Amplifiers – Miscellaneous – Amplifier protection means
Reexamination Certificate
2001-04-06
2002-07-09
Shingleton, Michael B (Department: 2817)
Amplifiers
Miscellaneous
Amplifier protection means
C455S115200, C455S117000, C330S298000
Reexamination Certificate
active
06417732
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to controllers for radio frequency (RF) power generators, and more particularly to controllers for RF power generators with reduced cable length sensitivity.
BACKGROUND OF THE INVENTION
Many radio frequency power generators include controllers that regulate RF output power and prevent amplifier damage due to load mismatch, excessive supply voltage, and excessive operating temperature. The controllers also minimize damage after failure of one or more of the power devices.
FIG. 1
shows a typical radio frequency (RF) power generator
10
that includes a power module
11
and a controller
12
. The power module
11
receives signals from RF exciter
14
, amplifies the signals, and delivers the signals to a load
16
. The power module
11
includes a driver
18
and a final amplifier
20
. The power module
11
receives DC power through a cable
24
that is coupled to a remote battery
26
with a ground return. The cable
24
may have substantial distributed impedance. The controller
12
includes an amplifier
30
, a frequency compensation capacitor
34
, and a buffer
38
. The controller
12
receives control inputs
40
and feedback signals
42
and produces a control voltage
44
that varies the gain of the driver
18
.
The controller
12
regulates output power during normal conditions and protects the power module
11
during abnormal conditions. The controller
12
employs negative feedback to diminish an error between the greatest feedback signal and a reference input that has been selected according to nominal operating levels of the feedback transducers. Feedback signals from the power module
11
include forward and reverse power signals
50
and
52
that are generated by RF detectors
54
and
56
. The detectors
54
and
56
are typically coupled to sampling arms of a directional coupler
60
. Other feedback signals include a temperature signal
62
from a thermistor
64
that is thermally coupled to the final amplifier
20
. Differential voltage feedback signals
66
and
70
are proportional to DC input current to the power module
11
(through a current-sampling resistor
72
). A drive signal
74
feeds back the drive current to the final amplifier
20
. A feedback signal
76
feeds back the control voltage
44
that is supplied to the driver
18
.
Under normal conditions, all of the feedback signals except the forward power signal
50
are small. The controller
12
increases the control signal
44
until the forward power signal
50
becomes approximately equal to a reference setpoint. Under abnormal conditions, the other feedback signals increase and exceed the forward power signal
50
. For example, the reverse power signal
52
increases when the load
16
becomes mismatched or is removed. Increasing the drive signal
44
without a corresponding increase in the forward power signal
50
indicates load mismatch or malfunction of the final amplifier
20
. Excessive control voltage for a given output power typically corresponds to a problem in the driver
18
. Low DC input current indicates load mismatch, a faulty driver
18
, or a faulty final amplifier
20
. High DC input current or a high final amplifier
20
temperature indicates that the controller
12
should reduce forward power demands on the power module
11
. When one or more of these conditions occur, the controller
12
reduces the drive signal
44
to the power module
11
to keep the largest feedback signal approximately equal to the reference setpoint.
Conventional methods for protecting the RF power generator
10
also typically employ a set of measured generator parameters and hard setpoint limits for each parameter. For example, maximum reflected power is limited to 600 watts (W), maximum power amplifier (PA) current is limited to 40 Amps (A), and maximum PA dissipation to 1800 W. This protection technique is effective in protecting the RF power generator
10
from adverse loads but does not give repeatable performance when a length of a cable between the RF power generator
10
and the load
16
is varied.
Referring now to
FIG. 2
, a simplified power generator control system
100
according to the prior art is illustrated. The RF power generator control system
100
includes a power module
102
, a RF sensor
104
, a load
106
, and a controller
108
. The power module
102
generates power module feedback signals
109
(such as PA supply current
110
and device temperature
114
). The RF sensor
104
generates RF sensor feedback signals
115
(such as forward and reverse power
116
and
118
). The power module feedback signals
109
, the RF sensor feedback signals
115
, and an external setpoint signal
120
are input to the controller
108
. The controller
108
generates a power module setpoint signal
124
that is input to the power module
102
. The power module setpoint signal
124
controls the forward power output by the power module
102
.
The basic control technique is to provide negative feedback signals from various detectors (such as the forward power
116
, the reverse power.
118
, the PA supply current
110
, and the device temperature
114
). During normal operation, all of the feedback signals except the forward power signal are relatively small. In this case, the controller
108
increases or decreases the power module setpoint signal
124
to regulate the forward power
116
of the power module
102
. Under mismatched load conditions, another feedback signal, for example the supply current
110
to the power amplifier in the power module
102
, dominates the forward power feedback
116
. This will cause the controller
108
to reduce the power module setpoint
124
. The power module
102
reduces the forward power delivered to the load
106
.
This control technique is effective in protecting the generator from adverse loads but does not give repeatable performance when the cable length L between the power module
102
(the RF sensor
104
) and the load
106
is varied. The change in the cable length L introduces a phase shift that may cause a high impedance load to be transformed into a low impedance load. The changes in the load impedance cause an increase or decrease in the current draw of the power amplifier in the power module
102
. The impedance shift causes the supply current limiting loop of the power amplifier to reduce or increase the power module setpoint
124
. This in turn causes the RF power generator control system
100
to deliver less or more power than the unphase-shifted case even though the voltage standing wave ratio (VSWR) has not changed.
In applications where repeatability is very important, such as semiconductor manufacturing, it may be very desirable to have a RF generator that has reduced sensitivity to cable length or load phase. For example, plasma delivery systems require precisely controlled conditions and repeatability. Some installations may have a longer distance from the chamber to the generator rack than others. Therefore, these systems will operate differently.
SUMMARY OF THE INVENTION
A radio frequency (RF) power generator system according to the invention includes a power generator that generates a RF power signal that is output to a load. The RF generator generates a forward power feedback signal and a reverse power feedback signal. A controller receives the forward power feedback signal and the reverse power feedback signal. The controller generates a setpoint signal that is output to the power generator. A setpoint modifier receives the forward feedback signal, the reverse feedback signal and an external setpoint signal. The setpoint modifier calculates a forward power limit based on the forward and reverse power feedback signals. The setpoint modifier and outputs a modified setpoint signal to the controller based on one of the forward power limit and the external setpoint signal.
In other features of the invention, the controller selects a lesser value between the forward power limit and the external setpoint signal. The power generator includes a RF sensor that generates the
Nasman Kevin P.
Radomski Aaron T.
ENI Technology Inc.
Shingleton Michael B
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