U.S. patent application number 09/837940 was filed with the patent office on 2001-11-08 for ac-dc converter.
Invention is credited to Hooijer, Christofher Daniel Charles, Marien, Petrus Cornelius Maria.
Application Number | 20010038545 09/837940 |
Document ID | / |
Family ID | 8171379 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010038545 |
Kind Code |
A1 |
Hooijer, Christofher Daniel Charles
; et al. |
November 8, 2001 |
AC-DC converter
Abstract
In an upconverter operating in the transient mode, an offset
signal is added to the signal at the current sensing pin of the
control IC. The upconverter generates a comparatively low THD even
if the supply voltage and/or the power supplied by the upconverter
are varied over a wide range.
Inventors: |
Hooijer, Christofher Daniel
Charles; (Eindhoven, NL) ; Marien, Petrus Cornelius
Maria; (Eindhoven, NL) |
Correspondence
Address: |
Jack E. Haken
Philips Electronics North America Corp.
580 White Plains Road
Tarrytown
NY
10591
US
|
Family ID: |
8171379 |
Appl. No.: |
09/837940 |
Filed: |
April 19, 2001 |
Current U.S.
Class: |
363/97 |
Current CPC
Class: |
H02M 1/4225 20130101;
Y02B 70/10 20130101 |
Class at
Publication: |
363/97 |
International
Class: |
H02M 003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 21, 2000 |
EP |
00201451.2 |
Claims
1. An AC-DC converter comprising input terminals which are to be
connected to the poles of a supply voltage source supplying an
alternating voltage and output terminals, rectifier means coupled
to the input terminals for rectifying the alternating voltage, an
inductive element coupled to the rectifier means, a buffer
capacitance coupled to the output terminals, a unidirectional
element coupled between the inductive element and the buffer
capacitance, a switching element coupled to the inductive element
for controlling a current through the inductive element, a control
circuit coupled to a control electrode of the switching element for
generating a periodic control signal for rendering the switching
element alternately conducting and non-conducting at a frequency f,
and provided with a first circuit part for generating a first
signal which is a measure of the instantaneous amplitude of the
current in the inductive element, and a second circuit part for
generating a second signal which is directly proportional to the
instantaneous value of the amplitude of the alternating voltage, a
comparator a first input of which is coupled to an output of the
first circuit part, a second input of which is coupled to an output
of the second circuit part, and an output of which is coupled to
the control electrode of the switching element, characterized in
that the control circuit additionally comprises a third circuit
part for generating an offset signal, and an adder circuit part for
combining the first signal and the offset signal, an output of
which is coupled to the first input of the comparator.
2. An AC-DC converter as claimed in claim 1, wherein the offset
signal has a constant amplitude.
3. An AC-DC converter as claimed in claim 2, wherein the third
circuit part comprises an ohmic resistance.
4. An AC-DC converter as claimed in claim 1, wherein the offset
signal is a periodic signal whose frequency is equal to the
frequency of the rectified alternating voltage.
5. An AC-DC converter as claimed in claim 4, wherein the amplitude
of the offset signal is at a local minimum when the amplitude of
the rectified alternating voltage is maximal.
6. An AC-DC converter as claimed in claim 1, wherein the inductive
element comprises an auxiliary winding, and the third circuit part
is coupled to said auxiliary winding and provided with a series
arrangement of a diode and two impedances, and a capacitive element
coupled to a junction point of the two impedances.
7. An AC-DC converter as claimed in claim 6, wherein the impedances
preferably comprise ohmic resistances.
8. An AC-DC converter as claimed in claim 1, wherein the third
circuit part is coupled to an output of the control circuit, and
the third circuit part is provided with a series arrangement of two
impedances and with a capacitive element coupled to a junction
point of both impedances.
9. An AC-DC converter as claimed in claim 8, wherein the impedances
comprise ohmic resistances.
