph shows an oscilloscope graph of the output voltage of the embodiment shown in FIG. 6 versus time.

DETAILED DESCRIPTION

[0022] The invention consists of an oscillating system similar in design to the Resonant Royer Oscillator as first espoused in U.S. Pat. No. 3,818,314. This design was modified and optimized for use in a DC-DC step up application and the fundamental schematic of the current invention is shown in FIG. 1.

[0023] The invention shown in FIG. 1 represents the core components of the Clamped Quasi-Resonant Inverter (CQR). The CQR consists of a number of systems: the oscillator, the clamp, the switches, the transformer, and the rectifier. LED2 indicates the presence of power to the unit, and LED1 indicates that the switching is activated.

[0024] In an embodiment of the present invention, the oscillator consists of a system similar to that depicted on the right hand side of FIG. 1, complimenting the resonant characteristics of the transformer, switches, inductor and tank capacitors shown in the center. The primary purpose of the oscillator is to activate one of the two switches, Q1 or Q5 at a time, in resonance with the natural driving frequency of the transformer 5. This resonance frequency is automatically adjusted by the VC of the primary tank capacitor C11 along with the primary of the transformer forming an L-C circuit. In an embodiment, the tank capacitor is of MLCC type with Class II dielectric with a (0V bias) nominal capacitance in the range of 10 F to 10 F. In an alternate embodiment, the tank capacitor may be film type with a (0V bias) nominal capacitance in the range of 10 nF to 10 F, although such an embodiment may be less efficient under some operating conditions.

[0025] The oscillating system is modulated by a voltage divider, consisting of resistors R10, R16, and R9. This voltage divider senses the voltage at the output of a bridge rectifier B2. When the output voltage exceeds a desired level, current begins to flow through an NPN bipolar junction transistor T3, relieving the voltage at the base of a similar NPN bipolar junction transistor T1, and allowing current to bypass T1 and flow directly through a low value resistor R5. In an embodiment of the present invention, resistor R5 has a value between 10-100 Ohms. This stops the oscillation of the system and pulls the gates of power transistors, Q1 and Q5 to ground. In an embodiment of the present invention, transistors Q1 and Q5 are Field Effect Transistors (FETs). Since the source of input voltage, 3 is supplied through the center tap on transformer 5 and both transistors Q1 and Q5 are shut down, current ceases to flow through the system.

[0026] The clamp's operation relies on the simple action of a voltage divider, which is amplified by the means of the gain on a transistor's base. When the voltage of the divider fed by the high voltage output 1 exceeds a certain level, the clamping mechanism shuts down the oscillator. The clamp performs its function through the following procedure: power is supplied to the low voltage input 3 where the current passes through the primary inductor 4 and into the center tap of the transformer 5. Simultaneously, current flows through resistors R3 and R4. This turns on the base of transistor T1 which also pulls down the base of PNP transistor T2, allowing a current to flow into the gates of Q1 and Q5. Due to miniscule differences in the structure of FETs Q1 and Q5 and miniscule differences in the resistance of R1 and R8, one of the two FETs will turn on slightly faster than the other, allowing for current to begin flowing through one FET first. In an alternate embodiment of the present invention, this asymmetry can be assisted by adding a resistor between the gate and source of transistor Q1 to induce a discrepancy between the transistors Q1 and Q5. This action immediately pulls down the gate of the opposing transistor through the gate diodes D1 or D4. A resonating system is created by the action of the power transistors Q1 and Q5, the low voltage tank capacitor C11 and the primary inductor 4. This oscillation continues until the desired output voltage at 1 is reached or the low voltage source 3 current is removed.

[0027] The desired output voltage is clamped by setting a variable resistor R16 between the value of resistor R9 and zero ohms, using the resistance of R15 as a backstop to prevent the resistance to ground from reaching zero. In one embodiment of the present invention, resistor R16 is an analog potentiometer having a resistance range between 0-10,000 Ohms and resistor R9 is a surface mount or through hole resistor having a resistance of 10,000 Ohms. In one embodiment of the present invention, resistor R10 is a surface mount or through hole high voltage resistor having a resistance of 1,000,000 Ohms. In an alternate embodiment of the present invention, R10 is a series of lower value resistors having a combined resistance in the neighborhood of 1,000,000 Ohms. In alternate embodiments of the invention, resistor values of R16, R9, and R10 may be scaled down or up proportionally, or adjusted individually to achieve the desired clamping behavior.

