Abstract:
An asymmetric waveform pulse generator comprises a metallic oxide semiconductor field effect transistor (MOSFET) bridge circuit, which includes a plurality of MOSFETs for inverting high voltage DC voltage to asymmetric waveform pulses. The asymmetric waveform pulse generator further comprises a pulse-width modulating (PWM) circuit for generating PWM signals, and a plurality of isolation driving circuits corresponding to the plurality of MOSFETs, for controlling switching on/off of the plurality of MOSFETs in the MOSFET bridge circuit based on the PWM signals generated by the PWM circuit. Each of the isolation driving circuits comprises an isolation transformer for isolating the MOSFET bridge circuit from the PWM circuit. A FAIMS ion detector employing the asymmetric waveform pulse generator is also disclosed.

Description:
TECHNICAL FIELD 
     The present disclosure is related to high-field asymmetric waveform ion mobility spectrometry (FAIMS) ion detectors, and more particularly to an asymmetric waveform pulse generator of the FAIMS ion detector. 
     BACKGROUND 
     With the development of human society, people have stronger and stronger expectation on living quality especially on environmental quality, which leads to a new research field for real time monitoring surrounding environment. Currently-used environment detecting apparatus have the shortcomings such as high cost, huge space occupation, low detecting efficiency, and so on, and could not meet people&#39;s daily use requirements. In comparison, ion detectors based on high-field asymmetric waveform ion mobility spectrometry (FAIMS) technology are become more and more popular in environmental detection, in that the ion detectors have the advantages of high sensitivity, fast detecting speed, wide detecting product ranges, small space occupation and low cost. Generally, the ion detectors are widely used in environmental monitoring, public security management, and so on. 
     FAIMS technology is formed based on Mason and McDaniel&#39;s experimental discovery result that ion mobility K is related to strength of electric field enforced thereon. Under a lower electric field strength, for example the strength of the lower electric field is lower than 11000 V/cm, the ion mobility K is not influenced by the lower electric field strength. However, when under a higher electric field, for example the strength of the higher electric field is higher than 11000 V/cm, the ion mobility K would change in accordance with the higher electric field strength in a nonlinear manner. Under a higher electric field, a relationship between the ion mobility K and the electric field strength E can be expressed as:
 
 K=K   0 *[1+α 1 ( E/N ) 2 +α 2 ( E/N ) 4   +L]   (1)
 
     Where, K 0  is a mobility of ion in the lower electronic field, α is ion mobility coefficient, E is the electric field strength, and N is gas density. Here if:
 
α( E )=[α 1 ( E/N ) 2 α 2 ( E/N ) 4   +L]   (2)
 
     The formula (1) could be simplified as:
 
 K=K   0 *[1+α( E )]  (3)
 
     According to formula (3), the ion mobility K is specific for each particular kind of ion, which makes, those ions having same or similar mobility in lower electric field strength could be isolated under higher electric field strength. 
     In practice, when loading an impulsive voltage with high frequency and asymmetric waveform on a pair of electrode panels that are placed face to face and subsequently form a narrow space, the narrow space would thus become an electric field. When airflow carrying ions flows through the narrow space in a first direction, the ions would vibrate along a second direction of the electric field. Under a composed speed of speeds in the first direction and the second direction, the ions with different mobility would be isolated from each other. The composed speed has an X-component in a direction along the narrow space and a Y-component in a direction vertical to the direction along the narrow space. Meanwhile, if another suitable direct current (DC) voltage is loaded on the pair of electrode panels, an electric field generated by the suitable direct current would act in an opposite direction to that of the Y-component on the ions and subsequently the Y-component speed of some particular ions would be set off Consequently, the particular ions only has the X-component speed, which would lead the particular ions move along the narrow space and eventually pass through the narrow space. At the same time, ions other than the particular ions would move to the electric panel under the combined effect of their X-component and Y-component speed. In this way, the particular ions would be checked out. 
     In said ion detecting process, a generator to generate the voltage with asymmetric waveform meeting requirements of the FAIMS is very important, in that the waveform would directly influence the performance of the FAIMS ion detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The details as well as other features and advantages of the embodiments are set forth in the remainder of the specification and are shown in the accompanying drawings. 
         FIG. 1  is an ideal asymmetric waveform required by a high-field asymmetric waveform ion mobility spectrometry (FAIMS) system. 
         FIG. 2  is a schematic diagram of a high frequency high voltage asymmetric waveform pulse generator according to a preferred embodiment. 
         FIG. 3  is a detailed circuit of a PWM circuit in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 4  is a detailed circuit of an isolation driving circuit in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 5  is a detailed circuit of a MOSFET half-bridge circuit in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 6  is a detailed circuit of a protection circuit in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 7  is a detailed diagram of a reference voltage circuit according to a preferred embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the embodiments. However, it will be apparent to those skilled in the art that other embodiments that depart from these specific details may also be practiced. In other instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present disclosure with unnecessary detail. 
     In the exemplary embodiment of the present disclosure, in a high-field asymmetric waveform ion mobility spectrometry (FAIMS) ion detector, a generator for generating high frequency and high voltage asymmetric waveform pulse is relatively important. An ideal asymmetric waveform is shown as  FIG. 1 , where the shadow areas on two sides of the X-axle are equal, that is:
 
