Method and apparatus for power control by frequency spreading

1 METHOD AND APPARATUS FOR POWER CONTROL BY FREQUENCY SPREADING

FIELD OF THE INVENTION

The present invention relates to control of power supplied to various devices such as LEDs using a frequency spreading technique.

BACKGROUND OF THE INVENTION

Various circuits are known to control the supply of power, voltage, and/ or current to electrical devices such as light sources of the LED and incandescent type. The supply of power is controlled for various reasons, such as dimming the light output of the light source. One technique used to control dimming of these types of light sources is pulse width modulation (PWM). In implementing the PWM technique the power supply produces a pulse whose time duration (width) is controlled during a fixed period of time. This is done repetitively. The pulse width relative to the fixed time is referred to as the duty cycle. This is shown in FIG. 1 in which the repetitive fixed period is between time points A and B and the power pulse P can have a duty cycle time of any percentage of the time between these two points. When PWM is used to control the output of a light source, the rate or frequency of the fixed time period repetition is selected so that the light source is turned on and off at a rate high enough so that the thennal lag effect of the filament inside of an incandescent lamp bulb or the elements of an LED and the persistence of vision of a human integrate out so that the human eye carmot perceive the light source turn on and off or flicker.

In the control of the lighting devices using PWM, the longer the duration of the power pulse P (that is, the larger its duty cycle), the brighter the light output will be. For example, if the pulse duty cycle is 50% of the fixed time period, the source light output will be about 50% of its rated value or, conversely, dimmed by 50%. By selecting the pulse P duty cycle using PWM, the dimming of the light source can be controlled to any desired level.

While control of the light source output can be successfully accomplished using PWM, a significant disadvantage exists. This is due to the fact that repetitive production of the power pulse P is often at a frequency such that its harmonics can cause interference with the radio frequency spectrum. Accordingly, a need exists to be able to supply power to devices (such as light sources, as well as other devices) in a manner such that the output of the devices can be controlled (such as dimming of a light source output), but without having the problem of possibly producing interference with the radio frequency spectrum.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the invention, a method and apparatus are provided for producing output power in a variable manner to control various types of devices, such as light sources. The method and apparatus of the invention operate to control the total energy delivered to the device during a fixed time period in a pseudo random manner. In a preferred embodiment a pseudo random code is used to control the energy supplied to the device during a fixed time period. A binary type pseudo random code is defined as one that produces a series of one and zero bits in a random manner but is detenninistic. In the invention, the pseudo random code is used so that when a one bit occurs the device is supplied power. Of course, the opposite approach can be used in that a zero bit will cause supply

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of power to the device. In operation, over a fixed time period or cycle, the pseudo random code will have a number of one bits whose total time of occurrence over a cycle will be equal to that of a duty cycle corresponding to the desired power output. For example, assuming that a light source such as an TED is being powered and a 50% light output or dimming is required, the number of one bits in the pseudo random code sequence will have a time duration that would correspond to the 50% duty cycle obtained using the PWM technique. The pseudo random code sequence is repeated over time at a rate such that there is no flicker perceived by the human. The invention can produce power output over a range corresponding to 0-100% by applying a pseudo random code sequence for each selected step within the range. For example, if the power output is to be selected in increments of 1%, there would be one hundred different pseudo random code sequences. Using PWM, this would correspond to different duty cycles from 1 to 100%. The different code sequences can be produced by algoritlnns that are stored in a memory (such as a ROM or read only memory) and then individually selected from the ROM and used to operate a circuit (such as a serial feedback shift register) that produces the one and zero bits that control the supply of the power to the device.

Supplying power in accordance with the invention significantly reduces or even eliminates the problem of interference with the radio frequency spectrum since the one bits that apply the power are nonnally spread out over the entire time of a power cycle and are of a shorter time duration than that of the single pulse produced using the PWM technique.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention become more apparent upon reference to the following specification and annexed drawings in which:

FIG. 1 is a drawing showing the application of the PWM technique;

FIG. 2 is a drawing showing a typical pseudo random code sequence; and

FIG. 3 is a schematic diagram of a circuit showing operation of a device using the frequency spreading technique of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a typical pseudo random code sequence that is produced by any suitable circuit, such as a shift register, during a fixed time periodbetween the pointsA and B. In FIG. 2, the dark vertical bars 12 correspond to the one bits of the code. In the code sequence a number of one bits can be produced sequentially together, such as shown by the wider dark bar 14. The lighter color vertical bars 16 correspond to the zero bits of the code. The zero bit bars 16 also can be of different width depending upon the number of zero bits that are produced sequentially in the code. The basic idea is that the total number of one bits that occur within the fixed period when all grouped together would correspond in time to the amount of energy that is to be supplied to the device over the fixed time period. To consider it from another point of view, the ratio of the number of one bits in a code sequence to the number of zero bits sets the power level. For example, an equal number of one and zero bits during a cycle would produce a 50% power output level or 50% dimming where the device being controlled is a light source.

Different levels of power can be produced by selecting different codes. For example, if a power level of 60% is desired, then a code having 60% of one bits over a cycle