Line frequency detector

A line frequency detector receives an input signal representing a power source and detects a line frequency of the power source based on the input signal. The line frequency detector includes a first band pass filter having a pass band centered at an upper end of an expected frequency range of the power source and a second band pass filter having a pass band centered at a lower end of the expected frequency range. The input signal is filtered by the first and second band pass filters, generating a first characteristic signal and a second characteristic signal. The line frequency detector determines a characteristic ratio between the first characteristic signal and the second characteristic signal, and maps the characteristic ratio to the line frequency of the power source.

BACKGROUND

The present disclosure relates to line frequency detectors and, in particular, to detecting the line frequency of a power source for a dimmable solid-state lighting device.

Solid-state lighting devices can be connected to a wall dimmer to provide a user with a way of varying the brightness of the lighting device. A wall dimmer typically receives electric power as a sinusoidal waveform (e.g., an AC power source at a line frequency of 50 Hz or 60 Hz) and performs phase cutting on the waveform to generate an input voltage for the lighting device. A controller in the solid-state lighting device measures the phase angle of the phase cut and causes a driver circuit to generate an output current at a corresponding power level for a solid-state light source (e.g., a light-emitting diode) in the lighting device.

One drawback to these types of dimmable solid-state lighting devices is that they are sensitive to changes in the line frequency. A change in line frequency can lead to a significant malfunction in the lighting device if the controller is unable to detect a change in frequency and make corresponding adjustments when generating the output current.

SUMMARY

A line frequency detector receives an input signal representing a power source and determines the line frequency of the power source. The input signal is sent through two band pass filters arranged in parallel. The pass band of the first filter is centered approximately at the high end of an expected range for the frequency of the input signal. The pass band of the second filter is centered approximately at the low end of the expected frequency range. In addition, both pass bands are narrow enough that any higher order harmonics of the input signal are substantially attenuated. Thus, the outputs of the two filters are sinusoidal signals at the frequency of the input signal.

The filtered signals are rectified and sent through low-pass filters to create two characteristic signals that both have substantially constant values. The frequency responses of the two band pass filters are configured so that the ratio between the two characteristic signals represents the line frequency. The line frequency detector calculates the ratio and uses the ratio to determine the line frequency of the power source.

In one embodiment, the line frequency detector is part of a controller for a solid-state lighting device, and the controller includes other components that monitor the output of the line frequency detector and generate a control signal for a driving circuit for the solid-state light source in the lighting device.

DETAILED DESCRIPTION

FIG. 1is a block diagram illustrating a line frequency detector100, according to one embodiment. In the illustrated embodiment, the line frequency detector100includes an analog-to-digital converter (ADC)104, a decimator108, band pass filters112, rectifiers116, low pass filters120, a characteristic ratio calculator124, and a mapping module128. At a high level, the line frequency detector100is a component of a controller in a solid-state lighting device. The detector100receives an analog input signal102that represents a power source and outputs the line frequency of the power source. The line frequency can then be used by other components of the controller.

The power source is a source of electrical power that oscillates at a line frequency. For example, if the power source is an alternating current source from a typical electrical power grid, the power source will have a line frequency between 50 Hz and 60 Hz and a line voltage between 100 V and 240 V. In other embodiments, the power source may be an engine-generator or a generator powered by some other means. In these embodiments, the power source may have a different line frequency than a power grid.

The power source can be transformed and distorted in several different ways before it is sent to the line frequency detector100as the analog input signal102. In one embodiment, the power source is rectified (e.g., with a bridge rectifier), and the analog input signal102is thus a rectified version of the power source. The power source may additionally be reduced to a lower voltage (e.g., with a transformer). Furthermore, the power source can be coupled to the lighting device via a wall dimmer. In this case, the analog input signal102can include distortion due to dimmer multi-firing or due to the phase cut introduced by the dimmer.