 V   +   ×t   2   =V   −   ×t   1  
 
     In order to generate said ideal waveform, in a preferred embodiment of the present disclosure, the high voltage high frequency asymmetric waveform pulse generator  200  comprises a MOSFET half-bridge circuit  203  for inverting high voltage direct current input into high voltage pulse. 
     As shown in  FIG. 2 , the high frequency high voltage asymmetric waveform pulse generator  200  according to the preferred embodiment of the present disclosure comprises a PWM (Pulse Width Modulation) circuit  201 , an isolation driving circuit  202 , a MOSFET half-bridge circuit  203 , a protection circuit  204  and a reference voltage circuit  205 . The PWM circuit  201  is used for providing negative and positive PWM signals with adjustable duty cycle, frequency and dead time. The isolation driving circuit  202  is used for controlling the MOSFETs in the MOSFET half-bridge circuit  203  respectively on or off according to the positive and negative PWM signals output from the PWM circuit  201 . The MOSFET half-bridge circuit  203  is used for inverting high voltage direct current input, such as 2 kV direct current, and outputting high voltage pulse, for example, in the form of square wave. The protection circuit  204  is used for protecting the MOSFETs in the MOSFET half-bridge circuit  203  either in static state or in dynamic state from being burnt by over-voltage or over-current. 
     The reference voltage circuit  205  provides various reference voltages for modules or chips in above-mentioned circuits. In a preferred embodiment of the present disclosure, the various reference voltages comprise 5V and 12V. For ensuring the isolation driving circuit  202  to efficiently drive the MOSFETs in the MOSFET half-bridge circuit  203 , in a preferred embodiment of the present disclosure, the reference voltage circuit  205  comprises linear transforming chips LT1084-12 and LT1084-5 provided by Linear Technology, to transforming main alternating voltage 220V to direct current voltages, such as 12V and 5V accompanying with suitably designed transforms and rectifier bridges. 
       FIG. 3  shows the PWM circuit  201  in a preferred embodiment of the present disclosure. In the preferred embodiment of the present disclosure, the PWM circuit  201  comprises a TL494CN chip produced by Texas Instruments company for generating pulse width modulation (PWM) signals, a NOT-gate circuit 74HC04 for reversing and reshaping the PWM signals generated by the TL494CN chip, and a dead zone generation circuit for generating positive and negative PWM signals to control the half bridge circuit  203 . 
     As shown in  FIG. 3 , the TL494CN chip is driven by 12V direct current voltage, and decoupled by a ceramic capacitor C 20 . A RC oscillator circuit composed of an adjustable resistor R 17  and a capacitor C 21  respectively connected to a RT pin and a CT pin of the TL494CN chip controls the frequency of the PWM signals, by changing resistance of the adjustable resistor R 17 , the frequency of the generated PWM signals could be adjusted correspondingly. In the embodiment, an adjustable resistor R 16  is connected between a reference voltage and a duty cycle (DTC) pin of the TL494CN chip, by changing the resistance of the adjustable resistor R 16  could change the duty cycle of the generated PWM signals. In a preferred embodiment of the present disclosure, because the PWM signals generated by the TL494CN chip are single-channel signals, for simultaneously controlling the upper and the lower arms of the MOSFET half-bridge circuit  203 , the generated signal-channel PWM signals are transmitted to a NOT-gate circuit, such as the 74HC04 type chip in  FIG. 3 , to generate two-channel PWM signals. In the preferred embodiment of the present disclosure, the NOT-gate circuit, i.e., the 74HC04 type chip comprises six NOT-gates. 
     Firstly, the single-channel PWM signals are input to a first NOT-gate by way of the pin  1 A of the NOT-gate circuit, wherein the output of the first NOT-gate is negative to the input thereof That means the output PWM signals by the first NOT-gate are negative to the single-channel PWM signals. In this embodiment, the PWM signals output by the first NOT-gate from the pin  1 Y is divided into two parts, one part enters into a sixth NOT-gate by way of a first dead circuit, and the other part is transmitted into a third NOT-gate and eventually input to a fourth NOT-gate by way of a second dead circuit. After the processing procedure, the polarity of the PWM signal PWM_H output from a pin  6 Y of the sixth NOT-gate is same to that of the single-channel PWM signals, and the polarity of the PWM signal PWM_L output from a pin  4 Y of the fourth NOT-gate is negative to that of the single-channel PWM signals. In this way, two-channel PWM signals with negative polarity are generated. 
