Integrated transient suppressor

Transient protection apparatus and circuitry that allows a circuit being protected to operate during the transient and after the transient. Two types are included, one being a blocking type for use in severe conditions when little is known about the source of the transients and protection must be provided, and the other being a shunt suppresser type for use where the impedance of the transient or other parameters known about the transient allows such use. For the blocking type, when a transient voltage is detected, a series component (Q-1) decouples prevents the transient from reaching the circuit being protected. A charge pump (A-1) is provided that powers the circuit being protected during the duration of the transient. A shunt suppressor type provides a zener threshold (Z1) detector with a power boost provided by a MOSFET or IGBT (Q-30. When low voltage power levels must be protected, a charge pump (4) to bias the gate source of FET's and/or MOSFET's is provided to offset for the gate/source voltage so that output voltages in the 3.5 to 4.5 volt range may be protected.

RELATED APPLICATIONS 
Priority is claimed under 35 U.S.C. .sctn. 119 (e) for the present 
invention from a Provisional Application Ser. No. 60/027,052 filed on Sep. 
26, 1996 of the same title and inventors as the present application. 
FIELD OF THE INVENTION 
The present invention relates generally to the protection of sensitive 
circuitry, especially integrated solid state circuitry from transient 
voltage spikes or voltage surges. More particularly, the present invention 
provide for transient voltage protection for computer and telecom 
circuitry by limiting the transfer of such voltage transients from 
propagating via the power supplies and the DC voltages to the protected 
circuitry. 
BACKGROUND OF THE INVENTION 
For decades solid state electronic circuits have been accompanied by a 
vulnerability to damage by transient voltage spikes or surges. Generally, 
voltage spikes are characterized by voltage levels which exceed normal 
levels for more than a few microseconds but less than a millisecond. 
Transient voltage surges are generally those excessive levels which may 
last more than a few milliseconds and as long as a few seconds. 
Transients are generally associated with 
a) Lightning 
b) equipment-generated transients, for example, inductive switching 
c) ESD (electrostatic discharge) 
The present invention is directed to protections against transients 
including both spikes and surges. Such transient might typically occur 
when nearby relatively high current loads are switched on and off, and 
from indirect effects from lightning, and from ESD voltages. Most prior 
art systems for computer and telecom applications have specifications for 
protection only against transient spikes. This may be due to the fact that 
excessive voltage excursions lasting more than a few seconds will usually 
cause permanent damage which cannot be avoided without extreme over design 
of the circuit. Another factor, as mentioned below, is that transient 
protection is not given high priority by circuit designers. 
Short-duration, excessive-voltage excursions or spikes can cause 
destructive breakdown of semiconductor device junctions and/or gate oxide 
layers. In the case of oxides, the damage is instantaneous and 
non-reversible since the discharge itself damages the oxide layer. In the 
case of reverse breakdown of PN junctions, the damage is caused primarily 
by over heating due to the power dissipated. The ability, of the PN 
junction to withstand the heat dissipated during a reverse breakdown, 
depends on the duration of the transient (and the voltage/current level) 
and the ability of the junction and the device and its packaging to 
dissipate heat. In some devices local high current density can be harmful 
as well. 
Since voltage transients occur in virtually all electronic applications 
protection against these transients is and has been a continuing need. 
However, in the electronics industry, transient suppressing is often an 
after thought that is poorly understood and poorly specified. Human 
engineer's often take "cook book" specifications without further 
investigation. For example, a "cook book" may specify that a protection 
device rated at 300 volt clamping will protect semiconductor circuitry 
that is rated at 350 volts, or that a 6.2 volt specified zener will 
protect a 5 volt power supply that a breakdown voltage of 300 volts will 
protect a semiconductor rated for 350 volts against transients, or that a 
6.2 volt-rated TVS diode will protect a power supply 5-volt output against 
over voltages. The numbers cited just above cannot be blindly accepted to 
guaranty protection. 
PRIOR ART TRANSIENT PROTECTION-BASIC TYPES 
Prior art transient suppression devices fall into the following categories 
1. Metal Oxide Varistors (MOV): These devices have been used for many years 
and depend on the cumulative breakdown voltage characteristic of 
intergranular Schottky barriers for providing clamping action that limits 
voltage transients. 
MOV's while relatively inexpensive, experience a degree of voltage rating 
degradation with repeated exposure to transients. Regardless of the degree 
of importance of this characteristic in a given application, it 
nevertheless has remained as a perceived liability within the industry in 
spite of MOV's having achieved widespread usage over the past 25 years. 
