Current monitor for field emission displays

A current measuring circuit for a field emission display includes a testing circuit coupled between a high voltage testing source and the display. The testing circuit includes a sampling circuit formed from a sampling impedance coupled in parallel with a high isolation switch. In one embodiment, the sample circuit is on the high voltage side of the testing source. In another embodiment, the sampling circuit is on the return (low voltage) side of the testing source. In normal operation, the switch is closed to provide the testing voltage directly to the display. During testing, the switch is open so that current flows through the sampling impedance. A sensing circuit coupled to the output of the sampling impedance determines a voltage change in response to opening of the switch. In response to a sensed voltage change, a microprocessor-based controller computes the current drawn by the display. A burn-in system includes a bank of displays within a burn-in oven each selectively coupleable to the testing circuit by respective switches. Another testing system includes separate supply and testing sources each coupled to displays by respective switches. The switches are opened and closed such that the displays are always coupled to one or both of the voltage sources to prevent the switches from being exposed to high voltage swings.

TECHNICAL FIELD 
The present invention relates to field emission displays and more 
particularly to current monitoring in field emission displays. 
BACKGROUND OF THE INVENTION 
Flat panel displays are widely used in a variety of applications, including 
computer displays. One type of device well-suited for such applications is 
the field emission display. Field emission displays typically include a 
generally planar substrate having an array of projecting emitters. In many 
cases, the emitters are conical projections integral to the substrate. 
Typically, the emitters are grouped into emitter sets where the bases of 
the emitters are commonly connected. The term emitters will be used herein 
to refer to single emitters or emitters grouped into sets. 
A conductive extraction grid is positioned above the emitters and driven 
with a voltage of about 30-120 V. The emitters are then selectively 
activated by providing a current path from the bases to the ground. 
Providing a current path to ground allows electrons to flow to the 
emitters by the extraction grid voltage. If the voltage differential 
between the emitters and extraction grid is sufficiently high, the 
resulting electric field extracts electrons from the emitters. 
The field emission display also includes a display screen mounted adjacent 
the substrate. The display screen is formed from a glass plate coated with 
a transparent conductive material to form an anode biased to about 1-2 kV. 
A cathodoluminescent layer covers the exposed surface of the anode. The 
emitted electrons are attracted by the anode and strike the 
cathodoluminescent layer, causing the cathodoluminescent layer to emit 
light at the impact site. The emitted light then passes through the anode 
and the glass plate where it is visible to a viewer. 
The brightness of the light produced in response to the emitted electrons 
depends, in part, upon the rate at which electrons strike the 
cathodoluminescent layer, which in turn depends upon the magnitude of 
current flow to the emitters. The brightness of each area can thus be 
controlled by controlling the current flow to the respective emitter. By 
selectively controlling the current flow to the emitters, the light from 
each area of the display can be controlled and an image can be produced. 
The light emitted from each of the areas thus becomes all or part of a 
picture element or "pixel." 
Anodes of such displays typically draw low currents on the order of just a 
few microamperes while operating at anode voltages of 1-2 kV. Measuring 
such low currents at such high voltages can require very expensive 
equipment. It can therefore be impractical to use available equipment to 
measure the anode current of field emission displays in a production 
environment where large numbers of field emission displays are tested 
repeatedly at reasonable speed and affordable cost. 
The current to a single anode can be determined by monitoring current flow 
through the low voltage side of the voltage supply providing the anode 
voltage. However, such an approach becomes undesirable where a single high 
voltage supply is used to test several displays, because it can be 
difficult to determine the portion of the current attributable to each 
individual display. 
SUMMARY OF THE INVENTION 
A current measuring circuit for measuring anode current in a field emission 
display includes a sampling impedance serially coupled to a supply voltage 
that provides anode current to the display. In one embodiment of the 
invention, the current measuring circuit includes a sampling circuit that 
contains the sampling impedance. The sampling circuit is serially coupled 
between the supply voltage and the display. The sampling circuit also 
includes a switch coupled in parallel with the sampling impedance and 
controlled by a control signal V.sub.CON. The switch is an electrically 
controlled switch having a high isolation, such as an optoisolator. 
