Electronically controllable capacitors using power MOSFET's

An electrically controllable variable capacitor includes the interelectrode capacitance of at least one power MOSFET, a capacitance connected in series with the MOSFET and having one terminal connected to the drain or source thereof (the series capacitance), and bias control circuitry for controlling the bias voltage applied to the MOSFET. The voltage rating of the MOSFET, the peak amplitude of the applied ac signal, and the value of the series capacitance determine the range of dc bias voltages over which the MOSFET can be operated, and hence the capacitance range of the variable capacitor. Such a variable capacitance is useful as a tuning capacitor in an electrodeless HID lamp ballast.

FIELD OF THE INVENTION 
The present invention relates generally to variable capacitors and, more 
particularly, to an electronically controllable variable capacitor using 
at least one power MOSFET. Such a variable capacitor is useful, for 
example, in an electrodeless high intensity discharge lamp ballast. 
BACKGROUND OF THE INVENTION 
Variable capacitors may be used for automatic circuit tuning. In low power 
circuits, for example, a varactor, which is a semiconductor device having 
a junction capacitance that varies with bias voltage, is often used for 
automatic tuning. For high power, high cost applications, mechanical 
actuators, e.g., stepper motors, with feedback may be used to control 
capacitance. Unfortunately, neither of these approaches is suitable for 
electrodeless high intensity discharge (HID) lamp ballasts, which are 
typically high power, low cost applications. 
In an electrodeless HID lamp, an arc discharge is generated by establishing 
a solenoidal electric field in a gas contained within an arc tube. The 
solenoidal electric field is created by the time-varying magnetic field of 
an excitation coil which is disposed about the arc tube. To maximize 
efficiency of an HID lamp, the degree of coil coupling between the 
magnetic field and the arc discharge must be maximized. Since the degree 
of coupling increases with frequency, electronic ballasts used to drive 
HID lamps operate at high frequencies in the range from 0.1 to 30 MHz, 
exemplary operating frequencies being 13.56 and 6.78 MHz. These exemplary 
frequencies are within the industrial, scientific, and medical (ISM) band 
of the electromagnetic spectrum in which moderate amounts of 
electromagnetic radiation are permissible; and such radiation is generally 
emitted by an electrodeless HID lamp system. 
Operation of an HID lamp ballast at the series resonant frequency of the 
load circuit maximizes power output. However, operation at a frequency 
slightly higher than the series resonant frequency of the load circuit 
maximizes ballast efficiency. Hence, for maximum efficiency, operation is 
slightly "off" resonance, and a specific ballast load resistance and phase 
angle are required. To this end, the impedance of the ballast load, 
including that of the arc discharge as reflected into the ballast load, 
must be matched to the required ballast load resistance and phase angle. 
As described in commonly assigned, U.S. Pat. No. 5,047,692 of J. C. 
Borowiec and S-A El-Hamamsy, issued Sept. 10, 1991, which is incorporated 
by reference herein, a capacitance connected in parallel with the 
excitation coil is needed to match the resistive component of the ballast 
load impedance, and a capacitance connected in series with the excitation 
coil is needed to obtain the proper phase angle. However, the ballast load 
impedance, and thus the matching conditions, for running and starting the 
lamp are different. In addition, the circuit is very sensitive to 
component variations because its quality factor (Q), when the lamp is 
running, is very high, e.g., 20-40. 
Accordingly, it is desirable to provide an automatically variable capacitor 
useful for tuning an electrodeless high intensity discharge lamp ballast 
as the output impedance thereof changes between starting and running 
conditions. In addition, it is desirable that such a variable capacitor be 
automatically tunable for matching the ballast impedance as it changes due 
to impedance variations in circuit components. 
SUMMARY OF THE INVENTION 
An electronically controllable variable capacitor comprises: the output 
capacitance of at least one power MOSFET, a capacitance connected in 
series with the MOSFET and having one terminal connected to the drain or 
source thereof (the series capacitance), and bias control circuitry for 
controlling the bias voltage applied to the MOSFET. The voltage rating of 
the MOSFET, the peak amplitude of the applied ac signal, and the value of 
the series capacitance determine the range of dc bias voltages over which 
the MOSFET can be operated, and hence the capacitance range of the 
variable capacitor. The value of the series capacitance is chosen so as to 
limit the peak amplitude of the ac signal level applied to the MOSFET, 
thereby avoiding conduction of the MOSFET's body diode and resultant 
significant power losses, and furthermore to enhance linearity of the 
capacitance versus voltage curve for the variable capacitor. 