     In a preferred embodiment of the present disclosure, the first dead circuit comprises a resistor R 10 , a diode D 10  and an adjustable resistor R 18  which are connected in parallel and between the pins  1 A and  6 A of the NOT-gate circuit, the second dead circuit comprises a resistor R 28 , a diode D 16  and an adjustable resistor R 27  which are connected in parallel and between the pins  3 Y and  4 A of the NOT-gate circuit. Changing the resistance of the adjustable resistor R 18  or R 27  respectively in the first and the second dead circuits could change the dead time of the first and second dead circuits respectively, so as to avoid simultaneously switching on/off the upper or lower arms of the half-bridge circuit  203 , and subsequently avoid conducting the high voltage direct current to the ground, thus improve the reliability of the high frequency high voltage asymmetric waveform pulse generator  200  of the present disclosure. 
     In the high frequency high voltage asymmetric waveform pulse generator  200  according to one preferred embodiment of the present disclosure, the power of the PWM signals PWM_H and PWM_L are relatively low as not to drive the MOSFETs in the MOSFET half-bridge circuit  203 , so that the power of the PWM signals PWM_H and PWM_L should be amplified to be able to switch on/off the MOSFETs in the MOSFET half-bridge circuit  203 . When peak voltage of the MOSFET half-bridge circuit  203  is up to 2 kV, and/or the frequency thereof is up to 200 kHz, those low voltage circuits, such as the PWM circuit  201 , would be seriously disturbed even destroyed by the peak voltage originating from the MOSFET half-bridge circuit  203 . For this reason, in the exemplary asymmetric waveform pulse generator  200  according to one of the present disclosure, the isolation driving circuit  202  is employed to eliminate the disturb of the peak voltage to the low voltage circuits, such as the PWM circuit  201 , consequently improve the quality of the PWM signals generated by the PWM circuit  201 . 
     Because the PWM signals to drive each MOSFET in the half-bridge circuit  203  should be isolated, as well as the upper and lower arms of the MOSFET half-bridge circuit  203  in a preferred embodiment of the present disclosure are structured with two MOSFETs connected in series, in this embodiment, total four channels of PWM signals isolated from each other are needed, as shown in  FIG. 5 . The isolation driving circuit  202  according the preferred embodiment of the present disclosure has the advantages of simple structure and high isolation, in that each MOSFET has an independent isolation driving circuit. 
     In a preferred embodiment of the present disclosure, the isolation driving circuit  202  of each MOSFET in the MOSFET half-bridge circuit  203  are same structured, for simplicity of description, the isolation driving circuit  202  of the MOSFET Q 1  would be detailed described in the following specification. Isolation driving circuits  202  of the other MOSFET Q 2 , Q 3  and Q 4  are same to that of the MOSFET Q 1 , and would not be described in detail. 
     In the preferred embodiment, the isolation driving circuit  202  of the MOSFET Q 1  comprises the IXDD409 type driver produced by IXYS company, the peak current of the IXDD409 type driver is up to 9 A, which can greatly fasten the speed of the MOSFET Q 1  being switch on so as to lower loss of the MOSFET Q 1 . The IXDD409 type driver is powered by 12V direct current voltage, and decoupled by the capacitor C 1 . For ensuring providing enough driving current to drive the MOSFET Q 1  switch on, in a preferred embodiment of the present disclosure, a capacitor C 5  is employed as power storage of the IXDD409 type driver. An EN pin of the IXDD409 type driver is the enable end and is connected to high level signal, an IN pin receives input PWM signals, two OUT pins output driving signals, voltage difference of positive and negative amplitude of which could be up to 12V. In a preferred embodiment of the present disclosure, a gate-break-over voltage of the employed IXYS 3N120 type MOSFET Q 1  is 3.5V, the 12V driving signals could ensure switching on of the MOSFET Q 1 . The driving signals output by the IXDD409 type driver pass through the capacitor C 2  and the isolation transformer T 1  so as to be isolated from the high voltage circuit, wherein a ratio of the isolation transformer T 1  is 1:1, detailed parameters of the isolation transformer T 1  is shown in Sheet 1. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Sheet 1 Spce of isolation transformet 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Input 
                   