[0028] Capacitor C4 is a smoothing capacitor designed to filter out high frequency noise from the output stage 1 and prevent undue effect on the sensitive base of transistor T3. In one embodiment of the present invention, capacitor C4 is a surface mount multi-layer ceramic capacitor having a capacitance of between 0.01 and 10 micro-Farads (F) and a voltage rating of over 5V. In alternate embodiments, the capacitor C4 may be of film, electrolytic, tantalum, polymer or electric double layer type, or any other future type of capacitor having a capacitance of 0.01-10 F and a voltage rating over 5V. In alternate embodiments of the invention, an alternate method of hysteresis may be employed using a Schmidt trigger, comparator or other system of transistors, inductors, discrete integrated circuits and/or microcontrollers to passively or actively feed back the output voltage into the logic which controls the primary side oscillating system thereby shutting down or varying the oscillation frequency of the power transistors to control the ultimate output voltage on the secondary side.

[0029] In one embodiment of the present invention, transistors T1 and T3 are of type bipolar NPN with a V_ce between 20 and 150V, and maximum collector current rating between 100 mA and 20 A. In an embodiment of the present invention, transistors T1 and T3 are Diodes Incorporated part number ZXTN19100CFFTA. In one embodiment of the present invention, transistor T2 is of type PNP with a voltage rating between V_ce between 20 and 150V, and a maximum collector current rating between 50 mA and 10 A. In an embodiment of the present invention, transistor T2 is a Diodes Incorporated part number ZXTP19100CFFTA.

[0030] The power transistors Q1 and Q5 in the present invention are shown as N-Channel Metal On Silicon Field Effect Transistors (MOSFETs). They are high current-rated standard level FETs designed to pass at least 50A of current through their Drain and Source during normal operation. Their gate charge must be sufficiently low so as to allow the delicate oscillator and clamping circuit to actuate the FETs at frequencies up to 100 kHz. Their reverse breakdown voltage must be sufficient to allow for spikes of up to 2 the input voltage (3) to be switched without causing breakdown of the FETs. The gates of the switches are connected to diodes D1 and D4 as well as resistors R1 and R8. The function of the resistors is to limit the inrush and outrush current to the switch gates. The function of the diodes is to facilitate oscillation as described above.

[0031] In one embodiment of the present invention, diodes D1 and D4 are general purpose silicon rectifier diodes with a reverse breakdown voltage of between 100-1000V and a forward current between 50 mA and 10 A. In one embodiment of the present invention, such diodes are of type 1N4148 general purpose rectifier diodes. In an alternate embodiment of the present invention, diodes D1 and D4 are Schottky diodes with reverse breakdown 50-250V and forward current rating between 50 mA and 10 A. In an alternate embodiment of the present invention, diodes are of type Schottky part number SL110PL-TP. This is in contrast to the popular Mazzilli ZVS circuit which uses Zener diodes with a reverse (Zener) breakdown voltage of 10-20V. In the present invention, such diodes are NOT designed to conduct current in their reverse direction and their breakdown voltage should be sufficient to prevent reverse breakdown.

[0032] In an embodiment of the present invention, gate resistors R1 and R8 are of similar values between 100-1000 Ohms. In an alternate embodiment of the present invention, resistors R1 and R8 are of different values slightly offset by 10-250 Ohms such to facilitate the system's oscillation at higher frequencies necessitating higher current to the power transistors' Q1 and Q5 gates. In an embodiment of the present invention, resistors R1 and R8 are surface mount resistors both rated at 330 Ohms.

[0033] In one embodiment of the present invention, power transistors Q1 and Q5 are of N-channel type FETs and have a minimum drain to source breakdown voltage of Vds>50V, and a pulsed drain current over 40A. In one embodiment of the present invention, such FETs have total Qg below 110 nC. In one embodiment of the present invention, such FETs are Texas Instruments CSD19505KCS 80 V N-Channel NexFET Power MOSFETs. In an embodiment, such FETs are affixed to heatsinks. In an alternate embodiment, the FETs are of DPAK, D2PAK or other surface mount package and are configured to dissipate heat through the internal and/or external copper layers of the printed circuit board substrate.