A variable capacitance in accordance with the present invention is useful 
as a tuning capacitor in an electrodeless HID lamp ballast.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a schematic diagram of an electrodeless HID lamp 10 and 
associated ballast 12, such as those described in U.S. Pat. No. 5,047,692 
of J. C. Borowiec and S-A El-Hamamsy, cited hereinabove. The HID lamp 
includes an arc tube 14 formed of a high temperature glass, such as fused 
quartz, or an optically transparent ceramic, such as polycrystalline 
alumina. By way of example, arc tube 14 is shown as having a substantially 
ellipsoid shape. However, arc tubes of other shapes may be desirable, 
depending upon the application. For example, arc tube 14 may be spherical 
or may have the shape of a short cylinder, or "pillbox", having rounded 
edges, if desired. 
Arc tube 14 contains a fill in which a solenoidal arc discharge is excited 
during lamp operation. A suitable fill, described in commonly assigned 
U.S. Pat. No. 4,810,938 of P. D. Johnson, J. T. Dakin and J. M. Anderson, 
issued on Mar. 7, 1989, comprises a sodium halide, a cerium halide and 
xenon combined in weight proportions to generate visible radiation 
exhibiting high efficacy and good color rendering capability at white 
color temperatures. For example, such a fill according to the Johnson et 
al. patent may comprise sodium iodide and cerium chloride, in equal weight 
proportions, in combination with xenon at a partial pressure of about 500 
torr. The Johnson et al. patent is hereby incorporated by reference. 
Another suitable fill, described in commonly assigned U.S. Pat. No. 
4,972,120 of H. L. Witting, issued Nov. 20, 1990 and incorporated by 
reference herein, comprises a combination of a lanthanum halide, a sodium 
halide, a cerium halide and xenon or krypton as a buffer gas. 
Electrical power is applied to the HID lamp by an excitation coil 16 
disposed about arc tube 14 which is driven by an rf signal via a ballast 
driver 18 and ballast 12. (For clarity of illustration, coil 16 is not 
shown in its operational position about arc tube 14.) A suitable 
excitation coil 16 may comprise, for example, a two-turn coil having a 
configuration such as that described in commonly assigned U.S. Pat. No. 
5,039,903 of G. A. Farrall, issued Aug. 13, 1991 and incorporated by 
reference herein. Such a coil configuration results in very high 
efficiency and causes only minimal blockage of light from the lamp. The 
overall shape of the excitation coil of the Farrall patent is generally 
that of a surface formed by rotating a bilaterally symmetrical trapezoid 
about a coil center line situated in the same plane as the trapezoid, but 
which line does not intersect the trapezoid. However, other suitable coil 
configurations may be used with the starting aid of the present invention, 
such as that described in commonly assigned U.S. Pat. No. 4,812,702 of J. 
M. Anderson, issued Mar. 14, 1989, which patent is hereby incorporated by 
reference. In particular, the Anderson patent describes a coil having six 
turns which are arranged to have a substantially V-shaped cross section on 
each side of a coil center line. Still another suitable excitation coil 
may be of solenoidal shape, for example. 
In operation, rf current in coil 16 results in a time-varying magnetic 
field which produces within arc tube 14 an electric field that completely 
closes upon itself. Current flows through the fill within arc tube 14 as a 
result of this solenoidal electric field, producing a toroidal arc 
discharge 20 in arc tube 14. The operation of an exemplary electrodeless 
HID lamp is described in commonly assigned Dakin U.S. Pat. No. 4,783,615, 
issued on Nov. 8, 1988, which patent is hereby incorporated by reference. 
As illustrated in FIG. 1, HID lamp ballast 12 comprises a Class-D power 
amplifier including two switching devices Q.sub.1 and Q.sub.2 connected in 
series with a dc power supply V.sub.DD in a half-bridge configuration. 
Switching devices Q.sub.1 and Q.sub.2 are illustrated as MOSFET's, but 
other types of switching devices having capacitive gates may be used, such 
as insulated gate bipolar transistors (IGBT's) or MOS-controlled 
thyristors (MCT's). Switching devices Q.sub.1 and Q.sub.2 are coupled to 
ballast driver 18 via input isolation transformers 22 and 24, 
respectively. In operation, the switching devices are driven alternately 
between fully on and fully off conditions such that one is conducting 
while the other one is turned off and vice versa. Hence, the Class-D 
ballast may be conveniently driven by a square wave signal. Alternatively, 
ballast driver 18 may comprise means for generating two out-of-phase 
sinusoidal signals, as described in commonly assigned U.S. Pat. No. 
5,023,566 of S-A El-Hamamsy and G. Jernakoff, issued Jun. 11, 1991 and 
incorporated by reference herein. 
A resonant load network is connected to the half-bridge at the junction 
between switching devices Q.sub.1 and Q.sub.2 and in parallel with 
switching device Q.sub.2. The resonant load network comprises the 
excitation coil 16 of HID lamp 10 and a tuning capacitor C.sub.p connected 
in parallel therewith, and a blocking/tuning capacitor C.sub.s connected 
in series with the parallel combination of coil 16 and capacitor C.sub.p. 
The parallel combination of capacitor C.sub.p and coil 16 functions as an 
impedance transformer to reflect the impedance of the arc discharge 20 
into the ballast load. Capacitor C.sub.s is used both for blocking dc 
voltage and for resonant circuit tuning. 
Capacitors C.sub.s and C.sub.p are chosen to ensure impedance matching for 
maximum efficiency. That is, these capacitors are chosen to ensure that 
the ballast load is designed for optimum values of resistance and 
impedance phase angle. As described in U.S. Pat. No. 5,047,692, cited 
hereinabove, the excitation coil of the HID lamp acts as the primary of a 
loosely-coupled transformer, while the arc discharge acts as both a 
single-turn secondary and secondary load. The impedance of the arc 
discharge is reflected to the primary, or excitation coil, side of this 
loosely-coupled transformer. To match the ballast load impedance for 
maximum efficiency, the parallel capacitor operates with the excitation 
coil to match the proper resistive load value, and the series capacitor 
acts with the combination of the excitation coil and parallel capacitance 
to yield the required phase angle. 
Preferably, both tuning capacitors C.sub.s and C.sub.p are variable to 
provide matching under starting and running conditions as well as for 
impedance variations in circuit components. FIG. 2 illustrates a 
dual-feedback control for a Class-D ballast, such as described in commonly 
assigned U.S. Pat. No. 5,118,997 of S-A El-Hamamsy, issued Jun. 2, 1992 
and incorporated by reference herein, useful for controlling the values of 
capacitances C.sub.s and C.sub.p to match the ballast load impedance under 
changing conditions. A phase control feedback loop 30 varies the value of 
series tuning capacitor C.sub.s to provide load phase angle control; and 
an amplitude control feedback loop 40 varies the value of parallel tuning 
capacitor C.sub.p to provide load amplitude control. In FIG. 2, L.sub.c 
represents the inductance and R.sub.c represents the resistance of coil 
16; L.sub.a represents the inductance and R.sub.a represents the 
resistance of the arc discharge; and k is the coupling coefficient between 
the coil and the arc discharge. 
In accordance with the present invention, an electrically controllable 
variable capacitor is provided which would be useful for implementing 
either or both tuning capacitors C.sub.s and C.sub.p of the HID lamp 
ballast of FIGS. 1 and 2. 
FIG. 3 illustrates one embodiment of an electrically controllable variable 
capacitor 50 according to the present invention with capacitor terminals 
52 and 54. Variable capacitor 50 comprises at least one power MOSFET 
M.sub.1, shown by way of illustration only as an N-channel power MOSFET, 
connected in series at its drain terminal d to a series capacitance 
C.sub.1. The gate terminal g and source terminal s of MOSFET M.sub.1 are 
connected together. Bias voltage control circuitry 56 is connected across 
the drain and source terminals of the MOSFET. Bias voltage control 
circuitry 56 comprises a low-pass filter 58 connected between the drain 
terminal d of the MOSFET and the inverting terminal 60 of a controlled 
current source shown as an operational amplifier 62. A reference voltage 
V.sub.ref is connected between the source terminal s of the MOSFET and the 
noninverting terminal 64 of controlled current source 62. 
The effective capacitance of variable capacitor 50 is taken between 
terminals 52 and 54 and comprises the series combination of series 
capacitance C.sub.1 and the output capacitance of the MOSFET. The output 
capacitance of the MOSFET comprises the sum of its drain-to-source 
capacitance and its drain-to-gate capacitance. Both of these 
interelectrode capacitances vary with the drain-to-source voltage V.sub.ds 
applied to the MOSFET. As the voltage V.sub.ds increases (e.g., becomes 
more positive for an N-channel MOSFET), the interelectrode capacitances 
decrease over normal operating biases, thus decreasing the total 
capacitance between terminals 52 and 54. 
Capacitance C.sub.1 performs several functions in the variable capacitor 
arrangement of the present invention. It blocks dc voltage between 
terminals 52 and 54 from being applied to the MOSFET. As a result, the dc 
bias voltage V.sub.ds across MOSFET M.sub.1 is separately controllable by 
circuitry 56. Moreover, as long as the capacitance C.sub.1 is not much 
larger than the output capacitance of the MOSFET, capacitance C.sub.1 
reduces the ac signal voltage applied between the MOSFET's drain and 
source terminals. This is important because the drain-to-source voltage 
V.sub.ds of the MOSFET must not become less than zero; otherwise, the 
integral body diode D.sub.1 (shown in phantom) of the MOSFET will become 
conductive, resulting in a significant power loss. As a result, the least 
positive dc bias that can be applied to MOSFET M.sub.1 is approximately 
equal to the peak amplitude of the ac signal swing across MOSFET M.sub.1 ; 
and the most positive dc bias that can be applied to MOSFET M.sub.1 is its 
maximum rated drain-to-source voltage V.sub.ds minus the peak amplitude of 
the ac signal voltage across MOSFET M.sub.1. Hence, the voltage rating of 
the MOSFET, the peak amplitude of the applied ac signal, and the value of 
the series capacitance C.sub.1 determine the range of dc bias over which 
the MOSFET can be operated. In addition, by limiting the peak amplitude of 
the ac signal applied to the MOSFET, the capacitance C.sub.1 also serves 
to enhance the linearity of the capacitance versus voltage curve for 
variable capacitor 50. 
Bias voltage control circuitry 56 is used to control the bias voltage 
V.sub.ds as follows. Low-pass feedback filter 58 blocks the high-frequency 
(e.g., rf) signal component of the drain-to-source voltage V.sub.ds and 
provides a voltage representing the dc bias component of V.sub.ds to the 
inverting input 60 of controlled current source 62. The difference between 
this voltage and the reference voltage V.sub.ref causes the current source 
to attempt to maintain the dc bias on MOSFET M.sub.1 at the value 
V.sub.ref. The ac output impedance of the bias voltage control circuitry 
is made very high at the signal frequency so that most of the signal 
current passes through the output capacitance of MOSFET M.sub.1. 
In alternative embodiments, other bias networks which have a high ac output 
impedance are used to bias MOSFET M.sub.1. For example, the voltage source 
V.sub.ref could be connected to the drain d of MOSFET M.sub.1 through an 
inductor, a resistor, or a series combination of an inductor and resistor. 
FIG. 4 shows a practical implementation of the current source 62 (FIG. 3) 
with voltage source V.sub.ref in series with a relatively large resistance 
and/or a relatively large inductance. For example, assuming a large 
resistance 2R.sub.B is used, the resistance 2R.sub.B has two purposes; one 
is to apply the bias voltage across the MOSFET's drain and source 
terminals, and the other is to provide a high impedance to limit rf 
current through the bias voltage source V.sub.ref. 
FIG. 5 shows an alternative embodiment of a variable capacitor according to 
the present invention wherein the fixed capacitor C.sub.1 of FIGS. 3 and 4 
is replaced with another MOSFET M.sub.2. The MOSFET's M.sub.1 and M.sub.2 
are arranged such that the anodes of their integral body diodes D.sub.1 
and D.sub.2 are connected together. The voltage V.sub.ref is applied with 
the polarity shown in order to reverse bias both body diodes. 
(Alternatively, the MOSFET's could be connected in the opposite direction, 
that is, with the cathodes of their body diodes connected together. When 
connected in this fashion, the polarity of V.sub.ref must be reversed.) 
The dc voltage V.sub.ref must be high enough so that when the rf voltage 
is in the forward direction across one of the MOSFET's, the body diode of 
the MOSFET does not turn on. If the capacitances of the MOSFET's were 
equal at all times, the dc voltage would have to be greater than or equal 
to one-half the peak amplitude of the rf voltage. However, the output 
capacitance of each MOSFET changes with rf voltage applied thereto. As the 
capacitance increases when the applied total voltage (rf and dc) is 
decreased, the voltage division across the two MOSFET's is not equal, but 
rather the voltage across the one having the larger output capacitance is 
smaller. Therefore, the applied dc voltage V.sub.ref can be somewhat 
smaller than one-half the peak amplitude of the rf voltage without causing 
any problems. At the other end of the rf voltage swing, the dc voltage 
plus the rf voltage across either MOSFET must not exceed the voltage 
rating of the device. Because the voltage division between the MOSFET's is 
unequal, the maximum applied dc voltage has to be somewhat less than the 
maximum voltage rating for the MOSFET minus one-half the peak amplitude of 
the rf voltage applied thereto. 
Either or both tuning capacitors C.sub.s and C.sub.p of the lamp ballast of 
FIGS. 1 and 2 can be implemented using a variable capacitor according to 
the present invention. By controlling the bias voltage V.sub.ref of such a 
variable capacitor, the circuit can be electronically tuned for optimum 
operation during both starting and running conditions and can be adjusted 
to account for variability in the other components in the circuit. 
FIG. 6 graphically illustrates exemplary capacitance versus dc bias voltage 
data for various applied ac voltages for the variable capacitor of FIG. 5. 
While the preferred embodiments of the present invention have been shown 
and described herein, it will be obvious that such embodiments are 
provided by way of example only. Numerous variations, changes and 
substitutions will occur to those of skill in the art without departing 
from the invention herein. Accordingly, it is intended that the invention 
be limited only by the spirit and scope of the appended claims.