Abstract:
Provided is system including a temperature regulator including at least on regulation component in communication with a semiconductor within a converter, and a peak detector in communication with the semiconductor within the converter configured to identify a maximum temperature of each semiconductor. Also provided is a method for regulating temperature change of semiconductor components including measuring a semiconductor temperature, determining a reference temperature when the semiconductor is energized, summing the first semiconductor temperature and the reference temperature to generate a first temperature sum, comparing the first temperature sum to a coolant temperature to generate a first temperature difference. The method also circulates a fluid configured within the system such that the first temperature difference is adjusted.

Description:
FIELD OF INVENTION 
     The present invention relates generally to extending the useful life of a power semiconductor. More specifically, the present invention relates to extending the useful life of a semiconductor when operating as an electronic switch in renewable energy applications. 
     BACKGROUND OF THE INVENTION 
     Renewable energy, harvested from natural phenomena such as sunlight and wind, has replaced conventional fuels in areas of energy service and power generation. Electrical power devices, such as semiconductors, convert energy obtained from renewable energy sources, such as solar power and wind power, from dc or variable frequency ac to utility (grid) frequency. Conversion is generally achieved by switching a plurality of semiconductors on and off at high frequency to synthesize sine wave voltages via pulse width modulation (PWM) techniques. 
     Three-terminal semiconductors, such as insulated-gate bipolar transistor (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), and thyristors are commonly used as switching devices in electrical applications. MOSFETs, used primarily in low voltage applications, may increase the current rating of a switch by placing several devices in parallel. Thyristors, used primarily in high voltage applications, require a finite time delay after the current in the thyristor has extinguished before the anode can again be positively biased and retain the thyristor in the off-state. IGBTs combine the performance in the on-state in high voltage devices with high impedance, thus making it a popular choice in power generation applications. 
     Transitioning each IGBT from the on-state to the off-state and vice versa results in heat production, as power is dissipated within each IGBT during switching events. The higher the current being switched on or off, the more losses, therefore the more heat. As current increases, the conduction losses within each IGBT also increase. Just as the power from renewable energy sources can often vary throughout a day, so can losses dissipated in the IGBTs. Therefore, the temperature of the IGBTs vary with changes in the output of the renewable energy sources. The thermal cycling, which may occur over minutes or hours, can result in mechanical stress and increased rates of IGBT failure over time due to material fatigue. 
     The life of an IGBT can be reduced due to thermal cycles. Thermal cycles, specifically in the baseplate and the bond wire of the switch, among others factors, results in device fatigue. Extreme changes between high and low temperatures decrease IGBT life more than small changes between high and low temperatures. 
     Prior attempts to increase fatigue tolerance within semiconductors are known in the art. For example, conventional approaches include variations of altering the surfaces of the semiconductor to prolong the life of the device. This approach, however, does not prevent cracks from expanding to critical sizes, for example, which can ultimately lead to fractures. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     Given the aforementioned problem, a need exists for systems and methods that prolong the life of a semiconductor by reducing temperature cycling due to variations in the renewable power generation. The systems and methods would prevent crack formations due to repetitive temperature cycles. 
     In the present technology, to keep IGBT heat losses from dropping in proportion to reductions in renewable power production, the converter may increase a PWM switching (chopping) frequency. The converter adjusts the PWM switching frequency inverse to the magnitude of the current. One in the art understands that an infinite variation in the PWM switching frequency is impractical. However, ranges of approximately two-to-one (2:1) to approximately ten-to-one (10:1) are feasible. 
     In one aspect, the present technology discloses a system for regulating temperature change of semiconductor components within a converter comprising (i) a temperature regulator comprising at least one regulation component in communication with at least one semiconductor within the converter, wherein the temperature regulator is in communication with at least one semiconductor within the converter and a power source, and (ii) a peak detector in communication with at least one semiconductor within the converter configured to identify a maximum temperature of each semiconductor. 
     The embodiments, for example, facilitate a reduction in temperature changes in the semiconductor. In some illustrious embodiments, a peak detector can be configured to more accurately identify temperature changes when the semiconductor conducts low current. 
     In other embodiments, a controller can be configured to alter the maximum temperature of the semiconductor, using a temperature regulator, to a temperature that increases useful life of the semiconductor. 
