Abstract:
A method for reducing power losses in a semiconductor switching device in a switching power converter by continuously monitoring an instantaneous power dissipation signal in the switching device and creating an appropriate correction signal to optimize that parasitic power dissipation. A multitude of strobed current and voltage measurement signals associated with the switching device are obtained over a time period using a pair of analog-to-digital converters (ADCs). These measurement signals are used to calculate the multitude of instantaneous power signals, then to derive an average power dissipation signal for a complete cycle and create the correction signal based on a comparison with a reference signal in memory. The correction signal can be, derived in a computer via an algorithm or a look-up table, and would preferably provide a relative adjustment in the timings of the switching device.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of power switching, and more particularly to a method for automatically controlling switching power losses. 
     BACKGROUND OF THE INVENTION 
     In applications having power semiconductors devices driving reactive loads, such as MOSFETs in voltage converters, a significant source of energy loss and attendant component overheating occurs as these devices change states. Due to the inability of inductances to instantaneously change currents, an On/Off switching of the power semiconductor creates a condition where a high voltage is impressed across the semiconductor switching device simultaneously with a high current being conducted through the switching device. This results in a large power transient that is proportional to the voltage, the current, and the amount of time that the condition exists relative to a cyclic time period. Such power transients reduce system efficiency and can often be damaging to the semiconductor device if not minimized. 
     Conventional design approaches to alleviating this switching stress on the power semiconductor has been to implement various “soft” switching circuits, such as zero voltage switching and zero current switching, where alternative circuit devices are used to sustain inductive currents during the switching transitions until the semiconductor device voltage is minimized. Exemplary techniques can involve the incorporation of one or more reactive “snubber/slope control” resistor-capacitor (RC) circuits to provide a transition charge to a main reactive element during the time that the semiconductor is changing states. During these switching transition times, energy is stored in the snubber circuits and is dissipated as heat during the remainder of a periodic cycle so as to be ready to provide an identical function on a following cycle. This allows the semiconductor device to change states at a minimal current condition, and, thus, with a minimal transient power loss. While this shifts the energy loss away from the semiconductor device to a heartier snubber component, it does not eliminate the loss, so the energy inefficiency still exists. 
     An alternative approach uses L-C resonant circuits that are excited to a steady-state resonance condition, wherein signal waveform changes are generated by the L-C circuit rather than by the semiconductor device. In the steady state operation of such an application, the semiconductor device can then be switched on exclusively in a low voltage/low power condition. As before, such a configuration requires the use of fixed components that are selected on the basis of a predetermined fixed operating frequency and a maximum transient power control requirement. With such predetermined solutions, system operation and efficiency cannot be optimized for variations in input voltage or output loading. 
     SUMMARY 
     An electronic apparatus and method for reduce switching power losses in a semiconductor power switching device in voltage converters by calculating a power dissipation signal using a multitude of instantaneous voltage and current samples. This calculated power signal is compared to a reference signal that is derived from previous data samples, and a correction signal is generated to dynamically modify the drive and/or timing parameters of the power switching device. The multitude of voltage and current samples are preferably measured using an analog-to-digital converter (ADC) and accumulatively stored in memory until used by a computing device to calculate the correction signal, which is preferably an adjustment in a T off  time for the switching devices. The constant monitoring of the power dissipation via the ADCs and continuous correction of the power dissipation optimizes the operation of the voltage converter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a circuit diagram of an exemplary power converter circuit according to the present invention. 
     FIG. 2 shows representative voltage and current waveform signals for a quasi-resonant converter over a single cycle time period, T. 
     FIG. 3 shows a detailed view of exemplary switching losses at a device turn off time period having individual sampling strobe times. 
     FIGS. 4 through 6 show the voltage and current waveforms shown in FIG. 2 under different times for t off . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to a preferred embodiment of the present invention, power dissipation in a power switching device of a voltage converter can be optimized by a continuous sampling of the voltage and current signals associated with the switching device in order to create appropriate adjustment to the drive signals of the switching device. In a conventional switching power converter, an input voltage signal is converted into one or more output voltage signals, typically at a different voltage level or levels from that of the input voltage signal. The converter typically employs one or more digital switching devices, which control a primary reactive energy storage element to implement the transformation. Converter operation over a representative time period, T, is characterized by current conduction in the switching device or devices during a T on  portion of T, wherein energy is stored in the reactive device, followed by a current non-conduction T off  portion of T, where the stored energy is transferred from the reactive element to a load. 
     FIG. 1 shows a circuit diagram of an exemplary power converter circuit  10  according to the present invention. In a preferred embodiment, converter  10  would be of a type known as quasi-resonant converters, wherein resonance voltage swings induced by the primary inductance, L p , of a transformer  12  and a resonance capacitance, C p ,  14  on a drive device  16  allow device  16  to be activated only when a low voltage is present across its terminals. This provides a low voltage across device  16  during On/Off transitions of a current signal through device  16 , thus minimizes the switching power losses. The duration of a conduction time, T on , is determined by a regulation device  18 , and is a function of an output voltage signal on voltage rail  20  and an input voltage signal on voltage rail  22 . 
