News Column

Patent Issued for Power Supply Circuit Having Switched Capacitor Units

July 23, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventors Otte, Rob (Eindhoven, NL); Peeters, Henricus Marie (Eindhoven, NL); Van Den Broek, Vincent Gerardus Petrus Cornelis (Oss, NL); Deppe, Carsten (Aachen, DE), filed on November 6, 2008, was published online on July 8, 2014.

The assignee for this patent, patent number 8773087, is Koninklijke Philips N.V. (Eindhoven, NL).

Reporters obtained the following quote from the background information supplied by the inventors: "A plurality of switched mode power supply topologies are known to the skilled person including the buck converter, the boost converter, the fly back converter, the buck boost converter and others. These circuits convert a DC input voltage into a desired output voltage by using at least one main switching element (which may be realized in a plurality of ways, as known to the skilled person, e.g. as a field effect transistor FET) and least one reactive element (i.e. a circuit element which can at least temporarily store energy, usually implemented as an inductor). In operation, the main switching element is continuously switched in a controlled manner, so that the input voltage is converted into an output voltage of desired voltage level.

"It is known to use a switched mode power supply to simultaneously drive a plurality of loads. These loads may be connected to the power supply e.g. in parallel fashion.

"An example of an application where multiple loads are to be supplied are lighting and display applications e.g. for video screens and projectors, that employ multiple light sources. These light sources, which are connected to the power supply circuit as loads may be e.g. LED, OLED, or laser diodes, e.g. of different color. It should be noted that in the present context the light sources may be single elements, such as single LEDs, but that the term 'light source' is also used for any array of single lighting elements, which may be connected e.g. in series or parallel. It is known to provide accurate dimming of the light sources by driving them in a pulsed manner. However, power supply circuits suited for use in display applications need to provide quite exact pulse shapes in order to obtain a correspondingly exact result. This is especially true for time-sequential display applications, where the light sources are driven rapidly in time-sequential manner to achieve color and intensity control, and/or the light is additionally modulated by a display device.

"For driving light sources in a pulsed manner, the required voltage or current (pulse height) may be fixed for one light source, but will differ between different light sources, e.g. of different color. Also, for some display applications it is known to drive the same light source sequentially with pulses of different height (i.e. voltage or current level).

"To drive the loads as described above, it is possible to provide a single, dedicated switched mode power supply for each load and each required voltage output level. However, the corresponding outlay is enormous.

"WO-A-2007-039862 describes a driver circuit arrangement for driving a plurality of individually switchable electrical subsystems, such as arrangements of LEDs. The subsystems are all connected in parallel to the same switched mode power converter. Each subsystem is comprised of an LED (the load) connected in parallel to a capacitor with a controllable subswitch for connecting the load to the capacitor. Further, each subsystem comprises a load switch which connects or disconnects the subsystem to/from the power converter. In operation, an external control unit selects the subsystems to be supplied with energy by closing the corresponding load switch. In contrast to prior circuits which do not have a subswitch between the capacitor and the load, it is then possible to control the load, e.g. in pulse width modulation mode by controlling the subswitch independently from the load switch.

"If multiple loads requiring different voltage levels are driven by the same switched mode power supply circuit, or if at least one load requires different voltage levels at different times, then the problem of voltage mismatch may occur: after supplying a first voltage level for a first pulse, the circuit will require some time to supply a different voltage level for a second, following pulse. This will lead to distortion of the pulse shape and subsequently worsened control results."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "It is an object of the present invention to provide a power supply circuit and a method for supplying electrical power well suited for driving loads in pulses with exact pulse shape.

"According to the invention, this is on one hand solved by a power supply circuit according to embodiments in which an output unit includes multiple switched capacitor units. This aspect of the invention is particularly advantageous to allow the use of a single switched mode converter for driving at least one load with different output levels.

"On the other hand, this is solved by a power supply circuit according to embodiments in which the power supply circuit includes multiple output units. This aspect of the invention allows using a single switched mode converter for a plurality of loads.