     In another aspect, the present technology discloses a method for determining temperature change of a semiconductor component within a converter comprising (i) measuring, by a temperature regulator, a first semiconductor temperature, at a junction located on the semiconductor component, (ii) calculating a reference temperature to determine a maximum temperature, when the semiconductor conducts high current, and a minimum temperature, when the semiconductor conducts low current, (iii) summing the first semiconductor temperature and the reference temperature to generate a first temperature sum, and (iv) comparing the first temperature sum to a coolant temperature to generate a first temperature difference. 
     In another aspect, the present technology discloses a method for modifying the temperature change of a semiconductor component within a converter comprising (i) measuring, by a temperature regulator, a first semiconductor temperature, at a junction located on the semiconductor component, (ii) calculating, a reference temperature, using a peak detector configured to determine a maximum temperature when the semiconductor conducts high current and a minimum temperature when the semiconductor conducts low current, (iii) summing, the first semiconductor temperature and the reference temperature, (iv) comparing, the first temperature sum to a coolant temperature to generate a first temperature difference, and (v) iterating, the first semiconductor temperature to generate a final semiconductor temperature, configured to adjust the first temperature difference according to power sent to the converter by a power source. 
     Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. 
         FIG. 1  is a block diagram illustrating an exemplary application for a variable frequency, in the form of a wind power converter system. 
         FIG. 2  is a schematic diagram illustrating an inverter in the exemplary application of  FIG. 1 . 
         FIG. 3  is a graph representing a mean time to failure of a semiconductor device of  FIG. 2  operating at different temperatures. 
         FIG. 4  is a block diagram of a controller of the data integration systems for  FIG. 1 . 
         FIG. 5  is a flow chart illustrating methods associated with the controller of  FIG. 4 . 
         FIG. 6  is a block diagram illustrating an exemplary application for a variable frequency, in the form of a solar power converter system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention is described herein with illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility. 
     Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms “first,” “second,” and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The term “or” is meant to be inclusive and mean either, any, several, or all of the listed items. 
     The use of “including,” “comprising,” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. The terms “circuit,” “circuitry,” and “controller” may include either a single component or a plurality of components, which are either active and/or passive components and may be optionally connected or otherwise coupled together to provide the described function. 
       FIG. 1  depicts an exemplary application in the form of a wind power system  100 , suitable for capturing power from wind using turbine blades  102 . The system  100  may include a gearbox  110  connected to a turbine rotor  104  of the turbine blades  102 . The gearbox  110  adapts the relatively low speed of the turbine rotor  104  with the relatively high speed of a generator  120 . 
     The generator  120  (e.g., an induction generator or synchronous generator) converts the mechanical power into electrical power. For example, the generator  120  illustrated in  FIG. 1  may be a doubly fed induction generator (DFIG), which includes a rotor winding  130  and a stator winding  140 . 
     In the illustrated embodiment of  FIG. 1 , the stator winding  140  is connected to a transformer  115 , which transfers electrical power through inductively coupled conductors to a suitable voltage level for an electrical grid. The rotor winding  130  may be connected to the grid by a converter and inverter module (e.g., a converter  150 ) which decouple mechanical and electrical frequencies (e.g., to enable variable-speed operation). 
     The converter and inverter module of the system  100  may include two three-phase converters  150 . One converter  150  may be connected to the rotor winding  130  and the second converter  150  may be connected to the grid by the transformer  115 . The ratio of the voltage produced by the stator winding  140  to the voltage produced by the rotor winding  130  known as the S-R ratio, can be used to determine qualitative factors about the converters  150 , such as, but not limited to, temperature within the converter. 
       FIG. 2  is an example configuration of the converter  150  including one or more semiconductors. The converter  150  may include a plurality of IGBTs  152  and power diodes  154 , each diode  154  being antiparallel to a respective IGBT  152 . Each IGBT  152  and diode  154  are joined to positive or negative DC lines and output lines I, II, or III with bond wires  156 . As seen in the example topology of  FIG. 2 , the output lines I, II, and III may output three-phase voltages v I , v II , and v III . Changes to the output current of the IGBTs  152 , used to produce a three-phase output current waveform at the output  158 , can result in power losses, resulting in a higher junction temperature at the IGBTs  152 . Such junction temperatures may result in mechanical strain and/or deformation of the bond wires  156 , shortening the lifespan of the converters  150 . 
     Cumulative damage (C), due to extreme high or low temperatures, determines semiconductor failure based on the number of thermal cycles and can be defined by the relationship 
               C   =         n   1       N   1       +       n   2       N   2       +   …   +       n   i       N   i           ,         
where n i  is the number of cycles at the i th  stress level and Ni is the number of cycles to failure at the i th  stress level.
 
