Abstract:
A circuit and method for electrostatic discharge testing using transmission line pulsing. A plurality of transmission line networks may be connected to a device under test, and each transmission line network may have different connected terminations. Switches may be used to select which transmission line networks are connected to the device under test, and which terminations, if any, are connected to transmission line networks.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/601,361, filed Feb. 21, 2012, which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Electrostatic discharge (ESD) is typically a high-voltage short-duration event, typically involving voltages in the range of hundreds to thousands of volts, and a typical duration of 10 to 150 nanoseconds. Devices such as electronic components, electronic circuits and systems, and larger systems such as automobiles, are commonly tested to confirm that they can withstand various standardized ESD events. For ESD testing, one common way to emulate ESD in a standard manner is to charge a floating transmission line to a pre-determined high voltage, and then discharge the transmission line into a Device Under Test (DUT). This method is called Transmission Line Pulsing (TLP).  FIG. 1A  illustrates a basic TLP test system  100 . A high voltage DC supply  102  charges a transmission line  106  through a high impedance (R s ) and a switch  104 . After the transmission line  106  is charged, the switch  104  connects one end of the transmission line to a DUT  108 . The transmission line  106  generates a single voltage pulse with a pulse width determined by the propagation time along the length of the transmission line. A termination impedance R L  matches the impedance of the transmission line  106  to prevent reflections. 
         [0003]    Some ESD events may be more complex than a single voltage pulse. For example, they may involve a positive pulse followed by a negative pulse. In  FIG. 1A , if the termination resistance R L  is eliminated (effectively infinite termination resistance), then a reflected pulse will be generated having an opposite polarity from the high voltage DC power supply. Alternatively, the termination resistance may be made a different impedance than the impedance of the transmission line and the voltage waveform will be partially reflected at the impedance discontinuity. 
         [0004]    In some test systems, the goal of ESD testing is to confirm that the DUT can withstand a standard emulated ESD voltage waveform. In other test systems, the goal is to characterize the impedance of a device or circuit that is intended to protect a system during ESD. A protective device (for example, a silicon controlled rectifier or thyristor) may turn on (called snapback) at a high voltage, and then conduct current at a reduced voltage (called the holding point). The voltage waveforms needed to characterize the impedance of a protective device at multiple current levels need to have a peak voltage sufficient to turn on the protective device and then “stair step” down to one or more holding point voltages. The resulting test system is called a multilevel TLP system.  FIG. 1B  illustrates an example multilevel TLP test system  110 . In  FIG. 1B , there are two transmission line sections ( 114 ,  116 ) of different lengths, in series, separated by a series resistor R SERIES . Both sections are pre-charged. When the switch  118  is closed, a pulse is generated in each section of the transmission line. The voltages are partially reflected in each section of the transmission line at each impedance discontinuity. The pulses and reflections add and cancel at the DUT to form a “stair-stepped” multi-level voltage pulse. There are many variations, with varying placements of the DUT, switches, and resistances. 
         [0005]    There are many different standards for ESD test systems for different product categories. There are standard systems that emulate ESD originating from a person (Human Body Model), standard systems for the automotive industry (ISO 10605, Zwickau), standard systems for electronic products such as cell phones, computers and televisions (IEC 61000-4-2), and standard systems specified by other standards bodies such as JEDEC, ESDA, and JIETA. Some ESD tests need complicated setups that are typically done only by the manufacturers of a final product, such as an automobile. This is a problem for vendors supplying parts or subsystems for multiple final product categories. The parts and subsystems vendors need to be able run a wide variety of different standard tests. There is a need for an ESD test system that can flexibly produce a wide variety of voltage waveforms consistent with a wide variety of industry ESD test standards. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1A  is a simplified block diagram schematic of an example prior art transmission line pulse system. 
           [0007]      FIG. 1B  is a simplified block diagram schematic of an example alternative prior art transmission line pulse system. 
           [0008]      FIG. 2  is a simplified block diagram schematic of an example embodiment of a transmission line pulse system. 
           [0009]      FIG. 3A  is a timing diagram illustrating an example voltage waveform generated by a transmission line in the system illustrated in  FIG. 2 . 
           [0010]      FIG. 3B  is a timing diagram illustrating an example voltage waveform generated by a second transmission line in the system illustrated in  FIG. 2 . 
