Abstract:
A method of operating a power controller includes assigning switching offset times to a plurality of channels of a multi-channel power controller, and adjusting a state change time of a selected channel by the switching offset time associated with the selected channel such that the selected channel and another of the plurality of channels multiple channels do not undergo the state change simultaneously.

Description:
BACKGROUND 
       [0001]    This disclosure relates to power control, and more specifically to a method of operating a multi-channel power controller. 
         [0002]    Power controllers are known that include a plurality of channels, each channel being connected to a load. When a channel is turned ON or OFF an EMI spike can be produced. If multiple channels are turned ON or OFF simultaneously the EMI spikes can aggregate, injecting current into system circuitry or increasing the peak EMI emissions from the product. 
       SUMMARY 
       [0003]    A method of operating a power controller includes assigning switching offset times to a plurality of channels of a multi-channel power controller, and adjusting a state change time of a selected channel by the switching offset time associated with the selected channel such that the selected channel and another of the plurality of channels multiple channels do not undergo the state change simultaneously. 
         [0004]    A method of operating a multi-channel power controller includes assigning switching offset times to channels of a multi-channel power controller, turning ON a selected channel at a first position in a half-cycle of an AC waveform, and turning OFF the selected one of the plurality of channels after a channel timer reaches a predefined turn OFF time. At least one of the turn ON time of step (B) or the turn OFF time of step (C) is adjusted by the switching offset time of the selected channel. 
         [0005]    A multi-channel power controller includes a plurality of power control channels. Each power control channel has a switching offset time such that at least one of a turn ON and a turn OFF time for each of the channels is staggered, and such that each channel is turned ON for an amount of time determined by at least one timer. 
         [0006]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  schematically illustrates a multi-channel power controller operable to implement a switching offset time for each of its power control channels. 
           [0008]      FIG. 2  schematically illustrates an example AC waveform. 
           [0009]      FIG. 3  schematically illustrates reverse-phase control dimming applied to the AC waveform. 
           [0010]      FIG. 4  schematically illustrates reverse-phase control dimming of the AC waveform with a switching time offset. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  schematically illustrates a multi-channel power controller  10  operable to implement switching offset time for each of its power control channels  12   a - n . Each channel  12  is coupled to a load  14   a - n . A master controller  16  is operable to selectively control a flow of AC current from AC mains  18  to each of the loads  14   a - n . The master controller  16  includes at least one zero cross detection circuit  20  and at least one timer  22 . The zero cross detection circuit  20  is operable to detect zero crossings in AC waveforms from the AC mains  18 . 
         [0012]    Each channel has a slave controller  24  and a fault detection circuit  26 . Each fault detection circuit  26  includes a current sensing resistor  28  and a current threshold comparison circuit  30 . A voltage across current sensing resistor  28  is proportional to an output current flowing to each load  14 . Comparison circuit  30  compares this output current to a threshold, and turns OFF the channel  12  in response to the output current exceeding the threshold. Each slave controller  24  is in communication with its fault detection circuit  26 , and is operable to implement commands received from the master controller  16 . 
         [0013]    The multi-channel power controller  10  further includes a wireless receiver  32  that is operable to receive wireless control signals  34  to turn one or more of the channels  12  ON or OFF, or to dim one or more of the channels  12 . In one example, the wireless signals  34  are received from a switch  36 . In one example the switch  36  is a self-energizing switch that is batteryless and is operable to harvest mechanical energy from a switch actuation to transmit the wireless signals  34 . Of course, wired signals, and switches that are not self-energizing could also be used. 
         [0014]    If multiple channels  12  are turned ON or OFF simultaneously, EMI from the channels  12  can aggregate, injecting current into one or more an adjacent channels, potentially tripping one of the fault detection circuits  26 . To minimize the effect of EMI, the master controller  16  implements a switching offset time for each of the channels  12 , as will be described in reference to  FIGS. 2-4  below. 
         [0015]      FIG. 2  schematically illustrates an example AC waveform  40  that has a plurality of zero crossings  42  and a period  44 . The duration of each half cycle is shown as time  46 , which is half of the period  44 . 
         [0016]      FIG. 3  schematically illustrates reverse-phase control dimming applied to the AC waveform  40 , which may be used to achieve dimming if one or more of the loads  14  was a lighting source. In reverse-phase control, a channel  12  is turned ON at a zero crossing  42 , when voltage is equal to zero, but are turned OFF at a higher non-zero voltage at time  50  such that a ratio of time  50  to time  46  corresponds to an amount of dimming. 
         [0017]      FIG. 4  schematically illustrates reverse-phase control dimming with a switching offset time for the multi-channel channel power controller  10 , in which a turn ON and a turn OFF time for each of the plurality of channels  12  are offset by implementing a switching delay time. However, as will be described below, a switching advance time would also be possible. At least one of the master controller  16  or one of the slave controllers  24  assigns a switching delay to each of the channels  12 . 
         [0018]    The switching offset time may be determined using equation #1 below. 
         [0000]        t   offset =( N− 1)*Δ t   min     —     offset   equation #1
 
