Abstract:
An electronic high frequency induction heater driver, for a variable spray fuel injection system, uses a zero-voltage switching oscillator that utilizes a full H-bridge topology wherein the semiconductor switches are synchronized within the bridge for function. The induction heater driver, upon receipt of a turn-on signal, multiplies a supply voltage through a self-oscillating series resonance, wherein one component of the tank resonator circuit comprises an induction heater coil magnetically coupled to an appropriate loss component so that fuel inside a fuel component is heated to a desired temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional of, and claims priority to the Apr. 22, 2011, filing date of, U.S. provisional patent application Ser. No. 61/478,350, entitled Synchronous Full-Bridge Power Oscillator, the entire content of which is incorporated herein by reference. 
     And this application is related to the following U.S. non-provisional patent applications filed on the same day as this application: 
     Synchronous Full-Bridge Power Oscillator with Leg Inductors, invented by Perry Czimmek, and identified by Ser. No. 13/332,501; 
     Synchronized Array Bridge Power Oscillator, invented by Perry Czimmek and Mike Hornby, and identified by Ser. No. 13/332,517; 
     Synchronized Array Power Oscillator with Leg Inductors, invented by Perry Czimmek and Mike Hornby, and identified by Ser. No. 13/332,521; 
     Variable Spray Injector with Nucleate Boiling Heat Exchanger, invented by Perry Czimmek and Hamid Sayar, and identified by Ser. No. 13/332,532; and 
     Adaptive Current Limit Oscillator Starter, invented by Perry Czimmek, and identified by Ser. No. 13/332,539. 
    
    
     BACKGROUND 
     Embodiments of the invention relate generally to heated tip fuel injectors, and more particularly, to controlling and driving an induction-heated fuel injector. 
     There is a continued need for improving the emissions quality of internal combustion engines. At the same time, there is pressure to minimize engine crank times and time from key-on to drive-away, while maintaining maximum fuel economy. Those pressures apply to engines fueled with alternative fuels such as ethanol as well as to those fueled with gasoline. 
     During cold temperature engine start, the conventional spark ignition internal combustion engine is characterized by high hydrocarbon emissions and poor fuel ignition and combustibility. Unless the engine is already at a high temperature after stop and hot-soak, the crank time may be excessive, or the engine may not start at all. At higher speeds and loads, the operating temperature increases and fuel atomization and mixing improve. 
     During an actual engine cold start, the enrichment necessary to accomplish the start leaves an off-stoichiometric fueling that materializes as high tail-pipe hydrocarbon emissions. The worst emissions are during the first few minutes of engine operation, after which the catalyst and engine approach operating temperature. Regarding ethanol fueled vehicles, as the ethanol percentage fraction of the fuel increases to 100%, the ability to cold start becomes increasingly diminished, leading some manufacturers to include a dual fuel system in which engine start is fueled with conventional gasoline and engine running is fueled with the ethanol grade. Such systems are expensive and redundant. 
     Another solution to cold start emissions and starting difficulty at low temperature is to pre-heat the fuel to a temperature where the fuel vaporizes quickly, or vaporizes immediately (“flash boils”), when released to manifold or atmospheric pressure. Pre-heating the fuel replicates a hot engine as far as fuel state is considered. 
     A number of pre-heating methods have been proposed, most of which involve preheating in a fuel injector. Fuel injectors are widely used for metering fuel into the intake manifold or cylinders of automotive engines. Fuel injectors typically comprise a housing containing a volume of pressurized fuel, a fuel inlet portion, a nozzle portion containing a needle valve, and an electromechanical actuator such as an electromagnetic solenoid, a piezoelectric actuator or another mechanism for actuating the needle valve. When the needle valve is actuated, the pressurized fuel sprays out through an orifice in the valve seat and into the engine. 
     One technique that has been used in preheating fuel is to inductively heat metallic elements comprising the fuel injector with a time-varying magnetic field. Exemplary fuel injectors having induction heating are disclosed in U.S. Pat. No. 7,677,468, U.S. Patent Application No&#39;s: 20070235569, 20070235086, 20070221874, 20070221761 and 20070221747, the contents of which are hereby incorporated by reference herein in their entirety. The energy is converted to heat inside a component suitable in geometry and material to be heated by the hysteretic and eddy-current losses that are induced by the time-varying magnetic field. 
