Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-11-07
2002-11-19
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S098000, C363S132000
Reexamination Certificate
active
06483722
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC/DC converter with a half bridge configuration, and the control method thereof.
2. Description of the Related Art
FIG. 6
shows a prior art example of a DC/DC converter that includes a series circuit of a MOSFET (Metal Oxide Semiconductor Field-Effects Transistor)
1
and MOSFET
2
, which are connected to a DC power supply
10
in parallel, and a series circuit of a capacitor
3
, a transformer primary winding
5
and a snubber capacitor
21
, which are connected to the MOSFET
2
in parallel respectively. The secondary side of a transformer
22
comprises 2 windings,
8
and
9
, and a rectifying and smoothing circuit which further comprises diodes
12
,
13
and capacitor
14
. In order to maintain a smoothed DC output voltage at a predetermined level, an output voltage detection circuit
17
and a frequency and phase control circuit
19
are disposed to perform feedback control. For the circuit to drive each gate of the MOSFET
1
and
2
, a high voltage driver IC (Integrated Circuit)
20
is used.
FIG. 7
shows the timing chart for the operation of the DC/DC converter shown in
FIG. 6
; the operation of which will now be described. At first, MOSFET
1
is turned ON in period [
1
], then the resonance current, by the capacitor
3
and the leakage inductance of the transformer
22
and the exciting current of the transformer
22
, flow via the DC power supply
10
to capacitor
3
, to transformer primary winding
5
, to MOSFET
1
, and the capacitor
3
is charged. At this time, the difference voltage VP
1
between the DC power supply voltage Ed and the voltage VC across the capacitor
3
is applied to the transformer primary winding
5
. The voltage VS
1
generated at the transformer secondary winding
8
is rectified and smoothed by the diode
12
and the capacitor
14
, and power is supplied to the load. The transformer secondary winding voltage VS
1
rises in proportion to the transformer primary winding voltage VP
1
(indicated by dotted lines in FIG.
7
). The diode
12
conducts when the transformer secondary winding voltage VS
1
reaches the output voltage Vo, and therefore the voltage VS
1
is clamped to the output voltage Vo. The difference voltage between the dotted lines and the solid lines in
FIG. 7
is applied to the leakage inductance of the transformer
22
.
In period [
2
], the transformer primary winding voltage VP
1
gradually drops, and when the voltage in proportion to the transformer primary winding voltage VP
1
becomes lower than the output voltage Vo, the diode
12
is blocked, and the diode current ID
12
becomes zero. In the MOSFET
1
, the exciting current of the transformer
22
, excited during period [
1
], flows continuously.
When the MOSFET
1
is turned OFF in period [
3
], the exciting current of the transformer
22
is commutated to the snubber capacitor
21
and the output capacitance of the MOSFET
1
and
2
, and the voltage VQ
1
across the MOSFET
1
gradually rises and the voltage VQ
2
across the MOSFET
2
gradually drops.
When the voltage VQ
2
across the MOSFET
1
reaches the DC power supply voltage Ed in period [
4
], the exciting current of the transformer
22
is commutated to the parasitic diode of the MOSFET
2
. At this time, by turning the MOSFET
2
ON, the resonance current and the exciting current of the transformer
22
flow via the capacitor
3
to MOSFET
2
, to transformer primary winding
5
, and the capacitor
3
is discharged. The difference voltage VP
1
between the DC power supply voltage Ed and the voltage across the capacitor
3
is applied to the transformer primary winding
5
, and therefore the transformer
22
is reset. At this time, the voltage generated at the transformer secondary winding
9
is rectified and smoothed by the diode
13
and the capacitor
14
, and power is supplied to the load.
Since operation in period [
4
] to [
6
] is the same as in period [
1
] to [
3
], a description thereof is omitted here.
By repeating the series of operations from period [
1
] to [
6
], power is supplied from the DC power supply
10
to the load.
Now the operation when the load is light will be described with reference to FIG.
8
. When the load is light, the frequency and phase control circuit
19
is adjusted so that the switching frequency does not increase, and the MOSFET
1
or MOSFET
2
is turned OFF when a predetermined time elapses after the transformer secondary current ID
12
and ID
13
become zero. The current IQ
1
and
2
, which flows through the MOSFET
1
and
2
becomes roughly equal to the exciting current of the transformer
22
.
In the above prior art, each MOSFET alternately switches from rated load to no load at a 50% duty, and controls the current to be supplied to the load by adjusting the voltage to be applied to the transformer primary winding with respect to the changes of the load using the output voltage detection circuit and the frequency and phase control circuit, so that output voltage becomes constant. With this method however, a value of the exciting current which flows through the exciting inductance of the transformer hardly changes from rated load to no load, so this exciting current becomes reactive current, loss is generated by the impedance in the circuit (e.g. the ON resistance of a MOSFET and the winding resistance of a transformer), and as a result, efficiency when the load is light drops.
Also the potential of the source terminal of the MOSFET, which is connected to the positive electrode of the DC power supply, is different from the potential of the source terminal of the MOSFET which is connected to the negative electrode of the DC power supply, so it is necessary to insulate signals which drive the MOSFET at the positive electrode side by a pulse transformer, or to use an expensive high voltage driver IC which has a level shift function, resulting in the system becoming large and costs increasing.
In view of the above, it would be desirable to control reactive current to be low and to lower costs by not using expensive components.
SUMMARY OF THE INVENTION
To overcome the deficiencies of conventional devices, a DC-DC converter according to the present invention is characterized in that serial circuits of two switching elements are connected in parallel between the positive electrode and the negative electrode of a DC power supply, a serial circuit of at least one capacitor and a transformer primary winding is connected to one of the switching elements in parallel, an ON/OFF signal is supplied to the switching element connected to the positive electrode side of the DC power supply from the tertiary winding of the transformer, the quaternary winding of the transformer is used for the power supply of a control circuit, the timing of the switching of positive
egative of the quaternary winding voltage is detected by the control circuit, and an ON/OFF signal is applied to the switching element connected to the negative electrode side of the DC power supply at this timing, so that half wave rectification or full wave rectification is performed on the positive
egative voltage generated at the secondary winding of the transformer, and DC output is obtained.
In the DC-DC converter, the switching element connected to the negative electrode side of the DC power supply turns ON when a short circuit prevention period has elapsed after the voltage of the transformer quaternary winding switches from positive to negative or from negative to positive, compares the reference voltage value, which increases in proportion to the time from the switching element ON or the voltage switching timing of the transformer quaternary winding, and when the reference voltage exceeds this voltage detection value, the switching element is turned OFF so that the DC output voltage becomes constant.
Furthermore, a predetermined offset can be provided so that the minimum value of the reference voltage b
Nishikawa Yukihiro
Nozawa Takeshi
Fuji Electric & Co., Ltd.
Rossi & Associates
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