                 Output 
               
               
                   
                   
                   
                   
                   
                 Voltage 
                   
                 Voltage 
               
               
                   
                 Dielectric 
                   
                 Frequency 
                 Specified 
                 Amplitude 
                 Pulse 
                 Amplitude 
               
               
                   
                 Strength 
                 Trigger 
                 Fp 
                 ∫udt 
                 V1 
                 Width 
                 V2 
               
               
                 Ratio 
                 V p (kV) 
                 Manner 
                 (Hz) 
                 (μVS) 
                 (V) 
                 t n (μS) 
                 (V) 
               
               
                   
               
               
                 1:1 
                 3.1 
                 pulse 
                 250k 
                 200 
                 15 
                 5 
                 15 
               
               
                   
               
             
          
         
       
     
     In a preferred embodiment of the present disclosure, the capacitors C 2  and C 4  is employed to block direct current signals, a Zener diode D 4  coupled o the isolation transformer T 1  is used to prevent the isolation transformer T 1  from outputting over-lower voltage. The gate of the MOSFET Q 1  is connected to a resistor R 6 , for reducing ring phenomenon and EMI. In a preferred embodiment of the present disclosure, the MOSFET Q 1  is an N-type transistor. In a switching-on procedure, driving signals are transmitted from the isolation transformer T 1 , through a resistor R 6 , and eventually to the gate of the MOSFET Q 1 , in a switching-off procedure, the driving signals are transmitted from the gate of the MOSFET Q 1 , through the diode D 1 , and eventually to the isolation transformer T 1 . The diode D 1 , with an anode connected to a gate of the MOSFET Q 1  and a cathode connected to the isolation transformer T 1 , is used for accelerating switching off of the MOSFET Q 1 , to ensure smooth switching between on and off of the MOSFET Q 1 , Q 2 , Q 3  and Q 4 . In  FIGS. 4 , D 3  and D 7  are Zener diodes, which are connected in series face to face so as to secure the driving signals transmitted to the gates of the MOSFETs not beyond specified voltage amplitude, to protect the MOSFETs from being destroyed. 
       FIG. 5  shows the MOSFET half-bridge circuit  203  in accordance with a preferred embodiment of the present disclosure. Under the effect of asymmetric high field, two panels of an FAIMS ion detector (not shown) according to the preferred embodiment of the present disclosure can be regarded as an equivalent capacitor, such as the capacitor C 1  illustrated in  FIG. 5 . The connection points VCC and GND are respectively connected to the positive and negative of the high voltage direct current input, the capacitor C 3  connected between the points VCC and GND is a high voltage storage capacitor, the resistor R 3  connected between the point VCC and the drain of the MOSFET Q 1  is a non-inductance resistor to limit the current. Gates of the MOSFET Q 1 , Q 2 , Q 3  and Q 4  are respectively connected to the isolation driving circuit  202  to receive the driving signals. The drain of the MOSFET Q 1  receives the high voltage direct current from the connection point VCC by way of the resistor R 3 , a source of the MOSFET Q 1  is connected to a drain of the MOSFET Q 2 , thus the MOSFET Q 1  and the MOSFET Q 2  are connected in series and collectively form the upper arm of the half-bridge circuit  203 . The source of the MOSFET Q 2  is connected to the drain of the MOSFET Q 3 , and the source of the MOSFET Q 3  is connected to the drain of the MOSFET Q 4 , thus the MOSFET Q 3  and the MOSFET Q 4  are connected in series and collectively form the lower arms of the half-bridge circuit  203 . A source of the MOSFET Q 4  is connected to the connection point GND. 