     In a practical (i.e. non-idealized) quasi-resonant converter operation, however, drive device  16  usually has to turn “on” in a condition of non-zero voltage. This can create a power transient during both the turn-on and turn-off transition times that occur in each cycle as the voltage and current traverse to their alternate quiescent values. To calculate and adjust these power transients, a pair of sampling circuits, such as analog-to-digital converter (ADC)  24  and ADC  26 , simultaneously sense a current magnitude through device  16  and a voltage magnitude across device  14  at a multitude of predetermined sample times, T s . Preferably, ADCs  24  and  26  use a common strobe to transfer this multitude of digital output voltage signals to a computing device  28 , which then calculates an average power dissipation value over a predetermined period. 
     This power metric can then be used to create a correction signal  30  that causes regulation device  18  to change in one or more drive parameter for device  16  in order to reduce the power dissipation. Correction signal  30  can take various forms, including, without limitation, adjusting the drive levels to device  14  to force faster transient switching time, or stretching T off  to increase period time T, thus lowering the average power and heating effects of the power transients. 
     Creation of correction signal  30  can be implemented using various methods as are well known in the art. For example, in a preferred embodiment according to the present invention, a stored power signal value, which was calculated in a previous time period, can be compared with the newly calculated value using an algorithm. A resultant from the algorithm could cause a timing change in the operation of converter  10 , such as increasing the “off” time, t off . Alternatively, a look-up table in memory holding corrective values and/or weighting coefficients could be employed alone or in combination with an algorithm or other computational means to create correction signal  30 . 
     It should be noted that in an exemplary converter having an operating frequency could be between 50 KHz to 200 KHz and a cyclic period, T, of 5 to 20 microseconds, a response time for creating correction signal  30  would not typically be from one cycle to the next, but rather over a longer period of time. Such a response time delay guards against closed-loop instabilities that can be caused by noise or other transient phenomena. Thus, a response time for generating correction signal  30  could preferably be in the millisecond range, a same time response as a normal voltage regulation feedback loop. 
     This slower response time requirement also greatly relaxes the performance requirements on computing device  28 , allowing for use of inexpensive microprocessors and micro-controllers, such as devices from the  8031  and  8051  families. However, this relaxed performance requirement does not prevent the use of more powerful computing devices, such as digital signal processors, personal computers, or handheld computing devices. In alternative embodiments, computing device  28  can be replaced by a digital or analog device having power calculating capabilities or by logical arrays containing predetermined values that produce a specific result when subjected to specific current and voltage input digital values, similar to a look-up table. 
     Further, rather than instantaneously calculating the power, digital sample signals from ADCs  24  and  26  can be temporarily stored in memory for either delayed calculation and correction operation or for transmission to alternate controllers. In this context, memory would preferably be a semiconductor device having volatile or non-volatile memory, and could consist of dynamic or static random access memory (DRAM and SRAM, respectively.) or electronically programmable memory, such as flash and electronically erasable, programmable read-only-memory (EEPROM). 
     FIG. 2 shows representative current and voltage waveform signals,  32  and  34 , respectively, for a quasi-resonant converter over a single cycle time period, T. At any instantaneous point in time, the instantaneous power is the product of the amplitude of the current and voltage signals  32  and  34 . Cyclic energy dissipation is the area contained within an envelope of such voltage/current product calculations over time. From FIG. 2 it can be seen that a cyclic dissipative power figure can consist of three principal components: a turn-on transition component  36 , an “On” component  38 , and a turn-off transition component  40 . On time and off time and period are labeled T on    42 , T off    44 , and T  46 , respectively. 
     In the exemplary waveforms shown in FIG. 2, turn-on transient component  36  has negligible power, since the voltage signal is close to zero. During T on    42 , voltage signal  34  across drive device  16  is very low, while the current signal  32  is ramping up. Instantaneous power during this time is similarly small, 1 volt×2 amps, or 2 watts for example. At turn-off time, however, the instantaneous power can be quite large, 200 volts×2 amps, or 400 watts, for example. The preceding quantities are exemplary only, and are not intended to be restricting. 
     Further, the waveform signals are also exemplary only, since different configurations produce different waveforms. For example, a power converter using a non-resonant, square wave or trapezoidal drive can have turn-on transient component  36  having significantly higher power dissipation, while turn-off transient component  40  can have negligible power dissipation. Alternative drive configurations can have equal power dissipation characteristics for transient components  36  and  40 . For all of these variations, the use of a correction signal  30  according to the present invention can reduce the average power dissipation and prevent destructive heating effects associated with switching converters. 
