Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-12-17
2004-05-18
Vu, Bao Q. (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S282000, C323S222000
Reexamination Certificate
active
06737846
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 circuitry for improving the output voltage regulation of switching power converters.
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 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 current drawn from the AC power line is in phase with the line voltage with minimum high-order harmonics. A typical and well-known implementation of an APFC converter is illustrated in
FIG. 1
(prior art). In
FIG. 1
, input voltage V
ac
is rectified by diode bridge D
1
and fed to a Boost converter that comprises input inductor L
in
, switch S
1
, high frequency rectifier D
2
, output filter capacitor C
O
and load R
L
. Power switch S
1
is driven by a high frequency control signal having duty cycle D
ON
so as to force input current i
ina
to follow the shape of rectified input voltage V
ivR
, in which case the power converter becomes essentially a resistive load to the ac power line; i.e., the Power Factor (PF) is unity.
The growing use of APFC converters is driven by the concern for the quality of AC 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 are the lower efficiency of power transmission, possible interference to other electrical units connected to the (same) power line and distorted shape of the line voltage. In the light of the practical importance of APFC converters, many countries have adopted, or are in the process of adopting, voluntary and mandatory standard and statutes, which set limits to the permissible current line harmonics injected by any given electrical equipment powered by the AC mains, in order to maintain a relatively high power-quality. Another advantage of an APFC converter is that it allows a better exploitation of the power level that is drawn from a given AC power line. Without Power Factor Correction, the current drawn from AC power line comprises a relatively high level of unwanted harmonics, which may be higher than the magnitude of the first harmonics of the current, the latter being the only component that contributes a real power to the load. Additionally, protection elements, such as fuses and circuit breakers, respond to the rms value of the current passing through them. Consequently, the rms value of the current limits the maximum power that can be drawn from the line. In Power Factor Correction equipment the rms current essentially equals the magnitude of the first harmonics of the current (due to lacking higher harmonics) and, hence, the power drawn from the line essentially reaches the maximum theoretical value. It is thus evident that the need for APFC circuits is widespread and that economical implementation 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.
Common APFC circuits operate in closed feedback configuration. For example, in the circuit illustrated in
FIG. 2
(prior art) the controller CONT samples the shape of the rectified power line voltage V
ivR
by voltage utilizing V
ac
—
ref
, which is obtained by voltage divider R
a
,R
b
. Namely, V
ac
—
ref
is used as the reference voltage for the desired shape of the input current. Controller CONT accepts also voltage V
se
, which is measured across R
se
and is identical to the input current when power switch Q
1
is on. Accordingly, controller CONT generates pulses D
ON
in order to drive power switch Q
1
such as to force inductor current i
ina
to follow reference voltage shape V
ac
—
ref
Current level is adjusted for any given load R
L
by monitoring output voltage V
O
by voltage divider R
1
, R
2
, and multiplying the reference signal V
ac
—
ref
by a factor that is correlated to the deviation from the desired output voltage level so as to adjust the effective reference signal to the load.
FIG. 3
(prior art) illustrates an improved construction of APFC converters, the improvement of which is obtained by applying a modified control strategy that does not require sensing the input voltage. In
FIG. 3
, V
a
is a cyclic pulsating voltage having maximal magnitude V
O
and duration T
OFF
, whenever Q
1
is in its non-conducting state. Consequently, the average value of V
a
is:
v
a
=
V
o
T
OFF
T
S
(
1
)
wherein T
S
is the PWM switching period.
Or:
V
a
=V
o
D
OFF
(2)
wherein
D
OFF
=
T
OFF
T
S
(
3
)
Similarly, whenever Q
1
is in its conducting state (i.e. during T
ON
), D
ON
is defined as:
D
ON
=
T
ON
T
S
(
4
)
The input voltage (V
ivR
) fed to the Boost converter is assumed to be of low frequency comparing to the switching frequency f
s
(f
s
=1/T
S
) and, hence, can be considered constant at least one switching period T
S
. Assuming proper functioning of the power converter, the average low frequency voltage across L
in
is essentially zero (or else the current will increase to very high magnitudes). This implies:
v
ivR
=v
av
(5)
wherein V
ivR
is the instantaneous low frequency component of V
ivR
. From (1):
v
ivR
=V
o
D
OFF
(6)
If DOFF is programmed according to the rule specified in equation (7):
D
OFF
=Ni
ina
(7)
wherein. N is a constant and i
ina
is the low frequency component of the input current (i
ina
), then:
v
ivR
=V
o
Ni
ina
(8)
or:
i
ina
=
v
ivR
v
o
N
(
9
)
Since C
O
is chosen to be adequately large, V
O
may be practically considered constant (i.e. the ripple of V
O
can be neglected). Therefore, according to (9), input current i
ina
follows input voltage V
ivR
. Consequently, the Power Converter looks resistive to the AC power source with an apparent input resistance (R
e
):
R
e
=NV
o
(10)
which implies that N controls the input resistance R
e
:
N
=
R
e
V
o
(
11
)
The value of the input resistance and hence the input current can thus be controlled by varying N. In practical applications, V
o
needs to be maintained constant even if the load (R
L
) varies. In the control scheme depicted in
FIG. 3
, the output voltage is maintained constant by closing a feedback loop on N, which can be expressed as:
N=R
s
V
e
K
PWM
(12)
wherein K
PWM
is a constant.
If V
od
(
FIG. 3
) deviates (in accordance with deviations in V
o
), from a predetermined value, as set by reference voltage V
ref
, error signal V
e
will adjust N so as to match R
e
to the power requirement of (new/changed) load R
L
.
A major problem with the voltage feedback scheme discussed above (which is common to all prior art APFC systems, is the presence of ripple in V
e
, which results from the pulsating current that is fed into the bulk capacitor C
o
through the APFC system. The ripple component on V
e
regulates N and hence T
OFF
, thereby distorting i
ina
. This problem could be overcome by extra filtering, e.g., by adding a large capacitor in parallel to R
2
. However, in this kind of solution output voltage V
o
will respond with severe voltage overshoots and undershoots to load changes since the system will be too slow in correcting N. Such a behavior could be dangerous as the voltage may exceed the acceptable and safe operating voltage range of the system.
Therefore, it is desirable to have APFC controllers that have the capacity to respond quickly to changes in their load without adding undesirable harmonics to the current. It is also desirable that the output signal of the voltage error amplifier in APFC controllers will be filtered, and will chang
Cooper & Dunham LLP
Green Power Technologies Ltd.
Vu Bao Q.
LandOfFree
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