       FIG. 3  is a graph illustrating a semiconductor lifetime curve where change in temperature of the semiconductor device from ON to OFF is shown along the x-axis. The number of cycles until failure is shown along the y-axis. 
     As seen in  FIG. 3 , reduction in a temperature change (ΔT) by approximately 10° C. improves the semiconductor life by a factor of two. For example, changing the ΔT from 80° C. to 70° C. improves the life from under 100,000 cycles to nearly 200,000 cycles. Similarly, reducing the ΔT from 70° C. to 60° C. improves the life from about 200,000 cycles to more than 400,000 cycles. 
     Referring back to  FIG. 1 , to assist in the regulation of ΔT within the converters  150 , the system  100  can include additional components, such as a peak detector  170  and a temperature regulator  180 , to control one or more operations of the converters  150 . 
     The peak detector  170  can be implemented as a series connection of a diode and a capacitor (not shown) outputting a direct current (DC) voltage equal to the peak value of an applied alternating current (AC) signal. An AC voltage source applied to the peak detector  170  charges the capacitor to the peak of the input. The diode conducts positive “half cycles,” charging the capacitor to the waveform peak. When the input waveform falls below the DC “peak” stored on the capacitor, the diode is reverse biased, blocking current flow from capacitor back to the source. Thus, the capacitor retains the peak value even as the waveform drops to zero. 
     The peak detector  170  can perform automatic measurements on multiple channels approximately simultaneously. The resultant measurements can be used to show data (e.g., plot in graphical form) or transfer the data to another program for alternate use (e.g., transferred to another program). The peak detector  170  can also detect positive peaks (e.g., maximum wavelength heights) or negative peaks (e.g., minimum wave height). The peak detector  170  is described in further detail below, in association with  FIG. 5 . 
     The temperature regulator  180  attenuates temperature variations within the converter(s)  150 . Increasing the PWM switching frequency keeps the IGBTs  152  from cooling down too much when the power source diminishes. 
     In some embodiments, when the speed of the turbine rotor  104  exceeds a normal operating range, additional regulation components, such as fans, heaters, and pumps (not shown), in conjunction with the temperature regulator  180 , allow temperature of a coolant to rise. When the power source disappears, as the coolant temperature rises, the PWM switching frequency can be reduced and an IGBT bridge within the converter  150  can be turned off. 
     In this situation, heaters can be used to increase the temperature of the coolant. Alternately, pump speed can be adjusted and/or operation of an anti-condensation system can be used to increase the temperature of the coolant. 
     In some embodiments, the temperature regulator  180  and/or the other regulation components (e.g., cooling fans and/or heaters), can be operated via one or more controllers, such as a controller  400  described in association with  FIG. 4  below. 
     The controller  400  can include a processor to monitor the PWM switching frequency by the converter  150  on the grid side. The controller  400  also includes a memory  410 . The memory  410  is configurable for storing several categories of software, firmware, and data including, applications  420 , a database  430 , an operating system (OS)  440 , and I/O device drivers  450 . 
     As will be appreciated by those skilled in the art, the OS  440  can be any operating system for use with a data processing system. The I/O device drivers  450  can include various routines accessed through the OS  440  by the applications  420  to communicate with devices, and certain memory components. 
     The applications  420  can be stored in the memory  410  and/or in a firmware (not shown) as executable instructions and can be executed by a processor  460 . 
     The applications  420  include various programs, such as an iterative temperature sequence  502  that, when executed by the processor  460 , process data received into the temperature regulator  180 . 
     The applications  420  may be applied to data stored in the database  430 , such as the specified parameters, along with data, e.g., received via the I/O data ports  470 . The database  430  represents the static and dynamic data used by the applications  420 , the OS  440 , the I/O device drivers  450  and other software programs that may reside in the memory  410 . 
     While the memory  410  is illustrated as residing proximate the processor  460 , it should be understood that at least a portion of the memory  410  can be a remotely accessed storage system, for example, a server on a communication network. The communications network can include a local area network (LAN), a metropolitan area network (MAN), or a wide area network (WAN), for example. 
     It should be understood that  FIG. 4  and the description above are intended to provide a brief, general description of a suitable environment in which the various aspects of some embodiments of the present disclosure can be implemented. While the description refers to computer-readable instructions, embodiments of the present disclosure can also be implemented in combination with other program modules and/or as a combination of hardware and software in addition to, or instead of, computer readable instructions. 
       FIG. 5  is a flow chart illustrating an exemplarily application executed by the controller  400 . The application can be configured to execute an iterative temperature sequence  502 , to whether the temperature of the IGBT  152  (seen in  FIG. 2 ) is within a predetermined temperature range. If the IGBT  152  is within the predetermined temperature range, the iterative temperature sequence  502  repeats. However, if the IGBT  152  is not within the predetermined temperature range, the PWM switching frequency of the converter(s)  150  are modified. 
     It should be understood that although the application is discussed in terms of PWM switching frequency. However, it is contemplated that the application may be executed using another type of switching frequency (e.g. a switching frequency that produces a smooth resultant waveform as perceived by the load). 
     It should also be understood that the steps of the application are not necessarily presented in any particular order and that performance of some or all the steps in an alternative order, is possible and is contemplated. 
     The software may be engaged through the controller  400  communicating an “engage response mode” instruction message through an interface, which sends the initial request to measure a starting temperature of the IGBT  152  within the converter  150 , as depicted in the iterative temperature sequence  502 . 
     The sequence  502  begins when the controller  400  measures a starting temperature, T 1 , of one or more IGBTs  152  within the converter at step  510 . The starting temperature of the IGBT can be measured through any known measuring device such as, but not limited to, thermometers and gauges. 
     The sequence  502  continues with the controller  400  determining a reference temperature, T ref , a desired value for the IGBT temperature, at step  515 . 
     In one embodiment, T ref  can be measured using the peak detector  170 . The peak detector  170  can be configured to determine a maximum temperature when the IGBT  152  conducts high current and a minimum temperature when the IGBT  152  conducts low current. The peak detector  170  may include a capacitor discharge rate that is gradual, also known as a slow-decay. Additionally, long periods of operation of the converter(s)  150  at a light load may also cause the slow-decay. The discharge rate includes a discharge time sufficient to reach a desired temperature. Since heating of the IGBT  152  is a byproduct of the converter load, T ref  can be a maximum or minimum (peak) temperature point resulting from operation of the converter(s)  150 . 
     The iterative temperature sequence  502  sums the starting temperature T 1  from step  510  with T ref  determined at step  515 , to generate a summed temperature, T sum , at step  520 . For example, during the initial temperature sequence, the summed temperature can be expressed as
 