     The source of the MOSFET Q 2  and the drain of the MOSFET Q 3  is connected at connection point A, that is, the connection pint A of the upper and lower arms of the half-bridge circuit  203  is one output of the asymmetric waveform pulse generator  200  of the present disclosure, and is connected to one of the two panels of the ion detector, so that the connection point A could be regarded as connected to one end of the equivalent capacitor C 1 . When the MOSFETs Q 1  and Q 2  in the upper arm are simultaneously switched on, electric potential of the point A is same to that of the connection point VCC, and when the MOSFETs Q 3  and Q 4  in the lower arm are simultaneously switched on, the electric potential of the point A is same to that of the connection point GND. In the preferred embodiment, the electric potential of the connection point GND is 0V. The adjustable resistor R 1  and R 2  are connected in series between the drain of the MOSFET Q 1  and the source of the MOSFET Q 4 . In other words, the resistor R 1  and R 2  are connected in series between positive and negative of the high voltage direct current input. A connection point B structured between the resistors R 1  and R 2  is another output of the asymmetric waveform pulse generator  200  of the present disclosure, and is connected to another one of the two panels of the ion detector, so that the connection point B could be regarded as connected to another end of the equivalent capacitor C 1 . By altering resistance of the resistors R 1  and R 2 , the electric potential of the connection point B could be changed between 0V and VCC continuously, accompanying with adjusting the frequency and duty ratio of the PWM signals that drives the MOSFETs, asymmetric waveform needed by the FAIMS would be generated between the equivalent capacitor C 1 , i.e., between the connection points A and B. In a preferred embodiment of the present disclosure, the N-type MOSFETs Q 1 , Q 2 , Q 3  and Q 4  are 3N120-type MOSFETs produced by IXYS company. 
     In general, when a pulse circuit works with high frequency, voltage and current loaded upon MOSFETs in the pulse circuit change very fast, which are prone to making the MOSFETs destroyed. Therefore, in a preferred embodiment of the present disclosure, a protection circuit is connected in parallel between the source and drain of each of the MOSFETs Q 1 , Q 2 , Q 3  and Q 4 , for absorbing overshoot voltage to prevent the MOSFETs Q 1 , Q 2 , Q 3  and Q 4  from being broken down due to over voltage, as well as reducing overshoot of rising edge of the output pulse by the asymmetric waveform pulse generator  200 , so as to output high quality asymmetric waveform. 
       FIG. 6  shows a protection circuit of the MOSFET Q 1 , other MOSFETs Q 2 , Q 3  and Q 4  have same structure of protection circuits to that of the MOSFET Q 1 , for the simplicity of description, the specification would only describe the structure of the protection circuit  204  of the MOSFET Q 1 . 
     When the MOSFET Q 1  is switched off, the high voltage direct current input loaded upon the drain of the MOSFET Q 1  charges the capacitor C 6  by way of the diode D 2 . When the MOSFET Q 1  is switch on, the power stored in the capacitor C 6  is discharged by way of the resistor R 5 . The diode D 2  is used to restrain peak voltage oscillation upon the drain of the MOSFET Q 1 , the diode D 2  is used to prevent the capacitor C 6  from discharging when the MOSFET Q 1  is switched on, so as to reduce the load of the MOSFET Q 1  when switched on. In a preferred embodiment of the present disclosure, power are transferred from the MOSFET Q 1  to the capacitor C 6 , and eventually depleted by the resistor R 5 , so as to protect the MOSFET Q 1 . In a preferred embodiment of the present disclosure, switch-on time of the MOSFET Q 1  is bigger than the discharging time of the capacitor C 6 , that is:
 
