U.S. patent application number 09/878655 was filed with the patent office on 2002-01-31 for complex resonant dc-dc converter and high voltage generating circuit driven in a plurality of frequency regions.
Invention is credited to Nagahara, Kiyokazu, Takahama, Masanobu.
Application Number | 20020012257 09/878655 |
Document ID | / |
Family ID | 18677772 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020012257 |
Kind Code |
A1 |
Takahama, Masanobu ; et
al. |
January 31, 2002 |
Complex resonant DC-DC converter and high voltage generating
circuit driven in a plurality of frequency regions
Abstract
A DC-DC converter includes a switching unit having a pair of
switching devices or transistors, a resonant circuit comprising a
capacitor, an inductor, and a primary coil of a converter
transformer which serves as an inductor, a smoothing and rectifying
circuit connected to a load in the secondary, an error amplifier
for the output voltage, and a variable oscillator for varying the
switching frequency according to the error voltage. The output of
the variable oscillator circuit is supplied to the switching
devices to provide a stabilized output voltage from the smoothing
and rectifying circuit. The switching frequency during light
loading is set at a frequency more than the resonant frequency
which is mainly defined by the inductance and the interwinding
capacitance of the secondary coil. The switching frequency to
stabilize the output voltage during light loading is much higher
than that during heavy loading, significantly reducing the exciting
current component during light loading to enhance the energy
conversion efficiency during light loading.
Inventors: |
Takahama, Masanobu;
(Saitama, JP) ; Nagahara, Kiyokazu; (Tokyo,
JP) |
Correspondence
Address: |
Jay H. Maioli
Cooper & Dunham LLP
1185 Avenue of the Americas
New York
NY
10036
US
|
Family ID: |
18677772 |
Appl. No.: |
09/878655 |
Filed: |
June 11, 2001 |
Current U.S.
Class: |
363/95 |
Current CPC
Class: |
Y02B 70/10 20130101;
H02M 3/337 20130101; H02M 3/33571 20210501; H02M 3/01 20210501 |
Class at
Publication: |
363/95 |
International
Class: |
H02M 003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2000 |
JP |
P2000-175992 |
Claims
What is claimed is:
1. A DC-DC converter comprising: switching means having a pair of
switching devices connected in series to form a bridge; a converter
transformer having a primary coil and a secondary coil which are
wound with a predetermined turns ratio for transferring to the
secondary coil the switching output provided for the primary coil
by the switching operation of said switching means; series resonant
means having a capacitor, an inductor, and the primary coil serving
as an inductor which are connected in series to a node between the
pair of switching devices, said series resonant means being
resonated at a first resonant frequency; parallel resonant means at
least having an equivalent capacitance in the primary equivalent to
the interwinding capacitance of the secondary coil, and the
inductance of the primary coil, said parallel resonant means being
resonated at a second resonant frequency higher than the first
resonant frequency; voltage supply means connected to the secondary
coil serving as a source for supplying an output voltage to a load;
and switching control means for varying the switching frequency of
said switching means according to variations in the voltage output
from said voltage supply means.
2. A DC-DC converter comprising: switching means having a pair of
switching devices connected in series to form a bridge; a converter
transformer having a primary coil and a secondary coil which are
wound with a predetermined turns ratio for transferring to the
secondary coil the switching output provided for the primary coil
by the switching operation of said switching means; series resonant
means having a capacitor, an inductor, and the primary coil serving
as an inductor which are connected in series to a node between the
pair of switching devices, said series resonant means being
resonated at a first resonant frequency; parallel resonant means at
least having an equivalent capacitance in the primary equivalent to
the interwinding capacitance of the secondary coil, and the
inductance of the primary coil, said parallel resonant means being
resonated at a second resonant frequency higher than the first
resonant frequency; voltage supply means connected to the secondary
coil serving as a source for supplying an output voltage to a load;
error detecting means for detecting variations in the output
voltage from said voltage supply means; variable oscillating means
for varying the switching frequency of said switching means
according to the detected variations; and switching drive means for
alternately turning on and off the pair of switching devices
according to the switching frequency, whereby the output voltage is
stabilized.