Description
[0001] The invention relates to an AC-DC converter comprising
[0002] input terminals which are to be connected to the poles of a
supply voltage source supplying an alternating voltage and output
terminals,
[0003] rectifier means coupled to the input terminals for
rectifying the alternating voltage,
[0004] an inductive element coupled to the rectifier means,
[0005] a buffer capacitance coupled to the output terminals,
[0006] a unidirectional element coupled between the inductive
element and the buffer capacitance,
[0007] a switching element coupled to the inductive element for
controlling a current through the inductive element,
[0008] a control circuit coupled to a control electrode of the
switching element for generating a periodic control signal for
rendering the switching element alternately conducting and
non-conducting at a frequency f, and provided with
[0009] a first circuit part for generating a first signal which is
a measure of the instantaneous amplitude of the current in the
inductive element, and
[0010] a second circuit part for generating a second signal which
is directly proportional to the instantaneous value of the
amplitude of the alternating voltage,
[0011] a comparator a first input of which is coupled to an output
of the first circuit part, a second input of which is coupled to an
output of the second circuit part, and an output of which is
coupled to the control electrode of the switching element.
[0012] Such an AC-DC converter is disclosed in U.S. 4,683,529. The
control circuit of the known AC-DC converter renders the switching
element conducting during a first time interval t-on, which is
substantially constant during each half period of the alternating
voltage supplied by the supply voltage source. During the first
time interval t-on, the current in the inductive element increases
substantially linearly. The value of t-on corresponds to the power
taken at the output terminals. As the value of t-on is
substantially constant during each half period of the alternating
voltage, the value of the current taken from the supply voltage
source, averaged over a period of the control signal, is
substantially proportional to the instantaneous amplitude of the
alternating voltage. It is thus achieved that the power factor of
the known AC-DC converter is comparatively high. During the second
time interval t-off, the current in the inductive element decreases
substantially linearly. In the known AC-DC converter, the control
circuit renders the switching element conducting again almost
immediately after the current in the inductive element has become
substantially equal to zero. This control of the switching element
is referred to as "transition mode". As the current in the
inductive element is substantially zero, the same applies to the
current through the unidirectional element. It is thus achieved
that, when the switching element becomes conducting, only a
comparatively small power dissipation occurs in the unidirectional
element. The frequency of the control signal is often chosen to be
comparatively high because this enables both the inductive element
and EMI filters, which are often arranged between the input
terminals and the rectifier means, to be chosen so as to be
comparatively small. As a result, the AC-DC converter is
comparatively small and inexpensive. However, if the power taken at
the output terminals decreases, or if the amplitude of the
alternating voltage supplied by the voltage supply source
increases, the value of t-on is reduced by the control circuit.
Also at such a low value of the power taken or at a comparatively
high value of the amplitude of the alternating voltage, the known
AC-DC converter operates in the transition mode, as a result of
which the frequency of the control signal increases. A drawback of
the known AC-DC converter resides in that, at a high frequency, the
majority of the known control circuits are insufficiently capable
of sufficiently accurately controlling the time interval t-on, so
that instabilities in the operation of the AC-DC converter may
occur. The quantity of power dissipated in the switching element
also is comparatively high at a comparatively high frequency of the
control signal.
[0013] It is an object of the invention to provide an AC-DC
converter which can operate in a stable manner over a large range
of the power taken and over a large range of the amplitude of the
alternating voltage supplied by the supply voltage source, and
which has a high power factor, a low THD and a low power
dissipation in the components.
[0014] To achieve this, an AC-DC converter of the type mentioned in
the opening paragraph is characterized in accordance with the
invention in that the control circuit additionally comprises a
third circuit part for generating an offset signal, and an adder
circuit part for combining the first signal and the offset signal,
an output of which is coupled to the first input of the
comparator.
[0015] It has been found that an AC-DC converter in accordance with
the invention can be used in a comparatively large range of the
amplitude of the alternating voltage and in a comparatively large
range of the power taken at the output terminals. Within these two
ranges, the power factor of the AC-DC converter is comparatively
high and the THD is comparatively low.
