Abstract:
Preferred embodiments of the present invention provide systems and methods that automatically correct the desired on-time of switching elements as the resonant frequency changes, so as to maintain the correct proportional value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 11/844,789, filed Aug. 24, 2007, which claims priority to Provisional application Ser. No. 60/840,046, filed Aug. 25, 2006. The entire contents of the above-identified applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods for controlling RF output power. 
     2. Discussion of the Background 
     To address a variety of applications, RF power supplies (e.g., RF power supplies used in Induction heating) must employ a method of varying RF output power. There are many ways to accomplish this. Some architectures simply supply a variable DC source to the RF output power stage of the power supply. The result has the advantage of a continuous (rather than discontinuous) RF output with a variable and easily measurable amplitude whose heating effects and harmonics are predictable. Disadvantages of this method are that it is costly, complex and generally requires twice the silicon (in power semiconductors) than is required by a direct off-line architecture. 
     Varying (controlling) the RF power level using a direct off-line architecture is less costly, however this approach presents a different set of challenges to the designer. Because the DC feed to the RF output power stage (or “final power stage”) cannot be directly controlled (having been simply derived directly from the AC line), another method of RF output regulation must be found. Commonly, Pulse Width Modulation (PWM) is chosen as means to accomplish this. 
     In PWM schemes, the final power stage is allowed to conduct during a controlled portion of each full RF cycle. Ignoring dead time between each half of the switching cycle, full power is generated when the output switching devices of the final power stage are allowed to conduct 100% of their allotted “on time,” while lowering the power output is done by reducing the amount of time that the output switching devices are allowed to conduct. At 0% power they conduct 0% of the time. 
     If the RF frequency does not change, power can be controlled by simply allocating an absolute amount of time that the switching devices are allowed to conduct, rather than a proportional value. 
     In order to maintain reliable operation without greatly increasing power device costs, RF induction power supplies must appropriately respond to varying loads. To maintain high efficiencies, Induction power supplies typically resonate a capacitor or heating coil in combination with the part to be heated. As the part is heated, the natural resonant frequency of the system is affected. To be cost effective and reliable, induction supplies must respond by sensing and adjusting (varying) the operating frequency to maintain resonant operation. 
     To maintain a constant output power, the specific amount of time that switching devices are allowed to conduct (i.e., the “on-time”) must be corrected for as resonant frequency (f r ) changes. More specifically, to maintain a constant power output, the switching devices must be allocated an on-time that is proportional to the cycle time (cycle time=1/f r ). 
     This can be done in a closed loop control circuit by sensing the output voltage and adjusting the switching devices on-time. Doing this affects resonant frequency sensing so that two loops are interactive and typically require compensation to slow their individual response. Another way to accomplish proportional on-time is through prediction, allowing the two loops to run independently, providing a much faster response to changing loads. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide systems and methods that automatically correct the desired on-time as the resonant frequency changes, so as to maintain the correct proportional value. 
     In some embodiments, the system includes: a voltage controlled oscillator configured to receive a tune voltage and configured to output a first signal having a frequency that corresponds to the tune voltage; a charge accumulating circuit; a voltage controlled current source configured to receive the tune voltage or a scaled tune voltage and configured to charge the charge accumulating circuit by outputting a current that corresponds to the received tune voltage or the received scaled tune voltage; a comparator configured to generate a second signal by comparing a voltage of the charge accumulating circuit to a pre-determined set point; and a circuit configured to receive the first signal and the second signal and configured to output a third signal that is a function of the first signal and the second signal, wherein the output of the circuit is coupled to the switching devices. 
     The system, in some embodiments, may also include an analog voltage scaling circuit, an edge detector circuit and a reset circuit, wherein the edge detector circuit and the reset circuit are configured to discharge the charge accumulating circuit at the beginning of each cycle of the first signal, and the analog voltage scaling circuit is configured to receive the tune voltage and output to the current source a scaled tune voltage. 
     The comparator may be configured to output a first voltage in response to the comparator determining that the voltage of the charge accumulating circuit is less than a pre-determined set point, and may be configured to output a second voltage in response to the comparator determining that the voltage of the charge accumulating circuit is greater than a pre-determined set point. The circuit that is configured to output the third signal may include or consist of a logical circuit that implements an AND function. And the charge accumulating circuit may include or consists of a capacitor. 
     In some embodiments, the method includes: producing a tune voltage; producing a first signal having a frequency that is dependent on the tune voltage; charging a charge accumulating circuit using the tune voltage or a scaled tune voltage; producing a second signal, wherein the step of producing the second signal comprises comparing a voltage of the charge accumulating circuit to a pre-determined set point; using the second signal to gate the first signal to the switching devices. The method may also include the act of discharging the charge accumulating circuit at the beginning of each cycle of said first signal, and using a scaling circuit to produce said scaled tune voltage. The step of producing the second signal may include: outputting a first voltage if the voltage of the charge accumulating circuit is less than a pre-determined set point; and outputting a second voltage if the voltage of the charge accumulating circuit is greater than the pre-determined set point. The step of using the second signal to gate the first signal to the switching devices may include: ANDing the first signal with the second signal to produce a third signal; and using the third signal to drive the switching devices. 
