Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-06-11
2002-07-09
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S132000
Reexamination Certificate
active
06418038
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
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
22
a
of a transformer
22
, which form a resonant circuit
20
.
A secondary coil
22
b
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.
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.
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
22
b
which is present at primary coil
22
a
would be parallel to the primary coil
22
a
. The interwinding capacitance which is present in the primary is indicated by Cp in FIG.
6
.
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
22
a
is combined with a parallel resonant portion comprising the inductor Lr, the inductance Lp, and the capacitor Cp.
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.
If a voltage for stabilization has been determined as depicted in
FIG. 7
, then, switching frequencies f
2
and f
4
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 f
1
and f
3
are obtained during heavy loading in the former and latter frequency regions, respectively.
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 f
1
to f
2
or Wb ranging from f
3
to f
4
. For example, the switching frequency is chosen to be within the frequency region Wa.
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.
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 f
2
during light loading and the switching frequency f
1
during heavy loading.
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
22
b
are set at 500 T, the number of turns in the primary coil
22
a
is 30 T in the converter transformer
22
. In this example, fs, f
1
, fp, and f
2
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.
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.
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
Accordingly, it is an object of the present invention to provide a bridge DC—DC converter having si
Nagahara Kiyokazu
Takahama Masanobu
Maioli Jay H.
Nguyen Matthew
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