[0016] Good results have been obtained using an AC-DC converter in
accordance with the invention wherein the offset signal has a
constant amplitude. If the offset signal has a constant amplitude,
then the AC-DC converter does not take power from the
voltage-supply source in the vicinity of the zero-crossings of the
alternating voltage. A constant amplitude for the offset signal can
be achieved in a comparatively simple manner in that the third
circuit part comprises an ohmic resistance.
[0017] Good results have also been achieved with embodiments of an
AC-DC converter in accordance with the invention, wherein the
offset signal is a periodic signal whose frequency is equal to the
frequency of the rectified alternating voltage. More particularly,
good results are achieved, particularly when the power taken at the
output terminals is comparatively small, if the amplitude of the
offset signal is at a local minimum when the amplitude of the
rectified alternating voltage is maximal. Such a form of the offset
signal can be achieved in a comparatively simple and very reliable
manner in that the inductive element comprises an auxiliary
winding, and the third circuit part is coupled to said auxiliary
winding and provided with
[0018] a series arrangement of a diode and two impedances, and
[0019] a capacitive element coupled to a junction point of the two
impedances.
[0020] The impedances preferably comprise ohmic resistances.
[0021] In a further embodiment of an AC-DC converter in accordance
with the invention, the third circuit part is coupled to an output
of the control circuit, and the third circuit part is provided with
a series arrangement of two impedances and with a capacitive
element coupled to a junction point of both impedances. Good
results have also been achieved by using this embodiment. The
impedances preferably comprise ohmic resistances.
[0022] These and other aspects of the invention will be apparent
from and elucidated with reference to the embodiment(s) described
hereinafter.
[0023] In the drawings:
[0024] FIG. 1 diagrammatically shows an example of an AC-DC
converter in accordance with the invention to which a load is
connected;
[0025] FIG. 2 shows a number of embodiments of a part of the
example shown in FIG. 1, and
[0026] FIG. 3 shows the form of the average current in the
inductive element, and the form of the offset signal for the
various embodiments shown in FIG. 2.
[0027] In FIG. 1, K1 and K2 denote input terminals which are to be
connected to a supply voltage source supplying an alternating
voltage. The input terminals K1 and K2 are coupled to respective
inputs of rectifier means DB which, in this example, are formed by
a diode bridge DB. Outputs of the diode bridge are connected to
each other by means of a series arrangement of ohmic resistances
RM1 and RM2. The series arrangement of ohmic resistances RM1 and
RM2 is shunted by a series arrangement of coil L, switching element
Q and ohmic resistance RCS. In this example, the coil L forms an
inductive element and is provided with an auxiliary winding L'. The
series arrangement of switching element Q and ohmic resistance RCS
is shunted by a series arrangement of diode D and capacitor C.
Diode D forms a unidirectional element and capacitor C forms a
buffer capacitance. A junction point of ohmic resistance RCS and
capacitor C is formed by a first output terminal K3. A junction
point of diode D and capacitor C is formed by a second output
terminal K4. Capacitor C is shunted by a series arrangement of
ohmic resistances RI1 and RI2. A load ZLD is connected to the
output terminals K3 and K4. A control circuit for generating a
control signal used to render the switching element alternately
conducting and nonconducting is formed by ohmic resistances RM1,
RM2, RZC, ZCOMP, RF, RCS, RI1 and RI2, auxiliary winding L',
capacitor CF and circuit parts III and IV. A first end portion of
the auxiliary winding L' is connected to the first output terminal
K3. A second end portion of auxiliary winding L' is connected, via
ohmic resistance RZC, to a first input of circuit part IV. In this
example, circuit part IV is formed by an IC, such as IC L6561 by ST
Microelectronics. A second input of circuit part IV is connected to
an output of circuit part VDC. Circuit part VDC is a direct current
source feeding the circuit part IV. A third input of circuit part
IV is connected to a junction point of ohmic resistances RM1 and
RM2. Ohmic resistances RM1 and RM2 form part of a second circuit
part for generating a second signal which is directly proportional
to the instantaneous value of the amplitude of the alternating
voltage. A fourth input of the circuit part IV is connected, via an
ohmic resistance ZCOMP, to a junction point of ohmic resistances
RI1 and RI2. A fifth input of circuit part IV is directly connected
to the junction point of ohmic resistances RI1 and RI2. A sixth
input of the circuit part IV is connected to output terminal K3.