           [0011]      FIG. 3C  is a timing diagram illustrating an example voltage waveform generated at the DUT in the system illustrated in  FIG. 2 . 
           [0012]      FIG. 4  is a timing diagram illustrating an example alternative voltage waveform generated by a transmission line in the system illustrated in  FIG. 2 . 
           [0013]      FIGS. 5A ,  5 B,  5 C, and  5 D are block diagram schematics illustrating alternative example embodiments of transmission line networks. 
           [0014]      FIGS. 6A ,  6 B, and  6 C are block diagram schematics illustrating alternative example embodiments of impedance networks. 
           [0015]      FIG. 7  is a flow chart of an example embodiment of a process for generating ESD test voltage waveforms. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 2  illustrates an example embodiment of a TLP system  200  that can generate a wide variety of waveforms. System  200  includes a high-voltage DC power supply  202  and networks of transmission lines ( 206 ,  208 ,  210 ,  212 ). A first switch  204  connects the power supply  202  to first ends of a portion of the networks of transmission lines. Switch  204  then connects the first ends of the portion of the networks of transmission lines to a DUT  216 . Second switches  214  determine which networks of transmission lines are charged by the power supply, and which networks of transmission lines are coupled to the DUT  216 . Third switches  218  determine which second ends of transmission line networks, if any, are connected to terminations. Note that the transmission line networks selected by the second switches  214 , with connected terminations selected by third switches  218 , are effectively connected in parallel. In the example of  FIG. 2 , there may also be an impedance  234  in series with the DUT  216 . A network may be an individual transmission line, or a network may comprise multiple transmission lines, as discussed in more detail in conjunction with  FIGS. 5A-5D . The transmission lines may have various lengths and various characteristic impedances. 
         [0017]    In the example illustrated in  FIG. 2 , each individual transmission line network can be connected to one of two terminations, or may be left open. A network of transmission lines (for example, network  212 ) may be connected to multiple terminations (for example, terminations  230  and  232 ) or may be left open. Termination  220  may be, for example, an impedance that matches the characteristic impedance of transmission line  206  to prevent reflections. Termination  222  may be, for example, an impedance that results in partial reflections in transmission line  208 . Leaving a switch  218  open for a transmission line leaves the end of the transmission line open (essentially infinite impedance) to provide a full magnitude reflection. The number and types of terminations are just examples for purposes of illustration. There may be any number of terminations and any variety of terminations available for each transmission line network. In addition, a transmission line network may be connected to multiple terminations. Terminations may also include, for example, capacitors, inductors, fixed or variable voltage sources, Zener diodes, variable resistances, non-linear resistances, and active devices. Impedance networks and/or active devices can then be used to generate oscillatory waveforms, or exponentially decaying waveforms, or other more complex waveforms. In  FIG. 2 , some illustrated examples include a capacitive network  224 , a voltage clamp  226 , a Zener diode  228 , a non-linear resistance  230 , and an active device  232 . 
         [0018]    In the example system  200  of  FIG. 2 , transmission lines may have different characteristic impedances. For example, in  FIG. 2 , transmission line  208  may have the same characteristic impedance as transmission line  206 , or transmission line  208  may have a different characteristic impedance than transmission line  206 . Transmission lines having different characteristic impedances provide additional flexibility in generating complex waveforms, particularly in transmission line networks (discussed in more detail in conjunction with  FIGS. 5A ,  5 B,  5 C, and  5 D). 
         [0019]    To further illustrate the function of the system  200  of  FIG. 2 , assume, for example, that transmission lines  206  and  208  are charged. Assume further that transmission line  206  is connected to termination  220  and that the impedance of termination  220  suppresses reflections for transmission line  206 . Assume further that transmission line  208  is connected to termination  222  and that the impedance of termination  222  suppresses reflections for transmission line  208 . Assume further that transmission line  208  is half the length of transmission line  206 . The resulting idealized waveforms are illustrated in  FIGS. 3A-3C . 
         [0020]    In  FIG. 3A , transmission line  206  generates a pulse  300  with no reflections. In  FIG. 3B , transmission line  208  also generates a pulse  302 , half the duration of the pulse generated by transmission line  206 , with no reflections. As illustrated in  FIG. 3C , at first the pulse from transmission line  208  adds to the pulse being generated by transmission line  206 . Then, when the pulse from transmission line  208  ends, the resulting voltage waveform  304  at the DUT takes a step down. In  FIG. 3C  a single step down is illustrated to simplify explanation, but other combinations, with partial reflections from multiple transmission lines, can generate a waveform having multiple steps. 
         [0021]    Alternatively, assume that only transmission line  206  is charged, and further assume that transmission line  206  is not connected to any termination. In  FIG. 4 , the resulting waveform  400  at the DUT has a positive pulse followed by a negative reflection. As a further alternative, a termination having an inductor and a capacitor in series can provide an oscillating waveform. As still another alternative, a termination having a resistor and a capacitor in series can provide an exponentially decaying waveform. 
         [0022]    In each of the previous examples, switches  214  connect a single transmission line, or multiple single transmission lines in parallel. In  FIG. 2 , element  212  depicts a network of transmission lines that may generate more complex waveforms. In each example, the various transmission lines may have various lengths.  FIGS. 5A-5D  illustrate examples of networks of transmission lines ( FIG. 2 ,  212 ) that may be selected by switches ( FIG. 2 ,  214 ).  FIG. 5A  illustrates two transmission lines  500  and  502  connected in parallel, and that parallel combination connected in series with another transmission line  504 .  FIG. 5B  illustrates two transmission lines  506  and  508  connected in series with a resistance  510  between them, and that combination connected in parallel with a transmission line  512 , and that combination connected in series with transmission line  514 .  FIG. 5C  illustrates a transmission line  516  connected in parallel to a transmission line  518 , and transmission line  518  has a DC voltage  520  connected to its outer conductor.  FIG. 5D  has multiple combinations of transmission lines just to illustrate that complex networks of transmission lines may be used to generate complex waveforms. In addition, as discussed above in conjunction with  FIG. 2 , transmission lines in networks may have different characteristic impedances. 
         [0023]    In the example of  FIG. 2 , there may be an optional impedance network  234  between the first switch  204  and the DUT. In the example of  FIG. 6A , an impedance network  234  may be just wires or a transmission line. In the example of  FIG. 6B , an impedance network  234  may be a resistive attenuator. In the example of  FIG. 6C , an impedance network  234  may be a more complex network of passive components. 
         [0024]    The two examples of  FIGS. 3C and 4  illustrate that a system as depicted in  FIG. 2  can generate a wide variety of voltage waveforms having a wide variety of pulse widths, multiple levels, and both positive and negative peaks. The examples of  FIGS. 5A-5D  illustrate that transmission lines can be combined into complex networks for generating complex waveforms. One system as depicted in  FIG. 2  can provide voltage waveforms replicating voltage waveforms generated by standard ESD test systems in multiple industries with vastly different requirements. For example, a voltage waveform as in  FIG. 3C  is useful for impedance characterization of an ESD protection device, and the test may be repeated with different impedances to provide additional holding points. In a specific example, a system substantially as in  FIG. 2 , with one transmission line for 300 nanosecond pulses, one transmission line for 200 nanosecond pulses, and two transmission lines for 40 nanosecond pulses, has been used to replicate ISO and IEC waveforms for on-wafer testing having four steps. As an additional example, a bipolar voltage waveform as in  FIG. 4  is useful for replicating ESD tests used in the automotive industry. In a specific example, a system substantially as in  FIG. 2  has been used to generate a bipolar 11-kilovolt waveform that replicated the results of a Zwickau ESD test. 
         [0025]    An additional advantage of a TLP system as in  FIG. 2  is that parallel transmission lines can deliver more current than a single transmission line or transmission lines in series. In particular, the current required by some ESD tests in the automotive industry exceeds the current that can be supplied by a typical single transmission line system. A typical single-line TLP system can provide about  10 A- 30 A, but a TLP system as in  FIG. 2  with four transmission lines can provide four times that amount of current. 
         [0026]      FIG. 7  illustrates a method  700  of ESD testing. At step  702 , first ends of a plurality of transmission line networks are coupled to a power supply. At step  704 , the first ends of the plurality of transmission line networks are switched to a device under test. 
         [0027]    While illustrative and presently preferred embodiments of the invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.