     The above and other features of embodiments of the present invention are described below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the specification, help illustrate various embodiments of the present invention. In the drawings, like reference numbers indicate identical or functionally similar elements. 
         FIG. 1 . is a block diagram illustrating a system according to an embodiment of the invention. 
         FIG. 2  is a flow chart illustrating a process according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a system  100  according to an embodiment of the invention. System  100  automatically allocates to switching devices (e.g., power transistors)  192  an on-time that is proportional to the cycle time. 
     According to an embodiment of the present invention, resonant frequency is constantly adjusted for in a closed loop that employs a voltage controlled oscillator (VCO)  102  that produces a drive signal  120 . A VCO tune voltage  101 , which is received by VCO  102  and controls the frequency of the drive signal  120  outputted by VCO  102 , is linearly proportional to operating (resonant) frequency. This tune voltage  101  is received by and used to control a current source  106  that charges a circuit  108  that accumulates charge. Circuit  108 , in preferred embodiments is a capacitor or includes one or more capacitors. Without limiting the invention, circuit  108  shall be referred to herein as “capacitor  108 .” As illustrated in  FIG. 1 , tune voltage  101  may be processed by an analog voltage scaling circuit  105  prior to being received by voltage controlled current source  106 . 
     Because the charging current  107  produced by current source  106  is proportional to the received tune voltage  101 , which is proportional to the resonant frequency (f r ), the charge rate of capacitor  108  is directly proportional to the resonant frequency. Thus, capacitor  108  is used to create voltage ramps  156 , the slope of which is directly proportional to the frequency of drive signal  120 . 
     As further illustrated in  FIG. 1 , edge detector circuit  110  in combination with reset/dead time generation circuit  111  function to discharge capacitor  108  (e.g., set the voltage of capacitor  108  to 0V at the beginning of each cycle). Thus, an increase in the frequency of drive signal  120  results in a greater capacitor  108  charging current, a greater slope to the linear voltage ramp  156 , and the same voltage at the end of the RF cycle. In other words, the slope of each voltage ramp  156  adapts with frequency to reach the same voltage level at the end of each cycle. In other words, as the rate capacitor  108  is current charged is proportional to the operating frequency (i.e., the frequency of the signal produced by VCO  102 ), the capacitor  108  reaches the same voltage level just before discharge, regardless of the operating frequency. 
     As illustrated in  FIG. 1 , a comparator  115  compares the voltage of capacitor  108  to a pre-set voltage value  117 . Comparator  115  is configured so that it outputs a first voltage (e.g., 0 volts) if the voltage of capacitor  108  is greater than or equal to the pre-set voltage value  117  and outputs a second voltage (e.g., 5 volts) if the voltage of capacitor  108  is less than the pre-set voltage value  117 . Thus, comparator generates a square wave signal  119  whose duty cycle is proportional to the pre-set voltage level  117 , regardless of the slope of the ramp. This signal  119  is used to gate (enable) the drive signal  120  to the power switching devices, thereby controlling their duty cycle in proportion to a desired set point, regardless of frequency. 
     The result yields a controllable on-time of the switching devices that automatically corrects for frequency operation, and avoids additional loop delays, complexity and cost. 
     Referring now to  FIG. 2 ,  FIG. 2  is a flow chart illustrating a process  200  according to an embodiment of the invention. Process  200  may begin in step  202  where a tune voltage (e.g., tune voltage  101 ) is provided to a VCO (e.g., VCO  102 ). In response to receiving the tune voltage, the VCO produces a drive signal (e.g., a square wave drive signal) having a frequency that is dependent on and proportional to the tune voltage. 
     In step  204 , the tune voltage is used to control a current source that charges a charge accumulating circuit (e.g., circuit  108 ). As described above, the step of using the tune voltage to control the current source may comprise scaling the tune voltage using a scaling circuit (e.g., an analog scaling circuit or other scaling circuit) and then using the scaled tune voltage to control the current source. 
     In step  206 , the charge accumulating circuit is discharged at the beginning of each cycle of the drive signal. In step  208 , a signal whose duty cycle is proportional to a set point is generated. The step of generating this signal includes comparing the voltage across the charge accumulating circuit to the set point. 
     In step  210 , the signal produced in step  208  is used to gate (enable) the drive signal  120 . For example, the signal produced in step  208  and the drive signal may be input to a logic “AND” circuit  191  to produce an output signal  195  that is a function of the signal produced in step  208  and the drive signal. 
     In step  212 , the signal produced in step  210  is used to drive the power switching devices  192 , thereby controlling their duty cycle in proportion to a desired set point, regardless of frequency. 
     While various embodiments/variations of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Further, unless stated, none of the above embodiments are mutually exclusive. Thus, the present invention may include any combinations and/or integrations of the features of the various embodiments. 
     Additionally, while the process described above and illustrated in the drawings is shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed simultaneously.