Patent Publication Number: US-6667869-B2

Title: Power control system and method for illumination array

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of patent application Ser. No. 09/512,575 filed on Feb. 24, 2000, now U.S. Pat. No. 6,349,023 which is fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to control systems for controlling power supplied to a dissipative/resistive load, and in particular, a power control system that protects an LED illumination array from reaching life-shortening or destructive temperature levels. 
     BACKGROUND INFORMATION 
     Sophisticated illumination systems and methods have been developed, for example, for use in the inspection of electronic components. One such illumination system, which is especially suitable for illuminating ball grid arrays (BGAs), which are commonly used in manufacturing electronic components, is disclosed, for example, in commonly-owned U.S. Pat. No. 5,943,125, which is fully incorporated herein by reference. U.S. Pat. No. 5,943,125 teaches the use of a ring-shaped light source, which includes a plurality of light emitting elements, such as light emitting diodes (LEDs). While this light source is designed especially for use in illuminating BGAs for inspection purposes, various configurations of LED arrays may be employed for a wide variety of illumination sources for a wide variety of inspection applications. 
     One drawback of using LED arrays as illumination sources, however, is that LEDs are dissipative (resistive) loads. As a dissipative/resistive load is powered, it will heat up. If the heat build up is allowed to progress uncontrolled, the temperature of the array may reach a destructive or life-shortening level. 
     Various systems and methods have been employed in the past to prevent dissipative/resistive loads from exceeding certain pre-defined life-shortening temperature levels. More sophisticated control systems have been employed as well to ensure that the peak and average temperatures of the LED array fall within safe limits. One such system controls the temperature of an LED array by enforcing a maximum pulse width of an LED power signal (during which the LED array is powered) and a minimum off time between pulses. This type of control system employs a simple digital circuit that generates a delay after each pulse. 
     A slightly more sophisticated prior art system computes an inter-pulse minimum delay based on the then-current pulse width. An even more sophisticated prior art system even takes the pulse repetition rate into account. 
     Since all of the prior art control systems are based on theoretical average thermal characteristics, they do not take into account the real-time, actual heat generation of an LED array. Therefore, a margin of safety must be factored into all prior art control systems. These built-in safety margins necessarily reduce the actual time of array illumination, which in turn limits the throughput of the inspection systems with which they are associated. 
     One solution to the problem with prior art control systems is to provide a power control circuit suitable for use in controlling dissipative/resistive loads (e.g., LED illumination arrays), which accurately models the heat being generated by the resistive load that it is controlling. In this manner, arbitrary, built-in safety margins can be eliminated, which provides an improvement in inspection system throughput. It also makes it possible to input a complex series of pulses of varying widths and intervals, such that power to the LED array could be arbitrarily switched without restriction, provided the modeled maximum temperature limit was not exceeded. 
     The control circuit discussed above, however, requires carefully calibrated and accurate low leakage analog components, especially when temperature calculations require a large ratio of charge (heating analog) to discharge (cooling analog) time constant. The analog control circuit for modeling temperature can thus be costly and requires careful layout and component selection. 
     Accordingly, there is a need for a power control system and method that models temperature with minimal or no analog components. 
     SUMMARY 
     According to one aspect of the present invention, a power control system for controlling power supplied from a power source to a resistive load to prevent the resistive load from exceeding a predetermined high temperature limit. A regulator circuit is coupled between the power source and the resistive load for supplying controllable power levels to the resistive load. The power control system comprises a pulse train generating circuit for converting power impulses received from the regulator circuit into a heating pulse train representing power flowing to the resistive load. A load temperature calculation circuit is coupled to the pulse train generating circuit. The load temperature calculation circuit includes digital logic for producing a temperature out value substantially representing a present temperature of the resistive load. 
     A temperature comparison circuit is coupled to the load temperature calculation circuit and the regulator circuit. The temperature comparison circuit selectively compares the temperature out value to at least one of a high temperature limit value and a base temperature value. The temperature comparison circuit causes the power source to be disconnected from the resistive load when the temperature out value reaches the high temperature limit value. The temperature comparison circuit causes the power source to be reconnected to the resistive load when the temperature out value reaches the base temperature value. 