 T   sum   =T   1   +T   ref  
 
where T 1  is the starting temperature. In subsequent sequence iterations, the summed temperature can be expressed as
 
 T   sum   =T   2, . . . ,n   +T   ref  
 
where T 2  is a first adjusted temperature of the IGBT  152  within the iteration sequence  502  and T n  is the temperature associated with an “n th ” iteration of the PWM switching frequency.
 
     The temperature difference (ΔT), between T sum  and the temperature of the coolant (T coolant ), is calculated at step  525 . Also determined within the iterative temperature sequence  502  is whether ΔT is within a predetermined range, at step  530 . The predetermined range is such that the life of the IGBT  152  will reduce failure due to temperature related failures as described above. For example, ΔT may be a value of 10° C. 
     Where ΔT is within the predetermined range (e.g., path  532 ), the controller  400  can return to step  510  were the temperature of the IGBT  152  is measured. Where ΔT is not within the predetermined range (e.g., path  534 ), the controller  400  can modify the PWM switching frequency of the IGBTs  152  within the converter  150  at step  540 . 
     The PWM switching frequency can be modified within the system  100  until a shut off event occurs. A shut off event may be any event that is predetermined to stop the PWM switching frequency of IGBTs  152 . For example, a predetermined passage of time. When a shut off event has occurred, the controller  400  will measure the effect of modifying the PWM switching frequency on the temperature of the IGBT  152 . Specifically, measuring an adjusted temperature, T 2  at step  510 . 
     The controller  400  can modify the PWM switching frequency and the iteration sequence until the ΔT is within the predetermined range. For example, controller  400  can include up to “n” iterations. 
     Alternate forms of renewable energy, such as a solar power system  101 , depicted in  FIG. 6  could utilize several embodiments of the disclosed invention. In these embodiments, solar energy is converted into electrical energy through the use of a battery  190  in connection with the converter  150 . 
     The system  101  can additionally contain a charge controller  195  to regulate the rate at which current is added to or drawn from the battery  190 . AC and/or DC isolation devices (not shown). Isolation devices allow disconnection of the solar power source (e.g., solar panel) in events such as electrical fault or device maintenance servicing. 
     It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.