Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2002-01-23
2003-06-03
Sherry, Michael (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S017000, C363S098000, C363S132000, C363S070000
Reexamination Certificate
active
06574125
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC—DC converter and a bi-directional DC—DC converter, and a method of controlling the DC—DC converter or the bi-directional DC—DC converter. More particularly, the present invention relates to a DC—DC converter used in a DC power source circuit for converting a power source voltage of a DC power source into a different DC voltage.
2. Description of the Related Art
For example,
FIG. 23
shows an example of a prior-art DC—DC converter used in a DC power source circuit.
FIG. 24
is a timing chart showing gate signals G for turning on/off respective switching elements Q
1
to Q
4
in the DC—DC converter.
The DC—DC converter shown in
FIG. 23
has a converter circuit portion
1
, a transformer Tr, a rectifier circuit
2
, and an LC smoothing circuit
3
. The converter circuit portion
1
includes two pairs of switching elements Q
1
, Q
4
and Q
2
, Q
3
(MOS-FETs) formed into full bridge configuration and connected to a DC power source E. The transformer Tr is connected to an output side of the converter circuit portion
1
. The rectifier circuit
2
is connected to a secondary output side of the transformer Tr and includes two pairs of diodes D
1
, D
4
and D
2
, D
3
. The LC smoothing circuit
3
is connected to an output side of the rectifier circuit
2
.
In the DC—DC converter, in the converter circuit portion
1
, the switching elements Q
1
, Q
4
and Q
2
, Q
3
are turned on/off alternately as shown in the timing chart of
FIG. 24
to thereby obtain an AC waveform output. The AC waveform output of the converter circuit portion
1
is transformed by the transformer Tr. The secondary output of the transformer Tr is rectified by the rectifier circuit
2
and smoothed by the LC smoothing circuit
3
to thereby generate a desired DC voltage.
Incidentally, when the load side of the transformer in the DC—DC converter is viewed from the input side of the transformer, the load is generally regarded as an inductive load (lagging load). In this case, voltages of the respective switching elements Q
1
to Q
4
, that is, drain-source voltages V
ds
and drain currents I
d
are shaped like the waveforms shown in FIG.
25
.
FIG. 26A
typically shows the wave forms of drain-source voltages V
ds
and drain currents I
d
of the switching elements Q
1
to Q
4
shown in FIG.
25
.
FIG. 26B
shows a turn-on switching loss P
1
, a turn-off switching loss P
3
and a conduction loss P
2
.
As shown in
FIG. 26B
, the loss in the switching elements Q
1
to Q
4
(MOS-FETs) is classified into the switching losses P
1
and P
3
and the conduction loss P
2
. The switching losses P
1
and P
3
are further classified into the turn-on switching loss P
1
and the turn-off switching loss P
3
. The turn-on switching loss P
1
is produced when each of the switching elements Q
1
to Q
4
is turned on. The turn-off switching loss P
3
is produced when each of the switching elements Q
1
to Q
4
is turned off. That is, the switching loss is produced when a drain-source voltage V
ds
is applied while a drain current I
d
flows in a short-time transient state in which each of the switching elements Q
1
to Q
4
is turned on/off. On the other hand, the conduction loss P
2
is an ohmic loss which is produced on the basis of ON-resistance and drain current when each of the switching elements Q
1
to Q
4
is turned on.
Incidentally, with respect to the switching loss in the case of an inductive load (lagging load), the turn-off switching loss P
3
is generally larger than the turn-on switching loss P
1
. The turn-on switching loss P
1
may not be produced if circuit constants can be set suitably.
To increase the switching frequency is effective means for reduction in size of the DC—DC converter because reduction in size of the transformer Tr can be attained when the switching frequency is increased. As the switching frequency increases, however, the switching loss increases in proportion to the switching frequency. Hence, to increase the switching frequency is not suitable means for reduction of the switching loss.
When an MOS-FET is used as each of the switching elements Q
1
to Q
4
, high-speed switching can be made because the MOS-FET can be turned on/off speedily compared with a bipolar transistor or an IGBT (Insulated Gate Bipolar Transistor). The MOS-FET, however, has the characteristic in which the ON-resistance of the MOS-FET increases in proportion to the 2.5
th
power of element withstand voltage compared with the bipolar transistor or IGBT in which the ON-voltage little increases even though the withstand voltage is high. The conduction loss in the MOS-FET is an ohmic loss determined on the basis of ON-resistance and drain current. Therefore, use of a high withstand voltage MOS-FET brings exponential increase of the conduction loss. Accordingly, this causes reducing of efficiency of the DC—DC converter.
Further, in the converter circuit portion
1
, the switching elements Q
1
to Q
4
are turned on/off so that a predetermined output voltage V is made to appear relative to an input voltage of the DC power source E. That is, because the pulse width of gate signals for turning on/off the switching elements Q
1
to Q
4
is constant, the ratio of output voltage to input voltage is kept constant. Hence, when the input voltage of the DC power source E fluctuates because of some cause, the output voltage V fluctuates. If such fluctuation in the output voltage V appears, stable electric power source to a load can be hardly performed.
Further, in the DC—DC converter, the converter circuit portion
1
provided on the left of the transformer Tr as shown in
FIG. 23
is used as the primary side and the rectifier circuit
2
provided on the right of the transformer Tr as shown in
FIG. 23
is used as the secondary side. Thus, electric power is supplied from the primary side to a load on the secondary side by electric discharge in the DC power source E. Hence, when the DC power source E includes a secondary battery such as a lead storage battery both dischargeable and chargeable, electric power conversion from the secondary side to the primary side exists for electrically charging the DC power source E as well as electric power conversion from the primary side to the secondary side. However, because the prior-art DC—DC converter can do nothing but electric power conversion from the primary side to the secondary side, the DC—DC converter was difficult to be applied to the purpose of electric power conversion from the secondary side to the primary side.
SUMMARY OF THE INVENTION
Therefore, a first object of the invention is to provide a DC—DC converter and a method of controlling the DC—DC converter in which reduction in switching loss is attained and in which low ON-resistance and low withstand voltage MOS-FETs can be used as switching elements.
A second object of the invention is to provide a DC—DC converter a method of controlling the DC—DC converter in which reduction in switching loss can be attained and in which a stable output voltage can be obtained.
A third object of the invention is to provide a bi-directional DC—DC converter a method of controlling the bi-directional DC—DC converter in which reduction in switching loss is attained and in which electric power conversion from the secondary side to the primary side can be performed as well as electric power conversion from the primary side to the secondary side.
In order to accomplish the objects above, the following means are adopted. According to the present invention, there is provided a DC—DC converter comprising: n sets of converter circuit portions for converting a power source voltage of a DC power supply into AC voltages, each of which includes two pairs of switching elements connected in full bridge configuration; and rectifier circuit portions provided on output sides of the converter circuit portions through transformers respectively. In the DC—DC converter, a switching phase of one switching element in each pair of switching elements included in each converter circuit
Hasebe Takaya
Kurio Nobuhiro
Matsukawa Mitsuru
Nakagaki Hitoshi
Finnegan Henderson Farabow Garrett & Dunner LLP
Laxton Gary L.
Nissin Electric Co. Ltd.
Sherry Michael
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