Of greater concern is that the MOV has limited clamping ability. This is 
illustrated by the fact that MOV's do not exhibit a sharp breakdown (or 
"knee") characteristic. In other words its dynamic impedance is relatively 
high, wherein break over occurs at a specified break over voltage, the MOV 
voltage can increase very substantially with high transient currents after 
the break over voltage. As a rule of thumb, an MOV can be expected to 
clamp a transient voltage to about 200% of the specified voltage when the 
source impedance is not well known. 
2. Zener diodes and related devices depend on the reverse breakdown 
characteristic of a PN junction for clamping action (General Semiconductor 
Industries produce a line of zener diode suppressors under the trademark 
"Transorb." 
Zener diode (PN junction) type devices, like MOV's, cannot be expected to 
clamp the voltage much better than 150%. Zener devices cannot handle the 
energy of an MOV but do not experience degradation with repeated 
transients. Their cost is somewhat higher than the MOV. Another limit of 
these device is that they are limited in the available voltage ratings 
3. Thyristors, including a variety of three layer (PNP), four-layer (PNPN) 
and five layer (NPNPN) devices which, when their breakdown voltage is 
exceeded, "switch on" from a high impedance state (off) to a low impedance 
state (on) which acts to clamp a voltage transient. Examples of these 
devices are marketed under the trademarks: "Sidactors", "Sidacs" and 
"Alternistor." These are essentially updated versions of an older product, 
known as "Bi-switch". 
Thyristors are excellent in that, upon triggering, they drop to a low 
impedance and therefore do not create substantial dissipation. Cost is 
modest. Unfortunately, the thyristor type device are available in only a 
limited number of voltages. Another limitation of thyristors in a DC 
circuit is that, once triggered, the thyristor stays on and results in 
blowing of a fuse or other circuit-opening function. In other words, 
unlike the devices of sections 1 and 2, above, the protected circuit can 
not "ride through" the transient when the thryistor is triggered. The 
circuit must shut off and reset. This inability to provide ride-through is 
a fundamental and often unacceptable limitation of thyristors. 
4. Gas tubes, which act in a similar fashion to the thyristors in having a 
voltage breakdown threshold, followed by switching to a low impedance 
state. 
Gas tubes function virtually the same way as the thyristor except that they 
can handle dramatically more current. They are most useful as primary 
protection from lightening where survival is more important than whether 
the end circuit stays functional. Gas tubes have a number of nuances which 
make them respond different to different kinds of transients. Simply said, 
gas tube suppressors are viewed as brute-force, relatively imprecise 
devices. They find little use on printed circuit boards. 
In a large number of cases (arguably the majority) the source impedance 
associated with a transient cannot be adequately defined. This means that 
a transient suppressor (as described above in sections 1, 2, 3, and 4 may 
be subjected to unlimited current. With a conventional shunt diode or MOV, 
this means that the ultimate clamping voltage cannot be predicted and 
therefore the degree of protection cannot be defined. In practice the 
protection is limited. 
There are cases where the voltage (a single transient or repetitive 
transients) could, in fact, exist for tens or hundreds of milliseconds, a 
situation where a conventional shunt suppressor's peak power capability 
would become less useful. For example, if a transient were to last for a 
good part of a second, a shunt diode or MOV would experience such power 
dissipation and heat rise as to destroy it. For example, 100 amps at 200 
volts equals 20,000 watts, such a power over virtually any time span will 
cause a temperature rise that will destroy most commercially available 
device, especially solid state. 
In either of the preceding cases, it is absolutely necessary, if 
ride-through is desired, to effectively limit the current to a safe level 
while at the same time keeping the output voltage at the prescribed level. 
In other words, the excessive voltage must be absorbed in a minimally 
dissipative way. 
When using shunt protection devices, such as MOV's or zener diodes, these 
devices may exhibit voltages of 200% (or more) of their rated specified 
voltage. This means that the components of the protected circuit must be 
rated at least at 200% of their typical operating ratings. For example, in 
a power supply circuit, if a shunt protection device is rated at 200 V, 
200 V rated MOSFETs and other active devices (and associated passive 
devices, e.g. capacitors, resistors, etc.) would have to be replaced by 
connotes rated at 400 V. Because the size and cost of a silicon power chip 
can go up as the square of the voltage rating for the same current rating, 
the negative implications of designing to 200% of the protection devices 
rating are enormous in a multi-semiconductor design. In a high wattage 
power supply, the extra rating could add tens of dollars to the material 
cost of a design. Often, because of cost pressures, transient protection 
is employed which is actually ineffective. 