In one embodiment, the band pass filters112and the low pass filters120are digital filters. In this case, the analog-to-digital converter (ADC)104and the decimator108operate together to transform the analog input signal102into a digital input signal110that is suitable for the remaining components112through128of the line frequency detector100. However, in another embodiment, the band pass filters112and low pass filters120are analog filters. In this case, the ADC104and decimator108are omitted from the line frequency detector100.

The ADC104samples the analog input signal102to convert the analog input signal102into a digital output106. Various methods of performing analog-to-digital conversion are widely known in the art and a detailed description thereof will be omitted for the sake of brevity.

In some embodiments, the ADC104is configured to sample the analog input signal102at a higher sampling rate, and the line frequency detector100includes a decimator108that generates the input signal110by downsampling the digital output106of the ADC104. The decimator108includes a low-pass filter (e.g., to prevent aliasing) whose output is connected to a downsampler.

An arrangement that includes both the ADC104and the decimator108is advantageous, but not required, in embodiments where the line frequency detector is part of a lighting controller integrated circuit because the digital output106of the ADC104can be sent to other digital logic on the lighting controller (e.g., logic for detecting the dimmer turn-on time) at the higher sampling rate. Meanwhile, the input signal110(at the lower sampling rate) is used for the digital logic112through128of the line frequency detector100, which reduces the coefficients for the filters112A,112B and allows the filters112A,112B to occupy less physical space.

In other embodiments, the decimator108is omitted from the line frequency detector100to save space and the digital output of ADC104is used as the input signal110for the digital logic112through128. This configuration is particularly useful, for example, if the line frequency detector100is embodied as a discrete integrated circuit. However, this configuration can also be used in an embodiment where the line frequency detector100is part of a lighting controller or in embodiments where the line frequency100is used as part of some other system.

After the ADC104and (optionally) the decimator108generate the digital input signal110, the remaining components112through128of the line frequency detector operate together to determine a line frequency130of the corresponding power source. For ease of description, these remaining components will be described in conjunction with the plots illustrated inFIGS. 2A-2D.

The band pass filters112A,112B perform band pass filtering on the input signal110to generate filtered signals114A,114B. In the embodiment illustrated inFIG. 1, the band pass filters112A,112B are implemented with digital logic. For example, the band pass filters112A,112B may be infinite impulse response (IIR) filters.

The center frequencies of the pass bands of the first and second band pass filters112A,112B are selected so that they are approximately the upper and lower ends, respectively, of the expected range of the first harmonic frequency of the analog input signal102. In embodiments where the input signal102has been rectified, the fundamental frequency of the input signal110is twice the line frequency (that is, equivalent to the first harmonic frequency of the analog input signal102), and the band pass fitters112A,112B are tuned to approximately the upper and lower ends of a frequency range that is twice the expected range of the line frequency. Meanwhile, in embodiments where the input signal102has not been rectified, the band pass filters112A,112B are tuned to the ends of a frequency range that matches the expected range of the line frequency.

A plot202of example frequency responses204A,204B for the two band pass filters112A,112B is shownFIG. 2A. In the example ofFIG. 2A, the frequency response204A of the first band pass filter112A has a pass band centered at approximately 130 Hz, and the frequency response204B of the second band pass filter112B has a pass band centered at approximately 90 Hz. Thus, if the input signal102has been rectified, the example configuration shown inFIG. 2Acan detect line frequencies between 45 Hz and 65 Hz. Furthermore, the pass bands of both filters112A,112B are configured to be narrow enough that any higher ordered harmonics are substantially attenuated. Thus, the filtered signals114A,114B are substantially sinusoidal signals with the same frequency as the input signal110.

As can be seen inFIG. 2A, the two frequency responses204A,204B cross each other at a crossing point frequency of approximately 110 Hz. If the frequency of the input signal110is at this crossing point frequency, then the amplitudes of the two filtered signals114A,114B will be the same. Meanwhile, if the frequency of the input signal110is higher than the crossing point frequency, then the amplitude of the first filtered signal114A will be greater than the amplitude of the second filtered signal114B. Similarly, if the frequency of the input signal110is lower than the crossing point frequency, then the amplitude of the first filtered signal114A will be less than the amplitude of the second filtered signal114B. In addition, it can be seen that the ratio between the amplitudes of the two filtered signals114A,114B will be different for any given frequency between the peaks of the two band pass filters112A,112B (e.g., between 90 Hz and 130 Hz). Thus, this ratio can be calculated and mapped to the corresponding line frequency, as described below.