During normal (non-testing) operation, the switch is closed to bypass the 
sampling impedance. The switch therefore transfers the supply voltage 
directly to the display. During testing, the switch is opened so that 
current flows through the sampling impedance. The current flowing through 
the sampling impedance causes a slight drop in voltage supply to the 
display; however, the slight voltage drop does not significantly affect 
the operation of the display. 
A sensing circuit is coupled to monitor the output voltage of the sampling 
impedance. The sensing circuit includes a capacitor and a test resistor 
serially coupled between the sampling impedance output and the reference 
potential. A voltage monitor is coupled to a node joining the capacitor 
and the test resistor. 
When the switch is opened, the capacitor couples the change in output 
voltage from the sampling impedance to the node. There, the voltage 
monitor detects the change in node voltage and provides an output signal 
to a microprocessor-based controller in response. The controller then 
calculates the current flowing to the display from the monitored node 
voltage. 
A burn-in system according to the invention includes a plurality of 
displays within a burn-in oven. Each of the displays is driven by a single 
supply line that is selectively coupleable to a supply voltage through a 
respective switch. A testing circuit is serially coupled to the voltage 
source to monitor current drawn by each of the respective displays that is 
coupled to the voltage source. Because the testing circuit uses only a 
single conductor for each of the displays, conductors occupy less space 
within the burn-in oven as compared to two-conductor systems. 
Consequently, additional space is freed so that the number of displays 
within the burn-in oven can be increased. 
One embodiment of a testing system according to the invention includes a 
testing voltage source and a supply voltage source that are selectively 
coupled through switches to the displays being tested. At first, the 
displays are all coupled to the supply voltage source. Then, one of the 
displays is coupled to the testing voltage source without removing the 
supply voltage source. Little or no current flows between the testing 
voltage source and supply voltage source, because the two voltage sources 
are maintained at substantially equal voltages. 
Next, one of the switches is opened such that the display being tested is 
isolated from the supply voltage source and receives current only from the 
testing voltage source. A testing circuit according to the invention 
determines the current drawn by the display. Then, the switch is closed so 
that the display is coupled to both the testing voltage source and the 
supply voltage source. Then the display is isolated from the testing 
voltage source. Because the display is continuously coupled to one or both 
of the testing voltage source and supply voltage source, the display and 
switches are not subjected to a large change in voltage. Consequently, the 
display and switches are less likely to be damaged by switching of high 
voltages.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 1A, a testing circuit 40 is serially coupled on a high 
voltage line 42 between a measuring power supply 44 and a field emission 
display 46. The power supply 44 is a high voltage supply that provides an 
input voltage V.sub.IN of about 1-2 kV to an anode 48 of the field 
emission display 46. The power supply 44 can be any suitable power supply 
having adequate voltage levels and stability. The field emission display 
46 is a known structure having an emitter substrate 47 positioned opposite 
a display screen 51 that includes the anode 48. A glass plate 53 carries 
the anode 48 and a cathodoluminescent layer 55. A video signal generator 
45, such as a television receiver, VCR, camcorder or computer, controls 
operation of the display 46 by providing a video image signal V.sub.IM to 
the emitter substrate 47. For testing, the video signal generator 43 sets 
the image signal V.sub.IM to a level corresponding to maximum illumination 
so that the current draw of the anode will be maximized. For color 
displays, this means that the red, green and blue components of the image 
signal will all be at their maximum levels to produce a "white" image. 
The testing circuit 40 includes a sampling circuit 50 that couples power 
from the power supply 44 to the field emission display 46. The sampling 
circuit 50 is formed from a sampling switch 52 and a sampling impedance 54 
that are coupled in parallel. The sampling switch 52 is preferably an 
optoisolator driven by a control signal V.sub.CON, although any 
selectively actuatable switching device having a sufficiently high 
isolation may be used. The sampling impedance 54 is preferably a resistor 
having an impedance of about 50 K.OMEGA.-1 M.OMEGA.. 
The testing circuit 40 also includes a sensing circuit 56 coupled between 
an output node 49 and a reference potential. The sensing circuit 56 is 
preferably formed from a high voltage capacitor 58 serially coupled at a 
test node 64 to a high impedance test resistor 60 having a resistance of 
approximately 10-20 M.OMEGA.. One terminal 57 of the capacitor 58 couples 
directly to the output node 49 of the testing circuit 40 and the other 
terminal couples to the test node 64. A voltage monitor 62 is coupled to 
the test node 64 between the capacitor 58 and test resistor 60 to monitor 
the voltage of the test node 64 relative to the reference potential. 
