Patent Publication Number: US-7710086-B1

Title: Modular high voltage power supply for chemical analysis

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of U.S. patent application Ser. No. 10/414,979 originally filed Apr. 16, 2003 now U.S. Pat. No. 7,161,334 entitled “MODULAR HIGH VOLTAGE POWER SUPPLY FOR CHEMICAL ANALYSIS” from which benefit of priority is claimed. 

   This invention was made with Government support under Contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights to the invention. 

   FIELD OF THE INVENTION 
   The present invention relates to high voltage power supplies such as those used in microfluidic systems for chemical analysis. 
   BACKGROUND 
   Microfluidics is the science dealing with manufacturing and operating devices and processes using very small volumes of liquids. Microfluidics systems operate at such small scales that the system behavior is dramatically altered. For example, capillary action changes the way liquid flows through microscopic-diameter tubes. 
   Microfluidics systems have diverse and widespread applications. Microfluidics systems include microchips or substrates that have networks that are connected by channels which have mesoscale dimensions, where at least one dimension is usually between 0.1 microns and SOD microns. Such substrates may be fabricated using photolithographic techniques similar to those used in the semiconductor industry, and the resulting devices can be used to perform a variety of sophisticated chemical and biological analytical techniques. Some of the processes that use this technology include ink jet printers, blood cell separation equipment, biochemical assays, chemical synthesis, genetic analysis, drug screening, surface micromachining, laser ablation and mechanical micromilling. 
   Many microfluidics systems use electrokinetic (EK) pumping of fluid. These applications generally require a high voltage at a low current. Typically large expensive laboratory power supplies are used for such microfluidics systems. Such laboratory power supplies are available from Bertran Associates of Hicksville, N.Y. and Stanford Research Systems (SRS) of Sunnyvale, Calif. The size and expense of such high voltage power supplies reduce the desirability of these power supplies for use in microfluidics systems that use EK pumping. 
   BRIEF SUMMARY 
   One embodiment of the present invention is a high voltage power supply comprising a DC-DC converter, and a voltage-controlled resistor. The high voltage power supply also includes a feedback circuit that controls the voltage-controlled resistor and DC-DC converter so as to regulate the voltage output of the high voltage power supply and to allow the high voltage power supply to source or sink current while maintaining the voltage output constant. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are diagrams of a microfluidics system that can use the high voltage power supply of the present invention. 
       FIGS. 2A-2C  are diagrams of a high voltage power supply of one embodiment of the present invention. 
       FIG. 3  is a diagram of a high voltage power supply of one embodiment of the present invention. 
       FIGS. 4A and 4B  are diagrams of a high voltage power supply module of one embodiment of the present invention. 
       FIG. 5  is a diagram illustrating the use of high voltage power supply modules of one embodiment of the present invention. 
       FIG. 6  is a diagram illustrating an exemplary input voltage versus output voltage curve of a high voltage power supply of one embodiment of the present invention. 
       FIG. 7  is a diagram illustrating signals sent from and received by a processor in one embodiment of the present invention. 
       FIG. 8  is a diagram that illustrates output voltage versus output current for a fixed voltage embodiment of the present invention. 
       FIG. 9  is a diagram of output voltage versus output current following a predetermined curve for one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIGS. 1A and 1B  illustrate the use of a high voltage power supply of one embodiment of the present invention used in a microfluidics system. The electrokinetic pumping of fluids for chemical analysis requires a high voltage with a low current. Looking at  FIG. 1A , the power supplies  102  and  104  are used to draw a sample  106  in a tube portion  105  between the power supplies,  108  and  110 . For example, power supply  102  can be set to 900V and power supply  104  can be set 0V, while power supplies  108  and  110  are set to 300V or floated. Once the sample  106  is loaded into the tube portion  105 , the power supplies  108  and  110  cause plug  107  to move down the tube  105 . For example, power supply  110  can be set to 5000V and power supply  108  can be set to ground, while the power supplies  102  and  104  are set to 250V or floated. This causes plug  107  to move toward power supply  110 . Based upon the weight or size of the components in the plug  107 , different components take different amounts of time to get to the sensor, such as laser  112  and detector  114 . The sensor will detect peaks as the components pass by. This gives an indication of the composition of the material in the plug  107 . The measurement resolution, sensitivity and repeatability are greatly affected by the plug shape and speed, which in turn are affected by the current flow and applied voltages. 