     The inductive fuel heater is useful not only in solving the above-described problems associated with gasoline systems, but is also useful in pre-heating ethanol grade fuels to accomplish successful starting without a redundant gasoline fuel system. 
     Because the induction heating technique uses a time-varying magnetic field, the system includes electronics for providing an appropriate high frequency alternating current to an induction coil in the fuel injector. 
     Conventional induction heating is accomplished with hard-switching of power, or switching when both voltage and current are non-zero in the switching device. Typically, switching is done at a frequency near the natural resonant frequency of a resonator, or tank circuit. The resonator includes an inductor and capacitor that are selected and optimized to resonate at a frequency suitable to maximize energy coupling into the heated component. 
     The natural resonant frequency of a tank circuit is fr=1/(2π√{square root over (LC)}), where L is the circuit inductance and Cis the circuit capacitance. The peak voltage at resonance is limited by the energy losses of the inductor and capacitor, or decreased quality factor, Q, of the circuit. Hard-switching can be accomplished with what are called half-bridge or full-bridge circuits, comprising a pair or two pairs of semiconductor switches, respectively. Hard-switching of power results in the negative consequences of switching noise, and high amplitude current pulses at resonant frequency from the voltage supply, or harmonics thereof. Also, hard switching dissipates power during the linear turn-on and turn-off period when the switching device is neither fully conducting nor fully insulating. The higher the frequency of a hard-switched circuit, the greater the switching losses. 
     The preferred heater circuit therefore provides a method of driving a heated fuel injector wherein switching is done at the lowest possible interrupted power. This heater circuit was disclosed in U.S. Pat. No. 7,628,340, Title: Constant Current Zero-Voltage Switching Induction Heater Driver for Variable Spray Injection. Ideally, energy should be replenished to the tank circuit when either the voltage or the current in the switching device is zero. It is known that the electromagnetic noise is lower during zero-voltage or zero-current switching, and is lowest during zero-voltage switching, this is the method of U.S. Pat. No. 7,628,340. It is also known that the switching device dissipates the least power under zero switching. That ideal switching point occurs twice per cycle when the sine wave crosses zero and reverses polarity; i.e., when the sine wave crosses zero in a first direction from positive to negative, and when the sine wave crosses zero in a second direction from negative to positive. 
     It is preferable to reduce the size of inductive components and in some cases, eliminate the impedance-matching transformer, while maintaining the minimum necessary connections to the inductive heater coil on the injector. The difficulties of this will be detailed in this specification in subsequent text. Embodiments of the invention continue to provide for the elimination of the hard-switching and its negative consequences, replace it with zero-voltage switching, and further apply this method in a full-bridge topology while advantageously eliminating the impedance matching transformer and overcoming the difficulties of alternative solutions. 
     Referring to  FIG. 3 , the description disclosed in U.S. Pat. No. 7,628,340 includes a center-tapped impedance matching transformer that those skilled in the art will be familiar with as to the additional cost this topology might present. 
     Referring to  FIG. 4 , elimination of the impedance matching transformer is possible and practical, but with the result that the induction heater coil uses a center-tap to satisfy the push-pull oscillator topology. This center-tap adds an additional conductor and pin to the inductively heated fuel injector. 
     Referring to  FIG. 5 , the further elimination and change of the topology of the push-pull oscillator follows from the  FIG. 4  topology, however, it uses two constant-current inductors of a relatively much higher inductance compared to the induction heater coil to allow sufficient current to flow through the impedance of the heater coil, otherwise the majority would flow only through the power switches. In order to maintain sufficient heater current, this tends to result in a small inductance of the heater coil rather than a reduction of constant-current inductance. This reduction of heater coil inductance tends to the point that parasitic inductance of the wire harness and connections may overwhelm that of the heater coil. Additionally, in order to obtain a small heater coil inductance, the number of coil turns is reduced for a given magnetic mass available, possibly sacrificing Ampere-Turns per Hertz and thereby detrimentally affecting the ability to perform induction heating. 
     BRIEF SUMMARY 
     Embodiments of the invention eliminate the impedance matching transformer. Further, embodiments of the invention eliminate the center-tap of the induction heating coil such that only two conductors are used for transmission of power. Additionally, embodiments of the invention force current sharing through the induction heating coil while allowing for flexibility and suitable inductance and ampere-turns of the induction heater coil. 
