Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2003-04-07
2004-11-09
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021180
Reexamination Certificate
active
06816393
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply wherein a current detection resistor is inserted between a switching element such as an FET and a primary side ground level and a current flowing into the switching element is limited based on a voltage generated between the both terminals of the current detection resistor.
In an RCC type switching power supply, the configuration shown in
FIG. 2
is proposed as a conventional technique which can prevent abnormal heat etc. even in the case of the short-circuit of a primary coil or a switching element. That is, supposing that a zener diode D
5
is eliminated, when a short-circuit of a primary coil L
1
or an internal short-circuit of an FET
5
occurs, an over-current flows through a current detection resistor R
1
. This over-current causes an open breakage of the current detection resistor R
1
before fusing of a fuse
3
. When the open breakage of the current detection resistor R
1
occurs, a high voltage exceeding 100 volt is applied to the base of a control transistor Q
2
through a diode D
1
. As a result, abnormal heat occurs at the control transistor Q
2
and so a firing may occur before melting of the fuse
3
.
Thus, although it is desirable to use as the current detection resistor R
1
a cemented resistor which is quite low in possibility of causing the open breakage, the cemented resistor is expensive. Therefore, conventionally, a metal film resistor which element cost is cheap is used as the current detection resistor R
1
, and the zener diode D
5
is connected in parallel with the current detection resistor R
1
. In this case, even if the primary coil L
1
or the zener diode D
5
is short-circuited and the open breakage of the current detection resistor R
1
occurs, the base voltage of the control transistor Q
2
is limited to the zener voltage of the zener diode D
5
. Thus, the abnormal heat of the control transistor Q
2
is suppressed. Therefore, since the fuse
3
melts before the breakage of the zener diode D
5
, the heat generation is stopped thereafter (first conventional technique).
Further, in a switching power supply, a technique for preventing the breakage etc. of a load caused by an over-voltage is proposed in the Unexamined Japanese Patent Application Publication No. Hei11-149320. That is, in this technique, an over-current fusing type resistor is inserted between a primary side positive power supply and a switching element. A thyristor is provided whose one end is coupled at a connection point between the switching element and the over-current fusing type resistor and the other terminal is grounded. Further, an over-voltage detection circuit is provided which shifts the thyristor from an off state to an on state when an output voltage exceeds a predetermined value. Thus, when the output voltage increases to an abnormal voltage due to an abnormality of the switching element etc., the thyristor becomes in an on state. As a result, the output voltage reduces to almost 0 volt. Further, a current sufficient for fusing flows through the over-current fusing type resistor. Thus, when the output voltage becomes a high value due to an abnormality of the switching element etc., since the over-current fusing type resistor fuses in a short time, a load is protected from the breakage due to an over-voltage (second conventional technique).
In a technique proposed in the Unexamined Japanese Patent Application Publication No. Hei9-266669, a fuse resistor is provided independently from a smoothing circuit constituted by a resistor and a smoothing capacitor. One terminal of the fuse resistor is coupled to the smoothing circuit and the other terminal thereof is coupled to a primary coil. Thus, a rush current to the smoothing capacitor at the time of turning-on of the power supply does not flow into the fuse resistor. That is, there is no fear that the fuse resistor melts by the rush current. Therefore, the fusing current value of the fuse resistor may be set to a value necessary for the protection of the load without taking the influence of the rush current into consideration, the reliability with respect to the protection for the load can be improved (third conventional technique).
However, when the first conventional technique is employed, the zener diode D
5
is required. Further, the zener diode D
5
is required to have a durability sufficient for not causing the breakage of the element itself until the fuse
3
melts. Thus, since the zener diode D
5
is required to be an element with a large rated watt, the zener diode D
5
is expensive.
When considering the second conventional technique in a view point of eliminating the problem of the first conventional technique, that is, when considering the second conventional technique in a view point of preventing without using an expensive element the generation of the abnormal heat at the time of the open breakage of the current detection resistor R
1
coupled between the source of the FET
5
and the primary side ground level, this technique is difficult to be applied to the switching power supply even only in a view point of using the fuse resistor since the fuse resistor is an expensive element. Further, since the thyristor is required to be provided additionally, the cost of the parts for the switching power supply increases further.
The third conventional technique is formed to have a circuit configuration intended not to flow the rush current into the fuse resistor. That is, when considering the third conventional technique in a view point of eliminating the problem of the first conventional technique, since the third conventional technique differs in the basic configuration of the switching power supply, the third conventional technique can not prevent the generation of the abnormal heat at the time of the open breakage of the current detection resistor R
1
coupled between the source of the FET
5
and the primary side ground level. Further, the third conventional technique is arranged to protect the load based on the rated power ratio of the resistor itself which is configured to easily case the open-breakage thereof.
SUMMARY OF THE INVENTION
The invention is made in order to solve the aforesaid problems of the conventional techniques an object of the invention is to provide a switching power supply which can prevent abnormal heat of a control transistor at the time of the short-circuit of a primary winding or an FET without using a zener diode which is an expensive element by differentiating the current values from which two resistors start the open-breakage, respectively; which can easily determine resistance values and rated watts of respective resistors used as a breaking resistor and a current detection resistor which are inserted in a path from the output point of a primary side positive power supply to the drain of the FET; and which can suppress the dissipation power of the breaking resistor to a minimum value even when surely causing the open-breakage of the breaking resistor without causing the open-breakage of the current detection resistor.
Further, another object of the invention is to provide a switching power supply wherein the breaking resistor is inserted at an arbitrary portion of the path from the output point of the primary side positive power supply to the drain of the FET and the breaking resistor is set to be open-broken more easily than the current detection resistor based on the relation of the resistance values and the rated watts between the breaking resistor and the current detection resistor, whereby abnormal heat of the control transistor can be prevented at the time of the short-circuit of the primary coil or the FET without using a zener diode which is an expensive element.
Furthermore, in addition to the aforesaid objects, still another object of the invention is to provide a switching power supply which can easily determine resistance values and rated watts of respective resistors used as the breaking resistor and the current detection resistor by forming each of the breaking resistor and the curre
Funai Electric Co. Ltd.
Osha & May L.L.P.
Patel Rajnikant B.
LandOfFree
Switching power supply does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Switching power supply, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Switching power supply will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3303988