[0034] The transformer 5 is a critical component of the system. Its size, form factor, inductance and turns ration are all critical to the function of the present invention. In an embodiment of the present invention, the transformer consists of a 4:4:150 turn IE-type ferrite core transformer with a center tap on the primary winding. The transformer must have sufficiently high gauge wire so as to pass at least 40 A of current through the primary when operating at its design frequency of 30-100 kHz. The transformer must be sufficiently isolated to prevent internal arcing between the layers. In an embodiment of the present invention, the transformer is HI-POT insulated to at least 1500V RMS for at least 60 s at 1 MHz. In an embodiment of the present invention, the inductance of the secondary winding is between 4-12 mH. In an embodiment of the present invention, the effective length of transformer core is between 15-75 mm. In an embodiment of the present invention, the core material is ferrite. In an alternate embodiment, the core material is Carbonyl SF. In an alternate embodiment, the core material is Permalloy. In an alternate embodiment, the core material is Sendust. In an alternate embodiment, the core material is laminated silicon-steel sheets. In an alternate embodiment, the transformer is air-cored. In an alternate embodiment, the transformer is cored with a ferromagnetic, paramagnetic, diamagnetic, dielectric or conductive material or combination of dielectric, conductive, ferromagnetic or diamagnetic or paramagnetic material(s) such as in a planar transformer. In an embodiment, the transformer is square or rectangular in shape. In an alternate embodiment, the transformer is circular or of an oblong circular-rhombus shape. In an embodiment, the transformer is sintered from a ceramic powder. In an embodiment, the windings are made from copper. In an embodiment, the insulation between the windings is polyimide or fluorocarbon based. In an embodiment, the insulation is PVC-based.

[0035] The rectifier B2 in the present invention is responsible for converting the high frequency AC signal into DC which can be stored by a capacitor in the output. The rectifier (and output capacitor as shown in FIG. 2) are both necessary in order for the clamp to properly regulate the operation of the device. If the rectifier AND output capacitor are not present, the CQR Inverter will be constantly active, and may overheat if not properly designed. The rectifier must have a sufficiently high breakdown voltage to rectify voltages above 500V. The rectifier must be capable of passing sufficiently high current through to the outputat least 2 A. The rectifier must be capable of ultra-fast recovery so as to rectify the high frequency output of the transformer 5. The rectifier must be capable of rectifying AC at frequencies over 10 kHz.

[0036] In an embodiment of the invention, the rectifier B2 may consist of a general purpose silicon bridge rectifier in a single four pin package format. In an alternate embodiment, the rectifier may consist of two, four, or more discrete semiconductor packages. In an alternate embodiment, the rectifier may consist of an array of silicon-carbide Schottky diodes which offer ultra-fast recovery, high voltage and high amperage capabilities. In an alternate embodiment, the rectifier may consist of fast recovery general purpose silicon diodes such as 1.2 kV/8A TO220 package diodes of part number STTH8S12D, or other fast recovery diodes with rated voltage 100V-10 kV or current above 100 mA (average rectified). In an alternate embodiment, the rectifier may be comprised of the body diodes. In an alternate embodiment, the rectification may be performed by actively switched MOSFETs, power BJTs or IGBTs. In an alternate embodiment, the rectifier may be actively or passively switched. In an embodiment of the present invention, the rectifier B2 is a general purpose bride rectifier of type KBL410-G with a 4 A/1 kV single phase rating. In an alternate embodiment of the present invention, the rectifier B2 is a bridge configuration of four discrete fast recovery diodes of type UF5408, each having a 3 A/1 kV single phase rating. In an alternate embodiment, the rectifier is of type GBU6K with a 6 A/800V single phase rating.

[0037] In an embodiment of the present invention, the primary inductor 4 has an inductance rating between 1-1000 micro-Henries (H) with a current rating over 4 A-DC. In an embodiment of the present invention, the primary inductor is a Bourns Inc. 2300LL-Series 100 H toroidal power inductor.