During normal operation, sampling switch 52 is closed and bypasses the 
sampling impedance 54 such that the voltage on the high voltage line 42 is 
connected directly to the anode 48. Consequently, the anode voltage 
V.sub.AN equals the entire input voltage V.sub.IN. The terminal 57 of the 
capacitor 58 also receives the entire input voltage V.sub.IN. The test 
resistor 60 references the remaining terminal of the capacitor 58 to 
ground. The capacitor voltage V.sub.C is thus equal to the input voltage 
V.sub.IN. 
For testing, control signal V.sub.CON (FIG. 2A) goes from low to high at 
times t.sub.1, to switch the state of the sampling switch 52 and initiate 
testing. Current from the power supply 44 flows through the sampling 
impedance 54 to reach the anode 48. The resulting voltage drop V.sub.Z 
across the sampling impedance 54 slightly lowers the anode voltage 
V.sub.AN from the input voltage V.sub.IN, as shown to exaggerated scale 
FIG. 2B. Consequently, the voltage at the test node V.sub.N is reduced by 
the voltage V.sub.Z compared to the normal operation of FIG. 1B, as the 
voltage across the capacitor V.sub.C cannot change instantaneously. 
The sampling impedance 54 is selected such that the voltage drop across the 
sampling impedance 54 is on the order of 1-20 V. Consequently, during 
testing, the anode voltage V.sub.AN shifts negligibly from the input 
voltage V.sub.IN of 1-2 kV. For example, for an input voltage of 1,500 V 
and a voltage drop across the sampling impedance 54 of 4 V, the anode 
voltage V.sub.AN will change from 1,500 V to 1,496 V as shown in FIG. 2B. 
Such a small variation in the anode voltage V.sub.AN does not 
significantly affect operation of the display 46. 
Rather than monitor the anode voltage V.sub.AN directly with a high voltage 
detector, the monitor 62 measures the change in the test node voltage 
V.sub.N to determine the anode current. The voltage across the sensing 
circuit 56 equals the anode voltage V.sub.AN. However, the voltage V.sub.C 
across the capacitor 58 does not change instantly. Instead, the capacitor 
voltage V.sub.C decreases very slowly due to the high impedance of the 
test resistor 60 so that the capacitor voltage V.sub.C remains 
substantially constant at 1500 V. 
The voltage V.sub.R across the test resistor 60 is equal to the anode 
voltage V.sub.AN minus the capacitor voltage V.sub.C and is also equal to 
the node voltage V.sub.N. Therefore, the voltage V.sub.R across the test 
resistor 60 jumps from 0 V to -4 V and then begins to decay gradually 
toward 0 V as the capacitor 58 discharges through the test resistor 60, as 
shown in FIG. 2C. The rate of decay of the resistor voltage V.sub.R will 
be determined by the RC time constant of the test resistor 60 and 
capacitor 58. The value of the test resistor 60 is very large, on the 
order of 10-20 M.OMEGA., so that the capacitor 58 discharges very slowly 
through the test resistor 60, as compared to the period of the switch 
control signal V.sub.CON. Consequently, the resistor voltage V.sub.R falls 
very slowly from 4 V and thus appears as a substantially constant voltage 
in FIG. 2C. 
The voltage monitor 62 detects the initial rise and gradual fall of the 
voltage V.sub.R across the test resistor 60 and provides a corresponding 
digital output to a microprocessor-based controller 70. The controller 70 
then calculates the voltage drop across the test resistor 60. The voltage 
monitor 62 directly indicates the magnitude of current flowing through the 
sampling impedance 54 according to Ohm's Law, because the peak change in 
the resistor voltage V.sub.R equals the voltage drop V.sub.Z across the 
sampling impedance 54. The voltage monitor 62 thus indicates the current 
draw of the field emission display 46. 