Ohmic resistance RCS is shunted by a series arrangement of
capacitor CF and ohmic resistance RF. Ohmic resistance RCS, ohmic
resistance RF and capacitor CF jointly form a first circuit part
for generating a first signal that is a measure of the
instantaneous amplitude of the current in the coil L. A seventh
input of circuit part IV is connected to an output of the first
circuit part formed by a junction point of ohmic resistance RF and
capacitor CF. A control electrode of the switching element Q is
connected to an output of circuit part IV. Circuit part III forms a
third circuit part for generating an offset signal. An output of
circuit part III is connected to the output of the first circuit
part. This connection forms an adder circuit part for combining the
first signal and the offset signal. Circuit part III also comprises
an input. Circuit part III may be embodied in different ways. Three
embodiments of circuit part III are shown in FIG. 2. Dependent upon
the embodiment of circuit part III, the input of circuit part III
is connected to another terminal in the AC-DC converter. Possible
connections are indicated in FIG. 1 by means of dotted lines. If
circuit part III is formed by an ohmic resistance R1, as shown in
FIG. 2a, then the input of circuit part III is connected to the
output of circuit part VDC. If the circuit part III is formed by
diode D1, ohmic resistances R1 and R2 and capacitor C1, as
indicated in FIG. 2b, then the input of circuit part III is
connected to a junction point of auxiliary winding L' and ohmic
resistance RZC. If the circuit part III is formed by ohmic
resistances R1 and R2 and capacitor C1, as indicated in FIG. 2c,
then the input of circuit part III is connected to the output of
circuit part IV.
[0028] The operation of the example shown in FIG. 1 is as follows.
If the input terminals K1 and K2 are connected to a supply voltage
source supplying an AC voltage, this AC voltage is rectified by the
diode bridge DB and the rectified AC voltage is present between the
outputs of the diode bridge DB. At the junction point of ohmic
resistances RM1 and RM2, and hence at the third input of circuit
part IV, there is a signal which is directly proportional to the
instantaneous value of the amplitude of the AC voltage. At the
junction point of ohmic resistances RI1 and RI2, there is a signal
which is directly proportional to the instantaneous value of the
amplitude of the voltage between terminals K3 and K4, i.e. the
amplitude of the output voltage. The signal present at the fourth
input of circuit part IV is also directly proportional to the
amplitude of the output voltage. From the signal present at the
fourth input of circuit part IV, a new signal is derived by a first
part of the circuit part IV, which new signal is inversely
proportional to the amplitude of the output voltage. A multiplier
circuit, which also forms part of circuit part IV, multiplies this
new signal by the signal present at the third input of circuit part
IV. The result of this multiplication forms the second signal. In
this example, the second signal, thus, is not only dependent on the
amplitude of the alternating voltage but also on the amplitude of
the output voltage. This second signal is present at the second
input of a comparator, which also forms part of circuit part IV. A
first input of this comparator is connected to the seventh input of
the circuit part IV. At this seventh input, there is a signal,
which is the sum of the first signal generated by the first circuit
part and the offset signal generated by circuit part III. An output
of the comparator is coupled to the output of circuit part IV. If
the circuit part IV has detected, via the auxiliary winding and the
first input, that the current in coil L has become substantially
zero, then the switching element Q is rendered conducting, provided
the second signal is larger than the signal present at the first
input of the comparator. This signal at the first input is
substantially equal to the offset signal since the coil current is
substantially zero. If the second signal is smaller than the offset
signal, the switching element is not rendered conducting. When the
switching element Q is conducting, a current flows through the coil
L and through the switching element Q. The amplitude of this
current increases linearly until the signal at the first input of
the comparator is approximately equal to the signal at the second
input. At that instant, the switching element Q is rendered
non-conducting via the output of the comparator. When the switching
element Q is non-conducting, the current through the coil L
decreases substantially linearly, and this current charges the
capacitor C. It has been found that, by virtue of the presence of
the offset signal, the AC-DC converter can be used in a
comparatively large range of the amplitude of the alternating
voltage and in a comparatively large range of the power taken at
the output terminals. Within these two ranges, the power factor of
the AC-DC converter is comparatively high and the THD is
comparatively low.
[0029] FIG. 3 shows, for the various embodiments of circuit part
III indicated in FIG. 2 and for various values of the power (P1, P2
and P3) taken at the output terminals, the form of the offset
voltage Va and the time-average value of the coil current IL as a
function of time over a time interval equal to a period T of the
alternating voltage.
[0030] FIG. 3a corresponds to the embodiment of circuit part III
shown in FIG. 2a. FIG. 3a shows that the offset signal has a
constant amplitude and that the time-average value of the coil
current is zero in the vicinity of the zero crossings of the
alternating voltage, so that no power is taken from the electric
mains.
[0031] FIG. 3b corresponds to the embodiment of circuit part III
shown in FIG. 2b. FIG. 3b shows that the offset signal has a
time-dependent amplitude. FIG. 3b also shows that the amplitude of
the offset signal also depends on the power taken at the output
terminals. The amplitude of the offset signal, at a maximum value
of the amplitude of the alternating voltage (t=0.25 T and t=0.75
T), decreases as the power decreases, while the amplitude of the
offset signal in the vicinity of the zero crossings of the
alternating voltage (t=0, t=0.5 T and t=T) increases as the power
decreases. The time-average value of the coil current is shown for
the various values of the power taken.
[0032] FIG. 3c corresponds to the embodiment of circuit part III
shown in FIG. 2c. FIG. 3c shows that, also for this embodiment, the
offset signal is at a local minimum when the amplitude of the
alternating voltage is maximal (t=0.25 T and t=0.75 T). This
minimum becomes smaller as the power taken at the output becomes
smaller. The offset signal reaches a maximum value in the vicinity
of the zero crossings of the alternating voltage (t=0, t=0.5 T and
t=T). The time-average value of the coil current is shown for the
various values of the power taken.
[0033] A practical embodiment of an AC-DC converter in accordance
with the invention, wherein circuit part III was embodied as shown
in FIG. 2b, was used in a ballast for feeding a low pressure
mercury vapor discharge lamp of the type TL5 (Philips) having a
rated power of 35 W. The effective value of the alternating voltage
was 230 V. If the power consumed by the lamp was approximately 20
W, then the frequency of the control signal varied (in dependence
upon the instantaneous value of the amplitude of the alternating
voltage) between 330 kHz and 380 kHz. In the case of a reduction of
the power consumed by the lamp to approximately 15 W, this
frequency range of the control signal varied between 410 kHz and
550 kHz. A further reduction of the power consumed by the lamp to
approximately 10 W resulted in a frequency range between 270 kHz
and 500 kHz, so that the average frequency of the control signal
was lower than in the case of a 15 W power consumption by the lamp.
It has also been found that throughout this range of power consumed
by the lamp, the THD was comparatively low and the AC-DC converter
met the EN55015 requirements for EMI and the EN61000-3-2
requirements as regards performance. In a separate experiment it
was found that, if the offset voltage was not added to the first
signal, stable operation of the AC-DC converter at a lamp power
consumption of 10 W was impossible.
* * * * *