Electric power conversion systems – Current conversion – Having plural converters for single conversion
Utility Patent
1998-07-24
2001-01-02
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S089000, C363S127000
Utility Patent
active
06169675
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a synchronous rectifying type DC-DC converter for supplying a load, such as an electric apparatus, etc., with substantially fixed DC electric power.
2. Description of the Related Art
A synchronous rectifying type DC-DC converter has hitherto been known as a converter for supplying a load, such as an electronic apparatus, etc., with substantially fixed DC electric power. FIG.
11
(
a
) is a diagram showing a circuit construction of the prior art synchronous rectifying type DC-DC converter. FIG.
11
(
b
) is a time chart showing an operation of a circuit element illustrated in FIG.
11
(
a
).
FIG.
11
(
a
) shows synchronous rectifying type DC-DC converters (hereinafter simply referred to as “converters”)
1
a,
1
b,
1
c
having the same construction connected in parallel to a load
2
. Herein, the converter
1
a
is exemplified, wherein a transformer T is provided on an input side of the converter
1
a.
A switching transistor Tr is provided on a primary side of the transformer T, and an energy generated on the primary side is discontinuously supplied to a secondary side (the converter
1
a
). One terminal a choke coil L is connected via a conductor to one terminal of a secondary winding M of the transformer T. The other terminal of this choke coil L is connected to an output terminal of the converter
1
a.
One terminal of a resistance Rs is connected to a conductor for connecting the secondary winding M to the choke coil L. The other terminal of the resistance Rs is connected to a gate of an enhancement type MOS-FET (a MOS type electric field effect transistor: hereinafter abbreviated to “FET”) Qs serving as a rectifying switch. A source of the FET Qs is connected to an output terminal of the converter
1
a
via a conductor. A drain of the FET Qs is connected to the other terminal of the secondary winding M via a conductor. Then, the FET Qs equivalently includes a parasitic diode DQs through which to connect a source to a drain thereof, and through which a direction from the source to the drain is set as a forward direction.
One terminal of a resistance Rf is connected via a conductor to a conductor for connecting the drain go the FET Qs to the other terminal of the secondary winding M. The other terminal of the resistance Rf is connected to a gate of an FET Qf serving as a fly wheel switch. A source of the FET Qf is connected to a source of the FET Qs via a conductor. A drain of the FET Qf is connected to a conductor for connecting the secondary winding M to the choke coil L. This FET Qf equivalently includes a parasitic diode DQf through which to connect a source to a drain thereof, and through which the direction from the source to the drain is set as the forward direction. Then, a smoothing capacitor C is connected in parallel to the secondary winding M posterior to the choke coil L, and the output terminal of the converter
1
a
is connected to the load
2
.
An operation of the above-described converter
1
a
will be explained with reference to FIG.
11
(
b
). A positive voltage and a negative voltage are, as shown in FIG.
11
(
b
), intermittently generated as a voltage VT on the secondary winding M of the transformer T with a given magnitude and a given period in accordance with the voltage application to the primary winding of the transformer T.
Herein, when the positive voltage VT is generated on the secondary winding M upon a turn-on of the switching transistor Tr, a gate voltage of the FET Qs rises, and the FET Qs is turned ON. At this time, the energy is accumulated in the choke coil L, then a smoothing capacitor C is charged with electric charge, and a thus smoothed fixed output is supplied to the load
2
.
Thereafter, the voltage VT becomes negative after the switching transistor Tr has been turned OFF. Then, the gate voltage of the FET Qs decreases with the result that the FET Qs is turned OFF, and the gate voltage of the FET Qf increases with the result that the FET Qf is turned ON. Further, the voltage VT becomes negative, and the energy accumulated in the choke coil L is thereby released. Then, the electric current flows through a closed circuit consisting of the choke coil L, the load
2
and the FET Qf.
Thereafter, the transformer T is magnetically reset, and the generation of the negative voltage VT ceases (the voltage VT comes to “0”), at which time the FET Qf is turned OFF with the decrease in the gate voltage of the FET Qf, and a resistance value of the parasitic diode DQf becomes smaller than a resistance value between the source and the drain of the FET Qf. The electric current thereby flows through a closed circuit consisting of the choke coil L, the load
2
and the parasitic diode DQf.
Thereafter, when the positive voltage VT is again produced on the secondary winding M, the FET Qs is turned ON, and the source/drain resistance value of the FET Qs gets smaller than the resistance value of the parasitic diode DQf. The parasitic diode DQf is thereby turned OFF. Then, the operations described above is repeatedly performed. Thus, the converter
1
a
supplies the load
2
with the fixed output by turning ON/OFF the FET Qs and the FET Qf in synchronization with the change in the voltage VT from positive to negative.
The respective converters
1
a,
1
b,
1
c
are operated in parallel and each supply the load
2
with the outputs. With this contrivance, even if any one of the converters
1
a,
1
b,
1
c
stops due to a breakdown, the load
2
can be supplied with the electric current in a proper range.
If a failure is caused in any one of the converters
1
a,
1
b,
1
c
(if, e.g., oscillations of the voltage VT stop), there must be a possibility in which the following problems might arise. For example, if the oscillations of only the converter
1
a
stop due to the breakdown, the electric current having flows from the converters
1
b,
1
c
toward the output terminal of the converter
1
a,
flows not through the load
2
but through the choke coil L of the converter
1
a.
Consequently, the gate voltages of the FET Qs and of the FET Qf rise. Therefore, the FET Qs and the FET Qf of the converter
1
a
are turned ON, and a large quantity of current flows into the converter
1
a
from the converters
1
b,
1
c.
In consequence, the electric current supplied to the load
2
becomes deficient, an it follows that the FET Qs and the FET Qf might be damaged by the large quantity of current.
SUMMARY OF THE INVENTION
It is a primary object of the present invention, which was contrived to obviate the problems given above, to provide a synchronous rectifying type DC-DC converter capable of reducing a possibility of a damage caused when executing parallel operations.
To accomplish the above object, the present invention adopts constructions which follow. To be specific, according to a first aspect of the invention, a synchronous rectifying type DC-DC converter, a plural pieces are connected in parallel to a load, for supplying the load with a predetermined direct current output, comprises a rectifying switch connected in series to a secondary winding of a transformer, a fly wheel switch connected in parallel to the secondary winding of the transformer and an auxiliary winding induced by a voltage generated on a primary winding of the transformer, and giving output having polarities opposite to each other to a control terminal of the rectifying switch and to a control terminal of the fly wheel switch.
According to the first aspect of the invention, during parallel operation of the plurality of DC-DC converters, when an output of a certain DC-DC converter stops, and even if outputs of other DC-DC converters are inputted to this DC-DC converter, a sneak voltage is not applied to the rectifying switch and the fly wheel switch. Therefore, the rectifying switch and the fly wheel switch are not turned ON, and the large electric current can be prevented from flowing into the DC-DC converter concerned.
According a second aspect of the invention, the DC-DC converter according to the first
Fuchigami Kazutoshi
Shimamori Hiroshi
Yamashita Shigeharu
Armstrong, Westerman Hattori, McLeland & Naughton
Fujitsu Limited
Sterrett Jeffrey
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