It is an object of the present invention to overcome the above mentioned 
dynamic impedance problem. In a corresponding manner, it is an object of 
the present invention to provide transient protection that allows the 
protected circuit to continue functioning during and after the transient. 
Another object of the present invention is to provide a suppressor of a 
size suitable for mounting along with other miniature electronic 
components on a printed circuit (PC) board, including surface mounting 
configurations. 
Yet another object of the present invention is to provide protection 
regardless of the source impedance. 
Still another object of the present invention is to provide and specify a 
transient voltage clamping level from which circuit components can be 
reliably specified. 
Yet another object of the present invention is to provide a protection 
circuit where transient voltages are blocked from reaching a circuit being 
protected, while still providing power to that circuit. 
Another object of the present invention is to provide a protection circuit 
that clamps or limits the voltage going to the circuit being protected 
with a characteristic that approaches that of an ideal zener diode--a near 
zero dynamic and static impedance in the area after the breakdown voltage. 
Yet another object of the present invention is to provide a clamping action 
and/or a blocking action wherein the high power, high current of the 
transient is directed to larger power dissipation devices while 
substantially not affecting the clamping and the blocking action of the 
protection circuitry. That is the protection apparatus provides very low 
dynamic and static impedance after clamping. 
SUMMARY OF THE INVENTION 
The above objects are met in an integrated electronic protection apparatus. 
The protection apparatus is arranged for accepting an input voltage and 
conveying an output voltage to a circuit being protected. The protection 
apparatus is designed and arranged for providing protection against 
incident input voltage transient spikes and surges from appearing at the 
output voltage. The protection apparatus comprises: a threshold device for 
providing a threshold voltage, means for defining a first voltage limit 
for the output voltage, the first voltage limit being substantially the 
maximum that the circuit can withstand, and under which the circuit can 
continue to operate as designed, wherein the first voltage limit is 
derived from and related to the threshold voltage, means for defining a 
second voltage limit, wherein the second voltage limit is related to the 
threshold voltage, means for detecting an input voltage rising above the 
second voltage limit, and responsive to said detecting, means for limiting 
said output voltage to said first voltage limit. 
In a preferred embodiment of the invention a power device is connected 
across the output voltage. The power device is normally off when the input 
voltage does not rise above the second voltage limit, and wherein the 
power device is designed and constructed to accept the entire current 
associated with the input voltage while maintaining the output voltage at 
or below the first voltage limit, and, responsive to the detecting, means 
for turning on the power device. In a preferred embodiment the power 
device comprises: a transistor, an FET, a MOSFET, an Insulated Gate FET 
(IGFET), or an IGBT. When an FET or equivalent is used to protect output 
voltages that are low, 3 to 5 volts, a charge pump is connected to offset 
the gate of the FET to increase the effective gate voltage so that first 
limit voltages down to the 3 to 5 volt levels at the output can be 
protected. 
In another preferred embodiment, a power device connects the input and the 
output voltages, wherein the power device is designed and arranged for 
withstanding the voltage levels of the transients. Responsive to the 
detecting, means are provided for turning off said power device decoupling 
the output voltage from the input voltage. A charge pump is connected to 
said output voltage, wherein the charge pump is designed and constructed 
to supply the circuit being protected with power during the transient or 
surge. The charge pump may be connected to receive power from the input 
voltage. 
The above objects are met in two preferred embodiments, one a shunt 
suppressor design and the second a blocker/charge pump design. 
In a large number of applications the source impedance associated with the 
transient cannot be adequately defined, and the duration of the transient 
can be up to several seconds long. If the current available from the 
transient source is unlimited virtually no shunt protection circuitry will 
provide practical protection for circuitry. In this instance a protection 
circuit that blocks or decouples the transient voltage from the circuit 
being protected can be used. In present invention provides a charge pump 
circuit that, when the transient is decoupled from the circuit being 
protected, can provide current to that circuit until the transient is 
over. 
The blocker/charge pump design provides for a series voltage regulator 
active device, a MOSFET or IGBT in preferred embodiments, that is 
conducting in a fully on state, when operating under normal condition. A 
charge pump is provided that is suitable for providing power to the 
circuit being protected for several seconds in a preferred embodiment. 
