Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-03-22
2002-02-19
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021040, C363S079000, C363S097000
Reexamination Certificate
active
06349046
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply circuit to be provided as a power supply for various electronic apparatus.
Switching power supply circuits employing switching converters such as flyback converters and forward converters are widely known. These switching converters form a rectangular waveform in switching operation, and therefore there is a limit to suppression of switching noise. It is also known that because of their operating characteristics, there is a limit to improvement of power conversion efficiency.
Hence, various switching power supply circuits using various resonance type converters have been previously proposed by the present applicant. A resonance type converter makes it possible to readily obtain high power conversion efficiency, and to achieve low noise because the resonance type converter forms a sine-wave waveform in switching operation. The resonance type converter has another advantage of being able to be formed by a relatively small number of parts.
FIG. 9
is a circuit diagram showing an example of a prior art switching power supply circuit that can be formed according to an invention previously proposed by the present applicant.
The power supply circuit shown in
FIG. 9
is provided with a full-wave rectifier circuit comprising a bridge rectifier circuit Di and a smoothing capacitor Ci. The full-wave rectifier circuit serves as a rectifying and smoothing circuit supplied with a commercial alternating-current power (alternating-current input voltage VAC) to provide a direct-current input voltage. The rectifying and smoothing circuit generates a rectified and smoothed voltage Ei whose level is equal to that of the alternating-current input voltage VAC multiplied by unity.
A voltage resonance type converter that includes a switching device Q
1
and performs switching operation by a so-called single-ended system is provided as a switching converter for interrupting the rectified and smoothed voltage Ei (direct-current input voltage) inputted from the rectifying and smoothing circuit.
The voltage resonance type converter in this case is externally excited, and a MOS-FET, for example, is used as the switching device Q
1
. A drain of the switching device Q
1
is connected to a positive electrode of a smoothing capacitor Ci via a primary winding N
1
of an insulating converter transformer PIT, while a source of the switching device Q
1
is connected to a ground on the primary side.
A parallel resonant capacitor Cr is connected in parallel with the drain and source of the switching device Q
1
. Capacitance of the parallel resonant capacitor Cr and leakage inductance obtained at the primary winding N
1
of the insulating converter transformer PIT form a primary-side parallel resonant circuit. The parallel resonant circuit performs resonant operation according to switching operation of the switching device Q
1
. Thus, the switching operation of the switching device Q
1
is of a voltage resonance type.
Also, a clamp diode (so-called body diode) DD is connected in parallel with the drain and source of the switching device Q
1
. The clamp diode DD forms a path of clamp current that flows during an off period of the switching device.
In this case, the drain of the switching device Q
1
is connected to an oscillating circuit
41
in a switching driver
10
B, which will be described next. An output of the drain supplied to the oscillating circuit
41
is used in switching frequency control to variably control an on period within one switching cycle, which will be described later.
The switching device Q
1
is driven for switching operation by the switching driver
10
B which is formed by integrating the oscillating circuit
41
and a driving circuit
42
, and the switching frequency of the switching device Q
1
is variably controlled for the purpose of constant-voltage control. Incidentally, the switching driver
10
B in this case is provided as a single integrated circuit (IC), for example.
The switching d river
10
B is connected to a line of the rectified and smoothed voltage Ei via a starting resistance RS. The switching driver
10
B starts operation by being supplied with the power supply voltage via the starting resistance Rs at the start of power supply, for example.
The oscillating circuit
41
in the switching driver
10
B performs oscillating operation to generate and output an oscillating signal. The driving circuit
42
converts the oscillating signal into a driving voltage, and then outputs the driving voltage to a gate of the switching device Q
1
. Thus, the switching device Q
1
performs switching operation according to the oscillating signal generated by the oscillating circuit
41
. Therefore, the switching frequency and duty ratio of an on/off period within one switching cycle of the switching device Q
1
is determined depending on the oscillating signal generated by the oscillating circuit
41
.
The oscillating circuit
41
changes the frequency of the oscillating signal (switching frequency fs) on the basis of the level of a secondary-side direct-current output voltage E
0
inputted via a photocoupler
40
, which will be described later. The oscillating circuit
41
changes the switching frequency fs and at the same time, controls the waveform of the oscillating signal in such a manner that a period TOFF during which the switching device Q
1
is turned off is fixed and a period TON (conduction angle) during which the switching device Q
1
is turned on is changed. The period TON (conduction angle) is variably controlled on the basis of the peak value of a parallel resonance voltage V
1
across the parallel resonant capacitor Cr. As a result of such operation of the oscillating circuit
41
, the secondary-side direct-current output voltage E
0
is stabilized, as will be described later.
The insulating converter transformer PIT transmits switching output of the switching device Q
1
to the secondary side of the switching power supply circuit.
As shown in
FIG. 11
, the insulating converter transformer PIT has an E-E-shaped core formed by combining E-shaped cores CR
1
and CR
2
made for example of a ferrite material in such a manner that magnetic legs of the core CR
1
are opposed to magnetic legs of the core CR
2
. A primary winding N
1
and a secondary winding N
2
are wound around a central magnetic leg of the E-E-shaped core in a state divided from each other by using a dividing bobbin B. Also, a gap G is formed in the central magnetic leg, as shown in
FIG. 11
, to provide loose coupling at a required coupling coefficient.
The gap G can be formed by making the central magnetic leg of each of the E-shaped cores CR
1
and CR
2
shorter than two outer legs of each of the E-shaped cores CR
1
and CR
2
. The coupling coefficient k is set for example to be k≈0.85 to provide a loosely coupled state, whereby a saturated state is not readily obtained.
As shown in
FIG. 9
, an ending point of the primary winding N
1
of the insulating converter transformer PIT is connected to the drain of the switching device Q
1
, while a starting point of the primary winding N
1
is connected to the positive electrode of the smoothing capacitor Ci (rectified and smoothed voltage Ei). Hence, the primary winding N
1
is supplied with the switching output of the switching device Q
1
, whereby an alternating voltage whose cycle corresponds to the switching frequency of the switching device Q
1
occurs in the primary winding N
1
.
An alternating voltage induced by the primary winding N
1
occurs in the secondary winding N
2
on the secondary side of the insulating converter transformer PIT. In this case, a secondary-side parallel resonant capacitor C
2
is connected in parallel with the secondary winding N
2
, and therefore leakage inductance L
2
of the secondary winding N
2
and capacitance of the secondary-side parallel resonant capacitor C
2
form a parallel resonant circuit. The parallel resonant circuit converts the alternating voltage induced in the secondary winding N
2
into a resonance voltage, whereby voltage reso
Frommer William S.
Frommer & Lawrence & Haug LLP
Han Jessica
Sony Corporation
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