Zero-cross detection circuit

Details Zero-cross detection circuit Zero-cross detection circuit

2002-12-20

2003-12-16

Le, Dinh Thanh (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

C363S089000, C327S451000

Reexamination Certificate

active

06664817

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zero-cross detection circuit for detecting a point at which an input alternating-current (AC) voltage crosses a predetermined voltage (0 V). More particularly, the present invention relates to a zero-cross detection circuit which is connected to a full-wave rectifying and smoothing circuit powered from a commercial AC power supply for full-wave rectification and smoothing, and which is also connected to a switching regulator for separating and stepping down the output from the full-wave rectifying and smoothing circuit to output a desired DC voltage.
2. Description of the Related Art
FIG. 10
is a circuit diagram of a power supply circuit using a commercial AC power input, in particular, showing a zero-cross detection circuit in the related art for detecting a point at which an input AC voltage crosses zero volts, and a rectifying and smoothing circuit and a switching regulator which are connected to the zero-cross detection circuit.
In
FIG. 10
, lines Line
1
and Line
2
are connected to a commercial AC power supply through a filter circuit (not shown). The full-wave rectifying and smoothing circuit is formed of diodes D
11
, D
12
, D
13
, and D
14
, and a smoothing capacitor C
11
.
In
FIG. 10
, the switching regulator which is the self-excitation type is formed of components indicated by Q
21
, Q
22
, C
21
, C
22
, C
31
, C
32
, D
31
, IC
31
, R
21
to R
27
, R
31
to R
35
, and PC
21
. The switching regulator is insulated by a transformer T
21
, and generates a constant voltage of +24 V.
The zero-cross detection circuit is formed of components indicated by Q
41
, C
41
, C
42
, D
41
, R
41
, R
43
, R
44
, R
45
, and PC
41
. In the zero-cross detection circuit, a low-voltage output terminal of the full-wave rectifying and smoothing circuit is connected to the emitter of the n-p-n transistor Q
41
, and the resistor R
43
is connected between the base and emitter of the transistor Q
41
. The resistor R
43
and the capacitor C
41
are connected in parallel with each other, and the resistor R
41
is connected between the capacitor C
41
and the line Line
1
.
A half-wave rectifying circuit is formed of the resistors R
41
and R
43
in the zero-cross detection circuit, and the diode D
13
, and the output of the half-wave rectifying circuit is applied between the base and emitter of the transistor Q
41
. If the potential of the line Line
1
is higher than the potential of the line Line
2
, a current flows in the resistor R
41
; otherwise, no current flows in the resistor R
41
. The resistances of the resistors R
41
and R
43
are set to suitable values so that the collector potential in the transistor Q
41
can substantially change according to the potential magnitude of the lines Line
1
and Line
2
. The high/low edges of the collector potential in the transistor Q
41
correspond to zero crossings, and a zero-cross signal ZEROX is transmitted to the secondary of the transformer T
21
via the photocoupler PC
41
. The capacitor C
41
is a capacitor for removing noise, and is not essential to the zero-cross detection circuit.
FIGS. 11A
to
11
C and
12
A to
12
C are signal waveforms of the components in the zero-cross detection circuit.
In
FIGS. 11A
to
11
C and
12
A to
12
C, the x-axis represents time.
FIG. 11A
shows the potential of the line Line
1
with respect to a ground GND,
FIG. 11B
shows the potential of the line Line
2
with respect to the ground GND, and
FIG. 11C
shows the difference in potential between the lines Line
1
and Line
2
.
FIG. 12A
shows a current flowing in the resistor R
41
,
FIG. 12B
shows an enlarged version of the y-axis in
FIG. 12A
, and
FIG. 12C
shows the phototransistor collector potential in the secondary of the photocoupler PC
41
, that is, the zero-cross signal ZEROX. In the secondary of the transformer T
21
, the voltage is stepped down from the output (+24 V) of the switching regulator to +3.3 V.
FIG. 13
shows another zero-cross detection circuit in the related art. The zero-cross detection circuits shown in
FIGS. 10 and 13
are different from each other in that the zero-cross detection circuit shown in
FIG. 13
further includes capacitors C
12
and C
13
. Specifically, in
FIG. 13
, the capacitors C
12
and C
13
are connected to the high-voltage output terminal and the low-voltage output terminal of the full-wave rectifying and smoothing circuit, respectively, and the node between the capacitors C
12
and C
13
is grounded.
In general, for the terminal noise suppression purpose, a capacitor (a so-called Y-capacitor) of approximately several thousand picofarads is connected between a commercial AC power supply line and a ground GND. The capacitors C
12
and C
13
are Y-capacitors. Although a Y-capacitor may be connected to an input terminal of a full-wave rectifying circuit, it is more effective for the terminal noise suppression purpose to connect a Y-capacitor to an output terminal of a full-wave rectifying circuit. The configuration shown in
FIG. 13
is often used.
In the circuit configuration shown in
FIG. 13
, if Y-capacitors (the capacitors C
12
and C
13
) have a small capacitance or if the commercial AC power supply exhibits a normal waveform, no problem occurs. However, if the Y-capacitors have a large capacitance or if the commercial AC power supply exhibits an undesirable waveform which is not normal, a problem occurs.
FIGS. 14A
to
14
C are signal waveforms of the components in the zero-cross detection circuit when the Y-capacitors in the circuit shown in
FIG. 13
have a relatively small capacitance. Since the waveforms indicating the potential of the line Line
1
with respect to the ground GND, the potential of the line Line
2
with respect to the ground GND, and the difference in potential between the lines Line
1
and Line
2
are the same as those shown in
FIGS. 11A
to
11
C, a description thereof is omitted.
FIG. 14A
shows a current flowing in the resistor R
41
,
FIG. 14B
shows an enlarged version of the y-axis in
FIG. 14A
, and
FIG. 14C
shows the phototransistor collector potential in the secondary of the photocoupler PC
41
, that is, the zero-cross signal ZEROX.
As is apparent from
FIGS. 14A
to
14
C, if the Y-capacitors have a relatively small capacitance, a zero-cross signal ZEROX can be successfully generated.
FIGS. 15A
to
15
C are signal waveforms of the components in the zero-cross detection circuit when the Y-capacitors in the circuit shown in
FIG. 13
have a relatively large capacitance. Since the waveforms indicating the potential of the line Line
1
with respect to the ground GND, the potential of the line Line
2
with respect to the ground GND, and the difference in potential between the lines Line
1
and Line
2
are the same as those shown in
FIGS. 11A
to
11
C, a description thereof is omitted.
FIG. 15A
shows a current flowing in the resistor R
41
,
FIG. 15B
shows an enlarged version of the y-axis in
FIG. 15A
, and
FIG. 15C
shows the phototransistor collector potential in the secondary of the photocoupler PC
41
, that is, the zero-cross signal ZEROX.
As is apparent from
FIGS. 15A
to
15
C, if the Y-capacitors have a large capacitance, the zero-cross signal ZEROX shown in
FIG. 15C
does not indicate a correct zero-cross point.
In
FIG. 15B
, the current flowing in the resistor R
41
rises in a sine-wave fashion at about 15 msec, thus causing a zero-cross point to be unsuccessfully detected.
This current flows in the Y-capacitors C
12
and C
13
towards the ground GND.
The potential of either the line Line
1
or Line
2
, whichever is lower, is used as the low-voltage output potential of the full-wave rectifying and smoothing circuit with respect to the ground GND. This exhibits a half-wave rectified waveform. The high-voltage output potential of the full-wave rectifying and smoothing circuit with respect to the ground GND is produced by adding the capacitance potential stored in the capacitor C
11
to the low-voltage output potential, and, if a DC component is removed

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