Amplifiers – Wheatstone bridge with amplifier in at least one arm
Reexamination Certificate
1997-12-19
2001-03-20
Mottola, Steven J. (Department: 2817)
Amplifiers
Wheatstone bridge with amplifier in at least one arm
C330S251000
Reexamination Certificate
active
06204729
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to control systems and methods for controlling inductive loads. More particularly, the invention relates to a low-noise and highly linear output stage for pulse-width-modulation (PWM) amplifiers.
2. Description of Related Art
Switching servo amplifiers are commonly used to supply drive current to inductive loads, such as linear, voice-coil, and DC motors. Such amplifiers often employ a type of output stage commonly known as H-bridge amplifiers, or simply “H-bridges. ”
FIG. 1
depicts a conventional H-bridge
100
. H-bridge
100
includes four transistors M
1
-M
4
configured to drive an inductive load
110
. Each transistor M
1
-M
4
has a corresponding diode D
1
-D
4
connected, in a reverse-current direction, from source to drain. Diodes D
1
-D
2
are typically fabricated integrally with respective transistors M
1
-M
4
. The gate voltages of transistors M
1
-M
4
are controlled by driver-amplifier circuits (not shown) of a conventional switching servo amplifier to alternate the direction of current flow through load
110
. Turning transistors M
1
and M
4
on and M
2
and M
3
off causes current to flow in one direction; turning transistors M
1
and M
4
off and M
2
and M
3
on causes current to flow in the other direction. Alternating between transistor pairs causes each terminal of load
110
to alternate between ground potential and the voltage level on power terminal +HV. The transistors are not switched simultaneously: some small delay ensures that transistors M
1
and M
2
(and similarly M
3
and M
4
) do not conduct at the same time.
Conventional H-bridges and their associated circuitry are well known. An explanation of their operation is therefore omitted for brevity. For further information explaining the operation of several conventional H-bridge configurations, see the following U.S. Patents, the contents of which are incorporated herein by this reference:
4,581,565 to Van Pelt, et al., issued Apr. 8, 1986;
4,851,753 to Hamilton, issued Jul. 25, 1989;
4,873,618 to Fredrick et al., issued Oct. 10, 1989;
5,552,683 to Dargent, issued Sep. 3, 1996; and
5,596,446 to Plesko, issued Jan. 21, 1997.
Conventional H-bridge circuits are too noisy and produce excessive distortion for some precision applications. One facet of this noise is a “ringing” of the voltage levels on load terminals for a time after one pair of transistors is switched on and the alternate pair is switched off.
FIG. 2
illustrates the ringing of an output voltage level on terminal
120
of conventional H-bridge
100
. The first segment of the waveform (e.g., T
0
to T
1
) represents the time during which transistors M
3
and M
2
are on, causing the voltage on terminal
120
to approach ground potential (e.g., zero volts).
Transistors M
3
and M
2
are switched off and transistors M
1
and M
4
switched on at time T
1
. Upon switching, the voltage on terminal
120
exceeds the supply voltage +HV for an instant due to the inductive “kick” of load
110
and associated leads. This noise spike is depicted as a spike
200
. Then, after some ringing
210
, the voltage on terminal
120
finally settles to the supply voltage +HV. H-bridge
100
also exhibits a noise spike
220
and ringing
230
at time T
2
when transistors M
3
and M
2
are switched on and transistors M
1
and M
4
are switched off.
Diodes D
1
-D
4
do much to limit the energy of spikes
200
and
220
and associated ringing
210
and
230
. Nevertheless, there remains a level of noise that is unacceptable for certain high-performance applications. There is therefore a need for a low-noise, highly linear output stage for pulse-width-modulation amplifiers.
SUMMARY
The present invention is directed to a highly linear PWM amplifier that exhibits low harmonic distortion. The amplifier includes four switching devices (e.g., power transistors) interconnected with each other and with an inductive load in an H configuration. Each transistor has a precision voltage clamp connected between the source and drain to suppress ringing oscillations. Two or more of the transistors also include ultra-high-speed transient-voltage suppressers connected in parallel with their respective voltage clamps. These transient-voltage suppressers do not have the clamping accuracy of the voltage clamps, but respond much more quickly to suppress noise spikes. An amplifier in accordance with the present invention therefore takes advantage of both the fast response time of the transient-voltage suppressers and the precise voltage-clamping characteristics of the voltage clamps.
This summary does not purport to define the scope of the invention. The scope of the invention is defined instead by the claims.
REFERENCES:
patent: 3525883 (1970-08-01), Iordanidis
patent: 4581565 (1986-04-01), Van Pelt et al.
patent: 4851753 (1989-07-01), Hamilton
patent: 4873618 (1989-10-01), Fredrick et al.
patent: 5144211 (1992-09-01), Daggett et al.
patent: 5552683 (1996-09-01), Dargent
patent: 5596446 (1997-01-01), Plesko
patent: 6072362 (2000-06-01), Lincoln
Halbert Michael J.
Klivans Norman R.
Mottola Steven J.
Nikon Research Corporation of America
Skjerven Morrill & MacPherson LLP
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