Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2001-08-16
2002-10-22
Berhane, Adolf Denske (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S089000, C323S222000
Reexamination Certificate
active
06469917
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of switching power converters. More particularly, the present invention relates to a method and apparatus for regulating the harmonics content of the current drawn from the power line boy electrical equipment and loads by utilizing Borderline Conduction Mode (BCD) of operation without sampling the input voltage. The present invention also relates to the electronic circuit design, physical construction and layout of such an apparatus.
BACKGROUND OF THE INVENTION
Currently, there are several types of converters, which are widely used for DC-to-DC, DC-to-AC, AC-to-DC and AC-to-AC power conversion. In some applications, the purpose of the power conversion schemes is to shape the input current seen at the input of the converter, in order to correct the power factor. For example, in a power converter known in the art as an Active Power Factor Correction (APFC) converter, the role of the converter is to ensure that the AC current drawn from the power line is in phase with the line voltage, with minimal level of high-order harmonics. A typical and well-known implementation of an APFC converter is illustrated in FIG.
1
. According to this implementation, the input voltage is rectified by diode bridge D
1
and fed into a Boost converter that comprises an input inductor L
in
, a switch S
1
, a high frequency rectifier (D
2
), an output filter capacitor (C
O
) and a load (R
L
). A power switch (S
1
) is driven by a high frequency control signal of duty cycle D
ON
, so as to force the input current (i
ina
) to follow the shape of the rectified input voltage (V
ivR
), in which case the power converter becomes essentially a resistive load to the power line; i.e., the Power Factor (PF) will be a unity.
The need for APFC converters is driven by the worldwide concern for the quality of the power line supplies. Injection of high harmonics into the power line and poor Power Factor (PF) in general, is known to cause many problems. Among these problems are the lower efficiency of power transmission, possible interference to other units connected to the power line, and distortion of the line voltage shape. In the light of the practical importance of APFC converters, many countries have adopted, or are in the process of adopting, voluntary and mandatory standards. These standards set limits to the permissible current line harmonics injected by any given equipment that is powered by an alternating current (AC) electrical power source, so as to maintain a high power-quality. Another advantage of an APFC converter is the increase in the power level than can be drawn from a given power line. Without Power Factor Correction, the effective (i.e., rms) current will be higher than the magnitude of the first harmonics of the current, the latter being the only component that contributes real power to the load. Additionally, protection elements such as fuses and circuit breakers respond to the rms current. Consequently, the rms current limits the maximum power that can be drawn from the line. In Power Factor Correction equipment, the rms current equals the magnitude of the first harmonic of the current (since the higher harmonics are absent) and hence, the power drawn from the line essentially reaches its maximum theoretical value. It is thus evident that the need for APFC circuits is widespread and that economical realization of such circuits is of prime importance. Cost is of great concern, considering the fact that the APFC is an add-on expense to the functionality of the original equipment in which the APFC converter is included. In the light of the above, physical construction methods of APFC that are economical to produce, and can be easily integrated in any given equipment, are highly desirable and advantageous.
Common APFC converters usually operate in one of three modes (with respect to the current passing through the main inductor Lin):
(1) Continuous Conduction Mode (CCM), in which the inductor current never drops to zero;
(2) Discontinuous Conduction Mode (DCM), in which the inductor current drops to zero for a portion of every switching cycle; and
(3) Borderline Conduction Mode (BCM), in which the inductor current rises immediately after it drops to zero.
The shape of the inductor current in CCM is depicted in
FIG. 2
, that of DCM in FIG.
3
and the shape of the inductor current in BCM is depicted in FIG.
4
. In these figures, T
ON
is the time during which the power switch S
1
(
FIG. 1
) is on, T
OFF
is the time during which the inductor (current) is in the discharge phase, T
S
is the switching period
(
T
S
=
1
f
S
,
f
S
=
switching frequency
)
,
I
L
in
is the inductor current, and I
pk
is the peak inductor current.
The most efficient mode of operation is CCM, since the rms current of the power switch S
1
is the lowest. However, reverse recovery of the main diode D
2
poses extra losses and EMI generation. Furthermore, implementing an APFC converter in CCM mode requires that L
in
is of high inductance value, making it bulky and costly. The DCM is the least desirable since the inductor rms current is the highest, which increases the power switch losses and makes the main inductor large in size, because the physical size of an inductor is proportional to the rms current that is expected to pass through it. A good compromise is, therefore, the BCM mode of operation. Implementing the BCM mode allows reduction of the inductor size, as well as the power switch losses. Furthermore, in a properly designed BCM converter, the voltage across the power switch will, by itself, drop to zero just after the inductor current reverses its direction due to the reverse current of the main diode. Turning the main power switch under zero voltage switching (ZVS) conditions reduces switching losses and hence increases the efficiency of the converter.
FIG. 5
represents a conventional realization of a BCM converter according to the prior art. The controller CONT receives the shape of the rectified power line voltage (V
ac
—
ref
) obtained via the voltage divider R
a
, R
b
from V
ivR
, which is used as the reference for the desired shape of the input current. The controller receives the voltage V
se
across R
se
, which is identical to the input current when the power switch Q
1
is conducting, and generates gate pulses D
ON
to the power switch Q
1
, so as to force the inductor current to follow the reference voltage shape. The current level is adjusted for any given load R
L
by monitoring the output voltage V
od
via the voltage divider R
1
, R
2
, and multiplying the reference signal V
ac
—ref
by the deviation from the desired output voltage level, so as to adjust the effective reference signal to the load. BCM operation is achieved by turning on the power switch Q
1
(i.e. Q
1
conducts) only after the inductor current reaches a zero level. One way to detect this instance is by an auxiliary winding L
2
that is coupled to the main inductor L
in
. The auxiliary winding L
2
produces a positive voltage V
tr
whenever the inductor current reaches zero. The same L
2
winding can also be used, together with D
3
, R
tr
and C
b
, to generate the auxiliary power supply +V
CC
required for the controller.
A major drawback of the prior art BCM converter is the need to sense the converter s input voltage, namely the line voltage after rectification. Due to the switching effects, the input voltage V
ivR
is normally noisy and is susceptible to interference that may distort the reference signal and hence the controlled input current. Furthermore, the extra contact required for sensing the input current increases the number of pins of a modular device, if built according to conventional BCM schemes.
U.S. Pat. No. 5,742,151 discloses a PFC converter that provides unity PF by sensing only a current in the PFC circuit and a DC supply voltage. In the disclosed technique, the feature of sensing the input voltage is not used. However, conventional methods that do not sample the input voltage cannot operate in the BCM, but only in CCM and, with some inf
Berhane Adolf Denske
Cooper & Dunham LLP
Green Power Technologies Ltd.
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