"Thus, both aspects of the invention deal with avoiding voltage mismatch in driving a load connected to a converter in a sequentially pulsed manner. While each aspect of the invention is advantageous separately, it should be emphasized that the aspects may well be combined, e.g. if among a plurality of loads one (or several) are required to be driven at subsequently differing voltage levels.

"According to the first aspect of the invention, there is provided a main power supply unit with a voltage input, at least one main switching element and at least one reactive element. This main power supply unit is a switched mode converter, where the switching element is controllable to deliver an output voltage or current. As known to the skilled person, the main power supply unit may comprise any of the known switched mode converter topologies and modes of operations.

"There is at least one output unit connected to the main power supply unit. The output unit comprises a load output, to which a load, such as e.g. an LED, OLED or laser diode may be connected. The output unit further comprises a load switching element connected to the load output to connect the main power supply unit to the load output, i.e. to switch the output voltage delivered by the converter on or off at the load output. While in principal other configurations could be used, it is preferred for the load switching element to be connected to the main power supply unit in series with the load output.

"According to the first aspect of the invention, the output unit further comprises a first and second switched capacitor unit. Each switched capacitor unit comprises at least a capacitor and a corresponding switching element. Again, while other configurations are possible, it is preferred for the switching element to be connected in series to the capacitor, and for the switched capacitor units to be connected in parallel to the load output and/or the load switching element.

"The switched capacitor units allow a mode of operation where the capacitor remains essentially charged to a predefined voltage level while it is not active, i.e. connected to the load output. If reconnected, the charged capacitor may then serve as a buffer element at the load output, instantaneously providing the desired voltage level upon activation of the output unit.

"This is especially advantageous to drive loads such as LEDs, OLEDs or laser diodes in a pulsed manner, e.g. in display applications. The pre-charged capacitor avoids otherwise necessary rise times at the start of the pulse and thus provides for very exact control, particularly in drive schemes where the voltage level remains constant during a pulse, or where at least the voltage at the end of a pulse is essentially equal to the voltage at the start of the pulse.

"In the first aspect of the invention, there are provided at least two switched capacitor units, which are both connected to the same load output, preferably in parallel. Since each capacitor unit is individually switchable by its capacitor switching element, they may be selectively activated. Thus, by keeping the inactive capacitor units substantially charged at different voltage levels, the same load output may be operated to instantaneously deliver exact pulsed outputs as described above, but at two different voltage levels.

"It should be noted that in the present context the terms 'essentially' charged and 'essentially' constant or equal relate to the fact that there may of course occur slight variations in the voltage--e.g. instantaneously after switching--but that it is preferred for the capacitors to remain charged, at least while disconnected from the load, to more than 90%, and particularly preferable 99% or more of the mean voltage during operation, i.e. during a voltage pulse.

"In the inventive method according to the first aspect of the invention the switching elements are operated to provide the load output with a required output voltage level. If a first voltage is required, then the first capacitor may be connected in parallel to the load output, and if a second output voltage level is required, the second capacitor will be connected in parallel to the load output. At the same time, it is preferred to operate the main power supply unit to deliver an output current corresponding to the desired output voltage level so that at the start of a pulse the capacitor delivers the desired voltage, but with the minimal necessary delay the converter then continues to deliver the voltage.

"Further, it is preferred that a capacitor currently not connected to the load output be switched off to essentially avoid discharge.

"According to a first aspect of the invention there is thus provided a circuit and operation method well suited to deliver exact current pulses at different voltage levels to the load output. There are a number of possible advantageous developments of the basic idea, to which the dependent claims relate.

"It is possible to connect not only one, but several output units to the main power supply unit. One or more of the further output units may comprise at least one switched capacitor unit, and it is also possible to provide further output units with two or more switched capacitor units. For example, there may be in total three loads present which are light sources of different color, e.g. red, green and blue, which are operated in sequential pulses, where the pulse height is different for the three light sources. Additionally, one light source--the one connected to the output unit with two switched capacitor units--may be driven with sequential pulses at different voltage levels. A corresponding driving scheme and circuit will be described in detail with regard to the preferred embodiment of the invention.