     According to one embodiment of the power control system, a pulse rate generator circuit including one or more oscillators generates heating and cooling pulse rates. An AND gate receives the heating pulse rate from the pulse rate generator circuit and receives a power control pulse from the regulator circuit. The heating pulse rate and the power control pulse cause the AND gate to output a heating pulse train such that the number of pulses out of the AND gate is proportional to the total energy delivered to the resistive load. 
     An up/down counter is coupled to the pulse rate generator circuit and receives the heating pulse train, which is applied to an up input of the up/down counter. The up/down counter outputs a temperature out value substantially representing a present temperature of the resistive load. A rate multiplier is coupled to the up/down counter and to the pulse rate generator circuit for generating a cooling pulse train, which is applied to a down input of the up/down counter. A temperature comparison circuit receives the temperature out value and provides a power control signal to the regulator circuit to disconnect or re-connect the power source. 
     According to one method of controlling power supplied from the power source to the resistive load, a heating pulse train representing power flowing to the resistive load is generated. Load temperature is modeled using digital logic and the heating pulse train to generate a temperature out value substantially representing a present temperature of the resistive load. The temperature out value is compared to a high temperature limit value, and the power source is disconnected from the resistive load if the temperature out value exceeds the high temperature limit value. The temperature out value is compared to a base temperature value, and the power source is re-connected to the resistive load if the temperature out value reaches the base temperature value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be better understood by reading the following detailed description, taken together with the drawings wherein: 
     FIG. 1 is a schematic functional block diagram of a power control system used to control power supplied to a resistive load, according to the present invention; 
     FIG. 2 is a flow chart illustrating a method of controlling power, according to the present invention; 
     FIG. 3 is a schematic diagram of a regulator circuit and a pulse train generator circuit used to control power supplied to a resistive load, according to one embodiment of the present invention; 
     FIG. 4 is a schematic diagram of a regulator circuit and a pulse train generator circuit used to control power supplied to a resistive load, according to another embodiment of the present invention; 
     FIG. 5 is a schematic diagram of a temperature calculation circuit, according to one embodiment of the present invention; 
     FIG. 6 is a schematic diagram of a pulse train generator circuit and a temperature calculation circuit, according to another embodiment of the present invention; 
     FIG. 7 is a schematic diagram of a rate multiplier used in the circuit shown in FIG. 6; and 
     FIG. 8 is a schematic diagram of a temperature comparison circuit, according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A power control system  10 , FIG. 1, according to one aspect of the present invention, is used to control power supplied from a power source  12  to a dissipative/resistive load  16 . In general, the power control system  10  includes a regulator circuit  20 , a pulse train generator circuit  24 , a temperature calculation circuit  28  and a temperature comparison circuit  32 . The power control system  10  uses digital differential analyzer (DDA) techniques to perform the analog computations described in the commonly owned U.S. Pat. No. 6,349,023 (Ser. No. 09/512,575), which is fully incorporated herein by reference. Exemplary embodiments of these circuits are described in greater detail below. One embodiment of the load  16  is a LED illumination array, although other types of dissipative/resistive loads are contemplated. 
     Referring to FIGS. 1 and 2, one method of controlling power supplied from the power source  12  to the load  16  using the power control system  10  is described. The pulse train generator circuit  24  converts power impulses  22  from the regulator circuit  20  into a heating pulse train  26  representing power flowing to the resistive load  16 , step  110 . The load temperature calculation circuit  28  models the load temperature using digital logic to generate a temperature out value  30  representing a present temperature of the load  16 , step  114 . 