     One or more embodiments use two pairs of complimentary pairs of power switching transistors in a full-bridge, or H-bridge, configuration. A deviation from a full-bridge driver is that the bridge is fed from a constant-current source inductor, and the load section of the conventional full bridge is replaced with a resonant tank circuit. Further deviation from a conventional full-bridge is an oscillator-synchronous inherent zero-switching topology that drives the gates of the complimentary pairs of transistors in alternating sequence of diagonal pairs. 
     Additionally, the tank-replenishment current passes through the induction heater coil, and the conventional center-tap current feed in the original push-pull oscillator, wherein only half the coil at a given cycle received the tank-replenishment current, is eliminated. Eliminating the center-tap in this way provides a minimum wire solution to driving the induction heater coil without an impedance matching transformer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified electrical schematic diagram showing a synchronous bridge oscillator with source inductor and without a transformer and without a center-tap in accordance with embodiments of the invention. 
         FIG. 2  is a simplified electrical schematic diagram showing a synchronous bridge oscillator with sink inductor and without a transformer and without a center-tap in accordance with embodiments of the invention. 
         FIG. 3  is a simplified schematic diagram depicting the prior art with a center-tapped impedance matching transformer in a push-pull oscillator topology. 
         FIG. 4  is a simplified schematic diagram depicting the prior art with a center-tapped induction heater coil and elimination of the impedance matching transformer in a push-pull oscillator topology. 
         FIG. 5  is a simplified schematic diagram depicting an alternative solution to the elimination of the impedance matching transformer and elimination of heater coil center-tap. 
         FIG. 6  is an alternative schematic layout of  FIG. 1 , with an added low-side control switch as a demonstration of an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Ideally, energy should be replenished to the tank circuit when either the voltage or the current in the switching device is zero. The electromagnetic noise is lower during zero-voltage or zero-current switching and is lowest during zero-voltage switching. The switching device dissipates the least power under zero switching. That ideal switching point occurs twice per cycle when the sine wave crosses zero and reverses polarity; i.e., when the sine wave crosses zero in a first direction from positive to negative, and when the sine wave crosses zero in a second direction from negative to positive. 
     Embodiments of the invention eliminate hard-switching and its negative consequences, and replace it with zero-voltage switching in a full-bridge configuration. The integrated functions of the synchronous full-bridge power oscillator heater driver of the invention will be explained with reference to  FIG. 1 , which is a simplified representation of a circuit in accordance with embodiments of the invention with many of the basic components not shown for clarity. Specific or general values, ratings, additions, inclusion or exclusion of components are not intended to affect the scope of the invention. 
     L 1  may be located inside a fuel injector. L 1  is an induction heater coil that provides ampere-turns for induction heating a suitable fuel-injector component. 
     A synchronous full-bridge power oscillator in accordance with embodiments of the invention may include R 1 , R 2 , D 1 , D 2 , Q 1 , Q 2 , Q 3 , Q 4 , L 2 , C 1  and L 1 . Q 1  and Q 2  are enhancement-type N-MOSFET (N-channel Metal-Oxide-Semiconductor Field-Effect Transistor) switches that alternatively connect tank resonator, C 1  and L 1 , circuit to ground and, when each is turned on in the respective state, enables current to flow through induction heater coil and ground. Q 3  and Q 4  are enhancement-type P-MOSFET (P-channel Metal-Oxide-Semiconductor Field-Effect Transistor) switches that alternatively connect tank resonator, C 1  and L 1 , circuit to the voltage supply sourced through L 2 , and with Q 1  and Q 2  in the appropriate state, enable current to flow through induction heater coil. The voltage supply may be a power supply, or in the case of a vehicle, a battery or an alternator, and is a source of potential energy to replenish energy lost in the oscillator. 