     Regardless of the circuit configuration employed, the sampling and correcting circuits and methods of the present invention can be used to reduce destructive heating effects from switching and conduction losses. By using an appropriate sensing configuration with ADCs  24  and  26  for simultaneously capturing current and voltage samples, a computing device  28  can make a running compilation of these samples over time and compute the energy dissipation over the entire period  46 . An appropriate correction signal  30  can then be generated and applied to the semiconductor drive device  16  to reduce this energy dissipation, and thus its heating effects, preferably via shortening or extending the T off  time period  44 . 
     Additionally, since a computing device  28  would preferably be employed for calculation and correction signal generation, additional environmental sensors that record influential ancillary parameters, such as ambient temperature, airflow, input, and output conditions could be used and monitored by that computing device  28 . This capability can be important in certain applications, since all of these parameters affect energy/heating accumulations and dispersions. For example, at higher ambient temperatures or low circulating air flow, less heat can be dissipated by the heat sinking of the switching device, and to prevent junction temperatures from reaching unacceptable levels, the power dissipation must be reduced to a lower operating level. 
     FIG. 3 shows an illustration of exemplary switching losses at a device turn off time period at selected sampling strobe times. Energy dissipation during such transitions will always be the area under a voltage-current product waveform. For example, current waveform  48  and voltage waveform  50  are multiplied at each sample time  52 ,  54 ,  56 , etc. Assuming a finite number of samples during the transition time being measured, a piece-wise linear approximation will produce a unique sequence of rectangular areas  58 ,  60 ,  62 , respectively, each of which represent a power amplitude that is maintained for the time duration between the samples. An exemplary energy calculation using normalized voltage, V max , and current, I max , could be V×I=(1% V max )×(100% I max ), or 0.01 V max I max  for rectangle  58  at strobe time  52 , (12.5% V max )×(88.5% I max ), or 0.11 V max I max  for rectangle  60  at strobe time  54 , 25% V max ×75% I max , or 0.19 V max I max  for rectangle  62  at strobe time  56 , etc. 
     Summing the rectangle areas gives the total energy dissipated between the beginning and end of the transition time period. This energy dissipation is then averaged over an entire period  46  to give the average power dissipation for this switching component that is governed by the equation 
     
       
           P   ave =(1/ T )Σ V   t   I   t   Δt   [1] 
       
     
     where P ave  is the average power dissipated in semiconductor driving device  14  due to this transition component, T is the period associated with a single cycle of operation, V t  and I t  are the instantaneous values of voltage and current measured at each sampling time t, and Δt is the sampling time interval. 
     An identical equation applies to all time intervals in the period, and a resulting average power dissipation is the sum of the individual average power dissipations, or 
       P   ave   =P   turn-on   +P   on   +P   turn-off   +P   off   [2] 
     where the individual terms are the average power dissipations associated with the transitions from off-to-on and on-to-off of drive device  14  as well as the quiescent on and off times. The time period T  46  is the sum of the time intervals associated with powers shown in equation 2, or 
     
       
           T=T   turn-on   +T   on   +T   turn-off   +T   off   [3] 
       
     
     It is recognized that the increase in period  46  decreases the operating frequency of the system and causes proportional impedance changes in any reactive elements. Further, to maintain a specific power output, an increase in T on    42  is required to provide the additional charge needed to sustain a load over the increased period  46 . Although on time  42  this is typically characterized by high currents at low voltage and producing lower average power dissipation in semiconductor switching device  14  by reducing the losses from the switching transients times  36  and  40 , alternative configurations can provide an opposite effect where the quiescent “on” condition is the higher power contributor to average power. The specific algorithm or method employed in the application must incorporate the effects of all such stimuli and associative relationships. 
     To provide a better understanding of the effects of adjusting t off  time  44 , FIGS. 4 through 6 show the voltage and current waveforms shown in FIG. 2 under different times for t off    44 . 
     In FIG. 4, a representative t off    44  of 2.9 microseconds shows a calculated power signal  58  having a transient pulse  70  resulting from a parasitic current pulse  72  in current signal  74  during a time of non-zero voltage. To compensate for this detected power component, correction signal exemplarily increases t off    44  to 3.58 microseconds as shown in FIG.  5 . The current spike is greatly reduced as shown by the absence of a transient power pulse in power signal  76 , due to a reduced current pulse  78  in current signal  80 . 
     Consider, further increases in t off    44  to a representative 4.11 microseconds as shown in FIG. 6, which, if uncorrected, can enable a parasitic current spike  82  on current signal  84  to appear coincident with an exemplary parasitic ringing  86  of voltage waveform  88 . This results in a power dissipation transient  90  in calculated power signal  92 . This increased power is exaggerated for instructional purposes, since correction signal  30  would have sensed a worsening power dissipation calculation and made corresponding adjustments in t off    44  before high power transient condition  90  occurred. 
     Although the above discussion discloses a method for a power switching device that impresses a voltage across an inductive element, a corollary description could describe a current switching device driving a capacitive load and such embodiment is intended to be within the scope of the present invention. 
     Numerous modifications to and alternative embodiments of the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the embodiments may be varied without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.