Electric power conversion systems – Current conversion – With starting arrangement
Reexamination Certificate
2002-11-05
2004-10-19
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
With starting arrangement
C363S021150, C323S901000
Reexamination Certificate
active
06807075
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a switched mode power supply comprising a start-up device for supplying electrical energy to the controller in a start-up phase. Further, the invention relates to a method for starting up such a switched mode power supply, a controller start-up device for use in such a switched mode power supply, and a display apparatus comprising such a switched mode power supply.
Switched mode power supplies are used in a wide range of electronic equipment. Examples of such electronic equipment include computing equipment, television and video equipment, as well as portable telecommunication devices. Switched mode power supplies convert a DC primary voltage, such as a battery voltage or a rectified AC line voltage, into one or more secondary voltages.
2. Description of the Related Art
U.S. Pat. No. 5,581,453 discloses a start-up circuit for a switched mode power supply. The power supply comprises a transformer which is switched by a switching transistor to be periodically connected to an input voltage. Periodic pulses for driving the switching transistor are provided by a power supply control circuit. To be able to start such a power supply, the power supply is equipped with a start-up circuit comprising a start-up resistor and a capacitor. The start-up circuit provides the power supply control circuit with initial electrical energy. In addition thereto, the power supply control circuit is supplied with energy from an additional winding on the transformer, the voltage on this winding being rectified and supplied to the power supply control circuit. During start-up of the power supply, the supply of the power supply control circuit is fully accounted for by the start-up circuit. The start-up resistor will charge the capacitor, causing the supply voltage of the power supply control circuit to rise. Initially, the voltage on the transformer is low and, consequently, the additional winding of the transformer does not contribute to the supply of the power supply control circuit. After a certain start-up time, the output voltage of the power supply starts to rise. Consequently, the voltage available on the additional winding of the transformer starts to rise. When the voltage on the additional winding is sufficiently large, the winding will take over the supply of the power supply control circuit from the start-up circuit.
Typically, the start-up resistor is dimensioned to provide only a current which is sufficient to start up the power supply control circuit. The normal operating current of the power supply control circuit is much higher. Although not disclosed in the above publication, this causes the arrangement to have a built-in overload protection. In case the output voltage of the power supply lowers, for example, because of an overload or a short circuit, the voltage supplied by the transformer will decrease and consequently, the voltage on the additional winding will decrease. This causes the supply voltage of the power supply control circuit to drop as the start-up resistor has been dimensioned such as to only provide a small start-up current, which is insufficient to power the power supply control circuit during normal operation. As a result, the switching will stop and the power supply will enter a start-up sequence, attempting a new start up.
A disadvantage of this power supply is that an additional winding in the transformer is needed. This complicates the design of the transformer. On the one hand, this winding needs to be magnetically well coupled to the secondary side of the transformer, thus it needs to be wound physically close to the secondary winding. On the other hand, the additional winding is connected to the primary side of the transformer. Consequently, the additional winding needs to be thoroughly insulated, to avoid safety problems with the insulation between the primary side and secondary side of the transformer. In general, a triple insulated wire is used. This makes the transformer relatively expensive, complex and difficult to manufacture.
Another disadvantage is that the start-up time is long. To reduce dissipation in the start-up resistor, the value of the start-up resistor is chosen to be as high as possible, however, at the cost of a slow start-up, because the high value of the resistor results in only a small start-up current available for charging the capacitor coupled therewith.
Related to the above disadvantage is another disadvantage, being that the start-up resistor will continue to dissipate electrical energy during the operation of the power supply. As power efficiency is a premium requirement in power supplies, this continuous power dissipation is undesirable. To make the power supply suitable for a wide input supply voltage range, the value of the start-up resistor has to be chosen low, so as to guarantee sufficient start-up current for the power supply control circuit during start-up at a minimum input supply voltage. However, this low value will cause a too large current through the resistor at high input voltages, and thus causes a large dissipation in the resistor. As explained above, also the requirements for a quick start-up of the power supply will tend to decrease the value of the start-up resistor, thus also increasing power losses.
In an attempt to overcome the disadvantage related to the power dissipation, U.S. Pat. No. 5,581,453 discloses a switching transistor, connected with its collector-emitter main current path in series with the start-up resistor. During start-up of the power supply, the switching transistor is in a conducting state, causing the resistor to supply a start-up current. During normal operation of the power supply, the switching transistor is in a non-conducting state, stopping the dissipation in the start-up resistor. A disadvantage of this set-up is that it easily leads to degradation of the switching transistor, as the switching transistor stops conducting by a reverse bias on the base-emitter junction. As is known, in regular transistor designs, only a few volts base-emitter reverse bias will irreversibly degrade the transistor by causing base-emitter breakdown. This condition can easily occur, for example, by a dip in the input voltage or by a raise of the voltage supplied by the additional winding, e.g., caused by load variances or manufacturing tolerances in producing the transformer. Also, it should be clear that this set-up, although attempting to solve the power dissipation problem, will not solve the above-mentioned problem associated with the third winding in the transformer.
SUMMARY OF THE INVENTION
An object of the invention is to remove, or at least reduce, the problems associated with the prior art. Another object of the invention is to simplify the design of a switched mode power supply.
A first aspect of the invention provides a switched mode power supply for conversion of an input voltage into at least one output voltage, comprising an inductive device for transforming the input voltage into the at least one output voltage; a switching device for periodically coupling the inductive device to the input voltage; a controller for controlling the switching of the switching device, and a start-up device for supplying electrical energy to the controller in a start-up phase, characterized in that the switched mode power supply further comprises a bypass-device coupled in parallel to the start-up device for bypassing the start-up device when the switched mode power supply is in regulation.
A second aspect of the invention provides a method of starting up a switched mode power supply comprising the steps controlling a start-up device to supply electrical energy to a switching controller of the switched mode power supply; and bypassing the start-up device when the switched mode power supply is in regulation.
A third aspect of the invention provides a controller start-up device for use in a switched mode power supply apparatus, the controller start-up device comprising an input port and an output port; and a start-up device coupling th
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