     C 1  and L 1  are the tank resonator capacitor and tank resonator inductor, respectively, of a resonant tank circuit. The resonant frequency of the tank circuit is fr=1/(2π√{square root over (LC)}), where L is the heater coil inductance L 1  and C is the capacitance of tank capacitor C 1 . The peak voltage in the tank circuit is set by V out =π*V in  where V in  is the supply voltage. The current level in the tank circuit is determined from the energy balance of 
     
       
         
           
             
               
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     The zero-switching power oscillator circuit is self-starting in oscillation, but may be forced into oscillation by selectively sequencing the switching of Q 1 -Q 4  in a full-reversing H-bridge strategy. The complimentary pairs, or here, the pairs of transistors that are flowing current between the MOSFET ‘drain’ and ‘source’ at the same time are Q 3  and Q 2  or Q 4  and Q 1 . It is not desirable to have Q 1  flowing current when Q 3  flows current, and likewise, it is not desirable to have Q 2  flowing current when Q 4  flows current. When Q 3  is flowing current, current passes through the induction heater coil and then through Q 2  to ground. When Q 4  is flowing current, current passes through the induction heater coil in the reverse direction as when Q 3  was flowing current, and then through Q 1  to ground, this is ‘full-reversal’ of current. 
     A MOSFET is a device that has a threshold for an amount of Coulomb charge into the gate, which is drain-source current-dependent. Satisfying the charge threshold enhances the device into an ‘on’ state. First and second gate resistors R 1 , R 2  supply the gate charging current to first and second legs of the H-bridge. R 1  supplies current to gates of Q 1  and Q 3 , R 2  supplies current to the gates of Q 2  and Q 4 , respectively, and R 1 , R 2  limit the current flowing into first and second gate diodes D 1 , D 2 , respectively. Q 3  and Q 4 , P-MOSFET conduct between drain and source when source is more positive than gate. Q 1  and Q 2 , N-MOSFET conduct between drain and source when source is more negative than gate. 
     The loading caused by the resistive and hysteretic loss of the heated component reflects back as a loss in the resonant tank circuit. That loss is replenished by current flowing from a current source inductor L 2  to the top bridge transistors, Q 3  and Q 4 .  FIG. 2  shows a variation with L 2  as a sink inductor from the bottom bridge transistors, Q 1  and Q 2 , those skilled in the art will understand this alternative embodiment as functionally interchangeable overall. Depending on the state of reversal of the H-bridge in which the current flows, the current will flow either through Q 3  or Q 4  and then through induction heater coil L 1 . Referring again to  FIG. 1 , L 2  supplies current to the tank circuit from the energy stored in its magnetic field. That energy is replenished from the supply voltage as a current that constantly flows into L 2  during operation of the synchronous full-bridge power oscillator. L 2  also provides transient separation of tank circuit from voltage source. 
     If current is flowing through Q 3 , as determined by the polarity of the sine wave half-cycle at that time, then the conduction to ground from Q 2  drain-to-source is pulling charge out of the gate of Q 3  and Q 1  through forward biased D 1 . Q 1  is also now not conducting and does not pull the gate charge out of Q 4  and Q 2  to ground through D 2 . Meanwhile R 1  draws current from the supply voltage. But the IR drop across R 1  cannot charge the gate of Q 3  and Q 1  with the gate shunted to ground by conduction through Q 2 . 
     When the sine wave crosses zero, then Q 3  becomes reverse biased and conducts through the internal intrinsic diode to reverse-bias D 1 . D 1  stops conducting current away from the Q 3  and Q 1  gate, and R 1  can charge the gate of Q 3  and Q 1 , which stops conduction in Q 3  and starts conduction in Q 1  to begin conducting current for the continuing sine half-cycle. Q 1  also pulls the gate charge out of Q 2  and Q 4  to ground through D 2  and holds Q 2  in a non-conducting state, which continues to allow R 1  to enhance Q 1 . And Q 4  conducts. 
     That process repeats as the sine wave alternates polarity, crossing zero in a first direction from negative to positive, and then in a second direction from positive to negative. This generates full-reversal of current in L 1 , the induction heater coil. Current continues to be replenished in the tank circuit from L 2 . An IGBT (Insulated Gate Bipolar Transistor) device can replace the N-MOSFET in this embodiment if the intrinsic diode of the N-MOSFET is represented by the addition of an external diode across the drain and source of the IGBT. 
     The foregoing detailed description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the description of the invention, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. For example, while the synchronous full-bridge power oscillator of the invention is described herein driving an induction heater coil for the heater in an internal combustion engine fuel injector, the driver may be used to drive other induction heaters in other applications. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.