Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2002-10-01
2003-09-09
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S309000
Reexamination Certificate
active
06617906
ABSTRACT:
TECHNICAL FIELD
The present invention relates to circuits, and more particularly to voltage clamping devices that enable the use of low-voltage devices in high-voltage circuits.
BACKGROUND OF INVENTION
In many applications, a high-voltage supply operates not only high-voltage devices, but also low-voltage devices as well. This situation is especially common in high-voltage integrated circuits. Most high-voltage integrated circuit processes offer both a wide variety of relatively compact low-voltage devices, and a smaller variety of larger high-voltage devices. The majority of a high-voltage integrated circuit consists of low-voltage circuitry. Therefore, some mechanism needs to be employed to protect the low-voltage components from excessive differential voltages. One common mechanism to protect a device against excessive voltage is a parallel-connected Zener diode (or a stack of such Zeners), popularly called a “Zener clamp”.
FIG. 1
illustrates a prior art high-voltage supply system
10
that employs a Zener clamp
16
. A high voltage V
HIGH
(e.g., 200-400 volts) is provided to system
10
and is employed to directly power high-voltage devices
12
and to indirectly power low-voltage devices
14
. A Zener clamp
16
is formed from Zener diodes D
1
and D
2
, and is connected in parallel across the low-voltage devices
14
. If the voltage across the low-voltage devices
14
exceeds the sum of the breakdown voltages of the Zener diodes D
1
and D
2
, these diodes clamp the voltage across the low-voltage devices
14
to V
LOW
. Depending upon the desired value of V
LOW
, the Zener clamp
16
may contain more or fewer series-connected Zener diodes. The Zener clamp of
FIG. 1
has limited application because it needs to be placed in series with some current-limiting element, such as a resistor R
1
. Many circuits require that the voltage across a device be clamped without restricting the current.
FIG. 2
illustrates an alternate prior art high-voltage supply system
20
that achieves this end.
In
FIG. 2
, the high-voltage supply system
20
employs one or more low-voltage devices
24
. A high voltage V
HIGH
(e.g., 200-400 volts) is provided to the system
20
and is employed to directly power high-voltage devices
22
and to indirectly power low-voltage devices
24
. The low-voltage devices
24
are protected by a cascode MOSFET transistor M
2
. The gate of M
2
is connected to a first Zener clamp
26
comprised of a Zener diode D
5
and a Zener diode D
6
. Zener clamp
26
is biased by current flowing through resistor R
2
, and also through either resistor R
3
or Zener diodes D
3
and D
4
, in a manner which will be explained below. If the Zener diodes D
5
and D
6
have a breakdown voltage V
Z
, then the voltage seen across the low-voltage devices
24
equals:
V
X
=V
IN
−V
GS2
if
V
IN
<2
V
Z
EQ. 1
V
X
=2
V
Z
−V
GS2
if
V
IN
≧2
V
Z
EQ. 2
A system employing a single cascode transistor suffices for voltages that do not exceed the breakdown voltage of the cascode transistor M
2
. For systems that do exceed this limit, a plurality of cascodes can be coupled in series.
In
FIG. 2
, a second cascode transistor M
1
is coupled between the high input voltage V
HIGH
and transistor M
2
, such that the second cascode transistor M
1
limits the differential voltage seen across the first cascode transistor M
2
. The gate of M
1
is connected to a second Zener clamp
28
comprised of Zener diodes D
3
and D
4
. Zener clamp
28
is biased by current flowing through resistor R
2
. Zener clamp
28
limits the voltage V
DS2
seen across transistor M
2
to
V
DS2
=V
GS2
; if
V
HIGH
<2
V
Z
EQ. 3
V
DS2
=[(
R
3
/(
R
2
+
R
3
)*(
V
HIGH
−2
V
Z
)]−
V
GS1
+V
GS2
; EQ. 4
if 2
V
Z
≦V
HIGH
<4
V
Z
+2
R
2
/
R
3
V
Z
V
DS2
=2
V
Z
−V
GS1
+V
GS2
; if
V
HIGH
≧4
V
Z
+2
R
2
/
R
3
V
Z
EQ. 5
Resistor R
3
provides a path for current to flow around Zener diodes D
3
and D
4
to bias Zener diodes D
5
and D
6
. At higher voltages, the current flowing through resistor R
3
is augmented by additional current flowing through Zener diodes D
3
and D
4
. The sum of both of these currents then flows through Zener diodes D
5
and D
6
to ground. Progressively higher voltages can be achieved by adding additional cascode stages. Furthermore, the number of Zener diodes in Zener clamps
26
and
28
can be decreased or increased to adjust to the needs of a specific application. Likewise, MOSFET transistors M
1
and M
2
can be replaced with bipolar junction transistors without significantly altering the operation of the circuit.
A stack of cascode stages such as those depicted in
FIG. 2
can be termed a “compliance stack”. Compliance refers to the ability of a system to adapt to externally imposed conditions. In this case, the circuit of
FIG. 2
has an enhanced ability to adapt to externally applied voltages due to the presence of the cascodes, or in other words, the cascodes provide voltage compliance.
The Zener clamp of
FIG. 1
does not provide voltage compliance, rather, it is the series current limiting element (R
1
in this case) that provides voltage compliance. The circuit of
FIG. 1
draws large amounts of current at higher voltages. The compliance stack of
FIG. 2
is superior to the Zener clamp of
FIG. 1
because the current drawn by the protected circuitry need not flow through the resistors, allowing very large resistances to be employed. Still, resistors R
2
and R
3
must draw some amount of current to overcome the effects of junction leakage, and the current they conduct increases at higher voltages. Since many modem integrated circuits are expected to operate on very small currents (<10 &mgr;A), it becomes difficult to simultaneously provide enough current at low voltages to bias the circuit, and yet to limit the current flow to acceptable levels at higher voltages. For this reason, systems of the sort illustrated in
FIG. 2
are unsuited for low-current applications.
SUMMARY OF INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some its aspects. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates to systems and methods for limiting voltage to low-voltage devices (e.g., amplifiers, current limiters and logic) in high-voltage applications (e.g. optical switching, high-voltage drivers, dimmers, and video displays) where a high-voltage supply feeds low-voltage devices. The systems and method employ voltage limiting devices (e.g., Zener clamps) to bias one or more cascode devices (e.g., MOSFET transistors). The cascode devices are serially connected from a high-voltage supply to a low-voltage node. A primary cascode and a primary voltage limiter cooperate with a primary current source to assure that the voltage provided to the one or more low-voltage devices is within the applicable voltage limits. Additional cascodes and voltage limiters may be added to ensure that the maximum voltage rating of the primary cascode device is not exceeded.
In one aspect of the invention, the voltage limiters are series-connected Zener diodes that bias a string of series-connected cascode devices. A string of series-connected current source devices (e.g., depletion MOSFET's) provide bias for the voltage limiters. Each of the current sources is set to conduct a current, I
MIN
, sufficient to bias a voltage limiter. The current drawn by the entire bias network, consisting of both the voltage limiters and their associated current sources, is substantially equal to the current drawn by one current source, or I
MIN
. Manufacturing variations, as well as the magnitude of the
Brady W. James
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Tran Toan
LandOfFree
Low-current compliance stack using nondeterministically... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Low-current compliance stack using nondeterministically..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low-current compliance stack using nondeterministically... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3031642