τ RC   &lt;t   on   (5)
 
     Here, τ RC  is a time constant of the RC circuit composed of the resistor R 5  and the capacitor C 6 , t on  is the switch-on time of the MOSFET Q 1 . 
     Because electronic features of the MOSFETs Q 1 , Q 2 , Q 3  and Q 4  are different from each other, divided voltages upon the MOSFETs Q 1 , Q 2 , Q 3  and Q 4  would be different from each other, the worst situation is the divided voltage upon one of the MOSFET Q 1 , Q 2 , Q 3  and Q 4  exceeds the value of permissible voltage of the corresponding MOSFET and destroys it. For this reason, in a preferred embodiment of the present disclosure, four equate resistors with same resistance are connected respectively in parallel to the sources and drains of the MOSFETs Q 1 , Q 2 , Q 3  and Q 4  in the upper and lower arms, such as the resistor R 7  electronically connected in parallel to the source and drain of the MOSFET Q 1 . In this way, the voltages loaded upon the MOSFETs Q 1 , Q 2 , Q 3  and Q 4  could reach to be same, so as to achieve protection to the MOSFETs Q 1 , Q 2 , Q 3  and Q 4 . 
     In a preferred embodiment of the present disclosure, the work voltages of the above-mentioned chips comprise 5V and 12V direct current (DC) voltages. For ensuring the isolation driving circuit  202  having enough power to fast drive the MOSFETs to switch on or off, linear transformer chips LT1084-12 and LT1084-5 produced by Linear Technology, a transformer T and a rectification bridge collectively form the reference voltage circuit  205 , to transform utility voltage, such as alternating current (AC) voltage 220V, to DC voltages, such as 12V and 5V. Detailed diagram of the reference voltage circuit  205  is shown in  FIG. 7 . 
     The interface P 1  in  FIG. 7  receives utility voltage, such as AC 220V. The utility voltage is then transformed by the transformer T to AC voltage 18V. The AC voltage 18V is subsequently rectified by the rectification bridge to DC voltage. Finally, the chips LT1084-12 and LT1084-5 respectively transform the rectified DC voltage as 12V and 5V DC voltages, which provide power to various chips in the asymmetric waveform pulse generator  200 . The most output current of the LT1084 series is up to 5 A, which could meet the requirements of the asymmetric waveform pulse generator  200  of the present disclosure. 
     In the preferred embodiment of the present disclosure, a peak voltage of the high frequency high voltage asymmetric waveform pulse generator  200  can reach to 2 kV, the frequency thereof is 200 kHz, and rising edge and falling edge of the asymmetric waveform pulse output by the pulse generator  200  are both within 10 ns, which means the quality of the asymmetric waveform is good. In addition, the structure of the pulse generator  200  is simple, wherein the frequency, duty cycle and dead time of the PWM circuit  201  are adjustable, and the power consumption is low, which collectively makes the asymmetric waveform pulse generator  200  meet the requirements of the FAIMS system. 
     While the foregoing description and drawings represent the preferred embodiments of the present disclosure, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the present invention as defined in the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and not limited to the foregoing description.