3. A DC-DC converter according to claim 1 or 2, wherein the turns
ratio of the primary coil to the secondary coil is chosen so that
the switching frequency is in a range of the first resonant
frequency to the second resonant frequency during heavy loading,
and exceeds the second resonant frequency during light loading, in
order to stabilize the output voltage.
4. A DC-DC converter according to claim 3, further comprising a
capacitor connected in parallel to the secondary coil, wherein the
switching frequency is defined by changing the capacitance of said
capacitor and the turns ratio of the primary coil to the secondary
coil.
5. A high voltage generating circuit comprising: switching means
having a pair of switching devices connected in series to form a
bridge; a converter transformer having a primary coil and a
plurality of secondary coils which are wound with a predetermined
turns ratio for transferring to the plurality of secondary coils
the switching output provided for the primary coil by the switching
operation of said switching means; series resonant means having a
capacitor, an inductor, and the primary coil serving as an inductor
which are connected in series to a node between the pair of
switching devices, said series resonant means being resonated at a
first resonant frequency to provide current resonance for the
switching operation; parallel resonant means at least having an
equivalent capacitance in the primary equivalent to a combination
of the interwinding capacitances of the plurality of secondary
coils, and the inductance of the primary coil, said parallel
resonant means being resonated at a second resonant frequency
higher than the first resonant frequency; high voltage supply means
for coupling a voltage multiplier rectifier circuit to each of the
plurality of secondary coils so as to connect them in series so
that a high voltage is supplied to a load; and switching control
means for varying the switching frequency of said switching means
according to variations in the high voltage output from said high
voltage supply means, whereby the high voltage output is
stabilized.
6. A high voltage generating circuit comprising: switching means
having a pair of switching devices connected in series to form a
bridge; a converter transformer having a primary coil and a
plurality of secondary coils which are wound with a predetermined
turns ratio for transferring to the plurality of secondary coils
the switching output provided for the primary coil by the switching
operation of said switching means; series resonant means having a
capacitor, an inductor, and the primary coil serving as an inductor
which are connected in series to a node between the pair of
switching devices, said series resonant means being resonated at a
first resonant frequency to provide current resonance for the
switching operation; parallel resonant means at least having an
equivalent capacitance in the primary equivalent to a combination
of the interwinding capacitances of the plurality of secondary
coils, and the inductance of the primary coil, said parallel
resonant means being resonated at a second resonant frequency
higher than the first resonant frequency; high voltage supply means
for coupling a voltage multiplier rectifier circuit to each of the
plurality of secondary coils so as to connect them in series so
that a high voltage output is supplied to a load; error detecting
means for detecting variations in the high voltage output from said
high voltage supply means; variable oscillating means for varying
the switching frequency of said switching means according to the
detected variations; and switching drive means for driving the pair
of switching devices according to the switching frequency, whereby
the high voltage output is stabilized.
7. A high voltage generating circuit according to claim 5 or 6,
wherein the turns ratio of the primary coil to the plurality of
secondary coils is chosen so that the switching frequency is in a
range of the first resonant frequency to the second resonant
frequency during heavy loading, and exceeds the second resonant
frequency during light loading, in order to stabilize the high
voltage output.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to bridge DC-DC
converters incorporating high voltage generating circuits of
cathode ray tubes (CRTs). More particularly, the present invention
relates to a bridge DC-DC converter in which the switching
frequency to stabilize an output voltage during light loading is
set at a frequency greater than the resonant frequency, which is
defined by the inductance of a primary coil and the interwinding
capacitance of a secondary coil of a converter transformer, so that
the switching frequency during light loading may be significantly
higher than that during heavy loading, whereby an exciting current
component which flows to the primary coil is reduced to
significantly enhance the energy conversion efficiency during light
loading.
[0003] 2. Description of the Related Art
[0004] In recent years, attempts have been made to use, as high
voltage generating circuits for generating a high voltage applied
to the anode of a cathode ray tube (CRT), asynchronous high voltage
generating circuits which uses a frequency asynchronous with the
horizontal scanning frequency as a switching frequency.
[0005] This is because asynchronous high voltage generating
circuits which use a switching frequency much higher than the
horizontal scanning frequency have several benefits compared to
high voltage generating circuits which use a switching frequency
synchronous with the horizontal scanning frequency. That is, the
circuit components constituting the asynchronous high voltage
generating circuit may be compact, and the cost of the overall
circuit can be reduced. Furthermore, the higher the switching
frequency, the lower the exciting current required. Thus, the
energy conversion efficiency can be enhanced.
[0006] Such an asynchronous high voltage generating circuit is
often implemented by, for example, a half bridge DC-DC converter.
FIG. 6 illustrates the conceptual structure of a half bridge DC-DC
converter 10. A dc power source 12 is connected to a switching unit
14 having a pair of switching devices, and the switching unit 14 is
connected to a series circuit comprising a capacitor Cr, an
inductor Lr, and a primary coil 22a of a transformer 22, which form
a resonant circuit 20.
[0007] A secondary coil 22b of the transformer 22 is connected to a
load 26 via a smoothing and rectifying circuit 24. The load 26 may
be a CRT. When the load 26 is a CRT, the smoothing and rectifying
circuit 24 may be implemented by a voltage multiplier rectifier
circuit, where a high voltage of on the order of 20 to 30 kV is
applied to the anode terminal of the CRT.
[0008] The high output voltage is supplied to an error detector 28,
where it is compared to a reference voltage and the error voltage
is supplied as a switching signal to a variable oscillator 30 to
output an oscillation frequency corresponding to the error voltage.
The oscillation frequency is supplied to the switching unit 14 via
a driver 32. Therefore, the switching frequency which is made
variable according to the load would achieve a stabilized output
voltage.
[0009] In this structure, the resonance of the resonant circuit 20
is used to transfer electromagnetic energy to the secondary of the
transformer 22 to provide a predetermined high output voltage HV.
Herein, interwinding capacitance Cs of the secondary coil 22b which
is present at primary coil 22a would be parallel to the primary
coil 22a. The interwinding capacitance which is present in the
primary is indicated by Cp in FIG. 6.
[0010] The relationship between the resonance characteristic when
the interwinding capacitance Cs is present in the primary and the
switching frequency is shown in FIG. 7. As in FIG. 6, in view of
the interwinding capacitance Cs, the resonant circuit 20 would be a
complex resonant circuit in which a series resonant portion
comprising the capacitor Cr, the inductor Lr, and the inductance Lp
of the primary coil 22a is combined with a parallel resonant
portion comprising the inductor Lr, the inductance Lp, and the
capacitor Cp.
[0011] The resonance characteristic is such that a first peak
provided by the series resonant portion, that is, a resonance curve
having a series resonant point Ps, is combined with a second peak
provided by the parallel resonant portion, that is, a resonance
curve having a parallel resonant point Pp. The high output voltage
profile is higher when the load 26 is light-loading, while the high
output voltage profile is lower when it is heavy-loading, thus
proving a different resonance characteristic depending upon
loading, i.e., heavy lording or light loading. That is, the
resonance characteristic is represented by a curve La during light
loading, while the resonance characteristic is represented by a
curve Lb during heavy loading. The resonance curve varies between
La and Lb depending upon load values, thereby providing a
stabilized output voltage.
[0012] If a voltage for stabilization has been determined as
depicted in FIG. 7, then, switching frequencies f2 and f4
corresponding to the predetermined voltage are obtained during
light loading in a frequency region higher than the series resonant
frequency fs and lower than the parallel resonant frequency fp, and
a frequency region higher than the parallel resonant frequency,
respectively. Switching frequencies f1 and f3 are obtained during
heavy loading in the former and latter frequency regions,
respectively.
[0013] The switching frequency of the half bridge DC-DC converter
10 is generally set higher than the resonant frequency fs
corresponding to the series resonant point Ps. In this case,
therefore, it is chosen to be within either frequency region Wa
ranging from f1 to f2 or Wb ranging from f3 to f4. For example, the
switching frequency is chosen to be within the frequency region
Wa.
[0014] The electric current which flows to the resonant capacitor
Cr and the resonant inductor Lr of the resonant circuit 20 shown in
FIG. 6 is a combination of the current component which is
transferred to the secondary and the current component which flows
only to the primary, namely, the exciting current component. The
exciting current component is a current component which does not
contribute to electromagnetic energy transfer. The lower the
switching frequency, the higher the amplitude of the exciting
current component, and energy dissipation increases accordingly, as
known in the art.
[0015] In the conventional DC-DC converter 10, as shown in FIG. 7,
the switching frequency is operable in the frequency region Wa
which is higher than the series resonant frequency fs. Here, there
are only a few differences between the switching frequency f2
during light loading and the switching frequency f1 during heavy
loading.
[0016] Specifically, for example, if the required high output
voltage is 32 kV, this voltage corresponds to a predetermined
voltage for stabilization, where when the turns of the secondary
coil 22b are set at 500 T, the number of turns in the primary coil
22a is 30 T in the converter transformer 22. In this example, fs,
f1, fp, and f2 are 50 kHz, 60 kHz, 65 kHz, and 260 kHz,
respectively. Therefore, the switching frequency during light
loading to provide stabilization at the predetermined voltage is 65
kHz, which is not significantly different from the switching
frequency of 60 kHz during heavy loading.
[0017] Of course, if the desired switching frequency is set to be
in the frequency region Wb higher than the parallel resonant
frequency fp, the switching frequencies during light loading and
heavy loading do not differ significantly.
[0018] Since the switching frequency during light loading is not
high relative to during heavy loading, the DC-DC converter 10 is
driven with large exciting current component, and a problem occurs
in that the energy conversion efficiency of the DC-DC converter 10
is not improved. This problem occurs in half bridge DC-DC
converters as well as in full bridge DC-DC converters.
SUMMARY OF THE INVENTION
[0019] Accordingly, it is an object of the present invention to
provide a bridge DC-DC converter having significant improvement in
the energy conversion efficiency in which the switching frequency
to provide a stabilized voltage particularly during light loading
is much higher than that in a conventional one.
[0020] To this end, according to one aspect of the present
invention, a bridge DC-DC converter includes a switching unit
having a pair of switching devices connected in series to form a
bridge, a converter transformer having a primary coil and a
secondary coil which are wound with a predetermined turns ratio for
transferring to the secondary coil the switching output provided
for the primary coil by the switching operation of the switching
unit, a series resonant circuit having a capacitor, an inductor,
and the primary coil serving as an inductor which are connected in
series to a node between the pair of switching devices, the series
resonant circuit being resonated at a first resonant frequency, a
parallel resonant circuit at least having an equivalent capacitance
in the primary equivalent to the interwinding capacitance of the
secondary coil, and the inductance of the primary coil, the
parallel resonant circuit being resonated at a second resonant
frequency higher than the first resonant frequency, a voltage
supply connected to the secondary coil for supplying an output
voltage to a load, and a switching control unit for varying the
switching frequency of the switching unit according to variations
in the voltage output from the voltage supply, whereby a stabilized
output voltage is obtained from the high voltage supply.
[0021] According to another aspect of the present invention, a high
voltage generating circuit includes a switching unit having a pair
of switching devices connected in series to form a bridge, a
converter transformer having a primary coil and a plurality of
secondary coils which are wound with a predetermined turns ratio
for transferring to the plurality of secondary coils the switching
output provided for the primary coil by the switching operation of
the switching unit, a series resonant circuit having a capacitor,
an inductor, and the primary coil serving as an inductor which are
connected in series to a node between the pair of switching
devices, the series resonant circuit being resonated at a first
resonant frequency to provide current resonance for the switching
operation, a parallel resonant circuit at least having an
equivalent capacitance in the primary equivalent to a combination
of the interwinding capacitances of the plurality of secondary
coils, and the inductance of the primary coil, the parallel
resonant circuit being resonated at a second resonant frequency
higher than the first resonant frequency, a high voltage supply for
coupling a voltage multiplier rectifier circuit to each of the
plurality of secondary coils so as to connect them in series so
that a high voltage is supplied to a load, and a switching control
unit for varying the switching frequency of the switching unit
according to variations in the high voltage output from the high
voltage supply, whereby the high voltage output is stabilized.
[0022] Preferably, the turns ratio of the primary coil to the
secondary coil is chosen so that the switching frequency ranges
from the series resonant frequency to the parallel resonant
frequency during heavy loading, and exceeds the parallel resonant
frequency during light loading.
[0023] In addition, an additional capacitor may be connected in
parallel to the secondary coil, of which the capacitance is chosen,
so that the switching frequency may be set.
[0024] By determining the resonance characteristic during light
loading in this manner, the switching frequency during light
loading may be set in a frequency region higher than the parallel
resonant frequency.
[0025] As a result, the switching frequency may be significantly
high during light loading to reduce the amplitude of the exciting
current during light loading, and the energy conversion efficiency
is thus improved.
[0026] The bridge DC-DC converter according to the present
invention is extremely suitably implemented as a high voltage
generating circuit for use in CRTs in which a load can constantly
vary in a range of heavy loading to light loading depending upon
content of pictures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a circuit diagram of a bridge DC-DC converter
according to an embodiment of the present invention;
[0028] FIG. 2 is a circuit diagram of a bridge DC-DC converter
according to the embodiment of the present invention in which a
voltage multiplier rectifier circuit which is connected to a CRT is
provided in the secondary of a converter transformer;
[0029] FIG. 3 is a waveform diagram according to the embodiment of
the present invention;
[0030] FIGS. 4A and 4B are graphs showing the relationship between
the resonant frequencies and output voltages according to the
embodiment of the present invention;
[0031] FIG. 5 is a schematic diagram of a bridge DC-DC converter
according to another embodiment of the present invention;
[0032] FIG. 6 is a circuit diagram of a conventional bridge DC-DC
converter; and
[0033] FIG. 7 is a graph showing the resonance characteristic of
the conventional bridge DC-DC converter.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0034] A bridge DC-DC converter according to the present invention
is described in conjunction with its illustrative embodiments.
[0035] FIG. 1 shows a half bridge DC-DC converter 10 according to
the present invention, as in the conventional example shown in FIG.
6.
[0036] A switching unit 14 is disposed in the primary of a
converter transformer 22 in the half bridge DC-DC converter 10.
[0037] The switching unit 14 includes a pair of switching devices,
or MOS transistors Q1 and Q2 in this embodiment, which are
connected in series, and switching signals Sa and Sb which are
alternately inverted are supplied to the respective gates thereof
from a driver 32, as will be described. The gate-source voltages
are illustrated in (A) and (B) of FIG. 3, respectively.
[0038] The driving voltage from a dc power source 12 is applied to
the pair of transistors Q1 and Q2. Diodes Dl and D2 for commutating
a resonant current are connected between the drains and the sources
of the pair of transistors Q1 and Q2, respectively. A primary coil
22a of a converter transformer 22 is connected to a node q between
the pair of transistors Q1 and Q2 via resonant circuit
components.
[0039] Specifically, a series resonant circuit comprising a
resonant capacitor Cr, a resonant inductor Lr, and the inductance
Lp of the primary coil 22a, which form a resonant circuit 20, is
formed between the node q and the primary coil 22a. A capacitor Cx
connected across the switching transistor Q2 is a
charging/discharging capacitor, and is used for partial resonance
when the switching transistors Q1 and Q2 are turned on or off.
[0040] A pair of secondary coils 22b and 22c which are connected in
series is wound in the secondary of the converter transformer 22,
and is connected to a pair of full-wave rectifier diodes D3 and D4,
respectively, followed by a smoothing capacitor Co, to form a
smoothing and rectifying circuit 24. The output voltage of the
smoothing and rectifying circuit 24 is applied to a load 26 in this
embodiment.
[0041] If the load 26 is the anode electrode of a CRT, as shown in
FIG. 2, multiple sets of secondary coils 22b to 22e and voltage
multiplier rectifier circuits which are connected in series are
provided so that the anode voltage may maintain a high voltage of
20 kV to 32 kV voltage as is known in the art. The voltages
provided for the secondary coils 22b to 22e are rectified by the
voltage multiplier rectifier circuit, and the resulting voltages
accumulate to boost the voltage so as to output a high output
voltage HV. In this case, the secondary coils 22b to 22e
parasitically contain interwinding capacitances Cs1 to Cs4,
respectively.
[0042] A low output voltage would be directly supplied to an error
amplifier 28 formed by an operational amplifier for comparison with
a reference voltage 29, as shown in FIG. 1. A high output voltage
would be supplied to the error amplifier 28 via a pair of
voltage-dividing resistors R1 and R2, where it is then compared
with the reference voltage 29, as shown in FIG. 2. The error
voltage which is a comparative voltage is supplied as a frequency
control voltage for a variable oscillator 30 for controlling the
frequency of an oscillation signal according to the input error
voltage. In this embodiment, the oscillation frequency may vary in
a range of 100 kHz to 260 kHz. The oscillation signal is supplied
to the driver 32, and is distributed as a pair of switching signals
Sa and Sb, which are alternately inverted, for suitably switching a
pair of switching transistors Q1 and Q2. The pair of switching
signals Sa and Sb are applied to the gates of the switching
transistors Q1 and Q2, respectively.
[0043] FIG. 3 is waveform diagram which illustrates the operation
of the components shown in FIG. 1, in which the pair of switching
signals Sa and Sb having a duty of approximately 50% allows the
pair of transistors Q1 and Q2 to be turned on/off repeatedly. In
response to the switching signals Sa and Sb, the drain-source
voltage of the first switching transistor Q1 exhibits a waveform
shown in (C), and, accordingly, currents Id shown in (D) and (E)
are alternately applied to the switching transistors Q1 and Q2,
respectively. A resonant current Ii indicated by the solid line of
(F) eventually flows to the resonant inductor Lr.
[0044] Using the resonant current Ii, electromagnetic energy is
transferred to the secondary of the converter transformer 22 so
that a current Ia depicted in (G) flows to the diode D3 when the
switching transistor Q1 is turned on, and a current Ib (=Ia)
depicted in (H) flows to the diode D4 when the switching transistor
Q2 is turned on. The currents Ia and Ib are smoothed and rectified
to output a predetermined commutated voltage HV from the smoothing
and rectifying circuit 24.
[0045] The resonant current Ii depicted in (F) is a combination of
the exciting current component depicted by the broken line and the
resonant current component depicted by the solid line, with shaded
portions defined therebetween representing a component transmitted
to the secondary. As described later, the less the exciting current
component, the higher the transmission efficiency to the secondary,
and the energy conversion efficiency is thus improved.
[0046] During heavy loading, as the anode current increases, the
high output voltage HV is reduced, and a feedback control is
carried out so as to lower the oscillation frequency. This allows a
lower switching frequency to be applied to the pair of switching
transistors Q1 and Q2, and more electromagnetic energy is thus
transferred to the secondary of the converter transformer 22. The
high energy transfer increases the high output voltage HV, so that
the high output voltage HV can be stabilized during heavy
loading.
[0047] At a light-load mode where less load is imposed, inversely,
the anode current decreases and the high output voltage HV
increases. An increase in the high output voltage HV makes the
error voltage higher, thus providing a control such that the
oscillation frequency may increase. Therefore, the switching
frequency increases. An increase in the switching frequency leads
to a reduction of the electromagnetic energy transferred to the
secondary of the converter transformer 22, to lower the high output
voltage HV. Therefore, stabilization in the high output voltage HV
is achieved.
[0048] In the half bridge DC-DC converter 10 which performs a
fundamental operation for voltage stabilization, when the converter
transformer 22 is used as a high-voltage conversion transformer,
each of the secondary coils 22b and 22c needs to be wound more than
the primary coil 22a. Therefore, the secondary coils 22b and 22c
parasitically contain the interwinding capacitances Cs1 and Cs2,
respectively.
[0049] As known in the art, in the primary, the interwinding
capacitances or parasitic capacitances Cs1 and Cs2 would be
equivalent to the interwinding capacitance which is connected in
parallel to the primary coil 22a. The equivalent capacitance which
is indicated by Cp shown by a broken line in FIG. 1 would be
connected in parallel to the inductor LP. Accordingly, the
resonance characteristic of the resonant circuit 20 is a
combination of a series resonance characteristic of a series
resonant portion comprising the capacitor Cr, and the inductors Lr
and LP and a parallel resonance characteristic of a parallel
resonant portion comprising the inductors Lr and Lp, and the
equivalent capacitance Cp.
[0050] As depicted in FIG. 4A, the resonance characteristic between
the switching frequency and the output voltage of the smoothing and
rectifying circuit 24 provides a twin-peak relationship in which a
series resonance curve having a first resonant point Ps is combined
with a parallel resonance curve having a second resonant point Pp.
In FIG. 4A, a curve Lc represents the resonance characteristic
during light loading, and a curve Ld represents the resonance
characteristic during heavy loading.
[0051] If an output voltage HV for stabilization is set at a
predetermined voltage shown in FIG. 4A, switching frequencies f1
and f2 corresponding to intersections between the predetermined
voltage and the resonance curves Lc and Ld, respectively, become
operational frequencies when the output voltage HV is stabilized.
As the load varies, the resonance curve varies in a range of Lc to
Ld according to the load variations. Therefore, desirably, the
switching frequency to stabilize the output voltage varies in a
range of f1 to f2.
[0052] In the present invention, the switching frequency to provide
a stabilized output voltage during light loading is set at the
frequency f2 more than the parallel resonant frequency fp.
Specifically, the interwinding capacitances Cs1 and Cs2 of the
secondary coils 22b and 22c, namely, the equivalent capacitance Cp,
are chosen so that the switching frequency when the output voltage
is stabilized is set at the frequency f2 more than the parallel
resonant frequency fp which is defined by the interwinding
capacitances Cs1 and Cs2, etc. The parallel resonant point Pp or
the parallel resonant frequency varies depending upon the
equivalent capacitance Cp, and as the value thereof increases, the
parallel resonance curve changes. Thus, a change in the series
resonance curve and the parallel resonance curve also influences
the curve in the vicinity of the bottom.
[0053] As the equivalent capacitance Cp gradually increases from
the state shown in FIG. 4A, the resonance characteristic curve Lc
exhibits that the bottom of the resonance characteristic, or the
frequency fa, is shifted above and exceeds the predetermined
voltage HV, as shown in FIG. 4B. Due to this shift, there is only
one frequency corresponding to the intersection between the
resonance characteristic curve Lc and the predetermined voltage HV,
so that the frequency f2 more than the parallel resonant frequency
fp can be thus set as the switching frequency to stabilize the
output voltage, as previously described.
[0054] According to a change in the equivalent capacitance Cp, the
resonance characteristic during heavy loading also varies such as
from the curve Ld shown in FIG. 4A to the curve Ld shown in FIG.
4B. Hence, the switching frequency f1 to stabilize the output
voltage during heavy loading is also shifted to a higher frequency
region.
[0055] The equivalent capacitance Cp, provided that the
interwinding capacitances Cs1 and Cs2 of the secondary coils 22b
and 22c are present in the primary, depends upon the turns ratio of
the primary coil 22a to the secondary coils 22b and 22c.
[0056] For example, it is assumed that a high output voltage is set
at 32 kV which is the predetermined voltage HV, and the switching
frequency is variable in a range of 100 kHz to 260 kHz. If the
total turns of the secondary coils 22b and 22c are 500 T, and the
turns of the primary coil 22a are 30 T, then, the resonance
characteristic having the curves Lc and Ld as shown in FIG. 4A is
obtained.
[0057] Meanwhile, the resonance characteristic if the turns of the
primary coil 22a are half the original, namely, 15 T, is shown in
FIG. 4B. Reducing the turns of the primary coil 22a by half the
original essentially implies that the equivalent capacitance Cp is
substantially doubled. In this case, therefore, the resonance
characteristic has a profile shown by the curve Lc during light
loading, and the resonance characteristic has a profile shown by
the curve Ld during heavy loading. Accordingly, the series resonant
point Ps and the parallel resonant point Pp are also shifted to
higher frequency regions. An experiment reported that the resonant
frequencies fs, f1, fa, fp, and f2 were 65 kHz, 110 kHz, 155 kHz,
190 kHz, and 260 kHz, respectively, as shown in FIG. 4B.
[0058] Furthermore, as is apparent from FIG. 4B, the switching
frequency f1 during heavy loading when the resonance curve Ld
intersects the predetermined voltage HV is 110 kHz, and the
switching frequency f2 during light loading when the resonance
curve Lc intersects the predetermined voltage HV is 260 kHz. It
should be noted that the switching frequencies f1 and f2 are 60 kHz
and 65 kHz, respectively, in the graph shown in FIG. 4A.
[0059] According to the present invention, therefore, the switching
frequency f2 to stabilize the output voltage during light loading
can be significantly increased more than usual.
[0060] Meanwhile, the amplitude Irpp of the exciting current
component which flows to the primary coil 22a is expressed as
follows.
Irpp=HV/{2.multidot.f2.multidot.{square root}{square root over
(Lp.multidot.Ls1)}}
[0061] where Ls1 denotes the inductance of the secondary coil 22b.
As is seen from the above expression, the amplitude of the exciting
current is inversely proportional to the switching frequency
f2.
[0062] Therefore, if the switching frequency f2 is shifted to a
higher frequency region, such as 260 kHz, the exciting current
component can be reduced to approximately one quarter the case
where the switching frequency f2 is 65 kHz.
[0063] As a result, the energy conversion efficiency of the
converter transformer 22 is significantly improved more than usual
by on the order of 6 to 7 W, as was observed in an experiment.
[0064] Components which define the resonance curves Lc and Ld
include the series resonant capacitor Cr, the resonant inductors Lr
and Lp, and the equivalent capacitor Cp shown in FIG. 1, and only
the equivalent capacitance Cp may be changed or, otherwise, the
values of any other components may be changed to obtain the
resonance characteristic shown in FIG. 4B.
[0065] The primary coil 22a and the secondary coils 22b and 22c of
the converter transformer 22 may be loosely coupled, with the
leakage inductance being used as the resonant inductance Lr.
[0066] FIG. 5 shows another embodiment of the bridge DC-DC
converter 10 according to the present invention.
[0067] In this manner, the operational frequency during light
loading is set at the frequency f2 more than the resonant frequency
fp which is mainly defined by the inductance Lp of the primary coil
22a, and the interwinding capacitances Cs1 and Cs2 of the secondary
coils 22b and 22c of the converter transformer 22. However, the
interwinding capacitances Cs1 and Cs2 may not provide sufficient
capacitance, and the resonance characteristic in which the output
voltage at the frequency fa in the vicinity of bottom exceeds the
predetermined voltage HV may not be expected. This inconvenience is
overcome by an embodiment illustrated with reference to FIG. 5.
[0068] Referring to FIG. 5, auxiliary capacitors Cs3 and Cs4 are
added to the secondary coils 22b and 22c of the transformer 22,
respectively. The equivalent capacitance Cp which is present in the
primary would thus increase, providing the resonance characteristic
as depicted by the curve Lc in FIG. 4B that the output voltage
level in the vicinity of the bottom frequency fa exceeds the
predetermined voltage HV. Other structure is the same as that in
FIG. 1, and a description thereof is therefore omitted.
[0069] The smoothing and rectifying circuit 24 may be configured
according to the output voltage. Therefore, a half-wave rectifying
circuit or a voltage multiplier rectifier circuit may be employed
as alternatives.
[0070] Although the present invention is embodied as a half bridge
DC-DC converter in the illustrated embodiments, it may also be
embodied as a full bridge DC-DC converter. Furthermore, although a
typical DC-DC converter is implemented in the present invention,
the present invention may also be applied to high voltage
generating circuits for use in CRTs, as is readily anticipated by
those skilled in the art.
[0071] If a high voltage generating circuit for use in CRTs is used
instead of a DC-DC converter such as a half bridge DC-DC converter,
it uses the switching frequency which is asynchronous with the
horizontal scanning frequency to stabilize the high output voltage,
so that the ripple (switching frequency component) of the output
voltage may appear on a screen in the form of undesired horizontal
fringes. One possible solution is to insert a resistor useful to
suppress the ripple in series into the transmission path of the
output voltage.
* * * * *