"It is preferred that the circuit comprises control means to control the switching elements, i.e. main switching element, load switching element and capacitor switching elements. Such a control means may be a single unit responsible for all switches, but it is also possible to divide the functionality of the control means between several units, e.g. one controlling the main switching element and another controlling load switching elements and capacitor switching elements. The units used as control means may be dedicated electric circuits, or a microcontroller or microprocessor programmed for the control task.

"It is especially preferred for the load switching element and the capacitor switching elements to be controlled synchronously, i.e. that upon activation of a load output both the corresponding load switching element and capacitor switching element are activated, and that upon deactivation both the load switching element and capacitor switching element are deactivated. By this operation, the inactive capacitor remains essentially charged to the desired voltage level, because no discharge occurs if it is switched off during deactivation of the output unit. In case of plural switched capacitor units per output unit, the load switching element is controlled synchronously with a selected one of the first and second capacitor switching elements, i.e. either one or the other.

"According to the second aspect of the invention, there is also provided a main power supply unit with a voltage input, at least one main switching element and at least one reactive element. This main power supply unit is a switched mode converter, where the switching element is controllable to deliver an output voltage or current. As known to the skilled person, the main power supply unit may comprise any of the known switched mode converter topologies and modes of operations.

"According to the second aspect, there are a plurality of output units connected to the main power supply unit. The output unit comprises a load output, to which a load, such as e.g. an LED, OLED or laser diode may be connected. The output units further each comprise a load switching element connected to the load output to connect the main power supply unit to the load output, i.e. to switch the output voltage delivered by the converter on or off at the load output. While in principal other configurations could be used, it is preferred for the load switching element to be connected to the main power supply unit in series with the load output.

"According to the second aspect of the invention, each output unit further comprises a switched capacitor unit. The switched capacitor unit comprises at least a capacitor and a corresponding switching element. Again, while other configurations are possible, it is preferred for the switching element to be connected in series to the capacitor, and for the switched capacitor units to be connected in parallel to the load output and/or the load switching element.

"According to the second aspect of the invention, the power supply circuit comprises a control means to control the load switching elements and the capacitor switching elements of the different output units such that in each output unit the load switching element is controlled synchronously with the capacitor switching element, i.e. that upon activation of a load output both the corresponding load switching element and capacitor switching element are activated, and that upon deactivation both the load switching element and capacitor switching element are deactivated. By this operation, the inactive capacitor remains essentially charged to the desired voltage level, because no discharge occurs if it is switched off during deactivation of the output unit.

"This ensures that in each output upon activation of the load (by connecting the load to the output voltage delivered from the main power supply unit) the capacitor is connected to buffer the output voltage and instantaneously deliver an output voltage at the desired output voltage level, even if the main power supply unit may take some time to adjust to the desired voltage level. On the other hand, by simultaneously switching off both switching elements, it is ensured that the capacitor remains essentially charged to the desired voltage level.

"Also for the second aspect, it is possible to connect not only one, but several output units to the main power supply unit as described above. Also the control means may be implemented as described above.

"According to the second aspect of the invention there is thus provided a circuit and operating method well suited to drive a plurality of loads with exact current pulses."

For more information, see this patent: Otte, Rob; Peeters, Henricus Marie; Van Den Broek, Vincent Gerardus Petrus Cornelis; Deppe, Carsten. Power Supply Circuit Having Switched Capacitor Units. U.S. Patent Number 8773087, filed November 6, 2008, and published online on July 8, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8773087.PN.&OS=PN/8773087RS=PN/8773087

Keywords for this news article include: Electronics, Laser Diodes, Koninklijke Philips N.V..

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Source: Electronics Newsweekly


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