Although the band pass filters112A,112B and their respective frequency responses204A,204B were described with reference to the plot202ofFIG. 2A, the frequency responses204A,204B shown in this plot202are merely exemplary. In other embodiments, the band pass filters112A,112B may be configured with different frequency responses as long as the ratio between the amplitudes of the two filtered signals114A,114B has the same characteristics. In other words, the crossing point frequency (shown inFIG. 2Aas 110 Hz) and the center frequencies (shown inFIG. 2Aas 90 Hz and 130 Hz) may have different values as long as the ratio between the amplitudes of the filtered signals114A,114B can be mapped back to a unique frequency of the input signal110.

Each filtered signal114A,114B is passed through a rectifier116A,116B and low pass filter120A,120B to generate a corresponding characteristic signal122A,122B. Together, each rectifier116and low pass filter120causes the corresponding characteristic signal122A,122B to settle to a substantially constant value that is proportional to the amplitude of the corresponding filtered signal112A,112B.

An example of two characteristic signals122A,122B is illustrated in the plot206ofFIG. 2B. In the example ofFIG. 2B, the input signal110had a frequency of approximately 94 Hz (which, since the input signal110is rectified, corresponds to a line frequency of 47 Hz in the power source). As can be seen in the plot202ofFIG. 2A, the value of the first frequency response204A at 94 Hz is approximately half the value of the second frequency response204B at 94 Hz. Thus, the first characteristic signal122A settles to a value that is approximately half the value of second characteristic signal122B.

The characteristic ratio calculator124calculates a characteristic ratio126between the two characteristic signals122A,122B. An example plot208of the characteristic ratio126is illustrated inFIG. 2C. In the embodiment ofFIG. 2C, the ratio126is calculated by dividing the first characteristic signal122A by the second characteristic signal122B. Thus, the ratio126shown in the example plot208settles to a value of approximately 0.5. In other embodiments, the characteristic ratio126can be calculated by dividing the second characteristic signal122B by the first characteristic signal122A or with some other formula that includes a ratio between the two characteristic signals122A,122B.

As described above, the ratio between the amplitudes of the filtered signals114A,114B corresponds to a unique line frequency within the expected range that is defined by the center frequencies of the band pass filters112A,112B. Since each characteristic signal122A,122B is proportional to the amplitude of the corresponding filtered signal114A,114B, the characteristic ratio126also corresponds to a unique line frequency within the expected line frequency range. As a result, the characteristic ratio126would change to a different value in response to any substantial change in the line frequency. For example, the characteristic ratio126in the example plot208would change to approximately 1.0 if the line frequency changes to 55 Hz, which corresponds to a frequency of 110 Hz in the rectified input signal110the crossing point frequency inFIG. 2A).

The mapping module128determines the line frequency corresponding to the characteristic ratio126. In one embodiment, the mapping module128uses a predetermined mapping function to determine the line frequency. An example plot210of a mapping function212between characteristic ratio126and line frequency is shown inFIG. 2D. In one embodiment, the mapping function212is generated by providing calibration signals (e.g., sinusoidal signals with a known frequency) as input to the line frequency detector100and recording the characteristic ratio that is generated for the calibration signal at each frequency. Regression analysis can then be performed to find a mapping function that represents the relationship between characteristic ratio and line frequency. For example, the mapping function212can be a third order polynomial function whose coefficients are calculated by performing a third-order polynomial regression. In another embodiment, the mapping function can be calculated analytically by calculating the ratio between the frequency responses of the two band pass filters at each of a plurality of frequencies within the expected range of the frequency of the input signal110.