When a transient of a given voltage level occurs, a threshold device 
becomes active turning off the regulator active device and so preventing 
the transient voltage from reaching the circuit being protected. The 
excess voltage is carried across the regulator device. During this time 
the charge pump provides current to the circuit being protected. The 
threshold device and the charge pump may be buffered and protected from 
the transient voltage. 
When a transient voltage reaches a specified voltage level, the threshold 
device exhibits a break over or knee characteristic that, when so 
activated, acts to turn off the regulator device. Whereupon the charge 
pump supplies current to the circuit being protected. In another preferred 
embodiment, the threshold device may be implemented with a comparator 
circuit as known in the art. 
When the source impedance of the transient is known and the value is such 
(with the voltage peak also known) that excessive peak currents will not 
occur, a suppressor that acts like an ideal threshold device (like a zener 
diode in a preferred embodiment) can be used to advantage. In this 
suppresor there is a threshold device that is buffered from the transient 
so that the threshold level can be relied upon. Circuitry is provided to 
substantially maintain the threshold level at the output to the circuit 
being protected, but with a larger current sinking capability. 
Other objects, features and advantages will be apparent from the following 
detailed description of preferred embodiments thereof taken in conjunction 
with the accompanying drawings in which:

DESCRIPTION OF PREFERRED EMBODIMENTS 
The preferred designs are based on the premise that the user must have 
"ride-through," which is herein defined to mean that the circuit being 
protected runs normally and provides the normal circuit functions without 
shutdown or interruption during and after the transient occurs. Where 
continuous nearby transient generation occurs, such ride-through can be 
critical (i.e. fork lift truck operation, computerized stop/start motor 
controls, sensitive logic controls which demand continual control, etc.) 
FIG. 1 illustrates an Integrated Transient Blocker (ITB) for severe 
applications where the nature of the source impedance, or the size, shape, 
duration or repetition rate of the transients cannot be comfortably 
defined or anticipated. 
In the circuit of FIG.1, Q-1 represents a series voltage regulator (MOSFET 
or IGBT) driven into partial conduction by R-1, enabling activation of 
charge pump A-1, which drives Q-1 into a total-on (short circuit) 
condition. In the presence of excessive voltage across the input voltage, 
Z-1 breaks down, reducing gate drive to Q-1 to a degree which causes Q-1 
current to drop and its drain-source voltage to increase to a level equal 
to the excessive voltage, while output voltage remains at a safe level. If 
the output voltage attempts to rise, the gate to source voltage is reduced 
and Q-1 is turned further off and absorbs the voltage increases. If the 
transient voltage disappears, Z-1 cease conduction and Q-1 reverts to a 
full-on condition. R-2 is a trimmed resistor that with R-1 and the zener 
Z-1 (plus Q-2 base emitter drop) sets the threshold voltage for the 
circuit of FIG.1. 
In a practical circuit, where the output could be rated at 4 amps at 300 
volts, the maximum instantaneous power across Q-1 would be 5 A at 500 V or 
2500 watts. Q-1 devices are available that can withstand such a power for 
a short time. In contrast if an MOV was place across the output voltage 2, 
the instantaneous current could be 100-200 amps or more resulting in 
instantaneous power of 50,000 watts or more for the same transient--a 20:1 
power difference even disregarding the poorer clamping of the MOV. 
In applications where nominal loads are no more than about 2 amps, the 
circuit of FIG. 1 can provide ride-through protection against transients 
up to 1000 volts, in an SMD (NOTE: Ed or Gary IS THERE A JEDEC EQUIVALENT 
NUMBER FOR THIS) package, for transients of indefinite duration and 
substantially zero source impedance. At higher maximum load currents, the 
ITB could provide the same protective capability with only a minimal 
amount of heat sinking to handle the steady state current through the 
regulator, generally characterized at about 1.5 watts per amp. 
The packaging for the ITB, in a preferred embodiment, is a two-chip set--a 
power chip and a monolithic driver chip making use of trimmed resistors 
for different clamp voltage specifications. For specific applications, the 
entire function could be monolithic. Similarly select circuit elements 
could be external in order to optimize particular characteristics or offer 
a degree of external programmability of the limiting output clamp voltage, 
for example, external potentiometer or voltage source. 
FIG. 2 illustrates another preferred embodiment, an Integrated Transient 
Shunt Suppresor (ITS), for applications where there is some control over 
source impedance and the size, shape, duration and repetition rate of the 
transients. If the transient has a duration, height, current or power 
level that are severe, as described above, the ITS may be insufficient for 
protection. 