[0038] The CQR inverter itself 8 is represented in FIG. 2 as a sub-system block in a larger system. From an overarching system perspective, the CQR Inverter 8 is connected to a large load capacitance 9 which is connected to a common ground as well as the low voltage input 3this common low voltage input ground is shown by the ground symbol in FIG. 2.

[0039] The load capacitance 9 may be any capacitive, resistive, or combination of capacitive or resistive elements in parallel or series. Inductive loads may also be possible. Assuming load 9 is a capacitive or resistive load connected between the high voltage output 1 and ground, the inverter 8 will attempt to maintain the positive terminal of load 9 at a set voltage as determined by the clamping mechanism of the individual inverter 8. This is useful in a capacitor charging application as the load capacitance 9 can be arbitrarily large.

[0040] In an alternate embodiment of the present invention as shown in FIG. 3, two or more inverters 8 may be assembled in a parallel configuration with their inputs 3, outputs 1 and grounds tied together to drive a common load 9. In such a configuration, care must be taken to set the individual inverters' 8 clamps to the same desired voltage, to avoid powering one inverter constantly while the other sits idle. This can be accomplished manually, digitally or via analog methods. If the clamps are set properly, the load power will be effectively distributed between the two or more inverters 8, thus multiplying the output current of the overall system by the number of parallel inverters 8. Such a configuration is useful to drive very large capacitive loads such as often required in defense or aerospace applications.

[0041] In an alternate embodiment as shown in FIG. 4, one or more CQR inverters 8 may be configured such that a load capacitance and load resistance are placed in parallel between the high voltage DC output 1 and ground. High voltage AC outputs 6 and 7 can then be tapped to provide power for a high voltage-high frequency application such as an arc welder where AC is desired and high frequency is preferable due to its ease in forming electrical arcs. In such an embodiment, the load 9 capacitance may be very small and the load resistance very large, providing an effective open circuit on the DC output, forcing all the current to flow as high frequency AC through an external load attached to AC outputs 6 and 7. In an alternate embodiment, the load 9 capacitance may be very large and the load 9 resistance used as a resistance or arc welder itself if the user desires a DC current for their welding application.

[0042] In yet another embodiment, FIG. 5 shows how the system may be controlled by a microcontroller 12 which is powered by an independent Low-Dropout Regulator (LDO) 11 or one of many similar regulated linear or switching power supplies known to those of ordinary skill. In an embodiment of the invention, the microcontroller may control a switch 15 which supplies current to an H-Bridge 10, depleting the charge in the capacitor 9 to drive an ultimate load 13. This simultaneously allows the H-Bridge 10 to begin inverting the DC output into AC, and signals the CQR 8 to begin charging the capacitive inverter load 9 back up to its setpoint voltage. The H-Bridge may switch DC at a desired frequency and with the desired number of phases to drive an inductive load or resistive ultimate load 13. This is especially useful in driving motors for household appliances or electric vehicles, or for coupling large DC power sources (solar-powered battery farms) to the AC power grid at a precisely defined frequency at a given phase angle as regulated by the microcontrollerthis may be advantageous in the case of a grid-tie inverter. The microcontroller may also act in such a fashion as to regulate the output voltage by means of a digitally controlled potentiometer in the place of potentiometer R16.

[0043] In another embodiment, FIG. 6 shows how the system in FIG. 5 may be connected to a battery or bank of batteries 14 such as in a solar, electric vehicle, or electric spacecraft propulsion application.

[0044] In a current embodiment of the invention, FIG. 9 shows an oscilloscope plot obtained by connecting a 6S Lithium Polymer battery at 25.2V to the low voltage input 3 of the present invention through a 0.1 Ohm shunt resistor which was probed by the oscilloscope to obtain the graph shown in the figure. The high voltage output 1 was connected to a 12 mF, 500V rated electrolytic capacitor sharing a common ground. FIG. 10 shows an oscilloscope plot obtained by connecting a 6S Lithium Polymer battery at 25.2V to the low voltage input 3 of the present invention, and connecting the high voltage output 1 to a 12 mF, 500V rated electrolytic capacitor sharing a common ground. The oscilloscope was connected to the positive terminal of the capacitive load 9 and shared a common ground with the capacitor and battery.

[0045] Substitute specification is submitted herein for consideration by the Office. The substitute specification contains no new matter.

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