     The temperature comparison circuit  32  compares the temperature out value  30  to one or more reference temperature values, such as high temperature limit value and/or a base temperature value, step  118 . When the power supply  12  is connected to the load  16 , the temperature out value  30  calculated by the temperature calculation circuit  28  increases. When the temperature out value  30  increases to reach the high temperature limit value, step  122 , the power source  12  is disconnected from the load  16 , step  126 . When the power supply  12  is disconnected from the load  16 , the temperature out value  30  calculated by the temperature calculation circuit  28  decreases. When the temperature out value  30  decreases to reach the base temperature value, step  122 , the power source  12  is re-connected to the load  16 , step  130 . To disconnect and connect the power source  12 , the temperature comparison circuit  32  sends a power enable signal  34  to the regulator circuit  20 . 
     In one embodiment, the regulator circuit  20  is a switching regulator with current feedback, which supplies controllable power levels to the load  16  such as a LED lighting array. One example is a switching regulator intended for battery charger applications. 
     One embodiment of a typical switching regulator circuit used in the present invention is shown in FIG.  3 . In this embodiment, a switch  50  connects the power source  12  and voltage is supplied to the load  16  across a power inductor  52 . A pulse width modulation (PWM) switch controller  54  is coupled to the switch  50 . The PWM switch controller  54  provides a pulse width control signal, which turns on the switch  50  for charging the inductor  52 . The pulse width of the control signal is proportional to the amount of energy delivered to the load  16 . A current sensing resistor  56  is coupled in series with the load  16  and registers a voltage proportional to the instantaneous current in the load  16 . An error amplifier  58  is coupled between the load  16  and current sensing resistor  56  and provides a feedback signal to the PWM switch controller  54 . 
     In this embodiment, the pulse train generator circuit  24  includes an AND gate  60  and a heating rate clock oscillator  62 . The clock oscillator  62  generates a heating pulse rate and preferably has a frequency much higher (e.g., by a factor of about 20 or more) than the regulator switching frequency. In one example, if the PWM switch controller  54  ran at 100 KHz, the clock oscillator  62  would run at 2 MHz. Other frequencies are possible for the clock oscillator  62  depending upon the desired accuracy of the temperature estimate and practical design considerations. Each switching regulator pulse causes the AND gate  60  to output a number of pulses proportional to the pulse width, thereby generating the heating pulse train  26 . Over time, the total number of pulses out of the AND gate  60  is proportional to the power source voltage applied to the inductor  52  and thus to the total energy delivered to the load  16 . Thus, the heating pulse train  26  represents heat flowing to the load  16 . The switching pulse is preferably resynchronized to the oscillator pulse rate for stable counting. 
     In one preferred embodiment, the pulse train generator circuit  24  adjusts the pulse rate according to the voltage across the inductor  52  to provide a more accurate measure of power to the load  16 . One way of making this adjustment is by varying the clock oscillator frequency using a voltage to frequency converter with a frequency control voltage based on the power source voltage minus the load voltage. One example of a voltage to frequency converter is a voltage controlled oscillator (VCO). Another example is a multivibrator in which the charging current or voltage connected to the R-C time constant charging circuit is proportional to the control voltage. 
     Another way to adjust the pulse rate according to the voltage across the inductor  52  is shown in FIG.  4 . In this exemplary embodiment, an accumulator  70  is coupled to a first AND gate  72 , which receives the power impulses  22  and heating pulse rate from oscillator  62 . A digital integer proportional to voltage (i.e., a voltage value) is added to the accumulator  70  on each oscillator or AND gate output pulse. Each time the accumulator  70  overflows, an output pulse is generated. The overflow output can be synched to the clock, for example, using second AND gate  74  that outputs the heating pulse train  26 . The resulting pulse train rate total better approximates total energy because it represents the product of voltage times the time it was applied to the inductor  52 . The voltage value can be derived by measuring the load voltage with an analog-to-digital converter and subtracting this value from the known or measured power source voltage. Other circuits for adjusting the pulse rate according to voltage across the inductor are also contemplated. 
     According to a further embodiment of the pulse train generator circuit  24 , the heating pulse train  26  is generated by applying the current sense voltage signal to a voltage to frequency converter. Voltage to frequency converters are well-known in the art. In one example, this method uses a multivibrator with a voltage controlled time constant and having a wide operating range and a control voltage proportional to the measured load current (e.g., the voltage drop across the current sensing resistor  56 ). This method of obtaining the heating pulse train  26  can be used with any type of power regulator including a simple power switch and voltage regulator. 
     One embodiment of the temperature calculation circuit  28  is shown in greater detail in FIG.  5 . This temperature calculation circuit can be used with any of the embodiments of the pulse train generator circuit  24  described above. The temperature calculation circuit  28  includes an up/down counter  80  coupled to the pulse train generator circuit  24 . An accumulator  82  is coupled to the up/down counter  80  and a cooling rate oscillator  84 . An AND gate  86  can be coupled to the accumulator  82  and the cooling rate oscillator  84  to synch the overflow output to the clock. 
     In operation, the heating pulse train  26  is applied to the UP input of the up/down counter  80 . The contents of the up/down counter  80  represent the load temperature rise above ambient (i.e., the temperature out value  30 ). The counter contents are added to the accumulator  82  and the output overflows from the accumulator  82  are applied to the AND gate  86  with a cooling pulse rate from the cooling rate oscillator  84  to generate a cooling pulse train  88  representing cooling. The cooling pulse train  88  output from the AND gate  86  is applied to the DOWN input of the up/down counter  80 . 
     The rate at which the addition occurs is preferably adjusted to model the cooling path time constant, while the rate of generating the UP pulse train is preferably scaled (e.g., using known methods) to represent the heating time constant. For example, the constant of proportionality of the numeric value in the counter  80  to the simulated load temperature is chosen. The rate of the heating pulse train is scaled to represent dq/(Rh★Cm), where dq is the quantum of energy represented by each pulse, Rh is the heating thermal resistance, and Cm is the thermal mass of the load. Similarly, the cooling rate is scaled so that each count also represents a quantum of heat flowing through the cooling path, which is proportional to the current temperature and inversely to the cooling thermal resistance Rc, i.e., dq=T/(Rc★Cm). The cooling rate oscillator  84  can be adjusted to be slower than the heating rate oscillator  62  by the ratio Rc/Rh. 
     Another embodiment of the pulse train generator circuit  24  and temperature calculation circuit  28  is shown in FIGS.  6 . In this embodiment, a pulse rate generator circuit including a single master clock oscillator  92  with additional rate multipliers  94  is used to generate the heating and cooling pulse rates. In this embodiment, the power input level can be derived by converting the power voltage minus the load voltage to a numeric value using an ADC. Alternatively, the heating pulse train can be generated directly by gating the heating rate frequency signal with a resynchronized version of the power switch pulse. One example of the rate multiplier  94  is shown in greater detail in FIG.  7 . The output rate is a function of the ratio of the numeric input value to the full scale accumulator value times the update enable rate. 
     A master timing circuit using the single clock oscillator  92  can also generate the switching frequency for the switching current regulator (as shown in FIG. 3) or for a switching voltage regulated power source (not shown). Although exemplary embodiments are shown and described herein, other embodiments of the pulse train generator circuit  24  and the temperature calculation circuit  28  employing known DDA techniques are contemplated. 
     The temperature comparison circuit  32  can be implemented using logic similar to that disclosed in pending application Ser. No. 09/512,575 or using any other type of logic known to those skilled in the art. One embodiment of the temperature comparison circuit  32  is shown in FIG.  8 . This embodiment of the temperature comparison circuit  32  includes a high temperature comparator  96  for comparing the temperature out value  30  to the high temperature limit value and a base temperature comparator  98  for comparing the temperature out value  30  to the base temperature limit value. 
     Accordingly, the power control system of the present invention controls power supplied to a resistive load to prevent the load from exceeding a high temperature limit using a circuit with fewer analog components. In particular, the power control system effectively determines power flowing to the load by converting switching regulator power impulses to a pulse train representing heating and models temperature using digital logic. 
     Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.