   In one embodiment, the high voltage power supply of the present invention is designed to have special features to help obtain optimal analysis results, and to diagnose chemistry and electrical problems when they occur. Additionally, the modularity and small size of one embodiment of the high voltage power supply of the present invention allows it to be used in a wide variety of applications and simplifies the reconfiguration of the experimental set up, thus saving time and money. 
   Although the system of the present invention can be useful for electrokinetic pumping in microfluidics systems, it also can be valuable for many other scientific and engineering applications, such as photomultiplier tube supplies, ion mobility spectroscopy, microseparations, and charged particle acceleration. 
   As used herein a “microfluidics system” includes one or more substrates (also commonly known as microchips and microwafers) that perform a variety of chemical and/or biological analytical operations, including, for example, protein separations, fluidic mixing, dispensing, valving, reactions, and/or detections involving very small sample sizes. The sample volumes can range in the nanoliters or less. Substrates can include microfluidic members that have integrated networks of microfluidic channels disposed therein. The substrate may include reaction cells, reservoirs, and other structures that are interconnected by the network. The substrate is preferably fabricated from glass, quartz, silicon or plastic by conventional techniques including LIGA (an acronym for the German for lithography, electroplating, and molding), deep x-ray lithography, silicon surface micromachining and lithography, electric discharge machining, and direct laser additive fabrication. Substrates are also commercially available. Substrates and related accessories are further described in U.S. Pat. Nos. 6,358,387 to Kopf-Sill et al., 6,224,728 to Oborny et al., 6,083,763 to Balch, and 5,846,396 to Zanzucchi et al., which are incorporated herein. 
     FIG. 2A  illustrates a high voltage power supply  200  of one embodiment of the present invention. The high voltage power supply  200  has a DC-DC converter  202  and a voltage controlled resistor  204 . In one embodiment, the DC-DC converter  202  and a voltage-controlled resistor  204  are connected in parallel. The DC-DC converter  202  can be for example an EMCO DC-DC converter such as a Q50 or Q10 model which are available from the EMCO High Voltage Corporation of Sutter Creek, Calif. DC-DC converters produce a high voltage output based on a low voltage input. The DC-DC converters are typically unregulated meaning that the current drawn from the output of the DC-DC converter can reduce the output voltage from its nominal value. The voltage-controlled resistor  204  can be a photo relay, such as the photo MOS relays available from the Aromat Corporation (Matsushita Electric Works America) of New Providence, N.J. 
   A feedback circuit  206  can control the DC-DC converter  202  and the variable voltage controlled resistor  204  so as to regulate the output of the high voltage power supply  200 . In one embodiment, the feedback circuit  206  alternately powers voltage-controlled resistor  204  or DC-DC converter  202 . When the high voltage power supply output is too high, the DC-DC converter is powered down and the voltage-controlled resister  204  is turned on to reduce the output at point  210 . When the high voltage power supply output is too low, the DC-DC converter is powered up and the voltage-controlled resistor  204  is turned off. 
   In one embodiment, feedback circuit  206  sets the output of the high voltage power supply  210  to be some function of the input voltage. In one embodiment; the feedback signal on line  212  is compared to the input voltage, VSET. If the feedback signal voltage is greater than VSET then the output of the feedback circuit adjusts voltage  215  so that voltage-controlled resistor  204  is powered up and thus reduces the output voltage. If the feedback signal is less than VSET then the DC-DC converter  202  is powered on thus raising the output voltage of the high voltage power source and thus the feedback signal. The feedback circuit  206  forces the feedback signal  212  to be approximately equal to VSET by changing the output voltage  210 . This results in the output on line  210  of the high voltage power supply to be a function of the input voltage. In one embodiment, a voltage divider  214  is used to set the feedback signal on line  212  to be a function of the output voltage. 
   The DC-DC converter  202  and the voltage-controlled resistor  204  allow for the high voltage power supply  200  to source or sink current. When the current is flowing into the output line  210 , the voltage at line  210  rises slightly above the desired voltage, causing an increase in the feedback signal in line  212 , which causes the voltage-controlled resistor  204  to turn on, lowering the output voltage until it is approximately equal to the desired voltage. When the high voltage power supply  200  sources current, the voltage at  210  drops slightly below the desired voltage causing a reduction in the feedback signal on line  212  which causes the DC-DC converter  202  to be powered up, resulting in a rise in the output voltage on line  210  until it is approximately equal to the desired voltage. 
   In one embodiment, the high voltage power supply  200  also includes a current sensor  214 . The current sensor  214  produces an output voltage signal proportional to the current that flows out of the high voltage power supply  200 . Operation detection unit  216  produces an indication of the feedback signal, which is a function of the output voltage of the high voltage power supply  200 . The example of  FIG. 2A  also includes a voltage follower transistor  218  positioned between the feedback circuit  206  and the DC-DC converter  202 , to provide a current sufficient to drive DC-DC converter  202 . 
   A photo relay, such as the photo MOS relay shown in  FIG. 2A , is one example of a voltage-controlled resistor. When current flows between pins  1  and  2  of the photo MOS relay, the photodiode between pins  1  and  2  emits light, which causes a current flow in the switches between pins  4  and  6  thus reducing the resistance between pins  4  and  6 . The optical coupling provides the required electrical isolation between the input and output of the voltage controlled resistor. 
     FIG. 2B  illustrates an embodiment which also uses an element  230 , such as a voltage-controlled resistor to set the output to float (disconnect from the DC-DC converter and the other voltage-controlled resistor). The float control circuit  232  sets the voltage-controlled resistor  230  on during normal operations; when float operation is needed, the voltage-controlled resistor  230  can be turned off. In one embodiment, the float control circuit  232  includes the same input as the feedback circuit  206 . This reduces the number of control inputs required for the high voltage power supply  200 . 
     FIG. 2C  illustrates examples of the details of a positive high voltage supply  200  of one embodiment of the present invention. The circuit of  FIG. 2C  generates a variable potential up to +5 KV as a function of the control voltage applied to the VSET pin. The circuit can source current, sink current, or float its output. Additionally with its bipolarity current monitor function and rapid feedback response (20 mS), the circuit can provide constant current operation when used with an external feedback system. A monitor function is included as a diagnostic aid. 
   Feedback circuit  206  is used to servo-control the output voltage of high voltage DC-DC converter  202  (when sourcing current) or voltage-controlled resistor  204  (when sinking current). In one embodiment, the output voltage is 2000 times the control voltage VSET minus an offset of 1.536V. In one embodiment, the feedback circuit has an operational amplifier that functions as the error amplifier for the feedback circuit. Unit  230  disconnects the entire circuit from the output line when VSET is less than 1.30V. With VSET between 1.30V and 1.536V, the output will be 1.536V. The voltage regulator of the float control circuit  232  generates a bias of 2.5V for the voltage-controlled resistor  204 , so that it does not begin to conduct until the error amplifier output is below 1.2V. This assures that the DC-DC converter  202  (which turns on with feedback circuit output VO=1.4V) and the voltage-controlled resister  204  are not on simultaneously. 
   The current sensor  214  drives an output signal ISNS to a voltage equal to 20,000 times the output current (in Amps) plus an offset of 2.048V. Zero output current is indicated by ISNS equal to 2.048V, +100 uA by 4.048V, and −100 uA by 0.048V. In one embodiment, float control circuit  232  includes an operational amplifier that serves as the float switch comparator. 
   Resistor  242  limits the output energy delivered during a short so that the sensitive board components are not damaged by latch-up during an accidental arc. 
   The circuit of the example of  FIG. 2C  uses three voltages: +5.3V for power, a reference of 4.096V (VREF), and a control voltage (VSET). A power voltage slightly higher than 5V is needed to assure a 5000V output from DC-DC converter  202  with rated load current up to 100 uA. The 4.096V reference can be eliminated from the interface if a reference circuit is included on the high voltage power supply module. Where many high voltage power supplies are used in the same system, a single reference on the motherboard can save space and power on each power supply module. 
   The schematic of  FIG. 2C  can be used to build a +5 KV or a +1 KV version of the circuit depending on the DC-DC converter model and the component values loaded on the PC board. Values in parentheses are to be used on the +1 KV version, in which case the output voltage is equal to 500 times VSET minus 2.048V. The DC-DC converter is a model Q10-5 in this case instead of a Q50-5. A memory such as a serial EEPROM allows a processor, such as a microprocessor, to identify the number and type of high voltage power supply modules in a system and adjust scaling factors and offsets accordingly. 
     FIG. 3  illustrates a negative high voltage power supply. In this example, an input voltage processing circuit  301  is used to process the input voltage into a range more conveniently used by the high voltage power supply  300 . In one embodiment, it is desirable that the DC-DC converter  302  is not turned on when the system is first powered up and the control input to the high voltage power supply is low. In one embodiment, the input voltage processing circuit  301  inverts the voltage VSET at the input of the feedback circuit about 2.048 volts. This means that a 4.048 volt value at the input processing circuit  301  produces a 0.048 volt input to the feedback circuit  306  and float control unit  332 . The DC-DC converter  302  is turned on when the feedback circuit output  306  is less than 3.9V. The voltage-controlled resistor  304  is turned on when the output of the feedback circuit is greater than 4.1 V. 
   Output voltages for the examples of  FIGS. 2 and 3  are given below. 
   Float Range
 
Vout=FLOAT for  V SET&lt;1.3V(±1000V &amp; ±5000V)
 
   Zero Volt Range
 
Vout=1.536V for 1.30≦1.536V(±5000V Version)
 
Vout=2.048V for 1.30 ≦V SET≦+2.048V(±1000V Version)
 
   Regulated Range
 
 V out=2.048V+500×( V SET−2.048V) for 2.048V≦ V SET≦4.048V(±1000V)
 
 V out=1.536V+2000×( V SET−1.536V) for 1.536V≦ V SET≦4.048V(±5000V)
 
The polarity of Vout will be positive for the circuit of  FIG. 2  and negative for the circuit of  FIG. 3 .
 
     FIG. 4A  illustrates a top view of a module containing a high voltage power supply of one embodiment.  FIG. 4B  illustrates a bottom view of the module of  FIG. 4A . The DC-DC converter shown on the module is an EMCO unregulated DC-DC converter. The components are mounted on a circuit board to form the module interconnecting the elements of the power supply. In the 5 KV version of one embodiment, the module is covered with high voltage insulating sealer. This is done to prevent arcing of the high voltage areas to other areas on the circuit board. Most silicone-based sealers, such as General Electric RTV 167 or the more commonly available General Electric Silicone II Gasket and Seal (GE283) will work for this application. Silicon-based high temperature automotive blue gasket seal will also work. 
   The module can be less than 2 in. (50.1 mm)×2 in. (50.1 mm)×2 in. (50.1 mm) in size. In the 5 KV embodiment, the module formed with a high voltage supply has a size of 1.45 in. (36.8 mm)×0.6 in. (15.2 mm)×1.0 in. (25.4 mm). In the 1 KV embodiment, the module has a size of 1.45 in. (36.8 mm)×0.6 in. (15.2 mm)×0.8 in. (20.3 mm). The small size of the module is an advantage in systems such as microfluidics systems, because the small size aids in the portability and ease of use of the system. 
     FIG. 5  illustrates modules  502 ,  504 ,  506  and  508  on a circuit board  512 . The circuit board  512  allows the interconnection of the modules  502 ,  504 ,  506  and  508  to the microprocessor  514 . In one embodiment, the modules plug into the circuit board for easy replacement and repair. Using a memory on each module can allow the processor  514  to determine which type of power supply is plugged into which location in the circuit board  512 . 
     FIG. 7  illustrates a microprocessor  700  with input and output signals. The inputs can include the feedback signal voltage, the output of the current sensor, as well as information from a memory on the module. Microprocessor  700  can be associated with A/D converters that can be internally or externally located. 
     FIG. 6  illustrates an example of the output voltage of a +1 KV high voltage power supply. The float region in this example is from 0 to 1.3 volts. Between 1.3 volts and 2.048 volts input, the output is 2.048 volts. Above 2.048 volts is the regulated region. In this example, the ideal curve is shown with a solid line and the dotted line shows an example of experimentally determined values. In one embodiment, the voltage offset and module ID are stored in the memory of the high voltage power supply. As shown in  FIG. 7 , the processor can receive identifiers and offsets from a memory on the module to allow a more accurate indication of the voltage curve. As shown in  FIG. 7 , the input and the feedback voltage and the current sensor output can be used to set the VSET. 
   Typically the VSET values are constant to produce a constant voltage output for different currents.  FIG. 8  illustrates the case where the output voltage is set to the constant value. 
   Any type of current/voltage transfer function can be produced with the external processor.  FIG. 9  shows an example of a voltage output/current output transfer function curve. The current sensor output can be used by the processor to calculate a new VSET. As the current changes, the input voltage is modified to fit the transfer function curve. 
   In one embodiment, the module can be controlled so that the VSET values are modified to keep the output current constant. As the output current rises, the input voltage, VSET, is decreased by the processor to correct the change in current. If the output current is decreased, the voltage output can be increased by the processor to bring the current back to the desired value. 
   One embodiment of the present invention uses a single power supply. In one example, a 4.096V reference is used, but a negative power supply rail is not used. In this example, a −5V supply is not needed—only a +5V (+5.3V) input voltage. One embodiment uses low current consumption for battery-power operation. This embodiment also has low heat dissipation. In a float mode, one embodiment only requires 80 μA from its 5.3V line. In one embodiment, the negative and positive polarity versions share a common electrical control interface. This makes the modules interchangeable in multi-module systems. 
   The foregoing description of preferred embodiments of the present invention has been provided for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to one of ordinary skill in the relevant arts. The embodiments were chosen and described in order to explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalence.