Abstract:
A high voltage switching power supply ( 10 ) for an X-ray/Gamma ray imaging camera provides high voltage switching and depolarization capabilities. The power supply includes a high voltage polarity switching and an image detector charge bleeding circuit ( 90 ) and is particularly useful with high energy radiation imaging cameras utilizing Cd—Te based detector substrates, especially substrates with blocked contacts, where charge accumulation in the detector material reduces imaging efficiency.

Description:
The present application claims the benefit of prior filed U.S. Provisional Application Ser. No. 60/433,457 filed 13 Dec. 2003, to which the present application is a regular U.S. national patent application. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of digital imaging of X-ray and gamma ray radiation. More specifically it relates to switching-type voltage power supplies for digital X-ray and gamma ray imaging devices. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a Dynamic Imaging Camera (DIC) or Scanning Camera (SCAN) having Cadmium-Telluride (Cd—Te, including Cd—Zn—Te) based radiation detectors for imaging X-ray signals. Cd—Te based detectors for imaging X-ray signals are known in the art, and are particularly useful for high energy radiation imaging. High energy radiation imaging include X-ray and gamma ray radiation in the 1.0 KeV and greater range. Typically, these detectors have a blocking contact on one face and pixel contacts on another face. In turn, the pixel contacts are bump-bonded to charge integrating application specific integrated circuits (ASICs). The blocking contact serves to reduce the dark or ambient current of the detector by a factor of 3 to 10. Lower dark currents mean higher sensitivity to incoming x-rays (signal). 
   However, a problem can exist in a Cd—Te detector having a blocking contact. Such contacts (e.g., Indium based blocking contacts) can polarize after a few seconds of operation, i.e., one to several hundreds of seconds. Polarization means that the detector starts to loose signal and the image loses definition or acuity and gets more blurry. Polarization happens due to gradual electric charge trapping inside the material bulk of the Cd—Te detector. Previously, one could not use a Cd—Te detector material in this X-ray imaging mode for more than a few seconds, due to the polarization effect. 
   Therefore, it would be useful in the field to have a means for preventing or overcoming the effect of electric charge trapping in high energy X-ray imaging systems utilizing Cd—Te based radiation detectors. 
   SUMMARY OF THE INVENTION 
   A Cadmium-Telluride (Cd—Te) based DIC detector typically requires a high voltage (HV) bias potential to operate properly. Unfortunately, such detectors can quickly accumulate an electrical charge and become polarized. The polarization charge offsets the HV bias potential and adversely affects operation and image quality of the camera imaging device. Once it becomes polarized, the detector unit requires a “refresh” action, i.e., the bleeding-off of the trapped or accumulated electric charge to depolarize the detector unit and restore operational efficiency of the device. 
   The present invention is a high voltage (HV) switching power supply for use with a high energy X-ray camera imaging device with a Cd—Te based detector (including a CdZnTe based detector). Such X-ray imaging cameras typically comprise a detector substrate bonded to a CMOS substrate and mounted to an interface/signal processing board, in combination with a power supply. The output from the camera is typically communicated to a computer for image processing. The depolarizing, switching HV power supply of the present invention is intended as a power supply for such an X-ray imaging camera. 
   The present HV switching power supply enables the use of Cd—Te detectors (especially those having blocking contacts) in dynamic imaging camera X-ray imaging systems and scanning camera/sensor imaging systems. The HV power supply is used to supply HV to the Cd—Te detector of the X-ray imaging system. The HV output of the HV power supply is switchable (on/off) at user defined intervals. For example, every few seconds the HV output of the switching power supply automatically cycles off for a few milliseconds and then very fast on again. When the HV output goes off, any electrical charge trapped at the detector(s) is able to bleed-off, which reverses or prevents polarization of the detector. This prevents the accumulation of electric charge and polarization of a Cd—Te detector having a blocking contact. The prevention of polarization allows continuous usage of a Cd—Te type detector DIC imaging device, and enables the use of such devices for inline imaging, e.g., in non-destructive testing or automated X-ray inspection systems. 
   The present depolarizing, HV switching power supply provides both high and low voltages useful for powering a Cd—Te base radiation detector, with or without a blocking contact. Typical low and high voltage requirements for Cd—Te base radiation detectors are known in the art. For example, low bias voltage requirements for the Cd—Te type detectors are on the order of +/−1.0V to +/−15.0V DC to operate the detector&#39;s internal circuits. The present HV switching power supply also provides an adjustable high bias voltage from +80 VDC to +450 VDC for driving the detector. 
   By using the present power supply that switches on/off the High Voltage as described herein, a plurality of dynamic imaging applications utilizing CdTe or CdZnTe detectors becomes possible. The applicant has developed already cameras that operate at 50 frames per second, 100 fps or even 400 fps. These cameras operate smoothly over many hours or indeed days without a need to manually refresh the detectors (by manually powering off, waiting and then switching the HV on again). The smooth, stable and uninterrupted operation in X-ray imaging applications is essential. Example applications where such DIC or SCAN cameras can be used included but are not limited to non destructive testing, inline inspection, automatic X-ray inspection, dental panoramic imaging, Computerized Tomography etc. 
   Additionally, while it was emphasized that the current invention is mostly suitable for CdTe or CdZnTe based detectors with a blocking contact, it can also have application in CdTe or CdZnTe detectors without blocking contact, but equipped with Platinum (Pt), Gold (Au) or other conventional contacts. Even such conventional contacts can create polarization after many minutes or hours and a power HV supply as described herein is ideal for the smooth and stable operation over many hours or indeed days. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 to 5  are schematic representations illustrating the present depolarizing/switching power supply and component circuits. 
       FIG. 6  (rising slope) displays the ramp-up and ramp-down waveforms of an actual Cd—Te semiconductor detector substrate where condition of the present HV power supply were: Ch 1 : HV_EN=5V, 50 μsec/div.; Ch 2 : Detector Voltage, 100 v/div. 
       FIG. 7  (falling slope) displays the ramp-up and ramp-down waveforms of an actual Cd—Te semiconductor detector substrate where condition of the present HV power supply were: Ch 1 : HV_EN=5V, 50 μsec/div.; Ch 2 : Detector Voltage, 100 v/div. 
       FIG. 8  (rising slope) demonstrates the flat slope produced by a pure capacitive load and the power supply current limiter, where: Ch 1 =HV_EN, and Ch 2 =Capacitor Voltage, 100 μsec, 100 v/Div. 
       FIG. 9  (falling slope) demonstrates the flat slope produced by a pure capacitive load and the power supply current limiter, where: Ch 1 =HV_EN, and Ch 2 =Capacitor Voltage, 100 μsec, 100 v/Div. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, the details of preferred embodiments of the present invention are graphically and schematically illustrated. Like elements in the drawings are represented by like numbers, and any similar elements are represented by like numbers with a different lower case letter suffix. 
     FIG. 1  generally illustrates the present depolarizing/switching power supply  10  of the present invention. In a preferred embodiment exemplified in  FIG. 1 , the present invention was externally supplied with +24V DC from a medical grade external power source (not shown). Of course, an alternative such a power source of an appropriate supply voltage is known to and selectable by one of ordinary skill in the art for practice in the present invention, either internally or externally. In this embodiment, the supply power requirement for the present switching power supply was on the order of about 10 W. 
   As illustrated in  FIG. 1 , the present HV switching power supply  10  comprised the combination of five main circuits: a control/conditioning circuit  18 , an internal power supply  30 , a low voltage power supply  50 , a high voltage power supply  70 , and a high voltage switch  90 . The conditioning/control circuit  18  has an external power source connection  20  and included a main power on/off switch  24 . The conditioning circuit  18  conditions the external power appropriately for use by the other circuits of the power supply  10 . A main operating voltage power connection  22  communicates the conditioned electrical power to the other circuits of the power supply  10 . Preferably, the present HV switching power supply  10  is electrically shielded, as is accomplishable by one of ordinary skill in the art. In a preferred embodiment, the switching power supply was housed in a metal casing (not shown), which casing was connected to a power supply ground J2-1 to attenuate or eliminate electromagnetic interference (EMI). Generally, the connectors utilized in the power supply are protected from electrostatic discharge (ESD) and filtered for EMI, and the DC voltage connections are protected from reverse polarity and voltage spikes. Additionally, the high voltage enable signal circuit is protected from over voltages and has a pull-down feature. The know-how to accomplish such protections in the present invention are known to and practicable by the ordinary skilled artisan. Preferably, shielded cable is utilized for current carrying conductors and coaxial cable for bias voltage conductors. 
     FIG. 2  is a schematic diagram of a preferred embodiment of an internal power supply circuit (int_supply)  30  practiced in the present HV switching power supply  10 : The internal power supply  30  provides low voltage power as required for the other circuits of the present switching power supply  10 . The internal power supply  30  comprises a linear regulator (N 5 )  32  and linear regulator circuit  34  adjusted to provide a +12V DC output  46 . An LM317 integrated circuit was utilized as the linear regulator  32  of the linear regulator circuit  34 . The internal power supply  30  also comprises a switching regulator (N 1 )  38  and switching regulator circuit  40 . The switching regulator circuit  40  provide the low negative voltage (e.g., −5V DC in the embodiment illustrated) as required in the other circuits of the HV switching power supply  10  at the negative bias voltage output  42 . A National LM2672 integrated circuit 260 kHz switching regulator in buck-boost configuration was utilized as the switching regulator  38 . A minimum drain for the negative bias voltage output  42  was provided by an LED  44 . 
     FIG. 3  is a schematic diagram of a preferred embodiment of a low DC voltage power supply circuit (power_supply)  50  practiced in the present HV switching power supply  10 . The power supply circuit  50  provides low DC voltages for operating external devices, like the detector unit(s), connected to the power supply  10 . In the exemplified preferred embodiment, the power supply circuit  50  comprised two switching regulator circuits  52  &amp;  53  to provide +6.7V and +5V respectively to operate detector elements. National LM2672s regulators  56  were used in the switching regulator circuits  52  &amp;  53  to respectively provide a +6.7 v output (Vout 1 )  58  and a +5V output (Vout 2 )  60 . The LM2672s regulators  56  of both switching regulator circuits  52  &amp;  53  were disposed in buck configuration. In this preferred embodiment, the +5V switching regulator circuit  53  had a green SMD LED  62  and the +6.7V switching regulator circuit  52  had a yellow angled LED  63  to differentiate and indicate the low voltage power supply  50  was turned on. 
     FIG. 4  is a schematic diagram of a preferred embodiment of a high voltage, switching power supply circuit (HV_supply)  70  of the present depolarizing/switching power supply  10 . The HV power supply circuit  70  provides a high voltage bias output  72  to the high voltage switch circuit  90  (see  FIG. 5 ). In the preferred embodiment shown, the HV switching power supply circuit  70  used a standard UC3842 current-mode PWM controller  76  operating in a boost configuration at about 75 kHz. A voltage feedback loop was used to adjust the voltage at high voltage bias output  72  to the desired level (about +350V in the present embodiment). The current feedback protected the switching power supply circuit  70  from short circuits and provided good transient response, improving HV bias rise time. 
   The voltage division utilized in the voltage feedback loop of the HV power supply circuit  70  was heavy and yielded a ripple voltage of 1–2 Vpp without compensation. A compensation circuit synchronized to the UC3842&#39;s oscillator circuit added an artificial ramp onto each current pulse. The compensation ensures the power supply did not skip pulses, and limited the voltage ripple to about 200 mVpp. A constant current load was used to provide about a 1.0 mA current drain for the power supply independent of the output voltage. 
