Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-01-23
2002-05-28
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S056020, C363S056050
Reexamination Certificate
active
06396714
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active clamp forward converter, and more particularly, to an active clamp forward filter having low levels of switching loss and conductance loss.
2. Description of the Related Art
FIG. 9
is a circuit drawing showing an example of the prior art of a single-transistor, forward active clamp circuit disclosed in Japanese Unexamined Utility Model Application, First Publication No. 4-72882. This circuit is equipped with a transformer
17
in which a first end of a primary coil is connected to the positive terminal of a direct current power supply
1
via an inductor
8
, and a second end of a primary coil is connected to the negative terminal of a direct current power supply
1
via a switching element
2
.
A capacitor
9
and a switching element
5
are additionally connected in series between the positive terminal of direct current power supply
1
and the second end of the primary coil of the transformer
17
. In addition, a diode
3
and a capacitor
4
are connected in parallel to the switching element
2
, and a diode
6
and a capacitor
7
are connected in parallel to the switching element
5
.
The anode side of a diode
18
is connected to a first end of the secondary coil of the transformer
17
, the anode side of a diode
19
and one end of a choke coil
20
are connected to a second end of the secondary coil of the transformer
17
, the cathode side of diode
18
and the cathode side of the diode
19
are connected to the positive side of an output connector, and the other end of the choke coil
20
is connected to the negative side of an output terminal. In addition, an output capacitor (smoothing capacitor)
21
is connected between the output terminals to which a load
22
is connected.
The following provides an explanation of the operation of the example of the prior art of FIG.
9
.
When switching the element
2
is controlled to on, an input voltage Vin of the direct current power supply
1
is applied to the inductor
8
and the primary coil of the transformer
17
, and a current rises from the inductor
8
towards the primary coil of the transformer
17
resulting in accumulation of excitation energy.
When the switching element
2
is controlled to off after a fixed amount of time, the current is maintained in the same direction by the accumulated excitation energy. Consequently, the capacitor
7
is discharged simultaneous to charging of the capacitor
4
, and the diode
6
takes on a forward direction bias and is turned on causing zero voltage to be held between the terminals of the switching element
5
.
During this time, the switching element
5
is controlled to on and zero voltage switching is performed.
Although the current from the inductor
8
towards the primary coil of the transformer
17
charges the capacitor
4
and the capacitor
9
, this current gradually decreases and finally inverts caused by resonance phenomena due to the inductance of the inductor
8
and transformer
17
and the capacitance of the capacitor
9
.
Subsequently, although the switching element
5
is controlled to off, the current from the primary coil of the transformer
17
towards the inductor
8
is maintained, and together with charging the capacitor
7
, charges the capacitor
4
to generate a forward direction bias in the diode
3
causing a zero voltage to be held between the terminals of the switching element
2
.
During this time, the switching element
2
is controlled to on, zero voltage switching of the main current is performed, and the voltage Vin of the direct current power supply
1
is applied to the inductor
8
and the primary coil of the transformer
17
.
As a result of repeating the above operation, the current flowing to the primary coil of the transformer
17
is controlled by zero voltage switching, and the voltage induced in the secondary coil is supplied to the load
22
after being rectified by the diodes
18
and
19
and smoothened by the choke coil
20
and the output capacitor
21
.
As has been described above, in this active clamp circuits, switching loss is attempted to be reduced by switching the switching element on with the zero voltage between terminals, and when off, delaying the rise of the voltage by the capacitors connected in parallel.
Here, the voltage Vin of the direct current power supply
1
is applied to the inductor
8
and the primary coil of the transformer
17
when switching the element
2
is on, while a charging voltage VcO of the capacitor
9
is applied in the reverse direction when the switching element
2
is off. However, since the time product of the applied voltage when the switching element
2
is on and off is 0 based on the conditions of magnetic flux equilibrium, the following equation is valid when the on duty factor is taken to be D:
Vin·D=VcO·
(1
−D)
Thus, the charging voltage VcO of the capacitor
9
becomes as follows:
VcO=Vin·D
/(1
−D
) (1)
In addition, a maximum voltage VswO applied to the switching element
2
or
5
becomes as follows:
VswO=Vin+VcO=Vin
/(1
−D
) (2)
As described above, the switching loss is reduced by a zero voltage switching or a zero current switching in the active clamp circuits. Though, in order to additionally reduce the loss caused by the on resistance of the FET (field effect transistor) used for switching element
2
, it is preferable to increase the windings ratio of the primary and secondary coils of the transformer
17
, decrease the current flowing to the switching element
2
, and set the ratio of the maximum time at which the switching element
2
is switched on to the switching cycle, namely a maximum on duty factor Dmax, to 0.5 or more.
However, in the active clamp circuit of the prior art shown in
FIG. 9
, as shown in equations (1) and (2), as the on duty factor D increases, the charging voltage VcO of the capacitor
9
or the maximum applied voltage VswO of the switching element increases.
For example, if the voltage Vin from the direct current power supply
1
is taken to be 360 V, even if the the maximum on duty factor Dmax is 0.6, the voltage VcO applied to the capacitor
9
becomes 1.5 Vin=540 V in the case the on pulse has widened to the maximum on time during a sudden change in output current. In addition, the maximum voltage VswO applied to the switching element
2
ends up becoming Vin+VcO=900 V.
Consequently, the problem was encountered in which the maximum on duty factor Dmax ends up being restricted by the withstand voltage of the switching element or capacitor. In addition, if the maximum on duty factor Dmax is increased, the FET having a high withstand voltage is required for use as the switching element
2
. In general, as the withstand voltage of the FET becomes higher, the on resistance of the FET also increases. Consequently, there was the problem of the conductance loss when the switching element
2
is on conversely increasing.
In addition, there was also the problem with respect to capacitor
9
in which, as the rated voltage becomes higher, the capacitor having a larger external shape is required.
The object of the present invention is to improve on these problems by providing an active clamp forward converter that reduces the maximum voltage applied to a switching element as well as the charging voltage of a capacitor, and allows the use of the switching element and a capacitor having lower withstand voltages, resulting in low on loss of the switching element and enabling the size of the capacitor to be made smaller as well as a wide control range for the on duty factor.
SUMMARY OF THE INVENTION
In order to solve the above problems, the active clamp forward converter as claimed in the present invention is equipped with:
a transformer having a primary coil of which one end is connected to a first contact, an inductor connected between the other end of the primary coil of this transformer and a second contact, a first switching element connected between the positive term
NEC Corporation
Vu Bao Q.
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