         [0019]    where N is a channel number;
       Δt min     —     offset  is a minimum switching offset time; and   t offset  is a switching offset time for a selected channel.       
 
         [0022]    As shown in equation #1, each switching offset time t offset  may be a multiple of the minimum offset time Δt min     —     offset . In one example, the minimum offset time Δt min     —     offset  is within the range of 4 μs-8 μs. Of course, other minimum offsets could be used. The minimum offset time Δt min     —     offset  is selected such that Δt min     —     offset  is short enough to prevent a viewer from detecting the offset. That is, the offset is not perceptible to a human, such that if all eight channels are turned ON or OFF at their staggered times a human would not detect the implementation of the offset. The minimum offset time Δt min     —     offset  also selected to be long enough so that an EMI spikes from multiple channels turning ON or OFF do not aggregate to trip a fault detection circuit  26  for one of the plurality of channels  12  or increase radiated or conducted EMI to unacceptable levels. 
         [0023]    In the example of  FIG. 4 , it is assumed that there are eight channels  12   a - h  in the power controller  10 . Of course, this is only an example, and other quantities of channels could be used. 
         [0024]    As shown in  FIG. 4 , channel  12   a  (first channel) is turned ON at time  60   a , which has an offset time of zero (t offset =(1−1)*Δt min     —     offset =0). Time  60   a  also corresponds to a zero crossing  42 . Channel  12   a  is turned OFF at time  60   b , resulting in a total ON time of  50  for the half-cycle. 
         [0025]    Channel  12   b  (second channel) is turned ON at time  62   a , which has an offset time of (t offset =(2−1)*Δt min     —     offset ). Channel  12   b  is turned OFF at time  62   b , which still results in a total ON time of  50  for the half-cycle. 
         [0026]    Channel  12   c  (third channel) is turned ON at time  64   a , which has an offset time (t offset =(3−1)*Δt min     —     offset ). Channel  12   b  is turned OFF at time  64   b , which still results in a total ON time of  50  for the half-cycle. 
         [0027]    Channels  12   d - h  are turned ON and OFF at times  66   a - b ,  68   a - b ,  70   a - b ,  72   a - b  and  74   a - b  respectively, such that each channel  12   d - h  still has a total ON time of  50  for the half-cycle. 
         [0028]    A turn OFF time for each of the eight channels  12   a - h  may be determined by the timer  22  in the master controller  16 , or may be determined by a timer in the slave controllers  34 , for example.  FIG. 4  illustrates each channel  12   a - h  turning ON and turning OFF in the same order, however it is understood that this would not be required and that other configurations would be possible. For example, channel  12  could be the first channel to turn ON and could be the last channel to turn OFF. 
         [0029]    Although a switching delay time has been described, it is understood that the offset could correspond to a switching advance time such that a switching time is advanced instead of delayed. For example, the offset time could be subtracted from a predicted future occurrence of an AC zero crossing such that the switching is performed in advance of the AC zero crossing instead of being performed after the AC zero crossing. 
         [0030]    Although an eight channel power controller has been discussed, it is understood that other quantities of channels could be used. Also, it is understood that reverse-phase control is only an example method of controlling a load, and that other methods, such as forward-phase control could be used. Additionally, although AC waveforms have been discussed, the offset switching described above could be useful in other pulse-width modulated power control schemes such as DC lighting control. 
         [0031]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.