Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2003-01-21
2004-05-18
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S056010
Reexamination Certificate
active
06738266
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switching power supply units, and more particularly, to a self-excited-oscillation resonance-type switching power supply unit that allows for power saving during a light load condition (during standby).
2. Description of the Related Art
A resonance-type switching power supply unit is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-187664.
This power supply unit performs a soft-switching operation, and thus achieves a zero-voltage switching operation by alternately turning on and off two switching elements. This can significantly reduce switching loss, which can achieve high efficiency at rated load.
FIG. 10
is a circuit diagram of the power supply unit of the related art.
In this power supply unit, switching elements Q
1
and Q
2
are turned on at zero voltage and the switching element Q
2
is turned off at near zero current, thereby reducing switching loss significantly. A secondary-side rectifying element Ds is turned on at zero current. Concurrently, the waveform of current therein exhibits a relatively sharp rise from zero current, reaches a peak point at which the rate of current change becomes zero, and returns to a point of zero current, thereby turning off the rectifying element Ds. Thus, the waveform of current flowing through the rectifying element Ds becomes a substantially rectangular shape, so that the peak current value can be kept low and the effective value of current can be reduced, thereby reducing conduction loss. Thus, these effects lead to a high efficiency operation.
Meanwhile, a switching power supply unit that is adapted to perform an intermittent oscillation operation at light load is disclosed in Japanese Unexamined Patent Application Publication No. 2000-350449.
This power supply unit is a one-switching-element type switching power supply unit, which is commonly called a “ringing choke converter”, and can achieve high efficiency by performing an intermittent oscillation at light load.
FIG. 13
is a circuit diagram of the power supply unit of the related art.
The above-described power supply units of the related art, however, have the following problems.
FIG. 11
is a waveform diagram illustrating operations, at light load, of the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 11-187664. Referring to
FIG. 11
, reference symbols S
1
and S
2
indicate signals representing ON and OFF states of the switching elements Q
1
and Q
2
, respectively; reference symbol Vds
1
indicates the waveform of a voltage across a capacitor C
1
; and reference symbol Id
1
indicates the waveform of a current flowing in the switching element Q
1
.
As shown, when the ON-time of the switching element Q
2
is substantially constant regardless of the load condition, energy stored in the transformer is partially regenerated into input power at light load. Thus, this switching power supply unit has a problem in that the circulating current is increased and the efficiency is reduced due to conduction loss. In
FIG. 11
, shaded areas represent regenerative current.
FIG. 12
is a waveform diagram illustrating operations when the ON-time of the switching element Q
2
is designed to be short at light load to decrease the circulating current. However, in this case, while the switching elements Q
1
and Q
2
perform a zero-voltage switching operation, there is a problem in that the efficiency is reduced, for example, since switching loss is increased in a drive circuit due to an increase in switching frequency.
This power supply unit can control the ON-time of the second switching element to be shorter than the ON-time at rated load (see
FIG. 12
) to reduce conduction loss at light load. Yet, the timing (turn-off timing) at which the ON-time of the second switching element ends is not set to a timing at which energy release from the secondary winding ends or is earlier than that timing. In other words, even when energy release from the secondary winding ends, the ON-time of the second switching element still continues. As a result, when the second switching element is turned off, current flows through a diode that is connected in parallel with the first switching element. This state is equal to a state in which the first switching element is virtually in a conduction state, which means that the first switching element cannot be turned off. This prevents the operation of the first switching control circuit from proceeding to an intermittent oscillation mode, which will be described later.
FIG. 14
is a waveform diagram illustrating operations, at light load, of the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 2000-350449. Referring to
FIG. 14
, reference symbol S
1
indicates a signal representing ON and OFF states of a switching element Q
1
, Vds
1
indicates the waveform of a voltage across the switching element Q
1
, and Id
1
indicates the waveform of current in the switching element Q
1
.
As shown, when intermittent oscillation is performed at light load, a large switching surge voltage is generated during intermittent oscillation. As a result, a high voltage switching is required, which can inhibit high efficiency. Additionally, the switching power supply unit has a problem in that an electronic apparatus that is connected thereto malfunctions due to a large output voltage ripple, thus requiring a filter circuit for removing the output voltage ripple.
In addition, the switching power supply unit has difficulty in achieving high efficiency due to switching loss, compared to a soft-switching power supply unit.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a self-excited-oscillation switching power supply unit that has a simple configuration with fewer components than conventional devices and that achieves high efficiency at light load, in such a manner that the operational modes are changed by adequately changing, at light load and at rated or heavy load, a time constant for determining the ON-time of ON-time control circuits in switching control circuits for controlling the ON and OFF operations of switching elements, and an intermittent oscillation operation is performed at light load.
According to a preferred embodiment of the present invention, a two-switching-element-type switching power supply unit includes a first switching control circuit and a second switching control circuit which have novel configurations.
Specifically, the second switching control circuit is preferably configured as follows.
The second switching control circuit includes a second On-time control circuit. At light load, the second ON-time control circuit controls the ON-time of a second switching element such that the ON-time of the second switching element becomes shorter that the ON-time at rated load and energy release from a secondary winding is completed after the ON-time of the second switching element ends.
According to a preferred embodiment of the present invention, at light load, the second ON-time control circuit controls the ON-time of the second switching element such that the energy release from the secondary winding is completed after the end of the ON-time of the second switching element. Also, before a voltage generated across the first drive winding causes the first switching element to be turned on, a first impedance circuit causes a first control transistor to go into conduction, to prevent the first switching element from being turned on and to thereby stop the switching operation. This allows for intermittent oscillation at light load, which has not been achieved by a two-switching-element self-excited-oscillation type switching power supply unit of the related art, by adding a small number of components.
According to preferred embodiments of the present invention, unlike the power supply unit disclosed in Japanese Patent Application Publication No. 11-187664, the second swi
Hosotani Tatsuya
Takemura Hiroshi
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