To obtain reliable current readings, the sampling switch 56 can be switched 
periodically. The voltage monitor 62 then synchronously detects the 
voltage V.sub.N at the test node 64. The duty-cycle of the control signal 
is preferably less than 5% to avoid a change in the equilibrium voltage of 
the test node 64. 
When the sampling switch 52 is closed or when the display 46 draws an 
undesirably high current, the voltage monitor 62 may receive undesirably 
high voltage spikes. To prevent damage to the voltage monitor 62, a pair 
of low pass filters 73, 75 are coupled in series with the sampling circuit 
50. The first filter 73 is coupled between the sampling circuit 50 and 
power supply 44 and the second filter 75 is coupled between the sampling 
circuit 50 and the display 46. The filters 73, 75 reduce unwanted voltage 
spikes to limit damage to the voltage monitor 48. For further protection, 
a Zener diode 79 is coupled between the test node 64 and the reference 
potential. The Zener diode 79 breaks down quickly to provide a current 
path to ground if a high voltage spike arrives at the test node 64. The 
Zener diode 79 thus prevents the high voltage spike from damaging the 
voltage monitor 62. 
FIG. 3 shows the testing circuit 40 of FIGS. 1A, 1B in a 
computer-controlled multi-display test station 74. The test station 74 
includes a bank 76 of field emission displays 46 mounted within a test 
apparatus 77, such as a burn-in oven or a field emission display lifetime 
tester. Each of the field emission displays 46 is coupled to the high 
voltage output of the power supply 44 by a respective sampling circuit 50 
that is controlled by the microprocessor-based controller 70. The voltage 
monitor 62 is selectively coupleable to the sampling circuits 50 through a 
switch bank 63. The controller 70 selectively actuates the switch bank 63 
to couple one of the displays 46 to the voltage monitor 62. Then, the 
controller 70 activates the corresponding sampling switches 52 to produce 
a voltage change of the respective output node 49 and receives the 
measured voltage from the voltage monitor 62 through a unity gain buffer 
65. The controller 70 then computes the current flowing into the 
corresponding field emission display 46 and stores the information. Then, 
the controller 70 actuates the switch bank 63 to couple the monitor 62 to 
the next field emission display 46 and switches the corresponding sampling 
switch 52 to activate the next field emission display 46. The controller 
70 then receives the corresponding voltage information through the 
corresponding buffer 65 and stores the information in its memory. The 
controller 70 repeats this process for each field emission display in the 
bank 76 at selected sampling intervals to evaluate the response of the 
field emission displays 46 over the time that the displays 46 are in the 
test station 74. 
Unlike the embodiment of the test station 78 of FIG. 4, described below, 
that includes a separate high voltage line for testing in addition to a 
high voltage line for non-testing operation, the test station 74 of FIG. 3 
utilizes only a single high voltage line per field emission display 46 
along with respective control lines 79 and a ground reference common to 
all of the field emission displays 46. The elimination of one high-voltage 
test line for each display 46 frees additional space within the test 
station 74 for testing additional displays 46. 
FIG. 4 shows another embodiment of a multi-display test station 78 in which 
a single monitor 62 monitors a single testing circuit 40 to detect the 
current through several displays 46, one at a time. In this embodiment, 
each of the displays 46 is coupleable to a main power supply 82 and a 
measuring power supply 44. During regular (non-testing) operation, 
respective isolation switches 84 are closed to couple the displays 46 to 
the main power supply 82, which is held at a constant voltage. During 
testing, one of the sampling switches 52 is closed, as shown for the third 
sampling switch 52 in FIG. 4. The closed sampling switch 52 couples the 
corresponding display 46 through a single testing circuit 40 to the 
measuring power supply 44. At this time, the selected display 46 is 
connected to both the measuring power supply 44 and the main power supply 
82. However, no current flows between the measuring power supply 44 and 
the main power supply 82, because the measuring power supply 44 and main 
power supply 82 are held at substantially identical voltages. 
Next, the isolation switch 84 corresponding to the selected display 46 is 
opened to isolate the selected display 46 from the main power supply 82. 
Then, the controller 70 determines the current draw of the display 46 
through voltage monitor 62 and testing circuit 40, as discussed above. 
Once the current draw of the selected field emission display 46 is 
measured, the isolation switch 84 is then closed to connect the display 46 
to the main power supply 82 once again. Then, the closed sampling switch 
52 is opened to isolate the selected display 46 from the measuring power 
supply 44. Throughout testing, the selected display 46 receives a 
continuous high voltage, because of the overlap of time during which the 
switches 52, 84 are closed. Consequently, the selected display 46 is 
protected against high voltage spikes which may occur when a high voltage 
is abruptly applied to the display 46. Contacts on the switches are 
protected because the voltage across the contacts when switching is very 
small. Thus, there is no arcing, and a high-voltage switching device need 
not be used. 
FIG. 5 shows another embodiment of the invention in which the sampling 
switch 52 is eliminated. In this embodiment, the high-voltage side of the 
power supply 44 is coupled directly to the anodes of ail of the displays 
46 within the test apparatus 77 and each of the displays 46 is coupled to 
a common ground reference. 
Unlike the previously described embodiments, the controller 70 controls the 
current flow through the displays 46 by controlling the image signal 
V.sub.IM supplied to the respective displays 46. To determine the current 
draw of selected display 46, the controller sets the red, green, and blue 
components of the image signal V.sub.IM to a "black" level, i.e., no red, 
green, or blue light is emitted. Then, the controller 70 sets the red, 
green, and blue components of the selected display 46 to their maximum 
values so that the selected display 46 will draw the maximum anode 
current. Because none of the other displays 46 in the test apparatus 77 
emits light, the anode currents of the remaining displays 46 will be 
substantially zero. Consequently, current drawn from the power supply 44 
will be attributable only to the anode current of the selected display 46. 
The low side of the power supply 44 is coupled to ground through a 
current-to-voltage converter 95 formed from a test resistor 97 and a 
buffer amplifier 99. The test resistor 97 provides a current path between 
the low side of the power supply 44 and ground and the buffer amplifier 99 
provides an output voltage to the voltage monitor 62 that corresponds to 
the current through the test resistor 97, amplified by the amplifier gain 
K. The output voltage of the current-to-voltage converter 95 indicates the 
anode current of the selected display 46. 
As with the above-described embodiments, the controller selectively 
activates each of the displays 46 and the voltage monitor 62 provides 
through the controller 70 an indication of each of the corresponding anode 
currents. 
In addition to the current-to-voltage converter 95, the embodiment of FIG. 
5 also includes a bypass switch 100 coupled in parallel with the test 
resistor 97. Unlike the previously described embodiments, the bypass 
switch 100 typically is not switched to provide a pulsed signal. Instead, 
the bypass switch 100 is open during testing so that all of the current 
flows through the test resistor 97. During non-testing operation, the 
bypass switch 100 is closed to provide a high capacity current path 
between the low side of the power supply 44 and ground. The high capacity 
current path allows several displays 46 to be active simultaneously by 
providing ample anode current to operate several displays 46. Moreover, 
the test resistor 97 can have a relatively large impedance to improve 
sensitivity of the current-to-voltage converter 95 because the test 
resistor 97 does not carry high current during either testing or normal 
operation. 
While the invention has been presented by way of exemplary embodiments, 
various modifications may be made without deviating from the spirit and 
scope of the invention. For instance, although the controller 70 is 
preferably microprocessor-based, other structures and methods may be used 
for controlling operation of the testing circuit 40, including manual 
operation. For example, the switches 52, 84 in the embodiment of FIG. 5, 
can be controlled manually or by the controller 70. One skilled in the art 
will also recognize that the testing method described herein can be 
altered such that the sampling switch 52 is normally open and is switched 
from open to closed during testing. In such an approach, the node voltage 
V.sub.N would rise by 4 V in response to activation of the sampling switch 
.OMEGA. rather than dropping by 4 V, as shown in FIG. 2C. Also, the 
testing circuit 40 and the video signal generator 45 can be implemented 
within the field emission display 46 to provide an indication of the anode 
current. Such an embodiment would be particularly applicable where the 
anode voltage or anode current is controlled through a feedback circuit. 
Further, the voltage monitor 62 can directly display the node voltage 
V.sub.N on a conventional display, or can record the node voltage V.sub.N 
as a conventional storage medium, such as a computer disk or chart 
recorder. Accordingly, the invention is not limited, except as by the 
appended claims.