DC-DC converter

Details DC-DC converter DC-DC converter

2002-12-27

2004-05-18

Riley, Shawn (Department: 2838)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific current responsive fault sensor

C361S041000, C324S509000

Reexamination Certificate

active

06738247

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application claims benefit of priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2001-398310, filed on Dec. 27, 2001, the entire contents of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC—DC converter with a plurality of transformers, and more particularly to a DC—DC converter composed of simple circuit structure that protects the electrical device from damages safely at the time of carrying out an abnormal test for short-circuiting or open-circuiting a transformer thereof.
2. Description of the Related Art
As shown in
FIG. 1
, a flyback type DC—DC converter having a plurality of transformers in order to increase a total capacity of the transformers has been conventionally known. According to the conventional DC—DC converter, primary windings T
1
a
and T
2
a
of transformers T
1
and T
2
are connected in series to a switching element Q
1
. One ends of secondary windings T
1
b
and T
2
b
of the transformers T
1
and T
2
are connected to anodes of diodes D
1
and D
2
, respectively. The other ends of the secondary windings T
1
b
and T
2
b
are commonly connected to a negative electrode of capacitor C
1
. Cathodes of the diodes D
1
and D
2
are commonly connected to a positive electrode of the capacitor C
1
. Auxiliary windings T
1
c
and T
2
c
of the transformers T
1
and T
2
are connected in parallel with each other.
Further, temperature fuses TF
1
and TF
2
are thermally coupled to the transformers T
1
and T
2
, and are fused at the time of reaching a specified fusing temperature. When a direct voltage is applied from a direct power source
1
, a current firstly flows from the direct power source
1
through a fuse F
1
and a starting resistance R
1
to a capacitor C
2
to increase a voltage of the capacitor C
2
. According to such state, a control circuit
7
is activated. A PWM signal repeating alternately a high level state and a low level state is transmitted from the control circuit
7
via the temperature fuses TF
1
and TF
2
to a gate of the switching element Q
1
.
The switching element Q
1
is on-off controlled by the PWM signal outputted from the control circuit
7
. Then, energy is sequentially induced from the primary windings T
1
a
and T
2
a
to the secondary windings T
1
b
and T
2
b
. The energy induced on the secondary windings T
1
b
and T
2
b
is rectified and smoothed by the diodes D
1
, D
2
and the capacitor C
1
, and then is supplied to a load
4
as an output voltage. At the same time, a voltage generated at both ends of the capacitor C
1
is detected by a voltage detection circuit
10
to generate a detection signal. The detection signal is fed back to the control circuit
7
and then the control circuit
7
controls an on period of the switching element Q
1
in order to supply a certain output voltage to the load
4
.
There are operational tests for DC—DC converter. One of the operational tests is an abnormal test for confirming safety of device by short-circuiting or open-circuiting a transformer thereof. As shown in
FIG. 2
, the abnormal test has four test items. It will be described how the temperature fuse protects the electrical device from damages in the abnormal test.
(1) Short of Primary Winding
As shown in FIG.
2
(
1
), a switch SW
1
as
is open in advance and is connected in parallel to the primary winding T
1
a
of the transformer T
1
. When the switch SW
1
as
is closed during an operation of the DC—DC converter, an inductance component L of the secondary winding T
1
b
of the transformer T
1
is minimized. In such condition, because energy is not induced from the primary winding T
1
a
to the secondary winding T
1
b
, a power supplied to the load
4
is generated only by the transformer T
2
. This gradually increases a temperature of the transformer T
2
during the operation of the DC—DC converter. The increased temperature of the transformer T
2
causes a temperature of the temperature fuse TF
2
coupled thermally to the transformer T
2
to reach a specified fusing temperature. At the specified fusing temperature, the temperature fuse TF
2
is fused. When the temperature fuse TF
2
is fused, a PWM signal transmitted from the control circuit
7
to the switching element Q
1
is interrupted, stopping an operation of the switching element Q
1
. This cuts off current flow supplied to the transformer T
2
. As a result, an increase in temperature of the transformer T
2
is stopped. The above-described operations can prevent the DC—DC converter from firing or smoking caused by the increased temperature of itself. Therefore, the electrical device is protected from damages. Incidentally, it should be noted that the fuse F
1
dose not exert any influences on the short test for the primary winding because the temperature fuse TF
2
is always fused prior to the fuse F
1
in this short test for absence of excess current.
(2) Short of Secondary Winding
As shown in FIG.
2
(
2
), a switch SW
1
bs
is open in advance and is connected in parallel to the secondary winding T
1
b
of the transformer T
1
. When the switch SW
1
bs
is closed during an operation of the DC—DC converter, an inductance component L of the primary winding T
1
a
of the transformer T
1
is minimized. In such condition, because energy is not induced from the primary winding T
1
a
to the secondary winding T
1
b
, a power supplied to the load
4
is generated only by the transformer T
2
. Similarly to the short of the primary winding, this gradually increases a temperature of the transformer T
2
during the operation of the DC—DC converter. The increased temperature of the transformer T
2
causes a temperature of the temperature fuse TF
2
coupled thermally to the transformer T
2
to reach a specified fusing temperature. At the specified fusing temperature, the temperature fuse TF
2
is fused. When the temperature fuse TF
2
is fused, a PWM signal transmitted from the control circuit
7
to the switching element Q
1
is interrupted, stopping an operation of the switching element Q
1
. This cuts off current flow supplied to the transformer T
2
. As a result, an increase in temperature of the transformer T
2
is stopped. The above-described operations can prevent the DC—DC converter from firing or smoking caused by the increased temperature of itself. Therefore, the electrical device is protected from damages. Incidentally, it should be noted that the fuse F
1
does not exert any influences upon the short test for the secondary winding because the temperature fuse TF
2
is always fused prior to the fuse F
1
in this short test for absence of excess current.
(3) Open of Primary Winding
As shown in FIG.
2
(
3
), a switch SW
1
ao
is closed in advance and is connected in series to the primary winding T
1
a
of the transformer T
1
. When the switch SW
1
ao
is opened during an operation of the DC—DC converter, a current flowing through the primary windings T
1
a
and T
2
a
of the transformers T
1
and T
2
is cut off. As a result, power supply to the load
4
is stopped.
(4) Open of Secondary Winding
As shown in FIG.
2
(
4
), a switch SW
1
bo
is closed in advance and is connected in series to the secondary winding T
1
b
of the transformer T
1
. When the switch SW
1
bo
is opened during an operation of the DC—DC converter, a power supplied to the load
4
is generated only by the transformer T
2
. This gradually increases a temperature of the transformer T
2
during the operation of the DC—DC converter. The increased temperature of the transformer T
2
causes a temperature of the temperature fuse TF
2
coupled thermally to the transformer T
2
to reach a specified fusing temperature. At the specified fusing temperature, the temperature fuse TF
2
is fused. When the temperature fuse TF
2
is fused, a PWM signal transmitted from the control circuit
7
to the switching element Q
1
is interrupted, stopping an operation of the switching element Q
1
. This cuts off current flow supplied to the transformer T
2
. As a result, an incr

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