News Column

Patent Issued for Overvoltage And/Or Electrostatic Discharge Protection Device

August 27, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventor Coyne, Edward (Ardrahan, IE), filed on March 30, 2012, was published online on August 12, 2014.

The patent's assignee for patent number 8803193 is Analog Devices, Inc. (Norwood, MA).

News editors obtained the following quote from the background information supplied by the inventors: "Electronic circuits can be damaged by electrostatic potentials, or just high voltage transients in general. This is particularly true for field effect transistors.

"Protection can, for example, be provided by placing reverse biased diodes between a node which is to be protected and power supply rails. As used herein, 'node to be protected' will refer to any one or more elements, conductors or terminals that are to be protected against electrostatic damage.

"A disadvantage with diodes is that they start conducting as soon as the necessary forward bias voltage, typically 0.7 volts, has been achieved. Thus if it is desired to protect against overvoltage events of positive and negative going polarity, then it is difficult to prevent the diodes from conducting when legitimately supplied voltages might fall outside the supply rail voltage of the device incorporating or attached to the node to be protected. Thus, it is desirable to set a 'protection voltage' at which protection is initiated independently of the supply voltage.

"A further disadvantage with diodes is that they exhibit a resistive current-voltage characteristic once they switch on. This means that the time taken to conduct an electrostatic discharge safely away might be unnecessarily long. Also since the rate of current flow is restricted by the resistance of the diode, the peak voltage experienced by the protected node might be unnecessarily high. This may be especially significant for cases in which the overvoltage events result from the presence of inductive components as opposed to electrostatic discharges. Diodes are not the only form of overvoltage protection that can be employed, but they serve to illustrate some of the issues that need to be overcome."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventor's summary information for this patent: "In an embodiment, there is provided an overvoltage protection device in combination with a filter, the overvoltage protection device having: a first node for connection to a node to be protected; a second node for connection to a discharge node; and a control node, and wherein the filter comprises at least one of: (a) a capacitor connected between the first node and the discharge node; (b) a capacitor connected between the control node and the discharge node; or an inductor in series with the first node.

"Preferably, the overvoltage protection device is operable to provide protection against overvoltage events of positive and negative polarity and an embodiment of the overvoltage protection device comprises: an NPN semiconductor structure defining: a first N-type region; a first P-type region; and a second N-type region; wherein one of the first or second N-type regions is connected to a terminal, conductor or node that is to be protected against an overvoltage event, and the other one of the first or second N-type regions is connected to a reference, and advantageously a field plate is in electrical contact with the first P-type region, and the field plate overlaps with but is isolated from portions of the first and second N-type regions.

"It is thus possible to provide a device that can be switched into a conducting state, either in response to an external stimulus or in response to internal electric fields, and that once it has switched on becomes low impedance to conduct the charge of the electrostatic discharge event or other overvoltage event away. Advantageously the device is provided as a lateral bipolar transistor.

"In one embodiment, the response of the device to an overvoltage event such as an electrostatic discharge is substantially independent of the polarity of the discharge event. Thus, for example, if the device has a trigger voltage of nominally 225 volts, then the device should remain in a high impedance state until a voltage of more than +225 volts occurs at the protected node or until a voltage of less than -225 volts occurs at the protected node.

"In order to achieve this, the doping concentration of the first and second N-type regions should be substantially equal. Additionally, when the conduction is to be triggered by punch-through mechanisms, then the distances between PN boundaries of the first and second N-type regions with the first P-type region, respectively, and internal punch through structures in the N-type regions should be about the same for each of the first and second N-type regions. However, if an asymmetric voltage response is desired, then the doping and/or distances can be varied with respect to each other.

"In one embodiment, the overvoltage protection device is arranged to temporarily latch on in response to an overvoltage condition. Thus, after the device is triggered to conduct, the device forces itself into a low impedance state irrespective of the amount of current being provided at a turn on terminal or region of the device.

"Such a latching function can be provided by fabricating a PNP transistor sharing N and P regions with the NPN transistor such that the transistors cooperate to turn each other on. Such a structure is known in the art as a silicon-controlled rectifier or SCR.

"Advantageously, although a silicon controlled rectifier will turn itself off once the current flow through it decreases to a small enough value, a switch off circuit can be associated with the overvoltage protection device to force the device to revert to a high impedance state once the voltage at the protected node has fallen below a turn off threshold.

"The switch off circuit may comprise active devices such as transistors operating under the influence of a timer to pull the base of the lateral NPN transistor to the emitter voltage of the transistor so as to switch the transistor into a high impedance state.

"Preferably the switch off circuit comprises passive impedances which control a split between current flow through the base region and emitter region of the lateral NPN transistor and this in turn controls a foldback voltage that the protection circuit exhibits.

"While described in the context of particular embodiments, the principles and advantages disclosed herein are applicable to alternative embodiments in which the P and N dopings are swapped, the descriptions of positive and negative voltage reversed, and the descriptions directions of current reversed."

For additional information on this patent, see: Coyne, Edward. Overvoltage And/Or Electrostatic Discharge Protection Device. U.S. Patent Number 8803193, filed March 30, 2012, and published online on August 12, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8803193.PN.&OS=PN/8803193RS=PN/8803193

Keywords for this news article include: Analog Devices Inc., Medical Device Companies.

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Source: Journal of Engineering


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