Where the source impedance is known and is of a value to prevent excessive 
peak transient currents, the circuit of FIG. 2 can be used, in another 
preferred embodiment, to reliably clamp the transient voltage to a 
specific level and exhibit virtually a zero dynamic impedance; i.e., act 
like an "ideal zener" even at currents up to 100 amperes or more. The 
zener diode Z1, which provides the trigger mechanism, is buffered from the 
transient by R2 and endures current changes in the milliamperes range. 
This maintains the zener Z1 close to its voltage breakdown specification 
or "knee". The input voltage and output voltage are shorted to each other 
and therefore always equal to each other. Once the input voltage, through 
the divider R1, R2 reaches a level of greater than the zener voltage plus 
the Q1 base emitter drop, Q1 will turn on and in turn supply base drive to 
Q2 . Q2 will drives Q3's gate. Z2 is specified to keep the gate/source 
voltage of Q3 below its maximum rating. 
At transient voltages just a few volts above breakdown, and with relatively 
high source impedances, the gate of Q3 need only be brought up to minimum 
threshold of 5-6 volts to achieve some Q3 conduction (and reduction in 
source-drain voltage) and resultant clamp voltage equilibrium. That is Q3 
acts to divert the transient current enough to maintain the output volt at 
a safe level. At high transient voltages with relatively low source 
impedances, the gate of Q3 may have to rise up to 15 volts or more to 
allow Q3 to rise to a sufficient level of conduction to maintain sharp 
clamping and limiting of the output voltage. 
The operation of the circuit in FIG. 2, however, is that the gate of Q4 
will automatically seek its own level to create an equilibrium condition 
forcing the source/drain voltage of Q3 to a to a level approximating the 
breakdown level of Z1 multiplied by R2/R1 plus the current through Z1 
times R2. Practical designs will maintain Z1 current small so that the 
load voltage can never exceed the zener voltage (plus the Q1 VBE diode 
drop) multiplied by the resistor divider ratio R2/R1. If the transient 
subsides, Z1 is no longer in its breakdown region, with Q1, Q2 and Q3 then 
turning off. 
The instantaneous power of Q1 can be characterized just as with an MOV or 
TVS diode. However, there is a significant difference in that the circuit 
of FIG. 2 provides clamping within a few percent of the calculated value 
from the Z1 breakdown voltage specification. 
The preferred embodiment circuit of FIG. 2 can be fabricated as a 
three-terminal device in a conventional TO-220 or TO-247 package in either 
a standard or surface mount configuration. The actual power element can be 
either a MOSFET or IGBT depending on the voltage or current ratings 
required. 
In one possible configuration, the protection apparatus of FIG. 2 would 
consist of a two chip set. There would be a standard power chip with all 
other elements being in a monolithic chip processed as known in the art, 
e.g., with junction isolation or dielectric isolation techniques. The 
monolithic portion could have a standard design with different clamping 
voltage specifications achieved by having an on-chip, laser-trimmed thin 
film resistor divider R1 and R2 to set the actual clamp voltage. This 
approach would permit economies of scale in the silicon processing, 
dramatically reduce in-process inventories and employ a high yield process 
to tailor the protection circuit to detailed reliable specifications. 
As a final extension of this product, a charge pump 4 circuit can be added 
to the monolithic portion to generate a supplemental voltage to raise the 
gate/source voltage of FET's and equivalents (MOSFET's, IGFET's, etc.) 
higher than the source voltage. This is needed for protection of low 
voltage circuits (for example, 3 to 5 volts). Field Effect Transistors 
(FET) of all types often require gate/source voltage higher than the 
voltages being protected. If an FET type with a gate/source turn-on 
voltage of 6 volts were used, there would insufficient gate voltage to 
drive Q3 into adequate conduction. A higher gate/source voltage is 
generated by the charge pump 4 to accommodate the low voltage 
applications. That charge pump is powered via R3 from the input. 
With the suggested circuit, employing a MOSFET or IGBT, very precise 
clamping of low voltage power supply overshoots (within 10%) can be 
achieved at currents of over 100 amps, in a surface mounted device. 
It will now be apparent to those skilled in the art that other embodiments, 
improvements, details and uses can be made consistent with the letter and 
spirit of the foregoing disclosure and within the scope of this patent, 
which is limited only by the following claims, construed in accordance 
with the patent law, including the doctrine of equivalents.