     FIG. 5  is a schematic diagram of a preferred embodiment of a high voltage switching circuit (HV_switch)  90  practicable in the present HV depolarizing power supply  10 . As shown in  FIG. 5 , the HV switching circuit  90  provides a high bias voltage at its HV voltage output  92  in response to the presence of an active high voltage enable signal at its signal input (HV-EN)  94 . 
   However, when the high voltage enable signal at the signal input  94  is inactive or disabled, the HV switching circuit  90  provides a −5V DC bias at its voltage output  92 . Additionally, when the high voltage enable signal at the signal input  94  goes inactive or is disabled, an FET sub-circuit in the H_switch  90  is cuts off the high voltage bias voltage. When the high voltage bias voltage is cutoff, the bias voltage at the HV voltage output  92  is pulled down to −5V, causing a reversal of the biasing current in the Cadmium-Telluride photo-conductor material. Reversing the biasing current in the photo-conductor material bleeds off the trapped electrical charge and de-polarizes the detector unit. 
   The Q 5  FET  100  is connected in series with the HV bias. It has a pull-up to the HV bias, resulting in an output of HV-Vgs in the steady-state. During the ramp-up, current flowing through resistor  102  causes a voltage differential. The Q 7  transistor  104  pulls the FET gate  100  closed when the set current limit is exceeded. This results in a triangular waveform for the HV bias voltage. 
   The Q 3  FET  106  pulls the Q 5  FET gate  100  down to −5V when open. This closes Q 5  FET  100  and reverses the bias voltage. Q 3  FET  106  has a similar current-limiter circuit as Q 5  FET  100 , resulting in a linear down slope. Opening and closing Q 3  FET  106  enables controlling the bias voltage. The voltage of the Q 3  FET gate  106  is controlled by the high voltage enable signal at the signal input  94 . When the high voltage enable signal at the signal input  94  is active (enabled or “pulled high”), an indicator LED  110  was lit. 
   The high voltage switching circuit  90  can be operated at a much higher switching frequency than in the illustrated embodiment. There is about a 50 μsec initial delay between low-to-high bias voltage transition and the beginning of the HV bias voltage ramp. The delay is likely caused by the large resistors used in pull-up circuit and the FET gate capacitance. Modification of the characteristics of these components could lessen the delay. It was intended in the present embodiment that the imaging system operate at about 50 frames per second, thus enabling dynamic imaging, the X-raying of moving objects. 
   HV bias voltage rise and fall time are determined by the bias voltage current limiter (see  FIG. 5 ) and the capacitance of the DIC detector unit. Approximate state change time (t) is given by:
 
 t =(350V* C )/5 mA
 
About 6–7% of the decay/growth time (t) is not current limited. Load resistance seen by the detector capacitance (C) was not known. An accurate result could be measured separately for each detector and power supply  10  combination. An approximated result has to be multiplied to estimate when the HV bias voltage has settled within 1% of steady state. See  FIG. 6 .
 
     FIGS. 6–9  display the ramp-up and ramp-down waveforms of an actual Cd—Te semiconductor detector unit. The unit under testing had four 1 cm 2 . A DIC  100  has a 25 cm 2  area. The falling slope exhibits undershoot, which appears to originate from the Cd—Te detector unit. The undershoot amplitude is limited by the power supply  10  current limiter of the HV switch  90 . The illustrated signal is almost a perfect 5 mA current-limited slope until the undershoot peaks. 
   Rise time is about 200 μsec, fall time is about 250 μsec. However, the detector takes several milliseconds to stabilize after the state change. Therefore, the ability of the power supply to ramp the high voltage is not the limiting factor in the depolarization process. The figures demonstrate the flat slope produced by a pure capacitive load and the power supply current limiter. The undershoot exhibited by the falling slope of the Cd—Te detector unit is not present with a capacitive load. 
   While the above description contains many specifics, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of one or another preferred embodiment thereof. Many other variations are possible, which would be obvious to one skilled in the art. Accordingly, the scope of the invention should be determined by the scope of the appended claims and their equivalents, and not just by the embodiments.