If the characteristic ratio calculator124is implemented as a digital logic block, the characteristic ratio126is a series of discrete digital values. In some embodiments, the mapping module128does not map each discrete value of the characteristic ratio126to a line frequency. Instead, the mapping module128in these embodiments is configured to periodically sample the characteristic ratio126and map each sampled value of the characteristic ratio126to a line frequency130. For example, the mapping may be configured to map every tenth value or every 100thvalue of the ratio126. Alternatively, the mapping may be configured to map the ratio126to a line frequency130at predetermined time intervals (e.g., every 0.5 seconds, or every 1.0 seconds).

After the mapping module128determines a line frequency130, the line frequency130can be used as an input to other portions of the lighting controller. For example, a dimming factor circuit on the controller can use the line frequency130in conjunction with other inputs to determine a desired degree of dimming and generate a corresponding control signal for a driver circuit that powers a light emitting diode.

In an alternative embodiment, the ADC104and decimator108are omitted, and the other components112through128of the line frequency detector100are implemented as analog components. In another alternative embodiment, a portion of the components112through128are implemented as analog components, and the remaining components are digital logic. For example, the band pass filters112A,112B, rectifiers116A,116B, and low pass filters120A,120B are implemented as analog components, the characteristic ratios122A,122B are digitized with ADCs, and the remaining components124,128are implemented as digital logic.

FIG. 3is a flow chart illustrating a process300for detecting a line frequency using the line frequency detector100, according to one embodiment. The line frequency detector100receives302an analog input signal102, and the ADC104and decimator108digitize and decimate304the analog input signal102to create a digital input signal110.

The digital input signal110is sent to two band pass filters112A,112B. The first band pass filter112A performs306A high band pass filtering306A and the second band pass filter112B performs306B low band pass filtering to create filtered input signals114A,114B. As described above, the filtered signals114A,114B will both be substantially sinusoidal signals that have the same frequency as the digital input signal110, and the ratio between the amplitudes of the two filtered signals114A,114B will vary based on the frequency of the input signal.

The rectifiers116A,116B rectify308A,308B the filtered signals114A114B to create rectified signals118A,118B, and the low pass filters120A,120B perform310A,310B low pass filtering on the rectified signals118A,118B to create two characteristic signals112A,112B. The combination of the rectification and low pass filtering causes the two characteristic signals118A,118B to settle to substantially constant values that are proportional to the amplitudes of the corresponding filtered signals114A,114B.

The characteristic ratio calculator124receives the characteristic signals112A,112B and calculates312a characteristic ratio126. For example, the characteristic ratio calculator124may divide the first characteristic signal112A by the second characteristic signal112B. Since the characteristic ratio126corresponds to a unique line frequency within the frequency range of the detector100, the mapping function128can then map314the characteristic ratio126back to the line frequency130. The detected line frequency130can then be used by other digital logic on the solid-state lighting controller.

The line frequency detector100and the corresponding method300described herein provide several advantages. First, since the band pass filters112A,112B pass the fundamental frequency but attenuate the higher order harmonics of the input signal110, most forms of distortion in the input signal110do not have any significant effect on the characteristic ratio126or the detected line frequency130. Thus, the line frequency detector100can reliably detect the line frequency of a power source even if the input signal102is subject to multi-firing or if the wall dimming unit introduces a phase cut into the signal102. In addition, the line frequency detector100is not affected by changes in the amplitude of the input signal104because the characteristic signals122A,122B are divided by each other to calculate the characteristic ratio126. As a result, the line frequency detector100can be used with power sources at various line voltages without any significant modifications.

The line frequency detector100in the embodiments described above is implemented as part of a controller in a dimmable solid-state lighting device. However, in other embodiments, the line frequency detector100can be part of a different system or device in which it would be advantageous to monitor the frequency of a signal. The line frequency detector100can also be implemented as a standalone integrated circuit that receives the analog input signal102at one or more external pins and outputs the line frequency130at one or more external pins. Alternative, some or all of the components104through128of the line frequency detector100can be implemented as discrete electronic components that are connected together in the manner shown inFIG. 1.

Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for a line frequency detector. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein.