Computer-aided design and analysis of circuits and semiconductor – Nanotechnology related integrated circuit design
Reexamination Certificate
2002-02-15
2004-08-24
Smith, Matthew (Department: 2825)
Computer-aided design and analysis of circuits and semiconductor
Nanotechnology related integrated circuit design
C716S030000, C716S030000, C323S248000, C363S075000, C363S082000, C363S093000
Reexamination Certificate
active
06782513
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to ferroresonant transformers, and more particularly to a method of designing a high power factor ferroresonant constant current source.
BACKGROUND OF THE INVENTION
The industry's choice of root-mean-square (rms) to measure AC voltage and current introduced the power factor concept. Power factor accounts for the discrepancy between kilovolt-ampere (kva) and kilowatt (kw), where kva is the multiplicative product of the rms current and voltage, and kw is the real power.
I
RMS
=
1
T
∫
0
T
i
2
(
t
)
ⅆ
t
(1)
V
RMS
=
1
T
∫
0
T
v
2
(
t
)
ⅆ
t
(2)
P
Watts
=
1
T
∫
0
T
i
(
t
)
·
v
(
t
)
ⅆ
t
(3)
PF
=
P
Watts
I
RMS
V
RMS
(4)
where i(t) and v(t) are the instantaneous current and voltage, respectively, and T is the period. The power factor can be from 0, for pure inductive or capacitive loads, to 1.0 for resistive loads—the higher the better. As can be seen from the above expressions, I
RMS
·V
RMS
=P
watts
only if the load is resistive. For non-linear loads, or inductive/capacitive loads, the real power P
watts
is less than I
RMS
·V
RMS
. It is important that an electrical load maintains a high power factor so as to reduce the current going to the load as well as the cost of cable and I
2
R losses.
Power factor has mostly been associated with inductive loads in which AC capacitors are incorporated to cancel the inductive current and correct any lagging power factor. The power factor in this case can be represented in terms of the cosine of the phase angle &agr; between the voltage and the current: PF=cos(&agr;). The above power factor is also known as displacement power factor. There is another form of power factor that is associated with non-linear loads such as, for example, rectified capacitive loads, and is referred to as distortion power factor.
Ferroresonant transformers typically have an input power factor of 0.98 to 1.0, and are used to buffer both displacement and distortion power factor loads. A controlled ferroresonant constant current source varies the output current by controlling the conduction angle of an inductive reactance X
L
that is magnetically coupled to the resonant capacitor. The inductive discontinuous current introduces both displacement and distortion power factor. The distortion power factor can be reduced to virtually zero by increasing the capacitor leakage reactance X
S
. As a result, displacement power factor is the only form of power factor present in a ferroresonant transformer, which is contributed by the control inductive reactance X
L
.
A constant voltage ferroresonant transformer has a high input power factor since the inductive current contributed by the control inductive reactance X
L
is kept to a minimum. A controlled ferroresonant constant current source, on the other hand, requires the voltage to change over a wider range in order to maintain a constant load current. The effect of this inductive current, contributed by X
L
, reflected on the primary may reduce the input power factor to less than 0.2 during minimum output current sensing.
As shown in FIG.
1
and as explained more fully in my U.S. patent application Ser. No. 09/904,997, now U.S. Pat. No. 6,426,610, a simplified equivalent circuit
10
of a controlled ferroresonant constant current source shows the control inductive reactance X
L
external to the core of the ferroresonant transformer. This equivalent circuit is useful in deriving the expression for power factor as set forth below in equation 5. As shown in FIG.
2
and as also explained more fully in my U.S. patent application Ser. No. 09/904,997, now U.S. Pat. No. 6,426,610, another equivalent circuit
100
of a controlled ferroresonant constant current source shows the control inductive reactance X
L
integrated into the core of the ferroresonant transformer. This equivalent circuit is useful for deriving the expressions for the resonant capacitor gain A to be explained hereinbelow with respect to equations 6 through 14. The equivalent circuits of
FIGS. 1 and 2
will briefly be explained prior to using them for deriving equations.
With reference to
FIG. 1
, the circuit
10
may be made to function as a constant current source by incorporating an output inductor, such as an output coil
12
and shunt
14
into the core of the ferroresonant transformer. In this instance, a control inductor
16
is employed externally of the transformer core.
As mentioned in my U.S. patent application Ser. No. 09/904,997, now U.S. Pat. No. 6,426,610, several factors were considered in developing an improved controlled ferroresonant constant current source. A linear inductor includes a steel core, a coil and an air gap. The inductance is determined by the core cross-sectional area, the number of turns, and the length of the air gap. As the power rating of a controlled ferroresonant current source increases, the resonant capacitance, capacitive current, and control inductive current increase, which requires the control inductor to have a lower value. To reduce the inductance of an inductor, the turns need to be reduced or the air gap increased. The cross-sectional area needs to be adjusted to maintain an acceptable maximum flux density. A large air gap poses serious thermal problems because of fringing flux, which cuts through the core laminations and the magnet wire at a high loss angle, producing eddy currents that overheat the inductor and reduce efficiency. Increasing the size of the magnet wire will further increase the magnitude of eddy currents and reduce efficiency.
Integrating the control inductor into the core of the ferroresonant transformer using magnetic shunts significantly reduces the gap loss heating effect. The air gap of the shunts is more effective in determining inductance and can be easily distributed into multiple air gaps of shorter lengths. If the control inductor is integrated with the transformer core, and the output inductor is external to the transformer core, then the inductor is subjected to the load voltage which may be extremely high in magnitude (i.e., 1000-5000V). A high voltage inductor requires a large number of turns with high electrical insulation between turns and layers. A large number of turns will also increase the resistive losses and reduce the efficiency.
It has been discovered that the controlled ferroresonant constant current source may be improved by integrating both the output inductor and the control inductor onto the core of the ferroresonant transformer while using standard EI laminations. In order for the controlled ferroresonant constant current source to operate, the control inductor must interface with the capacitor sub-circuit such that the currents are in phase.
With reference to the circuit
100
of FIG.
2
and as explained more fully in my U.S. patent application Ser. No. 09/904,997, now U.S. Pat. No. 6,426,610, it has been determined that drawbacks in integrating the output inductor and the control inductor are solved by creating two separate resonant sub-circuits—one to interface with the load inductor including the output coil
102
and the shunt
104
to provide maximum gain, and another to interface with the control inductor, including the control coil
106
and the shunt
108
to control the resonant gain.
The benefits of incorporating both the control inductor and the output inductor onto the transformer core are 1) complete isolation between all circuits; 2) simplified wiring between the transformer core and external components; 3) low inductance, high current chokes no longer a limiting factor to increasing the power rating of the current source since shunts have a wider inductance range; and 4) permits the use of standard laminations which simplifies the assembly process.
Returning now to our discussion on power factor, since the distortion power factor can be greatly reduced by the proper choice of X
S
, the input power factor is predominantly the
Kik Phallaka
McCormick Paulding & Huber LLP
Shape Electronics, Inc.
Smith Matthew
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